On the influence of thermomechanical treatments on shape memory alloys

Thermomechanical treatments (TMT) are used to modify the microstructure of austenite or martensite by introducing lattice defects and/or particles of a second phase. The temperature range extends between T greater than or equal to M-d (pure ausforming) and T less than or equal to M-f(pure marforming). In the intermediate range (M-f < T < M-d) a more complex behaviour is found. TMT are discussed in a systematic manner using NiTi-based and Cu-based alloys as examples. Transformation behaviour and mechanical properties of TM-treated alloys are compared with undeformed ones. The following effects can be observed for thermal transformation cycles: (al) lowering or raising of transformation temperatures, (a2) widening or decreasing of hysteresis, (a3) induction of multiple step transformation mechanisms, (a4) reduction of martensitic retransformability (e.g. plastic deformation of martensite = marforming). The mechanical properties are also highly affected: (bl) increase in pseudo yield stress, (b2) increase or decrease in shape memory strain, (b3) increase in conventional strength, (b4) increase in elongation at fracture (by hot rolling of austenite = ausforming). It is shown how tailor-made microstructures are obtained, which in turn provide the best bulk properties for specified applications of SMA in engineering.