Tensile deformation and fracture in high purity titanium: In situ observation by scanning electron microscopy

Abstract

The tensile deformation and fracture of high purity titanium have been studied by using a tensile stage in a scanning electron microscope. The experimental results show that the deformation behavior of polycrystalline titanium depends strongly on the grain orientations. After slipping becomes difficult by work hardening, deformation twinning was observed to occur and to cause slip line kinking. These kinks proved very stable against further deformation. Fracture was typically observed to occur as follows: inhomogeneous deformation at a grain boundary associated with a highly deformable grain formed a notch, which acted as a stress concentrator. Intersecting slip lines in the notch area were also found to produce microvoids. Incompatible deformation at the boundary caused still further local stress increase, sufficient to exceed the boundary fracture strength, and the crack grew. In the growth of the resulting macrocrack, the mechanism of growth supported the localized slip model of crack growth. However, where pre-existing shear bands were involved, microcracks nucleated ahead of the main crack and caused growth by linking. Structure-sensitive aspects of crack growth involving grain boundaries and deformation twins are also described.