Microstructure dependence of compactibility of rapidly solidified Ti-rich TiAl alloy powders produced by plasma rotating electrode process

The effect of initial microstructure on the compactibility at elevated temperatures in rapidly solidified Ti-rich TiAl alloy powders produced by the plasma rotating electrode process (PREP) has been investigated. There are two kinds of powders with respect to the microstructure. The first one has a surface relief of a martensitic phase and consists of twin-related α₂(Ti₃Al) lath platelets, which is referred to as M powder. The second one has a dendritic structure and consists of a single phase of α₂ with no microstructure other than the grain boundary, which is referred to as D powder, α₂+γ (TiAl) microduplex and α₂/γ lamellar structures are formed in M and D powders of the Ti-40 at%Al alloy by heat treatment at 1273 K, respectively. The microduplex structure consists of γ precipitate in the twin-related α₂ matrix with the usual orientation relationship. It is difficult to compact the D powder by hot pressing at 1273 K under 50 MPa for 14.4 ks. On the other hand, the M powder is compacted easily by hot pressing with the same condition. The twin-related α₂ and α₂ boundary changes to random ones and the α₂ and γ phases lose the usual orientation relationship in the duplex structure during the hot pressing. In other words, the low energy boundaries are changed to the high energy ones suitable for grain boundary sliding. Dislocations are scarcely observed inside of both the α₂ and γ crystal grains. It is concluded that the grain boundary sliding is a predominant deformation mode in the M powder during the hot pressing. The D and M powders in Ti-45 and 47 at%Al alloys show the same tendency as those in Ti-40 at%Al alloy during hot pressing. From the practical viewpoint the optimization of PREP conditions is also discussed to increase the number fraction of the M powder.