Correlation between spinodal decomposition and shape memory effect in γ-Mn alloys.

Mn-Cu合金中的γ相是无序的fcc固溶体，由马氏体转变为fct。这种变换是一种弱的一阶变换，适用于具有精确控制的执行器应用。该γ相处于亚稳态;在700 K下的热处理导致失稳分解成富Mn区和贫Mn区。采用高频感应炉或电弧熔炼法制备Mn-15at%Cu合金锭，经热轧/冷轧至0.2mm厚，在1173 K下均匀化，在673 K下进行盐浴等温时效，用DSC和电阻率法测定相变温度，研究了Mn-15at%Cu合金的热分解对形状记忆行为的影响。通过X射线衍射测量晶格参数的变化。为了表征合金的形状记忆行为，将样品在1173 K下热处理1800 s，并在673 K下在15 mm半径的陶瓷圆柱体的内表面上的约束下时效并淬火。然后在室温下将其压平（对应于0.7%的变形）。结果表明：（1）673 K时效显著提高了马氏体相变温度，相变温度的变化与电阻率的降低成正比。2）时效前形状恢复率约为30%，时效10^5s后形状恢复率提高到50%。老化样品表现出约30%的双向记忆。这一反常现象可以解释为反铁磁自旋排列即使在逆相变到fcc相后仍然存在。

The γ phase in Mn-Cu alloy, which is disordered fcc solid solution, transforms martensiticaly to fct. This transformation is a weak first order type and suitable for the use in actuator application with precise control. This γ phase is in a metastable state ; the heat treatment at 700 K leads to the spinodal decomposition into Mn-rich and Mn-depleted zones. In the present study the effect of the decomposition on shape memory behavior was investigated.Alloy ingots of Mn-15at%Cu were made by high frequency induction furnace or arc melting ; they were hot/cold rolled to the thickness of 0.2 mm and then homogenized at 1173 K.Isothermal aging was done at 673 K using a salt bath.Transformation temperatures were measured by DSC and electrical resistivity. Changes in the lattice parameter were measured by X-ray diffraction. To characterize the shape memory behavior of the alloy, a sample was heat treated at 1173 K for 1800 s and aged at 673 K under constrain onto the inner surface of a ceramic cylinder of 15 mm radius and quenched. It was then flattened at room temperature(corresponding to 0.7% deformation). Shape change of the sample was recorded during heating/cooling cycles to 493K.The following results were obtained :1)Martensitic transformation temperatures siginificantly increase by the aging at 673 K.The change in transformation temperature was proportional to the decrease in the resistivity. Their correlation can be well explained by a thermodynamic model which takes the compositionaly heterogenous structure as a results of spinolda decomposition into account.2)The shape recovery before the aging was about 30% : it increased to 50% by the aging for 10^5s. The aged sample exhibited two way memory of about 30%. This behaivor may be explained by that the antiferromagnetic spin arrangements remain even after the reverse transformation to the fcc phase.