The Microstructure And Formation Of The Directionally Solidified Al-Ni-Y Ternary Eutectic Alloy

Currently, the growth mechanism of the binary eutectic alloy has been researched,and some mature solidification models have already been built up. However, for theternary alloy, its solidification process is more complex as it involves competitivenucleation and growth of three phases, which make the investigation of ternary eutecticsolidification mechanism more difficult. In this study the downward directionalsolidification of the Al-Ni-Y ternary eutectic alloy was carried out. The microstructure andgrowth mechanism of the alloy was researched after it solidified.The downward directional solidification of the alloy with chemical composition ofAl-1.0Ni-3.0Y （at.%） was carried out at the pulling-down rates of1,2,4,5,10,15,30and50mm/min under the constant temperature gradient of5K/mm. The microstructure ofthe directionally solidified alloy and the stereoscopic topography of eutectic structure wasobserved by using optical microscopy （OM） and scanning electronic microscopy （SEM）,meanwhile its microstructure was analyzed quantitatively and its eutectic growthmechanism was analyzed. The crystal structure of the Al₃Y phase was simulated by theMaterial-Studio and ICSD software. Its evolution process and preferred orientation wereanalyzed.The results show that, the microstructure of the Al-Ni-Y ternary eutectic alloy wasconsisted of the white α-Al phase, gray Al₃Y phase, and black Al₂₃Ni₆Y₄phase （underoptical microscopy）. The microstructure of the alloy behaves the obvious characteristics ofdirectional growth after it solidified. With increase of the pulling-down rates, the gradualtransformation of the microstructure of Al-Ni-Y ternary eutectic alloy, from α-Al+Al₃Y+Al₂₃Ni₆Y₄ternary eutectic with the pulling-down rate of1mm/min to two eutecticsconsisted of the Al₂₃Ni₆Y₄+α-Al phase regular lamellar eutectic and Al₃Y phase+α-Alphase irregular chrysanthemum-like eutectic with the pulling down rate more than5mm/min, can be observed. When the pulling-down rate is increased to50mm/min, thecomplete chrysanthemum-like eutectic structure is gradually crushed and transformed toAl₃Y and α-Al phase dendritic eutectic. The microstructure of the Al-Ni-Y ternary eutectic alloy was analyzedcomprehensively. The instable growth model of the Al-Ni-Y ternary eutectic alloy wasestablished, and its growth mechanism was investigated. The microstructure evolutionmodel of the Al₃Y phase was established. The preferred orientation for the primarydendrite axis of the dendritic Al₃Y phase was pointed out,[1010],[0110],[1100],[1010],[0110],[1100].