Microstructural effects on the mechanical behavior of B-modified Ti-6Al-4V alloys

Small additions of B (<= 0.4 wt.%) to Ti alloys refine the as-east microstructure significantly and improve the alloys' mechanical performance. In this work, tensile, fracture and fatigue properties of the as-cast and hot isostatically pressed Ti-6Al-4V alloy with 0, 0.05, 0.10 and 0.40 wt.% B additions have been examined, with particular emphasis on identifying the microstructural length scale (grain size vs. lath size) that controls the mechanical properties of these alloys. Microstructural observations indicate an order of magnitude reduction in the prior beta grain size, d, as well as a significant reduction in the alpha lath size, lambda, with B additions. It was observed that d and lambda are correlated. With the refinement in the microstructure, the yield and ultimate tensile strengths, sigma(y) and sigma(u) respectively, increase whereas the fracture toughness, K-IC, decreases. Application of the Rice-Knott-Ritchie model for quasi-brittle cleavage fracture indicates that the reduction in K-IC with increasing B content is due primarily to the reduced lambda. Fatigue crack growth measurements show a gradual reduction in the threshold for fatigue crack propagation with sigma(y)root lambda dependence. (C) 2007 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.