Influence of cold work and heat treatment on the shape memory effect and plastic strain development of NiTi

The influence of cold work and heat treatment on the shape memory effect and plastic strain development is investigated for the thermally induced phase transformation of NiTi shape memory alloys (SMAs) under constant applied stress. Fully annealed SMA wire specimens of identical chemical composition are cold rolled to reduce the specimen width 10: 20, 30 and 40% of the initial wire diameter and then annealed at 300, 400 or 500 degreesC for 15 min. Thermally induced phase transformations under constant applied stress, at various levels up to 500 MPa, are performed to identify the effect of the cold work percentage and annealing temperature on the development of transformation and plastic strain in SMA specimens with one-way shape memory. Results show the maximum transformation strain is independent of cold work percentage and annealing temperature, and increased cold work, for similar annealing temperatures, raises the stress level for the onset of plastic strain and decreases the additional plastic strain development. Also, a reduction in annealing temperature, for similar cold work percentage, raises the stress level for the onset of plastic strain and decreases the additional development of plastic strain. In addition to the one-way SMAs described above, specimens with similar heat treatments are trained with two-way shape memory under a constant applied stress of 300 MPa. Increased cold work percentage is shown to reduce the plastic strain accumulation during the two-way training, as well as the amount of two-way strain developed in the specimens. A comparison between the untrained and trained specimens of identical cold work and annealing temperature shows a reduction in the maximum transformation strain. (C) 2001 Elsevier Science B.V. All rights reserved.