THERMAL EVIDENCE FOR THE STRUCTURAL INSTABILITY IN NI3AL ALLOYS

The thermal expansion coefficient (alpha) and calorimetric data were obtained as a function of temperature in order to clarify some of the significant variations reported with regard to these values in the literature. Stoichiometric and off-stoichiometric compositions of Ni3Al alloys (with and without boron addition) were investigated. Dilatometric experiments were performed on all the alloys and the alpha-values were estimated over the temperature range from ambient to 1000-degrees-C. Two runs were made on each sample under different initial conditions and differences in alpha-values were noticed. The results were analysed based on our earlier X-ray diffraction results. Additional isothermal dilatometric tests were also carried out and a significant volume change was noticed (0.45% contraction) when the alloy quenched from 1000-degrees-C was heated to 600-degrees-C and held for a long duration. The calorimetric data were obtained over the same range of temperature and enthalpy changes, though less distinct, were noticed at around 360, 660 and 900-degrees-C. The variations seen further augment our earlier results on the instability of the L1(2) structure and the existence of a structural transformation.