Study On The Axial Gradient Structural Characteristics And Tensile Behavior Of Ti6242 Titanium Allo

When designing metal materials,people usually hope to obtain a better combination of strength and ductility,but the traditional process must make a choice between strength and ductility.In recent years,many studies have confirmed that heterostructures or gradient structures can usually achieve simultaneous improvement in strength and ductility.In previous reports,researchers designed gradient samples with continuous changes of microstructure from surface to center.The microstructure of this sample is consistent in the direction parallel to the tensile axis,while there is heterogeneity in the direction perpendicular to the tensile axis.In this work,a unique gradient structure with various characteristics at the longitudinal direction also achieves an excellent combination of strength and ductility.Herein,multi-scale deformation behaviors are comprehensively investigated in this gradient structure with various V_fusing the mechanical properties test and microstructure characterization,supplemented by finite element analysis(FEA),to reveal the V_f-dependent strain hardening and the role of individual layers.The main results are as follows:(1)A gradient architecture is produced in flash-butt welded Ti6242 joints that possess an appropriate structure gradient ofα_p phase andβ_t structure with various characteristics at the longitudinal direction.(2)The axial gradient structure is the same as the radial gradient structure reported in many previous studies.Both of them promote the accumulation of additional geometrically necessary dislocations by activating multiaxial stress,which leads to a rapid increase in strength.And this additional work hardening is related to the volume fraction of the gradient layer.(3)There is a critical value,which is defined as the minimum volume fraction that the axial gradient structure can activate the interlayer constraint.When V_f is less than the critical value,the softer layer becomes the’short board’of the whole sample, and the strength of the sample cannot exceed the strength of the soft layer.When V_f increases above the critical value,the mechanical properties of the material will change significantly with the change of V_f.The critical values of different materials may be different and related to the influence range of the heterogeneous layer interface.The DIC test results show that the influence range of the stress-affected zone on the matrix is at least 1mm.The influence range of the double affected zone on the weld is at least 100μm.(4)The followed aspects allow the gradient sample(V_f=100%)itself to be more ductile: significantly thinnedβplates,theα_p/β_t boundaries replaced by a transition zone with βnano-plates gradually grown intoα_p,and the reduced property difference between deformedα_p andβ_t.The extra strain hardening induced by the mutual constraint between layers results in a further increase in the ductility.The substantial increase in interphaseαs/β_l boundaries,higher-density dislocations inα_p,and the extra strain hardening,evidenced by the increased back stress,are responsible for the increased strength in the gradient sample(V_f=100%).(5)The notched tensile test results further prove that each region has different mechanical properties.The plastic deformation ability of the double affected zone is the best,and it is least affected by notch damage.Theα_p/β_t phase interface,which is easy to cause stress concentration,’disappears’,which will significantly improve the ductility of the semi-equiaxed structure.In the weld zone,the widmanstatten structure itself is brittle,and the notch leads to a significant reduction in its ductility and strength.In the smooth specimen,the double affected zone as a plastic stable layer delays the failure of the weld,and the double affected zone also obtains additional work hardening due to the activation of multiaxial stress.(6)The finite element analysis results confirm that there is a stress-strain gradient near the interface between the gradient layer and the matrix,and the inconsistent transverse shrinkage activates the multiaxial stress near the interface.The heterogeneous deformation induced strengthening is also applicable in the axial gradient structure.