| Nickel-titanium alloys(NiTi)are widely used in aerospace,medical treatment,MEMS,new energy and other fields due to their excellent and rich physical properties(superelasticity,high energy dissipation,large latent heat,shape memory effect,etc.)in different forms.The microstructures greatly determine the mechanical and phase transation properties of the materials.The research on microstructural regulation and thermodynamic properties of NiTi alloys has always been one of the important fields of solid mechanics.As one of the most effective ways of microstructural regulation,grain size control has been paid more attention by many scholars.How to adjust and utilize the advantages of different grains to prepare new NiTi alloys to meet the new requirements of function and reliability has important practical significance.For example,problems such as poor bending fatigue life of dental pulp file,instability control of actuator and unpredictable damage of cardiovascular stent need to be solved.Some studys show that the grain refinement leads to weakening of the phase transiton characteristic,dissipation reduction and improvment of the mechanical cycle stability.However,the brittleness enhancement and hardening effect of nano materials lead to high sensitivity to defects,thus significantly reducing the toughness.Based on the goal of improving the application performance,this work mainly focuses on experiments,starting from the grain size control,carriing out materials’ preparation with different grain sizes and the characterization of microstructures.The effects of different grain size,size gradient and distribution on the thermo-mechanical coupled deformation and fracture behaviors induced by force and temperature are studied,and the corresponding mechanisms are revealed.The main research contents are as follows:Through innovative temperature-controlled gradient heat treatment process,the axial grain size gradient superelastic NiTi alloy was successfully developed,and the global thermo-mechanical coupling deformation behaviors at different rates under uniaxial tension were studied.The plate with grain size gradient along the thickness direction was also developed,and its deformation characteristics and fatigue properties under bending loading mode were investigated.The experimental results show that,under quasi-static loading,along the grain size reduction direction,the transformation stress monotonically increases,and the nucleation gradually transforms from local high strain gradient to uniformly distributed low strain gradient.However,the deformations in different regions are independent,and maintains their own grain size dependent characteristics.During dynamic loading,the coarse grain region shows multiple nucleation bands accompanied by high latent heat,while the nanocrystalline region still maintains the uniform deformation characteristics of highly suppressed phase transition and low latent heat release,forming a great temperature field gradient on the same material.The tensile and bending fatigue properties of NiTi alloys are not improved by the axial grain size gradient,but the fatigue life is reduced by the internal stress gradient.The grain size gradient distribution along the thickness direction significantly improves the bending fatigue life at lower deflections.The tensile stress is offset by the pre-compressive stress formed in the surface layer,while the nanocrystals prevent the movement and aggregation of dislocations from the interior to the surface layer.Grain size effect on temperature-induced two-way memory deformation under uniaxial tensile stress are studied.Actuation experiments in super wide temperature range(78-350K)were carried out by means of DC resistance heating and liquid nitrogen cooling,and the full strain-temperature curves of samples with different grain sizes were obtained.The influence of grain size on actutaion strain was analyzed from the change of microstructure and isothermal entropy.The free energy density model of "core-shell" crystalline-grain-boundary composite system was investigated to further explain the mechanism of grain size effect on transformation temperatures.It is found that the decrease of grain size increases the volume fraction of non-phase transformation region(grain boundary and phase boundary)and the intensity of inhibition of phase transition in grains,leading to the monotonic reduction of the actuation strain.The phase transiton changes from typical first-order to continuous one,resulting in the decrease of the slope of strain-temperature curve and the expansion of operating temperature range,which is also confirmed by the evolution law of the entropy change.The theoretical results of the crystalline-grain-boundary composite model show that the grain size changes the convexity of the free energy density,resulting in a monotonic decrease of the transformation temperature.The model captures the qualitative characteristics of the experimental results and explains the influence of grain size on actuation deformation.The effects of grain size and distribution on fracture toughness and behavior in the range of 10 nm to 30000 nm were investigated to seek a breakthrough in the contradiction between strength and toughness.Based on the characterization of microstructures at different scales and the analysis of the thermomechanical coupling behavior of crack tip localization,the grain size gradient network distribution materials were innovatively designed and prepared by means of synchronous asynchronous mixed rolling and heating at variable speed and temperature.It is found that the fracture toughness of the "coarse-grained-nanocrystalline" composite network structure with grain size gradient of20-7000 nm is 3-4 times of that of the uniform nanocrystallite,more than 30% of that of the coarse grain.The fracture toughness of uniform crystallite is mainly depended by the competition among phase transition toughening,internal stress intervention and effective yield stress.On the basis of phase transition toughening,the grain size gradient grid structure also contributes two new toughening mechanisms due to its networked distributed gradient nanocrystals and precipitate:One is that the bending of crack growth path changes the crack mode from mode I to mixed mode,leading to the increase of the actual critical fracture stress;Second,the micro-area composed of nanocrystals is distributed in a dot structure,which is equivalent to inclusion to some extent.The theoretical analysis results related to the elastic modulus,shape,size and distribution confirm that the micro-area reduces the stress intensity factor at the crack tip and plays a toughening role. |
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