In situ analysis of dendritic equiaxed microstructure formation in Al-Cu alloys by synchrotron X-ray radiography

This paper reports on experiments dedicated to equiaxed solidification carried out on Al-10 wt% Cu alloy at the European Synchrotron Radiation Facility (ESRF) in Grenoble-France. Equiaxed growth was achieved in nearly isothermal conditions and observed continuously in real time from the early stages of solidification to the developed grain structure by X-ray radiography. The length of primary dendrite arms was measured on several growing equiaxed grains as a function of time and it was found that dendrite arm evolution can be well fitted by the Kolmogorov-Johnson-Mehl-Avrami (KJMA) function. The growth of four primary dendrite arms of a single grain was then characterized. The dendrite arms behaved differently depending on their growth directions and the presence of grains in their vicinities. At the beginning, the dendrite arm directed upwards was growing faster than the ones growing below because of "self-poisoning" due to a gravity driven fluid flow. In the late stage of solidification, analysis on several couples of grains growing towards each other confirmed that solutal interaction was the main cause of growth being stopped.