Thermomechanical Treatments and Their Influence on the Microstructure and Stress/Strain Diagrams of NiTi Shape Memory Alloys

Thermomechanical fatigue of post-weld heat treated NiTi shape memory alloy wires

“…The R-phase was observed in the post-weld heat treated wires. The grain structure of the fusion zone was not changed by the heat treatment ( Figure 5), but the temperatures were sufficient to affect the dislocation density and structures [34]. The change in dislocation networks, or the presence of base metal in the DSC specimen was responsible for the observed R-phase peak..…”

“…At 87 °C the base metal maintained 94% ± 4% of the base metal UTS but had a reduced ductility, while the post-weld heat treated sample maintained 78% ± 1% of the base metal UTS and had a slightly increased ductility.. The heat affected and fusion zones of the welded samples experienced annealing temperatures which destroyed dislocations, , and increased grain size[34,[36][37][38] as shown inFigure 5. This coarsening of the microstructure resulted in the observed decrease in ultimate tensile strength and increase in ductility of the welded samples compared to the base metal.Application of the post-weld heat treatment was done to the entire gauge length of the wire.…”

“…No grain growth was observed in the fusion zone, and as discussed in the previous section, a change in the transformation temperatures indicated alteration of the dislocations in the material. This would have had minimal effect on the fusion and heat affected zones, however the remaining base material in the gauge length had a reduction in dislocation density[16,34,39], which contributed to the increased ductility and decreased UTS of the post-weld heat treated sample compared to the base material and welded samples. These changes in the post-weld heat treated samples also resulted in the formation of a clear martensite detwinning plateau, and a small initial R-phase plateau.…”

Grain growth and precipitation in an annealed cold-rolled Ni50.2Ti49.8 alloy

“…These thermomechanical treatments could be annealing following cold work [1e3], aging after solution treatment [4,5] and thermal and/or stress cycling [6,7]. Amongst these, low and moderate temperature annealing following appropriate cold deformation is one of the effective methods for controlling SMA properties [8,9]. Since these NieTi alloys are very susceptible towards solid-state amorphization by cold rolling [10e13], the amount of amorphization can be controlled by the percentage of cold work performed on these alloys.…”

Experimental validation of a one-dimensional model for monolithic shape memory alloys with multiple pseudoelastic plateaus

“…A final heat treatment of 400°C for 60 min in atmospheric air followed by a water quench was used to restore pseudoelastic properties. Annealing at 400°C reduces the overall number of dislocations present in the material (Prokoshkin et al, 2004). The as-received material after solutionizing, cold working and final heat treatment will be referred to as base material (BM) for the remainder of this study.…”