EP4022102A1 - Characterization method of formability properties of zinc alloy coating on a metal substrate - Google Patents

Characterization method of formability properties of zinc alloy coating on a metal substrate

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Publication number
EP4022102A1
EP4022102A1 EP20764085.5A EP20764085A EP4022102A1 EP 4022102 A1 EP4022102 A1 EP 4022102A1 EP 20764085 A EP20764085 A EP 20764085A EP 4022102 A1 EP4022102 A1 EP 4022102A1
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EP
European Patent Office
Prior art keywords
zinc
alloy coating
zinc alloy
phases
metal substrate
Prior art date
Legal status (The legal status is an assumption and is not a legal conclusion. Google has not performed a legal analysis and makes no representation as to the accuracy of the status listed.)
Pending
Application number
EP20764085.5A
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German (de)
French (fr)
Inventor
Yutao Pei
Masoud Ahmadi
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Rijksuniversiteit Groningen
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Rijksuniversiteit Groningen
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Publication of EP4022102A1 publication Critical patent/EP4022102A1/en
Pending legal-status Critical Current

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Classifications

    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C2/00—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor
    • C23C2/04—Hot-dipping or immersion processes for applying the coating material in the molten state without affecting the shape; Apparatus therefor characterised by the coating material
    • C23C2/06—Zinc or cadmium or alloys based thereon
    • C—CHEMISTRY; METALLURGY
    • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
    • C22C—ALLOYS
    • C22C18/00—Alloys based on zinc
    • C22C18/04—Alloys based on zinc with aluminium as the next major constituent
    • C—CHEMISTRY; METALLURGY
    • C23—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; CHEMICAL SURFACE TREATMENT; DIFFUSION TREATMENT OF METALLIC MATERIAL; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL; INHIBITING CORROSION OF METALLIC MATERIAL OR INCRUSTATION IN GENERAL
    • C23C—COATING METALLIC MATERIAL; COATING MATERIAL WITH METALLIC MATERIAL; SURFACE TREATMENT OF METALLIC MATERIAL BY DIFFUSION INTO THE SURFACE, BY CHEMICAL CONVERSION OR SUBSTITUTION; COATING BY VACUUM EVAPORATION, BY SPUTTERING, BY ION IMPLANTATION OR BY CHEMICAL VAPOUR DEPOSITION, IN GENERAL
    • C23C30/00—Coating with metallic material characterised only by the composition of the metallic material, i.e. not characterised by the coating process

Definitions

  • the invention relates to a method for the characterisation of formability properties of a zinc alloy coating on a metal substrate and a metal substrate comprising a zinc alloy coating.
  • Zinc-coated steels produced via hot-dip galvanization (HDG) process are regarded as the vital materials for household, construction and automotive industries.
  • the resultant ZnAIMg coatings offer superior corrosion resistance and friction/wear performance.
  • these recent hot-dip ZnAIMg coatings currently deliver rather low cracking resistance once subjected to forming processes.
  • Microstructural damages of ZnAIMg coatings lead to cracking in severely deformed areas and result in the formation of large cracks and subsequently deteriorate the in- service corrosion resistance.
  • EP33698308 for example, a zinc alloy plated steel sheet with good bending workability was disclosed, having a microstructure of a ZnAIMg coating with greater than 50% of the primary zinc phases having a preferred zinc orientation [0001]
  • JP2010255084 the crack resistance was improved by adding 0.005 to 0.2% by weight of nickel to a metal coating further comprising 1 to 10% by weight of aluminum and 0.2 to 1% by magnesium weight to alter the microstructure of the alloy.
  • a method for the characterisation of formability properties of a zinc alloy coating on a metal substrate is provided.
  • the zinc alloy coating containing one or more alloying elements selected from the group consisting of Mg, Al, Ni each with a content of at least 0.3 weight % and at most 10 weight %, optionally one or more additional elements selected from the group consisting of Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the metallic coating is less than 0.3 weight %, inevitable impurities, the remainder being zinc,
  • the zinc alloy coating having a microstructure comprising a primary phase and a binary eutectic and/or ternary eutectic phase
  • Electron Backscatter Diffraction EBSD
  • n crystallographic orientation-dependent strain hardening exponent
  • the crystallographic orientation-dependent strain hardening exponent (n) may be determined for all phases of the microstructures, of, e.g. primary zinc phase, binary eutectic phase and ternary eutectic phase.
  • the crystallographic orientation-dependent strain hardening exponent (n) is at least determined for the primary zinc phase.
  • the tendency for cracking of the zinc alloy is related to the micro ductility of the coating.
  • the method according to the invention is a solution to reveal and analyse which microstructural phases are most detrimental for crack initiation.
  • the crystallographic orientation-dependent strain hardening exponent (n) can be obtained directly from EBSD or via a combination of EBSD and nanoindentation tests.
  • EBSD is well known in the field and related experimental settings are described in IS013067:2011.
  • the zinc alloy has a hexagonal close-packed crystal structure (HPC). This HPC packing of the zinc alloy allows to determine the strain hardening exponent (n) with respect to the crystal orientation from EBSD measurements, which surprisingly provided a good indication for cracking.
  • the binary eutectic phase exhibited on average lower strain hardening exponents then the primary zinc phase or ternary eutectic phase, which corresponds to the general observation that the binary eutectic phase is more prone to cracking.
  • the cracking tendency between primary zinc phase with different strain hardening exponents can also be derived from the method according to the invention.
  • the crystallographic orientation-dependent strain hardening exponent (n) provides a way to describe the cracking tendency for all phases in the zinc alloy coating of the metal substrate, and shows that a microstructure with a low n value has a higher likelihood to show cracking.
  • the method according to the invention may be used for a zinc alloy coating on any metal substrate, e.g. a metal or metal alloy, but in most cases will be a steel strip or a steel blank.
  • a metal substrate e.g. a metal or metal alloy
  • BH Bake Hardening
  • D&l Drawn and Wall Ironing
  • a Schmid factor (m) of the zinc alloy coating microstructure is determined.
  • the Schmid factor (m) may be determined for all phases of the microstructures, e.g. primary zinc phases, binary eutectic phase and ternary eutectic phase.
  • the Schmid factor (m) is at least determined for all primary zinc phase. It was found by the inventors that the Schmid factor gave an additional indication for the cracking tendency of the coating, especially when considering the primary zinc phase under strain conditions. It was found that the primary zinc phase with a high Schmid factor can bear the imposed deformation without cracking. Hence the Schmid factor provides an additional indication for the cracking tendency of zinc alloys on a metal substrate under strain.
  • the Schmid factor was determined from the EBSD analysis.
  • the Schmid factor (m) map of the areas was calculated.
  • the principle zinc slip systems including all the equivalent crystallographic families were considered and the corresponding Schmid maps were calculated and plotted according to tensile principle stress tensor along loading direction. By this process, the Schmid factor (m) of each cracked and non-cracked zinc phase was determined.
  • an orientation angle (Q) between the c-axis of hep crystal of the primary zinc phase and the loading direction is determined.
  • the orientation angle (Q) was found to be a factor important for the deformation mechanisms of the zinc alloy.
  • This orientation angle (Q) can be obtained from the inverse pole figure (IPF) orientation maps generated on the EBSD data as well known to a person skilled in the art. Accordingly, for each primary zinc phase, orientation angle (Q) between the c-axis of HCP zinc crystal and the loading direction was determined. It was found that the orientation of the basal plane of the primary zinc phase was preferably oriented parallel to the tensile load direction, to reduce cracking.
  • the strain hardening exponent (n) is determined with nanoindentation tests (ISO 14577-4:2016).
  • the crystallographic orientation-dependant strain hardening exponent (n) associated with each indent was calculated using EBSD and raw nanoindentation data by the method described by Dao et al, Computational modeling of the forward and reverse problems in instrumented sharp indentation, Acta Mater. 49 (2001) 3899-3918. doi: 10.1016/S1359-
  • n 1.428x10- 8 0 4 -2.233x 10- 6 0 3 + 1.027x1 O 4 0 2 -4.631 x 1 O 4 0+O.29, wherein Q is ranging from 0 - 90 °, as determined with EBSD as described above.
  • the strain hardening exponent (n) of the primary zinc phases can be determined from Q, which significantly simplifies the method according to the invention as Q, and hence n can be easily acquired from EBSD measurements alone.
  • the variables a,b,c,d and e are characteristic for the zinc alloy coating and can be empirically determined from an indented sample. For each indented primary zinc phase, the orientation angle (Q) between the c-axis of HCP zinc crystal and the loading direction (surface normal for nanoindentation) as determined by EBSD was plotted versus the corresponding local strain-hardening index (n) as determined by nanoindentation. The procedure was repeated for all the indented zinc phases.
  • a metal substrate comprising a zinc alloy coating, the zinc alloy coating containing one or more alloying elements selected from Mg, Al, Ni each with a content of at least 0.3 weight % and at most 10 weight %, optionally one or more additional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the metallic coating is less than 0.3 weight %, inevitable impurities, the remainder being zinc, wherein the microstructure of the zinc alloy coating comprises primary zinc phases and a binary and/or ternary eutectic phase, wherein the primary zinc phases have a crystallographic orientation-dependant strain hardening exponent (n) of at least 0.29 as determined by the method as described above.
  • alloying elements selected from Mg, Al, Ni each with a content of at least 0.3 weight % and at most 10 weight %
  • additional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce
  • a metal substrate with a zinc alloy coating with a primary zinc phase with a higher n not only has a low tendency towards cracking, but may also rescue cracks initiated in other primary zinc phases or eutectic phases.
  • the primary zinc phases has a n of at least 0.29, preferably an n of at least 0.33, more preferably an n of at least 0.34, most preferably an n of at least 0.35.
  • a suitable upper limit for the n of the primary zinc phases is 1.00, as a primary zinc phase with a n above 1.00 would be unlikely to obtain according to standard production method.
  • the primary zinc phases of the zinc alloy coating has a Schmid factor m between 0.01 - 0.5, preferably between 0.33 - 0.5, more preferably between 0.34 - 0.5, most preferably between 0.35 - 0.5.
  • the cracking tendency of a primary zinc phase decreases with an increase in the Schmid factor.
  • At least 55% of the primary zinc phases of the zinc alloy coating have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 °, it was observed that primary zinc phases having a Q above 45° showed less cracking and were able to rescue initiated cracks in other phases.
  • the primary zinc phases preferably at least 55% have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 ° to ensure that the overall integrity of the zinc alloy coating is maintained, more preferably at least 65% of the primary zinc phases have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 °, most preferably at least 75% of the primary zinc phases have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 °.
  • the primary zinc phases of the zinc alloy coating have a crystallographic orientation-dependant strain hardening exponent n >0.33, preferably n>0.34, more preferably n>0.35. It was observed that primary zinc phases having n above 0.33 showed less cracking and were able to rescue initiated cracks in other phases. Without wishing to be bound by theory, it is believed that the different crystallographic orientation of primary zinc phases possess different magnitudes of local strain hardening exponent as a result of HCP mechanical anisotropy. Accordingly, if more than 75% of primary zinc phases in the zinc alloy have an unfavourable orientations with respect to loading direction, exhibiting n ⁇ 0.33, this will result in significant cracking during tensile/bending deformation;
  • the primary zinc phases preferably at least 75% have a n >0.33, preferably n>0.34, more preferably n>0.35 to ensure that the overall integrity of the zinc alloy coating is maintained, more preferably at least 80% of the primary zinc phases have a n >0.33, preferably n>0.34, more preferably n>0.35.
  • At least 55 % of the primary zinc phases have a Schmid factor m > 0.32, preferably m>0.33, more preferably m>0.35. It was observed that primary zinc phases having a m above 0.32 showed less cracking and were able to rescue initiated cracks in other phases.
  • the primary zinc phases preferably at least 55% have a m > 0.32, preferably m>0.33, more preferably m>0.35 to ensure that the overall integrity of the zinc alloy coating is maintained, more preferably at least 65% of the primary zinc phases have a m > 0.32, preferably m>0.33, more preferably m>0.35, most preferably at least 75% of the primary zinc phases have a m > 0.32, preferably m>0.33, more preferably m>0.35.
  • the primary zinc phases with a low Schmid factor (m ⁇ 0.32) lack a dislocation motion for plastic deformation, and therefore are more likely to experience cracking.
  • the zinc alloy coating comprises 5 to 35 % of a binary eutectic phase. It was also found by the inventors that most cracks initiated from the binary eutectic phase. The binary eutectic phase also showed a low n, on average below 0.10, showing a high tendency for cracking and a low tendency for rescuing cracks of neighbouring phases. Hence, the presence of binary eutectic phase is preferably maintained between 5 - 35%, more preferably between 5 - 20%, more preferably between 5 - 10%. In this range, initiated cracks in the binary eutectic phase can still be rescued sufficiently by the surrounding phases to maintain the overall integrity of the zinc alloy coating on the metal substrate.
  • the zinc alloy coating is free from binary eutectic phase. As most cracks initiate in the binary eutectic phase having a n well below 0.3, on average below 0.10, a zinc alloy coating free from binary eutectic phase is believed to have significantly less cracking.
  • the zinc alloy coating comprises 0.3 - 5 weight % Al and 0.3 - 5 weight % Mg, more preferably the zinc alloy coating comprises 1 - 2 weight % Al and 1-2 weight% Mg, as such a coating has improved corrosion resistance, stone chipping resistance as compared to conventional zinc coatings.
  • the zinc alloy coating is a hot dip coating.
  • the zinc alloy coating can be applied on the metal substrate in all common methods as known to a person skilled in the art
  • the metal substrate according to the invention preferably has preferably a zinc alloy coating made by hot-dip coating.
  • Hot-dip coating is well known to a person skilled in the art and has the advantage that it is a reliable and relatively cheap method to apply a zinc alloy coating to at least one side of the metal substrate.
  • the zinc alloy coating microstructure comprises phases with varying strain hardening exponents, wherein at least 70% of the phases with a crystallographic orientation-dependant strain hardening exponent n ⁇ 0.33 are surrounded by phases with a crystallographic orientation-dependant strain hardening exponent n > 0.33. It was found by the inventors that initiation of cracks in the alloy coating could either end up as a crack or could be rescued, leading to an overall acceptable coating layer. As elaborated already, the propagation of cracks involve phase boundaries. Hence, a crack could initiate in a phase with a low n, e.g. binary eutectic phase, and could propagate in a phase with a high n, e.g.
  • FIG. 1 Shows a schematic representation of microstructural cracking behaviour within ZnAIMg coatings.
  • FIG. 2 Shows Zn HCP crystal orientation with the loading parallel to the rolling direction (RD).
  • 2a shows Zn HCP crystal orientation favoured for cracking resistance, with a high m value (0.5)
  • 2b depicts Zn HCP crystal orientation favoured for cracking resistance, with a medium m value (0.32 - 0.45)
  • FIG 2 (c) shows Zn HCP crystal orientation leading to cracking.
  • FIG. 3 Depicts the EBSD results of a cracked area after 10% strain subjected to uniaxial tension, including image quality (IQ), IQ plus inverse pole figure (IPF), corresponding Schmid map and HCP crystals of the critical numbered phases.
  • IQ image quality
  • IPF IQ plus inverse pole figure
  • Schmid map HCP crystals of the critical numbered phases.
  • FIG. 4. Shows the principal zinc slip systems.
  • FIG. 5 Shows the n-value distribution for three samples (11, I2, C1) and their EBSD Image Quality Map
  • FIG. 1 Shows stage I of a cracking path that begun with a nucleation of tiny micro cracks in MgZn2 platelets of binary eutectic phase as the result of strain localization.
  • the binary eutectic phase has a low n of about 0.08 and is therefore prone to cracking.
  • stage (II) in FIG. 1
  • stage (II) in FIG. 1
  • the first circumstance is that the adjacent Zn phase exhibits an orientation close to [0001] perpendicular to the loading direction delivering Q > 60° and/or m > 0.32, in accordance to the claims.
  • the primary slip systems of Zn phase are activated by easy dislocation motion and subsequently ductile plastic deformation takes place instead of cleavage cracking (assuming the force direction is parallel to RD). Consequently, the crack is arrested in the primary zinc phase boundary.
  • the adjacent Zn phase possesses an orientation with HCP c-axis parallel to the loading direction (see Fig. 2c) and incorporates low m and n, serving as a favourable site for the propagating crack (e.g. cracked Zn phase No. 2 at stage III, Fig. 1).
  • FIG. 1 delivers the typical cracking mechanisms of ZnAIMg coatings; however, an individual crack may also form in a zinc phase with unfavorable orientation (m ⁇ 0.32) without prior nucleation in the binary eutectic phase.
  • the cracking behaviour of a zinc alloy coating can be easily measured and predicted by analysing n.
  • the other parameters, m and Q provide additional information to the cracking mechanism leading to a complete picture.
  • EBSD results were obtained according to the method according to the invention.
  • a metal substrate (HSLA grade steel) with a zinc alloy coating (Zn1.6AI1.6Mg) was prepared for nanoindentation and electron backscatter diffraction (EBSD).
  • EBSD electron backscatter diffraction
  • a square sample (1x1 cm) was cut from the coated metal sheets.
  • the surface of the zinc alloy coated samples was slightly mechanically polished using 1 pm diamond suspension and water-free lubricant on a Struers MD Nap disc for 3-5 minutes to obtain relatively smooth surface. Thereafter, the sample was flat ion milled for 15 minutes by means of an ion polisher (JEOL IB-19520CCP) in order to attain a good surface quality.
  • the in-situ SEM tensile and bending specimens were also prepared with the same procedure given above, before performing the tests.
  • Nanoindentation tests were conducted utilizing MTS Nano Indenter XP® with Berkovich tip using the continuous stiffness measurement (CSM) mode. Load controlled indentations with an applied force of 3 mN and spacing of 8 pm between the indents were performed on the coating microstructure.
  • CSM continuous stiffness measurement
  • EBSD measurements on the indented regions of the coating were conducted. For this, the sample was placed in a SEM chamber with 70° tilting angle. The region of interest was selected and a working distance of 12-15 pm was applied. A step size of 200 nm and accelerating voltage of 25 kV were used to acquire EBSD patterns. EBSD measurement was conducted on the cracked areas of specimens subjected to in-situ tensile and bending tests. The obtained EBSD results were analyzed by means of EDAX-TSL OIMTM Analysis 8 software. The image quality (IQ), inverse pole figure (IPF) orientation maps were generated on the EBSD data on the indented regions.
  • IQ image quality
  • IPF inverse pole figure
  • orientation angle (Q) between the c-axis of HCP zinc crystal and the loading direction (surface normal for nanoindentation) was determined and plotted versus the corresponding local strain-hardening exponent (n) of each labeled zinc phase which was achieved previously by nanoindentation.
  • the procedure was repeated for all the indented zinc phases.
  • a fourth order polynomial expression was fitted on the obtained cure to establish a relationship between Q and n.
  • FIG 4 shows the principle Zn slip systems including all the equivalent crystallographic families which were considered to calculate the corresponding Schmid maps, which was plotted according to tensile principle stress gradient along rolling direction (RD), as described also in R. Parisot, et al., Deformation and Damage Mechanisms of Zinc Coatings on Hot-Dip Galvanized Steel Sheets: Part I. Deformation Modes, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 35 A (2004) 797-811 and C. Tome et.al, The yield surface of hep crystals, Acta Metall. 33 (1985) 603-621.
  • RD tensile principle stress gradient along rolling direction
  • n 1.428xlO 8 0 4 -2.233xlO 6 0 3 + 1.027x10 4 0 2 -4.631xlO 4 0+O.29
  • Cracking quantification and fraction of phases were measured by image analysis It can been seen that the inventive samples 11 and I2, having at least 75% of primary phase crystals with an n > 0.33 leads to significantly less cracking, and shorter average crack length, than the sample C1 having less than 75% of the primary phase crystals with n > 0.33.

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Abstract

The present invention relates to a method for the characterisation of formability properties of a zinc alloy coating on a metal substrate and a metal substrate comprising a zinc alloy coating. The method for the characterisation of formability properties of a zinc alloy coating on a metal substrate, the zinc alloy coating containing one or more alloying elements selected from the group consisting of Mg, Al, Ni each with a content of at least 0.3 weight % and at most 10 weight %, optionally one or more additional elements selected from the group consisting of Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the metallic coating is less than 0.3 weight %, inevitable impurities, the remainder being zinc, the zinc alloy coating having a microstructure comprising a primary zinc phase and binary eutectic and/or ternary eutectic phases, wherein Electron Backscatter Diffraction (EBSD) is used to determine a crystallographic orientation-dependent strain hardening exponent (n) of the zinc alloy coating microstructure.

Description

Characterization method of formability properties of zinc alloy coating on a metal substrate
The invention relates to a method for the characterisation of formability properties of a zinc alloy coating on a metal substrate and a metal substrate comprising a zinc alloy coating.
Zinc-coated steels produced via hot-dip galvanization (HDG) process are regarded as the vital materials for household, construction and automotive industries. By addition of elements such as aluminum and magnesium to the conventional pure zinc coatings, the resultant ZnAIMg coatings offer superior corrosion resistance and friction/wear performance. Nevertheless, these recent hot-dip ZnAIMg coatings currently deliver rather low cracking resistance once subjected to forming processes. Microstructural damages of ZnAIMg coatings lead to cracking in severely deformed areas and result in the formation of large cracks and subsequently deteriorate the in- service corrosion resistance. Some studies have been undertaken to increase the cracking resistance of such coatings.
In EP3369838, for example, a zinc alloy plated steel sheet with good bending workability was disclosed, having a microstructure of a ZnAIMg coating with greater than 50% of the primary zinc phases having a preferred zinc orientation [0001]
In JP2010255084, the crack resistance was improved by adding 0.005 to 0.2% by weight of nickel to a metal coating further comprising 1 to 10% by weight of aluminum and 0.2 to 1% by magnesium weight to alter the microstructure of the alloy.
Although these patents provide some indication how to prevent cracking based on the microstructure, the formation mechanisms of these microstructural damages and hence, preventing measures, are not well understood.
This is hampered by the fact that a method to determine the formation mechanisms related to the microstructures is not yet well defined. Hence, there is a need for a method to assess the cracking tendency of zinc based coatings, especially ZnAIMg coatings, and to derive a metal substrate with a zinc alloy with a low tendency towards microcracking.
Therefore it is an objective of the invention to provide a method for the characterization of formability properties of a zinc alloy coating on a metal substrate.
It is another objective of the invention to provide a metal substrate with a zinc alloy coating with favorable formability properties. In a first aspect according to the invention there is provided a method for the characterisation of formability properties of a zinc alloy coating on a metal substrate
• the zinc alloy coating containing one or more alloying elements selected from the group consisting of Mg, Al, Ni each with a content of at least 0.3 weight % and at most 10 weight %, optionally one or more additional elements selected from the group consisting of Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the metallic coating is less than 0.3 weight %, inevitable impurities, the remainder being zinc,
• the zinc alloy coating having a microstructure comprising a primary phase and a binary eutectic and/or ternary eutectic phase,
• wherein Electron Backscatter Diffraction (EBSD) is used to determine a crystallographic orientation-dependent strain hardening exponent (n) of the zinc alloy coating microstructure.
The inventors surprisingly found that the method according to the invention providing a crystallographic orientation-dependent strain hardening exponent (n) as obtained from EBSD had a high correlation to the formation of crack initiation in the zinc alloy coating and therefore can be used as a measure for cracking. The crystallographic orientation-dependent strain hardening exponent (n) may be determined for all phases of the microstructures, of, e.g. primary zinc phase, binary eutectic phase and ternary eutectic phase. Preferably, the crystallographic orientation-dependent strain hardening exponent (n) is at least determined for the primary zinc phase. The tendency for cracking of the zinc alloy is related to the micro ductility of the coating. As micro ductility behaviours could be described with the strain hardening exponent (n) for all microstructures in the zinc alloy, the method according to the invention is a solution to reveal and analyse which microstructural phases are most detrimental for crack initiation. The crystallographic orientation-dependent strain hardening exponent (n) can be obtained directly from EBSD or via a combination of EBSD and nanoindentation tests.
EBSD is well known in the field and related experimental settings are described in IS013067:2011. The zinc alloy has a hexagonal close-packed crystal structure (HPC). This HPC packing of the zinc alloy allows to determine the strain hardening exponent (n) with respect to the crystal orientation from EBSD measurements, which surprisingly provided a good indication for cracking.
It was observed for example, that the binary eutectic phase exhibited on average lower strain hardening exponents then the primary zinc phase or ternary eutectic phase, which corresponds to the general observation that the binary eutectic phase is more prone to cracking. Moreover, the cracking tendency between primary zinc phase with different strain hardening exponents can also be derived from the method according to the invention. Hence, the crystallographic orientation-dependent strain hardening exponent (n) provides a way to describe the cracking tendency for all phases in the zinc alloy coating of the metal substrate, and shows that a microstructure with a low n value has a higher likelihood to show cracking.
It should be noted that in principle the method according to the invention may be used for a zinc alloy coating on any metal substrate, e.g. a metal or metal alloy, but in most cases will be a steel strip or a steel blank. For instance Interstitial Free (IF) steels and Bake Hardening (BH) steels for the manufacturing of automotive outer panels, or tinplate for Drawn and Wall Ironing (D&l) applications, for food and beverage packaging.
In a preferred embodiment a Schmid factor (m) of the zinc alloy coating microstructure is determined. The Schmid factor (m) may be determined for all phases of the microstructures, e.g. primary zinc phases, binary eutectic phase and ternary eutectic phase. Preferably, the Schmid factor (m) is at least determined for all primary zinc phase. It was found by the inventors that the Schmid factor gave an additional indication for the cracking tendency of the coating, especially when considering the primary zinc phase under strain conditions. It was found that the primary zinc phase with a high Schmid factor can bear the imposed deformation without cracking. Hence the Schmid factor provides an additional indication for the cracking tendency of zinc alloys on a metal substrate under strain. Without wishing to be bound by theory, the inventors believe that the Schmid factor is indicative of the principle slip system, a primary zinc phase with low Schmid factor serves as cracking site, whereas a primary zinc phase with high Schmid factor undergoes a deformation mechanism which stops the propagation of the cracks. This rescue mechanism was specifically observed for cracks propagating into the primary zinc phases.
The Schmid factor was determined from the EBSD analysis. The Schmid factor (m) map of the areas was calculated. For obtaining the Schmid factor (m) associated with each zinc phase, the principle zinc slip systems including all the equivalent crystallographic families were considered and the corresponding Schmid maps were calculated and plotted according to tensile principle stress tensor along loading direction. By this process, the Schmid factor (m) of each cracked and non-cracked zinc phase was determined.
In a preferred embodiment, an orientation angle (Q) between the c-axis of hep crystal of the primary zinc phase and the loading direction is determined. The orientation angle (Q) was found to be a factor important for the deformation mechanisms of the zinc alloy. This orientation angle (Q) can be obtained from the inverse pole figure (IPF) orientation maps generated on the EBSD data as well known to a person skilled in the art. Accordingly, for each primary zinc phase, orientation angle (Q) between the c-axis of HCP zinc crystal and the loading direction was determined. It was found that the orientation of the basal plane of the primary zinc phase was preferably oriented parallel to the tensile load direction, to reduce cracking.
In a preferred embodiment of the invention, the strain hardening exponent (n) is determined with nanoindentation tests (ISO 14577-4:2016). The crystallographic orientation-dependant strain hardening exponent (n) associated with each indent was calculated using EBSD and raw nanoindentation data by the method described by Dao et al, Computational modeling of the forward and reverse problems in instrumented sharp indentation, Acta Mater. 49 (2001) 3899-3918. doi: 10.1016/S1359-
6454(01)00295-6. As such the local mechanical properties of each individual phase within the coating could be determined.
In a preferred embodiment the strain hardening exponent (n) of the primary zinc phases is obtained from the orientation angle (0) via the following equation n=a04-b03+c02-d0+e, wherein
• 1.0x10 8 < a < 1.8x10 8
• 1.5x10 6 < b < 3x10 6
• 5x1 O 4 < c < 1.5x1 O 3
• 1 x104 < d < 8x1 O 4
• 0.27 < e < 0.32 preferably
• 1.2x10 8 < a < 1.6x10-8
• 2.0x10-® < b < 2.5x10-6
• 8x1 O 4 < c < 1.2x1 O 3
• 3x1 O 4 < d < 6x1 O 4
• 0.28 < e < 0.30 more preferably n =1.428x10-804-2.233x 10-603+ 1.027x1 O 402-4.631 x 1 O 40+O.29, wherein Q is ranging from 0 - 90 °, as determined with EBSD as described above.
By using this fourth order equation, the strain hardening exponent (n) of the primary zinc phases can be determined from Q, which significantly simplifies the method according to the invention as Q, and hence n can be easily acquired from EBSD measurements alone. The variables a,b,c,d and e are characteristic for the zinc alloy coating and can be empirically determined from an indented sample. For each indented primary zinc phase, the orientation angle (Q) between the c-axis of HCP zinc crystal and the loading direction (surface normal for nanoindentation) as determined by EBSD was plotted versus the corresponding local strain-hardening index (n) as determined by nanoindentation. The procedure was repeated for all the indented zinc phases. From these measurements, the exact numbers for the variables a, b, c, d and e can be obtained to establish a relationship between Q and n for the specific zinc alloy composition. After establishing the variables for the zinc alloy coating, n can be easily obtained from the samples by EBSD only.
In a second aspect according to the invention there is provided a metal substrate comprising a zinc alloy coating, the zinc alloy coating containing one or more alloying elements selected from Mg, Al, Ni each with a content of at least 0.3 weight % and at most 10 weight %, optionally one or more additional elements selected from among Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the metallic coating is less than 0.3 weight %, inevitable impurities, the remainder being zinc, wherein the microstructure of the zinc alloy coating comprises primary zinc phases and a binary and/or ternary eutectic phase, wherein the primary zinc phases have a crystallographic orientation-dependant strain hardening exponent (n) of at least 0.29 as determined by the method as described above.
It was found that a metal substrate with a zinc alloy coating with a primary zinc phase with a higher n not only has a low tendency towards cracking, but may also rescue cracks initiated in other primary zinc phases or eutectic phases. The primary zinc phases has a n of at least 0.29, preferably an n of at least 0.33, more preferably an n of at least 0.34, most preferably an n of at least 0.35. Although not particularly limited, a suitable upper limit for the n of the primary zinc phases is 1.00, as a primary zinc phase with a n above 1.00 would be unlikely to obtain according to standard production method.
In a preferred embodiment, the primary zinc phases of the zinc alloy coating has a Schmid factor m between 0.01 - 0.5, preferably between 0.33 - 0.5, more preferably between 0.34 - 0.5, most preferably between 0.35 - 0.5. The cracking tendency of a primary zinc phase decreases with an increase in the Schmid factor.
In a preferred embodiment at least 55% of the primary zinc phases of the zinc alloy coating have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 °, it was observed that primary zinc phases having a Q above 45° showed less cracking and were able to rescue initiated cracks in other phases. Hence, preferably at least 55% of the primary zinc phases have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 ° to ensure that the overall integrity of the zinc alloy coating is maintained, more preferably at least 65% of the primary zinc phases have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 °, most preferably at least 75% of the primary zinc phases have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 °.
In a preferred embodiment at least 55% of the primary zinc phases of the zinc alloy coating have a crystallographic orientation-dependant strain hardening exponent n >0.33, preferably n>0.34, more preferably n>0.35. It was observed that primary zinc phases having n above 0.33 showed less cracking and were able to rescue initiated cracks in other phases. Without wishing to be bound by theory, it is believed that the different crystallographic orientation of primary zinc phases possess different magnitudes of local strain hardening exponent as a result of HCP mechanical anisotropy. Accordingly, if more than 75% of primary zinc phases in the zinc alloy have an unfavourable orientations with respect to loading direction, exhibiting n <0.33, this will result in significant cracking during tensile/bending deformation;
Hence, preferably at least 75% of the primary zinc phases have a n >0.33, preferably n>0.34, more preferably n>0.35 to ensure that the overall integrity of the zinc alloy coating is maintained, more preferably at least 80% of the primary zinc phases have a n >0.33, preferably n>0.34, more preferably n>0.35.
In a preferred embodiment at least 55 % of the primary zinc phases have a Schmid factor m > 0.32, preferably m>0.33, more preferably m>0.35. It was observed that primary zinc phases having a m above 0.32 showed less cracking and were able to rescue initiated cracks in other phases. Hence, preferably at least 55% of the primary zinc phases have a m > 0.32, preferably m>0.33, more preferably m>0.35 to ensure that the overall integrity of the zinc alloy coating is maintained, more preferably at least 65% of the primary zinc phases have a m > 0.32, preferably m>0.33, more preferably m>0.35, most preferably at least 75% of the primary zinc phases have a m > 0.32, preferably m>0.33, more preferably m>0.35. The primary zinc phases with a low Schmid factor (m<0.32) lack a dislocation motion for plastic deformation, and therefore are more likely to experience cracking.
In a preferred embodiment the zinc alloy coating comprises 5 to 35 % of a binary eutectic phase. It was also found by the inventors that most cracks initiated from the binary eutectic phase. The binary eutectic phase also showed a low n, on average below 0.10, showing a high tendency for cracking and a low tendency for rescuing cracks of neighbouring phases. Hence, the presence of binary eutectic phase is preferably maintained between 5 - 35%, more preferably between 5 - 20%, more preferably between 5 - 10%. In this range, initiated cracks in the binary eutectic phase can still be rescued sufficiently by the surrounding phases to maintain the overall integrity of the zinc alloy coating on the metal substrate.
In an alternative embodiment, the zinc alloy coating is free from binary eutectic phase. As most cracks initiate in the binary eutectic phase having a n well below 0.3, on average below 0.10, a zinc alloy coating free from binary eutectic phase is believed to have significantly less cracking.
In a preferred embodiment the zinc alloy coating comprises 0.3 - 5 weight % Al and 0.3 - 5 weight % Mg, more preferably the zinc alloy coating comprises 1 - 2 weight % Al and 1-2 weight% Mg, as such a coating has improved corrosion resistance, stone chipping resistance as compared to conventional zinc coatings.
In a preferred embodiment the zinc alloy coating is a hot dip coating. Although the zinc alloy coating can be applied on the metal substrate in all common methods as known to a person skilled in the art, the metal substrate according to the invention preferably has preferably a zinc alloy coating made by hot-dip coating. Hot-dip coating is well known to a person skilled in the art and has the advantage that it is a reliable and relatively cheap method to apply a zinc alloy coating to at least one side of the metal substrate.
In a preferred embodiment the zinc alloy coating microstructure comprises phases with varying strain hardening exponents, wherein at least 70% of the phases with a crystallographic orientation-dependant strain hardening exponent n < 0.33 are surrounded by phases with a crystallographic orientation-dependant strain hardening exponent n > 0.33. It was found by the inventors that initiation of cracks in the alloy coating could either end up as a crack or could be rescued, leading to an overall acceptable coating layer. As elaborated already, the propagation of cracks involve phase boundaries. Hence, a crack could initiate in a phase with a low n, e.g. binary eutectic phase, and could propagate in a phase with a high n, e.g. a primary zinc phase. In case the phases with a n below 0.33 are surrounded by phases with a n of at least 0.33, it was found that the propagation of the crack is halted, thereby rescuing the crack formation and maintaining an overall good zinc alloy coating.
The invention is now further described based on the figures and non-limiting examples.
FIG. 1. Shows a schematic representation of microstructural cracking behaviour within ZnAIMg coatings.
FIG. 2 Shows Zn HCP crystal orientation with the loading parallel to the rolling direction (RD). 2a shows Zn HCP crystal orientation favoured for cracking resistance, with a high m value (0.5), 2b depicts Zn HCP crystal orientation favoured for cracking resistance, with a medium m value (0.32 - 0.45) and FIG 2 (c) shows Zn HCP crystal orientation leading to cracking.
FIG. 3. Depicts the EBSD results of a cracked area after 10% strain subjected to uniaxial tension, including image quality (IQ), IQ plus inverse pole figure (IPF), corresponding Schmid map and HCP crystals of the critical numbered phases.
FIG. 4. Shows the principal zinc slip systems.
FIG. 5. Shows the n-value distribution for three samples (11, I2, C1) and their EBSD Image Quality Map
FIG. 1. Shows stage I of a cracking path that begun with a nucleation of tiny micro cracks in MgZn2 platelets of binary eutectic phase as the result of strain localization. The binary eutectic phase has a low n of about 0.08 and is therefore prone to cracking. As the deformation proceeds, these micro-cracks coalesce and grow mostly perpendicular to binary eutectic platelets (e.g. stage (II) in FIG. 1). When the crack reaches the interfaces between the primary Zn phases and the binary eutectic phase, there might be two situations according to the in-situ evaluations performed earlier. The first circumstance is that the adjacent Zn phase exhibits an orientation close to [0001] perpendicular to the loading direction delivering Q > 60° and/or m > 0.32, in accordance to the claims. In this situation, the primary slip systems of Zn phase are activated by easy dislocation motion and subsequently ductile plastic deformation takes place instead of cleavage cracking (assuming the force direction is parallel to RD). Consequently, the crack is arrested in the primary zinc phase boundary. If the adjacent Zn phase possesses an orientation with HCP c-axis parallel to the loading direction (see Fig. 2c) and incorporates low m and n, serving as a favourable site for the propagating crack (e.g. cracked Zn phase No. 2 at stage III, Fig. 1). In particular, by increasing of Q (the orientation angle of HCP c-axis with respect to the loading direction) from 0 to 90°, cleavage becomes less dominant, and principle ductile deformation mechanisms including basal dislocation slip effectively activate and prevent crack propagation. It should be stated that, FIG. 1 delivers the typical cracking mechanisms of ZnAIMg coatings; however, an individual crack may also form in a zinc phase with unfavorable orientation (m < 0.32) without prior nucleation in the binary eutectic phase. Hence, by the method according to the invention, the cracking behaviour of a zinc alloy coating can be easily measured and predicted by analysing n. The other parameters, m and Q provide additional information to the cracking mechanism leading to a complete picture.
FIG 3. Depicts the EBSD results of a cracked area after 10% strain subjected to uniaxial tension, including image quality (IQ), IQ plus inverse pole figure (IPF), corresponding Schmid map and HCP crystals of the critical numbered phases. From the EBSD results it can be observed that Zn phases with low m and low n experienced transgranular cracking during tensile test. For instance, phase no. 1 having c-axis almost parallel to loading direction or RD (0=2.1°), exhibited a very low strain hardening exponent (n=0.289) and also low Schmid factor (m=0.03), oriented unfavourably and consequently experienced cracking. On the other hand, phase no. 3 possessing strain hardening exponent (n=0.368), and high Schmid factor (m=0.44), exposed as a favourable orientation to accommodate plastic deformation without enduring cracking. In addition, the formed cracks in phases no. 1 and 2, are arrested at the boundaries of adjacent phases with high m-factor.
These EBSD results were obtained according to the method according to the invention. A metal substrate (HSLA grade steel) with a zinc alloy coating (Zn1.6AI1.6Mg) was prepared for nanoindentation and electron backscatter diffraction (EBSD). For this purpose, a square sample (1x1 cm) was cut from the coated metal sheets. The surface of the zinc alloy coated samples was slightly mechanically polished using 1 pm diamond suspension and water-free lubricant on a Struers MD Nap disc for 3-5 minutes to obtain relatively smooth surface. Thereafter, the sample was flat ion milled for 15 minutes by means of an ion polisher (JEOL IB-19520CCP) in order to attain a good surface quality. The in-situ SEM tensile and bending specimens were also prepared with the same procedure given above, before performing the tests.
Nanoindentation tests were conducted utilizing MTS Nano Indenter XP® with Berkovich tip using the continuous stiffness measurement (CSM) mode. Load controlled indentations with an applied force of 3 mN and spacing of 8 pm between the indents were performed on the coating microstructure.
In order to obtain the equation to correlate theta with n, more than 200 indentations in randomly selected three regions were applied in order to sufficiently capture all the existing phases of the microstructure (primary zinc, binary eutectic and ternary eutectic phases) with a good statistical representation. For each indent on the coating microstructure a label was assigned. Strain hardening exponents (n) associated with each labeled indent were then calculated using raw nanoindentation data as described above.
EBSD measurements on the indented regions of the coating were conducted. For this, the sample was placed in a SEM chamber with 70° tilting angle. The region of interest was selected and a working distance of 12-15 pm was applied. A step size of 200 nm and accelerating voltage of 25 kV were used to acquire EBSD patterns. EBSD measurement was conducted on the cracked areas of specimens subjected to in-situ tensile and bending tests. The obtained EBSD results were analyzed by means of EDAX-TSL OIM™ Analysis 8 software. The image quality (IQ), inverse pole figure (IPF) orientation maps were generated on the EBSD data on the indented regions. Accordingly, for each primary zinc phase, orientation angle (Q) between the c-axis of HCP zinc crystal and the loading direction (surface normal for nanoindentation) was determined and plotted versus the corresponding local strain-hardening exponent (n) of each labeled zinc phase which was achieved previously by nanoindentation. The procedure was repeated for all the indented zinc phases. Ultimately, a fourth order polynomial expression was fitted on the obtained cure to establish a relationship between Q and n. For the alloy coating of the sample containing Zn, 1.6% Al and 1.6% Mg, it was found that a=1.428x1CT8, b=2.233x1CT6, c=1.027x1C>-4, d=4.631 x1C>-4 and e=0.29, giving the relation n to theta as n=1.428x1O 804 -2.233x10 603+ 1.027x10 402-4.631 x 1 O 40+O.29
Separate EBSD analysis was performed on the cracked areas of microstructure undergone bending and tensile test. In addition to image quality (IQ) and inverse pole figure (IPF) maps, the Schmid factor (m) map of the areas was calculated. For obtaining the Schmid factor (m) associated with each zinc phase, the principle zinc slip systems including all the equivalent crystallographic families were considered and the corresponding Schmid maps were calculated and plotted according to tensile principle stress tensor along rolling direction (RD). By this process, the Schmid factor (m) of each cracked and non-cracked zinc phase was achieved.
Having the HCP zinc crystal orientation of each cracked and non-cracked phases after bending/tensile tests by EBSD analysis, Q values were calculated for associated phases. The Q values were determined with respect to tensile loading, parallel to RD. Afterwards, each measured Q value was used to calculate corresponding n-values using the expression as defined above.
FIG 4 shows the principle Zn slip systems including all the equivalent crystallographic families which were considered to calculate the corresponding Schmid maps, which was plotted according to tensile principle stress gradient along rolling direction (RD), as described also in R. Parisot, et al., Deformation and Damage Mechanisms of Zinc Coatings on Hot-Dip Galvanized Steel Sheets: Part I. Deformation Modes, Metall. Mater. Trans. A Phys. Metall. Mater. Sci. 35 A (2004) 797-811 and C. Tome et.al, The yield surface of hep crystals, Acta Metall. 33 (1985) 603-621. The obtained results for primary zinc phases on a metal substrate with a zinc alloy coating after being subjected to tensile and bending according to method of the invention is shown in the table below. Table 1 Detailed crystallographic orientation and micro-mechanical information of all labeled phases.
Strain
Area Zn phase Average phase Orientation Schmid
Test hardening Cracked no. no. orientation angle, Q (°) factor, m exponent, n
Tensile 1 1 (6 Ϊ2 6 ϊ)[1 0 Ϊ 0] 2.1 0.03 0.289 Yes
1 2 (3 69 ϊ)[11 9 2 3] 14.5 0.25 0.298 Yes 1 3 (37 1 362)[3 5 2 22] 85.3 0.44 0.368 No 1 4 (5 41 15) [84Ϊ 49 5] 63 0.42 0.335 No
Bending 1 1 (35 32 3 3)[9 T2 21 2] 8.5 0.14 0.292 Yes
1 2 (0 1 Ϊ 23)[11 6 Ϊ71] 86.4 0.49 0.371 No
2 1 (69 15 5)[ϊΐ 9 2 3] 19.1 0.29 0.304 Yes
2 2 (10 12 22 ϊ)[6 5 ϊ 22] 77 0.47 0.346 No
3 1 (411 7 3) [6 1 5 0] 8.4 0.14 0.292 Yes
3 2 (41 3 0)[414Ϊ0 3] 15.3 0.26 0.299 Yes
3 3 (2 11 Ϊ3 2) [10643] 22.1 0.3 0.309 Yes
3 4 (5 41 0)[3 69 2] 12.6 0.21 0.296 Yes
It can be derived from the table that primary zinc phases with a theta above 45, a Schmid factor above 0.32 and/or a strain hardening exponent above 0.33 do not crack. Next, three zinc coated steel samples (11, I2 and C1) were prepared with hot dip galvanising at a temperature of 460 - 470 °C. The wiping speed for sample C1 was 20 times higher than for sample 11 and I2. The samples were analysed with EBSD to obtain n, m and Q values and were subjected to a tensile test until a true strain of 0.1. q-values, varying from 0 - 90 ° with respect to tensile direction were obtained by post-processing of EBSD results. The n-values of the grains and the distribution (as shown in FIG. 5.) were obtained via the equation n =1.428xlO 804 -2.233xlO 603+ 1.027x10 402-4.631xlO 40+O.29 Cracking quantification and fraction of phases were measured by image analysis It can been seen that the inventive samples 11 and I2, having at least 75% of primary phase crystals with an n > 0.33 leads to significantly less cracking, and shorter average crack length, than the sample C1 having less than 75% of the primary phase crystals with n > 0.33.

Claims

1. A method for the characterisation of formability properties of a zinc alloy coating on a metal substrate,
• the zinc alloy coating containing one or more alloying elements selected from the group consisting of Mg, Al, Ni each with a content of at least 0.3 weight % and at most 10 weight %, optionally one or more additional elements selected from the group consisting of Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the metallic coating is less than 0.3 weight %, inevitable impurities, the remainder being zinc,
• the zinc alloy coating having a microstructure comprising a primary zinc phase and binary eutectic and/or ternary eutectic phases,
• wherein Electron Backscatter Diffraction (EBSD) is used to determine a crystallographic orientation-dependent strain hardening exponent (n) of the zinc alloy coating microstructure, wherein an orientation angle (Q) between the c-axis of hep crystal of the primary zinc phase and the loading direction is determined, and wherein n is determined from the orientation angle (0) via an equation n =a04 -b03+ c02-d0+e, wherein
■ 1.0x10 8 < a < 1.8x10 8 ■ 1.5x10 6 < b < 3x10 6 ■ 5x1 O 4 < c < 1.5x1 O 3 1 x104 < d < 8x1 O 4 ■ 0.27 < e < 0.32,
2. The method according to claim 1 , wherein a Schmid factor (m) of the zinc alloy coating microstructure is determined.
3. The method according to claim 1 or 2, wherein the strain hardening exponent (n) is determined with nanoindentation tests.
4. The method according to any of the claims 1 - 3, wherein a, b, c, d and e of the equation n =a04 -b03+ c02-d0+e are determined by plotting the orientation angle (Q) as determined by EBSD versus the corresponding local strain-hardening index (n) as determined by nanoindentation.
5. The method according to any of the claims 1 - 4, wherein the strain hardening exponent (n) of the primary zinc phases is obtained from the orientation angle (Q) via the following equation n =a04 -b03+ c02-d0+e, wherein
• 1.2x10 8< a < 1.6x10 8 · 2.0x10-® < b < 2.5x10-6
• 8x1 O 4 < c < 1.2x1 O 3
• 3x1 O 4 < d < 6x1 O 4
• 0.28 < e < 0.30 more preferably n =1.428c10_8q4 -2.233x1 O_603+ 1.O27c1O 402-4.631c1O- 40+O.29.
6. A metal substrate comprising a zinc alloy coating, the zinc alloy coating containing one or more alloying elements selected from the group consisting of Mg, Al, Ni each with a content of at least 0.3 weight % and at most 10 weight %, optionally one or more additional elements selected from the group consisting of
Si, Sb, Pb, Ti, Ca, Mn, Sn, La, Ce, Cr, or Bi, wherein the content by weight of each additional element in the metallic coating is less than 0.3 weight %, inevitable impurities, the remainder being zinc, wherein the microstructure of the zinc alloy coating comprises primary zinc phases and a binary and/or ternary eutectic phase, wherein the primary zinc phases have a crystallographic orientation-dependant strain hardening exponent (n) of at least 0.29, as determined by any of the claims 1 - 5, and wherein at least 75% of the primary zinc phases have a crystallographic orientation-dependant strain hardening exponent n > 0.33.
7. The metal substrate comprising a zinc alloy coating according to claim 6, wherein the primary zinc phases have a Schmid factor m between 0.01 - 0.5.
8. The metal substrate comprising a zinc alloy coating according to claim 6 or 7, wherein at least 55% of the primary zinc phases have a Q > 45 °, preferably Q > 60 °, more preferably Q > 65 °.
9. The metal substrate comprising a zinc alloy coating according to any of claims 6 - 8, wherein at least 55 % of the primary zinc phases have a Schmid factor m > 0.32, preferably m>0.33, more preferably m>0.35.
10. The metal substrate comprising a zinc alloy coating according to any of the claims 6 - 9, wherein the zinc alloy coating comprising 5 to 35 % of a binary eutectic phase.
11. The metal substrate comprising a zinc alloy coating according to any of the claims 6 - 9, wherein the zinc alloy coating is free from binary eutectic phase.
12. The metal substrate comprising a zinc alloy coating according to any of the claims 6 - 11, wherein the zinc alloy coating comprises 0.3 - 5 weight % Al and 0.3 - 5 weight % Mg.
13. The metal substrate comprising a zinc alloy coating according to any of the claims 6 - 12, wherein the zinc alloy coating is a hot dip coating.
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