CRYSTAL-STRUCTURE CHANGES IN NI3AL AND ITS ANOMALOUS TEMPERATURE-DEPENDENCE OF STRENGTH

The existence of a structural transformation in Ni3Al alloys establish ed earlier through X-ray diffraction, dilatometry and TEM investigatio ns are summarised. The results obtained are discussed through a model proposed. The L1(2) structure appears to transform to another L1(2) or to a DO22 structure during heating. Such a transformation starts at a round 700 degrees C and seems to complete around 1100 degrees C. In th e temperature range 700-1100 degrees C both phases coexist. This cause s a tetragonal distortion of the L1(2) lattice giving rise to a tweed morphology in TEM observations, The flow stress studies indicate that the anomalous strengthening behaviour is not only due to the dislocati on kind of mechanism as proposed in the literature but also due to the structural changes noticed in this work.