Microstructure Evolution And Phase Selection In The Initial Transition Zone Of Directionally Solidified NiAl-V Eutectic Alloys

The properties of the material are closely related to the microstructure,and the initial unsteady solidification process determines the final solidification form of the structure in a great extent.Therefore,it is significant to study the organizational evolution in the initial transition zone.In this paper,unidirectional solidification of NiAl-V system eutectic alloy was taken as the research object,and the change of microstructure morphology in the initial transition zone and the influence on microstructure growth in the steady-state zone were studied by controlling the pumping rate.At the same time,the competitive growth mechanism between primary phase and eutectic microstructure was studied by calculating the interfacial growth temperature.Here,NiAl-32V subeutectic alloy,NiAl-39V eutectic alloy and NiAl-43V supereutectic alloy were prepared by high temperature gradient directional solidification device.The microstructure evolution and the competitive growth relationship between primary phase and eutectic microstructure during solidification were studied.The main conclusions are as follows:The initial transition zone of NiAl-32V subeutectic alloy is two stages under the condition of experimental pumping rate??=6¹50?m/s?.As the solidification distance increases,the microstructure in the first stage is transformed into cellular primary?_NiAl+??+??eutectic,and then into dendritic primary?_NiAl+??+??in the second stage.In the steady-state region,the primary phase of the former was expanded prior to eutectic growth,and the dendrites grew finer and finer with the growth rate increasing.The solid-liquid interface experienced the changes of cell and dendrite.NiAl-39V eutectic alloy at low speed growth rate??=6?m/s?,its microstructure in the form of short-range ordered lamellar eutectic.in the growth rate of?=10¹50?m/s,the microstructure in the form of eutectic cells;and with the increase of the growth rate,the number of eutectic cells increases,the average diameter of cells decreases,the lamellar spacing decreases.The solid-liquid interface experienced the changes of the plane and the cell.The initial transition zone of NiAl-43V eutectic alloy will undergo a phase of evolution,that is,low-speed withdrawing rates rate??=2⁶?m/s?.The initial transition region of NiAl-43V hypereutectic alloy will undergo stage evolution of from Plane crystal to cellular crystal.The initial transition zone is composed of two phases,namely,cellular primary?_V+??+??eutectic zone and dendritic?_V+??+??eutectic zone.Medium-speed withdrawing rates??=10⁶0?m/s?by the strip primary?_V phase,cellular primary?_V+??+??eutectic,dendrite?_V+??+??eutectic of three stages.Under the high-speed withdrawing rates??=90¹50?m/s?,the strip primary?_V gradually disappeared and evolved into the latter two stages of medium speed.In the steady-state region,the growth of?_V phase is faster than that of eutectic microstructure,and with the increase of growth rate,the primary?_V dendritic gradually becomes finer and finer.The solid-liquid interface experienced the changes of cell and dendritic.At the same time,the symmetry of?_V alloy coupled eutectic growth zone is obtained by B-H type theoretical model.Based on the NiAl-35V and NiAl-43V as validation object,it was found that at the initial stage of solidification when the growth rate of NiAl-35V and NiAl-43V was 4um/s and 2 um/s respectively,there was competition between the primary phase and eutectic growth.Until the primary phase disappeared when the growth distance was 32mm and 37 mm,only the coupling symbiotic eutectic microstructure existed.The experimental results agree well with the theoretical ones.