Fundamental Investigations of Plasticity in High Strength Nanostructured Aluminum Alloys

摘要

Cryomilled aluminum alloys are being developed for aerospace applications. These high-strength aluminum alloys are currently targeted for low-temperature rocket applications. This research focused on the fundamental strengthening mechanisms in these alloys. The primary sources of strengthening in cryomilled aluminum alloys are dispersion of nanocrystalline particles and ultrafine grain size that form during the cryomilling stage. Optimized thermo- mechanical processing of cryomilled powder results in an ultrafine grain size material with nanocrystalline particles. This study investigated the temperature dependence of strength and ductility in these alloys. The theoretical models predict strengthening at low temperatures and decrease of flow stress with decreasing grain size because of diffusional flow and grain boundary sliding at elevated temperatures. The results from this study show discrepancies with theoretical models. Significant work hardening was observed at room temperature in both Al-Ti-Cu and Al-Mg alloys. The strain hardening exponents suggest contributions from both direct and indirect work hardening mechanisms. At elevated temperatures, the stress-strain rate data for Al-Mg alloys can be best described by a power law constitutive relationship with a stress exponent of 5 and threshold stress. However, the activation energy is very high even after compensating for threshold approach. This indicates that dissolution of Al3Mg2 precipitates influence the deformation kinetics. (6 figures, 8 refs.).