Effects of B and W alloying additions on the formation and stability of lamellar structures in two-phase γ-TiAl

Abstract

The effects of additions of 140 appm B and 0.5 at.% W on the formation and stability of fully-lamellar structures in a Ti-47Al base alloy were studied. The fully-lamellar structure was formed by heat-treatment at 1400°C for 1 h (in vacuum) followed by furnace cooling. Stability was studied by aging these alloys for 168 h at 800, 1000 and 1200°C and making quantitative measurements of lamellar structure parameters (i.e. average lamellar spacing, α2-α2 spacing) using transmission electron microscopy (TEM) and hardness measurements. The B addition alone refines the lamellar structure relative to the binary TiAl alloy, but causes fragmented and discontinuous α2 lamellae. By contrast, W+B addition refines the lamellar structure and produces more uniform and continuous α2 lamellae. The as-heat-treated Ti-47Al+W+B alloy is harder than the other two alloys. Aging for 168 h at 800°C does not change the fine initial lamellar microstructure of any of these alloys. Aging at 1000°C significantly coarsens the Ti-47Al alloy, causes some coarsening in the Ti-47Al+B alloy, but has little effect on the lamellar structure of the Ti-47Al+W+B alloy. Aging at 1200°C causes discontinuous coarsening and complete loss of the initial, fine, lamellar microstructure in all three alloys. Aging in vacuum produces a subsurface ‘damage region’ consisting of an α(Ti3Al) surface layer with another layer of coarsened γ microstructure underneath. At 1000°C, this layer is greatest in the binary alloy and least in the Ti-47Al+W+B alloy. Specimens were 5.5% cold-forged and then aged at 1000°C to evaluate the effects of subsurface deformation caused during cutting of the specimens. Prior cold-deformation caused considerable recrystallization in the binary alloy, but almost none in the Ti-47Al+W+B alloy. The additions of W and B to a Ti-47Al alloy refine the fully-lamellar structure obtained by heat-treatment, and make it more resistant to lamellar coarsening (and recrystallization if cold deformed) during aging at 800–1000°C. © 1997 Elsevier Science Limited. All rights reserved.