Influence of architecture and temperature on the critical strain for serrated flow in additively manufactured Inconel 718 lattices

Abstract

This work aims to investigate the influence of architecture and testing temperature (T) on the critical strain for serrated flow (ε_c) in Inconel 718 additively manufactured lattices. Three BCC lattices with different strut and cell dimensions were fabricated by laser powder bed fusion (LPBF) and they were tested in uniaxial compression at 25, 300, 450 and 600°C at an initial strain rate of 10⁻³ s⁻¹. Serrated flow was observed in the three BCC lattices at T ≥ 300 °C and ε_c was measured for each lattice architecture and temperature. At a fixed T ε_c is inversely proportional to the lattice relative density and for each investigated lattice architecture ε_c exhibits the lowest value at 450°C. Finite element modeling (FEM) was utilized to calculate the local stress distributions during uniaxial compression of the BCC lattices, revealing that the onset of serrated flow requires an activation volume fraction of material (V_f*) to be subjected to a local stress exceeding a threshold stress (σ_th). The values of V_f* and σ_th at 300, 450 and 600°C were calculated from the FEM simulations of the BCC lattices and they were used to accurately predict ε_c in an LPBF-manufactured FCCXYZ lattice at similar testing conditions. Our results suggest that the inverse relationship between ε_c and the lattice relative density is explained by the fact that lighter lattices require higher nominal strains to reach V_f*. Conversely, the variation of ε_c with temperature is attributed to changes in V_f*, as σ_th remains essentially constant at the investigated temperatures.