MECHANICAL ALLOYING AND THERMAL-STABILITY OF AL67TI25M8 (M=CR, ZR, CU)

By means of X-ray diffraction (XRD), transmission electron microscopy (TEM) and differential scanning calorimetry (DSC) measurements, the se quence of the metastable phase formation and thermal stability of the mechanically alloyed Al67Ti25M8 (M = Cr, Zr, Cu) have been studied sys tematically. It was found that with the addition of Cr and Zr, nanocry stalline ordered Ll(2)-Al3TiM compounds were obtained after 40 h of mi lling. Subsequent annealing in the differential scanning calorimeter c auses a grain growth process of the nanocrystallites. With the additio n of Cu, the final product was nanocrystalline disordered Al(Ti,Cu) so lid solution. Two types of instability upon thermal annealing for the nanocrystalline disordered Al(Ti,Cu) solid solution were observed: an instability against the grain growth; and an instability against L1(2) chemical ordering. The grain growth and L1(2) chemical ordering seem to occur simultaneously at temperatures in the range 400-700 degrees C . Our experimental results suggest that Cr and Zr may be promising all oying additions to stabilize the L1(2) structured Al3Ti compound for f urther application. (C) 1997 Elsevier Science S.A.