Effect of ball milling conditions on the microstructure and the transformation behavior of Ti−Ni and Ti−Ni−Cu shape memory alloy powders

Ti−50Ni and Ti−40Ni−10Cu (at.%) shape memory alloy poweders have been fabricated by the ball milling method. Their alloying behaviour and transformation behaviour were investigated by means of optical microscopy, electron microscopy, X-ray diffractometry and differential scanning calorimetry. As-milled Ti−Ni powders fabricated with a milling time of less than 20 hrs were a mixture of pure elemental Ti and Ni, and therefore we were unable to obtain alloy powders because the combustion reaction between Ti and Ni occurred during heat treatment. Since those fabricated with a milling time of more than 20 hrs were a mixture of Ti-rich and Ni-rich Ti−Ni solid solution, it was possible to obtain alloy powders without a combustion reaction during heat treatment. Clear exothermic and endothermic peaks appeared in the cooling and heating curves, respectively, in DSC curves of 20 hr and 30 hr milled Ti−Ni powders. On the other hand, in DSC curves of 1 hr, 10 hrs, 50 hrs and 100 hrs, the thermal peaks were almost discernible. The optimum ball milling time for fabricating Ti−Ni alloy powders was 30 hrs. Ti−40Ni−10Cu alloy powders were fabricated successfully by the optimum ball milling conditions deduced from Ti−50Ni alloy powders.