Training and two-way shape memory in NiTi alloys:: influence on thermal parameters

The two-way shape memory effect (TWSME) was studied in a near equiatomic commercial alloy. A training procedure based on a constant load applied in the temperature range of the parent <----> martensite transformation was carried out on NiTi wires. The efficiency of the method was determined from deformation-temperature measurements by MTA at different training stress and number of cycles. A maximum of two shape memory strain was obtained for a stress training of 115 MPa, independently of number of training cycles. A correlation was established between the TWSME arisen and the permanent strain generated during the training. The A(s) and A(f) transitions present a positive shift and the M-s and M-f a negative one with increasing training stress. All the transitions temperatures decrease with the training cycling. In the trained material, the P --> M and M --> P temperatures and the latent heat of these conversions undergoes a strong decrease with increasing training stress, with a strong asymmetry between the forward and the reversed transitions. The changes of these thermal parameters as a function of the training parameters were studied on a thermodynamic frame. (C) 2004 Elsevier B.V. All rights reserved.