Fatigue Fracture Behavior Of Biomedical Nb-Ta-based Solid Solution Alloys

Vascular stent implantation has become a mature operation for the coronary atherosclerotic heart disease.As vascular stent will suffer from alternate loading in the physiological solution environment,the corrosion fatigue resistance is a decisive factor for long-term service safety.The vascular stents commonly used in clinical application are mainly permanent metal stents,such as 316L stainless steel,Co-Cr alloy,and nickeltitanium alloys,which generally contain ferromagnetic elements Fe,Co,Ni,etc.,resulting in artifacts on MRI images.In addition,the ion release of toxic elements such as Ni,Co,Cr can induce allergic reactions in the body.Nb-60Ta-2Zr alloy showed excellent biocompatibility and MRI compatibility,and the tensile properties basically meet the requirements of vascular stents,but its quasi-static tensile fracture mechanism and fatigue fracture behavior in room temperature air and simulated physiological solution environment still need to be studied.High-entropy alloys(HEAs)or medium entropy alloys(MEAs),consisted with multiple principal elements,have attracted great attention due to their promising properties.This concept can be applied in the biomaterial implant development with biocompatible elements,such as Nb,Ta,Ti,Hf,Zr,etc.,which might present better mechanical properties,corrosion resistance and biocompatibility than subsistent biomedical alloys.The as-cast NbTaTiHf entropy alloy possesses good room temperature plasticity and can be potentially used as biomedical material.However,the dependence of the mechanical properties,as well as the deformation and fracture mechanism on the processing history and microstructure has not been well understood,let alone the fatigue property under cyclic loading.The quasi-static mechanical properties of Nb-60Ta-2Zr basically meet the requirements of the cardiovascular stent with σ0.2 of～344 MPa,and plastic elongation of～28%.The fracture mechanism shows grain boundaries decohesion induced by inconsistent plastic deformation of adjacent grains during necking,followed by microvoid coalescence process.The fatigue strength of Nb-60Ta-2Zr in air is 300MPa,higher than the σUTS of Nb-1Zr,comparable to the 316L SS,in the applicable range for cardiovascular stent.The crack initiated intergranularly and then propagated with a mixed mode.Finely distributed tiny impurity particles can be also observed on some grain boundaries both on the crack initiation site and stable propagation region.The grain boundary strength plays a dominant role under both quasi-static and cyclic loading.In simulated physiological solution,the Nb-60Ta-2Zr alloy manifested a low uniform corrosion rate and a stable passivation state up to 1.2VSCE.Different from the pitting corrosion of 316L stainless steel and the transpassive dissolution of Co-Cr alloy,no localized corrosion was detected due to the dense and stable passive film on the surface of Nb-60Ta-2Zr alloy.The fatigue performance of Nb-60Ta-2Zr does not deteriorate in the simulated physiological solution.The fracture surface morphology showed no difference with that in air,with no corrosion trace on the surface.During fatigue tests,the metal ion releasing was below the detection limit.The great corrosion fatigue resistance performance,together with the toxic-element-free composition,makes it quite promising as candidate for stent material.The as-cast NbTaTiHf MEA consists of a single BCC phase with an equiaxed structure and an average grain size of 134 μm.Cast pores were found evenly distributed in the ingot with equivalent diameter about 15 μm.After hot-rolled and annealed at 1000℃,1300℃ and 1400℃,the average grain sizes of the alloy were 19 μm,51 μm and 64 μm,respectively.Tensile tests showed that annealing at 1000℃ can obtain better matching of strength,plasticity and work hardening ability with σ0.2 of 852 MPa,σUTS of 866 MPa,the total elongation of～20%and the uniform elongation of～5%.With the increase of annealing temperature,the tensile fracture mode undergoes a transition from dimple fracture to dimple and intergranular mixed fracture,and then to total intergranular brittle fracture.The fatigue tests of NbTaTiHf MEA showed that,compared with the as-cast state,the fatigue strength of the alloy was increased from 200 MPa to 390 MPa after hotrolled and recrystallized at 1000℃,and the fatigue ratio was increased from 0.24 to 0.45.The fracture morphologies showed that the fatigue crack of the as-cast sample initiated at the near-surface void defects,while the fatigue crack of the recrystallized sample initiated from the surface,showing grain boundary characteristics.The as-cast alloy was found to display a fatigue threshold,ΔKth of～5.4 MPa√m which decreased to ΔKth～4.2 MPa√m after recrystallization;the Paris exponent m was roughly between 2.7 and 3.3.The fatigue crack propagation of the alloy with both processing histories was associated predominantly with transgranular fracture.Compared to the recrystallized alloy,the crack path of the as-cast alloy possessed more twists and turns,which was believed to cause roughness-induced crack closure.In was found that in the crack initiation stage,the fatigue behavior of the alloy is mainly affected by defects such as holes,and the elimination of holes and grain refinement treatment improvs the alloy’s resistance to fatigue crack initiation.While in the crack propagation stage,the difference in grain size plays a major role,which means the larger the grain size,the more tortuous the crack growth path,and the higher the ΔKth.