Study On The Large Hysteresis And Its Regulation Mechanism Of NiTiNb Shape Memory Alloys

Shape memory alloy(SMA)undergoes reversible martensitic transformation and outputs shear strain,thus exhibiting shape memory effect and superelasticity,when induced by external thermal or stress fields et al.Hysteresis is often observed during transformation,because the strain output usually lags behind the change of external fields.Transformation hysteresis is one of the key physical properties and of great importance for the application of SMA.For examples,a wider thermal hysteresis means a wider service or storage temperature range for SMA-made parts relying on shape memory effect,and a larger stress hysteresis enables superelastic SMA with higher energy dissipating capacity for anti-seismic purposes.By now,the physical nature of transformation hysteresis is not very clear.It is generally believed that the hysteresis is mainly determined by two microscopic factors.The first determinant is the geometric compatibility between the parent and martensite phases(athermal part).High geometric compatibility promises low local interfacial stress,and thus reduces the irreversibility of crystal structure,e.g.the generation of dislocations,during transformation,so that small hysteresis is promised.In other words,lowering the geometric compatibility is effective to enlarge transformation hysteresis.However,this significantly increases irreversible crystal defects and thus weakens the transformation cycling stability of SMA.Therefore,it is challenging to obtain SMA possessing large hysteresis and good cyclic stability simultaneously.The second determinant for hysteresis is the lattice friction during the migration of parent/martensite interface(thermal part).Various studies have shown that this part is controlled by the transformation resistance.The thermal part is expected to be strengthened by enhancing the transformation resistance,and large hysteresis is thus obtained.More importantly,this part is barely related to the irreversibility of crystal structure,which may allow SMA to possess large hysteresis and good cyclic stability simultaneously.Based on the above,several single-phase NiTiNb-based alloy wires are fabricated by a series of processing processes such as melting,forging,wire-drawing and subsequent annealing.Their thermally-and stress-induced transformation behaviors are systemically studied by a series of in-situ techniques.It is shown that large hysteresis and good cyclic stability have been obtained in thermal and stress fields.In addition,the contributions of the athermal and thermal parts are detailly analysed basing on an existing physical model,and the physical natures of large hysteresis and good cycling stability are preliminarily clarified in the alloys.The main results are as follows:(1)Large intrinsic thermal hysteresis up to 100 K is obtained in NiTiNb2 alloy,which is 2-3 times of that of Ni Ti alloys,and is comparable to that of dual-phase NiTiNb alloy.The thermally-induced transformation behaviors of NiTiNb2 and equi-atomic Ni Ti specimens with different grain sizes(annealing temperatures)are systematically studied and compared.It is found that the thermal hysteresis of NiTiNb2specimens enlarges significantly with grain refinement,but that of Ni Ti specimens is almost grain-size-independent.An existing kinetic model based on dislocation theory is modified,and is found to fit the thermal hysteresis well.It is detected that NiTiNb2specimens contain multi-oriented nanodomains,whose crystal structure is in between those of parent and martensite phases and volume fraction increases during cooling,before transformation.It is suggested that the multi-oriented nanodomains formed upon cooling may enhance the transformation resistance,thus strengthening the thermal part of thermal hysteresis.(2)Good cycling stability of transformation is achieved in NiTiNb2 specimens with small grain size.The effect of grain size on cycling stability is systematically studied.It is found that the cycling stability improves with grain refinement.The transformation geometric compatibility parameters of samples with different grain sizes are calculated.It is found that the geometric compatibility improves with decreasing of grain size.It is basically clarified that the improvement of cycling stability is due to the weakening of the athermal part.It is preliminarily proved that the weakening of the athermal part results from the improved geometric compatibility,which may be related to the intermediate crystal structure and increasing volume fraction during cooling of nanodomains.(3)Large and cycling-stable absorbed energy/stress hysteresis is achieved in NiTiNb4 specimens containing nanodomains.The absorbed energy of NiTiNb4 is up to32.6 MJ/m~3 at 77 K after 30 cycles,which is at least 150%larger than that of reported SMA.Based on the results of a series of in-situ cooling experiments and the understanding of transformation hysteresis in(1)and(2),two basic criteria for designing SMA with large and cycling-stable absorbed energy are proposed and verified.Criterion 1,weaken the athermal part,which can be achieved by improving the geometric compatibility and thus inhibiting the formation of dislocations during transformation,so as to ensure good cycling stability.Criterion 2,strengthen the thermal part,which can be achieved by increasing the resistance of interface migration,in order to increase the absorbed energy/stress hysteresis.(4)The effects of grain size,alloying elements and pre-stress on the stress hysteresis of NiTiNb alloys are systematically studied.It is found that grain size has almost no effect on stress hysteresis,which is different from the grain size effect on thermal hysteresis.Nb addition and pre-stress have no obvious effect on stress hysteresis,while doping Fe reduces stress hysteresis significantly.