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

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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.