Experimental Study On Shape Memory Effect And Mechanical Properties Of NiTiNb Alloys

Numerous studies have shown that NiTiNb-SMA can effectively strengthen the structure actively.Previous studies have shown that NiTiNb-SMA needs to be pre-deformed in a low temperature environment to obtain a larger recovery stress.However,the experimental research of this research group has shown that NiTiNb-SMA pre-deformed at room temperature can also obtain a larger recovery stress,and can meet the ambient temperature requirements of the use state.Based on this finding,this paper speculates that the heat treatment process of NiTiNb-SMA is a key factor affecting the pre-deformation temperature and recovery performance.Therefore,this paper conducts a systematic experimental study on the thermodynamic properties of the hot-drawn state and the high-temperature annealed state of NiTiNb-SMA,and discusses the effect of heat treatment on the shape memory effect and mechanical properties of NiTiNb-SMA.In addition,after the NiTiNb-SMA is heated to obtain the recovery stress,it will be inevitably subjected to the automobile loads and temperature effect during the service period of the structure,so that the NiTiNb-SMA and the bridge structures jointly bear the alternating and cyclic loads.However,the research on the stability of NiTiNb-SMA recovery stress and the secondary tensile mechanical properties under service is still in its infancy,so this paper also compares the performance of hot-drawn and high-temperature annealed NiTiNb-SMA under cyclic loads,the main research content and conclusions are as follows:(1)The microstructure of NiTiNb-SMA is observed by metallographic microscope and electron probe,and the composition of NiTiNb-SMA component phase is analyzed by scanning electron microscope.The results show that compared with the hot-drawn NiTiNb-SMA,the high-temperature annealed NiTiNb-SMA structure is more uniform and refined,and the ratio of Ni/Ti in NiTi matrix phase decreases.(2)This paper optimizes the low-temperature cold-drawing device of shape memory alloy(patented invention patent: a low-temperature cold-drawing device of shape memory alloy,patent number: ZL 2018 1 0823722.5,ranking second),the optimized device meets the requirements of temperature and loading method in this article tests.(3)This paper systematically studies the recovery stress magnitude,recovery stress stabilization stress,secondary tensile mechanical properties and deformation mechanism during constrained phase transformation of hot-drawn NiTiNb-SMA.The results showed that the hot-drawn SMA prestrained at room temperature(10?)by 12% to 16% can obtain a recovery stress above 450MPa;The recovery stress of the SMA increases with the increase of the heating temperature and tends to be stable when the heating temperature reaches 180 °C;it is not almost affected by the heating rate during the heating process;it decreases rapidly as the amount of memory strain pre-recovery(loss)increases.The hot-drawn SMA undergoing different pre-strain temperatures or pre-strains has no significant difference in the stability of the recovery stress under cyclic loading;in addition,within the range of the yield strain of the steel bar,the SMA recovery stress has a smaller fluctuation range and better stability.Although the recovery stress attenuation value gradually increases with the increase of the additional strain amplitude,the area formed by the additional strain loading and unloading curve increases significantly,showing good energy dissipation capacity and superelasticity.Ambient temperature has a significant effect on the performance of SMA.The recovery stress and the corresponding additional strain amplitude in zero stress state,secondary tensile yield strength and ductility of SMA at-30 ? environment are significantly reduced;The deformation mechanism is the coexistence of elastic deformation and plastic deformation.(4)The thermodynamic characteristics of the hot-drawn state and the high-temperature annealed state of NiTiNb-SMA are compared.The results show that the heating treatment process is a key factor affecting the mechanical properties,pre-deformation temperature and recovery performance of SMA.That is,after 850 ? high temperature heating treatment,the ductility of SMA has increased,and the stability of the recovery stress at temperature changes has improved.However,the high temperature annealed SMA's ultimate strength,yield strength,recovery stress magnitude,and under the secondary tensile load,it does not show stress strengthening phenomenon,and the required pre-deformation temperature is more severe.It needs a pre-deformation temperature of about-60 ? to obtain a large recovery stress,so the hot-drawn SMA is more suitable as a material for structural strengthening.