Method for realizing low-temperature strengthening and toughening of TRIP high-entropy alloy

Abstract

The invention discloses a method for realizing low-temperature strengthening and toughening of TRIP high-entropy alloy, which comprises the following steps: S1, low-temperature pre-stretching: carrying out low-temperature pre-stretching on the TRIP high-entropy alloy until the TRIP high-entropy alloy is stretched to a Luders strip deformation end point or a stretching stress-strain curve stress platform end point, and then unloading; and S2, heat treatment is conducted, specifically, heat preservation treatment of the TRIP high-entropy alloy is conducted between the room temperature and the melting point temperature, and then rapid cooling is conducted through a cooling medium. According to the method for achieving low-temperature strengthening and toughening of the TRIP high-entropy alloy, low-temperature pre-stretching and heat treatment are adopted, the high-entropy alloy with two phase structures and two grain size characteristics is prepared, the low-temperature yield strength can be remarkably improved through a fine grain structure, residual dislocation and back stress caused by deformation of the two grain sizes, and the low-temperature strengthening and toughening of the TRIP high-entropy alloy can be achieved. And the deformation coordination between different grain sizes and phase structures can also ensure that the low-temperature plasticity is not reduced, so that the TRIP high-entropy alloy has high strength and high plasticity in a low-temperature environment, and the TRIP high-entropy alloy shows excellent low-temperature mechanical properties.

Description

Technical Field

[0001] The invention relates to the technical field of low-temperature engineering structures, and in particular to a method for achieving low-temperature toughening of TRIP high-entropy alloys. Background Art

[0002] With the development of advanced manufacturing, the demand for metal materials with excellent mechanical properties in extremely low temperature environments is becoming more and more urgent. They play an important role in the manufacture of key components in deep space, deep sea, and polar exploration. In recent years, high entropy alloys have attracted widespread attention as a new type of structure. Unlike traditional single-principal metal materials (such as steel, aluminum alloys, titanium alloys, etc.), the alloy elements present nearly equal atomic proportions, showing the characteristics of multiple principal components. It is precisely because of this unique chemical composition that high entropy alloys usually have excellent mechanical properties and corrosion resistance. Based on the composition characteristics of high entropy alloys, through the regulation of stacking fault energy, researchers have successively developed a series of high entropy alloys with TRIP (transformation induced plasticity) effect. Not only do they not show ductile-brittle transition at low temperatures, but they also show better plasticity at low temperatures due to phase transformation.

[0003] In the prior art, compared with the currently widely used high-strength steel, the low-temperature yield strength of most TRIP high-entropy alloys is still relatively low, which seriously affects their use as load-bearing components in the field of cryogenic engineering. In the field of metal materials, commonly used strengthening methods include solid solution strengthening, deformation strengthening, fine grain strengthening, dispersion strengthening, etc., most of which improve material strength by hindering dislocation movement, but this often leads to a decrease in plasticity and toughness, that is, an inverted relationship between strength and toughness.

[0004] Therefore, although the traditional strengthening method can significantly improve the low-temperature strength of TRIP high-entropy alloy, it also greatly reduces the low-temperature plasticity and toughness, and easily induces brittle fracture, making it difficult to ensure its safe use as a key component material in the field of cryogenic engineering. Therefore, how to effectively improve the low-temperature strength of TRIP high-entropy alloy without reducing plasticity, that is, to achieve low-temperature strengthening and toughness, is the key to the widespread application of this alloy in the field of cryogenic engineering structures. Summary of the invention

[0005] The purpose of the present invention is to provide a method for achieving low-temperature toughening of TRIP high entropy alloys, so as to achieve both high strength and high plasticity of TRIP high entropy alloys in a low-temperature environment, so that the TRIP high entropy alloys exhibit excellent low-temperature mechanical properties.

[0006] To achieve the above object, the present invention provides a method for achieving low temperature toughening of TRIP high entropy alloy, comprising the following steps: S1. Low-temperature pre-stretching: The TRIP high-entropy alloy is pre-stretched at low temperature to the end point of the Luders band deformation or the end point of the stress platform of the tensile stress-strain curve, and then unloaded; S2. Heat treatment: The TRIP high entropy alloy is kept warm between room temperature and melting point, followed by rapid cooling using a cooling medium.

[0007] Preferably, in S1, the mixing entropy of the TRIP high entropy alloy is greater than or equal to 1R, and below 273K, the TRIP high entropy alloy has the following characteristics: Luders band deformation occurs, or a stress plateau appears in the tensile stress-strain curve.

[0008] Preferably, in S1, the tensile strain rate is 10 -5 ~10 -1 / s.

[0009] Preferably, in S1, the unloading is between the plastic strain value corresponding to the end point of the Luders band deformation increasing or decreasing by 5% of the plastic strain value, and the unloading is between the plastic strain value corresponding to the end point of the stress platform of the tensile stress-strain curve increasing or decreasing by 5% of the plastic strain value.

[0010] Preferably, in S1, the TRIP high entropy alloy is in a cast state or obtained through a thermomechanical treatment.

[0011] Preferably, in S1, the temperature range of the low-temperature pre-stretching is between 77K and 273K.

[0012] Preferably, before S1, the TRIP high entropy alloy needs to be kept warm in a low temperature environment for more than 10 minutes.

[0013] Preferably, in S2, the cooling medium is one of water, quenching mineral oil or air.

[0014] Preferably, in S2, the insulation time of the insulation treatment is between 20 minutes and 90 minutes.

[0015] Preferably, in S2, the temperature of the heat preservation treatment is between 0.3Tm and 0.5Tm.

[0016] Therefore, the present invention adopts the above-mentioned method for achieving low-temperature toughening of TRIP high entropy alloy, and the beneficial effects are as follows: (1) Since TRIP high entropy alloys often exhibit Luders band deformation characteristics or stress platform characteristics in the tensile stress-strain curve at low temperatures (below 273K), the present invention adopts low-temperature pre-stretching to low-temperature stretch the TRIP high entropy alloy with the above characteristics to the end point of the Luders band deformation or near the end point of the stress platform (within the range of 5% plastic strain). At this time, the alloy undergoes a large-area phase change (such as the transformation of the face-centered cubic FCC structure to the body-centered cubic BCC structure or the close-packed hexagonal HCP structure). Not only does the crystal structure of the phase change region change, but a large number of dislocations are distributed, while the non-phase change region still maintains the original crystal structure and has a low dislocation density.

[0017] (2) The present invention adopts heat treatment to partially recover the dislocations in the phase transformation region, and the phase transformation region and the non-phase transformation region will undergo recrystallization to varying degrees, thereby preparing a high-entropy alloy with two phase structures and two grain size characteristics. The fine-grained structure, residual dislocations and back stress caused by the deformation of the two grain sizes can significantly improve the low-temperature yield strength of the high-entropy alloy, and the deformation coordination between different grain sizes and phase structures can also ensure that the low-temperature plasticity of the high-entropy alloy is not reduced, thereby achieving the effect of simultaneously improving the low-temperature strength and plasticity.

[0018] (3) Compared with the solid solution strengthening, deformation strengthening, grain refinement and dispersion strengthening induced by conventional thermomechanical treatment, the multi-level heterogeneous structure formed by further processing the deformation characteristics of the TRIP high entropy alloy in the present invention can simultaneously achieve the strengthening and toughening of the high entropy alloy, rather than improving the strength at the expense of plasticity.

[0019] (4) The traditional strengthening method has limited strengthening effect on low-strength alloys (yield strength below 1000 MPa). The combined processing method of low-temperature pre-stretching and heat treatment adopted in the present invention still has significant strengthening effect on high-strength high-entropy alloys (yield strength above 1000 MPa).

[0020] (5) The present invention can be superimposed on traditional strengthening methods (solid solution strengthening, deformation strengthening, fine grain strengthening, dispersion strengthening, etc.), which means that the strength of the TRIP high entropy alloy strengthened by traditional strengthening methods can be further improved by the method proposed in the present invention without reducing the plasticity, thereby achieving low-temperature toughening of the TRIP high entropy alloy.

[0021] The technical solution of the present invention is further described in detail below through the accompanying drawings and embodiments. BRIEF DESCRIPTION OF THE DRAWINGS

[0022] Figure 1 Schematic diagram of tensile stress-strain curves of TRIP high entropy alloys in Example 1 and Comparative Example 1 of a method for achieving low-temperature toughening of TRIP high entropy alloys according to the present invention; Figure 2 Schematic diagram of the microstructure of Example 1 and Comparative Example 1 of a method for achieving low-temperature toughening of TRIP high-entropy alloys of the present invention, wherein (a) is a schematic diagram of the grains of the sample of Comparative Example 1, (b) is a schematic diagram of the BCC phase and FCC phase of the sample of Comparative Example 1, (c) is a schematic diagram of the grains of the sample of Example 1, and (d) is a schematic diagram of the BCC phase and FCC phase of the sample of Example 1. DETAILED DESCRIPTION

[0023] The technical solution of the present invention is further described below through the accompanying drawings and embodiments.

[0024] Unless otherwise defined, technical or scientific terms used in the present invention shall have the common meanings understood by one having ordinary skills in the field to which the present invention belongs.

[0025] Comparative Example 1 Sample: Composition: Fe 62 Cr 13 Ni 10 A TRIP high entropy alloy of Si7Al3Co5 (atomic percentage) is obtained by heat preservation at 1200°C for 6 hours after casting, and then by 85% rolling at room temperature and heat preservation at 800°C for 10 minutes to obtain the TRIP high entropy alloy before treatment.

[0026] Embodiment 1 The TRIP high entropy alloy obtained in the above comparative example 1 was subjected to low temperature pre-stretching at 77 K, and the tensile strain rate was 10 -3 / s. Unloading at the end point of the Luders band (i.e., unloading at the position of 10% plastic strain). The unloaded sample is subjected to a heat preservation treatment at a temperature between 0.3 and 0.5 times the melting point temperature (Tm) of the sample alloy. In this embodiment, the unloaded sample is kept at 700°C for 60 minutes, and the cooling method is water cooling to obtain the treated TRIP high entropy alloy.

[0027] Depend on Figure 2 It can be seen that before treatment, the TRIP high entropy alloy of comparative example 1 is a two-phase structure, namely, body-centered cubic (BCC) and face-centered cubic (FCC) structures, and the grain size distribution is relatively uniform; after treatment, the TRIP high entropy alloy of embodiment 1 is still a two-phase structure. Although the ratio of the two phases has not changed significantly, the grain structure in the two phases is a bimodal structure, that is, there are two sizes of large grains and small grains. The yield strength of the TRIP high entropy alloy of comparative example 1 at 77K is about 1200MPa. After treatment, the yield strength of the TRIP high entropy alloy of embodiment 1 is increased by about 200MPa, and the elongation at break is increased by about 5%, such as Figure 1 It shows that the method proposed in the present invention can significantly improve the low-temperature strength of TRIP high-entropy alloy without reducing plasticity, thereby achieving the effect of improving low-temperature mechanical properties.

[0028] Embodiment 2 For the composition of Fe 59 Cr 13 Ni 18 Al 10 (atomic percentage) of TRIP high entropy alloy, which is obtained after casting, 85% rolling at room temperature and keeping at 800°C for 10 minutes.

[0029] The TRIP high entropy alloy was pre-stretched at 77K at a strain rate of 10 -4 / s, unload at the end point of the stress platform of the low-temperature tensile stress-strain curve (i.e., unload at the 10% plastic strain position). The unloaded sample is kept at 700℃ for 50min and cooled by water.

[0030] The TRIP high entropy alloys before and after treatment are both two-phase structures, the grain size distribution before treatment is relatively uniform, and the grain structure of the TRIP high entropy alloy after treatment is a bimodal structure. The yield strength of the TRIP high entropy alloy at 77K before treatment is about 1250MPa. After treatment, the yield strength of the TRIP high entropy alloy is increased by about 200MPa, and the elongation at break does not change, indicating that the method proposed in the present invention can significantly improve the low-temperature strength of the TRIP high entropy alloy without reducing plasticity, and achieve the effect of low-temperature toughening.

[0031] Embodiment 3 For the composition V 10 Cr 10 Fe 45 Co 30 A TRIP high entropy alloy of Ni5 (atomic percentage) is obtained by casting, then undergoing 60% rolling at 1373K, keeping at 1473K for 120 min (water cooling), 50% rolling at room temperature and keeping at 1123K for 60 min (water cooling).

[0032] The TRIP high entropy alloy was pre-stretched at 77K at a strain rate of 10 -3 / s, unload at the end point of the stress platform of the low-temperature tensile stress-strain curve (i.e., unload at the 20% plastic strain position). The unloaded sample is kept at 800℃ for 60min and cooled by water.

[0033] The TRIP high entropy alloys before and after treatment are both single-phase face-centered cubic structures with relatively uniform grain size distribution. The TRIP high entropy alloys after treatment are face-centered cubic and body-centered cubic dual-phase structures with a dual-mode grain structure. The yield strength of the TRIP high entropy alloy at 77K before treatment is about 680MPa. After treatment, the yield strength of the high entropy alloy is increased by about 300MPa, and the elongation at break does not change, indicating that the method proposed in the present invention can significantly improve the low-temperature strength of the TRIP high entropy alloy without reducing plasticity, and achieve the effect of low-temperature toughening.

[0034] Therefore, the present invention adopts the above-mentioned method for realizing low-temperature toughening of TRIP high-entropy alloy, and the TRIP high-entropy alloy is stretched at low temperature to the end point of the Luders band deformation or the end point of the stress platform (within the range of 5% plastic strain), at which time the alloy undergoes a large-area phase change (such as the transformation of the face-centered cubic structure to the body-centered cubic structure or the close-packed hexagonal structure), and the phase change region not only changes the crystal structure, but also distributes a large number of dislocations, while the non-phase change region still maintains the original crystal structure and the dislocation density is low. The subsequent heat treatment causes the dislocations in the phase change region to partially recover, and the phase change region and the non-phase change region will undergo different degrees of recrystallization, thereby preparing a high-entropy alloy with two phase structures and two grain size characteristics at the same time, and the fine grain structure, the residual dislocations and the back stress caused by the deformation of the two grain sizes can significantly improve the low-temperature yield strength of the high-entropy alloy, and the deformation coordination between different grain sizes and phase structures can also ensure that the low-temperature plasticity of the high-entropy alloy is not reduced, thereby achieving the effect of simultaneously improving the low-temperature strength and plasticity.

[0035] Finally, it should be noted that the above embodiments are only used to illustrate the technical solution of the present invention rather than to limit it. Although the present invention has been described in detail with reference to the preferred embodiments, those skilled in the art should understand that they can still modify or replace the technical solution of the present invention with equivalents, and these modifications or equivalent replacements cannot cause the modified technical solution to deviate from the spirit and scope of the technical solution of the present invention.

Claims

1. A method for achieving low temperature toughening of TRIP high entropy alloys, characterized in that: The following steps are involved: S1. Low-temperature pre-stretching: The TRIP high-entropy alloy is pre-stretched at low temperature to the end point of the Luders band deformation or the end point of the stress platform of the tensile stress-strain curve, and then unloaded; S2. Heat treatment: The TRIP high entropy alloy is kept warm between room temperature and melting point, followed by rapid cooling using a cooling medium.

2. A method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: In S1, the mixing entropy of the TRIP high entropy alloy is greater than or equal to 1R, and below 273K, the TRIP high entropy alloy has the following characteristics: Luders band deformation occurs, or a stress plateau appears in the tensile stress-strain curve.

3. A method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: In S1, the tensile strain rate is 10 -5 ~10 -1 / s.

4. A method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: In S1, the unloading is between the plastic strain value corresponding to the end point of the Luders band deformation increasing or decreasing by 5% of the plastic strain value, and the unloading is between the plastic strain value corresponding to the end point of the stress platform of the tensile stress-strain curve increasing or decreasing by 5% of the plastic strain value.

5. The method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: In S1, the TRIP high entropy alloy is in a cast state or obtained through a thermomechanical treatment.

6. A method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: In S1, the temperature range of the low-temperature pre-stretching is between 77K and 273K.

7. The method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: Before S1, the TRIP high entropy alloy needs to be kept warm in a low temperature environment for more than 10 minutes.

8. The method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: In S2, the cooling medium is one of water, quenching mineral oil or air.

9. The method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: In S2, the insulation time of the insulation treatment is between 20 minutes and 90 minutes.

10. The method for achieving low temperature toughening of TRIP high entropy alloy according to claim 1, characterized in that: In S2, the temperature of the heat preservation treatment is between 0.3Tm and 0.5Tm.

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