Investigation On Damage Tolerance Behavior Of Ti-24Nb-4Zr-8Sn Alloy

Ti-24Nb-4Zr-8Sn(wt.%,abbreviated as Ti2448)alloy is a metastable β-type multifunctional titanium alloy.Through continuous transformation mechanism controlled by nanoscale concentration separation,it possesses both good mechanical properties(such as high strength and low modulus)and outstanding functional properties(such as non-linearity,super-elasticity,and tunable thermal expansion across a wide temperature range),that help it show broad application prospects in the field ofbiomedical implants and smart devices.Preliminary study has showed that the alloy possesses good high cycle and low cycle fatigue properties,while the internal mechanism is still unclear.Also,the fatigue damage mechanisms in metals so far are almost based on linear elascticity and cannot be appropriate to materials with nonlinear elasticity.This work investigates the damage tolerance behavior,including notch fatigue,fatigue crack growth and fracture toughness performance,of Ti2448 alloy with nonlinear-elasticity and explores its behavior difference with linear-elastic metals in order to try to lay foundation for the establishment of fatigue damage mechanism in metallic materials with nonlinear elasticity.The notch fatigue behavior of hot-rolled Ti2448 alloy was studied.Results show that the alloy possesses high strength-to-modulus(σ/E)ratio reaching about 1/50 and relatively weak fetigue notch sensitivity(q)at the same time,which exceeds significantly the typical relationship between aiE and q of other metallic materials exhibiting linear elasticity.Distinct from flat surface in conventional linear-elastic metals,Ti2448 alloy demonstrates extremely deflected mountain-shape fracture section with surface roughness reaching about 60%of sample diameter.The fetigue notch sensitivity of Ti2448 alloy is lower than linear-elastic Ti-6Al-4V alloy.According to theoretical analysis and DIC(Digital Image Coorelation)experimental results in single notched plate sample,this can be explained by the intrinsic nonlinear elasticity and its induced stress relief at the notch root in an adaptive manner of higher applied stress generates stronger stress relief.Anisotropic fatigue crack growth behavior exists in as-forged Ti2448 alloy.For R-L(R and L means radial and axial direction of forged bar respectively)and R-R orientatio n,their fatigue crack propagatio n threshold red uce with the inc rease of stress ratio and macro-straight fatigue crack can be observed although with micro-level crack deflection,whose behavior being similar with linear-elastic metals.While for L-R orientation,its fatigue crack propagation comes up macro-scale crack deflection and obviously deviate the nominal plane of crack extension.This special behavior is mainly related to the following factors:crack closure level of L-R orientation is higher than the other two,which can help remarkably reduces the effective stress intensity factor around crack tip and impedes the growth of fatigue crack;besides,only L-R orientation shows nonlinear elasticity under load direction,which forms obvio us gradient elastic filed around crack tpp and may lead to macro crack deflection.In order to futher verify the retardation effect of nonlinear-elasticity on fatigue crack growth,hot-rolled alloy with much obvious nonlinearity was used to machine single notched plate sample and conduct fatigue test.Results show multiple crack deflection and bifurcation behavior with a much greater extent,which helps Ti2448 alloy significantly extend its fetigue crack growth lifetime and finally display better overall fetigue property than linear-elastic Ti-6A1-4V alloy and 316L stainless steel.During almost the entire valid crack extension stage in J-R resistance curve of fracture toughness,the J-integral elastic component of as-forged Ti2448 alloy is always higher than the plastic component.Elastic part still holds dominant position although after obvious crack growth occurred.This behavior is significantly different from the crack extension dominated by plastic portion in conventional metallic materials but is similar with some super-elastic polymer materials,which is a special toughening mechanism dominated by elasticity.Crack iniation toughness and crack growth toughness along various orientations are different.L-R orientation possesses outstanding crack growth toughness because of larger fracture energy absorbed by its tortuous and rough fracture surface.Nonlinear elastic Ti2448 alloy is a kind of material with integration of structure and function property,its overall performance on strength and damage tolerance after modulus normalization is superior to most conventional metals with linear elasticity.Further and deeper studying of its damage torlerance behavibr will likely provide probable solution for the inverse relationship between strength and damage tolerance property in metals,but also lay solid foundation for the establishment of mechanical damage mechanism in metallic materials with nonlinear elasticity.