Study On Phase Transformations And Creep Behaviors Of Multiphase Ni₃Al-Based Intermetallic Alloy

Due to the introduction of?-Ni Al phase with B2 structure during solidification,multiphase Ni₃Al-based intermetallic alloys exhibit excellent temperature capacity,low mass density and outstanding weldability,thus possessing a wide range of application prospects and attracting lots of research focuses.In this dissertation,the multiphase microstructure evolutions and creep behaviors during solidification,heat treatment and serving processes of a novel multiphase Ni₃Al-based intermetallic alloy with high Fe and Cr contents were systematically studied.The multiphase formation sequences during solidification of this alloy were analyzed,and the as-cast multiphase structure was characterized in detail.The results showed that this explored alloy has a complex as-cast phase configuration,which is mainly composed of primary??+?dendrite,interdendritic?phase and less minor phases like??-envelope,carbides(Cr₂₃C₆ and HFC),etc.The total volume fraction of varied-type??phases in the as-cast structure was calculated to be an uncommonly high level of 80?vol.%.The effects of cooling rates following high temperature supersolution treatment on the morphologies of??+?dendrite,interdendritic?phase and??-envelope structure were investigated,and the as-cast microstructure of this alloy was proved susceptible to the supersolution cooling rate variants.The primary and secondary??phase in??+?dendrite showed strong dependence on the supersolution cooling rate,whose sizes exhibited logarithmic relationship between the cooling rates.The generation of microtwins and precipitation of minor phases in the interdendritic?-matrix were tailored by cooling rates,the substructure and width of microtwins were varing with different cooling methods.Three types of minor phases were observed in?phase:the plate-like??precipitates,the semi-spherical?-Cr particles and the rod-like Cr₃C₂carbides.Otherwise,the decrease of cooling rate can promote the formation and widening of the??-envelope structure.The influences of high-temperature annealing treatments on the multiphase hardening configuration and creep properties of this alloy were explored.The steady-state creep rate of this alloy at 800 ^oC was principally controlled by the primary??size in??+?dendrite,while the accelerated creep behavior was mainly affected by?phase.With the growth of primary??size from 0.48um in the as-cast state to 0.64?0.91um in the annealed state,the steady-state creep rate of this alloy was significantly reduced.The interdendritic?phase occurred to aggregate and coarsen during the high-temperature annealing periods while staying in constant volume fraction,but the variants of size and distribution of?phase showed limited impact on the creep rupture life of this alloy.The evolution of??-envelope and intersected plate-like??(??_P)phase in the interior of interdendritic?phase during long-term aging period was studied.It is proved that the(??+?)/??-envelope/?phases were bonded by strong/weak coherent/semi-coherent interfaces.The widening mechanism of??-envelope was?(Ni Al)+?(Ni)???(Ni₃Al),and the??-envelope widening exerted double-edged influence on the creep properties of this alloy.The precipitation of adequate intersected??_P phase can effectively inhibit the dislocation proliferation and crack initiation in the brittle interdendritic?phase during creep,contributed to improved creep properties.The creep mechanism of steady-state creep stage at medium temperature range800?840 ^oC/220?300 MPa of this alloy was determined to be dislocation-shearing,but it exhibited a“Four-stage”creep character which was uncommon to Ni-based superalloys.During the long-term aging and creep deformation processes,three types of??rafting structures were formed in this alloy,whose resistance to creep deformation was ranked as follows:bimodal R-type??rafting structure>R-type??rafting structure>N-type??rafting structure.