KR101910744B1 - Medium-entropy alloys with excellent cryogenic properties - Google Patents

Abstract

The present invention relates to a medium-entropy alloy with much more improved cryogenic mechanical properties of a conventional FCC-based high-entropy alloy and a competitive price. The medium-entropy alloy according to the present invention comprises: 6-15 at% of Cr; 50-64 at% of Fe; 13-25 at% of Co; 13-25 at% of Ni; and the remaining including inevitable impurities. The medium-entropy alloy includes a metastable FCC phase and has excellent cryogenic mechanical properties by having strain induced transformation from the FCC phase to a BCC phase occurring in plastic deformation of alloys.

Description

[0002] Medium-entropy alloys with excellent cryogenic properties < RTI ID = 0.0 >

The present invention relates to medium-entropy alloys (MEAs) which are excellent in cryogenic mechanical properties. The present invention relates to medium-entropy alloys (MEAs) having excellent cryogenic mechanical properties, To induce modified organic phase transformation during cryogenic deformation, thereby achieving excellent cryogenic mechanical properties.

High-entropy alloys (HEAs) are multi-element alloys obtained by alloying five or more constituent elements at a similar rate, instead of the major element constituting the alloy. The entropy alloy has a high entropy of mixing in the alloy, so that a single phase structure such as a face-centered cubic (FCC) or a body-centered cubic (BCC) .

Especially, in the case of the Co-Cr-Fe-Mn-Ni series high entropy alloy, it has excellent cryogenic properties, high fracture toughness and corrosion resistance.

Two important factors in designing such a high entropy alloy are the composition ratio of the constituent elements of the alloy and the constituent entropy of the alloy system.

The first is the composition ratio of the entropy alloy. The entropy alloy should be composed of at least five elements, and the composition ratio of each alloy constituent element is defined as 5 to 35 at%. In addition, when an element other than the main alloy constituent element is added in the production of the high entropy alloy, the addition amount thereof should be 5 at% or less.

In general, alloys are classified into high entropy alloys, medium-entropy alloys (MEAs) and low-entropy alloys (LEAs) according to the compositional entropy (ΔS _conf ) 1] and the constituent entropy value obtained by [1].

[Formula 1]

(R: gas constant, X _i : mole fraction of i element, n: number of constituent elements)

[Formula 2]

In the case of Co ₂₀ Cr ₂₀ Fe ₂₀ Mn ₂₀ Ni ₂₀ (at%) alloy, which is a representative cryogenic FCC type entropy alloy, the price of the added alloying element is high and the price competitiveness is low. Therefore, despite its excellent cryogenic properties, there is a limit to replacing existing steel materials with marine plants, LNG materials, cryogenic tanks, and marine / marine materials.

Therefore, for the industrialization of high entropy alloys, it is essential to secure price competitiveness through control of alloy elements and to realize excellent cryogenic characteristics at the same time.

U.S. Patent Application Publication No. 2002/0159914

1. B. Gludovatz, et al., "A fracture-resistant high-entropy alloy for cryogenic applications ", Science, 345 (2014) 1153-1158.

It is an object of the present invention to develop an alloy which reduces the content of a relatively expensive alloy element in place of a conventional Co-Cr-Fe-Mn-Ni alloy to secure price competitiveness, To provide a trophic alloy capable of realizing excellent mechanical properties.

In order to solve the above-described problems, the present invention provides a method of manufacturing a steel sheet comprising 6 to 15 at% of Cr, 50 to 64 at% of Fe, 13 to 25 at% of Co, 13 to 25 at% of Ni and the remaining unavoidable impurities Alloy.

Further, the middle trophy alloy according to one embodiment of the present invention includes a metastable FCC phase at room temperature, and the metastable FCC phase is transformed into a BCC phase at the time of cryogenic deformation, and an organic phase transformation occurs, thereby improving the mechanical properties of the alloy .

In the middle trophy alloy according to the present invention, the content of Fe, which is an inexpensive alloying element, is increased up to 50 to 64 at%, thereby reducing the amount of expensive elements such as Co, Cr, and Ni and thus ensuring price competitiveness. Has a tensile strength of 1024 MPa or more at a cryogenic temperature of 77 K and an excellent elongation of 47% or more.

Further, the mid-trophy alloy according to one embodiment of the present invention comprises a metastable FCC phase at room temperature (298K), wherein the metastable FCC phase is deformed to a BCC phase at cryogenic temperatures, induced phase transformation, resulting in improved cryogenic mechanical properties.

FIG. 1 shows X-ray diffraction (XRD) measurement results of a ternary alloy of Co-Cr-Fe-Ni based on Comparative Examples 1 and 2 and Examples 1 to 4 of the present invention.
Fig. 2 shows the results of tensile tests at room temperature (298K) of a ternary alloy of Co-Cr-Fe-Ni based on Comparative Examples 1 and 2 and Examples 1 to 4 of the present invention.
3 shows the results of tensile tests at a cryogenic temperature (77 K) of Co-Cr-Fe-Ni based tough alloy according to Comparative Examples 1 and 2 and Examples 1 to 4 of the present invention.
4 is a graph showing the results of Electron Backscatter Diffraction (EBSD) analysis of the phase change of the Co-Cr-Fe-Ni based trophy alloy according to Example 3 of the present invention at room temperature and low temperature deformation.

Hereinafter, a method according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings, but the present invention is not limited to the following embodiments. Accordingly, it is obvious that those skilled in the art can variously change the present invention without departing from the technical idea of the present invention.

The present inventors have studied to obtain mechanical properties in a cryogenic environment at the same time as enhancing the price competitiveness of highly entropy alloys having excellent mechanical properties in a cryogenic environment. As a result, it has been found that the content of Fe, which is a low cost element, It is found that when the content of the alloying elements other than Fe is remarkably increased compared to the entropy alloy, the stability of FCC and BCC is changed, and the modified organic phase transformation is induced between the deformations and excellent cryogenic mechanical characteristics can be obtained.

In particular, when alloys are designed to include a metastable FCC phase at room temperature, the metastable FCC phase may undergo deformation in the BCC phase during deformation in a cryogenic environment, thereby improving the cryogenic mechanical properties And reached the present invention.

Thus, in the present invention, it was judged that the metastable state phase was transformed into a stable state at the corresponding temperature by causing the modified organic phase transformation in the plastic deformation process, and these phases were all defined as normal phase.

The core trophy alloy according to the present invention has an alloy composition containing 6 to 15 at% of Cr, 50 to 64 at% of Fe, 13 to 25 at% of Co, 13 to 25 at% of Ni and unavoidable impurities .

Further, the middle trophy alloy according to the present invention may include a metastable FCC phase at room temperature, and at the time of modification, the metastable FCC phase may be transformed into a BCC phase.

When the Cr content is less than 6 at%, the FCC phase is stabilized. When the Cr content exceeds 15 at%, the BCC phase is stabilized. Therefore, the Cr content is preferably 6 to 15 at%. In addition, since it is more advantageous to improve the cryogenic mechanical properties to form the metastable FCC phase, the content of chromium (Cr) is more preferably 7.5 to 12.5 at%.

When the iron (Fe) is less than 50 at%, the FCC phase is stabilized. When the iron (Fe) exceeds 64 at%, the BCC phase is stabilized. Since it is more advantageous to improve the cryogenic mechanical properties to form the FCC phase in the normal state, the content of iron (Fe) is more preferably 55 to 62.5 at%.

When the cobalt (Co) is less than 13 at%, the FCC phase is stabilized. When the cobalt (Co) exceeds 25 at%, the BCC phase is stabilized, so 13 to 25 at% is preferable.

When the nickel (Ni) content is less than 13 at%, the BCC phase is stabilized. When the nickel (Ni) content exceeds 25 at%, the FCC phase is stabilized.

When at least one selected from the group consisting of molybdenum (Mo) and aluminum (Al) substituted for cobalt (Co) is substituted, the FCC phase is stabilized when it is less than 13 at%, and the BCC phase is stabilized when it exceeds 25 at% , And 13 to 25 at%.

When the content of manganese (Mn) is less than 13 at%, the BCC phase is stabilized. When the content exceeds 25 at%, the FCC phase is stabilized. Therefore, it is preferably 13 to 25 at%.

In general, interstitial elements such as C and N in metal alloys are incorporated into the metal matrix to enhance the strength of the alloy due to the strengthening effect during the transformation of the metal. However, when at least 1 atomic percent of C and N is added in excess of 1 at%, the FCC phase is stabilized. Therefore, in order to induce a metastable FCC phase and utilize the effect of modified organic phase transformation, .

These unavoidable impurities are components other than the above-described alloying elements and are unavoidable components that are inevitably incorporated into the raw material or the manufacturing process.

In addition, the above-mentioned trophic alloy may be composed of a metastable FCC phase at room temperature, or a mixed phase of a metastable FCC phase and a BCC phase, and it is preferable that the metastable FCC phase fraction is high in terms of improvement of tensile strength and elongation. The fraction of the metastable FCC phase is preferably at least 50%. However, the fraction of the metastable FCC need not necessarily be more than 50%.

In addition, the above-mentioned trophic alloy may have a tensile strength of 500 MPa or more at room temperature (298K) and an elongation of 50% or more.

In addition, the above-mentioned trophic alloy may have a tensile strength of 1,000 MPa or more at a cryogenic temperature (77K) and an elongation of 40% or more.

[Examples 1 to 4]

Trophies in the middle  Manufacture of alloys

First, Co, Cr, Fe, and Ni metals having a purity of 99.9% or more were prepared.

The thus prepared metal was weighed so as to have the mixing ratio shown in Table 1 below.

Raw material mixing ratio (at%) Co Cr Fe Ni Example 1 17.50 10.00 55.00 17.50 Example 2 16.25 10.00 57.50 16.25 Example 3 15.00 10.00 60.00 15.00 Example 4 13.75 10.00 62.50 13.75

After charging the prepared raw metal into the crucible at the above ratios, 1550 占 폚 and dissolved. Using a mold, a rectangular parallelepiped having a thickness of 7.8 mm, 150 g, a width of 33 mm, a length of 80 mm, and a thickness of 7.8 mm Shaped alloy ingot was cast.

To remove the oxides formed on the surface of the cast alloy, surface grinding was performed and the thickness of the polished ingot was 7 mm.

A 7 mm thick ingot polished to a surface was subjected to a homogenization heat treatment at a temperature of 1100 ° C. for 6 hours, followed by cold rolling from a thickness of 7 mm to a thickness of 1.5 mm.

The cold rolled alloy sheets were further annealed at 800 ° C for 10 minutes.

[Comparative Examples 1 and 2]

Preparation of alloys for comparative example

An alloy for comparative example was prepared according to the composition of Table 2 below in the same manner as in Example.

Raw material mixing ratio (at%) Co Cr Fe Ni Comparative Example 1 14.50 5.00 66.00 14.50 Comparative Example 2 12.50 10.00 65.00 12.50

An alloy ingot was cast in the same manner as in Example, and homogenization heat treatment was performed at a temperature of 1100 캜 for 6 hours in the same manner as in Example, followed by cold rolling from 7 mm to 1.5 mm.

In addition, cold rolled alloy sheets were annealed at 800 DEG C for 10 minutes in the same manner as in Example.

Component analysis result

The actual components of the alloy prepared according to Comparative Examples 1 and 2 and Examples 1 to 4 annealed were analyzed using EDS, and the results are shown in Table 3 below.

EDS analysis composition (at%) Co Cr Fe Ni Comparative Example 1 14.34 5.10 66.29 14.27 Example 1 17.37 10.52 55.58 16.53 Example 2 16.16 10.21 57.41 16.22 Example 3 14.54 10.68 60.89 13.89 Example 4 13.55 10.27 62.55 13.63 Comparative Example 2 12.23 10.81 65.31 11.65

As shown in Table 3, the actual composition slightly deviates from the initial raw material mixing ratio, but it can be said to be almost the same level considering the purity of the raw material and the impurities that can be incorporated into the manufacturing process. In all of the examples, it is confirmed that the composition is contained in the composition ranges of Cr: 6 to 15 at%, Fe: 50 to 64 at%, Co: 13 to 25 at%, and Ni: 13 to 25 at% I could.

XRD analysis result

Fig. 1 shows the XRD measurement results of the alloys of Comparative Examples 1 and 2 and Examples 1 to 4 subjected to annealing at room temperature.

XRD measurements were performed after polishing in the order of sandpaper 600, 800, and 1200, followed by electrolytic etching in 8% perchloric acid to minimize phase transformation due to deformation during polishing of the specimen.

As a result, as shown in FIG. 1, in the case of Comparative Example 1, the BCC phase was formed. In the case of Examples 1 to 4, the metastable FCC phase was dominant. In Comparative Example 2, the BCC phase was main and the FCC phase And a small amount of phase was observed.

That is, as the content of Fe was increased and the content of Co and Ni was lowered, the stability of the FCC phase was lowered. As a result, a metastable FCC phase was formed in the range of Examples 1 to 4. In Comparative Examples 1 and 2, Fe was added in an amount of 65 at% or more, indicating that the FCC phase was no longer in a quasi-stable state but became an unstable state and the BCC phase was stabilized relatively.

Tensile test results

2 and 3 and Table 4 below show the tensile test results of the alloys annealed according to Comparative Examples 1 and 2 and Examples 1 to 4 of the present invention at room temperature (298K) and cryogenic temperature (77K), respectively.

Psalter Room temperature Cryogenic (77K) Yield strength
(MPa) The tensile strength
(MPa) Elongation
(%) Yield strength
(MPa) The tensile strength
(Mpa) Elongation
(%) Comparative Example 1 850 975 24 1336 1455 33 Example 1 280 550 68 615 1024 126 Example 2 274 568 86 543 1164 118 Example 3 226 534 98 526 1508 82 Example 4 228 787 67 620 1649 47 Comparative Example 2 579 996 26 1110 1516 30

As can be seen from Figs. 2 and 3 and Table 3, the tensile properties at room temperature of the tungsten alloy produced according to Examples 1 to 4 of the present invention were as follows: yield strength 226 to 280 MPa, tensile strength 534 to 787 MPa, elongation 67 To 98%.

On the other hand, the tensile properties at a cryogenic temperature show a very excellent cryogenic tensile characteristic with a yield strength of 526 to 620 MPa, a tensile strength of 1024 to 1649 MPa, and an elongation of 47 to 126%.

On the other hand, the tensile properties at room temperature of the tropospheric alloy prepared according to Comparative Examples 1 and 2 are such that most of the initial crystal structure is composed of the BCC structure, so that there is no effect of strengthening and elongation increasing according to the modified organic phase transformation between room temperature and cryogenic tensile strain, Due to the BCC structure, tensile yield strength and tensile strength at room temperature and cryogenic temperature are high, but elongation is low and brittleness is obtained.

In particular, in the case of the alloy according to Example 3 containing a large amount of metastable FCC phases, the cryogenic tensile properties have excellent tensile strengths of 526 MPa, tensile strengths of 1508 MPa, and elongation ratios of 82% Respectively.

In addition, in the trophy alloy according to the present invention, when the content of Cr and Fe is maintained, the content of Co is replaced with the content of Co by at least one selected from Mo and Al, It was confirmed that low temperature ductility and stiffness occurred due to the transformation of organic phase during effect transformation.

In addition, in the middle eutectic alloy of the present invention, even when the content of Cr and Fe is kept and the content of Ni is replaced with Mn to replace the content of Ni with the content of Ni, the deformation Organic phase transformation occurred and low temperature ductility and rigidity were confirmed.

Further, it was further confirmed that when the at least one of C and N is incorporated in the metal matrix as the interstitial element, the strength of the alloy due to the solid solution strengthening effect is further confirmed.

Modified organic phase transformation

4 is a graph showing the results of EBSD analysis of the phase change at room temperature and cryogenic temperature of the middle trophy alloy according to Example 3 of the present invention.

As shown in Fig. 4, the pre-strain Example 3 alloy is composed of a sub-stable FCC phase containing a very small amount of BCC phase, and the phase fraction of BCC phase is remarkably increased after room temperature (298K) and cryogenic (77K) You can do that. In particular, after cryogenic deformation, a phase transformation occurs on the FCC over the entire region to the BCC phase, and such a phase transformation greatly contributes to the improvement of the cryogenic mechanical properties as shown in Fig.

Therefore, the cryogenic mechanical property is preferably 50% or more of the phase fraction on the pre-strain FCC.

Before transformation
(volume%) After transformation (298K)
(volume%) After deformation (77K)
(volume%) Comparative Example 1 91.26 93.96 98.99 Example 1 0.34 15.07 28.46 Example 2 0.38 20.26 36.12 Example 3 0.41 27.68 56.71 Example 4 25.68 62.23 85.08 Comparative Example 2 87.81 89.20 94.87

Table 5 shows the results of ferritescope measurement of the BCC phase fraction (vol%) before and after the transformation of the alloy prepared according to Comparative Examples 1 and 2 and Examples 1 to 4 of the present invention at room temperature and cryogenic temperature.

As shown in Table 5, the alloys of Examples 1 to 3 contain a small amount of BCC phase before deformation, and it can be confirmed that the BCC phase fraction increases due to the phase transformation between normal temperature and cryogenic temperature deformation. In addition, the stability of the BCC phase of the alloy of Example 4 was higher than that of Examples 1 to 3, and the BCC phase contained 25.68 at%, and the BCC phase fraction increased due to the phase transformation between the normal temperature and the cryogenic temperature . The alloys of Comparative Examples 1 and 2 contained BCC phases of 91.26 at% and 87.81 at%, respectively, before the strain, because the stability of the BCC phase was significantly higher than those of Examples 1 to 4, and due to the phase transformation between room temperature and cryogenic deformation It can be confirmed that the fraction of the BCC phase increases.

Claims (13)

An alloy consisting of Cr: 6 to 15 at%, Fe: 50 to 64 at%, Co: 13 to 25 at%, Ni: 13 to 25 at% and the remaining unavoidable impurities. Wherein a phase transformation takes place. The method according to claim 1,
Wherein said modified organic phase transformation is characterized by occurring on a metastable FCC. The method according to claim 1,
The content of Cr is from 6 to 12.5 at%. The method of claim 3,
Wherein the content of Fe is 55 to 62.5 at%. The method according to claim 1,
Wherein the Co is substituted by at least one selected from Mo and Al. 6. The method according to claim 1 or 5,
Wherein said Ni is replaceable with Mn. The method according to claim 1 or 4,
Wherein said at least one trout alloy comprises at least one of C and N in an amount of less than 1 at% relative to the total at% of said trophic alloy. The method according to claim 6,
Wherein said at least one trout alloy comprises at least one of C and N in an amount of less than 1 at% relative to the total at% of said trophic alloy. The method according to claim 1,
Wherein said deformation occurs at a temperature below room temperature (298K). 3. The method of claim 2,
Wherein the phase fraction of the metastable FCC phase is greater than or equal to 50%. The method according to claim 1,
Wherein the trophic alloy is a mixed phase of a BCC phase and a metastable FCC phase, or a metastable FCC phase. The method according to claim 1,
Wherein said trophic alloy has a tensile strength of at least 226 MPa at room temperature (298 K) and an elongation of at least 67%. The method according to claim 1,
Wherein said trophic alloy has a tensile strength at cryogenic temperature (77K) of at least 1024 MPa and an elongation of at least 47%.

Images