Study On The Rapid Solidification Microstructure Of Deeply Undercooled Eutectic Alloy Melts

The formation of anomalous eutectic widely observed in solidification of undercooled eutectic melt,and have been attracting much attention during the past decades.As a result,many theories have been proposed for the anomalous eutectic formation,among with the point of view that anomalous eutectics form due to the remelting of the primary eutectics in temperature recalescence were widely accepted.However,what affect on the remelt of primary solid and then the formation of anomalous eutectics has not systematically studied so far.In addition,Metastable phases often form during rapid solidification of undercooled melts but,they generally transform into stable phases during further cooling to room temperature,which the study on their structure,properties and more importantly application impeds.In this dissertation,the remelt fraction of primary eutectics during temperature recalescence as a function of melt undercooling and alloy property was first theoretically analyzed.The theoretical predication for anomalous eutectic formation was verified by solidify Ag-Cu,Ni-P and Pd-P eutectic alloy at different undercooling.The Co-B alloys were undercooled and solidified on the emphasis of metastable phase selection.The following findings have been achieved.Base on the theoretical model of eutectic growth in bulk undercooled melts（LZ Model）,an expression of the remelted fraction of primary solid during recalescence was derived,and the effects of solute redistribution coefficient k,liquidus slope m and eutectic composition C_e on the extent of remelting were evaluated.It is indicated that the remelted fraction of primary solid not only increases with increasing melt undercooling but also changes with the characteristic parameters of eutectic phase diagrams.For eutectic alloys with either larger equilibrium solute distribution coefficients or gentle liquidus slopes,the primary eutectic is highly supersaturated with solute and more prone to remelting,and easy to transit into anomalous eutectic.Otherwise,it is not easy to form anomalous eutectic.An extreme case is when the solute redistribution coefficient k is close to zero,for example,the eutectic phase of stoichiometric compounds or phases with very large of liquidus slope,the remelted is not occur on them.While the supersaturation of solute in each eutectic phase is closely related to the eutectic composition,however,the remelted fraction is not very sensitive to the eutectic composition.Among the three representative binary eutectic alloys Ag-39.9at.%Cu,Ni-19.6at.%P and Pd-16.0at.%P-their eutectic products are solid solution-solid solution,solid solution-stoichiometric intermetallic compound and stoichiometric intermetallic compound-stoichiometric intermetallic compound,respectively.The Ag-39.9at.%Cu eutectic alloy,solidified into anomalous eutectic at larger undercooling with both of the two eutectic phaseα-Ag andβ-Cu remelted,and neither of the phase shows a strong preferred orientation in the anomalous eutectic.For the Ni-19.6 at.%P eutectic alloy,only theα-Ni partialy remelted,but the stoichiometric intermetallic phaseβ-Ni₃P is not.As a result,theα-Ni crystals are not preferred to orientate,butβ-Ni₃P shows highly oriented in the anomalous eutectic.For the Pd-16.0at.%P eutectic alloy,whose eutectic consists of two stoichiometric intermetallic compounds,any anomalous eutectic is not formed,since no remelting occurs in the primary solid.These experimental results agree well with the theoretical prediction.The primary lamellar（rod）eutectic structure was successfully remained to room temperature without remelted during solidification by using thin samples.When the obtained samples were rapidly heated in a thermal simulation machine up to different temperature below the equilibrium eutectic temperature,it was found that for Ag-39.9at.%Cu alloy,eitherα-Ag orβ-Cu as they were heated near to the equilibrium eutectic temperature,and neither of them shows a strong preferred orientation in the anomalous eutectic.For the Ni-19.6 at.%P eutectic alloy,only theα-Ni phase in the thin sample was remelted.As a result,theα-Ni phase has no preferred orientation,but theβ-Ni₃P shows highly oriented in the anomalous eutectic.For the Co-B alloy around the eutectic composition Co-18.5at.%B,a critical undercooling⁶0 K was revealed.When the alloy melts solidify below the critical undercooling,the solidification goes on according to the equilibrium phase diagram,and the solidification structures consist of stableα-Co and Co₃B phases.Above the critical undercooling,the metastable Co₂₃B₆ phase forms instead of the stable Co₃B phase and can be retained to room temperature if the cooling rate is greater than 25 K/min.The crystal structure of the metastable Co₂₃B₆ phase was determined to be face centred cubic and of the Cr₂₃C₆-type structure with a lattice parameter,measured by Rietveld refinement method,of a=10.4912（1）?.The melting point of Co₂₃B₆ phase is 1348 K,and below the temperature of 1208 K,it transforms by a eutectoid reaction toα-Co+Co₃B.The metastable Co₂₃B₆ phase exhibits good stability in the temperature range 1208-1348 K.Below the eutectoid reaction temperature,the TTT curve associated with the isothermal decomposition of Co₂₃B₆ is of the classic C-type with a nose temperature of 1023 K,and an upper and lower limit of 1208 K and 923 K,respectively.The apparent activation energy of isothermal decomposition of the metastable Co₂₃B₆ is 213.6 KJ/mol in the temperature range of 923-1123 K,but decrease to 44.9 KJ/mol in the temperature range1123-1208 K.A metastable phase diagram of Co-B that consist of a metastable eutectic reaction L→Co+Co₂₃B₆ under the equilibrium eutectic line was constructed according to the experimental result.The metastable eutectic temperature is 1343 K and the metastable eutectic composition is Co-19.6at.%B.