Effect of mechanical alloying on the solid state reaction processing of Ni-36.5 at.% Al alloy

The solid state reaction of the mechanical alloyed NiAl elemental powders has been studied using differential scanning calorimetry, X-ray diffraction and transmission electron microscopy. NiAl compacts containing 36.5 at.% Al are within the shape memory range and were prepared by mechanical alloying for various periods. In specimens that were mechanical alloyed for less than 2 h, the grain sizes of Ni and Al were above 200 and 100 nm respectively and a two steps solid-state reaction took place during heating. In specimens which were mechanical alloyed for longer than 2 h, the corresponding grain sizes of Ni and Al were below 200 and 100 nm and in this case, a three step solid state reaction took place during heating. Thus the initial state of the alloy system before heating plays an important role in the phase reactions.