Tailoring Of Grain Morphology And Elastocaloric Properties Of Cu-Al-Mn Shape Memory Alloy Microwires

Based on the elastocaloric effect of shape memory alloys(SMAs),the elastocaloric refrigeration technology is expected to become a new refrigeration technology to replace the traditional vapor compression refrigeration with its advantages of high efficiency,environmental protection and economy.Cu-based shape memory alloys with high elastocaloric properties and low cost have become the most promising shape memory alloys besides Ni-Ti based alloys.In this paper,Cu-Al-Mn alloy micowires with excellent elastocaloric effect at room temperature were successfully prepared by glass coating method.The relationship between grain morphology and elastocaloric properties of Cu-Al-Mn micowires was studied by adjusting the grain morphology.The way to improve the superelastic and elastocaloric properties of Cu-Al-Mn micowires from the perspective of grain design was clarified.After quenching at 1073 K and aging at 423 K,a single?phase with thermoelastic martensitic transformation characteristics was obtained in Cu₇₁Al₁₈Mn₁₁(at.%)alloy.For the superelasticity of fine-grained Cu₇₁Al₁₈Mn₁₁microwires,it was found that the critical stress was up to 180?200 MPa,the superelastic transformation strain was only 2%?3%.Moreover,a small residual strain was obtained after unloading,and the stress hysteresis was as high as 100MPa.When the abiemnt temperature was 303 K,the adiabatic temperature change of fine-grained microwires was 3?5 K under the uniaxial stress of 300?350 MPa,but the reversibility was poor.The temperature change during loading and unloading was asymmetric,and the microwires brook after 50 cycles.Cu₇₁Al₁₈Mn₁₁ microwires with bamboo-liked grains were prepared by high temperature annealing.Compare to the fine-grained microwires,the transition critical stress of bamboo-grained microwires were greatly reduced and the superelastic transforamtion strain was increased.The stress-induced isothermal entropy change was 10.1 J/kg·K at room temperature.The two-way shape memory effect test showed that the stress hysteresis of the bamboo-grained microwires did not change with the increase of the applied stress.The elastocaloric tests show that the microwires was uniform and reversible and it can achieve 5 K temperature change under a uniaxial stress of 150 MPa.However,the stability of the elastocaloric cycle was still not good enough.In order to enhance cyclic stability of elastocaloric effect,the bamboo-grained Cu₇₁Al₁₈Mn₁₁ microwires was continuously cold drawn at room temperature to refine the grains and improve the yield strength.Meanwhile,the texture was introduced into the microwires to improve the phase transition coordination,making it can withstand higher external stress,which in turn improves the elastocaloric cycling stability of microwires.The study showed that the textured oligocrystalline microwires had a?101?texture parallel to the microwire axis after cold drawing,the grain size was reduced to less than 100?m,the critical stress of phase transition was increased,and the transition strain was slightly decline.The results of the elasto caloric test showed that an average adiabatic temperature change of 5.4 K can be obtained at room temperature,and a stable temperature change of about 3 K can be maintained during 275elastocaoloric cycles.However,a clear non-uniform distribution of adiabatic temperature change was observed on the microwire,where the highest part can reach 7.1 K,but the lowest part was only 2.9 K.The non-uniform elastocaloric effect originated from the strain localization during the phase transformation process,which was related to the grain mophology of the oligocrystalline microwires.Although the elastocaloric cycle stability of the Cu₇₁Al₁₈Mn₁₁ microwires were improved by cold drawing,however,the martensitic transformation was restrained due to the introduction of more grain bound ary area,resulting in lower adiabatic temperature change.In order to obtain large adiabatic temperature change and stable elastocaloric cycle at the same time,a new Cu_71.1Al_17.2Mn_11.7microwire with high elastocaloric properties was prepared by adjusting the grain orientation,microwires diameter,chemical composition and other parameters based on the bamboo-liked grain mophology.Compared with the previous results,a large reversible adiabatic temperature change of 11.9 K was directly measured at room temperature,and a large stress-indudced isothermal entropy change of about 21 J/kg·K was obtained in the temperature range of 90 K.A stable temperature change of 5.6 K was obtained in 200 elastocaloric cycles by controlling the external stress.Finally,this paper systematically summarizes the strategy of preparing Cu-Al-Mn alloy microwires with high elastocaloric properties.We propose a method from the perspective of grain structure design,which provides a theoretica l basis for the subsequent preparation of excellent Cu-based elastocaloric materials.