Structural Transformation And Superelastic Mechanism Of Ni-Fe-Ga-Co And Co-Ni-Ga Shape Memory Alloys

Shape memory alloys(SMAs)relying on stress-induced reversible martensitic transformation can produce superelasticity that is far greater than the actual elastic limit(generally less than 1%)of traditional metal materials.Widely studied SMAs such as Ni-Ti can produce nonlinear elastic strains up to 8%upon loading and unloading.At the transformation point,the free energies of austenite and martensite are equal,but the reorganization of the structure needs to cross the potential barrier,which leads to the difference in the transformation critical stressσcrA→M and σcrM→A.Therefore,the superelastic curve exhibits significant stress hysteresis,which is an inherent feature of the first-order phase transformation and is unavoidable.Hysteresis not only causes mechanical energy dissipation but also results in the premature structural and functional damage of in-service SMAs.In order to realize low power consumption,high accuracy and high sensitivity of sensors and actuators,the fields of aerospace and micro-electromechanical systems have put forward an urgent application requirement for high-performance SMAs with near zero-hysteresis and ultra-high elasticity over a wide temperature range.However,the existing methods to regulate superelastic hysteresis have gradually encountered technical bottlenecks.Based on a new principle to design novel SMAs is the only way to cross the valley of death of high-performance functional responses.Ni-Fe-Ga-Co and Co-Ni-Ga single crystals display excellent mechanical behaviors with high yield strength,large superelastic strain,and wide superelastic temperature range.However,strong crystal anisotropy,coarse grain of the ingot,and weakened grain boundaries cohesion seriously hinder the application of that polycrystalline SMAs.The core research content of this thesis is to propose a new material design method,develop superelastic alloys with narrow or even zero stress hysteresis,and establish a theoretical basis for the minimization of energy dissipation in SMAs.In this paper,we successfully fabricated Ni-Fe-Ga-Co and Co-Ni-Ga single-crystal microwires with[001]A orientation in the axial direction by the Taylor liquid drawing technique.By adjusting the composition and heat treatment process,a new type of structural fluctuation can be introduced into the austenite matrix,so that the related superelastic responses can be regulated.By utilizing advanced material characterization methods such as high-energy X-ray diffraction,neutron scattering,and high-resolution scanning transmission electron microscopy,the physical mechanism underlying the high-performance elastic behavior of the Ni-Fe-Ga-Co and Co-Ni-Ga alloys was revealed.In Ni45Fe18Ga27Co10(at.%)alloy,the superelasticity with narrow stress hysteresis is realized by adjusting the degree of atomic order.In the L21-ordered Ni43Fe18Ga27Co12 alloy,the nonhysteretic superelasticity based on the expansion and shrinkage of a single martensite plate is realized by eliminating the nucleation energy.The above-mentioned superelasticity of Ni-Fe-Ga-Co alloy originates from the strong first-order martensitic transformation.Inspired by the continuous phase transition,we propose a method to construct novel supercritical elasticity in SMAs.We believe that if we established an unprecedented microstructure consisting of atomic-level entanglement of ordered and disordered crystal structures,it can be manipulated to tune the superelasticity.The entangled structures play a key role in the strengthening of this crystal against plastic deformation and suppression of the first-order martensitic transformation.After ingenious alloy design and thermal treatment,the entangled state(L21+ω)has been realized in Ni-Fe-Ga-Co alloy.The in-situ neutron diffraction experiment revealed the structural evolution of the ω phase during the aging process,and the TEM analysis gave direct crystallographic evidence for the existence of the ωphase at the atomic scale.Astonishingly,Ni35Fe18Ga27Co20 alloy exhibits a maximum non-hysteretic superelasticity of about 15.2%,and non-hysteretic superelasticity of about 11%was achieved over a wide temperature range from-150℃ to 150℃ with very small temperature dependence.The in-situ synchrotron X-ray diffraction measurements demonstrate that the non-hysteretic elasticity originates from a stress-induced continuous lattice distortion,so we denote this particular type of elasticity as supercritical elasticity.Based on the new concept of supercritical elasticity,we further constructed entangled structures in Co-Ni-Ga alloys to achieve supercritical elasticity.A large amount of Co in Co-Ni-Ga alloy can promote the formation of ω phase in metastable austenite matrix when the sample is quenched from high temperature or aged at low temperature.Due to the construction of ω+B2 coherent structure,supercritical elasticity of up to 13.2%was achieved in Co52Ni18Ga30 alloy.The in-situ synchrotron X-ray diffraction measurements demonstrate that the non-hysteretic elasticity also comes from continuous lattice distortion.In addition,Co-Ni-Ga alloy displays exceptional stability of supercritical elasticity under 50,000 mechanical cycles at a 7%tensile strain amplitude,which is superior to that of the Ni-Fe-Ga-Co supercritical elastic alloy.The core innovation in this thesis is to realize supercritical elasticity in SMAs through the design of entangled structures,which broadens the research area of SMAs.Furthermore,the proposed material design strategy may bring out other unexpected behaviors in condensed matter physics.