Non-Equilibrium Containerless Solidification of Al-Ni Alloys

Abstract

More than 90% of all metallic materials are manufactured starting from their liquid state. Designing the solid structure produced during solidification can have major savings in downstream processing. Rapid solidification yields significant enhancement in properties through refined microstructure, reduced microsegregation and the formation of metastable phases. To control the microstructure obtained from rapid solidification and attain desired properties, understanding of effects of processing parameters, in particular cooling rate and undercooling on microstructure evolution is required. In the case of peritectic reaction this understanding is lacking. In this dissertation, the effect of cooling rate on the peritectic reactions occurring in the binary Al-Ni system is investigated. Impulse Atomization technique was used to produce rapidly solidified particles of Al-36 wt%Ni and Al-50wt%Ni. The effect of cooling rate on the microstructure evolution and phase fractions achieved after solidification was studied. Also, porosity formation in the atomized particles was investigated and the effect of processing parameters on the amount and distribution of porosity was analyzed. For characterization, neutron diffraction, X-ray micro-tomography, electron and optical microscopy were utilized. The results showed that in both Al-36 wt%Ni and Al-50 wt%Ni, cooling rate has a significant effect on the formation of microstructure, phase fractions and metastable phase formation. It was shown that at different cooling rate regimes different mechanisms are responsible for the changes observed in the phase fractions. Using X-Ray tomography, multiple nucleation sites were observed in large particles, while smaller particles contained only a single nucleation site. Also, porosity within the particles was quantified and the distribution of porosity with regard to the nucleation site and cooling rate is discussed. The distribution of porosity within the small particles and large particles was found to be significantly different. Quantitative analysis of the micro-tomography images revealed that the volume percent of porosity decreased with increasing cooling rate. Also, it was found that for higher cooling rates, porosity tends to form closer to the periphery of the particles, whereas at lower cooling rates the pores are more randomly distributed.