Origin of non-uniform plasticity in a high-strength Al-Mn-Sc based alloy produced by laser powder bed fusion

Abstract

• The origin of non-uniform plasticity of the Al-Mn-Sc based alloy produced by laser powder bed fusion has been clarified. • The non-uniform plasticity of the alloy originates from those microstructural areas containing fine and equiaxed grains. • The absence of work hardening in fine-grain areas is attributed to the lack of mobile dislocations and grain boundary particles. • Quantitative strength contributions from solutes, grain boundaries and secondary precipitates in bimodal grain microstructure have been identified. The Al-Mn-Sc-based alloys specific for additive manufacturing (AM) have been recently developed and can reach ultrahigh strength and adequate elongation. However, these alloys commonly exhibit non-uniform plasticity during tensile deformation, which is a critical issue hindering their wider application. In this work, the origin of this non-uniform plasticity of the alloys produced by laser powder bed fusion (LPBF) has been systematically investigated for the first time. The results show that the loss of uniform plasticity in the alloy originates from microstructural regions containing equiaxed fine-grains (FGs) (∼650 nm in size) at the bottom of the melt pools. In micro-tensile tests, the strength of these FG regions can reach a peak of ∼630 MPa. After this, an apparent yield drop occurs, followed by rapid strain softening. This FG behavior is associated with intermetallic particles along grain boundaries and a lack of uniform mobile dislocations during deformation. The columnar coarse-grain (CG) regions in the remaining melt pools show uniform plasticity and moderate work hardening. Furthermore, the quantitative calculations indicate that the solid solution strengthening in these two regions is similar. Nevertheless, secondary Al 3 Sc precipitates contribute to ∼260 MPa strength in the FG, compared to 310 MPa in the CG due to their different number density. In addition, grain boundary strengthening can reach 230 MPa in the FG region; nearly double the CG region value. .