Microstructure Evolution And Growth Mechanism Of Faceted Peritectic Phase During Directional Solidification Of Al-Ni Peritectic Alloys

In this study, Al-Ni peritectic alloys exhibiting peritectic reactionL+Al₃Ni2Al₃Ni in which the peritectic Al₃Ni phase is an intermetallic compoundwith nil solubility, have been chosen for investigation. Growth of Al₃Ni phasedirectly from the melt, nucleation of Al₃Ni phase as a peritectic phase,microstructure evolution and phase selection of Al-Ni peritectic alloys duringdirectional solidification, and growth mechanism of Al₃Ni phase as a peritecticphase have been investigated using differential scanning calorimetry （DSC）analysis and a Bridgman-type directional solidification apparatus.Al-6at%Ni alloy has been selected to investigate the growth mechanism ofAl₃Ni phase as the primary phase precipitating from the melt. During directionalsolidification, at a growth rate of5m/s and a temperature gradient of30K/mm,Al₃Ni phase precipitates from the melt directly, exhibiting faceted growth interfaceand complex3-D morphology. Nucleation and growth of Al₃Ni as a peritectic phaseduring continuous solidification process of Al-Ni peritectic alloys have beenanalyzed. The DSC analysis results show that when extrapolating the cooling ratesto0℃/min, the degenerated nucleation temperature of solid solution type peritecticphase is equal to the equilibrium peritectic temperature, while a great amount ofundercooling has been observed for Al₃Ni phase. The microstructure analysis showsthat the primary Al₃Ni2phase can act as a sound heterogeneous nucleation substrate.The Al₃Ni phase layer envelopping the primary Al₃Ni2phase exhibits facetedgrowth interface and is composed of several grains.The preparation of the initial solid/liquid interface during directionalsolidification on which the growth starts is a critical step, which consists of meltingand thermal stabilization under a temperature gradient. Experiments on Al-Ni andCu-Sn peritectic alloys consisting of melting followed by thermal stabilizationranging from0to2h have been carried out in a Bridgman-type furnace. In thedirectional melting process, due to the temperature gradient imposed on the rod, amushy zone is created between the complete liquid zone and non-molten solid zoneparallel to the temperature gradient. With the thermal stabilization time increase,the volume fraction of the liquid phase in the mushy zone decreases, and the initialsolid/liquid interface moves downward parallel to the temperature gradient. After2h thermal stabilization, the initial solid-liquid interface is corrugated, whichdeparts from the theorectical prediction. A solute diffusion model under atemperature gradient has been built up to explain the above experimental observations.The consequence of the initial solid/liquid interface morphology on themicrostructure evolution during subsequent directional solidification process of Al-Ni and Cu-Sn alloys has been investigated by changing the thermal stabilizationduration. It is found that if the initial solid/liquid interface is non-planar, even whenthe experimental parameters such as temperature gradient and growth velocitymaintaining planar growth interface are met, the growth interface is non-planar.However, if the initial solid/liquid interface is planar, a planar growth interface canbe obtained. When the experimental conditions maitaining planar growth interfaceare not met, whether the initial solid/liquid is planar or not doesn’t have obviousinfluence on the subsequent microstructure evolution during directionalsolidification.Systematic directional solidification experiments have been performed on Al-Ni peritectic alloys. During directional solidification of Al-18at%Ni alloy, withpulling rates of1m/s and8m/s, with freezing distance increase, the precipitatingsolid phase from the liquid at the quenching solid/liquid interface transforms fromprimary Al₃Ni2phase to peritectic Al₃Ni phase. Based on the principle of soluteconservation and Fick’s diffusion law, solute redistribution behavior ofintermetallic compound with nil solubility range during planar directional growthhas been analyzed, which has been used to explain the experimental observations indirectionally solidified Al-18at%Ni alloy.During directional solidification of Al-25at%Ni alloy, with pulling rate rangingfrom5to500m/s, primary Al₃Ni2phase precipitates from the liquid firstexhibiting dendrite morphology, and peritectic Al₃Ni phase forms at a locationbelow the peritectic interface. At growth rate of5,10, and20m/s, a great volumefraction of primary Al₃Ni2phase is consumed below the peritectic reactiontemperature. With growth rate beyond20m/s, the change of Al₃Ni2volumefraction changes a little below the peritectic reaction temperature, which indicatesthe growth mechanism of the peritectic Al₃Ni phase is direct solidification from theliquid. Volume fractions of each phase in the transverse sections with differenttemperatures have been evaluated for the sample at pulling rate of20m/s. PrimaryAl₃Ni2phase dissolution is much greater than that should be consumed by peritecticreaction and transformation which is calculated based on traditional peritecticsolidification theory.During dendritic solidification of Al-Ni peritectic alloys under a temperaturegradient, it is observed that a thick peritectic layer forms on the front edge of thesecondary dendrite arm of the primary Al₃Ni2phase, while there is almost no peritectic Al₃Ni phase on the back edge of the secondary dendrite arm. Thisobservation is explained satisfactorily by a new version of secondary dendrite armmigration caused by temperature gradient zone melting （TGZM） during peritecticsolidification, which involves both primary and peritectic phases. A divorcedperitectic reaction model caused by TGZM in directional solidification process hasbeen proposed, which can satisfactorily explain the fast primary Al₃Ni2dissolutionduring directional solidification.