The martensitic phase transition in Ni-Al: experimental observation of excess entropy and heterogeneous spontaneous strain

Experimental studies of the martensitic phase transition of Ni63.5Al36.5 by means of differential scanning calorimetry and x-ray diffraction show a coexistence interval of 30 K for the austenite and martensitic phase. Within a smaller interval of 10 K near the upper limit of the stability of the martensitic phase no strain broadening of x-ray diffraction lines was observed for either phase. With decreasing temperature, diffraction profiles broaden, with a maximum reached when the sample is fully transformed into the martensitic phase. This observation is analyzed in terms of heterogeneous strain broadening due to imperfect microstructural self-accommodation of the martensitic grains. No strain broadening occurs when the martensitic phase is not fully interconnected but forms isolated islands inside an austenite matrix. It is concluded that large stress fields result from the coalescence of martensitic regions in the sample. The calibration of the excess entropy from the measurements of the specific heat confirms the Landau potential of Salje et al (2007 Appl. Phys. Lett. 90 221903). Imperfect self-accommodation does not contribute to a measurable increase of the entropy of the sample