Cube Planes Research Articles

Dissociation of superdislocations in single crystal L1 0TiAlV compounds

Single crystals of Ti 44Al 54V 2(L1 0) have been deformed to 0.6%–0.8% in uniaxial compression at room temperature, 500, 873 and 1073 K, respectively. The dissociated configurations of superdislocations ( b =«011] ) in the deformed alloys have been studied under weak beam conditions to determine the temperature dependendence of the configurations. The study shows that the dissociated configurations remained co-planar from room temperature to 500 K, but became non-planar at 873 K, in which the superpartial with an antiphase boundary cross-slipped onto a cube plane. In addition to the dissociations in the octahedral and cube planes, superdislocations with a screw character form superkins whose height increases with increasing deformation temperature. The apparent activation energy for the deformation in the range of 500–1073 K was found to be 0.05 eV. Based on the characteristics of deformation and dislocation structure in this alloy, it is proposed that two different deformation mechanisms operate at the corresponding temperature ranges, i.e. room temperature to 650 K and 650 to 1073 K.

Octahedral cross-slip in Ni3(Al, Ti) deformed at −196°C

Abstract A single crystal of L12 long-range-ordered Ni3(Al, Ti) has been deformed in compression at −196°C. The dislocation structure in the (111) planes is predominated by edge superlattice dislocations which are arranged in dipoles and bundles of dipoles. The density of screw superlattice dislocations is low. They are mainly dissociated on octahedral (1ī1) and (111) planes. This provides strong evidence for octahedral (1ī1) cross-slip. Cube (010) cross-slip was only observed locally presumably assisted by the help of localized internal stresses. It is assumed that, at very low temperatures, cross-slip of screw dislocation from octahedral (111) to cube (010) planes cannot be thermally activated. Altogether the results are consistent with the fact that the increase in the flow stress is strongly correlated to cross-slip of screw superlattice dislocations from (111) to (010) planes.

Large 〈110〉-segmented helical dislocations in natural diamond

Abstract Helical dislocations composed of very straight 〈110〉 line segments (helical shape due to climb from original screw orientation) have been observed by transmission electron microscopy in a region of diamond where growth had occurred on non-faceted, near-{100}-orientation surfaces. Of the six 〈110〉 segments forming each helix turn, the two that are pure edge segments normal to the helix axial Burgers vector are about ten times longer than the other segments, thereby producing strong flattening of the helix in the cube plane containing the helix axis. One of 30 such helices counted in a thinned specimen area of 3 × 104 μm2 was studied in detail. Its major and minor diameters were 5 and 0·6 μm respectively. The helix axis was marked by a line of vesicles, diameters from about 20 to about 130 nm, producing matrix strain contrast attributed to fluid under pressure within the vesicles. Diffraction contrast analysis showed the helix to be of interstitial character, and the vacancy volume emitted to form ...

Microstructures in Ll2 Titanium Trialuminides Containing Iron

The microstructures of the L1 2 titanium trialuminides at low temperatures were studied using a number of single crystals with various Al-Ti-Fe compositions, all of which lie in the nominal single phase L1 2 Field at 1200 °C. Five different second phases were found to be in equilibrium with the L1 2 matrix, namely , (Al,Ti) 3 Fe, Al 3 Ti, Al 2 FeTi, Ti 2 NAl and Al 2 Ti+Fe. Small volume fractions of the first two phases are often seen in compounds containing relatively low Ti contents. The Al 2 FeTi, a so-called T phase, was observed at relatively high Fe contents. The Ti 2 NAl does not seems to be sensitive to the Al-Ti-Fe composition, it exists to some extent in all the alloys used in this study. Ti 2 NAl and the interface with the L1 2 matrix are found to be brittle and provide the sites for crack initiation. The Al 2 Ti+Fe phase has been observed in many compounds containing high Ti contents (>25 at.%), and has a large hardening effect. Like binary Al 2 Ti, the phase possesses a tetragonal structure of the Ga 2 Hf-type, and forms plates on the cube planes of the LI 2 matrix. The crystallographic relation between the Al 2 Ti and the Ll 2 matrix was determined to be (100)p//(100) m .and (010)p//(010) m . Porosity is also commonly seen in these alloys, and has a very destructive impact on ductility.

Kink mobility and flow stress behaviour of L12, alloys

Abstract Implications are investigated of a deformation mechanism founded on the idea that kinks on screw dislocations are capable of transporting Kear—Wilsdorf configurations. In this mechanism, which takes into account microstructural features observed experimentally, Kear—Wilsdorf configurations need not transform into a glissile core prior to movement, as envisaged in previous treatments. The total friction stress that should be overcome in order to activate kink slip depends upon kink distribution; this stress is shown to increase as the bending of Kear—Wilsdorf configurations in the cube plane increases. The consistency between the present kink dragging mechanism and selected mechanical properties of L12, alloys is discussed.

Plastic deformation of an iron-modified titanium trialuminide alloy

Mechanical properties and dislocation structures have been studied from room temperature to 700°C in a titanium trialuminide alloy modified to the composition Al 5Ti 2Fe. In compression the material shows extensive ductility with the yield stress increasing above 300–400°C, but in bend testing it is very brittle. At low temperatures undissociated 〈110〉 dislocations are mobile and deformation is controlled by Peierls effects and by extensive work hardening. At intermediate temperatures many dislocations dissociate into mobile superdislocations giving rise to serrations in the stress-strain curve; undissociated segments appear immobile because of a solute-associated core relaxation. At high temperatures dislocations are dissociated as superdislocations, mobile on both octahedral and cube planes, with cross slip between these planes bringing about high temperature strengthening. The low ductility in bend testing may be related to the high work hardening as well as the intrinsic cleavage weakness of the material.

Anin situstudy of cube glide in the γ′-phase of a superalloy

Abstract An in situ study of the plastic deformation of single crystals of the γ′-phase of a CMSX2 superalloy (L12 structure) has been performed between 120 and 1150 K, in order to analyse the role and the mechanisms of glide of superdislocations in {001} cube planes. Extensive cube glide and dislocation sources are observed from very low-temperatures (140 K) to high-temperatures (1150 K), for a tensile axis close to ⟨110⟩. Glide on cube planes is controlled by a locking—unlocking mechanism acting on screw dislocations, in the whole temperature range. This mechanism is described in detail and compared with that presented earlier for prismatic glide in beryllium. It corresponds to a series of cross-slip events, on super partial dislocations which are alternately extended in one intersecting octahedral plane, and in the cube plane. On the other hand, non-screw dislocations interact with small clusters or solute atoms. The explanation for the activation of cube glide at very low-temperatures is given in part II of this study.

An investigation of the mechanisms that control intermediate temperature creep of Ni3Al

In this study, constant stress creep tests were combined with detailed transmission electron microscopy in order to characterize and explain the intermediate temperature creep properties of Ni3Al. It was observed that octahedral glide, the mechanism associated with the anomalous yielding behavior of this alloy, is exhausted during primary creep. Primary creep does not lead to steady state creep but is instead followed by inverse creep. TEM observations indicate that the Kear-Wilsdorf locks that are formed during primary creep do lead to the exhaustion of octahedral glide, but that given sufficient time and temperature these cross-slipped segments are able to bow out and glide on the cube cross-slip plane. This dislocation generation, and subsequent glide, on the (010) plane is the basis for the observation of inverse creep in this alloy. Dislocation motion on the cube plane is a thermally activated process and as such is able to explain the strong temperature dependence that was observed for the intermediate temperature creep strength of Ni3Al.

X-ray topographic observations of 180° magnetic domain walls in (001) plates of nearly perfect iron-silicon alloy single crystals

Abstract Contrary to generally-held views, 180° magnetic domain walls lying in the cube planes normal to (001)-orientation single-crystal plates of Fe + 3wt%Si can be detected by diffraction contrast on X-ray projection topographs by use of conventional experimental arrangements. Conditions for good observations are explained with illustrations. The dominant contrast appears at intersections of the 180° Bloch walls with the X-ray exit surface of the specimen and is generated by small localized deformations at these outcrops. The contrast is at best very weak, being only about 10−2 of that generated by 90° domain walls.

Mechanical properties and dislocation structure in L12long-range ordered Ni3Fe

Abstract Although Ni3Fe belongs to the group of L12 long-range ordered intermetallics the critical resolved shear stress (CRSS) does not depend strongly on the deformation temperature. Dipoles and bundles of dipoles near edge orientations are characteristic features in the dislocation structure. Few screw superlattice dislocations are observed. There are striking similarities to the dislocation structure of pure f.c.c. metals or dilute solid solutions after low and medium plastic deformation. Cross-slip of screw superlattice dislocations onto cube (010) planes plays a minor role and is not regarded to be the rate-controlling factor for dislocation motion. The reason for the absence of cube cross-slip is the high activation enthalpy necessary to constrict widely dissociated Shockley partials to form unit screw superlattice partials.

Exhaustion of a[101](111) slip to explain the strength anomaly in Ni3(Al, Ti)

Abstract The dislocation structure of Ni3(Al, Ti) deformed in the temperature range where the flow stress increases with increasing temperature (room temperature up to 450°C) has been investigated. Long straight screw superlattice dislocations transformed into Kear-Wilsdorf locks have been observed after room temperature deformation. Also frequently observed are dislocations curved on the cube (010) cross slip planes. With increasing temperature the tendency for cube cross slip and dislocation movement on cube planes increases. This is consistent with thermally assisted exhaustion of primary octahedral slip by cube cross slip which is responsible for the flow stress anomaly. Dislocations of the primary cube slip system (±a[110](001)) are activated at deformation temperatures already below the temperature of the flow stress maximum. Their density increases markedly with increasing deformation temperature. Dislocation interactions of octahedral (111) and cube (001) slip could also contribute to the flow str...

The interaction of non-glissile superdislocations and a coherent twin boundary in ordered Cu3Au

Abstract The glide behaviour of superdislocations at a coherent twin boundary in ordered Cu3Au was examined in a transmission electron microscope by dynamic and static experiments for the case when superdislocations are sessile in the boundary. Possible schemes for dissociation of a superdislocation in the boundary were analysed geometrically. The interacting superdislocations were for a part of their length still present in the twin. The leading superpartial of each superdislocation dissociated into a superpartial in the matrix and a residual Shockley partial in the boundary of glissile type. The trailing superpartial remained undissociated in the boundary. The superpartial in the matrix glided on a cube plane, and a ribbon of antiphase boundary connected to the boundary was left in its trail. The cube slip occurs as a result of firstly a maximal resolved shear stress for the observed slip system and secondly the geometric criteria for slip applied to all possible slip systems in the matrix. The Schmid factors for the slip systems in the matrix could be calculated by assuming a uniform tensile axis in the foil. The tensile axis was deduced from the observed slip systems in the twin. This method could be applied to both dynamic and static experiments. The observed cracking along coherent twin boundaries appears to be the result of a thin-foil effect and is not representative for mechanical properties of bulk crystals.

Dislocation loops on cube planes in long range ordered Cu3Au

Long range ordered Cu3Au exhibits like other L12 ordered alloys an anomalous increase in the yield strength with increasing temperature. Dislocations which have an antiphase boundary APB fault on the cube plane play a main role in theoretical models that have been proposed to explain this anomaly. In this investigation two types of loops are studied that occur on cube planes. From the measured dissociaton widths of the superlattice dislocations the value of the APB energy can be determined. Different values have been achieved for octahedral and cube planes; they can be compared with previous measurements and with the theoretical models.Single crystals of Cu3Au were grown under vacuum and subsequently annealed to achieve a high degree of order and a large domain size. The crystals were deformed in tension at room temperature. For the TEM investigation discs were cut parallel to the primary octahedral glide plane and to the cube cross slip plane. The preparation of the thin foils is rather difficult because of the large gold content.

Evolution of dislocation structures and cyclic behaviour of a 316L-type austenitic strainless cycled in vicuo at room temperature : Gerland, M., Mendez, J., Violan, P. and Saadi, B.A. Mater. Sci. Eng. A Oct. 1989 A118, (1–2), 83–95

To quantify the effects of interactions between various microstructure attributes on fatigue life in the high cycle fatigue (HCF) regime, we have proposed a new microstructure-sensitive extreme value statistical framework. This framework couples the extreme value distributions of certain fatigue indicator parameters (FIPs) or response functions to the correlated microstructure attributes that exist at the extreme value locations of these FIPs. We demonstrate the application of this statistical framework to investigate the microstructure-sensitive fatigue response of the PM Ni-base superalloy IN100 at 650 °C. To accomplish this task, we construct statistical volume elements (SVEs) used to compute the local response for 200 instantiations of IN100. These SVEs are constructed and simulated via the finite element method with crystal plasticity constitutive relations. The results of the simulations are used to explore extreme value statistics of the FIPs for these microstructures. The extreme value distributions of the Fatemi–Socie FIP are fit with high confidence by the Gumbel distribution and are defined in a representative nature with as few as 25 simulated microstructure instantiations (i.e., SVEs). The extreme value marked correlation functions of the apparent Schmid factor based on the geometry of the slip systems relative to the loading direction indicate that cube slip may be important to fatigue crack formation in this material system. This supports previous experimental observations of fatigue crack formation and microstructurally small fatigue crack growth along cube planes in IN100 in grains that are unfavorably oriented for octahedral slip at elevated temperatures.

The Dissociation of Sessile Superdislocations at a Coherent Twin Eoundary in Ordered CU3AU During Plastic Deformation

AbstractThe glide behaviour of superdislocations at a coherent twin boundary in ordered Cu3Au was examined in a transmission electron microscope for the case when the superdislocations are sessile in the boundary. Possible schemes for dissociation of a superdislocation in the boundary were analysed geometrically. The leading superpartial of each superdislocation dissociated into a superpartial in the matrix and a residual Shockley partial in the boundary of glissile type. The trailing superpartial remained undissociated in the boundary. The superpartial in the matrix glided on a cube plane, and a ribbon of APB connected to the boundary was left in its trail. The cube slip occurs as a result of (i) a maximal resolved shear stress for the observed slip system and (ii) the geometric criteria for slip applied to all possible slip systems in the matrix. The Schmid factors for the slip systems in the matrix could be calculated by assuming a uniform tensile axis in the foil. The tensile axis was deduced from the observed slip systems in the twin.

Deformation mechanisms in ordered Cu2NiZn: The role of elastic and antiphase boundary energy anisotropies

Abstract The ordered Cu2NiZn alloy shows a change in antiphase domain structure according to the ordering temperature. At low annealing temperatures there are isotropic domains and at high temperatures there are cube-oriented domains characteristic of highly anisotropic boundary energies. The material is always elastically anisotropic and hence it is possible, within the same alloy, to compare the influences of the two types of anisotropy on dislocation microstructures and deformation behaviour. At low temperatures, and for disordered materials, strengthening is dominated by dipole and multipole formation and tangled dislocation arrays. At elevated temperatures the ordered alloys show extensive glide on planes near {110} as well as some on {001}. For the material with cube-oriented domain boundaries there is some evidence to support point blocking of dislocations by cross-slip onto cube planes, leading to a higher work hardening at intermediate temperatures. Glide on {110} planes is favoured by the elasti...

On the mechanism of cross slip in Ni3AI

The mechanical properties of Ll2 intermetallic alloys have been previously described by models based on the assumption that cube cross slip is the rate-limiting step. In this study, it was demonstrated that the cube cross-slip event must be reversible under a change in loading direction. This observation allows the cross-slip models to remain consistent with cyclic deformation data. Additionally, this observation was used as a critical test of the available cross-slip models. It was demonstrated that the rate-limiting step cannot be a total cross-slip event, in which both α/2(110) superpartial dislocations cross slip to the cube plane. Conversely, the limited cross-slip event proposed by Paidar, Pope, and Vitek (PPV) was demonstrated to be consistent with the reversibility constraint. This lends additional experimental support to the PPV model.

Temperature effect on superdislocation dissociation on a cube plane in Ni3Al

Abstract Temperature dependence of the apparent antiphase boundary (APB) energy on cube planes is studied using weak-beam determination of dissociation distances of superdislocations originating from deformation at 20,650 and 850°C. It is difficult to assess how closely the observed dependence applies to the true cube ABP energy because of the uncertainty associated with lattice resistance to the motion of superpartials. It is estimated that the true energy of conservative APB on {001} is liable to decrease with temperature as much as 30% between room temperature and 850°C.

Weak-beam study of superlattice dislocations moving on cube planes in Ni3(Al, Ti) deformed at room temperature

Abstract The dislocation structure resulting from the compression of single crystals of Ni3(Al,Ti) at room temperature has been studied using transmission electron microscopy imaging methods. Deformed samples of e = 1% and e = 9% true strain were sliced parallel to (111) and (010) planes. Weak-beam imaging methods reveal the (010) planes to be the dissociation planes of the screw superlattice dislocations. Transition of superlattice dislocations from (111) onto (010) planes by the Kear-Wilsdorf mechanism was identified. Remarkable dislocation movement on (010) planes was found. The flow stress is interpreted as a dislocation interaction on octahedral and cube planes. Measurements of the antiphase-boundary energies on (111) and (010) planes yielded values of γ111 = γ010 =250±30mJm−2

Slip Systems in Ni3(Al, Ti) At Temperatures Above the Peak in Flow Stress

ABSTRACTDislocations in single crystals of Ni3(Al, Ti) deformed at temperatures above the peak in flow stress have been studied by the TEM “weak-beam” technique. <110> dislocations on the primary cube plane are mostly of edge character, and they have been observed to transform into “super” Lomer- Cottrell locks. <100> dislocations, by contrast, are principally of 45° character. They are believed also to become immobilized by dissociation on {111} planes. Properties of both dislocations on cube planes are discussed and are related to the deformation behaviour of L12ordered intermetallic alloys.