Hot Deformation Behavior Of NiTinol And Its Application In Tube Extrusion

The unique shape memory effect and desirable mechanical properties of NiTinol have seen them being widely used in aerospace and medicine,but hot working to break the structure of as-cast ingots is needed to ensure sufficient ductility and a significant shape memory effect and has been a hot research focus recently.The hyperbolic-sine constitutive equation,which has been widely used for evaluating the plastic flow behavior,was constructed in lots of NiTinol.The effect of the chemical composition and deformation conditions on the hot deformation behavior was considered in some reports.However,the systematic research of NiTinol hot deformation is still limited.The shortcomings are exited in both the hot deformation behavior study and its application in hot workings.Firstly,all the researches declared that the dynamic recrystallization(DRX)will be occurred during the NiTinol hot deformation,but the DRX mechanism is still under the debate as all the discontinuous DRX(DDRX),continuous DRX(CDRX)and geometric DRX were reported in previous studies.Secondly,there are no clear reports for guarding the hot extrusion and no available industrial produces for the growing demand of NiTinol tube.The current research program reports the findings which aim to:(1)do a systematic research on hot deformation behavior.It includes(a)confirming a universal expression to describe the plastic behavior,(b)identifying the DRX mechanisms of NiTinols,(c)comparing and checking the effects on hot deformation behavior and microstructure evolution.(2)find an industrial hot extrusion parameter with controlled microstructure by virtue of processing map and Finite Element Method(FEM)simulation.The main conclusions are as follow:Three kinds of NiTinol alloy,a signal TiNi phase alloy(Ti50Ni47Fe3(at.%)),a Ni-rich NiTinol(TiNi50.8)and the Ti44Ni47Nb9 alloy which contains the eutectic phase,are selected for this study,since comparison can present the effect of Ni-rich hard participations and the second soft phase or eutectic phase in the NiTinol.Hot compression tests(?50%height reduction)at different temperatures(750/800?1050 0C)and different strain rates(0.01?10 s-1)covering the range of typical hot working deformation conditions are conducted in a Gleeble-3500 thermo-mechanical simulator.The work-hardening rate is induced to analyze the stress-strain curves.The critical stress ?c,peak stress ?p and the dynamic recovery saturation stress ?sat are measured and can be specified approximately by the expressions:?sat?1.12?p and ?c,?0.86?p for all of 3 NiTinols.The data are fitted to an Arrhenius model to describe the relationship between peak stress and the Z parameter.There are some notable differcence among 3 NiTinols including the behavior of stress-strain curves,stress declining and deformation activation energy were found and discussed.The solution drug of Ni element in TiNi50.8 and Nb element in TiNiNb alloy and the P-N force of Nb participation should be response for this.Additionally,with the help of electron backscatter diffraction(EBSD),the development of recrystallizing grain boundaries is tracked in deformed grains with different grain orientation spread(GOS)values.CDRX is subsequently confirmed by evidence of the conversion of low-angle boundaries(LAGBs)into high-angle boundaries(HAGBs),and the schematic model has been estimated.While,grain boundary sliding(GBS)and HAGB migration are therefore considered secondary softening mechanisms under high-Z and low-Z conditions in both binary TiNi alloys(TiNiFe alloy and TiNi50.8 alloy),and a DDRX-CDRX transition line is drawn by the Z parameter in both NiTinols.The CDRX is confirmed as the dominant DRX mechanism in most of deformation conditions of all of 3 NiTinols.With checking the relationship between the Z parameter and the subgrain size,the DRX mechanism and the secondary mechanism of 3 NiTinols,the effect of solution Ni element in the Ni-rich TiNi alloy and Nb participation and eutectic phase on the microstructure evolution are discussed.Based on the power dissipation efficiency analysis and observation of microstructure,the processing map is constructed with 2 peak region(DRX dominated region)which indicates 2 hot working windows for hydraulic forging and extrusion,respectively.Then Deform 3D simulation is used for optimizing NiTinol tube extrusion operating parameters in the extrusion working window.During the extrusion,the metal flow and the effects on the strain/stress/temperature field were discussed.The friction factor between billet and the die plays an important role to reduce heterogeneous their distribution and is considered to be the prior parameter for the optimization.Finally,the experimental extrusions have been done for verifying the previous analysis and the optimization.The microstructure evolution of the TiNiFe and TiNi50.8 extrusion has been carefully tracked and analyzed.It shows an unified and general understanding of microstructure evolution during the extrusion.The mechanical properties of extruded tube are tested which exhibits the same levels compared with the cold rolled one.Generally,a systematic research of hot deformation of NiTinol has been done.The hot deformation behavior,the microstructure evolution and the effect of chemical component on them have been studied.Based on these results,the tube extrusion has successfully achieved with the optimization of processing map and FEM simulation.It brings some further understanding of hot deformation behavior and new sights on the DRX of NiTinol,and solves some basic problems of NiTinol tube extrusion operating and optimization.