The effect of grain boundary normal on predicting microcrack nucleation using fracture initiation parameters in duplex TiAl

In a prior study of a duplex Ti–48Al–2Cr–2Nb, Simkin et al. [B.A. Simkin, M.A. Crimp, T.R. Bieler, Scripta Mater. 49 (2003) 149], 11 cracked and 11 intact boundaries between γ grains were analyzed to determine why some boundaries that were similarly stressed cracked, and others did not. This led to a geometrically based fracture initiation parameter that considered the tensile stress axis and the deformation systems in both grains. In order to further examine the physical basis of this fracture initiation parameter, we have measured the grain boundary orientation of the previously studied boundaries. In contrast to the expectation that incorporation of the grain boundary orientation into this parameter would improve the ability to discriminate between cracked and intact boundaries, the orientation of the grain boundary normal was found to be statistically less significant than the orientation of the Burgers vectors of the most highly stressed twinning systems, quantified by | b ˆ tw ⋅ t ˆ | , where t ˆ describes the tensile axis direction. Grain boundary microcracking was found to be most likely when b ˆ tw was in the range of 30–50° from the tensile axis. Combining the Schmid factor with | b ˆ tw ⋅ t ˆ | and the effect of ordinary dislocation slip accommodation provides a simple and effective predictor of the propensity for microcrack nucleation.