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

doi:10.1007/s40195-017-0675-3

Abstract

Abstract:

The effect of γ-phase on two-way shape memory effect (TWSME) of polycrystalline Ni₅₆Mn_25-_x Co _x Ga_18.9Gd_0.1 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 Ni₅₆Mn₂₁Co₄Ga_18.9Gd_0.1 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.

Key words: Shape memory alloys, Ni-Mn-Ga alloys, Thermomechanical training, Second phase

Wei Zhang, Xin Zhang, Qian Wang, Qing-Suo Liu. Gamma-phase influence on shape memory properties in Ni-Mn-Co-Ga-Gd high-temperature shape memory alloys[J]. Acta Metallurgica Sinica (English Letters), 2018, 31(5): 471-476.

Figures/Tables 5

Fig. 1 Secondary electronic images (a x = 1; b x = 2; c x = 4; d x = 8) and e XRD patterns of Ni56Mn25-x Co x Ga18.9Gd0.1 alloys

Fig. 2 DSC curves of Ni56Mn25-x Co x Ga18.9Gd0.1 alloys

Fig. 3 Strain recovery characteristic curves of Ni56Mn25-x Co x Ga18.9Gd0.1 alloys under different pre-strain. The blue arrows represent recovery strain after being heated to 600 °C for 1 min, and the purple arrows represent TWSME

Fig. 4 Effects of thermal cycle number on TWSME in Ni56Mn25-x Co x Ga18.9Gd0.1 alloys. The cycle temperature ranges from 25 to 600 °C

Fig. 5 Schematic diagram showing the effect of γ-phase on TWSME (arrow denotes martensitic variants, yellow area denotes internal stress field and gray area denotes stress field surrounding the soft γ-phase)

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