Production of high performance materials through powder consolidation and mechanical alloying using high-pressure torsion

In this review paper, high-pressure torsion (HPT) is introduced as a processing tool for powder consolidation. It is shown that not only HPT can refine the grain size to the submicrometer and/or nanometer range in bulk metallic materials but also can achieve consolidation of metallic and ceramic powders including mechanical alloying, synthesis of intermetallics, allotropic phase transformation and fabrication of metal-matrix composites. Supersaturation of Fe in ultrafined grained Al is achieved in the Al-Fe system where Fe solubility in Al is as little as 0.052 mass%. Allotropic phase transformation of Ti and Zr occurs from a phase to a phase during operation of HPT because of high pressure in comparison with the conventional ball milling process. Nanograined NiAl and TiAl intermetallics form by HPT processing including successive annealing at temperatures as low as ∼0.35T_m where T_m is the melting temperature of the intermetallics. Fabrication of composites is feasible with nano ceramic particles dispersed in the metal matrix. Finally, applications of HPT to ceramics such as Al₂O ₃ and ZrO₂ demonstrate that it is possible to introduce plastic strain into their powders and thus to make their consolidation at reduced temperatures.