Preparation of nanocrystalline TiAl powder by the HDDR process using mechanical grinding in a hydrogen atmosphere

Abstract

Titanium aluminides (TiAl and Ti[sub 3]Al) are potential high temperature structural materials for aerospace applications owing to their low density, high specific strength and oxidation and corrosion resistance. However, their low ductility at room temperature prevents their use and efforts are being made to increase ductility by alloying additions, grain refinement, the heat treatment to change the microstructures and so on. However, these efforts have had only limited success. Generally, the grain refinement is a useful technique to enhance ductility of metallic materials. Recently, it has been demonstrated that the coarse-grained materials are converted into very fine grained powder by the HDDR (hydrogenation, disproportionation, desorption and recombination) process in the Nd-Fe-B permanent magnets. The first step of the HDDR process is the decomposition of the intermetallic compound A[sub 1[minus]x]B[sub x], where A and B is a hydride forming and non-forming metal respectively, into hydride AH[sub x] and the other B-rich A[sub 1[minus]x[minus]y]B[sub x] compound (or pure metal B) by hydrogenation. The second step is realloying between A, produced by the decomposition of AH[sub x], and the B-rich A[sub 1[minus]x[minus]y]B[sub x] compound. TiAl consists of hydride forming metal (Ti) and hydride non-forming metals (Al), but it does not absorb hydrogen andmore » does not decompose into TiH[sub 2] and the Al-rich TiAl. Consequently, the grain refinement by the conventional HDDR process is impossible in the case of TiAl. In the present work, the authors try to refine the grain size of TiAl by the modified HDDR process using mechanical grinding (MG) in a hydrogen atmosphere.« less