DISLOCATION DISSOCIATIONS AND FAULTENERGIES IN Ni_3Al ALLOYS DOPED WITH PALLADIUM

Dislocation structures in polycrystalline Ni 3Al alloy doped with palladium deformed at room temperature have been investigated by transmission electron microscopy. The structure consists mainly of dislocations dissociated into a /2〈011〉 super partials bounding an anti phase boundary (APB). Dislocations dissociated into a /3〈112〉 super Shockley partials bounding a superlattice intrinsic stacking fault (SISF) are also common debris. The majority of the SISFs are truncated loops, i.e. the partials bounding the SISF are of similar Burgers vector. These faulted loops are generated from APB coupled dislocations, according to a mechanism for formation of SISFs proposed by Suzuki et al , and recently modified by Chiba et al . The APB energies for {111} and {010} slip planes are measured to be 144±20 mJ/m 2 and 102±11 mJ/m 2 respectively, and the SISF energy has been estimated to be 12 mJ/m 2 in this alloy. It is concluded that the dislocation structure in Ni 74.5 Pd 2Al 23.5 alloy deformed at room temperature is similar to that in binary Ni 3Al, and the difference in fault energies between these two alloys is small. Thus, it seems unlikely that the enhancement of ductility of Ni 74.5 Pd 2Al 23.5 results from only such a small decrease of the ordering energy of the alloy. SISF bounding dislocations also have no apparent influence on the ductilization of Ni 74.5 Pd 2Al 23.5 alloy.

Atomistic Simulations of the Effect of Helium on the Dissociation of Screw Dislocations in Nickel

The interactions of He with dissociated screw dislocations in face-centered-cubic （fcc） Ni are investigated by using molecular dynamics simulations based on an embedded-atom method model. The binding and formation energies of interstitial He in and near Shockley partial cores are calculated. The results show that interstitial He atoms at tetrahedral sites in the perfect fee lattice and atoms occupying sites one plane above or below one of the two Shockley partial cores exhibit the strongest binding energy. The attractive or repulsive nature of the interaction between interstitial He and the screw dislocation depends on the relative position of He to these strong binding sites. In addition, the effect of He on the dissociation of screw dislocations are investigated. It is found that Fie atoms homogeneously distributed in the glide plane can reduce the stacking fault width.

Stacking fault,dislocation dissociation,and twinning in Pt_(3) Hf compounds:a DFT study

The Pt3Hf compound plays a decisive role in strengthening Pt-Hf alloy systems.Evaluating the stacking fault,dislocation dissociation,and twinning mechanisms in Pt3Hf is the first step in understanding its plastic behavior.In this work,the generalized stacking fault energies(GSFE),including the complex stacking fault(CSF),the superlattice intrinsic stacking fault(SISF),and the antiphase boundary(APB) energies,are calculated using firstprinciples calculations.The dislocation dissociation,deformation twinning,and yield behavior of Pt3Hf are discussed based on GSFE after their incorporation into the Peierls-Nabarro model.We found that the unstable stacking fault energy(γus) of(111)APB is lower than that of SISF and(010) APB,implying that the energy barrier and critical stress required for(111)APB generation are lower than those required for(010)APB formation.This result indicates that the a<110> superdislocation will dissociate into two collinear a/2<110> superpartial dislocations.The a/2<110> dislocation could further dissociate into a a/6<112> Shockley dislocation and a a/3<211> superShockley dislocation connected by a SISF,which results in an APB→SISF transformation.The study also discovered that Pt3 Hf exhibits normal yield behavior,although the cross-slip of a a/2<110> dislocation is not forbidden,and the anomalous yield criterion is satisfied.Moreover,it is observed that the energy barrier and critical stress for APB formation increases with increasing pressure and decreases as the temperature is elevated.When the temperature rises above 1400 K,the a/2<110> dislocation slipping may change from the {111} planes to the {100} planes.

Effective Stacking Fault Energy in Face-Centered Cubic Metals

As a typical configuration in plastic deformations, dislocation arrays possess a large variation of the separation of the partial dislocation pairs in face-centered cubic（fcc） metals. This can be manifested conveniently by an effective stacking fault energy（SFE）. The effective SFE of dislocation arrays is described within the elastic theory of dislocations and verified by atomistic simulations. The atomistic modeling results reveal that the general formulae of the effective SFE can give a reasonably satisfactory prediction for all dislocation types, especially for edge dislocation arrays. Furthermore, the predicted variation of the effective SFE is consistent with several previous experiments, in which the measured SFE is not definite, changing with dislocation density. Our approach could provide better understandings of cross-slip and the competition between slip and twinning during plastic deformations in fcc metals.