Deformation of an Alloy with a Lamellar Microstructure: Experimental.

The deformation behavior of individual Widmanstatten colonies comprised of aligned lamellae of ductile phases has been investigated. Based on the alpha-beta Ti alloy, Ti-8Al-1Mo-1V, this study shows the existence of a large 2X variation in the critical resolved shear stress for yielding of individual colonies. Schmids Law is not obeyed except for prism slip parallel to the beta lamellae. In addition, colonies with a high yield stress exhibit a high work hardening rate and fine, uniform slip, while colonies with a low yield stress deform by planar, non-uniform slip. This behavior appears to be independent of, slip system basal, prism, or pyramidal and of microstructure alpha-beta vs. alpha-alpha martensite. The experimental behavior is correlated to several colony orientation parameters including the stress axis, slip plane, slip direction, and orientation of the alpha-beta interface. The yield stress of a colony is found to increase as the slip direction of the dominant macroscopic slip plane tends toward being normal to the alpha-beta interface. These results indicate that the macroscopic flow behavior of colonies comprised of ductile lamellae depends on the ability of a slip system, once activated in the softer phase, to shear through the harder phase. The data also indicates that the interaction stresses at the phase interfaces are not a principle factor controlling macroscopic yielding. Finally, the alignment of a slip system in the alpha-phase with a potential slip system in the beta-phase lamellae does not appear to affect the yield stress strongly. Author