Study Of Microstructure Evolution During Heat Treatment Of Cold-deformed NiTiFe Shape Memory Alloy

As a typical kind of NiTi-based shape memory alloy(SMA),NiTiFe SMA is widely used in pipe coupling of oil system in aerospace and aviation fields.For the pipe coupling which is used in important fields,better microstructures and higher performance of it must be required.As a consequence,it is necessary to investigate the control of microsturctures and performance of NiTiFe SMA.In the present study,experiments were applied to investigate the behaviors of static recrystallization(SRX)during annealing of NiTiFe SMA which is subjected to severe plastic deformation induced by cold canning compression.In addition,the processes of severe plastic deformation induced by cold canning compression and subsequent SRX were simulated by coupling crystal plasticity finite element method(CPFEM)with cellular automaton(CA)method.By means of combination of experiment and simulation,the influences of three processing parameters,including compressive deformation degree,annealing temperature and holding temperature of annealing,on the SRX microstructures of NiTiFe SMA were investigated.Consequently,the effect mechanisms of processing parameters on the SRX of NiTiFe SMA subjected to severe plastic deformation were revealed.In addition,a new SRX simulation method which considers inhomogeneous distribution of dislocation density was explored.Theses investigations lay important significance for studying the controlling methods of microstructures and performance of NiTiFe SMA.The results show that due to the inhomogeneously distributed dislocation density being used as driving force of nucleation,the coupling model,which is established on the basis of canning compression simulation performed by CPFEM and SRX simulation performed by CA,is more closed to the real experimental conditions.As a consequence,the model is able to simulate the SRX process of NiTiFe SMA which is subjected to severe plastic deformation induced by cold canning compression.At the holding time of 4min,the error between the simulation value and the experimental one of SRX fraction is in the range of 0.5%-1.05%.However,at the holding time of 1min,the error between the simulation value and the experimental one is able to reach at up to 56.24%.This phenomenon is attributed to the fact that the specimen experienced heating process after it was put into the furnace,and the starting time of SRX is incapable of being measured.Furthermore,the course of SRX is too fast.With the time prolonged,the simulation results tend to be consistent with the experimental ones,which indicates that the model realizes the predicted simulation function.It can be known form experimental results that the grain size of SRX increases with the increase in the annealing time,and the time of complete SRX and the yield stress decrease with the increase in the annealing time.However,the annealing time has little influence on the recrystallization texture.The greater compressive deformation degree leads to the finer SRX grains,the shorter time of complete SRX,the lower yield strength and the stronger recrystallization density.The longer the holding time is,the coarser SRX grains,lower yield strength and the stronger recrystallization texture are obtained.It can be noted from simulation results that the dynamics curves of SRX simulation obtained based on the coupling model in the present study agree with JMAK theory,where the Avrami constant n is determined as 2.8,which is in the range of 2.3-3 and is more closed to 3.This value is a very ideal Avrami constant,which indicates that the current simulation model for SRX is accurate and reliable.It can be known from the law obtained from simulation of the influence of processing parameters on the NiTiFe SMA that the influences of compressive deformation degree,annealing temperature and holding time of annealing on the volume fraction and grain size of SRX are in consistent with those obtained from experiments.In other words,the greater the compressive deformation degree leads to the finer SRX grains and the left shift of SRX fraction curve,and the evaluated annealing temperature results in the coarser SRX grains and the left shift of SRX fraction curve,while the longer annealing time causes to the coarser SRX grains and larger SRX fraction.