Metastable Dendrite Morphologies in Aluminum Alloys

Cubic metallic alloys generally grow along 〈100〉 directions due to the anisotropy of the solid–liquid interfacial energy. Under rapid solidification conditions, dendrites may deviate from 〈100〉 and develop unusual morphologies. Here, Al-alloy droplets (Al-4.5Cu, Al-10Si, Al-1.9Fe, Al-33Cu, all in wt.%) were rapidly solidified using Impulse Atomization to study the microstructures forming at different cooling rates and undercoolings. Growth morphologies of Al-4.5Cu droplets were characterized using x-ray micro-tomography and EBSD. Al-dendrites were found to grow along either 〈100〉 or a more unusual 〈111〉 depending on the solidification conditions. Also, a transition from 〈111〉 to 〈100〉 in the same droplet was observed. These uncommon growth directions were also observed in other Al-alloys. In Al-1.9Fe droplets, a change in dendrite growth direction from 〈100〉 to 〈111〉 was observed, while 〈110〉 growth directions were detected in Al-10Si samples. These experimental observations will be related to their solidification conditions using Solidification Continuous Cooling Transformation diagrams.