Grain boundary localized damage in hexagonal titanium

Abstract

Fully recrystallized hexagonal titanium tubes were processed through industrial cold pilgering. Two different pilgering sequences, generalized as case I and case II, were used. Both pilgering processes involved identical effective pilgering strains, but they had different strain modes. Case I, for example, used a pilgering pass with more (∼2.5 times) deviation from ideal plane strain compression. Both fully pilgered tubes showed similar grain fragmentations, misorientations, and tensile properties. However, their crystallographic textures and residual stresses significantly differed. Further, the inner surfaces of case II tube showed localized intergranular damage. Direct experimental observations, combining interrupted tensile tests with high-resolution electron backscattered diffraction (HR-EBSD), revealed that the damage originated only at select grain boundaries. In particular, dislocation pile-up plus stress localization appeared to be responsible for the grain boundary damage initiation. Microstructurally, this originated from the relative difference in elastic moduli between neighboring grains and the Luster-Morris parameter, representing slip transfer across the grain boundary.