Experimental investigation of the dependence of void nucleation and growth on initial microstructure in high-purity titanuim under tension
To which limits can a material be deformed before it fails? Identifying the root cause of void nucleation and growth could allow expansion of those limits. For example, what is the role of the initial material microstructure? And can this be altered such that fewer voids nucleate or grow at a given degree of deformation to postpone total failure?
To answer some of these questions, researchers from University of New Hampshire and Los Alamos National Laboratory carried out an experimental investigation of failure mechanisms in relation to the initial microstructure in high-purity titanium samples. The initial 3D microstructure was measured using lab-based DCT, while absorption CT was used to visualize voids at various levels of deformation attained by interrupted tensile testing. Finally, EBSD mapping was used to characterize the deformed microstructure in 2D.
Grain size rather than grain orientation
Two specimen types with different texture relative to the tensile direction were investigated. The results showed no correlation between early void formation and initial grain orientations. In addition, the failure locations (marked by red arrows) were found to be unrelated to the formation of the first voids. Instead, failure was observed to begin from a surface crack in the severely-deformed necked regions. It was shown that necking occurred near the largest grain in the gauge sections. This indicates that failure is likely related to the decrease in strength of larger grains, causing strain to concentrate in their vicinity. Finally, during crack propagation through the samples, the vast majority of void nucleation and growth was found at the tip of the failure-causing crack.
