Synergistic effect of joint addition of Sb+Mn on high temperature strengthening in Al–4Cu heat-resistant alloy

The synergistic effect of joint addition of Sb + Mn on high temperature strengthening in Al–4Cu heat-resistant alloy was studied by sole and joint addition of Sb and Mn and tensile test at 300 °C. The microstructure evolution of the precipitation particles during the LTHE treatment (a long-time heat exposure at 300 °C for 100 h) was characterized by TEM-SAED and Elements-Mapping to reveal the high temperature strengthening mechanisms. It is found that the joint addition of trace Sb + Mn leads to an overflowing superimposition effect that significantly increase the values of σ0.2 at 300 °C in T5 and LTHE states. In Al–4Cu alloy, Sb micro-alloying can promote the precipitation of θ′ phases, however, during the LTHE treatment the phase transformation from θ′ to θ cannot be suppressed by it. Elements-Mapping images of Mn element strongly demonstrate that, during the LTHE treatment, Mn atoms are increasingly enriching in front of the growth interface of θ′ particles. This enrichment hinders the coarsening of θ′ particles and hence suppress the phase transformation from θ′ to θ during the LTHE treatment. It is the synergistic effect of Sb-induced promotion on the precipitation of θ′ particles and Mn-induced suppression on the coarsening of θ′ particles and phase transformation from θ′ to θ that leads to high density of fine θ′ particles still existing in the Al matrix after the LTHE treatment which still significantly strengthen the matrix. Therefore, the alloy with the joint addition of Sb + Mn has the highest value of σ0.2 at 300 °C in LTHE state among the studied alloys.