New insights on tensile plasticity of ultrafine-grained metallic materials

The tensile properties of TiNi43.5Fe6.5 alloy samples having different grain sizes (0.16, 0.35, 1.7, 2.3, and 3.9m) and fabricated by severe plastic deformation and annealing were investigated. It was observed that both the strength and the elongation of the alloy increase with a decrease in the grain size until the average size reaches 1.7m. However, for average grain sizes smaller than 1.7m, the elongation decreases continuously with further grain refinement. On the other hand, the strain-hardening rate does not decrease with the decrease in plasticity but instead increases slightly. The poor ductility of the ultrafine-grained TiNi43.5Fe6.5 alloy is accompanied by a high degree of strain hardening. This newly observed ductility behaviour of the ultrafine-grained TiNi43.5Fe6.5 alloy is elucidated by characterising the intragranular and grain boundaries.