Molecular Dynamics Study Of Dislocation-twin Interactions And The Mechanical Behaviour Of Nanotwins In Pure Titanium

Titanium(Ti)and its alloys are frequently applied in aerospace engineering,chemical industry,and medical implantation due to the combination of resistance to corrosion,light weight and high strength,and great biocompatibility.Compared to face-centred and bodycentred cubic metals,they have fewer slip systems that can be activated,resulting in low plasticity which is a challenge for processing and application of Ti alloys.Deformation twinning is another significant mechanism of plastic deformation,and it has been shown that dislocation-twin interaction plays an important role in determining the toughness and strength of Ti and Ti alloys.Twin boundaries can impede dislocation movement,causing dislocations to pile up at the twin boundary to produce stress concentrations,which may lead to twin boundary migration,secondary twin,stacking faults,etc.In this paper,we perform molecular dynamics simulations to reveal the microstructural evolution of the interaction between the basal<a>dislocation pileup and {1124}<2243>,{1011}<1012>,{1012}<1011>and {1013}<3032>twin boundaries in α-Ti.The{1012}<1011>,{1011｝<1012>and ｛1124}<2243>nanotwinned Ti with different twin boundaries spacing is loaded in tension and compression to elucidate the nature of tensilecompressive asymmetry and the effect of twin boundaries spacing on the mechanical properties,and to establish the link between macroscopic mechanical properties and microscopic deformation patterns.The main work and progress throughout the text are as follows:(1)The outcome of dislocation-twin interactions is closely related to the number of dislocations and the type of twin.For {1124} twin,after the first dislocation glides on the twin boundary,(1121)secondary twin is generated in the adjacent grain,and as successive dislocations enter the same location,subsequent dislocations climb along the(1124)twin boundary and pile up due to the hindering effect of residual dislocations,contributing to the expansion of the nucleation region and increase in thickness of the(1121)secondary twin;for{｛1011} twins,dislocation piling forms a disordered region at the interaction region and expands to produce stress concentrations,leading to the formation of {1011} pyramidal stacking fault within the adjacent grain;for {1012} twins,as the dislocation pileup interacts with the twin boundary,basal/prismatic interface continues to form and expand;for {1013} twin,the first two dislocations pile up to produce {1011} pyramidal stacking fault,but the latter two dislocations enter and lead to the nucleation and expansion of {1011} secondary twin.(2)Different types of nanotwinned Ti have significantly different mechanical properties and plastic deformation mechanisms due to the different twin boundaries they contain,resulting in different dislocation nucleation capabilities on the twin boundaries and types of dislocationtwin interactions.For {1012}<1011>nanotwins,the basal dislocation is more likely to nucleate at the twin boundaries.The plastic deformation mechanism of the material under the tensile loadings is dominated by basal partial dislocation slip and face-centered cubic phase transformation,where the yield strength of the model increases with decreasing twin boundaries spacing,whereas under the compressive loadings,the plastic deformation mechanism of the material is dominated by a combination of basal partial dislocation slip and twin boundary migration.For {1011}<1012>nanotwins,the twin boundaries are more stable and it is difficult to nucleate a large number of dislocations at the twin boundaries.Under tensile loading,the plastic deformation mechanism of the material is dominated by basal incomplete dislocation slip and crack nucleation extension,while under compressive loading,the plastic deformation mechanism of the material is dominated by both basal incomplete dislocation slip and twin boundary migration;for {1124}<2243>nanotwins,the interaction between the twin boundary and dislocation can generate secondary twin and as the secondary twin expands and interacts with the {1124} twin boundary to generate basal dislocations,a cycle of dislocation ? twin transitions is formed,providing substantial ductility without sacrificing strength.Under the tensile loadings,the plastic deformation mechanism of the material is dominated by dislocationtwin and twin-twin interactions,while under the compressive loadings,the plastic deformation mechanism of the material is mainly based on basal partial dislocation slip,with the dislocationtwin interaction products being mainly basal dislocations.On the one hand,the simulation results will enrich and expand the basic theory of plastic deformation of Ti metal,on the other hand,it is important for optimising the plastic processing process and developing Ti alloy materials with high forming properties.