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 n ickel-rich boron-doped Ni3Al in order to confirm our earlier observati ons on the existence of a structural transformation in these alloys. T he 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 around 700 degrees C and seems to complete around 1100 degrees C and appears to be of a con tinuous type. In the temperature range 700-1100 degrees C both phases coexist, which causes a tetragonal distortion of the L1(2) lattice giv ing rise to a tweed morphology in TEM observations. The transformation mechanism involves a periodic modulation of lattice constant. The cal culated values for this periodicity (expressed as a number of unit cel ls in the [1 0 0] direction) obtained from X-ray diffraction (L = 62, 70, 74) matched well with that (L = 65) obtained using TEM.