Quantifying the Evolution of Nanomechanical Response and Local Constitutive Relationship of Melt Pool in Additively Manufactured Ti6Al4V Composites via Nanoindentation Evaluation

The evolution of size and distribution of second-phase reinforcements will cause significant local stress gradient in metal matrix composites fabricated by additive manufacturing and post heat treatment. However, current studies focused on macroscopic mechanical performance, which is unable to quantitatively unravel the local structural stability and nanomechanical responses of composites and its correlation with the evolution of melt pool regions. Here, the local deformation behavior and nanomechanical response of TiBw-reinforced Ti6Al4V composites fabricated by electron beam powder bed fusion and post-annealing treatment have been investigated via instrumented nanoindentation. Distinct microstructural evolution of melt pool boundary (MPB) and melt pool center (MPC), such as the width of melting layer and the variation of TiBw in size and distribution, was observed in the composites as annealing temperature increased from 800 °C to 1100 °C. Relative to the narrow dispersion of nanohardness and elastic modulus for MPB regions, the load-displacement data obtained from MPC region showed a clear transition from low to high dispersion at the critical temperature of 950 °C, which is consistent with macroscopic tensile results. This transition implied a microstructural change of recrystallization and precipitation of nano-TiBw, and the migration of boron atoms from MPB into MPC regions. In addition, the characteristic parameters of nanohardness and elastic modulus were statistically analyzed by Weibull distribution model. The local stress–strain relationship and strain hardening exponent was also determined by indentation inverse algorithm and dimensional analysis. Our findings provide a microscopic perspective on the understanding of macroscopic strain hardening and structural stability of metallic composites upon deformation.