NiTi-Nb原位复合材料的准线性超弹性变形

Quasi-Linear Superelasticity Deformation in an In Situ NiTi-Nb Composite

据文献报道,Nb纳米线增强NiTi记忆合金复合材料可展现超常的准线性超弹特性。为揭示该准线性超弹特性的产生和变形机制,通过真空感应熔炼、锻造、拔丝方法原位合成了NiTi-Nb复合材料丝材。TEM显微分析表明,Nb纳米线沿丝材轴向平行分布在纳米晶NiTi基体中。该材料在经历一次9%的预变形后会展现准线性超弹特性,其屈服强度达1.7 GPa,表观Young's模量约34 GPa,准线性超弹性应变接近5.5%。同步辐射高能X射线原位拉伸实验结果表明,准线性超弹性的产生与以下2点原因有关:(1)复合材料经历预变形后,Nb纳米线和NiTi基体间会产生耦合力,再次加载时,NiTi所受的耦合拉应力可以将局部区域应力诱发马氏体相变所需的外应力降低到零附近,并且耦合力越大,加载初期的相变速率越高,经过适当的预变形后,加载初始就能够持续发生高速率相变;(2) NiTi中耦合拉应力呈梯度分布,使相变应力-应变曲线不再是常见的“平台型”,转变为“硬化型”斜线。

In the past decade,a unique composite system consisting of Nb nanowire and NiTi shape memory alloy matrix has attracted considerable attention.One of the works published in Science proposed that the NiTi-Nb composite has superior properties,including high strength(1.65 GPa),low Young's modulus(25.8 GPa),and quasi-linear superelasticity(6.4%).In particular,given the quasi-linear superelasticity of this composite,(1)continuous stress-induced martensitic transformation occurred even at the beginning of tensile loading,which indicated that the external stress required to start the transformation was reduced to almost zero;(2)the transformation(stress-strain)curve is a“hardening type”rather than a“plateau type,”with apparent Young's modulus of 25.8 GPa,and(3)the amount of quasi-linear superelasticity deformation is 6.4%,which is higher than that of conventional binary NiTi alloy.This work focused on the quasi-linear superelasticity property.Thus,an in situ NiTi-Nb composite was prepared by vacuum induction melting,hot forging,and wire drawing.Microscopic analysis showed that Nb nanowires were distributed in parallel inside the nanocrystalline NiTi matrix along the wire axial direction.Quasi-linear superelasticity was obtained after 9%pre-deformation,with a yield stress of 1.7 GPa,apparent Young's modulus of 34 GPa,and quasi-linear superelasticity deformation of~5.5%,which is similar to the result proposed in Science.In situ synchrotron XRD measurements were conducted to analyze the effect of pre-deformation on the coupling effect between NiTi and Nb nanowire.The origin and deformation mechanism of the quasi-linear superelasticity were systematically studied.Results revealed that coupling tensile stress in NiTi,which was generated by pre-deformation,increased gradually with the increase of the pre-deformation strain,thereby providing a driving force for stress-induced martensitic transformation.The external stress required to start the transformation could be reduced to almost zero in some local areas as a result of the coupling tensile stress.The initial velocity of transformation increased with the increase of the coupling tensile stress in NiTi.Therefore,a continuous transformation with relatively high velocity was obtained even at the beginning of tensile loading after a proper pre-deformation.Furthermore,the gradient distribution of coupling tensile stress inside B2-NiTi led to the“hardening-type”transformation(stress-strain)curve.