Research On The Martensitic Transformation And Elastocaloric Effects In Cold-drawn Ni-Ti Wires

NiTi alloy is the most widely used shape memory material with the most exceptional performance owing to its excellent shape memory effect,superelasticity and mechanical properties.Elastocaloric refrigeration is a new type of solid-state refrigeration technology using stress-induced martensitic transformation through loading and unloading cycles.Among the materials possessing giant elastocaloric effects,NiTi alloys have received extensive attention due to its excellent properties.In the present paper,continuous NiTi alloy microwires with uniform diameter and good surface quality were prepared by cold drawing process combining with pulse electrical current annealing treatment.The effects of heat treatment on the composition,phase,microstructure and martensitic transformation behaviors of microwires were studied.The corresponding elastocaloric properties of R phase and B19' phase transformations of microwires with microcrystalline and nanocrystalline obtained through different heat treatment were analyzed by studying their superelasticity and twoway shape memory effect.Different heat treatment processes were carried out on microwires with a diameter of 170?m obtained after cold drawing,the microstructures revealed that after solution treated at 1000? for 60 min,the grains grew to 5-15?m in diameter(microcrystalline),while the grain size remained 20 nm to 40 nm in diameter(nanocrystalline)after annwaling at 450? for 30 min to 180 min.The solution treated NiTi microwires were aged at 450? for different time from 15 min to 2400 min,then differential scanning calorimetry(DSC)analysis results showed that transformation temperature of B19'phase increased quickly with aging time especially after 300 min,simultaneously the temperature change during transition also reduced.However,B19' transition was suppressesd in annealed nanocrystalline microwires.It was found that after aging at 300? for 60 min to 300 min,the R phase transition temperature increased with aging time in annealed nanocrystalline microwires,and two-step R phase transformation behaviors occurred in wires aged for 60 min and 120 min.A dynamic mechanical analyzer(DMA)was used to investigate the superelasticity and two-way shape memory effects corresponding to R pahse and B19' phase transformation respectively.The temperature hysteresis of R phase transformation is less than 5?,stress hysteresis is less than 10 MPa and stable superelasticity associated with R phase transitions can be obtained only after 3 circulations.Whereas the temperature hysteresis of B19' transformation is larger than 60?,the stess hysteresis is more than 200 MPa.Therefore R phase has a much better cycle stability.Both R and B19' phase transformation can produce significant elastocaloric effcts.Under a stress of 300 MPa,the isothermal entropy change brought by R phase transition is about 20J/kg·?,while B19' pahase transformation can produce 60-80 J/kg·?.what's more,the refrigeration capacity of B19' transformation is calculated to reach 1921.1J/kg,and the operational temperature window is much wider than R phase trasition.But due to the larger hysteresis,the B19' phase transition behaves much worse elastocaloric effcts reversibility.Compared with microcrystalline NiTi wires,nanocrystalline wires contain little Ni4Ti3 precipitates so the internal stress is rather small resulting the decrease of required driving force for R and B19' phase transformation.The low content of precipitates reduces the elastic strain energy during forward transformation,making symmetry between the forward and reverse R phase transition better than that of microcrystalline wires.In addition,nanocrystalline wires behaves considerably high yield strength,which makes the superelasticity operational temperature window of B19' transformation lager than 70?,maximum entropy change greater than 80J/kg·?,the operational temperature window between 30? and 60? and a refrigeration capacity near 2000J/kg.While the superelasticity operational temperature window of B19' transformation in microcrystalline is only 10?,the maximum entropy change is about 60 J/kg·? and the refrigeration capacity is less than 400J/kg.Taken together,the overall performance of nanocrystalline wire is significantly better than that of microcrystalline wires.