| As a functional material with shape memory effect(SME),shape memory alloy(SMA)has been widely used in engineering field.NiTi-based shape memory alloy and Fe-based shape memory alloy are the two most mature shape memory alloys at present.Shape memory alloy pipe coupling is the most successful example of shape memory alloy for engineering applications,which are mainly used in pipeline joints in aerospace,petrochemical and nuclear industries.Because of the high strength,large recovery strain,high resilience and high corrosion resistance,NiTi-based shape memory alloy pipe coupling becomes the most mature one.However,the NiTi-based shape memory alloy pipe coupling has a narrow phase transformation hysteresis and it is difficult to manufacture.Although the Fe-based shape memory alloy pipe coupling has the advantages of high strength,low cost,good hot workability and wide phase transformation hysteresis.However,the recoverable strain and the restoring force of Fe-based shape memory alloy pipe coupling are small,and the corrosion resistance is low.In view of the advantages and disadvantages of the two kinds of pipe coupling,a new concept of composite pipe with dissimilar shape memory alloys is proposed innovatively in this paper.The NiTi-based shape memory alloy pipe and the Fe-based shape memory alloy pipe are combined to form a composite pipe with the inner layer of NiTi-based shape memory alloy pipe and the outer layer of Fe-based shape memory alloy pipe by means of extrusion.It combines the advantages of shape memory alloy and makes up for their shortcomings.It is helpful to widen the engineering application scope of shape memory alloy pipe joint.In this paper,FeMnSiCrNi and NiTiNb shape memory alloys are used as research carriers.The constitutive equations and processing maps of two kinds of shape memory alloys are constructed by combining mechanical property experiments,microstructure characterization experiments,finite element numerical simulation and cellular automata simulation.In this paper,it provides a scientific theoretical basis for the industrialization of extrusion forming of dissimilar shape memory alloy composite pipe to clarify the flow behavior of plastic deformation at high temperatures,to optimize the processing parameters,to reveal the evolution law of microstructure and to study the extrusion formability of dissimilar shape memory alloy composite pipe.The main research results in the paper are as follows:The stress-strain curves of FeMnSiCrNi shape memory alloy at 850?1000? and strain rate of 0.0005?0.5s-1 were obtained.Based on the Arrhhenius constitutive model,the constitutive equation of high-temperature plastic deformation of FeMnSiCrNi shape memory alloy based on strain compensation is constructed.The constitutive equation can accurately predict the flow behavior of FeMnSiCrNi shape memory alloy subjected to plastic deformation at high temperature.The hot workability of FeMnSiCrNi shape memory alloy was studied.The stable region of FeMnSiCrNi shape memory alloy during hot working is determined as follows.When the strain rate is less than 0.0008s-1,the deformation temperature range is 850?925?.When the strain rate range is 0.001?0.03s-1,the deformatin temperature range is 925?1000?.The high-temperature plastic deformation of FeMnSiCrNi shape memory alloy has an important influence on its microstructure.In all the FeMnSiCrNi shape memory alloy samples subjected to high-temperature plastic deformation,? austenite,? martensite,austenite twin and stacking faults can be observed.There are variants of ? martensite,which has an orientation relationship of[110]?//[2110]with y austenite,and the content of ? martensite increases with the increase of deformation temperature.Stacking faults play an important role in the formation of ? martensite and austenite twin.Based on the mechanism of Shockley partial dislocations slip,intrinsic stacking faults and extrinsic stacking faults,the formation mechanism of ? martensite and austenite twin is revealed.Based on the stacking fault energy and quenching stress,the formation correlation of ? martensite with austenite twin is discussed.In addition,it can be concluded that the formation of ? martensite and austenite twin is mutually inhibited.The stress-strain curves of NiTiNb shape memory alloy at 850?1000? and strain rate of 0.0005?0.5s-1 were obtained.Based on the Arrhhenius constitutive model,the constitutive equation of high-temperature plastic deformation of NiTiNb shape memory alloy based on strain compensation is constructed.The constitutive equation can accurately predict the flow behavior of NiTiNb shape memory alloy subjected to plastic deformation at high temperatures.NiTiNb shape memory alloy is characterized by dynamic recovery and dynamic recrystallization during plastic deformation at high temperatures.With the increase of deformation temperature,subgrain boundary and dislocation density decrease.The hot workability of NiTiNb shape memory alloy was studied.The stable region of NiTiNb shape memory alloy during hot working is determined as follows.When the strain rate range is 0.0003?0.01s-1,the deformation temperature range is 750?840?.When the strain rate is 0.016?0.1s-1,the deformation temperature range is 930?1000?.Based on the Arrhenius constitutive model of FeMnSiCrNi and NiTiNb shape memory alloys,the isothermal extrusion of dissimilar shape memory alloy composite pipe is simulated by rigid viscoplastic finite element method.The effects of different deformation temperatures(800?,900?,1000?),different friction coefficients(0.1,0.15,0.2),different thickness reductions(0.2mm,0.4mm,0.6mm)and different cone angles of bottom die(60,90,120 degrees)on extrusion formability of dissimilar shape memory alloy composite pipes are studied.It can be found that the deformation zone of the dissimilar shape memory alloy composite pipe is always in a three-dimensional compressive stress state during extrusion forming,and the deformation of the inner pipe is obviously higher than that of the outer pipe With the increase of thickness reduction,friction coefficient and bottom die angle,the coordination of inner and outer pipe becomes worse.The cellular automata model of FeMnSiCrNi shape memory alloy is constructed,and the simulation program of cellular automata is developed.The dynamic recrystallization of FeMnSiCrNi shape memory alloy during compression deformation at high temperature is simulated by cellular automaton method.The evolution of flow stress,grain size and dislocation density with deformation temperature and deformation rate during dynamic recrystallization of FeMnSiCrNi shape memory alloy subjected to compression deformation at high temperatures is revealed.The relationship between macroscopic process variables and microscopic variables during plastic deformation of FeMnSiCrNi shape memory alloy at high temperatures is established by coupling finite element simulation and cellular automata simulation.The microstructural evolution of FeMnSiCrNi shape memory alloy in different deformation zones during isothermal extrusion of dissimilar shape memory alloy composite pipe is simulated.It can be found that the grain size of dynamic recrystallization is refined with the increase of plastic strain in the deformation zone. |
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