Micromechanical behavior study of α phase with different morphologies of Ti-6Al-4V alloy by microindentation

In order to investigate four typical morphologies of α phase, including Platelet α, α case, Equiaxed α, and Martensite α, microindentation tests of Ti-6Al-4V under different heat treatment conditions were performed with various indentation loads from 50mN to 4900mN and two loading speeds of 9.6841mN/s and 103.7053mN/s by a sharp Berkovich indenter. The microhardness H, young's modulus E, initial yield stress σ_y and strain hardening exponent n were obtained by means of Oliver-Pharr method and reverse analysis algorithms. The results show that H of four typical morphologies decreases with the increase of indentation load. When loads are above around 2000mN, H of Equiaxed α is the largest. However, when loads fall down below 450mN, H of α case is the largest. This phenomenon can be explained by indentation size effects (ISE) and degree of ISE can be indicated by variable hH*. E of four typical morphologies also decreases with the increase of indentation load, which can be explained by damage during indentation. Among four morphologies, damage could be easily generated and spread within Equiaxed α. But it is difficult for α case. Moreover, σ_y and n remain constant with different loads in four morphologies. The working hardening effect becomes weaker as n becomes larger.