Method for preparing superelastic Ni-Mn-Ti alloy with high porosity and adjustable porosity based on vacuum oxygen uptake control pressureless sintering

Abstract

The invention discloses a method for preparing a superelastic Ni-Mn-Ti alloy with high porosity and adjustable porosity based on vacuum oxygen uptake control pressureless sintering, and relates to the technical field of solid refrigeration. The preparation method comprises the following steps: preparing high-purity spherical Ni-Mn-Ti powder by adopting an electrode induction melting gas atomization method; a vacuum oxygen uptake control system containing high-purity Ti powder and Mn powder is constructed, and a low-oxygen partial pressure closed environment is formed through physical isolation and gas control; and placing the sealed sample in a tubular furnace for pressureless sintering. According to the method, a continuously adjustable porous structure with the porosity of 60-90% can be formed through direct sintering under the condition of no external pressure, Mn volatilization and Ti oxidation are effectively inhibited, and composition stability is kept. The obtained alloy has an L21 ordered austenite structure, reversible martensite phase transformation can be achieved without subsequent heat treatment, excellent hyperelasticity, low heat lag and a remarkable elastic heat effect are shown, and a new way is provided for preparation of a high-performance elastic heat refrigeration material.

Description

TECHNICAL FIELD

[0001] The present application relates to the technical field of solid refrigeration, and in particular to a method for preparing a super-elastic Ni-Mn-Ti alloy with high porosity and adjustable porosity based on vacuum oxygen absorption control and pressureless sintering. BACKGROUND

[0002] With the increasing seriousness of global climate change, developing solid refrigeration working substances that are efficient and have zero global warming potential has become one of the key areas for addressing climate change. The elastocaloric refrigeration technology is considered to be the most promising solid refrigeration technology due to its low energy consumption and environmental friendliness. The core principle of the elastocaloric refrigeration technology is that the material undergoes a solid-state phase transition under stress induction, accompanied by significant thermal effects. Ni-Mn-based Heusler alloys, as a new type of elastocaloric refrigeration material, have attracted widespread attention due to their low phase transition driving force, low phase transition hysteresis, and multi-field coupling. However, traditional Ni-Mn-based Heusler alloys (such as Ni-Mn-Ga, Ni-Mn-Sn, Ni-Mn-In, and Ni-Mn-Sb) have high brittleness, which greatly limits their application in practical applications.

[0003] To overcome this problem, Ni-Mn-Ti alloy, as a new type of Heusler alloy composed of pure transition metals, exhibits stronger metallic properties and good mechanical properties. Since the alloy significantly enhances the mechanical properties by sharing d-orbital electrons between atoms, it becomes an excellent candidate material for elastocaloric refrigeration working substances. However, existing research on the preparation of nickel-manganese-titanium alloys still focuses on two approaches: one is to use high-purity constituent elements to prepare bulk alloys under an argon atmosphere through arc melting; the other is to prepare alloys through arc melting and then perform directional solidification with the aid of heat treatment. However, the alloys prepared by these two methods lack effective adjustment in terms of density (porosity), and porosity (i.e., specific surface area) plays a crucial role in solid refrigeration materials. Proper porosity can provide more interfaces for heat transfer, and thus has an important influence on the refrigeration performance.

[0004] Patent CN120442994A discloses a Ni-Mn-Ti-Si alloy material, a preparation method, and applications thereof. The method uses raw material proportioning, vacuum arc multiple melting, and combines annealing and rapid water cooling to prepare a Ni-Mn-Ti-Si elastocaloric refrigeration alloy block. This method belongs to a typical melt metallurgy process, and the obtained material is a dense ingot, and does not involve powder atomization, sintering, or porosity control, so it cannot be used to prepare a Ni-Mn-Ti porous material with a controllable pore structure.

[0005] ​Patent CN120425179A discloses a method for preparing a porous Ni-Mn-Ti alloy with linear elastic martensitic phase transition. The method constructs a porous structure by powder annealing, powder mixing, sintering, and an additional pore forming step after sintering, and relies on multiple stress relief annealing to improve the martensitic phase transition hysteresis. The process does not achieve natural pore formation during the sintering stage, and the pore structure needs to be obtained through additional secondary processing, and the composition volatilization and oxygen content during sintering are not controlled. Therefore, although this technical route can prepare a porous Ni-Mn-Ti alloy, it does not have the characteristics of "directly obtaining adjustable porosity and maintaining stable composition during the sintering process" required by the present invention.

[0006] Patent CN115991975A discloses a high-performance high-temperature Ni-Mn-Ti-B solid-solid phase change heat storage material and its preparation method. The method prepares a Ni-Mn-Ti-B ingot by vacuum arc melting or induction melting, and realizes solid-solid phase change heat storage performance by high-temperature long-time annealing. The method is completely based on melt metallurgy and heat treatment process, and the obtained material is a high-density block, which does not involve powder preparation and pressureless sintering process, and has no pore structure regulation ability. The process cannot meet the needs of constructing a porous structure and maintaining the stability of the chemical composition through sintering.

[0007] Therefore, there is an urgent need for a preparation method that is simple, can directly form a controllable pore structure during sintering, and can effectively maintain the stability of the composition and phase structure of Ni-Mn-Ti alloy. SUMMARY

[0008] The present invention is to solve the problems of traditional electric arc melting and directional solidification method, such as difficulty in realizing porosity regulation, composition segregation, high temperature oxygen pollution, large sintering stress, and inability to obtain reversible martensitic phase transition without heat treatment, and further proposes a method for preparing a super-elastic Ni-Mn-Ti alloy with high porosity and adjustable porosity based on vacuum oxygen absorption control pressureless sintering.

[0009] The technical solution adopted by the present invention to solve the above problems is: The present invention proposes a method for preparing a super-elastic Ni-Mn-Ti alloy with high porosity and adjustable porosity based on vacuum oxygen absorption control pressureless sintering, which includes the following steps: Step 1: Preparation of high-purity nickel-manganese-titanium powder: Ni-Mn-Ti powder is prepared by electrode induction melting gas atomization method, and the chemical composition is Ni 49.5 Mn 32.5 Ti 18 (atomic percentage), and the powder particle size is 15-53 μm; Step 2, constructing a vacuum oxygen absorption control system: the Ni-Mn-Ti pre-alloy powder prepared in step 1 is placed in a crucible and, together with high-purity Ti powder as an oxygen absorber and high-purity Mn powder as a vapor pressure compensator, is placed in a quartz tube, the three being physically isolated; the quartz tube is vacuumed, washed with inert gas, back-filled with inert gas and sealed after the process, forming a closed sintering environment with low oxygen partial pressure; Step 3, pressureless sintering: the sealed quartz tube is placed in a tube furnace for pressureless sintering, and during the sintering process, the Ti powder and Mn powder are arranged in different zones to play their respective roles, the oxygen partial pressure is controlled, and the Mn evaporation is compensated, finally obtaining a porous Ni-Mn-Ti alloy with a porosity of 60-90% and super-elasticity and reversible martensitic phase transformation.

[0010] Further, in step 1, the electrode induction melting gas atomization method for powder preparation includes the following steps: under vacuum suspension conditions, melt the Ni-Mn-Ti master alloy rod, and additionally add 5 wt.% of manganese to compensate for the loss of manganese evaporation at high temperatures; at the same time, by adjusting the coupling depth of the induction coil and the suspension height to stabilize the superheat, the composition segregation in the suspension melting stage is reduced; the prepared Φ50×450 mm rod is introduced into the conical induction coil at a rotation speed of 10-20 r / min and a descending speed of 40-60 mm / min under a high-purity inert atmosphere, and the metal liquid stream is formed in a suspended and crucible-free state; the metal liquid stream is atomized into spherical droplets and rapidly solidified by high-pressure inert gas, and a double-stage inert gas curtain is used to isolate secondary oxidation during the atomization process.

[0011] Further, in step 2, the quartz tube is provided with a necked section, and the Ti powder zone, the Ni-Mn-Ti powder zone and the Mn powder zone are physically separated into three independent functional zones.

[0012] Further, in step 2, the vacuuming and inert gas treatment process is as follows: the quartz tube is vacuumed to a pressure lower than 1×10 -4 Pa, then 3-5 cycles of high-purity argon gas with a purity of 99.999% are circulated for washing, and then back-filled with 10 -1 bar argon and sealed with an acetylene flame.

[0013] Further, the sintering temperature is maintained at 1300-1400 K, the holding time is 4-12 h, and the heating rate is 5-10 K / min.

[0014] Further, in step 3, during sintering, the Ti powder is placed at the high-temperature end and the Mn powder is placed at the lower-temperature end.

[0015] Further, in step 3, after sintering is completed, the sample is naturally air-cooled in the furnace, and the finally obtained porous Ni-Mn-Ti alloy has a stable L21 ordered austenite structure, and can exhibit reversible martensitic phase transition without subsequent annealing.

[0016] The beneficial effects of the present application are: 1. The present application realizes the porous nickel-manganese-titanium alloy material which is difficult to obtain by traditional process through establishing the synergistic process of EIGA powdering, vacuum oxygen absorption control and pressureless sintering. The obtained alloy has stable composition, controllable porosity, and extremely low internal stress in the sintering process, and does not appear cracking and deformation; 2. The porous Ni-Mn-Ti alloy prepared by the present application exhibits clear and stable reversible martensitic phase transition (M s is 243-245 K), low phase transition thermal hysteresis (23-27 K), excellent superelasticity (compression strength reaches 540 MPa) and significant elastocaloric effect (adiabatic temperature change reaches-4.97 K) without any subsequent heat treatment.

[0017] 3. The present application discards the steps of press forming and complex heat treatment in the traditional process, avoids the internal stress and cracking problems caused by pressurization and rapid cooling, and provides an efficient and feasible preparation method for the application of nickel-manganese-titanium alloy in the field of solid refrigeration. BRIEF DESCRIPTION OF DRAWINGS

[0018] Figure 1 It is the characterization of the performance of the nickel-manganese-titanium powder produced by electrode induction melting gas atomization method in the embodiment of the present application; Figure 2 It is the micro-morphology feature of the sample prepared by pressureless sintering in the embodiment of the present application; Figure 3 It is the martensitic phase transition curve of the sample prepared by pressureless sintering in the embodiment of the present application; Figure 4 It is the compression performance curve of the sample prepared by pressureless sintering in the embodiment of the present application; Figure 5 It is the stress performance curve and elastocaloric performance diagram of the sample prepared by pressureless sintering in the embodiment of the present application. DETAILED DESCRIPTION

[0019] The present application proposes a new pressureless sintering preparation route for Ni-Mn-Ti elastocaloric alloy, which forms a synergistic effect through three steps of EIGA high-purity spherical powder preparation, vacuum oxygen absorption control system and pressureless sintering to form a porous structure, solves the long-term technical bottlenecks such as difficult realization of porosity control, easy segregation of composition, serious high-temperature oxygen pollution, large sintering stress and inability to obtain reversible martensitic phase transition without heat treatment by traditional arc melting and directional solidification method. The specific steps are as follows: Step 1: Preparation of high-purity nickel-manganese-titanium powder To obtain Ni with martensitic phase transformation characteristics near room temperature 49.5 Mn 32.5 Ti 18 This invention utilizes Electrode Induction Gas Atomization (EIGA) technology to produce alloy powder, and introduces novel composition compensation and melting stabilization measures suitable for Ni-Mn-Ti based on existing EIGA technology. First, Ni-Mn-Ti master alloy rods are melted under vacuum suspension conditions, with an additional 5 wt.% of metallic manganese added to compensate for the volatilization loss of Mn at high temperatures. Simultaneously, superheat is stabilized by controlling the coupling depth and suspension height of the induction coil, reducing compositional segregation during the suspension melting stage. The resulting Φ50×450 mm rods are introduced into a conical induction coil at a rotational speed of 10-20 r / min and a descent speed of 40-60 mm / min under a high-purity inert atmosphere, melting into a molten metal stream under suspension and crucible-free conditions.

[0020] The molten metal is atomized into spherical droplets by high-pressure inert gas in a clean environment without crucibles, nozzles, or refractory material contact, and then rapidly solidifies under controlled high-speed cooling. This invention constructs a two-stage inert gas curtain within the atomization chamber, effectively isolating secondary oxidation and maintaining extremely low oxygen partial pressures for the Ti and Mn components during solidification. Finally, spherical powder with a particle size of 15-53 μm is obtained through sieving, providing an ideal forming basis for subsequently constructing adjustable porosity structures without applying external pressure.

[0021] Step 2: Vacuum Oxygen Absorption Control System To address the common problem of phase structure destruction caused by Mn volatilization and Ti oxidation during high-temperature sintering of Ni-Mn-Ti alloys, this invention constructs a ternary synergistic oxygen control system combining vacuum, oxygen absorber, and vapor pressure balance. Specifically, high-purity Ni-Mn-Ti powder prepared by EIGA is placed in a zirconia crucible, while high-purity Ti powder and Mn powder are placed at different axial positions inside a quartz tube, maintaining physical isolation among the three: Ti powder acts as an oxygen absorber at high temperatures, continuously consuming residual oxygen; Mn powder forms a stable vapor pressure during the heating stage to compensate for the volatilization loss of the powder; and the alloy powder is kept in an independent sintering zone with extremely low oxygen partial pressure and controlled vapor pressure, thereby avoiding compositional drift and suppression of martensitic phase transformation.

[0022] Building upon this, the present invention further constructs an oxygen-controlling isolation structure inside the quartz tube. By setting a constricted section inside the quartz tube, the titanium powder region, the powder region, and the manganese powder region are divided into three independent functional areas. This physical isolation structure restricts direct contact between solids while allowing gas to diffuse slowly and controllably at high temperatures, ensuring that the effects of the oxygen absorber and vapor pressure compensator on the powder region are both continuous and stable. Finally, the quartz tube is passed through a <10 -4After high-vacuum evacuation at 10 Pa, perform 3-5 cycles of 99.999% high-purity argon gas circulation purging to remove adsorbed oxygen; then refill to 10 Pa. -1 Argon gas is used in a bar and an acetylene flame is used to seal the quartz tube, thereby creating a closed sintering environment with extremely low oxygen partial pressure and compositional self-balancing capability.

[0023] Step 3: Pressureless sintering to construct a porous nickel-manganese-titanium alloy structure The sealed samples were placed in a tube furnace for pressureless sintering. The sintering temperature was maintained at 1300-1400 K for 4-12 h, with a heating rate of 5-10 K / min. During sintering, this invention employs a staggered temperature zone arrangement: Ti powder is placed at the high-temperature end to preferentially absorb oxygen in the initial heating stage, while Mn powder is placed at the lower-temperature end to form stable vapor pressure compensation in the high-temperature stage. This staggered release method makes the oxygen partial pressure and Mn activity more stable during sintering, avoiding component drift caused by local oxidation or Mn volatilization, and ensuring the chemical and phase structure stability of Ni-Mn-Ti powder during sintering.

[0024] Due to the high packing characteristics of spherical powder and the ultra-low oxygen environment, the powder can spontaneously undergo neck growth and pore structure evolution without external pressure, achieving a continuously adjustable porosity of 60-90% without introducing residual stress. Natural air cooling after sintering avoids phase transformation impact, resulting in a porous Ni-Mn-Ti alloy with a stable L21 ordered austenitic structure. It exhibits reversible martensitic phase transformation, low hysteresis, and excellent cycling stability without subsequent annealing.

[0025] like Figure 1 As shown, Figure 1 (a) shows the macroscopic morphology of nickel-manganese-titanium powder prepared by electrode induction melting gas atomization method. The powder is generally spherical with a smooth surface, and no obvious hollow powder or satellite powder is observed, indicating that the powder morphology is uniform.

[0026] Figure 1 (b) shows the powder particle size distribution curve, and the powder's D... 10 =11.06 μm, D 50 =19.28 μm, D 90 =32.51 μm, with a narrow particle size distribution range, suitable for subsequent sintering process of stacking and shaping.

[0027] Figure 1 (c) is the X-ray diffraction pattern of the powder at room temperature. The diffraction peaks correspond to the L21 ordered austenite structure, and no other impurity phases were detected.

[0028] Figure 1 (d) shows the DSC curve of the powder in the range of 273–1673 K. It can be seen that the melting point corresponding to the melting peak is about 1383 K. This temperature can be used as a reference for the process window of pressureless sintering.

[0029] like Figure 2 As shown in (a)-(e), the pore structure evolution of porous nickel-manganese-titanium alloys under different sintering temperatures and holding times is illustrated. With the increase of sintering conditions, the necks between powders gradually increase, the particle outline changes from clear to blurred, and the pores gradually evolve from a connected state to a dispersed and semi-closed state, reflecting the continuous improvement of the sintering degree.

[0030] The corresponding density changes in Table 1 are... Figure 2 The microstructural changes shown are consistent: at lower temperatures and shorter times, the density is lower, and the pores form an interconnected network; as the temperature increases or the time is prolonged, the density increases significantly, and the pores gradually become isolated. Both of these characteristics indicate that the pressureless sintering process of this invention achieves controllable adjustment of porosity, and the density can be continuously controlled within the range of 60-90%.

[0031] Table 1. Composition (mean ± standard deviation) and density of structures under different sintering processes

[0032] Figure 3 DSC phase transformation curves of porous nickel-manganese-titanium alloys under different sintering conditions are presented. Clear endothermic and exothermic peaks of the martensitic phase transformation are observed in all samples. M… obtained by the tangent method… s M f A s A f The temperatures are listed in Table 2. As can be seen from the curve shapes and the data in the table, even with significant differences in sintering temperature and density, the phase transformation start and end temperatures of each sample remained within a similar range. s Approximately 243-245 K, A f The K value is approximately 267-271 K, and the phase transition range and thermal hysteresis are also basically the same.

[0033] Figure 3 The phase transformation peaks are stable with minimal shift in position. As shown in Table 2, changes in sintering temperature and porosity did not lead to significant alterations in the phase transformation characteristics. This indicates that the porous nickel-manganese-titanium alloy prepared in this invention maintains good compositional stability and structural consistency under pressureless sintering conditions, without any phase transformation temperature drift caused by element volatilization or segregation. Figure 3 Together with the results in Table 2, this invention effectively avoids component loss during sintering by using high-purity spherical powder and a vacuum oxygen absorption control system, ensuring that samples with different densities maintain consistent martensite-austenite reversible phase transformation behavior. This provides a key basis for obtaining stable elasto-thermal effects in materials without heat treatment.

[0034] Table 2. Characteristic martensitic phase transformation temperatures, temperature ranges, and thermal hysteresis (K) of structures under different sintering processes.

[0035] Figure 4 The stress-strain curves of the porous nickel-manganese-titanium alloy prepared in this invention under uniaxial compression conditions are shown. The maximum compressive stress is finally reached near 12% strain, and the maximum compressive strength is 540 MPa, indicating that the material maintains good load-bearing capacity and deformation stability during compression.

[0036] like Figure 5 As shown, Figure 5 The cyclic loading curve in (a) further demonstrates that the stress-strain response of the material is stable during multiple loading processes. Figure 5 (b) shows that temperature changes occur during each loading process, with the maximum adiabatic temperature change of -4.97 K occurring under a compressive stress of 450 MPa. The above description is merely a preferred embodiment of the present invention and is not intended to limit the present invention in any way. Although the present invention has been disclosed above with reference to preferred embodiments, it is not intended to limit the present invention. Any person skilled in the art can make some modifications or alterations to the above-disclosed technical content to create equivalent embodiments without departing from the scope of the present invention. Any simple modifications, equivalent substitutions, and improvements made to the above embodiments without departing from the scope of the present invention, based on the technical essence of the present invention and within the spirit and principles of the present invention, shall still fall within the protection scope of the present invention.

Claims

1. A method for preparing a high-porosity and tunable-porosity superelastic Ni-Mn-Ti alloy based on vacuum oxygen absorption controlled pressureless sintering, characterized in that, Includes the following steps: Step 1: Preparation of high-purity nickel-manganese-titanium powder: Ni-Mn-Ti powder was prepared by electrode induction melting gas atomization method, and its chemical composition is Ni 49.5 Mn 32.5 Ti 18 The powder particle size is 15-53 μm; Step 2: Constructing a vacuum oxygen absorption control system: Place the Ni-Mn-Ti pre-alloy powder prepared in Step 1 in a crucible, and place it together with high-purity Ti powder as an oxygen absorber and high-purity Mn powder as a vapor pressure compensator in a quartz tube, keeping the three physically isolated; after evacuating the quartz tube and purging it with inert gas, refill it with inert gas and seal it to form a closed sintering environment with low oxygen partial pressure; Step 3: Pressureless sintering: The sealed quartz tube is placed in a tube furnace for pressureless sintering. During the sintering process, the Ti powder and Mn powder are arranged in different regions to play their roles by staggering the temperature zones, controlling the oxygen partial pressure and compensating for Mn volatilization, and finally obtaining a porous Ni-Mn-Ti alloy with a porosity of 60-90% and superelasticity and reversible martensitic phase transformation.

2. The method according to claim 1, characterized in that, In step 1, the electrode induction melting gas atomization method for powder preparation includes the following steps: melting Ni-Mn-Ti master alloy rods under vacuum suspension conditions, and adding an additional 5 wt.% of metallic manganese to compensate for the volatilization loss of Mn at high temperature; simultaneously, stabilizing the superheat by adjusting the coupling depth and suspension height of the induction coil to reduce component segregation during the suspension melting stage; the obtained Φ50×450 mm rods are introduced into a conical induction coil at a rotation speed of 10-20 r / min and a descent speed of 40-60 mm / min under a high-purity inert atmosphere, melting to form a molten metal flow in a suspended and crucible-free state; the molten metal flow is atomized into spherical droplets by high-pressure inert gas and rapidly solidified, and a two-stage inert gas curtain is used to isolate secondary oxidation during the atomization process.

3. The method according to claim 1, characterized in that, In step 2, a constricted section is provided inside the quartz tube to physically separate the Ti powder region, the Ni-Mn-Ti powder region, and the Mn powder region into three independent functional areas.

4. The method according to claim 1, characterized in that, In step 2, the vacuuming and inert gas treatment process is as follows: the quartz tube is evacuated to a pressure below 1×10⁻⁶. -4 Then, perform 3-5 cycles of high-purity argon gas (99.999% purity) circulation purging, followed by refilling to 10 Pa. -1 Argon gas was used in the bar and sealed with an acetylene flame.

5. The method according to claim 1, characterized in that, In step 3, the sintering temperature is maintained at 1300-1400 K, held for 4-12 h, and the heating rate is 5-10 K / min.

6. The method according to claim 1, characterized in that, In step 3, during sintering, Ti powder is placed at the high-temperature end and Mn powder is placed at the lower-temperature end.

7. The method according to claim 1, characterized in that, In step 3, after sintering, the sample is naturally air-cooled in the furnace. The resulting porous Ni-Mn-Ti alloy has a stable L21 ordered austenitic structure and can exhibit a reversible martensitic phase transformation without subsequent annealing.

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