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 sequence of the metastable phase formation and thermal stability of the mechanically alloyed Al67Ti25M8 (M=Cr, Zr, Cu) have been studied systematically. It was found that with the addition of Cr and Zr, nanocrystalline ordered L12-Al3TiM compounds were obtained after 40 h of milling. Subsequent annealing in the differential scanning calorimeter causes a grain growth process of the nanocrystallites. With the addition of Cu, the final product was nanocrystalline disordered Al(Ti,Cu) solid 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 L12 chemical ordering. The grain growth and L12 chemical ordering seem to occur simultaneously at temperatures in the range 400–700°C. Our experimental results suggest that Cr and Zr may be promising alloying additions to stabilize the L12 structured Al3Ti compound for further application.