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 815 °C and a strain rate of 0.1 s Ϫ1 in Ti-6Al-4V with a colony-␣ microstructure. Cavities nucleated preferentially in the ␣-␤ interface along prior-␤ 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 caxis 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 triaxiality. Estimates of the local strains and stress states were then incorporated into a 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.

Hot-torsion testing was used to establish the cavitation behavior of a typical alpha/beta titanium alloy, Ti-6Al-4V, with a colony microstructure, during simple-shear deformation. For this purpose, sections of deformed specimens were examined by optical metallography, and by scanning and orientationimaging microscopy (OIM). It was found that cavity nucleation occurred along prior beta boundaries as well as at triple points; in particular, most cavities nucleated along boundaries perpendicular to the axial direction of the specimen. Extensive growth was observed for cavities surrounded by both hard and soft orientations, with the soft colonies accommodating more of the imposed strain. At high degrees of deformation, dynamic globularization of the colony microstructure adjacent to the cavities was also observed. In addition, the metallographic observations revealed that the cavities did not grow in an equiaxed mode, but in an elliptical manner. A tensor describing the cavity-growth rate along the axial, radial, and hoop specimen directions was determined using measurements of individual cavity sizes. The cavity-growth behavior in torsion was compared to previous observations from hot-tension tests. This comparison indicated that the rate of cavity growth in shear was approximately one-tenth that in uniaxial tension. This finding is in broad agreement with models predicting the variation of the cavity-growth rate as a function of the ratio of the mean stress to the hydrostatic stress.

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Notched-tension tests were conducted on Ti-6Al-4V samples with a colony-alpha microstructure to determine the effect of the stress state on cavitation during hot working of alpha/beta titanium alloys. The experiments were complemented by finite-element-method analyses to establish the local stress state, strain, and damage factor for different areas within each sample. A critical damage factor, defined by a modified Cockcroft-and-Latham criterion, was found to be applicable for the prediction of cavity initiation for the different notch geometries. Measured cavity growth rates were also correlated to the stress state (i.e., ratio of mean-to-effective stress, M / e ) and compared to predictions of prior models. Model predictions showed reasonable agreement with measurements at low levels of stress triaxiality, but exhibited some deviations at higher values of M / e . The differences were attributed to differences in the properties of the present material and those assumed in deriving the models as well as the neglect of cavity interaction in one of the models. The results were summarized in terms of a processing map, which delineates the initiation of cavities and their size as a function of the stress state and effective strain.