Forged HITEMAL: Al-based MMCs strengthened with nanometric thick Al2O3 skeleton

Abstract

Bulk Al-Al2O3 metal matrix composites (MMCs) named HITEMAL (high temperature aluminum) were fabricated in situ by forging compaction of five different types of gas-atomized commercial purity Al powders with a mean particle size in the range of 1-9μm. As-forged HITEMAL consisted of (sub)micrometric Al grains (matrix) decorated with nanometric thick amorphous Al2O3 (a-Al2O3) skeleton. Low-angle grain boundaries (LAGBs) free of Al2O3 were located in the Al grain interior. The Al grain size and the portion of LAGBs increased with the increase in the relative powder surface area. As-forged HITEMAL shows excellent thermal stability up to 400°C for 24h. Annealing at temperatures ≥450°C led to crystallization and morphological transformation from a-Al2O3 skeleton to nanometric γ-Al2O3 particles. Owing to the pinning effect of Al2O3 phase, no Al grain growth took place during annealing up to 500°C. HITEMAL showed attractive mechanical properties especially when tested at 300°C (yield strength up to 220MPa, Young's modulus up to 58GPa). Despite the presence of a nearly continuous a-Al2O3 skeleton along adjacent Al grains, forged HITEMAL materials had reasonable room temperature elongation of 7-26%. HITEMAL's elongation decreased as the Al grain size decreased and with increased testing temperature. The loss in elongation (uniform and total) was attributed to the inhomogeneous flow, which occurred due to high densities of high angle grain boundaries (dislocation sinks) and small content of LAGBs. The strength of HITEMAL stemmed from grain boundary mediated strengthening mechanisms. The results showed a positive deviation from the Hall-Petch plot, which is typical behavior of ultrafine-grained metals. Transformation of a-Al2O3 skeleton to γ-Al2O3 particles led to deterioration of the HITEMAL strength and Young's modulus.