| The exploration of intrinsic relationships between deformation micromechanisms and mechanical properties and the exploitation of novel alloy systems with excellent mechanical properties are important methods to promote the development and application of cubic metals.In cubic metals,dislocation slip and deformation twinning are basic mechanisms for plastic deformation.The generalized stacking fault energy(GSFE)provides vital information at atomic scale for the investigation of the dissociation and slip of dislocations and the deformation twinning.For the past few years,it has been found that the alloy composition and the applied pressure have significant influences on GSFE,and therefore these factors could also impact the deformation behaviors of cubic metals,such as the intermetallic compounds with the B2 structure and the face centered cubic(FCC)medium/high-entropy alloys(MEAs/HEAs).However,few reports have been performed about the effects of alloy compositions or pressures on the deformation behaviors of B2 alloys.Although previous experiments demonstrate that the operation of deformation twinning could synchronously improve the strength and ductility of MEAs and HEAs,it still remains elusive how the alloy composition and the pressure impact the competition between dislocation slip and twinning as well as the mechanical properties of alloys in details.Aimed at better understandings of the deformation behaviors of B2 alloys and providing the theoretical guidance for the composition optimization and the manipulation of mechanical properties of MEAs and HEAs,the two above alloys were selected as subjects,and the molecular static and dynamic simulations and first principles calculations were adopted with the pointcut of GSFE.In this case,we systematacially discussed the influence of compositions and pressures on the dissociation and slip behaviors of<111>screw superdislocations in B2 alloys and analyzed the effects of composition,pressure and temperature on the deformation mechanisms and mechanical properties of MEAs and HEAs.The main results are as follows:1.The research on the dissociation and slip behaviors of<111>screw superdislocations in B2 alloys indicates that with the change of the type and composition of alloy,the dissociation configuration of screw superdislocation on {110}planes and the Burgers vectors of partial dislocations have continuous variations,and it transforms from the two-fold configuration with two screw 1/2<110>superpartial dislocations into the triple configuration with three mixed partials dislocations.Based on this,the generalized dissociation configuration of screw superdislocation was proposed where the consecutive adjustment of Burgers vectors was permitted for partial dislocations.In view of this,the energy minimization criterion was developed which can accurately predict the dissociation behavior of screw superdislocations.It is confirmed that the spontaneous optimization of system energy is the fundamental principle that determines the variation of dissociation configurations with the alloy composition.Furthermore,the dissociation configuration and the transformation of core structures are the crucial factors that have important effects on the slip and cross-slip behaviors of screw superdislocations.The energy minimization criterion could predict the energy barrier of core transformation,and therefore it can be utilized for evaluating the cross-slip ability of different dissociation configurations.2.The effects of pressure on the dissociation and yield behaviors of<111>screw superdislocation were discussed in FeAl.With the decrease of pressure,the dissociation configuration of screw superdislocation has a continuous transformation from the screw two-fold configuration to the mixed triple configuration.Meanwhile,the nonplanar characteristics of core structures are enhanced,and thus the yield strength increases evidently.The yield behavior is dominated by {112} slip at high pressures,whereas only {110} slip occurs at low pressures.Based on the simulation results,the modified energy minimization criterion ? was developed for the accurate prediction of dissociation behaviors of screw superdislocations at different pressures.The generalized cross-slip mechanisms were proposed:in the screw two-fold configuration,the leading and trailing superpartial dislocations successively cross slip onto {112} plane without the occurrence of dislocation constrictions;the transformation into two-fold configuration is the basic mechanism for the cross-slip of triple configurations.Considering the effects of shear stresses,the modified energy minimization criterion ? was constructed which can quantitatively assess the cross-slip ability of screw superdislocations at different pressures and also contributes to the in-depth understandings for the yield behaviors of screw superdislocations at complicated stresses.3.The composition plays an important role in the deformation mechanism and mechanical properties of FCC MEAs and HEAs.The basic principles for the optimization and design of alloy composition were proposed:the FCC phase stability is improved by reducing the free energy of FCC phase,the shear modulus G is increased to enhance the yield strength,and the stacking fault energy(SFE)is reduced to promote deformation twinning and increase the strain hardening ability and plasticity.Moreover,the poisson ratio v and the ratio of bulk modulus B to shear modulus G,i.e.B/G can be used to assess the toughness.For CoCrNi alloys,the rise of Co content causes the increase of shear modulus G,and the increment of Cr content leads to the increases of B/G and v.The reduction of Ni content with the increase of Co or Cr content results in the obvious decrease of SFE,and thus it can promote the operation of deformation twinning and the enhancements of comprehensive mechamical properties.The addition of Al,Cu,Ti or Mo with a content of 0?10 at.%leads to the decrease of FCC phase stability for CoCrFeMnNi alloy.While the Mo content is 2 at.%,SFE decreases and the shear modulus G is raised,which contributes to the operation of twinning and the synchronous enhancements of strength and plasticity.4.The transition of hexagonal close-packed(HCP)and FCC phase stability is the inherent reason for the enhancement of twinning propensity with the rise of pressure for CoCrFeMnNi HEA:at high pressures,the inscrease of HCP phase stability leads to the reduction of SFE,and thus it promotes the nucleation of deformation twins by the overlapping of wide SFs or the successive emission of Shockley partial dislocations on adjacent {110} planes;furthermore,high pressures could cause the phase transformation from FCC to HCP,and twins and SFs are the intermediate microstructures of phase transformation.However,the effects of pressure on the SFE and twinning propensity of CoCrNi MEAs are slight.In addition,the temperature is the important factor that plays a significant role in the twinning propensity and the mechanical properties of CoCrNi and CoCrFeMnNi MEAs and HEAs.At cryogenic temperature,both the two alloys have strong twinning propensities;with the decrease of temperature,the modulus G,v and BIG all increase.As a result,the deformation at cryogenic temperature can significantly improve the strain hardending ability,plasticity,yield strength and toughness. |
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