MARTENSITE PRECURSOR OBSERVATIONS USING THERMAL-EXPANSION - NI-AL

Measurements of thermal expansion, in the temperature range 2-300 K, were made along the three [100] directions of the cubic phase of a martensitically transforming Ni0.625Al0.375 single crystal. Anisotropic and anomalous expansion for the three different directions were observed at temperatures about 115 K above the transformation temperature, of approximately 85 K. Optical observation showed that the onset temperature for the anomalous expansion is consistent with the temperature at which surface tilts appear and that the anisotropic expansion is due to the preferential selection of orientation of the martensite variants in the crystal. The amplitude of the observed anisotropy varies with applied stress which suggests that the relative fractions of the variants in the crystal with preferential orientation are stress dependent.