High Temperature Scandium Containing Aluminum Alloys Subjected to Equal Channel Angular Pressing

Abstract

Precipitation hardenable aluminum alloys are well-known for their high strengthto-weight ratio, good thermal stability, electrical conductivity, and low cost. Al-Sc alloys micro-alloyed with rare earth and transition metal elements can be strengthened for ambient and high temperature applications. This is primarily due to the precipitation of coherent, hard, finely distributed, and coarsening resistant, nanosized Al3(X) trialuminide precipitates. In addition to precipitation strengthening, equal channel angular pressing (ECAP) can be applied to further enhance mechanical properties of this system by microstructure modification. In this work, the effect of ECAP on microstructure modification, precipitate evolution, and mechanical response of a high temperature aluminum alloy with microadditions of Er, Sc, Zr, V, Si was investigated. Combined strengthening with yield strength up to ~180 MPa was achieved after aging to peak hardness followed by grain refinement through ambient temperature ECAP using route 4Bc. Subsequently, a different processing approach of ECAP after homogenization was also carried out. Tensile results showed only a slight improvement of about 2-5% in yield strengths of peak-aged followed by ECAP (PA-ECAP) alloy as compared to homogenized followed by ECAP (H-ECAP) alloy. Mechanical tests combined with calorimetry studies and scanning/transmission electron microscopy confirmed the occurrence of dynamic precipitation during ambient temperature ECAP of Al-Er-Sc-Zr-V-Si in homogenized condition. Hence, it was established that ECAP can significantly influence the kinetics and distribution of precipitates in these alloys. Furthermore, pre- and post- ECAPed alloys were subjected to annealing heat treatments. The variations in microhardness after annealing heat treatments at different temperatures highlighted the important role nanoprecipitates play in maintaining microstructure stability of Al-Er-Sc-Zr-V-Si before and after ECAP. Microstructure evolution during static annealing (without the application of load) and dynamic annealing (with applied load) was also studied using interrupted high temperature tensile tests followed by electron backscatter diffraction (EBSD) analysis. Results showed that there is a difference in deformation mechanism for H-ECAP and PA-ECAP. Among the two processing routes, although the magnitude of static and dynamic grain growth in H-ECAP condition was found to be higher than PA-ECAP condition, it showed superior elevated temperature strength and ductility. Lastly, electrochemical characteristics of Al-Er-Sc-Zr-Si with micro-additions of Group 5 transition elements (V, Nb, or Ta) added individually and then exposed to saline media. There is slight increase in activity of Al-Er-Sc-Zr-Si after addition of any of Group 5 elements (V, Nb, or Ta) which justifies their addition to improve ambient and elevated temperature mechanical properties. The order of mechanical strength is Al-Er-Sc-Zr-Si-V > Al-Er-Sc-Zr-Si-Nb > Al-Er-Sc-Zr-Si-Ta > Al-Er-Sc-Zr-Si