2012 Volume 53 Issue 6 Pages 1138-1147
The Ti3Al (α2 phase)-based Ti–22Al–11Nb–2Mo–1Fe (in at%) alloy is a derivative of Ti2AlNb (O phase)-based orthorhombic Ti–22Al–27Nb (in at%) and was designed to reduce Nb content and thereby material cost. In the present study, two microstructure modification methods, a minor addition of boron (B) and a thermo-mechanical treatment in the (B2 + α2) two-phase region, were applied to refine the microstructure of this alloy and thus to improve its ductility. The prior B2 grain size of the as-cast ingot was drastically reduced, from 600–1000 to 100–200 µm with the addition of 0.1 mass% B, and thereby a refined full lamellar microstructure was obtained. The duplex microstructure consisting of spherical α2 phases and lamellar microstructure area was formed by the thermo-mechanical treatment, which was much finer in the B-modified alloy than in the B-free alloy. The B-modified alloy with a refined full lamellar or duplex microstructure showed higher tensile ductility, as well as higher high cycle fatigue strength and superior creep properties compared to those of the B-free counterpart. The fatigue crack initiated neither from the TiB/matrix interface nor from the TiB itself, but rather from the matrix area of the alloy. Thus, it was confirmed that the addition of a small amount of B exerts a favorable effect as a whole on the mechanical properties of the present Ti–22Al–11Nb–2Mo–1Fe alloy.