Study On Phase Transformation Behavior And Functional Properties Of Co-Ni-Ga Shape Memory Alloys

As a typical type of metal intelligent material,shape memory alloy has attracted extensive attention from researchers due to its unique functionalities,such as shape memory effect,superelasticity,and elastocaloric effect.The Co-Ni-Ga shape memory alloys,which have a large adjustable range of phase transformation temperature,narrow stress hysteresis,low temperature sensitivity,and widetemperature-range superelasticity,are expected to be used in intelligent sensing,driving and other fields at extreme conditions.However,the Co-Ni-Ga alloy system owns high intrinsic brittleness because of the weak cohesion of grain boundaries and few movable slip systems,and in particular,polycrystalline counterparts are prone to brittle fracture under a considerably small external stress,which seriously restricts the development and practical application of this alloy.In order to overcome this problem,three parts of researches were performed.1)On one hand,the strategy of multi-element doping and texture control was carried out for realizing collaborative strengthening in the Co-Ni-Ga polycrystalline alloy to enhance its mechanical properties and the cyclability of functional properties,and the correlation between its microstructure and functional properties were deeply studied.2)On the other hand,in order to obtain bamboo-like oligocrystalline or even single-crystal wires,a series of Co-Ni-Ga microwires were fabricated by Taylor-Ulitovsky technique,which can therefore reduce or eliminate the stress/strain mismatch at grain boundaries upon deformation.3)Moreover,nano-scale heterostructure can also be introduced to further strengthen the matrix to improve the functional properties.Meanwhile,their microstructure morphology,phase transformation characteristics and related functional properties were also systematically investigated.Firstly,the Co49Ni21Ga30 alloy was doped with Cu and B.Texture control was then supplemented on the basis of multi-element alloying.The(Co47.1Ni22.9Ga30)96Cu3.8B0.2 alloy with<001>B2 preferred orientation,columnar grain structure and two ductile precipitates in the matrix and on grain boundaries was prepared.Due to the synergistic strengthening in this alloy,its mechanical properties have been significantly improved.The yield strength is 1203 MPa and the fracture strain is 8.8%,which are better than the common Co-Ni-Ga alloys(500 MPa,6.8%).On this basis,the alloy shows excellent superelastic responses,with a fully recoverable strain of 2.5%,a low driving force of 99 MPa,narrow stress hysteresis of 33.5 MPa and mechanical cycle stability of more than 20,000 cycles.Moreover,the alloy possesses stable elastocaloric effect at room temperature.The adiabatic temperature change during unloading is about-3.2 K,while the transformation strain is only about 1.5%,and especially after 20,000 mechanical cycles,the functional properties show no apparent degradation.More importantly,the fatigue resistance of superelasticity and the elastocaloric effect is better than that of the previous monocrystal counterparts.Furthermore,for decreasing or even eliminating the stress/strain mismatch at grain boundaries upon deformation,Co-Ni-Ga microwires with different components were prepared by a Taylor-Ulitovsky method.As a result,bamboo-like polycrystalline microwires were successfully fabricated based on the composition of(Co49Ni21Ga30)97Cu3.It was found that the microwire exhibits large tensile superelasticity in the temperature range of 223-373 K,and the superelastic recoverable strain is 5%,which is the best in Co-Ni-Ga polycrystalline alloys.The microwire also owns one-way shape memory effect,and 3%strain output can be obtained in the range of 123-353 K.More importantly,Co49Ni21Ga30 single-crystal microwires were also achieved,and their wide-temperature-range superelasticity and two-way shape memory effect were then studied.The results show that the Co49Ni21Ga30 microwire owns single-austenitic monocrystal structure,with nearly<001>B2 crystal orientation along the wire axis.Under the tensile mode,it is known that in the ultra-wide temperature range of 223-773 K,the microwire shows excellent superelastic behaviors,and their tensile recoverable strain is more than 8%.Moreover,due to the introduction of internal stress during the rapid watercooling process,the microwire also exhibits a perfect inherent two-way shape memory effect,and the output strain is up to 6.3%.After superelastic training,the two-way shape memory strain can reach 6.8%.On the basis of single-crystal wires,we engineered nano-scale heterostructure inside the wires.The linear elastic behavior of the Co46Ni24Ga30 martensitic wire was studied.It is found that the Co46Ni24Ga30 wire is dominated by single martensitic phase,which shows a single-crystal structure with[001]L10 orientation along the axis of the wire.Further HAADF results show that there exists obvious nano-scale lattice distortion in this studied sample,which may derivate from the local strain or the fluctuation of fault energy caused by the compositional undulation.This heterostructure can induce ’supercritical elasticity’ based on the higher-order transformation.Combined with the intrinsic elasticity of the martensitic phase of Co-Ni-Ga alloys,excellent tensile quasi-linear elasticity has been successfully achieved in the macroscopical martensitic Co46Ni24Ga30 monocrystal wire in the temperature range of 123-323 K.The linear elasticity possesses a large elastic strain(5%),low elastic modulus(10-40 GPa),narrow stress hysteresis(～9.5 MPa)and a high energy storage efficiency(97.9%)at room temperature.Meanwhile,the linear elasticity of this wire also has no obvious degradation after 90,000 tensile cycles,showing extraordinary cyclic stability,which is ascribed to the severe restriction and dispersion of dislocations by the undulating heterogeneous nanostructure.Insitu HEXRD results exhibits that the linear elastic behavior is generated from the large continuous distortion of the martensitic crystal lattice under the applied stress.According to the above results,different strategies have been adopted to overcome the defects of high intrinsic brittleness of Co-Ni-Ga alloys successfully.Eventually,the cyclic stability of superelasticity and elastocaloric effect was significantly enhanced in polycrystalline bulks;In the single crystal microwires,ultra-wide-temperature large superelasticity and excellent two-way shape memory effect were obtained;The most important one is to induce ’supercritical elasticity’based on continuous lattice distortion,by constructing the undulating heterogeneous nanostructure.Coupled with martensitic elasticity,a large linear elasticity with excellent fatigue resistance was achieved.These not only promote the development and practical application of Co-Ni-Ga alloy,but also provide a new idea for the design of novel high-performance shape memory alloys.