Research On The Microstructure And Mechanical Behavior Of Metastable Zirconium-based Alloys

In recent years,material scientists have developed a series of low-modulusβ-type Zr-based alloys with better magnetic compatibility with human tissues as alternative materials for hard tissue replacements,for the purpose of solving the issues of stress shielding and magnetic artifacts caused by the high elastic modulus and high magnetic susceptibilities ofβ-type Ti alloys those were used in the biomedical field.Although the low Young’s modulus of these newly developedβ-type Zr alloys with low magnetic susceptibilities can mitigate the risk of implant failure to some extent,it is still higher than that of human bone.More importantly,the physical mechanisms underlying the low Young’s modulus in theseβ-type Zr alloys are not yet known.In addition,due to the limitations of conventional characterization techniques,most previous studies on the mechanical behavior of Zr-based alloys have focused on the effect of monotypic martensite（i.e.,α′orα″martensite）on the microstructures and mechanical properties,and few studies have paid attention to the mechanical behavior of Zr-based alloys that may involve bothβ→α′andβ→α″transformations during quenching or deformation.In particular,the correlation between the mechanical behavior of multiphase Zr-based alloys containingβphase as well asα′andα″martensite and martensitic transformation characteristics remains to be clarified.To address the problems mentioned above,two kinds of metastableβ-type Zr-based alloys with a combination of low Young’s modulus and low magnetic susceptibility（Zr-12Nb-4Sn and Zr-4Mo-4Sn）,and a multiphase Zr-based alloy composed ofβparent phase as well asα′andα″martensitic phase（Zr-30Ti-7Nb-4Sn）were prepared based on a rational composition design and appropriate thermal-mechanical treatment process.The physical mechanisms underlying the ultra-low Young’s modulus of the Zr-12Nb-4Sn and Zr-4Mo-4Sn alloys were systematically elucidated by in-situ synchrotron radiation（SXRD）technique and the Eshelby-Kroner-Kneer elastoplastic self-consistent（EPSC）model.Furthermore,transmission electron microscopy（TEM）and backscattered electron diffraction（EBSD）techniques were employed to explore the microstructure of two types of martensitic phases（α′andα″）in Zr-30Ti-7Nb-4Sn alloy.The detailed deformation mechanism for this multiphase Zr-based alloy was elucidated by investigating the martensitic transformation characteristics during the different deformation procedures and its correlation with the mechanical behavior of the Zr-30Ti-7Nb-4Sn alloy.The main research conclusions are as follows:Both the Zr-12Nb-4Sn and Zr-4Mo-4Sn consist of a singleβparent phase after solution quenching,and they both undergo linear elastic deformation followed by plastic deformation during tensile deformation.The Young’s modulus and magnetic susceptibility of these two alloys are significantly lower than those of most currently available biomedical alloys.In-situ SXRD and EPSC model results show that both metastableβ-type Zr-12Nb-4Sn and Zr-4Mo-4Sn alloys exhibit much lower shear moduli C′and C₄₄ compared to binaryβ-type Ti-based alloys,implying that theβ-phase stability in Zr-12Nb-4Sn and Zr-4Mo-4Sn alloys always remains at a much lower level with respect to{110}<11?0>shear（C′）and in{110}<110>shear(C₄₄),which is the intrinsic reason for the ultra-low Young’s modulus of metastableβ-type Zr-12Nb-4Sn and Zr-4Mo-4Sn alloys.The solution-treated Zr-30Ti-7Nb-4Sn alloy is composed ofβparent phase andα′andα″martensitic phases,and the two types of martensite in this alloy exhibit different characteristics and micromorphology:α′martensite with hexagonal structure possesses a typical acicular morphology and an orthorhombicity value of 1.00;α″martensite with orthorhombic structure exhibits a lath-like morphology that contains（110）_α″-type internal twinning structures and an orthorhombicity value of 0.97.In addition,the multiphase Zr-30Ti-7Nb-4Sn alloy exhibits a distinct“stress-plateau”in the stress-strain curve during the single tensile test.In-situ SXRD results indicate that the“stress plateau”is mainly attributed to the combined effect of the extensiveβ→α″SIM transformation and the reorientation of pre-existingα″martensitic variants.In contrast toα″martensite,theα′martensite experiences only intrinsic elastic deformation throughout the tensile process,and no SIM transformation occurs betweenα′martensitic phase andβparent phase.During the pre-straining process,the multiphase Zr-30Ti-7Nb-4Sn alloy exhibits different deformation behaviors and martensitic transformation characteristics in two tensile loading-unloading cycles.In the first tensile loading-unloading cycle,the Zr-30Ti-7Nb-4Sn alloy exhibits“double yielding”deformation behavior owing to the combined effect of theβ→α″SIM transformation and the reorientation of pre-existingα″martensitic variants,during which the SIM transformation occurs extensively and intensely in a narrow stress range.In the second tensile loading-unloading cycle,Zr-30Ti-7Nb-4Sn alloy exhibits a“non-linear”deformation behavior dominated by the elastic elongation and elastic recovery ofα″,α′andβphases as well as the almost completely reversibleβ?α″SIM transformation,and the SIM transformation starts to occur slightly in a homogenous and continuous manner.This implies that the deformation behavior of the Zr-30Ti-7Nb-4Sn alloy is largely influenced by the martensitic transformation characteristics.During the tensile loading process after the pre-straining treatment,the elastic elongation ofα″,α′andβphases and theβ→α″SIM transformation occurred simultaneously in the Zr-30Ti-7Nb-4Sn alloy.In addition,due to the retardation effect of dislocations introduced by the pre-straining process,the slightβ→α″SIM transformation occurred homogeneously and continuously,resulting in the near-linear elastic deformation behavior and excellent mechanical properties of the Zr-30Ti-7Nb-4Sn alloy.