Effects Of Nb Content And Heat Treament On The Superelasticity Of Porous NiTi(Nb) Shape Memory Alloys

Recently, porous NiTi based shape memory alloys (SMAs) have attracted increasinginterests in biomedical hard tissue implants and energy-absorbing materials, owing to theirexcellent superelasticity, damping characteristics, biocompatibility and bio-mechanicalcompatibility. In this study, the effect of Nb content and heat treatment on the microstructure,phase transformation behavior, mechanical properties and superelasticity behaviors of porousNiTi(Nb) SMAs prepared by the low-pressure sintering and vacuum sintering weresystematically investigated.The theoretical analysis shows that when the porosity of porous NiTi SMAs is less than31.4%, its superelasticity behavior exhibit a obvious ramp stress-induced plateau in thestress-strain curves, while when the porosity of porous NiTi SMAs is more than31.4%or thepore is not a close or rounded pore, it will exhibit a linear superelasticity behavior, i.e. nostress plateau. The experimental results concluded that the critical porosity of thestress-induced platform in the stress-strain curve of the porous NiTi SMAs is between30%and34%, which is highly coincided with the theoretical result.The experimental results shows that the existing forms and morphology of Nb element inporous NiTiNb SMAs, as well as pore shapes, martensitic transformation behaviors andsuperelastic behavior are completely different for two preparation methods and sinteringtemperatures. For the samples prepared by low pressure sintering temperature at1050℃, theyshow rounded and closed pores, the dissolved Nb in NiTi matrix were increased, moreovereutectic structure (β-Nb and NiTi) and β-Nb were increased and roughened with theincreasing of Nb content. The recoverable strain and linear recovery strain in porous NiTiNballoys are first decreased and then increased, which is the typical wide hysteresis SMAs,especially the porous Ni46Ti43Nb11SMA exhibite good linear superelasticity (maximal about4%complete strain). However for the samples by vacuum sintering temperature at1125℃,they show irregular pores, the phases of porous NiTiNb SMAs consist B2phase,(Ti,Nb)2Niphase and β-Nb, except eutectic structure. Now, the porous NiTiNb SMAs exhibit narrow hysteresis transform behavior.It is also found that, on the mechanical properties and superelasticity of the porousNiTiNb SMAs can be significantly improved after solid solution and ageing treatment. AfterST+AG heat treatment, the porous Ni46Ti43Nb11alloy prepared by low pressure sinteringexhibits excellent linear superelasticity behaviors, the superelasticity strain at roomtemperature can reach as high as more than5%and the elastic limit reaches900MPa, which is3times of the porous NiTi alloy with the same porosity; However, after ST+AG heattreatment, the superelasticity behaviors of the porous Ni48Ti48Nb4alloys prepared by vacuumsintering is similar wit the porous NiTi SMAs. The superelasticity of the porous Ni48Ti48Nb4alloys also can be reach about5%at room temperature.