TEM and high temperature X-ray diffractometric studies on the structural transformations in Ni₃Al

High-temperature X-ray diffraction and TEM studies were conducted on nickel-rich boron-doped Ni₃Al in order to confirm our earlier observations on the existence of a structural transformation in these alloys. The results obtained are discussed through a model proposed. The L₁₂ structure appears to transform to another L₁₂ or to a DO₂₂ structure during heating. Such a transformation starts at around 700 °C and seems to complete around 1100 °C and appears to be of a continuous type. In the temperature range 700–1100 °C both phases coexist, which causes a tetragonal distortion of the L₁₂ lattice giving rise to a tweed morphology in TEM observations. The transformation mechanism involves a periodic modulation of lattice constant. The calculated values for this periodicity (expressed as a number of unit cells in the [100] direction) obtained from X-ray diffraction (L = 62, 70, 74) matched well with that (L = 65) obtained using TEM.