A low dislocation density nanometer cantor alloy and a preparation method thereof

Abstract

The application belongs to the field of material preparation and specifically relates to a kind of low dislocation density nano Cantor alloy and its preparation method.The application comprises the following steps: (1) multi-axial forging: the initial Cantor alloy is subjected to multi-axial forging treatment; (2) homogenization treatment: uniform large-size grain structure is obtained by high-temperature long-time annealing; (3) cryogenic rolling: the homogenized Cantor alloy is subjected to cryogenic rolling, and the grains are refined by low-temperature induced twinning and large strain; (4) high-temperature instantaneous annealing: the Cantor alloy with nanometer-scale grains and low dislocation density is obtained by high-temperature instantaneous annealing. The low dislocation density characteristics of the prepared microstructure are beneficial to theoretical research and future structural design of new alloys, and the material utilization rate is high. The characteristic structure, i.e. microstructure, can be significantly refined to nanoscale in the entire volume, and uniform nanocrystalline grains are obtained.

Description

Technical Field

[0001] This invention belongs to the field of materials preparation, specifically relating to a low dislocation density nano-Cantor alloy and its preparation method. Background Technology

[0002] High entropy alloys (HEAs) have attracted widespread attention from scientists due to their excellent properties. Among the vast HEAs system, Cantor (CoCrFeNiMn) alloy stands out for its excellent ductility and low-temperature performance. According to the Hall-Petch relation, the strength of conventional metals increases with decreasing grain size. Therefore, alloys with UFG microstructure have higher mechanical strength compared to coarse-grained alloys (CG). For example, in the journal Materials Science & Engineering A, Vol. 705, September 29, 2017, pp. 411-419, Shahmir et al. achieved an ultimate tensile strength of ~990 MPa by refining the grain size of Cantor alloy through equal channel angular extrusion (ECAP). Numerous studies have shown that the strength of HEAs is directly related to the lamellar thickness and grain size. When the lamellar thickness is above submicron, the relationship between the material's strength and thickness satisfies the generalized Hall-Petch relation: the yield strength is proportional to the reciprocal of the square root of the thickness. However, when the thickness of the sheets decreases to the nanometer level, the constitutive relationship between the strength and thickness of the material has not yet been fully studied.

[0003] Currently, there are various methods for preparing ultrafine-grained high-entropy alloys. For example, Chinese patent CN201410545199.6 discloses a method for preparing high-strength and tough ultrafine-grained high-entropy alloys using powder metallurgy. However, it is well known that powder metallurgy is expensive and can only produce small samples, which cannot meet the needs of preparing larger samples in industrial production. In the field of high-entropy alloys, researchers have used traditional methods such as intense plastic deformation (SPD), high-pressure torsion (HPT), and equal channel extrusion (ECAP) to refine grains and obtain ultrafine-grained or nanocrystalline high-strength high-entropy alloys. However, the traditional SPD method has strict requirements for the preparation process and, similar to powder metallurgy, can only produce small-sized samples, making it difficult to achieve large-scale industrial production and application. Summary of the Invention

[0004] The purpose of this invention is to provide a low dislocation density nano-Cantor alloy and its preparation method.

[0005] The technical solution to achieve the objective of this invention is: a method for preparing low dislocation density nano-Cantor alloys, comprising the following steps:

[0006] Step (1) Multi-directional forging: The initial Cantor alloy is subjected to multi-directional forging treatment;

[0007] Step (2) Homogenization treatment: A uniform large-size grain structure is obtained by high-temperature long-time annealing;

[0008] Step (3) Deep cryogenic rolling: The homogenized Cantor alloy is subjected to deep cryogenic rolling, which drastically refines the grains through low-temperature induced twinning and large strain.

[0009] Step (4): High-temperature instantaneous annealing: Cantor alloy with nanoscale grains and low dislocation density is obtained by high-temperature instantaneous annealing.

[0010] Furthermore, the composition of the Cantor alloy is: equimolar ratios of Co, Cr, Fe, Mn and Ni.

[0011] Further, step (1) specifically involves: placing the Cantor alloy in an annealing furnace and holding it at 1200±10℃ for 20±2 minutes to obtain a uniform coarse-grained structure; then performing three consecutive drawing operations under an air forging hammer to remove defects in the pre-melted alloy.

[0012] Furthermore, step (2) specifically involves: keeping the temperature at 1100±10℃ for 2±0.1h.

[0013] Furthermore, the total reduction in step (3) cryogenic rolling is 75%-95%.

[0014] Furthermore, the total reduction in step (3) cryogenic rolling is 90%.

[0015] Furthermore, step (3) deep cryogenic rolling specifically involves immersing the homogenized Cantor alloy in liquid nitrogen for 3-10 hours to achieve a uniform deep cryogenic temperature.

[0016] The Cantor alloy, having reached cryogenic temperature, was subjected to repeated cryogenic rolling in a two-roll mill for both hot and cold rolling. The rolling speed was 35 Hz, with a reduction of 0.2 mm per pass. After each pass, the sample was immersed in liquid nitrogen for 10-30 minutes to ensure the cryogenic temperature.

[0017] Further, step (4) specifically involves: placing the deep-cold rolled Cantor alloy in an induction coil heating furnace for rapid annealing at 900-1000℃ for 25-45 seconds and then cooling it by air cooling.

[0018] Furthermore, the high-temperature instantaneous annealing temperature is 975℃, and the annealing time is 30s.

[0019] A low dislocation density nano-Cantor alloy was prepared using the method described above, resulting in an average grain size of less than 800 nm in the low dislocation density nanocrystalline structure.

[0020] This invention first obtains a uniform equiaxed coarse-grained structure through multi-directional forging and high-temperature long-term homogenization annealing, ensuring uniform deformation during subsequent deformation processes; secondly, liquid nitrogen rolling is used because the stacking fault energy of Cantor alloys is typically 21-25 mJ / m at room temperature (25°C). 2 Therefore, at room temperature, the plastic deformation of Cantor alloys is generally carried out by dislocation movement and climbing, and twinning only occurs at a large degree of deformation. As is well known, the stacking fault energy of materials decreases with decreasing external temperature. Therefore, rolling at liquid nitrogen temperature is chosen, changing the previous single dislocation movement to dislocation movement + deformation twinning behavior. The high-density twinning structure further refines the deformed structure. At 90% deformation, shear bands are also generated. The combined effect of multiple plastic deformation mechanisms results in a fine nanocrystalline + nanosheet structure in the deformed Cantor alloy. The severe deformation introduces a large amount of deformed structure, providing a recrystallization driving force for subsequent annealing. Finally, a high-temperature instantaneous annealing treatment is used to produce a uniform bulk nanostructured Cantor alloy. The principle of this treatment is that the high dislocation density generated by rolling is annihilated at high temperature, while short-term holding inhibits the coarsening of nanosheets and prevents recrystallized grain growth. The final result is a single-phase austenitic Cantor alloy with a clean, low dislocation density nanocrystalline microstructure throughout the entire volume.

[0021] Compared with the prior art, the significant advantages of this invention are:

[0022] (1) The Cantor alloy prepared by the present invention through four parts, namely multi-directional forging, high temperature long time, deep cold rolling and high temperature instantaneous process, has a uniform nanocrystalline structure; its low dislocation density characteristics of microstructure are beneficial to theoretical research and structural design of future new alloys.

[0023] (2) The nanocrystalline Cantor alloy prepared by the present invention has high material utilization and can significantly refine its characteristic structure, i.e., the thickness of the sheet, to the nanoscale within a large volume, thereby obtaining uniform nanosheets. Attached Figure Description

[0024] Figure 1 This is a schematic diagram of the process for preparing a Cantor alloy microstructure with low dislocation density according to the present invention; wherein: 1 is the initial Cantor alloy, 2 is a heat-insulating barrel containing liquid nitrogen, 3 is a two-roll mill for hot and cold rolling, 4 is a cryogenically rolled sample, 5 is a box-type annealing furnace, 6 is a sample after warm rolling, and 7 is an induction coil heating furnace.

[0025] Figure 2 This is a bright-field electron transmission microscope (TEM) image of the uniform nanosheet Cantor alloy obtained by the preparation method of the present invention in Example 1.

[0026] Figure 3 The image shows an EBSD image of the low dislocation density nanocrystalline Cantor alloy obtained by the preparation method of the present invention in Example 1.

[0027] Figure 4 This is a grain size statistics diagram of the low dislocation density nanocrystalline Cantor alloy obtained by the preparation method of the present invention in Example 1.

[0028] Figure 5 The image shows the engineering stress-strain curve of the low dislocation density nanocrystalline Cantor alloy obtained by the preparation method of the present invention in Example 1. Detailed Implementation

[0029] The present invention will now be described in further detail with reference to the accompanying drawings.

[0030] All the following implementation examples use commercially available Cantor alloys as the initial base material, the chemical composition of which is shown in Table 1. The base material is processed into a plate sample of a predetermined thickness (specific thickness is shown in the examples) and then processed according to the preparation method of this example. The equipment used in the embodiments of this invention includes a hot and cold two-roll mill, a box annealing furnace, and an induction coil heating furnace. For the specific preparation process, please refer to [link to relevant documentation]. Figure 1 The flowchart shown is for the preparation method of the present invention. Depending on the parameters of the preparation method, the specific preparation process can be found in [reference needed]. Figure 1 The flowchart of the preparation method of the present invention is shown. Depending on the parameters of the preparation method, Example 1 is constructed as described below.

[0031]

[0032] Example 1

[0033] The initial sample size was 50mm × 15mm × 15mm (length × width × thickness). The processing flow for the Cantor alloy material with a low dislocation density nanocrystalline microstructure prepared in this embodiment is as follows:

[0034] (1) The initial sample is surface treated and the edges are ground, which is beneficial to the cleanliness of the Cantor alloy sample surface and the bite of the rolls during multi-directional forging and subsequent repeated rolling.

[0035] (2) The initial Cantor alloy was placed in an annealing furnace and held at 1200℃ for 20 min to obtain a uniform coarse-grained structure. Then, three consecutive forging steps were performed under an air forging hammer, and each step required forging in all three directions (X, Y, Z). The final forged sample size was 30mm × 30mm × 200mm.

[0036] (3) The forged sample was subjected to high temperature and long-term homogenization treatment, and kept at 1100℃ for 2h to obtain a uniform equiaxed crystal structure with a grain size of about 180μm.

[0037] (4) The Cantor alloy that has undergone multi-directional forging and homogenization treatment is placed in a liquid nitrogen insulated tank for a long time (3h-10h) to achieve a uniform cryogenic temperature in liquid nitrogen.

[0038] (5) The stainless steel sheet that has reached cryogenic temperature is placed in a two-roll mill for repeated cryogenic rolling. The roll speed is 35 Hz, and the reduction is 0.2 mm per pass. After each pass, the sample is immersed in liquid nitrogen for 10-30 minutes to ensure the cryogenic temperature. The total reduction is 90%.

[0039] (6) The deep cold rolled Cantor alloy is placed in an induction coil heating furnace for rapid annealing at 950°C for 30 seconds, and the cooling method is air cooling.

[0040] EBSD images of the Cantor alloy with low dislocation density nanocrystals obtained in Example 1 are shown below. Figure 2 As shown, it is uniform throughout the entire volume.

Claims

1. A method for preparing a low dislocation density nano-Cantor alloy, characterized in that, Includes the following steps: Step (1) Multi-directional forging: The initial Cantor alloy is subjected to multi-directional forging treatment; Step (2) Homogenization treatment: A uniform large-size grain structure is obtained by high-temperature long-time annealing; Step (3) Deep cryogenic rolling: The homogenized Cantor alloy is subjected to deep cryogenic rolling, which drastically refines the grains through low-temperature induced twinning and large strain. Step (4): High-temperature instantaneous annealing: Cantor alloy with nanoscale grains and low dislocation density is obtained by high-temperature instantaneous annealing; The composition of Cantor alloy is: Co, Cr, Fe, Mn and Ni in equimolar ratio; Step (1) specifically involves placing the Cantor alloy in an annealing furnace and holding it at 1200±10℃ for 20±2 minutes to obtain a uniform coarse-grained structure; then performing three consecutive drawing operations under an air forging hammer to remove defects in the pre-melted alloy. Step (2) specifically involves: maintaining the temperature at 1100±10℃ for 2±0.1h; The total reduction in step (3) cryogenic rolling is 75%-95%; Step (3) Deep cryogenic rolling specifically involves immersing the homogenized Cantor alloy in liquid nitrogen for 3-10 hours to achieve a uniform deep cryogenic temperature; The Cantor alloy that has reached the cryogenic temperature is placed in a two-roll mill for repeated cryogenic rolling at a rolling speed of 35 Hz. Each pass reduces the temperature by 0.2 mm. After each pass, the sample is placed back into liquid nitrogen for 10-30 minutes to ensure the cryogenic temperature. Step (4) is as follows: the deep-cold rolled Cantor alloy is placed in an induction coil heating furnace and rapidly annealed at 900-1000℃ for 25-45s, and then cooled by air cooling.

2. The method according to claim 1, characterized in that, In step (3), the total reduction in cryogenic rolling is 90%.

3. The method according to claim 2, characterized in that, The high-temperature instantaneous annealing temperature is 975℃, and the annealing time is 30s.

4. A low dislocation density nano-Cantor alloy, characterized in that, Prepared using the method described in any one of claims 1-3, the average grain size in the low dislocation density nanocrystalline structure is refined to below 800 nm.

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