Atomic Simulations On The Dislocation Yield Behaviors In Body-centered Cubic Metals

Dislocations,ubiquitous line defects in crystalline materials,are of key importance in studying the mechanical properties of deformed crystals.It has been demonstrated that 1/2<111>screw dislocations are responsible for the low temperature plastic deformation in body-centered cubic(BCC)metals.The non-planar core of 1/2<111>screw dislocation brings about their specific plastic behaviors,differring from that of the compacted metals,such as face-centered cubic metals.Due to the nanometric size of the dislocations,as well as the limitation of spatial and temporal resolution of the existing experimental methods,multi-scale simulations have become the effective tools to explore the properties of dislocations,especially in the understanding of microscopic mechanism.In this paper,the slip geometry and yield behaviors of different dislocation configurations in BCC iron,molybdenum and niobium are studied by molecular statics(MS)and dynamics(MD)simulations for further understanding 1/2<111>screw dislocations and even plastic behaviors of BCC metals.For a single 1/2[111]screw dislocation in BCC iron,the effects of non-glide stress on the yield stress and the activated slip systems for differently oriented crystals were studied when the crystals were loaded by double-shear strains.It was found that the activated slip system is associated with both crystal orientation and the change of the core structure with applied strains.The yield stress can be well reproduced by the yield criterion which was proposed based on bond-order potentials for 1/2<111>screw dislocations.Because ?23 is the predominant non-glide stress,combined with previous studies which studied the the effect of all other non-glide stresses but ?23,the effect of non-glide stresses on the motion of 1/2<111>screw dislocation can be fully demonstrated.In addition,the influence of ?23 on activated slip systems at finite temperatures was preliminarily investigated.For the {110} hexagonal dislocation networks(HDN)in BCC molybdenum,by three types of loading modes,the general yield behaviors of the HDN was explored from different aspects,such as the critical stresses to move the network along any direction on the {110} plane that the HDN belongs to,the partitions of the stress states based on the cooperative motion of two or three sets of component dislocations,the conditions under which the steady motion of the HDN takes place and so on.Associated with experimental phenomena,it was found that the characteristic that the motion directions of the HDN are changeable,a result of the natural transition to the steady motion,can explain why there is no fixed dead band for the(011)anomalous slip in these BCC metals.For BCC niobium,the interaction and reaction of two non-parallel,non-coplanar 1/2<111>screw dislocations were studied.Firstly,the relationship between the external shear stress needed for the dislocation's intersection and the dislocation spacing were expIored by molecular statics simulations as well as the dislocation elasticity theory.At finite temperature,the interaction mechanism of the above dislocation configurations was studied.The evolution of the dislocation configurations is the intersection of the two dislocations,reaction to form a[001]binary junction,formation of the edge dipole and finally release of the two dislocations.The dislocations are released in two ways,either by pinching off a 1/2[111]dislocation loop,or by dragging the jog,produced duo to the junction decomposition,to the crystal surface.Analogous models of the two release processes have been established theoretically or deduced experimentally,this study confirms both processes from the perspective of atomic simulation.The effect of temperature and strain-rate on the interaction process was discussed as well.