Effect of oriented γ′ precipitates on shape memory effect and superelasticity in Co-Ni-Ga single crystals

This paper reports on the effect of dense dispersions of nanometric γ′ precipitates on the shape memory and superelastic response of Co49Ni21Ga30 single crystals. The particles are grown by ageing at 620 K with or without applied stress, which leads to oriented single-variant or non-oriented four-variant precipitates with a habit plane on one or four 〈1 1 1〉B2-type planes, respectively. The non-oriented precipitates strongly decrease the martensitic transformation temperatures, enhance the transformation hysteresis and decrease the recoverable strain. These effects are significantly reduced in the case of oriented particles. The effects are correlated with difficulties in accommodating the L10 martensite within the irregular stress fields generated by the non-oriented precipitates, which cause a micromodulation of the martensitic structure. In turn, the collective effect of the stress fields around the oriented precipitates improves the martensite accommodation, which is reflected in higher transformation temperatures, lower hysteresis, larger transformation strain and smoother σ-ε curves during the stress-induced transformation plateau, in comparison to non-oriented particles. The samples containing nanometric γ′ particles present several stages in the (σ, T) diagram and lower values of the slope dσ/dT, compared to the precipitate-free material. This fact has been related to the temperature dependencies of the irreversible dissipation energy and transformation strain. The nanometric precipitates strengthen the B2 matrix and enlarge the temperature range for superelasticity above 570 K. In addition, the particles difficult the detwinning process of the stress-induced martensite, which produces microplastic deformation by slip and tiny martensite plates are retained around the particles after the mechanical tests.