Experimental Study Of Spall Damage Characteristics Of NiTi Shape Memory Alloys

NiTi alloy is a representative shape memory alloy(SMA)due to its high sensitivity to temperature and stress resulting from complex phase transformations.It exhibits a strong macro-microstructure correlation.While NiTi SMA’s phase transformation,shape memory effects,and thermodynamic properties related to super-elasticity have been studied extensively under quasi-static loading conditions,research on plastic deformation,interlaminar strength,and damage mechanisms under high strain rate loading conditions is not sufficient.Therefore,conducting research on the mechanical properties of NiTi SMA under dynamic loading has important scientific significance,as well as providing assistance in the safe design and service of its related key components under high-speed impact.This thesis explores the interlaminar cracking and damage characteristics of NiTi SMA alloy by controlling its microstructure through pre-processing of the NiTi samples,subjecting the samples with different initial states to one-dimensional plate dynamic impact loading,and using multiple microscopic characterization methods to analyze the microstructure morphology of the soft-recovered samples after impact loading.The data on interlaminar strength and characterization results are then combined to study the plastic deformation and interlaminar damage mechanism of NiTi SMA under dynamic impact loading.Furthermore,NiTi alloy was prepared by selective laser melting(SLM)process,and the sample with the optimal combination of process parameters was analyzed by a series of microstructural characterization and mechanical testing under quasi-static and dynamic conditions,and its mechanical performance was compared with forging NiTi alloy to explore the mechanical performance law of additive manufacturing NiTi SMA.The main conclusions and innovation points of this study are as follows:(1)When NiTi SMA alloy is subjected to an impact load exceeding 5 GPa,a phase transformation occurs from the B2 austenite phase to the B19’ martensite phase,then reverses to the B2 phase through intermediate R-phase during unloading,and residual Rphase is found.The interlaminar strength and elastic limit of NiTi B2 auste NiTic phase are higher than those of the B19’ martensitic phase.The pore generated by interlaminar cracking nucleates inside the grain,indicating that the interlaminar damage form of NiTi SMA alloy is transgranular fracture.Deformation twinning plays an important role in impact loading,where austenite twinning dominates the deformation behavior of the B2 phase,and reversible martensite twinning may exist in mixed-phase NiTi or require higher impact pressure conditions to induce its occurrence.(2)The mechanical properties of NiTi SMA prepared by SLM forming not only depend heavily on its process parameters but also on the relative relationship between the stress loading direction and the printing direction,and the quasi-static tensile performance and dynamic interlaminar strength of NiTi SMA samples prepared by current SLM technology are lower than those of traditional forging NiTi alloy materials.