Effect of precipitates on the stress–strain behavior under compression in polycrystalline Ni–Fe–Ga alloys

The stress–strain behavior under compression has been studied in three Ni55−xFe18+xGa27 (x = 0, 1, 1.5 at.%) polycrystalline alloys with different distributions of γ phase precipitates. All the alloys present the superelastic effect at temperatures above the austenite-finish temperature and the mechanical tests confirm the linear dependence of the critical stress with temperature, as described by Clausius–Clapeyron-type equation. The ‘plateau’ in the stress–strain curves, associated to the stress-induced transformation, presents an increasing slope as samples contain a higher amount of inter-granular particles. This hardening effect is due to the enhanced difficulty to accommodate the deformation between adjacent grains. Intra-granular precipitates increase still further the hardening effect, as a consequence of the difficulties found by the growing martensitic plates to overcome the precipitates. Furthermore, the maximum observed stress-induced strain is notably reduced in samples containing higher fractions of γ precipitates. These precipitates also have an effect in the deformation of martensite, increasing the stress needed to rearrange the variants.