Effects of volume fraction of tempered martensite on dynamic deformation properties of a Ti-6Al-4V alloy having a bimodal microstructure

Abstract

The effects of the volume fraction of tempered martensite on the tensile and dynamic deformation properties of a Ti-6A1-4V alloy having a bimodal microstructure were investigated in this study. Five microstructures having various tempered-marten site volume fractions were obtained by varying heat-treatment conditions. Dynamic torsional tests were conducted on them using a torsional Kolsky bar. The test data were analyzed in relation to microstructures, tensile properties, and adiabatic shear-band formation. Under a dynamic loading condition, the maximum shear stress increased with increasing ternpered-martensite volume fraction, whereas the fracture shear strain decreased. Observation of the deformed area after the dynamic torsional test indicated that a number of voids initiated mainly at alpha-phase/tempered-martensite interfaces, and that the number of voids increased with increasing : martensite volume fraction. Adiabatic shear bands of 6 to 10 mum in width were formed in the specimens having lower martensite volume fractions, while they were not formed in those having higher martensite volume fractions. The possibility of adiabatic shear-band formation was explained by concepts of absorbed deformation energy and void initiation.