Phase Transformation and Recovery Stress of Ni₄₇Ti₄₄Nb₉ Alloy During Constrained Heating and Cooling

The phase transformation and recovery stress of pre-deformed Ni₄₇Ti₄₄Nb₉ alloy are systematically investigated during constrained heating, cooling and thermal cycling. Recovery stress is determined by a phase transformation, thermal expansion, and contraction. Reverse martensitic transformation occurs during constrained heating and then results in a sharp increase in the recovery stress. Meanwhile, the wide phase transformation hysteresis shrinks during thermal cycling. A few martensitic transformations gradually occur across a wide temperature range during constrained cooling, resulting in a negative decreasing stage or a wide plateau of recovery stress. When cooled further to a critical value of 20 °C, the recovery stress and temperature follow the Clausius–Clapeyron equation, and large amounts of martensitic transformations appear, resulting in the sharp decrease of the recovery stress.