Molecular Dynamics Simulations Of The Plastic Deformation Mechanisms In α-ti

Titanium(Ti)and its alloys play an important role in the field of aerospace,biomedical and chemical industries because of their high strength,superior biocompatibility and strong corrosion resistance.However,the poor ductility and formability at room temperature limit their large-scale application for α-Ti alloys.In α-Ti and its alloys,the formation and evolution of defects during plastic deformation have a significant effect on the mechanical properties of them.Therefore,in this dissertation,the plastic deformation behavior and mechanism of pure Ti and Ti-Al solid solution alloys with the single phase are investigated at the atomic scale using a research method mainly based on molecular dynamics(MD)simulations and combined with experimental characterization,providing a theoretical basis for designing the internal micro structure of highperformance titanium alloys.The main results are shown as follows:(1)The effects of different loading directions on the deformation mechanism of single-crystal pure Ti are illustrated.When the loading direction is along[2110],[0110]and[0001]directions of single-crystal,the deformation mechanism are the strain induced phase transformation,prismatic slip and the activation of pyramidal slip and {1012} twinning,respectively.FCC structures that satisfy the Prismatic-type({1010}HCP‖{110}FCC,<1120>HCP‖<110>FCC)and Basal-type({0001}HCP‖{111}FCC,<1120>HCP‖<1110>FCC)orientation relationships with HCP structure can be obtained by activating the phase trainstion of"hexagonal close-packed(HCP)structure→body centered cubic(BCC)structure→face centered cubic(FCC)structure" and the slip of Shockley partial dislocation,respectively.(2)The mechanism of HCP→FCC phase transformation induced by the phase interface and twin tip are revealed.During plastic deformation,the incoherent region at the P-type HCP/FCC phase interface favors the preferential nucleation of Shockley partial dislocations,leading to the induced HCP→FCC phase transformation at the phase interface.Simultaneously,Shockley partial dislocations interact with the phase interface,resulting in the formation of the localized {111} coherent twin boundaries at the HCP/FCC phase interface.During plastic deformation,the twinning dislocations(TDs)b4=(4γ2-9)/(3+4γ2)<1012>at the{1011} twin tip can be dissociated under the state of stress concentration,forming the Shockley partial dislocations.The slip of these Shockley partial dislocations induces the HCP→FCC phase transformation at the{1011} twin tip.(3)The behavior and mechanism of {1012},{1011} and {1121}twinning in α-Ti are revealed during the plastic deformation,and the effect of the addition of Al elements on {1012} and {1011} twinning are illustrated.(ⅰ)In pure Ti and Ti-Al alloys,the migration of {1012} twin boundaries is dominated by the slip of TDs b2=(3-γ2)/(3+γ2)<1011>during plastic deformation.The addition of Al elements reduces the critical resolved shear stress(CRSS)required for TDs nucleation,thus promoting the nucleation of TDs b2 on twin boundaries.(ⅱ)In pure Ti and Tisat.%Al alloys,the migration of {1011} twin boundaries are dominated by the slip of TDs b2=(4γ2-9)/(6+8γ2)<1012>±1/6<1210>and the slip of TDs b2 and b4 during plastic deformation,respectively.The addition of Al elements can promote the generation of immobile interfacial defects,causing the stress concentration on the twin boundaries,thus promoting the nucleation of TDs.(ⅲ)In pure Ti,the nucleation and growth of {1121} twins are carried out the slip of partial dislocation with the Burgers vector of 1/36<1126>M and TDs b1=1/6(4γ2+1)<1126>±1/2<1100>during plastic deformation,respectively.In addition,{1121} twins can be induced by the dissociation of the {1122}TDs or the interaction of the {1122} twin boundary with the<a>dislocation,forming the {1122}-{1121} double twins.(4)The relationship between {1011} twin boundary and crack propagation and the effect of Al addition on crack propagation are revealed.While crack propagates along the {1011} TB,the HCP→FCC phase transformation can be induced at crack tips,thus hindering the crack propagation at high strain rates in pure Ti.After decreasing the strain rate,the crack propagates rapidly along the TB and makes the material fracture,because only a small amount of basal stacking faults can be generated.After the addition of Al element,a higher volume fraction of FCC phase generated around the crack tip at the TB can be obtained,which effectively releases the stress concentration at the crack tip,blunts the crack tip,inhibits crack propagation on the TB,and thus enhancing the plasticity and toughness of the Ti alloy.