Tensile and High Cycle Fatigue Properties of a Minor Boron-Modified Ti-22Al-11Nb-2Mo-1Fe Alloy

The Ti3Al (alpha(2) phase)-based Ti-22Al-11Nb-2Mo-1Fe (in at%) alloy is a derivative of Ti2AlNb (0 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 + alpha(2)) 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 mu m with the addition of 0.1 mass% B, and thereby a refined full lamellar microstructure was obtained. The duplex microstructure consisting of spherical alpha 2 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. [doi:10.2320/matertrans.M2012043]