Microstructure Evolution And Grain Refinement Mechanism Of Highly Undercooled Binary Alloys

By applying glass fluxing method and cyclical superheating under the high vaccum and protection of Ar gas,bulk Co-20 at.%Pd and Ni-20 at.%Cu alloy melts were deeply undercooled.Factors that influence the fluxing effect and control conditions that influence obtaining stable undercooling were discussed.Conbined with analysis of theoretical models,the microstructure evolution and grain refinement phenomenon of highly undercooled Co-20 at.%Pd and Ni-20 at.%Cu alloys were systematically investigated.Based on design of a quenching system,recrystallization mechanism of highly undercooled Ni-20 at.%Cu alloy was systematically investigated.Recrystallization of solidification microstructures of bulk highly undercooled Ni-20 at.%Cu alloys was realized.The main contents are in the following text:1.Applying glass fluxing method and cyclic superheating,Co-20 at.%Pd alloys were undercooled by 340 K.When the alloy melt was undercooled beyond 265 K,hypercooling was achieved in reality.In the achieved undercooling range,two grain refinement events were occurred in the solidification microstructure of Co-20 at.%Pd alloys.When undercooling was between 50 K and 265 K,the grain refinement was attributed to dendrites remelting resultant from chemical superheating.When the undercooling was between 265 K and 280 K,the grain refinement mechanism of the alloy was transformed from dendrite remelting to stress-induced recrystallization mechanism.By using transmission electron microscopy technique,it was found that there were a lot of dislocation networks,stacking faults and small angle grain boundaries in the as-quenched alloys.These substructures indicate that when the undercooling is larger than 280 K,the grain refinement of Co-20 at.%Pd alloys was totally attributed to recrystallization mechniasm.2.As the initial undercooling was increased,the solidification microstructures of the Ni-20 at.%Cu single phase alloys evolved as follows: “coarse dendtrite—granular grains—directional fine dendtrites—granular grains”.The first granular grain resulted from the remelting of the parmary dendtrites.By theory calculation of nucleation,growth and stress accumulation during recrystallization in combination of TEM observation of dislocation morphology in the second granular grains of highly undercooled Ni-20 at.%Cu alloys,the stress-induced recrystallization mechanism of the second grain refinement events was proved.3.A stress accumulation model was developed on the base of a two phase model and a stress accumulation model.This model can semiquantitatively evaluate the stress accumulated in the rapidly solidified microstructures.When the initial undercooling exceeds a critical undercooling,solidification contraction and thermal strain induced liquid phase flow between the dendtrites will induce stress accumulation in the pramarily formed dendtrites,this stress will drive the breakup and plastic deformation of the dendtrites,and will be stroed in the form of strain energy in the pieces of the dendtrites,this strain energy will provide the thermodynamic driving force for the following recovery and recrystallization processes.5.Highly undercooled Ni-20 at.%Cu alloy melts were quenched by Ga-In liquid before recalescence.It was found that there were lots of plastical substructures in the as quenched alloys,e.g.Dense dislocation networks.Further anealling these quenched alloys under isothermal conditions,it was first found that recrystallization occured in the microstructures of the quenched alloys.However,annealing the naturally cooled alloys under the same conditions,the microstructures almost did not change,which indicated that the plastic strain energy of the naturally cooled alloys was far less than that in the quenched alloys.Obviously,these experimental results revealed the recrystallization mechanism of the grain refinement happening in the high undercooling range of the undercooled single phase alloys.6.The microstructures of the quenched Ni80Cu20 alloys with different undercoolings were investigated.It was found that the alloy sample had grain size degree gradient along the direction of the axis of the alloy samples.As the undercooling increased,the grain sizes in the microstrcutures at the top of the samples decreased gradually.The microstructures of quenched Ni80Cu20 alloy under isothermal conditions were investigted.Along the grain size gradient direction,as the depths of the alloys increase,the fraction of the small angle grain boundary decreased gradually,and however,the fraction of twin boundary increased gradually,these are typical characteristics of recrystallization transformation,this indicate that the quenched alloy recrystallized under isothermal conditions.