KR102041885B1 - High entropy alloy and method for manufacturing the same - Google Patents

Abstract

Disclosed are a high strength high entropy alloy having nano grains at low strain and a method of manufacturing the same.
Method for producing a high entropy alloy according to the invention (a) the initial alloy material at 1000 ~ 1200 ℃ for 1 to 24 hours Homogenizing annealing; And (b) rod-rolling the homogenized annealed initial alloy material to produce a high entropy alloy having cross twins and secondary micro twins formed therein as a microstructure, wherein the initial alloy material is weight percent. Co: 5 to 35%, Cr: 5 to 35%, Fe: 5 to 35%, Mn: 5 to 35%, Ni: 5 to 35%.

Description

HIGH ENTROPY ALLOY AND METHOD FOR MANUFACTURING THE SAME

The present invention relates to a high entropy alloy and a method for producing the same, and more particularly, to a high strength high entropy alloy having nanocrystal grains by cryogenic rolling at a low deformation amount and a method for producing the same.

High Entropy Alloy (HEA) is a grain alloy mainly composed of five or more elements, and is an alloy having no brittleness even in an intermediate composition. A high entropy alloy is usually a face centered cubic (FCC), a crystal structure in which particles are arranged so that particles are centered on each side of a cube, or a body centered cubic lattice, a spatial structure in which particles are coordinated at the vertices and centers of a cube. (Body Centered Cubic, BCC).

The high entropy alloy has a feature of maximizing the entropy effect than the mixed enthalpy. Specifically, the high entropy alloy stabilizes the single phase of the face-centered cubic lattice, increases the degree of freedom of the microstructure, and the lattice deformation is large, thereby increasing the solid solution strengthening effect. In addition, it is difficult to diffuse to form a nano-precipitated phase growth is suppressed, there is a feature that increases the diversity and stability of the microstructure. Furthermore, it has been reported that both the strength and toughness are excellent and the banana curve, which is a relationship between strength and elongation, can also be deviated.

Background art related thereto is A fracture-resistant high-entropy alloy for cryogenic applications, Vol. 345, 6201 (2014) 1153-1158, which discloses a high entropy alloy exhibiting high strength and high toughness at cryogenic temperatures. It is. However, the high entropy alloy has an excellent ductility effect at cryogenic temperatures, but has a disadvantage of weak strength.

On the other hand, in order to increase the strength of the high entropy alloy, it is effective to make the crystal grains ultrafine, and special processing techniques that can impart a large amount of deformation may be applied. For example, there is a method of applying a high strain amount using a SDP (Severe deformation process) such as an ECAP (equal channel angular pressing) to refine the grains.

However, the processing method capable of imparting most of the rigidity processing is not only very limited in size or shape of the specimen to be manufactured, but also low in production efficiency. Therefore, it is impossible to manufacture a high-strength material having high practical practicality by a rigid plastic working method.

It is an object of the present invention to provide a high entropy alloy having excellent strength and ductility at cryogenic temperatures.

Another object of the present invention is to provide a method for producing the high entropy alloy.

Method for producing a high entropy alloy according to the present invention for achieving the above one object is (a) the initial alloy material at 1000 ~ 1200 ℃ for 1 to 24 hours Homogenizing annealing; And (b) rod-rolling the homogenized annealed initial alloy material to produce a high entropy alloy having cross twins and secondary micro twins formed therein as a microstructure, wherein the initial alloy material is weight percent. Co: 5 to 35%, Cr: 5 to 35%, Fe: 5 to 35%, Mn: 5 to 35%, Ni: 5 to 35%.

The homogenized annealed initial alloy material may comprise a cubic microstructure.

The bar rolling may be performed at cryogenic temperatures of -100 ~ -200 ℃.

The bar rolling may be performed with a deformation amount of 0.5 to 2.

The rod rolling is carried out in the rolling of the 8 ~ 11 pass using a ball roller having a circular ball (caliber), the size of the ball gradually moves in the direction of the initial alloy material relative to the size of the first ball May be decreasing.

Step (b) includes the steps of (b1) forming a primary twin, wherein a cross twin is formed; And (b2) forming a secondary fine twin inside the line of the crossing twin.

The average size of the microcrystalline grains by the cross twins and the secondary micro twins may be 50 to 150 nm.

The high entropy alloy may have a single phase structure of face centered cubic.

High entropy alloy according to the present invention for achieving the above another object is 5% to 35% by weight, 5% to 35% Cr, 5% to 35% Fe, 5% to 35% Mn: 5% to 35%, 5% Ni to 5%. 35% and comprises cross twins as microstructures.

The crossing twin may include a primary twin and a secondary fine twin formed in a line of the primary twin.

The average size of the microcrystalline grains by the cross twins and the secondary micro twins may be 50 to 150 nm.

The high entropy alloy may have a single-phase structure of face centered cubic structure.

The high entropy alloy may have a yield strength of 1500 MPa or more at room temperature and an elongation of 8% or more.

In the high entropy alloy according to the present invention, the inside of the grains can be effectively segmented by the improvement of twinning activity by using the rolling process in the multi-axis direction at cryogenic temperature, thereby promoting the miniaturization.

As a result, the nanostructures are microstructured at low strains without the rigidity processing, so the productivity is excellent and the ultra high strength characteristics of the high entropy alloy can be exhibited. In addition, the high-entropy alloy prepared by rolling at cryogenic temperatures is improved in strength and elongation compared to room temperature or rigid plastic (SDP) method, and thus is particularly suitable for materials used in extreme environments such as cryogenic environments.

1 is a flow chart showing a manufacturing method of a high entropy alloy according to the present invention.
2 is a schematic diagram of a multipass ball roller according to the present invention.
Figure 3 is a cross-sectional view of the high-entropy alloy and the photograph (a) comparing the high-entropy alloy after the initial alloy material and cryogenic rod rolling (condition: -196.15 ℃, rolling reduction 64%, 11 passes, deformation amount 1) according to the present invention (b).
4 is an XRD pattern of the initial alloy material, RTCR material, CTCR material.
5 is a photograph of the high-entropy alloy after cryogenic bar rolling (conditions: -196.15 ℃, 75% reduction, 12 passes, strain 1.4) using the initial alloy material according to the present invention.
6 is an EBSD inverse pole figure (IPF) map photo (left) and IQ (image quality) map photo (right) of RTCR material (a) and CTCR material (b).
7 is a TEM analysis result of the CTCR material.
8 shows the elongation of the initial alloy material, RTCR material, CTCR material, SPD high press torsion (deformation amount> 10) material and cryogenic sheet rolling material (sheet rolling, -196.15 ° C, 80% reduction, 1.6 deformation) This shows the yield strength.
9 is a microstructure of the cryogenic rod rolled (CTCR) high entropy alloy of FIG.
FIG. 10 is a microstructure of the cryogenic plate rolled high entropy alloy of FIG. 8. FIG.

Advantages and features of the present invention and methods for achieving them will be apparent with reference to the embodiments described below in detail with the accompanying drawings. However, the present invention is not limited to the embodiments disclosed below, but may be implemented in various forms. It is provided to fully convey the scope of the invention to those skilled in the art, and the present invention is defined only by the scope of the claims. Like reference numerals refer to like elements throughout.

Hereinafter, a high entropy alloy and a method of manufacturing the same according to a preferred embodiment of the present invention will be described in detail with reference to the accompanying drawings.

The present invention is to provide a high-entropy alloy and a method for manufacturing the same, which can exhibit ultra-high strength and high elongation properties by having nano grains microstructured without rigid plastic processing.

1 is a flow chart showing a manufacturing method of a high entropy alloy according to the present invention. Referring to Figure 1, a method for producing a high entropy alloy according to the present invention comprises the step of homogenizing annealing the initial alloy material (S110) and rolling using a multi-pass ball roller (S120).

Homogenizing annealing the initial alloy material (S110)

The initial alloy material is in the state of as cast, which is in the state of being primarily processed. The microstructure of the initial alloy material has a coarse columnar structure (grain size <300 µm) in a cubic system. The cubic system is a crystal having three virtual axes having a center of crystal orthogonal to each other and the same length, also called a cubic system.

In order to remove the fine segregation of Mn in the initial alloy material for 1 to 24 hours at 1000 ~ 1200 ℃ It is preferable to carry out.

Generally, the presence of Mn segregation weakens the stability of the FCC locally, causing ferrite precipitation or martensite transformation at grain boundaries during deformation. This secondary phase and FCC phase interface promotes cracking during deformation. In addition, Mn fine segregation lowers the strain homogeneity of the matrix, thereby reducing the ductility. This phenomenon occurs during solidification of the alloy because the melting temperature of Mn is relatively low. Therefore, in order to secure excellent workability of the present material, it is preferable to remove Mn fine segregation by homogenizing annealing at 1000 to 1200 ° C. for 1 to 24 hours.

The initial alloy material is 5% to 35% by weight, 5% to 35% Cr, 5% to 35% Fe, 5% to 35% Mn: 5% to 35%, and 5% to 35% Ni. The initial alloy material may further include a small amount of impurities.

If the Co content is less than 5%, the phase may become unstable, and if it exceeds 35%, manufacturing cost and formation of an intermediate phase may be possible.

When the Cr content is less than 5%, physical properties of the alloy, such as corrosion resistance may be lowered, and when the Cr content exceeds 35%, the formation of an intermediate phase may be possible.

When the Fe content is less than 5%, it is inefficient in terms of manufacturing cost, and if it exceeds 35%, the phase may become unstable.

When the Mn content is less than 5%, it is inefficient in terms of manufacturing cost, and when it exceeds 35%, an oxide may be formed during the manufacturing process while the phase becomes unstable.

If the Ni content is less than 5%, the phase becomes unstable and if it exceeds 35%, it is inefficient in terms of manufacturing cost.

When it is out of the composition range of the high entropy alloy, it is difficult to obtain a solid solution having an FCC single phase, and therefore it is preferable to satisfy the composition range given above.

Rolling using a multi-pass ball roller (S120)

Subsequently, the homogenized annealed initial alloy material is bar-rolled to produce a high entropy alloy having cross twins and secondary micro twins formed therein as a microstructure.

In the present invention, as shown in Figure 2, it is preferable to perform the bar rolling of 8-11 pass using a ball roller having a circular ball (caliber). In the multi-pass ball roller, 8 to 11 circular holes having different diameters are formed at the interface where the upper roller and the lower roller are in contact with each other. At this time, the circular diameter of the first position is 11.9mm, the circular diameter of the second position is 11.3mm, it can be seen that the circular diameter of the last 11 positions is 7.2mm. As such, it is preferable that the size of the ball is gradually reduced in the direction in which the initial alloy material moves based on the size of the first ball, and as it passes through 8 to 11 round holes having different diameters, all directions of the circular arrangement That is, compressive stress may be generated in the multi-axis direction to form cross twins of the high entropy alloy.

In addition, when using the multi-pass ball roller, it is preferable to perform the bar rolling under the condition that the maximum area reduction (AR) is 64% and the total deformation amount is 0.5 to 2 at -100 to -200 ° C, which is a cryogenic temperature. Do. For example, the reduction ratio may be 40 to 64%, and the total deformation amount may be 0.5 to 1. The reduction ratio (%) is the difference between the diameters of the raw materials before rolling and after rolling, and is equal to the reduction ratio (%). The total amount of deformation may be calculated as ln (initial specimen cross-sectional area / cross-sectional area after deformation).

As the bar rolling of the present invention is performed under the above conditions, the crystal grains are effectively segmented while cross twins are formed into the microstructure of the alloy. Crossover twins are formed by the primary twins, and secondary microtwins are formed inside the crossover twinning lines. At very low temperatures, twins are more active, resulting in secondary microtwins that are generally less likely to occur, and microstructures become finer when the secondary microtwins occur inside the lines of the crossed twins. The average size of the microcrystalline particles formed by twins may be 50 to 150 nm, preferably 50 to 100 nm. In this way, through the cryogenic ball roller processing to improve the activation of twins and maximize the grain refining effect, it is possible to implement the high strength of the alloy.

The reason why the microstructure is formed is that the bar rolling of the present invention is a multiaxial deformation and cross twinning occurs, whereas the general rolling is a biaxial rolling and deformation in one direction so that twins are formed in only one direction.

As such, the high-entropy alloy having microstructured cross twins and secondary micro twins formed therein can also be made of alloys in bulk rod state without defects on the surface and inside. In addition, the high entropy alloy may have a single-phase structure of a face centered cubic structure.

The high entropy alloy according to the present invention includes Co: 5 to 35% by weight, Cr: 5 to 35%, Fe: 5 to 35%, Mn: 5 to 35%, and Ni: 5 to 35%. The tissue is characterized by including a cross twin.

As discussed above, the crossing twins may comprise very thin secondary microtwins formed within the lines of the crossing twins. The average size of the microcrystalline grains formed by the secondary micro twinning may be 50 to 150 nm.

The high entropy alloy may have a single phase structure of a face centered cubic structure.

The high-entropy alloy that satisfies this configuration has a yield strength of 1500 MPa or more and an elongation of 8% or more at room temperature (25 ± 5 ° C.), and thus may have very high strength and an appropriate elongation.

3 to 11, the initial alloy material was prepared as follows.

Co ₂₀ Cr ₂₀ Fe ₂₀ Mn ₂₀ Ni ₂₀ (Co _19.97 Cr _20.43 Fe _19.78 Mn _19.54 Ni _20.28 ) An initial alloy material having a composition was prepared. Then, homogenization annealed at 1100 ° C. for 24 hours. Subsequently, in order to maintain the cryogenic temperature during the bar rolling at the cryogenic temperature, the annealed initial alloy material was immersed in liquid nitrogen for about 10 minutes and then proceeded.

In Figures 3 to 11 RTCR material refers to the alloy rolled bar at a condition of 64% reduction, 11 passes, strain 1 at 25 ℃, CTCR material is 64% reduction, 11 passes, strain 1 at -196.15 ℃ Refers to a bar-rolled alloy as a condition.

The composition analysis was measured with an energy dispersive spectrometer mounted on a scanning electron microscope (7100F, JEOL).

Tensile properties were evaluated at a strain rate of 10 ⁻³ s ⁻¹ and at room temperature. The specimens used for the tensile test had a gauge length of 10 mm and a diameter of 2.5 mm (ASTM-E8). Tensile test pieces were machined from the core of the produced rods. The microstructure was evaluated by electron back diffraction measurement (EBSD, model: Helios NanoLabTM 600, FEI) at 15 kV acceleration voltage and 50 nm step size, and the viewing direction is a vertical cross section with respect to the longitudinal direction of the material. In addition, the microstructure was evaluated by electron microscopy (TEM, model: JEM 2100F, JEOL) operating at an acceleration voltage of 200 kV. TEM samples were prepared by a focused ion beam (model: Quanta 3D FEG, FEI). The crystal structure of the alloy was measured by X-ray diffraction (XRD) measurements using an MXP21VAHF diffractometer with CuKα radiation (model: D / Max-2500VL / PC, RIGAKU).

Figure 3 is a cross-sectional view of the high-entropy alloy and the photograph (a) comparing the high-entropy alloy after the initial alloy material and cryogenic rod rolling (condition: -196.15 ℃, rolling reduction 64%, 11 passes, deformation amount 1) according to the present invention (b). Referring to FIG. 3, an alloy of a bulk rod having a diameter of 7.5 mm and a length of 300 mm after cryogenic rod rolling was prepared from a cylindrical initial alloy having a diameter of 12.5 mm, and the straightness was good and the surface and the inside were cracked. It shows that a healthy bar was produced without this.

4 is an XRD pattern of the initial alloy material, RTCR material, CTCR material. The XRD pattern shows that only the FCC phase peak was detected, indicating that the initial alloy material is a completely FCC single phase.

5 is a photograph of the high-entropy alloy after cryogenic bar rolling (conditions: -196.15 ℃, 75% reduction, 12 passes, strain 1.4) using the initial alloy material according to the present invention. As the bar rolling conditions are 12 passes, it shows cracks inside and on the surface. This means that it is preferable to give a processing amount of 11 passes or less, a maximum reduction ratio of 64%, and a strain of 1 or less during rod rolling.

6 is an EBSD inverse pole figure (IPF) map photo (left) and IQ (image quality) map photo (right) of RTCR material (a) and CTCR material (b). Multipath ball rolling has greatly changed the initial microstructure. The RTCR material in (a) has twin lines intersecting each other from 100 to 600 nm, and because twins are very fine, each twin cannot be clearly identified in the IPF map, but in the IQ map the twin lines are parallel You can see that it consists of twin lines. The CTCR material in (b) showed a twin form similar to that of RTCR material, but substantially more twins were observed. EBSD result, the total length of the twin lines per unit area in the case of RTCR material was calculated to 1.07㎛ ^-1, if CTCR material was calculated to 3.69㎛ ^-1.

Therefore, the formation of twins was maximized at cryogenic temperatures compared to the room temperature, and the segmentation of the microstructure was increased. Secondary twins were formed inside the lines of the crossing twins, and the grains were refined.

In addition, referring to Figure 6 (b), the relatively dark gray color means the presence of high density dislocation, which means that the dislocation slip occurred with the twins deformed during the bar rolling process. Strength enhancement by dislocation accumulation arises from the interaction of dislocations that interfere with dislocation motion by generating internal stresses. As the number of dislocations increases, the strength of the alloy increases. Alloy materials with high density dislocations are expected to affect strength enhancement in addition to grain refinement.

Figure 7 shows the results of TEM analysis of CTCR materials ((a) TEM, (b) BF image, (c) DF, (d) DF image at matrix, (e) DF image at twin A, (f) DF image at twin B).

In FIG. 7A, twinning lines were observed to form cross twins. In spite of the fact that the activation of twins decreased significantly as the matrix narrowed, secondary microtwins were formed within the matrix interface by the primary twins. In (b) and (c), the diffraction pattern (DP) combined with the dark field image shows that the twin twin variants of the 12 possible twin variants in the face-centered cubic structure are activated to cause the shape of the cross twin.

Referring to (d) to (f), sheet rolling is applied with compressive stress only in the direction perpendicular to the sheet and mainly activates twins in a single direction. On the other hand, in the case of using a multipass ball roller, all twin deformations can be formed during CTCR since compressive stress is generated in all directions of the circular array. The size of each secondary twin is 5-15 nm and shows very fine characteristics.

The microcrystalline size by the secondary twins was calculated using the intercept method. For example, after drawing several lines in any direction, the number of lines satisfying the TB was counted and the ratio of the line length to the number of lines was calculated. As a result of the calculation, the CTCR material was 93nm at the maximum, and the RTCR material was the size of the microcrystalline grains at the maximum 711nm. In general, SPD technology reduces the size of the microstructure to less than 100nm, it can be seen that in the present invention induced the formation of ultrafine particles of 30 ~ 100nm size using CTCR.

FIG. 8 shows an initial alloy material, RTCR material, CTCR material, rigid plastic processing (SPD high press torsion, deformation amount> 10, * H. Sharhmir et al., MSEA 676 (2016) 294-303) material and cryogenic sheet rolling material (sheet It shows the elongation and yield strength of rolling, -196.15 ℃, rolling reduction 80%, strain 1.6).

Referring to Figure 8, it shows that the yield strength of the CTCR material significantly increased. Cryogenic rod rolling process according to the present invention increased the yield strength of the initial alloy material up to 458% showed a very high value of 1548MPa. In addition, moderate elongation at break of up to 10% was shown. The yield strength of the CTCR material was approximately 500 MPa over that of the RTCR material, and this result demonstrates a high efficiency that lowers the deformation temperature of the alloy to -196.15 ° C.

In addition, the CTCR material has a higher yield strength than the rigid plastic material (SPD), but shows no significant difference. The elongation at break was 2.5 times higher for CTCR materials.

In addition, the CTCR material showed higher yield strength and elongation than the cryogenic sheet rolled material. These results suggest that rod rolling using a multipass ball roller can obtain better tensile properties than using cryogenic sheet rolling or a rigid plastic working method.

FIG. 9 is a microstructure of the cryogenic rod rolled (CTCR) high entropy alloy of FIG. 8, and FIG. 10 is a microstructure of the cryogenic plate rolled high entropy alloy of FIG. 8. In the CTCR material, the cross twins are formed by the deformation in the multi-axis direction, whereas the cryogenic sheet rolling is subjected to compressive stress only in the direction perpendicular to the sheet, so that deformation of the single direction occurs mainly, so that the parallel twins are formed.

Table 1 shows the results of the specimen properties according to the number of passes in the bar rolling (-196.15 ℃).

Cross-sectional reduction rate = (initial cross-sectional area-cross-sectional area after deformation) / initial cross-sectional area × 100

True strain = ln (cross section of initial specimen / cross section after deformation)

TABLE 1

Referring to Table 1, as the amount of processing is given, the specimen diameter decreases, and the cross-sectional reduction rate tends to increase. In particular, the specimens given 11 passes of processing amount were superior in yield strength to 1548 MPa and exhibited an elongation of approximately 10% compared to other specimens. This is a result of maximizing the grain refining effect is easy to implement a higher strength, can exhibit a high cross-sectional reduction rate.

Table 2 shows the results of grain refinement and dislocation accumulation to calculate yield strength.

TABLE 2

Referring to Table 2, in the RTCR material, the grain refinement was 237 MPa, and the potential accumulation was 408 MPa. In the CTCR material, grain refinement was 710 MPa and dislocation accumulation was 440 MPa. In CTCR materials, grain refinement had a greater effect on yield strength than dislocation accumulation, indicating that a large amount of twins played a crucial role in achieving higher yield strength by further refinement of grains.

As described above, in the present invention, by rod-rolling at an extremely low temperature using an initial alloy material, high-entropy alloys having high strength and high elongation can be manufactured by forming grain refinement by cross twins.

In addition, it is excellent in productivity because it has nanostructured microstructures at low strain even without rigid plastic processing, and is suitable as a material used in extreme environments such as cryogenic environments.

Although the embodiments of the present invention have been described above with reference to the accompanying drawings, the present invention is not limited to the above embodiments and can be manufactured in various forms, and having ordinary skill in the art to which the present invention pertains. It will be understood by those skilled in the art that the present invention may be embodied in other specific forms without changing the technical spirit or essential features of the present invention. Therefore, it should be understood that the embodiments described above are exemplary in all respects and not restrictive.

Claims (13)

(a) Initial alloy material at 1000-1200 ° C. for 1-24 hours Homogenizing annealing; And
(b) bar-rolling the homogenized annealed initial alloy material to produce a high entropy alloy having cross twins and secondary micro twins formed therein as a microstructure;
The rod rolling is rolled in the multi-axis direction at cryogenic temperatures of -100 ~ -200 ℃,
The initial alloy material is
Method for producing a high entropy alloy, characterized in that by weight% Co: 5 to 35%, Cr: 5 to 35%, Fe: 5 to 35%, Mn: 5 to 35%, Ni: 5 to 35% .
The method of claim 1,
The initial alloying material of step (a) is a method for producing a high entropy alloy, characterized in that it comprises a cubic microstructure.
delete The method of claim 1,
The bar rolling is a high entropy alloy manufacturing method, characterized in that carried out with a deformation amount of 0.5 ~ 2.
The method of claim 1,
The rod rolling is performed by rolling 8 ~ 11 pass using a ball roller having a circular ball (caliber),
The size of the ball is a method of producing a high entropy alloy, characterized in that gradually decreasing in the direction in which the initial alloy material moves based on the size of the first ball.
The method of claim 1,
Step (b)
(b1) forming a primary twin to form a cross twin; And
(b2) forming a secondary fine twin in the line of the crossing twin; a high entropy alloy manufacturing method comprising a.
The method of claim 6,
The method for producing a high entropy alloy, characterized in that the average size of the microcrystalline grains by the cross twins and the secondary fine twins is 50 ~ 150nm.
The method of claim 1,
The high entropy alloy has a single phase structure of a face centered cubic structure.
By weight% Co: 5 ~ 35%, Cr: 5 ~ 35%, Fe: 5 ~ 35%, Mn: 5 ~ 35%, Ni: 5 ~ 35%
A high entropy alloy comprising cross twins as a microstructure.
The method of claim 9,
Wherein said cross twins comprise a primary twin and a secondary fine twin formed within the line of said primary twin.
The method of claim 10,
High-entropy alloy, characterized in that the average size of the micro-grains by the cross twins and the secondary micro twins is 50 ~ 150nm.
The method of claim 9,
The high entropy alloy is a high entropy alloy, characterized in that it has a single-phase structure of the face-centered cubic structure.
The method of claim 9,
The high entropy alloy is a high-entropy alloy, characterized in that the yield strength is at least 1500MPa at room temperature, the elongation is 8% or more.

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