Influence of Alloying Element Addition on Cu–Al–Ni High-Temperature Shape Memory Alloy without Second Phase Formation

In this study, the effects of rare earth Gd and Fe elements on the microstructure, the mechanical properties and the shape memory effect of Cu-11.9Al-3.8Ni high-temperature shape memory alloy were investigated by optical microscopy, scanning electron microscopy, X-ray diffraction and compression test. The microstructure observation results showed that both Cu-11.9Al-3.8Ni-0.2Gd and Cu-11.9Al-3.8Ni-2.0Fe-0.2Gd alloys displayed the fine grain and single- phase fll1 martensite, and their grain size was about several hundred microns, one order of the magnitude smaller than that of Cu-11.9Al-3.8Ni alloy. The compression test results proved that the mechanical properties of Cu-11.9Al-3.8Ni alloy were dramatically improved by alloying element additions due to grain refinement and solid solution strengthening, and the compressive fracture strains of Cu-11.9Al-3.8Ni-0.2Gd and Cu-ll.9Al-3.8Ni-2.0Fe-0.2Gd were 12.0% and 17.8%, respectively. When the pre-straln was 10%, the reversible strains of 5.4% and 5.9% were obtained for Cu-11.9Al-3.8Ni- 0.2Gd and Cu-ll.9Al-3.SNi-2.0Fe-0.2Gd alloys after being heated to 500 ℃ for 1 rain, and the obvious two-way shape memory effect was also observed.

Gamma-phase influence on shape memory properties in Ni-Mn-Co-Ga-Gd high-temperature shape memory alloys

The effect of γ-phase on two-way shape memory effect（TWSME） of polycrystalline Ni56Mn25-xCoxGa18.9Gd0.1 alloys was investigated. The results show that an appropriate amount of ductile γ-phase significantly enhances the TWSME. The largest TWSME of 1.4% without training is observed in N（56Mn21Co4Ga18.9Gd0.1 alloy, and this value is increased to 2.0% after thermomechanical training. The as-trained TWSME decays over the first five thermal cycles and then reaches a stable value as the number of cycles further increasing. Only the degradation of 0.2% is observed after 100 thermal cycles. The better TWSME and thermal stability are ascribed to the stable extra stress field formed by the plastically deformed γ-phase.

Effect of Annealing Temperature on the Shape Memory Properties of Cold-Rolled Dual-Phase Ni-Mn-Ga-Gd Alloy

The influence of annealing temperature on the shape memory effect （SME） and recovery ratio of cold-rolled dual-phase Ni58Mn25Ga16.9Gdo.1 high-temperature shape memory alloy were investigated. The results showed that the SME can be improved by appropriate annealing. SME of 6.0% and recovery ratio of 98% were observed in the rolled alloy at 550 ℃ annealing, which were larger than that of the samples annealed at other temperatures. The annealing temperature was in between the recrystallization temperatures of the martensite and 7 phase, which leads to the work-hardening state of 7 phase kept unchanged during annealing process, and therefore, the critical slip stress of y phase increased. Thus, the plastic deformation of ~ phase reduced during the compression process, and the SME increased.