Transmission electron microscopy of the rapid solidification microstructure evolution and solidification interface velocity determination in hypereutectic Al-20at.%Cu after laser melting

The evolution of rapid solidification (RS) microstructure and solidification interface velocity has been studied experimentally by in-situ transmission electron microscopy and postmortem characterization for hypereutectic Al-20at.%Cu (37 wt.%Cu) after laser melting. Four morphologically distinct regions formed via growth modes changing from theta-Al2Cu phase dendrites to eutectic cell growth, possibly alpha-Al dendrite growth (alpha-cell), banded growth, and alpha-Al plane front growth. Tendency for faceting and low capacity for solute trapping limited the theta-phase dendrite growth to solidification interface velocity v < 0.07 m/s. Consequently, formation of pro-eutectic micro-constituent was suppressed, and RS microstructure formation was dominated by eutectic, alpha-cell, and banded morphology grains for the Al-20at.%Cu alloy. Eutectic growth operated for interface velocity of 0.1 m/s <= v < 0.3 m/s, with a transition from regular lamellar, 2-lambda and 1-lambda mode to a dense irregular morphology dominated by alpha-phase at v = 0.3 m/s. Interface temperature calculations indicated feasibility of alpha-cell growth mode for 0.3 m/s <= v < 0.7 m/s. Banded growth occurred for 0.7 m/s <= v < 1.3 m/s. Plane front alpha-phase growth was evident for interface velocities v >= 1.3 m/s. Previous work on rapid solidification microstructure development in hypereutectic Al-Cu alloys reported a regime of alpha-cell growth subsequently to eutectic and prior to transition to banded growth for Al-19at.%Cu (36 wt.%Cu), while for Al-22at.%Cu (40 wt.%Cu) eutectic growth transitioned directly to alpha-plane front growth without emergence of a banded regime. Based on the current study the disappearance of the banded growth regime occurs for a composition larger than 20at.%Cu

Automated summary

The evolution of rapid solidification microstructure and solidification interface velocity of hypereutectic Al-20at.%Cu alloy after laser melting has been studied experimentally. It was found that the formation of microstructure was dominated by eutectic, alpha-cell, and banded morphology grains, and the growth modes changed with increasing interface velocity.