Direct observation of spatially isothermal equiaxed solidification of an Al–Cu alloy in microgravity on board the MASER 13 sounding rocket

Abstract

For the first time, isothermal equiaxed solidification of a metallic alloy has been observed in situ in space, providing unique benchmark experimental data. The experiment was completed on board the MASER 13 sounding rocket, launched in December 2015, using a newly developed isothermal solidification furnace. A grain-refined Al–20wt%Cu sample was fully melted and solidified during 360s of microgravity and the solidification sequence was recorded using time-resolved X-radiography. Equiaxed nucleation, dendritic growth, solutal impingement, and eutectic transformation were thus observed in a gravity-free environment.Equiaxed nucleation was promoted through application of a controlled cooling rate of −0.05K/s producing a 1D grain density of ~6.5mm−1, uniformly distributed throughout the field of view (FOV). Primary growth slowed to a visually imperceptible level at an estimated undercooling of 7K, after which the cooling rate was increased to −1.0K/s for the remainder of solidification and eutectic transformation, ensuring the sample was fully solidified inside the microgravity time window. The eutectic transformation commenced at the centre of the FOV proceeding radially outwards covering the entire FOV in ~3sMicrogravity-based solidification is compared to an identical pre-flight ground-based experiment using the same sample and experiment timeline. The ground experiment was designed to minimise gravity effects, by choice of a horizontal orientation for the sample, so that any differences would be subtle. The first equiaxed nucleation occurred at an apparent undercooling of 0.6K less than the equivalent event during microgravity. During primary equiaxed solidification, as expected, no buoyant grain motion was observed during microgravity, compared to modest grain rotation and reorientation observed during terrestrial-based solidification. However, when the cooling rate was increased from −0.05K/s to −1.0K/s during the latter stages of solidification, in both 1g and micro-g environments, some grain movement was apparent due to liquid feeding and mechanical impingement of neighbouring grains.