Formation and recovery behavior of hetero-nanostructured metals processed by high-pressure torsion

Nanocrystalline metals processed by high-pressure torsion (HPT) are often inhomogeneous in microstructure and composition. This presentation displays the formation of a hetero-nanostructure in an Al-Mg alloy synthesized by HPT and the recovery behavior during heating of the formed nanostructure using high-energy synchrotron X-rays. Moreover, it demonstrates the heterogeneous microstructural evolution upon heating of an HPT-processes nanostructured high-entropy alloy using laser-scanning confocal microscopy. Earlier studies demonstrated the successful formation of a homogeneous nanostructure when separate Al and Mg were processed concurrently by stacking them in order of Al/Mg/Al through HPT under 6 GPa for a severe shear strain of γ ≈ 4000 at room temperature. The mechanical bonding by HPT resulted in the synthesis of a nanostructured Al alloy with grain sizes of 30-40 nm in a Mg-supersaturated metastable state. While lab-scale X-ray diffraction analysis reveals significant broadening in chemical composition, micro-beam synchrotron high-energy X-ray diffraction (HEXD) analysis displays the position-sensitive transition to form a metastable nanostructure solid solution alloy from the dissimilar metals during HPT. Thus, the HEXD measurement reveals the strain-dependent phase transformations from separate f.c.c. Al-rich and h.c.p. Mg-rich phases at the locations having γ 2500 to a single f.c.c. Al supersaturated phase in solid solution, with an average Mg concentration of ~15 at.% at γ 2500. The supersaturated Al solid solution alloy after HPT is further tested upon heating up to 850K to examine the microstructural recovery and phase evolution at a local region by in-situ heating micro-beam HEXD. Besides the general microstructural evolution with increasing temperature, nucleation and dissolution of several different intermetallic phases are apparent from the evaluation of peak profiles at varying temperatures. While such micro-scale evaluation, including HEXD, may miss the opportunity to identify the heterogeneous microstructural recovery, inhomogeneous microstructural relaxation on the macro-scale is monitored in an HPTprocessed CoCrFeNiMn high-entropy alloy using in-situ heating laser-scanning confocal microscopy. The examined length scale is not for identifying nucleated phases but for observing macro-scale heterogeneous microstructure relaxation with increasing temperature, which is contributed to the different shear directions influenced by HPT processing. These novel techniques complement each other and other in- or ex-situ testing methods, particularly when nanocrystalline metals experience microstructural and compositional transformations with varying temperature and time in a hierarchical manner. The application of these novel diffraction techniques at specific testing conditions provides a significant opportunity to understand the position-, time- and temperature-resolved heterogeneous microstructural evolution in nanostructure materials processed by HPT. In addition, the outcomes of the series of studies emphasize the importance of the methodologies and the development of characterization techniques for further in-depth exploration in the research field of severe plastic deformation.