Microstructural characterization of a rapidly solidified ultrahigh strength Al94.5Cr3Co1.5SCe1 alloy

The microstructure of a rapidly solidified Al94.5Cr3Co1.5Ce1 alloy has been examined in detail by means of high resolution transmission electron micro scopy (HRTEM) and atom probe field ion microscopy (APFIM). In the as-quench ed microstructure, nanoscale particles of a solute-enriched amorphous phase and an Al-Cr compound are dispersed in randomly oriented fine grains of al pha-Al (similar to 200 nm). The interface between the Al grains and the amo rphous particles is not smooth but irregular with atomic protrusions and co ncavities, suggesting that interfacial instability occurs during the solidi fication process. Nanoscale amorphous particles are formed as a result of s olute trapping within the rapidly grown Al grains. After annealing at 400 d egrees C for 15 minutes grain growth occurs, and the interface of the Al gr ains is smoothed. The amorphous region trapped within the grains is crystal lized to an Al-Cr compound, but no icosahedral phase has been confirmed. Th e APFIM results have revealed that Cr and Ce atoms have a similar partition ing behavior, i.e., they are rejected from the alpha-Al phase and partition ed into the trapped amorphous regions. On the other hand, Co atoms are not partitioned between the two phases in the as-quenched state but are partiti oned into the alpha-Al grains in the annealed alloys being rejected from th e Al compounds and finally form Al-Co compounds. Based on these microstruct ural characterization results, the origins of high strength of this alloy a re discussed.