Macro/Micro Deformation Behavior And Mechanism Of Pure Titanium Under Uniaxial/Biaxial Stress State

The macroscopic and microscopic mechanical behaviors of titanium alloys under uniaxial stress have been extensively studied.However,during the forming process,titanium alloys often undergo complex stresses such as biaxial and triaxial stresses,resulting in significant differences in mechanical behavior and deformation mechanisms compared to those under uniaxial stress.Currently,research on the mechanical behavior and deformation mechanisms of titanium alloys under complex stress states is limited.α titanium alloys and α+β titanium alloys are widely used in engineering applications.Conducting research on the macro and micro deformation behaviors and mechanisms of titanium alloys with the α phase under complex stress states can greatly enrich the plastic deformation theory of α titanium alloys and α+βtitanium alloys.Commercially pure titanium(CP-Ti),as a special type of α titanium alloy,possesses advantages such as low density,high specific strength,and good corrosion resistance,making it widely applicable in fields such as aerospace,transportation,and the chemical industry.Therefore,considering both scientific research and engineering applications,this thesis comprehensively investigates the macroscopic and microscopic mechanical behaviors and deformation mechanisms ofα titanium under complex stress states using experimental and simulation methods.CP-Ti is selected as the test material to achieve a thorough understanding of the subject and cater to the needs of both scientific research and engineering applications.The mechanical anisotropy behavior and intrinsic mechanisms of CP-Ti were studied using crystal plasticity simulation,and the influence of stress direction on slip and twinning activities was analyzed.The findings revealed that the crystal plasticity model,considering the material’s initial texture,effectively captured the macroscopic mechanical behavior under different stress directions,thus confirming the correlation between mechanical anisotropy and T-texture.Compared with the TD stress,the prismatic <a> slip activity increases while the basal <a> slip and tensile twinning activities decrease under RD stress,resulting in lower yield stress along the RD direction during tensile deformation.The presence of T-texture under TD tensile stress is conducive to the occurrence of tensile twinning.The macro and micro deformation behavior of CP-Ti under complex stresses was studied using biaxial tensile testing,and the influence of stress states on dislocation slip and deformation twinning activities was analyzed.Then,the differences in macro and micro mechanical behaviors of CP-Ti under different stress states were explained.With the increase of stress triaxiality,prismatic<a> slip activity decreases while pyramidal <c+a> slip and tensile twinning activity increase.This was attributed to a decrease of TD stress component on the prismatic planes and an increase of that on the c-axis and pyramidal planes with increasing stress triaxiality.When the stress state changes from uniaxial to equi-biaxial,the increase in deformation resistance of CPTi is related to the decrease of prismatic slip with low critical resolved shear stress(CRSS)and the increase of twinning and pyramidal slip with high CRSS.The fracture behavior of CP-Ti with typical T-texture under equi-biaxial tension were significantly distinct from that under uniaxial tension.For uniaxial tension,cracks originated from the pronounced strain-localized regions,forming a normal fracture.For biaxial tension,a similar normal fracture proceeded firstly,followed by a shear fracture along the 45° direction to RD.The distribution of normal stress and shear stress on the potential secondary fracture path was obtained using a two-dimensional finite element model.The equivalent traction structural stress on the potential fracture path was calculated,and the reason for the secondary fracture along the oblique direction in the cruciform specimen under biaxial tension was explained by combining the anisotropic yield stress of the material.It was confirmed that the shear stress along the 45° direction significantly contributed to the secondary shear fracture,and the generation of the shear stress was related to the mechanical anisotropy caused by the T-texture.In order to comprehensively analyze the intergranular deformation behavior of CP-Ti,in-situ tensile experiments were conducted to analyze the microstructural evolution during the deformation process of CP-Ti.With increasing strain,multiple slip systems were activated within the grains.The interaction between intragranular dislocation slip and twin thickening enhanced the stress concentration within the grains.The slip-slip,slip-twin,and twin-twin transmission behaviors between neighboring grains contributed to the coordination of local strain and facilitated the plastic deformation of the material.Quantitative calculations of the geometric compatibility factor confirmed that the observed nucleation and growth of twins near grain boundaries were attributed to the prismatic slip activity within adjacent grains.The influence of grain refinement on the micro-deformation mechanism of CP-Ti was investigated.It was found that no deformation twinning occurred during the tensile deformation of grain-refined CP-Ti.Stress concentration,which is caused by dislocation accumulation at grain boundaries,may activate dislocation slip in adjacent grains,but there are also cases where dislocation accumulation at grain boundaries does not activate dislocation slip in adjacent grains.The stress and strain inside the grains exhibit a banded distribution,and there is a certa in correlation between the locations of stress concentration and strain concentration.This is because when the local stress concentration exceeds the CRSS of slip or twinning system,slip or twinning is initiated,resulting in plastic deformation in the l ocal area.As a result,the local stress decreases during the plastic deformation process,and the local strain increases.