Cavitation during hot tension testing of Ti-6A1-4V

Cavity initiation, growth, and coalescence phenomena during the hot tension testing of Ti-6Al-4V with several different transformed beta-microstructures were established to obtain an understanding of the failure process during subtransus hot working. Samples with either a colony-type microstructure (containing grain-boundary alpha) or a martensitic, acicular alpha-microstructure were pulled to failure at strain rates between 0.01 and 3 s(-1) and various subtransus temperatures ranging from 540 to 955 degrees C. Metallographic analysis of sectioned tension samples with colony microstructures revealed that cavity initiation occurred at very low strains (less than or similar to 0.2) irrespective of strain rate at temperatures below 900 degrees C due to an apparent incompatibility of deformation between the grain-boundary alpha-layer and the Widmanstatten side plates. Initiation strains were somewhat higher (similar to 0.2-0.5) for samples with the acicular alpha-microstructure tested at temperatures below 900 degrees C; however, failure was still intergranular. Subsequent cavity growth was more rapid for colony microstructures than that for the martensitic alpha-microstructure at these temperatures. Above 900 degrees C, however, substantially higher cavity initiation strains and lower cavity growth rates were obtained for both microstructures. These latter behaviors were ascribed to the presence of a large volume fraction of ductile beta-phase and moderate values of the strain-rate sensitivity of the flow stress, respectively. (C) 1998 Elsevier Science S.A. All rights reserved.