Magnetization associated with reversible phase transformation or rearrangement of martensite variants of two kinds of shape memory alloys under the coupling of tensile stress were investigated.One is the austenitic Ni_(46)Mn_(28)Ga_(20)Co_(3)Cu_(3)micro wire with the [001] preferred orientation,which exhibits enhanced cyclic stability and large fully recoverable strain(> 8%) due to the stress-induced reversible martensitic transformation at room temperature.The other is the Ni_(54)Mn_(24)Ga_(22)microwire with ferromagnetic martensitic phase,which has preferential orientation and also exhibits large tensile strain.Based on the improved mechanical properties,the strain-magnetization effect of the two kinds of microwire under the coupling of orthogonal magnetic field and tensile stress was performed and the results indicate that the magnetization decreases with the increase of tensile strains.Furthermore,the magnetization mechanism related to the magnetostructural evolution under stress-magnetic coupling was discussed.This study provides a new way for smart magnetic microwires for novel non-contact and non-destructive detection.
基于磁热效应的磁制冷技术可以更好满足能源高效消耗和转化的要求.以工程化视角来看,制备磁热复合材料是结合理想磁热性能、高热导率和良好力学性能的有效方式.在本文中,采用激光等离子烧结技术(SPS)制备了Ni-Mn-Ga/Cu磁热复合材料,并将其与利用传统方式制备的材料进行了对比.本文详细研究了Ni-Mn-Ga/Cu的磁学性质,发现其磁热效应优于热压烧结及微米/纳米化的对应材料体系.同时,该磁热复合材料具有11.2 W m^(-1)K^(-1)的优良热导率.本文使用Hasselman–Johnson模型并对其进行优化,探究了热导率与复合材料微观组织的关系.与电弧炉熔炼样品相比,不同SPS烧结温度制备的磁热复合材料的力学水平得到提高,其最小断裂应力和断裂应变分别为340 MPa和4%.此外,利用基于拓展线性Drucker–Prager模型的有限元模拟方法,阐明了Ni-Mn-Ga/Cu磁热复合材料的失效机制.以上实验和模拟结果丰富了磁热材料领域的相关知识,并促进了磁制冷技术面向实际应用的进一步发展.
In this study,the effect of transverse magnetic field-assisted directional solidification(MFADS)on the microstructures in Ni-Mn-Ga alloys has been investigated.The results show that the magnetic field is capable of inducing transversal macrosegregation perpendicular to the magnetic field,causing the emergence of martensite clusters in the austenite matrix.Moreover,the magnetic field alleviates the microseg-regation on a dendritic scale and promotes the preferred growth of austenite dendrites.On the basis of the above investigation,several special samples are designed using the MFADS to study the crystallographic evolution and mechanical behavior during thermal/stress-induced martensite transformation.The martensite cluster in the austenite matrix is used to investigate the martensite transformation and growth under cooling-heating cycles.The crystallographic relationship and phase boundary microstructure between martensite and austenite have been characterized.In addition,the microsegregation on a dendritic scale can significantly influence the martensite variant distribution,corresponding to the performance during compressive circles based on the analysis of the deformation gradient tensor.The stress-induced superelasticity is closely dependent on orientation,well explained from the perspective of different resolved shear stress factors and correspondence variant pair formation transformation strain.The crystallographic evolution has been characterized during in-situ stress-induced transformation.The findings not only deepen the understanding of martensite transformation and mechanical behavior under a thermal/stress field in Ni-Mn-Ga alloys but also propose a promising strategy to obtain microstructure-controllable functional alloys by MFADS.
