Stabilisation of Martensite During Training of Cu-Al-Ni Single Crystals

Abstract
Cu-Al-Ni single crystals with composition 82.6Cu-13.4Al-4Ni (wt%) were heat treated at 800°C for 30 min and subsequently water quenched to room temperature. The samples with 3 mm diameter and length between 100 and 200 mm had a longitudinal orientation close to the [001] direction of the β-phase at high temperature. The martensite start temperature after the initial heat treatment was M_s=93°C. Training was performed by loading the sample above A_f, followed by cooling under constant load. After removal of the load the sample was heated to recover its original shape. Repeating this cycle several times introduces the two-way memory effect. Several strains involved in the process were measured. It was found that a large residual deformation (RD) persisted after cycling between the training temperatures. Most of this residual deformation was recovered by overheating the sample. The non recoverable residual strain was observed to increase with increasing load. From the overheating experiments and differential scanning calorimetry measurements, it becomes evident that the total residual deformation is due to two contributions : the stabilisation of martensite and a plastic deformation.