Influence of microstructure heterogeneity on the tensile response of an Aluminium alloy designed for laser powder bed fusion

Aluminium alloys designed for additive manufacturing with Zr addition exhibit heterogeneous microstructures from the grain scale down to the atomic scale. Bimodal grain structures with submicron equiaxed grains near the melt pool boundaries and columnar grains at the melt pool interiors are observed. The regions consisting of submicron grains are interconnected in 3D. The tensile response of these alloys often shows the presence of a stress plateau or a yield drop phenomenon near the yield strength but the underlying mechanisms still need to be clarified. Herein we investigate the tensile response of a new Al-4Mn-3Ni-2Cu-1Zr alloy in various conditions: after a stress relief (SR=300°C/4h) as well as after ageing at 400°C for 1h, 4h (peak-aged) and 96h (overaged). Based on the plot of the work hardening rate vs true stress, we identify specific characteristics allowing to appreciate the evolution of the elastoplastic transition depending on the heat treatment. The presence of microyielding and a stress plateau near the macroscopic yield strength are evidenced. Microscale DIC based on the local contrast provided by the microstructure decorated by a high fraction of intermetallic particles turns out to be the relevant scale to clarify the mechanisms involved in the elastoplastic transition. Microyielding is related to the regions near the melt pool boundaries and to the columnar grains. The latter yields at lower stress in comparison with the submicron grains due to a reduced contribution of grain boundary strengthening. The stress plateau is attributed to the low work-hardening capacity of the submicron grains.