Inverse Optimization and Design of Thermo-Mechanical Processing Paths for Improved Formability of Mg Alloys

Commercial Mg alloys’ demand in aerospace and automotive applications have significantly increased due to its lightweight properties and higher mechanical strength than its aluminum counterparts. Processing Mg alloys into functional grades for industrial use, could reduce our carbon footprint by increasing the fuel efficiency of all modes of transportation. However, Mg alloys’ use in such applications is hindered by the lack of formability due to its lack of active slip systems at room temperature and increased twinning effects that promotes tension-compression yield asymmetry for the material. Therefore, forming Mg alloys into complex geometrical shapes such as a car bumpers, can lead to premature failures of the material at room temperature during forming. Multiple studies were conducted on Mg alloys to reduce the CRSS difference between basal and non-basal slip systems, but a different approach was considered in this study, instead of mitigating the difference in CRSS, steps were taken to engineer the anisotropy of the material using texture alterations to engineer the formability of these alloys. Equal Channel Angular Pressing (ECAP) has been used to alter the texture of Mg alloys to maximize formability. A relationship between texture anisotropy to ductility was considered that related Lankford coefficient measurements or R-value measurements to a single invariant parameter called the anisotropy effect on ductility (AED) parameter. Using AED, we can relate mechanical R-value measurements of ECAP materials to formability. To find the best route with highest formability a novel inverse optimization method was used to automatically derive ECAP routes from the plasticity properties of Mg alloys. The Visco-Plastic Self Consistent (VPSC) crystal plasticity model was used to simulate and predict mechanical properties and textures. New Optimization methods were used to calibrate the model with experimental results which revealed further improvements were required to model ECAP with good accuracy. With the addition of grain fragmentation and Hall-Petch effect to further improve the VPSC crystal plasticity model, texture, and mechanical property predictions after ECAP was conducted at a much higher accuracy than what was done before. This in turn improved the novel inverse optimization method in predicting new ECAP routes with superior formability

Structure and Growth of Core–shell Nanoprecipitates in Al–Er–Sc–Zr–V–Si High-temperature Alloys

Lightweight Sc-containing aluminum alloys exhibit superior mechanical performance at high temperatures due to core–shell, L12-ordered trialuminide nanoprecipitates. In this study, the structure of these nanoprecipitates was studied, using different transmission electron microscopy (TEM) techniques, for an Al–Er– Sc–Zr–V–Si alloy that was subjected to a two-stage overaging heat treatment. Energy-dispersive X-ray spectroscopy of the spherical Al3(Sc, Zr, Er ,V) nanoprecipitates revealed a core–shell structure with an Sc- and Er-enriched core and a Zr-enriched shell, without a clear V outer shell. This structure is stable up to 72% of the absolute melting temperature of Al for extended periods of time. High-angle annular dark-field scanning TEM was used to image the {100} planes of the nanoprecipitates, demonstrating a homogeneous L12-ordered superlattice structure for the entire nanoprecipitates, despite the variations in the concentrations of solute atoms within the unit cells. A possible growth path and compositional trajectory for these nanoprecipitates was proposed using high-resolution TEM observations, where different rod-like structural defects were detected, which are considered to be precursors to the spherical L12-ordered nanoprecipitates. It is also hypothesized that the structural defects could consist of segregated Si; however, this was not possible to verify with HAADF-STEM because of the small differences in Al and Si atomic numbers. The results herein allow a better understanding of how the Al–Sc alloys’ core–shell nanoprecipitates form and evolve temporally, thereby providing a better physical picture for future atomistic structural mappings and simulations

Equal Channel Angular Pressing of a Newly Developed Precipitation Hardenable Scandium Containing Aluminum Alloy

Precipitation hardenable aluminum alloys are well-known for their high strength-to-weight ratio, good thermal stability, electrical conductivity, and low cost. Equal channel angular pressing (ECAP) is proven to further improve the mechanical properties of metallic alloys through microstructure modification. In this work, ECAP of a recently developed, precipitation hardenable, cast Al–Er–Sc–Zr–V–Si alloy in peak-aged condition by route 4Bc was carried out to create an alloy with ultra-fine grain structure. The combined effect of grain refinement and precipitation on the tensile behavior and thermal stability of the ECAPed alloy is reported here. Improvement in yield strength and lack of strain hardening in ECAPed alloy were as expected. Microhardness contour plots with a narrower spread indicated enhancement in microstructural homogeneity after four ECAP passes as compared to the peak-aged condition. The variations in microhardness after annealing heat treatments at different temperatures highlighted the important role precipitates play in maintaining microstructure stability up to 250 °C in the ECAPed material