The massive transformation in Ti-Al alloys: Mechanistic observations

The massive α→γ_m transformation, as observed using analytical transmission electron microscopy, in Ti-49Al, Ti-48Al-2Nb-2Mn, Ti-55Al-25Ta and Ti-50Al-20Ta alloys is described. Conventional solution heating and quenching experiments have been combined with the more rapid quenching possible using electron beam melting in order to provide further insight into the early stages of the transformation in these alloys. It is shown that the γ develops first at grain boundaries as lamellae in one of the grains and that these lamellae intersect and spread into the adjacent grain in a massive manner. Consequently, there is no orientation relationship between the massive gamma (γ_m) and the grain being consumed whereas there is the expected relation between the γ_m and the first grain which is inherited from the lamellae. It is further shown that the γ_m grows as an f.c.c. phase after initially growing with the L1₀ structure. Furthermore, it is shown that the massive f.c.c. phase then orders to the Ll₀ structure producing APDB-like defects which are actually thin 90° domains separating adjacent domains that have the same orientation yet are out of phase. The advancing γ_m interface tends to facet parallel either to one of its four {111} planes or to the basal plane in the grain being consumed by impinging on existing γ lamellae. Thin microtwins and α₂ platelets then form in the γ_m presumably due, respectively, to transformation stresses and supersaturation of the γ_m with titanium for alloys containing ∼48% Al; indeed, there is a local depletion in aluminium across the α₂ platelets as determined using fine probe microanalysis.