Patent Description:
High-entropy alloys (hereinafter, HEAs), which are multi-element alloys obtained by alloying similar proportions of five or more constituent elements without the main elements constituting the alloys (for example, general alloys such as steel, aluminum alloys, titanium alloys, etc.), are metallic materials that have a single-phase structure (e.g., face-centered cubic (FCC), body-centered cubic (BCC)) in which an intermetallic compound or intermediate phase is not formed due to high entropy of mixing within the alloys.

In particular, Co-Cr-Fe-Mn-Ni based HEAs have excellent cryogenic properties, high fracture toughness, and corrosion resistance, and are thus in the limelight as a material applicable to extreme environments.

An important factor in designing these HEAs is the composition ratio of the elements that constitute the alloy.

With regard to the composition ratio of HEAs, a typical HEA should consist of at least five major alloy elements, and the composition ratio of each alloy constituent element is defined as <NUM>-<NUM> at%, and if an element other than the main alloy constituent elements is added, the addition amount should be less than <NUM> at%.

However, in the recent years, the definition of HEAs has also been expanded, including the introduction of Fe<NUM>Mn<NUM>Co<NUM>Cr<NUM> HEA, etc..

Meanwhile, it is known that the existing Co-Cr-Fe-Mn-Ni based HEA has excellent cryogenic properties through generation of a large number of deformation twins at a cryogenic temperature. <NPL> discloses adding V as a further component in a Co-Cr-Fe-Mn-Ni based HEA resulting in 10V15Cr5Mn35Fe10Co25Ni alloy. <NPL> discloses Co-Cr-V-Fe-Ni based face-centered cubic HEA consisting of <NUM> at% Co, <NUM> at% Cr, <NUM> at% V, <NUM> at% Fe, and <NUM> at% Ni.

The object of the present invention is to provide an alternative transformation-induced plasticity high-entropy alloy, which mainly consists of FCC phase and is capable of achieving more improved mechanical properties at a cryogenic temperature of -<NUM>, compared to previously reported HEAs having an FCC single-phase.

To achieve the above object, the present invention provides a transformation-induced plasticity high-entropy alloy comprising the features of claim <NUM>.

Further, the present invention provides a method for preparing a transformation-induced plasticity high-entropy alloy comprising the features of claim <NUM>.

A high-entropy alloy (HEA) according to the present invention, as in the existing quinary HEAs, can provide a single-phase FCC structure by having a quaternary or quinary HEA composition that essentially contains Co, Cr, Fe, and V, and optionally containing Ni.

Additionally, unlike Co-Cr-Fe-Mn-Ni based HEAs, a HEA according to the present invention causes transformation-induced plasticity at a cryogenic temperature of -<NUM>, and thus has a more excellent tensile strength, ductility, and fracture properties at a cryogenic temperature of -<NUM>, than conventional single-phase HEAs.

Hereinafter, the present invention will be described in detail with regard to HEAs according to preferred embodiments of the present invention and a method thereof by referring to the accompanying drawings, but the present invention is not limited to these embodiments.

<FIG> shows phase equilibrium information on an alloy according to mole fractions of the alloy, as a cobalt (Co) content changes in a composition, where iron (Fe) is fixed at <NUM> at%, chromium (Cr) is fixed at <NUM> at%, and vanadium (V) is fixed at <NUM> at%, whereas cobalt (Co) is contained in an amount of X at% and nickel (Ni) is contained in an amount of <NUM>-X at%.

As shown in <FIG>, it was confirmed that when cobalt (Co) and nickel (Ni) were substituted in 45Fe-10Cr-10V (values are in unit of at%), it is confirmed that an FCC single-phase region is expanded as the cobalt (Co) content is decreased. This means that it is possible to obtain a HEA which has a microstructure stably and mainly consisting of an FCC phase at <NUM> or higher, when <NUM> at% of iron (Fe), <NUM> at% of chromium (Cr), <NUM> at% of vanadium (V), and at most <NUM> at% of cobalt (Co) are added while cobalt (Co) and nickel (Ni) are substituted.

<FIG> shows the stability of an FCC phase with respect to a BCC phase through thermodynamic calculations, as a cobalt (Co) content changes at <NUM> in a composition where the iron (Fe) is fixed at <NUM> at%, the chromium (Cr) is fixed at <NUM> at%, and the vanadium (V) is fixed at <NUM> at%, whereas cobalt (Co) is contained in an amount of X at% and nickel (Ni) is contained in an amount of <NUM>-X at%.

As shown in <FIG>, when nickel (Ni) is substituted with cobalt (Co) in 45Fe-10Cr-10V (values are in unit of at%), the Gibbs free energy difference between the BCC phase and the FCC phase is increased as the molar ratio of cobalt (Co) is increased, and the stability of the BCC phase is increased. This means that when deformation is applied, such an increase acts as a driving force to cause a phase to be transformed from the FCC phase to the BCC phase.

<FIG> shows phase equilibrium information on an alloy according to mole fractions of the alloy as an iron (Fe) content changes in a composition, where the chromium (Cr) is fixed at <NUM> at%, the vanadium (V) is fixed at <NUM> at%, and the cobalt (Co) is fixed at <NUM> at%, whereas the iron (Fe) is contained in an amount of X at% and nickel (Ni) is contained in an amount of <NUM>-X at%.

As shown in <FIG>, when iron (Fe) and nickel (Ni) are substituted in 10Cr-10V-30Co (values are in unit of at%), it is confirmed that an FCC single-phase region is expanded as the iron (Fe) content is decreased, and it can be seen that the iron (Fe) content be preferably in an amount of <NUM> at% or less so as to maintain the FCC single-phase.

<FIG> shows the stability of an FCC phase with respect to a BCC phase through thermodynamic calculations, as an iron (Fe) content changes at <NUM> in a composition where the chromium (Cr) is fixed at <NUM> at%, the vanadium (V) is fixed at <NUM> at%, and the cobalt (Co) is fixed at <NUM> at%, whereas the iron (Fe) is contained in an amount of X at% and nickel (Ni) is contained in an amount of <NUM>-X at%.

As can be expected in <FIG>, it is desirable that the iron (Fe) content be in an amount of <NUM> at% or more, in consideration of a driving force required for transformation from an FCC phase to a BCC phase.

Through the results shown in <FIG>, the present inventors have found that, by heat-treating an alloy having a composition with the above components and the content ranges thereof, a HEA, which mainly consists of an FCC phase and in which the Gibbs free energy of the body-center cubic structure (BCC) is smaller than that of the face-centered cubic structure (FCC), can be obtained, and such an alloy can significantly improve mechanical properties thereof at a cryogenic temperature because at least a part of the alloy is transformed from the FCC phase to the BCC phase when the alloy undergoes deformation at a cryogenic temperature of -<NUM>, and thereby have completed the present invention.

The HEA according to the present invention is developed in accordance with the alloy designing principle described above, and is characterized in that the HEA essentially contains Co, Cr, Fe, and V, and optionally contains Ni, and mainly consists of an FCC phase, wherein transformation-induced plasticity from an FCC phase to a BCC phase occurs when plastic deformation is applied at a cryogenic temperature of -<NUM>.

The HEA according to the present invention, consists of <NUM>-<NUM> at% of Co, <NUM>-<NUM> at% of Cr, <NUM>-<NUM> at% of V, <NUM>-<NUM> at% of Fe, and <NUM>-<NUM> at% of Ni, and the remaining unavoidable impurities in an amount of <NUM> at% or less.

The reason why the content ranges of the alloy elements constituting the alloy are determined as described above is as follows.

When the Co content is less than <NUM> at% or greater than <NUM> at%, transformation-induced plasticity may not occur or a phase in which the FCC phase is dominant may not be obtained. Therefore, the Co content is preferably in a range of <NUM>-<NUM> at%, and more preferably <NUM>-<NUM> at%.

When the Cr content is less than <NUM> at%, the corrosion resistance is decreased; however, when the Cr content exceeds <NUM> at%, the price is increased. Therefore, the Cr content is preferably in a range of <NUM>-<NUM> at%, and more preferably <NUM>-<NUM> at%.

When the Ni content is equal to or greater than <NUM> at%, transformation-induced plasticity may not occur. According to the invention, the Ni content is <NUM>-<NUM> at%. When the Ni content is <NUM> at%, a complete FCC single-phase may not be obtained by the heat treatment at <NUM>. Therefore, in order to achieve an FCC single-phase structure by the heat treatment at <NUM>, the Ni content is more preferably in a range of <NUM>-<NUM> at% (exclusive of <NUM>).

When the Fe content is less than <NUM> at% or greater than <NUM> at%, transformation-induced plasticity may not occur or a phase in which the FCC phase is dominant may not be obtained. Therefore, the Fe content is preferably in a range of <NUM>-<NUM> at%, and more preferably <NUM>-<NUM> at%.

When the V content is less than <NUM> at%, the solid-solution strengthening effect decreases; however, when the V content exceeds <NUM> at%, the price is increased. Therefore, the V content is preferably in a range of <NUM>-<NUM> at%, and more preferably <NUM>-<NUM> at%.

The unavoidable impurities are components other than the above-described alloy elements, which are raw materials or components unavoidably incorporated during the preparation process, and the impurities are included in an amount of <NUM> at% or less, preferably <NUM> at% or less, and more preferably <NUM> at% or less.

Additionally, the transformation-induced plasticity HEA according to the present invention is characterized by mainly consisting of an FCC phase, and the fraction of the FCC phase is <NUM>% or greater, and may consist of an FCC single-phase.

Additionally, the transformation-induced plasticity HEA according to the present invention is characterized in that phase transformation, in which at least part of the FCC phase before deformation changes to a BCC phase during a deformation process, occurs at a cryogenic temperature (-<NUM>). Here, all of the FCC phases may be changed to BCC phases.

Additionally, the transformation-induced plasticity HEA according to the present invention may preferably have a tensile strength of <NUM> MPa or greater and has an elongation of <NUM>% or greater, at room temperature (<NUM>).

Additionally, the transformation-induced plasticity HEA according to the present invention may preferably have a tensile strength of <NUM>,<NUM> MPa or greater and has an elongation of <NUM>% or greater, at a cryogenic temperature (-<NUM>).

Additionally, in the transformation-induced plasticity HEA according to the present invention, a difference between an impact energy at room temperature (<NUM>) and an impact energy at a cryogenic temperature (-<NUM>) may be <NUM>% or less.

Additionally, the transformation-induced plasticity HEA according to the present invention can be prepared through the following steps of (a) to (c) :.

In the homogenization step, when the temperature for homogenization treatment is lower than <NUM>,<NUM>, the homogenization effect is insufficient; however, when the temperature for homogenization treatment is higher than <NUM>,<NUM>, the heat treatment costs become excessive. Therefore, the temperature for homogenization treatment is in a range of <NUM>,<NUM> to <NUM>,<NUM>. When the time for homogenization treatment is less than <NUM> hours, the homogenization effect is insufficient; however, when the time for homogenization treatment exceeds <NUM> hours, the heat treatment cost becomes excessive. Therefore, the time for heat treatment is in a range of <NUM> to <NUM> hours.

In the annealing, when the temperature for annealing treatment is lower than <NUM>, it is not possible to achieve complete recrystallization; however, when the temperature for annealing treatment is higher than <NUM>,<NUM>, grain coarsening becomes more severe. Therefore, the temperature for annealing treatment is in a range of <NUM> to <NUM>,<NUM>. When the time for annealing treatment is less than <NUM> minutes, it is not possible to achieve complete recrystallization; however, when the time for annealing treatment is greater than <NUM> hour, the heat treatment cost becomes excessive. Therefore, the time for annealing treatment is in a range of <NUM> minutes to <NUM> hour.

The cooling at steps (a) and (c) may be performed through water quenching, but is not particularly limited as long as a microstructure, which is required after each cooling treatment, can be achieved.

First, Co, Cr, Fe, Ni, and V metals having a purity of <NUM>% or more were prepared. The metals thus prepared were weighed so as to have a mixing ratio shown in Table <NUM> below.

The raw material metals prepared at the above ratio were charged into a crucible, dissolved using vacuum induction melting equipment, and an alloy ingot in a rectangular parallelepiped shape (thickness: <NUM>, width: <NUM>, and length: <NUM>) was cast. The cast ingot (thickness: <NUM>) was subjected to homogenization heat treatment at a temperature of <NUM>,<NUM> for <NUM> hours, followed by water quenching, as shown in <FIG>.

To remove oxides formed on the surface of the homogenized alloy, surface grinding was performed. The thickness of the ground ingot was <NUM>, and cold rolling was performed such that the thickness thereof changes from <NUM> to <NUM>.

Additionally, each of the cold-rolled alloy sheets was subjected to annealing treatment by heating at <NUM> for <NUM> minutes to maintain the FCC phase, followed by quenching to maintain the FCC phase at room temperature.

<FIG> shows the results of XRD measurement of the alloys at room temperature according to Examples <NUM> to <NUM> and Comparative Example prepared according to the process described above.

To minimize the phase transformation caused by the deformation of a sample during the grinding of the sample, the XRD measurement was performed after performing the grinding in the order of sandpaper Nos. <NUM>, <NUM>, <NUM>, and <NUM>, followed by electrolytic etching in <NUM>% perchloric acid.

In <FIG>, "<NUM> Ni", "<NUM> Ni", "<NUM> Ni", and "<NUM> Ni" indicate alloys according to Example <NUM>, Example <NUM>, Example <NUM>, and Comparative Example, respectively. The same applies to the drawings following <FIG>.

As observed in <FIG>, it was confirmed that all the alloys according to Example <NUM>, Example <NUM>, and Comparative Example consist of FCC single-phases by XRD analysis.

On the other hand, it was shown that the alloy according to Example <NUM> mainly contained FCC phase and small amount of BCC phase. This is consistent with what is predicted from the equilibrium phase diagram of <FIG>, and if the annealing temperature is higher than <NUM>, the alloys can be prepared to have an FCC single-phase, as is the case with the alloys according to Examples <NUM> and <NUM>.

<FIG> shows the fractions of a BCC phase in the microstructure after the tensile tests of the HEAs, which were prepared according to Examples <NUM> to <NUM> and Comparative Example at room temperature and at a cryogenic temperature (-<NUM>), according to Ni content.

As shown in <FIG>, in the case of Example <NUM>, about <NUM>% of phase transformation was achieved even when a tensile test performed at room temperature, whereas the amount of phase transformation was <NUM>% in Example <NUM>, very low to be <NUM>% in Example <NUM>, and <NUM>% in Comparative Example.

In contrast, in the case of a tensile test performed at a cryogenic temperature (-<NUM>), the amounts of phase transformation were <NUM>% in Example <NUM>, <NUM>% in Example <NUM>, <NUM>% in Example <NUM>, and <NUM>% in Comparative Example, respectively. Further, it was confirmed that as the content of Ni became smaller, the phase transformation from an FCC phase to a BCC phase occurred more actively.

<FIG> and Table <NUM> show the tensile test results of the alloys of Examples <NUM> to <NUM> and Comparative Example of the present invention at room temperature (<NUM>) and a cryogenic temperature (-<NUM>).

As shown in Table <NUM>, the HEAs according to Examples <NUM> to <NUM> of the present invention, at room temperature, showed a yield strength of <NUM> MPa to <NUM> MPa, a tensile strength of <NUM> MPa to <NUM> MPa, and an elongation of <NUM>% to <NUM>%, and the HEA according to Comparative Example showed a yield strength of <NUM> MPa, a tensile strength of <NUM> MPa, and an elongation of <NUM>%, thus showing no significant difference compared to those of Examples <NUM> to <NUM>.

Meanwhile, the HEAs according to Examples <NUM> to <NUM> of the present invention, at a cryogenic temperature, showed a yield strength of <NUM> MPa to <NUM> MPa, a tensile strength of <NUM>,<NUM> MPa to <NUM>,<NUM> MPa, and an elongation of <NUM>% to <NUM>%, and the HEA according to Comparative Example showed a yield strength of <NUM> MPa, a tensile strength of <NUM> MPa, and an elongation of <NUM>%, thus showing lower mechanical properties compared to those of Examples <NUM> to <NUM>. Such a result demonstrates that the Comparative Example shows a significant difference compared to Example <NUM> that exhibits mechanical properties similar to those of Comparative Example at room temperature. These differences are assumed to be due to the transformation-induced plasticity.

Additionally, the HEA according to Example <NUM>, at a cryogenic temperature, showed a high tensile strength of <NUM>,<NUM> MPa, and good elongation of <NUM>%, which proves that the HEA according to Example <NUM> has high strength and good elongation. The HEAs of Examples <NUM> and <NUM>, at a cryogenic temperature, showed a fairly high tensile strength of <NUM>,<NUM> MPa to <NUM>,<NUM> MPa, and very high elongation of <NUM>% to <NUM>%%, which proves that these HEAs have very high values in terms of tensile strength and elongation, respectively.

<FIG> shows the comparison results of the tensile strength and elongation at a cryogenic temperature of the HEAs (herein indicated as 'star' mark) according to Examples <NUM> to <NUM> of the present invention and other HEAs reported previously.

As shown in <FIG>, the tensile strength and elongation of the HEAs according to Examples <NUM> to <NUM> of the present invention were extremely high thus exhibiting excellent characteristics compared to any conventional alloys or HEAs.

<FIG> shows the results of the Charpy impact test performed under the conditions from room temperature to a cryogenic temperature. In the Charpy impact test, sub-sized samples with a thickness of <NUM> were used.

Claim 1:
A transformation-induced plasticity high-entropy alloy, consisting of <NUM>-<NUM> at% of Co, <NUM>-<NUM> at% of Cr, <NUM>-<NUM> at% of V, <NUM>-<NUM> at% of Fe, <NUM>-<NUM> at% of Ni, and inevitable impurities in an amount of <NUM> at% or less,
the transformation-induced plasticity high-entropy alloy is <NUM>% or more an FCC phase at room temperature of <NUM>,
wherein transformation-induced plasticity, in which at least part of the FCC phase changes to a BCC phase, occurs at a cryogenic temperature of -<NUM>.