D.P. Pope

A theory of the anomalous yield behavior in L12 ordered alloys

It has been generally accepted that in many L12 ordered alloys the observed flow stress increase with increasing temperature is due to thermally activated cross slip from the (111) primary slip plane to the (010 cross slip plane. Using the results of recent atomistic studies of screw dislocations in L12 structures, a functional form for the activation enthalpy of cross slip of a 12 [101] (111) dislocation has been derived. This activation enthalpy is principally controlled by the following four phenomena: 1. (i) The difference in antiphase boundary energy on (111) and (010) planes 2. (ii) the resolved shear stress on (010) 3. (iii) the difference of the energy of the superpartial pair due to a stress induced compression or extention of the superpartial splitting before a jump and an equilibrium splitting afterwards 4. (iv) the nature of the core dissociation on {111} planes before and after the cross slip event, the so-called Escaig effect (i)-(ii) lead to a result that can be reduced to the form assumed by Takeuchi and Kuramoto [Acta metall. 21, 415 (1973)] but (iii) and (iv), which have not been included by Takeuchi and Kuramoto, predicts a tension/compression asymmetry of the CRSS that is also orientation dependent and is a prominent feature of the anomalous yield stress behavior. It is shown that recent experimental data can be explained by the model, and furthermore, the model predicts that a maximum tension/compression asymmetry will occur for orientations near [011], a result which has been tested in the following paper in this journal. A model for unpinning is also developed which explains the shape of the CRSS vs temperature curve of [111] oriented samples.

Dissociation and core structure of 〈110〉 screw dislocations in L12 ordered alloys I. Core structure in an unstressed crystal

Abstract The core structure of 〈110〉 screw dislocations in ordered alloys with the L12 structure has been studied using computer simulation techniques. Dislocations lying on both {111} arid {100} planes in stress-free crystals were studied using three different interatomic potentials corresponding to different antiphase boundary (ABP) and complex stacking fault (CSF) energies on {111} planes. The superlattice intrinsic stacking fault (SISF) energy on {111} planes was held constant in all calculations. When the APB energy is not too high, the dislocation dissociates on the {111} plane into two ½〈110〉 superpartials separated by APS. The cores of the superpartials are planar and similar to those of dislocations in f.c.c. materials. When the APB energy is high, the dislocation dissociates on {111} into two ⅓〈112〉 superpartials separated by SISF, the cores of which can be highly non-planar. On {100} planes the dislocation always dissociates into two ½〈110〉 superpartials, the cores of which are non-planar, spre...

The asymmetry of the flow stress in Ni3(Al,Ta) single crystals

Abstract Flow stress measurements were performed on single crystalline Ni 3 (Al, Ta) as a function of temperature, orientation, strain rate and sense of the applied uniaxial stress to check the predictions of the Paidar et al. model [ Acta metall. 32 , 435 (1984)]. It was found that the critical resolved shear stress (CRSS) for (111)[101] slip depends not only on the test temperature and orientation of the samples, as other investigators have previously observed, but also on the sense of the applied stress. The orientation dependence of the tension/compression asymmetry, including the regions where the asymmetry is a maximum (positive), a minimum, and where it disappears, is as predicted by the model. The applied stress changes the activation enthalpy of cross slip primarily through its effect on constricting the Shockley partials during cross slip and only secondarily on directly promoting (111) to (010) cross slip. A maximum attainable CRSS for (111)[101] slip, the saturation stress, is also in agreement with the model. It was also found that the CRSS for (111)[101] slip is strain rate independent, but the CRSS for (001)[101] slip shows a strong positive strain rate dependence. The temperature at which the peak in the flow stress vs temperature curve occurs increases with increasing strain rate and decreases with increasing ratio of RSS on (001)[110] divided by that on (111)[101]. When the deformation occurs by (001)[110] slip the stress-strain curve exhibits clearly defined, continuous yield points.

Dissociation and core structure of 〈110〉 screw dislocations in L12 ordered alloys II. Effects of an applied shear stress

Abstract The effects of an applied shear stress on [101] screw dislocations in L12 ordered alloys has been studied by computer simulation techniques. If the dislocation dissociates on the (111) plane into two ½[101] superpartials separated by antiphase boundary (APB), and if the superpartial cores are also spread on the (111) plane, then the Peierls stress for motion on (111) planes is low. However, when the dislocation dissociates into a ⅓[211] and a ⅓[1 12] superpartial on a (111) plane separated by superlattice intrinsic stacking fault (SISF), then the Peierls stress for motion on this plane is very high. When the dislocation dissociates on the (010) plane into two ½[101] superpartials separated by APB, motion on this plane is never observed. The Peierls stress is again high since the superpartials always move on one of the {111} planes at stresses which have to be high enough to produce APB on this plane. The results of these calculations suggest, that there should be two classes of L12 ordered alloys...

Planar faults in the L12, lattice Stability and structure

Abstract The stability of various planar faults on the {111} and {100} planes of A3B alloys having the L12 structure is investigated using two methods. First, the crystallography of the faulted lattice is examined for indications of which types of faults are expected to be stable on the basis of symmetry. It is shown that superlattice intrinsic stacking fault, SISF, on the (111) plane with a fault vector, f, of 1/3[121] must always be stable. These same considerations show that the antiphase boundary, APB, can be stable with a fault vector of 1/2[110] +f′ where f′ lies along [112]. Similarly the complex stacking fault, CSF, con be stable with a fault vector of 1/6[211] + f′, whore f′ lies along [211]. In both cases f′ may be zero. Thus the prediction of the hard sphere model for the SISF is correct, but it is not necessarily correct for the APB and CSF. Secondly, the γ surfaces (i.e. the dependence of fault energy on displacement) for the {111} and {100} planes are calculated using a series of central for...

Plastic flow of Pt3Al single crystals

Abstract Compressive flow stress measurements have been performed on single crystals of the Ll2 alloy Pt3Al at temperatures ranging from near liquid He to 1080 K. Three different orientations of the compressive axis, near −[001], [123] and [111] have been investigated. At low temperatures, a dramatic increase of the flow stress with decreasing temperature, similar to that commonly observed in b.c.c. metals, has been found for all orientations studied. Also, unlike other f.c.c.-related materials, two distinct slip systems were observed, depending on the sample orientation. Two-surface slip trace analysis showed that only the cube slip systems operated at all test temperatures in specimens with [111] and [123] compression axes. On the other hand in the samples with the [001] compresive axis the primary octahedral slip system operated at low temperatures, and at higher temperatures, both octahedral and cube slip occurred. It is proposed that the rapid increase of the yield stress at low temperatures is in both cases a consequence of the sessile configuration of the screw dislocation core. While screws are always sessile in the case of {010} slip in Ll2 alloys, for {111} slip this is only so if the superdislocations are dissociated with superlattice intrinsic fault between the superpartials.

Deformation twinning in intermetallic compounds—the dilemma of shears vs. shuffles

In this paper the geometric description and general theory of mechanical twinning are reviewed, the twins in general lattices and superlattices are summarized, and the kinematic process by which mechanical twins form is revisited. A case study of mechanical twinning of HfV2 + Nb, a C15 (cubic) Laves phase, is presented and the synchroshear of selected atomic layers is proposed to explain the physical process of twin formation. If the twins form in this way, then long shear vectors and/or atomic shuffles are not really necessary.

Mechanical behavior of Ni3Al: Effects of environment, strain rate, temperature and boron doping

Abstract The mechanical behavior of polycrystalline Ni3Al was studied as functions of environment, strain rate, temperature and boron doping. We find that the ductility of boron-free Ni3Al increases with increasing strain rate, increasing (or decreasing) temperature from room temperature, and decreasing amounts of water vapor in the test environment. The present results are consistent with our earlier conclusion that moisture-induced environmental embrittlement is a major cause of the room-temperature brittleness of Ni3Al. We also find in the present study that boron suppresses environmental embrittlement in Ni3Al; however, its efficacy depends on the amount added. At a level of 100 wppm, it quite effectively suppresses environmental embrittlement, with ductility remaining high and insensitive to strain rate in both air and water. At a level of 50 wppm, however, boron alleviates environmental embrittlement only in air—in water the ductility is strongly strain-rate sensitive. Boron also affects the fracture mode, tending in general to promote transgranular fracture if present in sufficient quantity.

Core structure and motion of 〈110〉 screw dislocations in L12, alloys with unstable antiphase boundaries on {111} planes

Abstract The cores of the individual superpartials which comprise a [10l] screw superdislocation dissociated on the (111) and (010) planes in L12 ordered materials have been investigated using computer simulation techniques for the case where antiphase boundaries (APBs) on (111) planes are unstable. The superdislocation dissociates in this case on the (010) plane into two 1/2[101] superpartials separated by an APB, and on the (111) plane into a 1/3[112] and a 1/3[211] superpartial separated by a superlattice intrinsic stacking fault (SISF). In both cases the corn of the superpartials are non-planar, and therefore sessile. The cores of the 1/2[71bar;01] superpartials. separated by an APB on (010) planes, are spread into the (111) and the (l11) planes. The 1/3〈112〉 superpartials separated by an SISF on the (111) plane divide into edge and screw components, one above the other: the former component is spread into the (111) plane, but the latter is again spread into both the (111) and (111) planes. The motion...

Planar faults and dislocation dissociations in body-centred-cubic-derivative ordered structures

Abstract The stability of faults on. {110} and {112} planes in the B2, DO3 and L21, ordered structures is investigated in several ways. Crystal symmetry considerations, using Neumanns principle, are used to discover which faults may be stable on symmetry grounds, i.e. which faults correspond to stationary points on the γ-surface. The restoring force on one half of the faulted crystal is also calculated for certain special faults which are not required to be stable on symmetry grounds. A and a section of the {110} γ-surface and a section of the {112} γ-surface is calculated for the B2 lattice using two different potentials. It is found that certain faults can be stable with fault vectors which deviate slightly from those predicted on the basis of the hard-sphere model. Possible dislocation dissociation schemes are discussed.