Classifications

• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
  • C22C30/02—Alloys containing less than 50% by weight of each constituent containing copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C19/00—Alloys based on nickel or cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C30/00—Alloys containing less than 50% by weight of each constituent
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/42—Ferrous alloys, e.g. steel alloys containing chromium with nickel with copper
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/44—Ferrous alloys, e.g. steel alloys containing chromium with nickel with molybdenum or tungsten
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/52—Ferrous alloys, e.g. steel alloys containing chromium with nickel with cobalt
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C38/00—Ferrous alloys, e.g. steel alloys
  • C22C38/18—Ferrous alloys, e.g. steel alloys containing chromium
  • C22C38/40—Ferrous alloys, e.g. steel alloys containing chromium with nickel
  • C22C38/58—Ferrous alloys, e.g. steel alloys containing chromium with nickel with more than 1.5% by weight of manganese
• • C—CHEMISTRY; METALLURGY
  • C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
  • C22C—ALLOYS
  • C22C2200/00—Crystalline structure

Definitions

• This invention relates to corrosion resistant austenitic steel alloys and in particular, to a multi-principal element, high entropy, corrosion resistant alloy that includes nitrogen.
• alloying elements such as chromium (Cr), molybdenum (Mo), and nitrogen (N) improve corrosion resistance of steel alloys, particularly resistance to localized attack in chloride containing environments.
• the degree of corrosion resistance can be predicted by a pitting resistance equivalent number (PREN).
• PREN pitting resistance equivalent number
• Other elements, such as tungsten, copper, and vanadium have been proposed as beneficial alloying additions for corrosion resistance.
• Cr and Mo are strong ferrite formers and can lead to the formation of sigma phase and chi phase which adversely affect both pitting resistance and mechanical properties.
• austenite formers such as nickel, cobalt, and copper may be added to the alloys.
• This practice has led to the use of nickel- base and cobalt-base alloys for the most severely corrosive environments.
• the addition of N is known to be generally beneficial to both corrosion resistance and strength, but nitrogen solubility and the unwanted precipitation of nitrides, especially at grain boundaries, limits the total amount of nitrogen that can be added. Nitrogen solubility becomes increasingly limited as nickel and cobalt contents increase.
• alloys there are nickel-base and cobalt-base alloys that include significant amounts of Mo.
• a high Mo content is stabilized by either a high nickel content or a high cobalt content.
• Most of those alloys do not contain a positive addition of N.
• Alloy N-155 which is sold under the registered trademark MULTIMET® has the following nominal composition in weight percent: 20% Ni, 20% Co, 20% Cr, 3% Mo, 2.5%W, 1.5% Mn, 1% Nb+Ta, 0.15% N, and 0.1% C.
• the balance of the alloy is iron and usual impurities.
• Those alloys have essentially a single base element such as iron, nickel, or cobalt.
• Alloy design has traditionally not considered the contributions of the mixing entropy to alloy phase stability because the mixing entropy is relatively low in systems with a single base element. Because they do not have a single base element, high entropy alloys (HEA) employ configurational entropy to affect the stability of solid structural phases within the alloy.
• HEA are composed of a single solid solution phase or a mixture of solid solution phases. With the exception of a few studies, the solid solution phases have either a body centered cubic (BCC) or a face centered cubic (FCC) structure.
• BCC body centered cubic
• FCC face centered cubic
• HEA typically consist of at least three elements in equiatomic or close to equiatomic proportions to maximize the configurational entropy.
• the basic principles derived from the above-listed rules overlap with the Hume-Rothery rules relating to solid solution formation in alloys and are suitable starting point for designing an alloy with a solid solution structure.
• the mixing enthalpy should not be too negative or too positive in order to avoid the formation of intermetallic phases and to avoid phase separation.
• the atomic size difference between the constituent elements should be minimized to prevent lattice strain. Further, the mixing entropy should be maximized.
• the electronegativity of the constituent elements should be similar among the principal elements.
• the solid solution phase that forms is also related to the valence electron concentration (VEC). Guo et al.
• a single-phase FCC structure is predicted when VEC is greater than about 8
• a single-phase BCC structure is predicted when the VEC is less than about 6.87
• a mixed FCC/BCC structure is predicted when 6.87 ⁇ VEC ⁇ 8.
• a multi- principal element, corrosion resistant alloy having the following composition in weight percent:
• the alloy also includes the usual impurities found in corrosion resistant alloys intended for the same or similar use.
• one or both of W and V may be substituted for some or all of the Mo.
• the alloy provides a solid solution that is substantially all FCC phase, but may include minor amounts of secondary phases that do not adversely affect the corrosion resistance and mechanical properties provided by the alloy.
• the alloy also comprises from at least about 0.10% N up to the solubility limit.
• the elements are selected to provide the following combination of parameters; -6 kJ/mol ⁇ AHmix ⁇ 0 kJ/mol;
• the valence electron concentration is greater than about 7.80.
• alloy according to the present invention may comprise or may consist essentially of the elements described above, throughout the following specification, and in the appended claims.
• percent and the symbol “%” mean percent by weight or percent by mass, unless otherwise indicated.
• the drawing is a graph of Rockwell C hardness (HRC) as a function of cold working percent for Example 5 of the alloy according to this invention.
• a CoCrNiMnFe base alloy By using the foregoing parameters in the design of multi-element alloy, corrosion resistant alloy, it is believed that higher amounts of elements such as molybdenum, tungsten, and vanadium, can be included in a CoCrNiMnFe base alloy to provide an FCC solid solution structure that is substantially free of undesired secondary phases.
• the alloy also includes a small amount of N as an interstitial element.
• An equiatomic or near-equiatomic composition comprising a combination of Cr, Mn, Fe, Co, and Ni provides the multi-element base of the high entropy alloy according to this invention.
• the combination of base elements is chosen because it meets the constraints for HEA outlined about. Interstitial elements such as N have not been studied extensively within the HEA design constructs and may require novel design
• AHmix as an average term should be avoided in order to properly design an alloy in which nitride formation does not occur.
• Relatively large additions of Mo, W, or V in conjunction with N at or close to its solubility limit provides a novel alloy system with potentially superior corrosion resistance compared to the known Fe-base, Ni-base, and Co-base stainless steel alloys.
• Nickel and cobalt are present in the high entropy alloy of this invention to help stabilize the preferred FCC phase.
• Nickel and cobalt also benefit the desired single phase nature of the alloy by reducing the precipitation of undesirable ordered phases such as sigma ( ⁇ ) and mu ( ⁇ ) phases in the solid solution. In this way nickel and cobalt benefit the ductility provided by the alloy.
• Nickel and cobalt are relatively expensive elements and so their contents are limited to control the cost of making the alloy of this invention.
• Chromium contributes to the general and localized corrosion resistance provided by this alloy. It is also believed that chromium helps to increase the solubility of nitrogen in the alloy. Too much chromium adversely affects the mechanical properties (e.g., ductility) and corrosion resistance by promoting the precipitation of ordered phases, like sigma and/or Chromium nitrides.
• the alloy also contains about 4 to about 20 atomic percent (at. %) or at least about 8% up to about 28% weight percent of molybdenum to benefit the alloy's resistance to localized corrosion such as pitting corrosion. Too much molybdenum promotes the precipitation and stabilization of topologically close packed phases which adversely affects the corrosion resistance and mechanical properties. Like chromium too much molybdenum adversely affects the ductility and processability of the alloy because it forms sigma phase at relatively high temperatures. Tungsten and/or vanadium can be substituted for some or all of the molybdenum on an equiatomic basis.
• Manganese is present in the alloy of this invention because it benefits the solubility of nitrogen in the solid solution of the alloy. Too much manganese reduces the solidus temperature of the alloy which can adversely affect the intergranular strength during hot working.
• Iron contributes to the high entropy of mixing (ASmix) that characterizes this alloy and helps to stabilize the desired single phase FCC structure of the alloy. Iron is also present as a substitute for some of the nickel and/or cobalt to help limit the cost of producing the alloy. Similar to chromium and molybdenum, too much iron can result in the precipitation of sigma phase which adversely affects the ductility of the alloy and its processability. At least about 0.10% nitrogen is also present in this alloy as an interstitial element. The addition of nitrogen helps to further stabilize the FCC phase and benefits the localized corrosion resistance provided by the alloy. As an interstitial element nitrogen also contributes to the good mechanical properties provided by the alloy such as its yield strength and tensile strength.
• Nitrogen may be present up to its solubility limit in the alloy, but preferably is limited to not more than about 1.00% in this alloy.
• the alloy according to the present invention may also include copper to benefit the stability of the FCC phase structure.
• too much copper reduces the solidus temperature of the alloy which can result in incipient intergranular liquation during hot working of the alloy.
• An alloy in accordance with this invention provides very good resistance to corrosion, especially pitting corrosion.
• the alloy is characterized by having a pitting resistance equivalent number (PREN) of at least 50 where the PREN is defined as follows:
• the alloy is characterized by a PREN of at least about 65 and better yet at least about 70.
• the elements that constitute the alloy of this invention are selected to provide the following combination of parameters;
• the valence electron concentration (VEC) is greater than about 7.80.
• ASmix is mainly affected by the number of main elements in the alloy and their concentrations. Preferably, a minimum of five equiatomic elements provide a ASmix that results in a stabilized alloy micro structure. In the five-element embodiment of the alloy it is expected that ASmix will be not more than about 13- 13.5 J/K mol. However, in the copper-containing embodiment it is expected that ASmix will be greater than 13-13.5 J/K mol.
• AH m ix is determined by the chemical affinity of the constituent elements and is preferably as close to zero as practicable to allow the entropy to manage the stability of the alloy.
• the parameter ⁇ is related to the difference in atomic size of the constituent elements. In this alloy, molybdenum is the largest atom and is the one that most affects the value of ⁇ .
• Valence electron concentration is the number of total electrons in the valence band including the "d" electrons.
• Cobalt and nickel have the higher VEC's, 9 and 10 respectively, than the other elements. However, since this is an alloy, the VEC is calculated as
• the alloy according to this invention provides a VEC greater than 8.0.
• the ingots contained mainly a solid solution consisting essentially of an FCC structure with some interdendritic secondary phase(s).
• the 40-lb ingots were homogenized, forged to 0.75" square bars, and then solution annealed at 2250 F for 2.5 hrs. followed by water quenching. It was determined that the alloy had a solid solution structure consisting substantially of the FCC phase in the solution-annealed-and-quenched condition.
• CPT Critical pitting temperature
• Cyclic polarization potentiodynamic testing was performed based on ASTM Standard Test Procedure G61. Voltage values at the knee of the curve, at 50 ⁇ /cm 2 , and at 100 ⁇ /cm 2 were measured for two sets of samples prepared from solution annealed 0.75" square bars. The results of the potentiodynamic pitting tests are shown in Table 3 below including the pitting potentials and the repassivation potentials in millivolts (mV).
• the very high ductility provided by the alloy is the very high ductility provided by the alloy as demonstrated by the high elongation values set forth in Tables 6 and 7.
• the percent elongation provided by the alloy is up to 73% at room temperature which compares very favorably to 58% elongation provided by the known stainless steels.
• more important is the capability to provide that level of ductility even at cryogenic temperatures without adversely affecting the tensile strength provided by the alloy as shown in Table 7.
• this alloy provides excellent cold processability as demonstrated by its cold work hardening capability.
• the alloy is able to provide a Rockwell C-scale hardness (HRC) of about 37 after about 30% cold work, where the percent cold work is defined by the equation below: Initial Area— final Area

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• 2018
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