CN116005061B - Magnetic control memory alloy with gradient tissue structure and controllable magnetic performance and preparation method thereof - Google Patents
Magnetic control memory alloy with gradient tissue structure and controllable magnetic performance and preparation method thereof Download PDFInfo
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Abstract
The invention discloses a magnetic control memory alloy with gradient tissue structure and adjustable magnetic performance and a preparation method thereof, which introduces magnetic fields with different intensities into Ni 42 Co 8 Mn 39 Sn 11 In the alloy heat treatment process, the morphology of the edge and the core of the sample is effectively regulated, the surface layer is a precipitated second phase dense region in the obtained gradient tissue structure, the thickness is about 0.15-0.25mm, and the central layer is a martensite sheet layer between austenite grains and grains (in). The preparation technology fully utilizes the advantages of conventional heat treatment and strong magnetic field, and simultaneously obtains the tissue structure with optimized mechanical property and magnetic property under the same sample condition. The preparation parameter range is wider, and the preparation method has great advantages for mass production.
Description
Technical Field
The invention belongs to the technical field of alloy materials, and particularly relates to Ni with a gradient tissue structure and adjustable magnetic performance 42 Co 8 Mn 39 Sn 11 A method for preparing a magnetic control shape memory alloy.
Background
The magnetic control shape memory alloy is used as an intelligent material, can generate memory effect, magnetocaloric effect, magnetic resistance effect and the like through martensitic transformation and reverse phase transformation under the control conditions of magnetic field, temperature and the like, integrates the sensing effect and the driving effect, and has wide application prospect in the fields of aerospace, mechano-electronics, automobile energy, biological medicine and the like. In recent years, byThe Ni-Mn-Sn system Heusler shape memory alloy with the macroscopic strain driven by the external reinforced magnetic field becomes a research hot spot, and can induce larger magnetically induced strain and driving stress by taking the inherent magnetization difference delta M of the martensite austenitic phase as a phase change driving force, and simultaneously, the giant magneto-thermal and giant magneto-resistance effects are accompanied. But the alloy has highly ordered L2 1 Intermetallic phases tend to undergo rapid crack propagation when subjected to an applied load. By introducing fourth kind of elements Co, fe, etc., solid solution FCC type second phase is formed, which has obvious improvement effect on mechanical properties of alloy, but excessive second phase can obstruct L2 1 The martensitic transformation of the structure. Therefore, how to control the content and the morphology distribution of the second phase has important significance for comprehensively playing the advantages of two phases and optimizing the magnetic performance and the mechanical performance of the alloy. The introduction of the gradient structure can obtain an alloy structure with martensite/austenite and a second phase on the same sample, so that a proper local structure can be conveniently selected according to different requirements of magnetic properties and mechanical properties in the practical application process. Conventional heat treatments can only obtain a uniform austenite/martensite structure without introducing a second phase. By introducing a heat treatment means of strong magnetic field, local segregation can be regulated by influencing element distribution, thereby regulating the structure and performance, which technique has been proved effective in superalloy treatment (Li C, guo G, yuan Z, et Al. Journal ofAlloys and Compounds,2017, 720:272-276), while application of magnetic field in directional solidification has also been demonstrated to cause Ni-Mn-Ga, al-Cu, etc. alloys to form a cyclic gradient structure by causing solute diffusion and segregation (Hou L, daiY, fautrelleY, et Al, acta materials, 2020,199:383-396;Zhong Q,Zhong H,Han H,et al.Journal ofMaterials Science)&Technology,2022, 99:48-54). In conclusion, the strong magnetic field heat treatment technology is hopeful to control element diffusion, is used as a new technology for generating a gradient structure, and can regulate and control the thickness of two phases through factors such as magnetic field intensity, treatment temperature and the like.
Disclosure of Invention
The invention successfully designs a magnetic control shape memory alloy with dense second phase gamma phase on the outer surface layer and with a lamellar thickness of about 0.2-0.5mm and austenite grain and martensite gradient structure and adjustable magnetic property through high temperature heat treatment under the condition of strong magnetic field. Under the condition of unchanged element content, different magnetic field parameters are adjusted, and the prepared alloy structure is in a gradient structure formed by martensite sheet layers/austenite grains, wherein the outer surface layer of the alloy structure is dense gamma phase, and the center layer is in a gradient structure formed by martensite sheet layers/austenite grains. The thickness of the alloy sheet and the grain size of the core part change with the intensity of the magnetic field, but the gradient structure still keeps good stability and is not destroyed.
In order to achieve the above purpose, the technical scheme of the invention is as follows:
1. firstly, designing a target component organization, wherein the specific thought is as follows:
first, the common Ni can be known through element regulation 50-x Co x Mn 39 Sn 11 The system has the advantages that as the Co content increases, namely x increases, the solid solution second phase content in the alloy continuously increases, the austenite and martensite magnetization difference delta M also continuously increases, and the mechanical property is improved. However, at the same time, the increase of x can reduce the martensitic transformation temperature of the alloy to be lower than the room temperature, when x=9 and 10, the martensitic transformation characteristics disappear, and the magnetic entropy, the refrigerating capacity and the like of the alloy are required to be completed through the martensitic transformation and the reverse transformation process of the martensitic transformation at the room temperature, so that the factors are combined, and when x=8, the alloy has more second phases and maintains the martensitic transformation characteristics at the room temperature and the new phase magnetization difference delta M of the parent phase, so that Ni is selected 42 Co 8 Mn 39 Sn 11 Alloys (FIG. 1, ref: cong D Y, roth S, schultz L. Acta materials, 2012,60 (13-14): 5335-5351.). As the conventional smelting as-cast alloy has more defects and uneven elements, a single structure is obtained by long-time heat treatment, but a second phase is eliminated, the element segregation can be controlled by the strong magnetic field heat treatment, so that the material structure is regulated and controlled, different element enrichment occurs at the edge and the center of the material, different-phase structures are generated, the gradient material can be formed to have both martensite and austenite phases, and at the moment, the Co content of the whole austenite grain component is reduced due to the precipitation of the Co-rich second phase at the edge, therebyThe magnetic performance is further improved compared with that of a uniform tissue.
2. And preparing a component alloy master ingot. The method comprises the following steps:
1) Proportioning materials
Weighing clean pure metals Ni, co, mn and Sn according to the requirement;
2) Smelting preparation
Placing Ni particles at the bottom of a crucible of a smelting furnace, uniformly covering the upper part with Co, mn and Sn particles, and removing oxygen in a furnace chamber;
3) Smelting
Smelting alloy raw materials in a smelting furnace under vacuum, and cooling to obtain Ni 42 Co 8 Mn 39 Sn 11 Magnetic control shape alloy.
Further, the step 1) specifically comprises:
1.1 Metal Ni, co, mn and Sn; polishing to remove surface oxide skin, and cleaning with ultrasonic waves; the metals Ni, co, mn and Sn; the purity of the raw materials is more than or equal to 99.999wt.%;
1.2 Respectively weighing pure metals Ni, co, mn and Sn according to the atomic ratio of 42:8:39:11.
Further, the step 2) specifically comprises:
placing Ni particles at the bottom of a crucible of a smelting furnace, and using Co, mn and Sn; uniformly covering the particles above, sequentially pumping with a mechanical pump and a molecular pump, and filling protective gas until oxygen in the furnace chamber is removed;
further, the step 3) specifically comprises:
3.1 Firstly melting a pure Ni ingot in a furnace by using an arc melting method, removing residual O, melting alloy raw materials in the melting furnace into alloy liquid, and cooling to obtain an alloy ingot; 2 to 3 mass percent of Mn raw material is additionally added in the smelting process, so that serious volatilization of Mn element caused by high temperature is compensated; the cooling mode adopts a copper mold furnace for natural cooling; induction melting may also be used; the subsequent pouring process can also be adopted to prepare samples with different sizes;
3.2 Homogenizing the alloy cast ingot obtained in the step 3.1) to obtain as-cast Ni 42 Co 8 Mn 39 Sn 11 And (3) alloy.
3. Preparation of gradient tissue was performed. The method comprises the following steps:
1) Sealing tube
To cast Ni 42 Co 8 Mn 39 Sn 11 And (5) placing the polished alloy into a quartz tube for oxyhydrogen flame vacuum sealing.
2) Magnetic field heat treatment
Placing the sample into a magnetic field heating device, raising the temperature to the temperature of a single austenite phase region, preserving heat under a strong magnetic field, and then quenching the sample to obtain Ni with a gradient tissue structure and adjustable magnetic properties 42 Co 8 Mn 39 Sn 11 A magnetic control shape memory alloy.
Further, the step 1) specifically comprises:
1.1 As-cast Ni 42 Co 8 Mn 39 Sn 11 Polishing to remove oxide skin, grinding into cubic block with size of about 20mm×8mm×8mm, placing into quartz tube, and vacuumizing to 10 -3 And (3) filling high-purity argon gas below Pa, heating the middle position of the quartz tube to locally soften the quartz tube, and kneading to obtain the sealed quartz tube containing the sample.
Further, the step 2) specifically comprises:
2.1 Placing the sealed quartz tube into a large quartz tube, uniformly heating the large quartz tube in a high-temperature resistance heating furnace, inserting the large quartz tube into a hole of a high static magnetic field device, applying a magnetic field parallel to the axial direction of a sample while heating, setting the magnetic field parameter to be 3T-10T, preserving heat for 72 hours at 930-960 ℃ under an external magnetic field, then rapidly taking out a sample, smashing the quartz tube, and quenching the sample. Polishing the surface layer of the sample, removing the oxide layer to obtain Ni with gradient tissue structure and adjustable magnetic property 42 Co 8 Mn 39 Sn 11 A magnetic control shape memory alloy.
Compared with the prior art, the invention has the following beneficial technical effects:
1. the invention selects Ni 42 Co 8 Mn 39 Sn 11 The alloy is subjected to magnetic field heat treatment to obtain a second phase gamma phase with a dense outer surface layer, the second phase is about 0.15-0.25mm in size, and the central layer is made of a gradient tissue material of austenite grains and martensite sheets, which is completely different from a single-phase structure obtained by conventional heat treatment, and the Co content of the central layer is reduced due to precipitation of the second phase, so that compared with the conventional heat treatment, the alloy can obtain higher phase transformation driving force delta M, higher magnetic entropy, refrigerating capacity and lower phase transformation heat hysteresis loss delta T. The outer layer is composed of a second phase which is basically dendritic and short rod-shaped, and has high magnetization intensity and excellent mechanical property. So the magnetic field heat treatment realizes further optimization of the comprehensive performance.
2. The invention breaks through the idea of the traditional high-temperature heat treatment regulation structure in the Ni-Co-Mn-Sn system, utilizes the coupling of the effect of strong magnetic field regulation element segregation and heat treatment homogenization structure, realizes the preparation of two types of structures on the same sample of the Ni-Co-Mn-Sn system alloy, and selects different structural parts according to the requirements of actual magnetic property and mechanical property. And the performances of the two structures can be continuously adjusted through the adjustment of the intensity of the external magnetic field. The practical experimental preparation method is flexible and various, and can be produced in a large scale.
3. The gradient structure obtained by the strong magnetic field heat treatment method has stronger stability, can obtain gradient structure by quenching after 72h heat preservation in the magnetic field range of 3T-10T and the temperature range of 930 ℃ to 960 ℃, and can not separate out other miscellaneous phases in the two-layer structure to deteriorate alloy performance, thus having certain fault tolerance for the condition that actual production deviates from technological card parameters.
Drawings
FIG. 1 is Ni 50-x Co x Mn 39 Sn 11 The martensitic transformation temperature and paramagnetic-ferromagnetic transformation temperature of the alloy are shown as the Co content changes.
FIG. 2 is a three-dimensional schematic diagram of a gradient structure obtained by the present invention, (a) a gradient structure surface layer structure after magnetic field heat treatment; (b) schematic gradient structure after being cut along the section.
FIG. 3 is an SEM photograph of a cross-section of example 1 of the present invention after 3T magnetic field heat treatment.
Fig. 4 is an SEM tissue photograph and EDS result of the surface after the 3T magnetic field heat treatment of example 1 of the present invention.
FIG. 5 is a plot of magnetization M as a function of ambient temperature T after a 3T magnetic field heat treatment according to example 1 of the present invention.
FIG. 6 shows the magnetization M after the 3T magnetic field heat treatment according to example 1 of the present invention as a function of the external magnetic field strength H.
Wire (C)
Fig. 7 is an SEM tissue photograph of a cross section of example 2 of the present invention after 10T magnetic field heat treatment.
FIG. 8 is an SEM tissue photograph and EDS result of the surface after 10T magnetic field heat treatment of example 2 of the present invention.
FIG. 9 is a graph showing the magnetization M after 10T magnetic field heat treatment according to example 2 of the present invention.
FIG. 10 is a plot of magnetization M as a function of external magnetic field strength H after 10T magnetic field heat treatment in accordance with example 2 of the present invention.
FIG. 11 is Ni after non-magnetic heat treatment 42 Co 8 Mn 39 Sn 11 SEM tissue picture of magnetic control shape memory alloy.
FIG. 12 is Ni after non-magnetic heat treatment 42 Co 8 Mn 39 Sn 11 The magnetization M of the magnetic control shape memory alloy changes along with the external temperature T.
FIG. 13 is Ni after non-magnetic heat treatment 42 Co 8 Mn 39 Sn 11 The magnetization M of the magnetic control memory alloy changes along with the external magnetic field strength H.
FIG. 14 shows examples 1 and 2 of the present invention and non-magnetic heat treatment of Ni 42 Co 8 Mn 39 Sn 11 Temperature T of martensitic transformation for comparison M A variation graph.
FIG. 15 shows examples 1 and 2 of the present invention and non-magnetic heat treatment of Ni 42 Co 8 Mn 39 Sn 11 Comparative magnetization difference ΔM change plot
FIG. 16 shows examples 1 and 2 of the present invention and non-magnetic heat treatment of Ni 42 Co 8 Mn 39 Sn 11 Comparative martensitic transformation and inverse transformation temperature difference delta T thereofA drawing.
FIG. 17 shows examples 1 and 2 of the present invention and non-magnetic heat treatment of Ni 42 Co 8 Mn 39 Sn 11 A graph of the comparative magnetic entropy deltas.
FIG. 18 shows examples 1 and 2 of the present invention and non-magnetic heat treatment of Ni 42 Co 8 Mn 39 Sn 11 Comparative cooling capacity RC change graph.
Detailed Description
In order that the above-recited objects, features and advantages of the present invention will be more clearly understood, a more particular description of the invention will be rendered by reference to specific embodiments thereof which are illustrated in the appended drawings. It should be noted that, without conflict, the embodiments of the present invention and features in the embodiments may be combined with each other.
Unless defined otherwise, all technical and scientific terms used herein have the same meaning as commonly understood by one of ordinary skill in the art to which this invention belongs. The terminology used herein in the description of the invention is for the purpose of describing particular embodiments only and is not intended to be limiting of the invention.
The embodiment of the invention comprises two parts:
the first part is to determine an alloy system and successfully prepare Ni with gradient structure and adjustable excellent magnetic property by adopting a high-temperature heat treatment method under a strong magnetic field 42 Co 8 Mn 39 Sn 11 The magnetic control shape memory alloy comprises the following specific steps:
in the design stage: step 1.1.1 early stage first, the common Ni is known through element regulation 50 -xCo x Mn 39 Sn 11 The system has the advantages that as the Co content increases, namely x increases, the solid solution second phase content in the alloy continuously increases, the austenite and martensite magnetization difference delta M also continuously increases, and the mechanical property is improved. However, at the same time, the increase of x can reduce the martensitic transformation temperature of the alloy to be lower than the room temperature, when x=9 and 10, the martensitic transformation characteristics disappear, and the magnetic entropy, the refrigerating capacity and the like of the alloy are required to be completed through the martensitic transformation and the reverse transformation process thereof at the room temperature, thereby combining the factorsConsidering that when x=8, the alloy has more second phases while maintaining the martensitic transformation characteristics close to room temperature and a larger difference Δm in magnetization of the new phase of the parent phase, ni is selected 42 Co 8 Mn 39 Sn 11 Alloy (fig. 1). As the conventional smelting as-cast alloy has more defects and uneven elements, a single structure is obtained by long-time heat treatment, but a second phase is eliminated, through the strong magnetic field heat treatment, element segregation can be controlled, so that the material structure is regulated and controlled, different element enrichment occurs at the edge and the center of the material, different-phase structures are generated, a gradient material can be formed, and the gradient material can have both martensite and austenite phases, and at the moment, the Co content of the whole austenite grain component is reduced due to precipitation of the Co second phase rich at the edge, so that the magnetic property of the magnetic property phase structure is further improved compared with that of the uniform structure.
By heat treatment under 3T-10T strong static magnetic field, we obtain Ni with gradient structure and adjustable excellent magnetic property 42 Co 8 Mn 39 Sn 11 The magnetic control shape memory alloy has a surface layer of dense second phase layer of about 0.15-0.25mm thick, and a central layer of austenite grains and martensite sheet.
In the preparation stage, in the step 1.2.1, according to the component proportion constructed as above, the corresponding mass percentage of each metal element is calculated through the atomic percentages of Ni, co, mn and Sn alloy components, and raw materials are weighed, wherein the purity of each metal element is more than 99.999 wt%.
Step 1.2.2 is to put the raw materials into a water-cooled copper mold crucible of a vacuum melting furnace, put Ni with low melting point element at the bottom of the crucible of the melting furnace and cover with Co, mn and Sn elements, and then vacuumize the furnace chamber to 1X 10 by using a mechanical pump and a molecular pump - 3 Pa, then filling Ar gas as protective gas, and repeating the vacuumizing process for 3 times to ensure that oxygen in the furnace chamber is completely discharged.
Step 1.2.3 firstly, utilizing a pre-placed Ti ingot to strike an arc and smelting the Ni ingot for 3-5 minutes to absorb trace oxygen possibly remained in a furnace chamber, observing that all materials are normal, starting alloy smelting, and preliminarily smelting alloy raw materials in a smelting furnace into alloy liquid. And after the alloy liquid is cooled, the manipulator is used for overturning and smelting for the second time. And adding magnetic stirring in the repeated overturning smelting process in the third and fourth times. And finally, checking the fluidity and uniformity of the alloy in the fifth and sixth smelting processes. Smelting for 3-5 minutes each time. Finally obtain as-cast Ni 42 Co 8 Mn 39 Sn 11 。
Step 1.2.4 As-cast Ni 42 Co 8 Mn 39 Sn 11 Polishing to remove oxide skin, placing in quartz tube, and vacuum-keeping quartz tube at 10 by molecular pump -3 Pa, then filling high-purity argon, and simultaneously heating the middle part of the quartz tube to locally soften the quartz tube by heating, and then kneading to obtain the vacuum packaging quartz tube for sealing the sample.
And 1.2.5, placing a thermocouple in a high-temperature resistance heating furnace, applying a magnetic field at the same time, calibrating temperature, determining output power, placing a sealed quartz tube into a large quartz tube after the temperature is stable, placing the large quartz tube into the high-temperature resistance heating furnace for uniform heating, inserting the large quartz tube into a hole of a high static magnetic field device, keeping the heating temperature at 930-960 ℃, applying a magnetic field parallel to the axis direction of a sample, setting the magnetic field parameter to be 3T-10T, preserving heat for 72h under the magnetic field, rapidly taking out the quartz tube for quenching after the heat preservation time is over, and smashing and taking out the sample. Polishing the surface layer of the sample to remove the oxide layer to obtain Ni with gradient structure 42 Co 8 Mn 39 Sn 11 A magnetic control shape memory alloy.
The second part is a second phase layer with a dense outer surface layer, the central layer is a microstructure of martensite and austenite phases for analysis and performance test, and the second part is specifically carried out according to the following steps:
step 2.1, firstly, cutting an alloy by utilizing a wire electric discharge machine, testing a metallographic specimen, and cutting the metallographic specimen into block specimens with flat and parallel upper and lower surfaces.
And 2.2, embedding the alloy sample obtained by cutting, sequentially polishing by using 600# abrasive paper, 1500# abrasive paper, 2500# abrasive paper and 4000# abrasive paper, and polishing the alloy sample until the surface roughness to be measured is reduced to be less than 1.0 mu m.
Step 2.3 Using electropolishing with a polishing agent of 10% HClO by volume 4 The solutions were subjected to microscopic tissue observation using an Olympus metallographic microscope and a Scanning Electron Microscope (SEM), respectively.
And 2.4, cutting the cylindrical samples with the diameters of 3 and the heights of 3 on the outer surface layer and the core layer in a layering manner, polishing the cylindrical samples to be smooth, measuring M-T and M-H curves by using a physical property integrated measurement system (PPMS), and analyzing the M-T and M-H curves.
Example 1:
a gradient-structure magnetic control shape memory alloy with a second phase gamma phase, an outer surface layer of which is dense, a central layer of which is a martensite phase and an austenite phase, wherein the atomic ratio of an alloy system is Ni: co: mn: sn=42: 8:39:11, the magnetic field parameter of the magnetic field heat treatment is 3T, and the specific preparation method comprises the following steps:
polishing an oxide layer on the surface of pure metal by using sand paper, placing the metal into an ultrasonic cleaner, cleaning the metal with alcohol, calculating the mass ratio of the Ni-Co-Mn-Sn alloy according to the atomic ratio, and then weighing corresponding mass of Ni, co, mn, sn elements with purity of more than 99.999wt.% by using a balance.
The raw materials are put into a water-cooled copper mold crucible of a vacuum melting furnace, ni with low melting point is placed at the bottom of the crucible when the raw materials are placed, and the interior of a furnace chamber is vacuumized to 1 multiplied by 10 by a mechanical pump and a molecular pump in sequence -3 Pa, and filling protective gas Ar gas, the vacuumizing process needs to be repeated 2 times to ensure that oxygen in the furnace chamber is completely discharged.
Firstly, striking an arc by utilizing a pre-placed Ti ingot and smelting the Ti ingot for 3-5 minutes to absorb trace oxygen possibly remained in a furnace chamber, and then, fully melting alloy raw materials by utilizing arc smelting and preliminarily obtaining the alloy ingot.
And turning over the cooled alloy ingot and smelting again, and adding magnetic stirring in the third and fourth repeated turning over smelting processes. The alloy overturning smelting process needs to be repeated for at least five times, and each time is not less than 3 minutes.
Taking out the master alloy to determine the burning loss rate, and after meeting the conditions, taking out as-cast Ni 42 Co 8 Mn 39 Sn 11 Polishing to remove oxide skin, placing in a quartz tube, filling high-purity argon into the quartz tube,and meanwhile, heating the top of the quartz tube, heating and softening the quartz tube, and kneading to obtain the sealed quartz tube.
Placing a thermocouple in a high-temperature resistance heating furnace, applying a magnetic field at the same time, calibrating temperature, determining output power, placing a sealed quartz tube in a large quartz tube after the temperature is stable, placing the large quartz tube in the high-temperature resistance heating furnace for uniform heating, inserting the large quartz tube into a hole of a high static magnetic field device, heating at 960 ℃, applying the magnetic field parallel to the axis direction of a sample, keeping the magnetic field at 3T, keeping the temperature for 72h under the magnetic field, rapidly taking out the quartz tube after the heat preservation time is over, smashing the quartz tube, and quenching the sample. Polishing the surface layer and removing the oxide layer.
And cutting the alloy by using a wire electric discharge machine, and cutting the metallographic specimen into a sheet shape with flush upper and lower surfaces.
And inlaying the alloy sample obtained by cutting by using a sample inlaying machine, polishing by using 600# abrasive paper, 1500# abrasive paper, 2500# abrasive paper and 4000# abrasive paper, and polishing the sample until the surface roughness to be measured is reduced to below 1.5 mu m.
Using electropolishing, the polishing agent was 10% HClO by volume 4 The solutions were subjected to microscopic tissue observation using an Olympus metallographic microscope and a Scanning Electron Microscope (SEM), respectively. Fig. 3 shows the microstructure of example 1, from which it can be seen that the microstructure is composed of a second phase whose outer surface layer is dense, whose central layer is composed of martensite and austenite phases, whose second phase size is 0.15mm, and which is free from significant impurity phase formation, and fig. 4 shows a second phase enriched layer of the surface layer of example 1, in which six surfaces are basically all second phase dendrites different in size, and a small amount of austenite phase is present between dendrites, and three-dimensional schematic diagrams are shown in fig. 2, and it is further confirmed by EDS composition analysis that the second phase is rich in Co, mn, ni, sn, and the austenite phase is rich in Ni, sn. Since the second phase region contains a relatively large amount of Co-rich solid solution phase, the martensitic transformation characteristics will not be apparent, but the precipitation will reduce the core Co content, and thus the magnetization behavior of the core region will be positively affected. Therefore, cylindrical samples with the diameter of 3mm and the height of 3mm are cut from the center part and polished to be smooth, and the change curves of the magnetization of the cylindrical samples under different external magnetic fields at different temperatures are measured by a physical property integrated measurement system (PPMS) and used for the tablePhase change is characterized. Fig. 5 shows the magnetization M curves of example 1 at different temperatures, and it can be seen that, at the test magnetic fields of 0.1T,0.5T,1.0T,4.0T and 8.0T, the alloy undergoes a phase transformation from ferromagnetic austenite to paramagnetic martensite in the cooling phase, and undergoes a phase transformation from paramagnetic martensite to ferromagnetic austenite in the heating phase, which corresponds to the occurrence of a distinct martensitic transformation and reverse transformation thereof, respectively. And in the range of the test magnetic field, the phase transition temperature fluctuates within the range of 200K-320K, which is relatively close to the room temperature. As the test magnetic field increases, a kinetic trapping effect is created such that the phase transition temperature decreases. FIG. 6 shows the magnetization M curves for example 1 at different external magnetic field strengths H (0-1.5T), with test temperatures being 250K,275K,300K,325K and 350K, respectively, and with a portion of the test temperature points covering the martensitic transformation temperature range. It can be seen that as the temperature increases, the paramagnetic martensite in the alloy undergoes a reverse transformation to produce more ferromagnetic austenite, resulting in a faster reaction with a higher saturation magnetization from 32emu/g at 250K to 86emu/g at 325K, and a higher transformation, and then as the temperature continues to increase to 350K, the saturation magnetization decreases to 73emu/g, at which time the alloy has completed a reverse transformation of martensite, all of which is the austenite phase, and the austenite phase undergoes a curie transformation (ferromagnetic-paramagnetic) with an increase in temperature, resulting in a decrease in saturation magnetization. The optimal temperature range available is 250K-325K, the alloy undergoes martensitic transformation and its reverse transformation near room temperature, and a higher magnetic entropy (9.5J/kg/K), refrigerating capacity (510J/kg) and low transformation thermal hysteresis (28K-34K) results from the fact that the core Co content is reduced by precipitation of the outer layer second phase.
Example 2:
a gradient-structure magnetic control shape memory alloy with a second phase gamma phase, an outer surface layer of which is dense, a central layer of which is a martensite phase and an austenite phase, wherein the atomic ratio of an alloy system is Ni: co: mn: sn=42: 8:39:11, the magnetic field parameter of the magnetic field heat treatment is 3T, and the specific preparation method comprises the following steps:
polishing an oxide layer on the surface of pure metal by using sand paper, placing the metal into an ultrasonic cleaner, cleaning the metal with alcohol, calculating the mass ratio of the Ni-Co-Mn-Sn alloy according to the atomic ratio, and then weighing corresponding mass of Ni, co, mn, sn elements with purity of more than 99.999wt.% by using a balance.
The raw materials are put into a water-cooled copper mold crucible of a vacuum melting furnace, ni with low melting point is placed at the bottom of the crucible when the raw materials are placed, and the interior of a furnace chamber is vacuumized to 1 multiplied by 10 by a mechanical pump and a molecular pump in sequence -3 Pa, and filling protective gas Ar gas, the vacuumizing process needs to be repeated 2 times to ensure that oxygen in the furnace chamber is completely discharged.
Firstly, striking an arc by utilizing a pre-placed Ti ingot and smelting the Ti ingot for 3-5 minutes to absorb trace oxygen possibly remained in a furnace chamber, and then, fully melting alloy raw materials by utilizing arc smelting and preliminarily obtaining the alloy ingot.
And turning over the cooled alloy ingot and smelting again, and adding magnetic stirring in the third and fourth repeated turning over smelting processes. The alloy overturning smelting process needs to be repeated for at least five times, and each time is not less than 3 minutes.
Taking out the master alloy to determine the burning loss rate, and after meeting the conditions, taking out as-cast Ni 42 Co 8 Mn 39 Sn 11 Polishing to remove oxide skin, placing in a quartz tube, filling high-purity argon gas into the quartz tube, heating the top of the quartz tube, heating and softening the quartz tube, and kneading to obtain the sealed quartz tube.
Placing a thermocouple in a high-temperature resistance heating furnace, applying a magnetic field at the same time, calibrating temperature, determining output power, placing a sealed quartz tube in a large quartz tube after the temperature is stable, placing the large quartz tube in the high-temperature resistance heating furnace for uniform heating, inserting the large quartz tube into a hole of a high static magnetic field device, applying the magnetic field parallel to the axis direction of a sample while heating at 930 ℃, keeping the magnetic field strength at 10T, keeping the temperature for 72h under the magnetic field, rapidly taking out the quartz tube after the temperature is stable, smashing the quartz tube, and quenching the sample. Polishing the surface layer, removing the oxide layer,
and cutting the alloy by using a wire electric discharge machine, and cutting the metallographic specimen into a sheet shape with flush upper and lower surfaces.
And inlaying the alloy sample obtained by cutting by using a sample inlaying machine, polishing by using 600# abrasive paper, 1500# abrasive paper, 2500# abrasive paper and 4000# abrasive paper, and polishing the sample until the surface roughness to be measured is reduced to below 1.5 mu m.
Using electropolishing, the polishing agent was 10% HClO by volume 4 The solutions were subjected to microscopic tissue observation using an Olympus metallographic microscope and a Scanning Electron Microscope (SEM), respectively. Fig. 7 is a microstructure of example 2, from which it can be seen that the microstructure is composed of a second phase whose outer surface layer is dense, whose central layer is composed of martensite and austenite phases, whose second phase size is 0.25mm, without significant impurity phase formation. Fig. 8 is a diagram showing the second phase enriched layer of the surface layer of example 2, in which six surfaces are basically second phase dendrites having different sizes, a small amount of austenite phase is present between dendrites, and three-dimensional schematic diagram is shown in fig. 2, and it is further confirmed by EDS composition analysis that the second phase is rich in Co, mn, ni, sn, and the austenite phase is rich in Ni, sn. Since the second phase region contains a relatively large amount of Co-rich solid solution phase, the martensitic transformation characteristics will not be apparent, but the precipitation will reduce the core Co content, and thus the magnetization behavior of the core region will be positively affected. Therefore, cylindrical samples with the diameter of 3mm and the height of 3mm are cut from the core, polished to be smooth, and measured by a physical property integrated measurement system (PPMS), the change curves of the magnetization intensity of the cylindrical samples under different temperatures and different external magnetic fields are used for representing phase change. Fig. 9 shows the magnetization M curves of example 2 at different temperatures, and it can be seen that, at the test magnetic fields of 0.1T,0.5T,1.0T,4.0T and 8.0T, the alloy undergoes a phase transformation from ferromagnetic austenite to paramagnetic martensite in the cooling phase, and undergoes a phase transformation from paramagnetic martensite to ferromagnetic austenite in the heating phase, which corresponds to the occurrence of distinct martensitic transformation and reverse transformation thereof, respectively. And under different test magnetic fields, the phase transition temperature fluctuates within the range of 180K-280K and is relatively close to the room temperature. As the test magnetic field increases, a kinetic trapping effect is created such that the phase transition temperature decreases. FIG. 10 shows the variation curves of magnetization M of example 1 under different external magnetic field intensities H (0-1.5T), wherein the test temperatures are respectively 200K,250K,275K,300K and 350K, and the temperature points cover the martensitic transformation temperature range. It can be seen that the alloy at 200K and 250K has a magnetization of less than 20emu/g at 1.5T saturation due to its mainly paramagnetic state due to the high martensite content, and the temperature risesWhen 275K is reached, the saturation magnetization is rapidly increased to 80emu/g, which indicates that the martensite is reversely transformed into ferromagnetic austenite, the reaction is faster, the transformation amount is more, and the magnetization curves are not completely overlapped when the temperature is increased and decreased, thereby indicating that hysteresis loss exists to a certain extent in the martensite transformation process. Then when the temperature continues to rise to 300K, the saturation magnetization increases slowly, but still continues to increase to 110emu/g, which means that the martensite reverse phase transformation process is still continuing and the ferromagnetic austenite content is still increasing, and when the temperature continues to rise to 350K, the saturation magnetization starts to decrease, and the paramagnetic austenite is generated by Curie transformation (physical transformation) corresponding to the ferromagnetic austenite. The optimal temperature range available is 275K-300K, the alloy undergoes martensitic transformation and its reverse transformation near room temperature, and a higher magnetic entropy (17J/kg/K), refrigerating capacity (700J/kg) and low transformation thermal hysteresis (31K-39K) results from the fact that the core Co content is reduced by precipitation of the outer layer second phase.
Comparing the SEM tissue patterns of examples 1 and 2 with the tissue pattern obtained by the non-magnetic field heat treatment, as shown in fig. 11, it was found that the non-magnetic field heat treatment was similar to the reported experimental data in the literature, i.e., the second phase was completely eliminated, only the martensite/austenite single-phase structure was present, and the martensite and austenite grains tended to coexist due to the transformation temperature approaching room temperature. As shown in FIG. 12, compared with the magnetization curves (M-T and M-H), it can be seen that the phase transition temperature in the no-magnetic field state is 180K-270K, which is closer to room temperature. As shown in FIG. 13, the magnetization M change curves are measured under different external magnetic field intensities H (0-1.5T), the test temperatures are respectively 223K,238K,258K,273K and 283K, and the inside of the alloy is more in the paramagnetic state due to the fact that the martensite content is more at 223K, so that the magnetization at 1.5T is only 40emu/g, and the saturation magnetization is rapidly increased to 83emu/g when the temperature is increased to 238K, which means that the martensite is reversely transformed into ferromagnetic austenite at the moment, the reaction is faster, the transformation amount is more, then the saturation magnetization is slowly increased when the temperature is continuously increased, but is still continuously increased until the saturation magnetization is increased to 97emu/g when the temperature is continuously increased to 283K, which means that the reverse transformation process is continuously performed in the temperature range. Therefore, the available optimal temperature range is 223K-283K, the alloy generates martensitic transformation and reverse transformation near room temperature, and the magnetic entropy (3J/kg/K) and the refrigerating capacity (190J/kg) are weak because no outer layer Co-rich phase is precipitated and the martensitic Co content is high. From fig. 14 to 17, the magnetic properties of the core of the sample after the heat treatment of the magnetic field are more advantageous, which is represented by an increase in the martensitic transformation temperature (fig. 14), an increase in the magnetization difference (i.e., an increase in the transformation driving force, fig. 15), a decrease in the transformation thermal hysteresis (fig. 16), an increase in the transformation magnetic entropy (fig. 17), and an increase in the refrigerating capacity RC (fig. 18). Therefore, the excellent magnetic performance is obtained by adjusting the morphology of the gradient structure by utilizing the strong magnetic field heat treatment technology.
Finally, it should be noted that the above-mentioned embodiments are merely for illustrating the technical solution of the present invention and not for limiting the same, and although the present invention has been described in detail with reference to the preferred embodiments, it should be understood by those skilled in the art that modifications and equivalents may be made to the technical solution of the present invention without departing from the spirit and scope of the technical solution of the present invention.
Claims (5)
1. The preparation method of the magnetic control memory alloy with the gradient tissue structure and the controllable magnetic performance is characterized by comprising the following steps:
smelting in the step (1):
smelting raw materials of Ni, co, mn and Sn with the atomic ratio of 42:8:39:11 in a smelting furnace under high vacuum condition, and cooling to obtain as-cast Ni 42 Co 8 Mn 39 Sn 11 A memory alloy, wherein 2 to 3 mass percent of Mn raw material is additionally added in the smelting process;
and (2) carrying out heat treatment on the strong static magnetic field:
subjecting the as-cast Ni obtained in step (1) 42 Co 8 Mn 39 Sn 11 The memory alloy is placed in a quartz tube and vacuumized to 10 -3 Filling high-purity inert gas Ar under Pa, and then sealing the quartz tube under vacuum by utilizing oxyhydrogen flame; then the quartz tube with the sample is put into a magnetic field heating device, the heating temperature is increased to 930 ℃ to 960 ℃ of single-phase austenite,applying magnetic fields with different intensities of 3T-10T, preserving heat for 72h, and quenching the sample in the quartz tube with water to obtain Ni with gradient tissue structure and adjustable magnetic properties 42 Co 8 Mn 39 Sn 11 A magnetic control shape memory alloy;
the outer surface layer of the magnetic control memory alloy is a dense second phase, the thickness of the outer surface layer is 0.15-0.25-mm, and the central layer is an austenite grain layer and a martensite sheet layer.
2. The method for preparing the magnetic control memory alloy with the gradient tissue structure and the controllable magnetic performance according to claim 1, wherein the second phases of the outer surface layer of the magnetic control memory alloy are in a branch-shaped or short-rod-shaped morphology growing from outside to inside, and martensite sheets in the central layer are distributed in austenite grains or among the austenite grains.
3. The method for preparing the magnetic control memory alloy with the gradient tissue structure and the controllable magnetic performance according to claim 1, wherein the controllable magnetic performance of the magnetic control memory alloy comprises a phase transition temperature T M Phase change magnetic entropy delta S M Phase transition heat stagnation delta T H And refrigeration capacity RC, and phase transition temperature T M At 180-320K, the phase transition entropy delta S M The temperature is regulated within the range of 9.5-17J/kg/K and the refrigeration coefficient RC is regulated within the range of 510-700J/kg, and the phase change thermal hysteresis delta T H 28-39K.
4. The method for preparing the magnetic memory alloy with the gradient tissue structure and the controllable magnetic property according to claim 1, wherein the step (1) is specifically:
polishing and removing surface oxide skin of metal raw materials Ni, co, mn and Sn, and cleaning by ultrasonic waves; the purities of the raw materials of the metals Ni, co, mn and Sn are all more than or equal to 99.999 and wt percent;
respectively weighing pure metals Ni, co, mn and Sn according to an atomic ratio of 42:8:39:11;
placing Ni particles at the bottom of a crucible of a smelting furnace, uniformly covering the upper part with Co, mn and Sn particles, removing oxygen in a furnace chamber, melting alloy raw materials in the smelting furnace into alloy liquid by using an arc smelting method, and cooling to obtain an alloy cast ingot, wherein the cooling mode adopts natural cooling in a copper mold furnace;
homogenizing the obtained alloy cast ingot to obtain as-cast Ni 42 Co 8 Mn 39 Sn 11 And (3) alloy.
5. The method for preparing the magnetic memory alloy with the gradient tissue structure and the controllable magnetic property according to claim 1, wherein the step (2) is specifically:
subjecting the as-cast Ni obtained in step (1) 42 Co 8 Mn 39 Sn 11 Polishing the memory alloy to remove oxide skin, grinding into cubic blocks with the size of 20mm multiplied by 8mm, placing into a quartz tube, vacuumizing to 10 -3 Under Pa, recharging high-purity inert gas Ar, then intensively heating the middle part of the quartz tube by utilizing oxyhydrogen flame, heating and softening the quartz tube, and kneading to obtain a sealed quartz tube containing a sample;
placing the sealed quartz tube into a large quartz tube, uniformly heating the large quartz tube in a high-temperature resistance heating furnace, inserting the large quartz tube into a hole of a high static magnetic field device, applying a magnetic field parallel to the axial direction of a sample while heating, setting the magnetic field parameter to be 3T-10T, preserving heat at 930-960 ℃ under the magnetic field to be 72h, then rapidly taking out, crushing the quartz tube, and quenching a sample; polishing the surface layer of the sample, removing the oxide layer to obtain Ni with gradient tissue structure and adjustable magnetic property 42 Co 8 Mn 39 Sn 11 A magnetic control shape memory alloy.
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