Effect of powder state on spark plasma sintering of TiAl alloys
Résumé
Two sets of TiAl-based powders were consolidated by spark plasma sintering (SPS) using identical processing conditions. Both were prealloyed powders obtained by atomization, but one was additionally mechanically grinded by high energy ball milling. Both presented a metastable, almost-alpha structure, but atomization results in a coarse-grained microstructure, whereas milling induces an ultrafine-grained one. Differential thermal analysis during continuous heating showed that phase transformation towards equilibrium (gamma + alpha(2)) structure starts at lower temperature in the case of the mechanically milled powder. Densification during SPS processing also takes place 200 degrees C below for the mechanically milled powders. Higher kinetics associated with mechanical grinding is attributed to the presence of a high density of defects favoring nucleation. However, it is also proposed that the difference of initial structures results in different deformation modes during sintering. For the ultrafine-grained mechanically milled powder, densification takes place by grain boundary sliding assisted by surface/grain boundaries diffusion, whereas conventional plastic deformation by gliding and twinning is active for the coarse-grained as-atomized powder. Moreover, both structures after full densification were heterogeneous at fine scale. But, for the first one this hetero-structure originates from the mechanical milling process, while for the other, it results from the heterogeneity of the deformation during densification.