An experimental and theoretical investigation of the effect of local colony orientations and misorientation on cavitation during hot working of Ti-6Al-4V

Orientation in aging microscopy was used to determine the effect of local crystallographic texture on the size of cavities formed during hot tension testing at 815degreesC and a strain rate of 0.1 zeta(-1) in Ti-6Al-4V with a colony-alpha microstructure. Cavities nucleated preferentially in the alpha-beta interface along prior-beta grain boundaries that were perpendicular to the tension axis, adjacent to colonies with a (hard) c-axis colony orientation parallel to the tension axis. Cavity growth was most rapid at locations where 20 to 40 pct of the area surrounding the cavity also had colonies with soft orientations (with the c-axis inclined to the tension axis). The constraint of the hard orientations and the strongly incompatible anisotropic deformation by prism and basal slip in the softer orientations appeared to facilitate cavity nucleation and growth in these local regions. To interpret these observations, a simple model was developed to quantify the effect of the misorientation between neighboring colonies on the partitioning of strain between them and the development of a local stress trixiality. Estimates of the local strains and stress states were then incorporated into plasticity-controlled cavity-growth model to estimate the cavity-growth rate, and thus cavity sizes. Predicted cavity sizes following initiation were very sensitive to the local strain and the hydrostatic stress through its effect on the cavity-growth parameter. The model was successful in differentiating growth rates according to local values of the Taylor factor.