Dislocation Glide Process Research Articles

The effect of grain size and temperature on the superplastic deformation behavior of a 7075 Al alloy

The high-temperature deformation behavior of a 7075 Al alloy has been investigated within the framework of a recently proposed internal-variable theory for structural superplasticity (SSP). The flow curves were obtained by performing a series of load relaxation tests for specimens with various grain sizes, at temperatures ranging from 445 °C to 515 °C. The overall flow curves were then separated into two parts, according to the respective physical mechanisms, viz., the grain-boundary sliding (GBS) and the accommodating dislocation glide processes, contrary to the conventional approach which uses a single power-law relation. These individual curves were then analyzed based on the internal-variable theory. Much valuable information has been obtained in this way, providing new physical insight as well as a more comprehensive understanding of SSP. The GBS curve could be described as a Newtonian viscous flow, signified by the power-index value of Mg=1.0 for this alloy. The unresolved issue of threshold stress is also clarified and identified as a critical stress required for the GBS. The role of grain refinement is found to shift the grain-matrix deformation (GMD) curve into a higher stress and strain-rate region, while the GBS curve into a lower stress and higher strain-rate region along the respective characteristic scaling line to bring both curves into a common flow-stress region, in which the GMD and GBS can operate simultaneously, resulting in the usual superplastic deformation behavior.

Steady-state creep behavior of a Ti-25al-10Nb-3v-lMo alloy

Creep tests were conducted on Ti-25Al-10Nb-3V-1Mo alloy in the the temperature range of 913 - 1093 K at stresses ranging from 40 to 600 MPa. The creep behavior of the Ti3Al alloy under these testing conditions revealed three different stress exponent regimes. In the temperature range of 1033 to 1093 K at low applied stress levels, the stress exponent was equal to 1.5. At the intermediate stress range (103<σ/E<3x10-3), a stress exponent of 3.3 was exhibited indicating that the creep deformation was controlled by a viscous dislocation glide process As the applied stress increase, the stress exponent changed from 3.3 to 4.4 The activation energy for creep was equal to 288 kJ/mole in the region where viscous dislocation glide was the dominant deformation mechanism (n=3.3) In view of the diffusion data, the rate-controlling species in the viscous glide region was assumed to be Ti lattice diffusion

Microstructural study of optically degraded ZnCdSe quantum wells

The defect structure of optically degraded ZnCdSe quantum wells was investigated using transmission electron microscopy. The defects were composed of the dislocation dipoles with a Burgers vector of b=−(a/2)[101] inclined at 45° to the (001) plane. The dislocation dipoles consist of two segments aligned along the [11̄0] direction and the [120] direction. The [11̄0] dipole segments lying in the (111̄) plane were developed by the recombination-enhanced dislocation glide process, while the [1̄2̄0] dipole segments lying in the (2̄11) plane were developed by the recombination-enhanced dislocation climb process. Both processes operate simultaneously.

An internal variable theory of structural superplasticity

A new approach for structural superplasticity (SSP) is attempted in this study based on the internal deformation variables. The basic mechanisms of SSP are thought here to consist of interface sliding (IS), i.e. grain or phase boundary sliding (GBS/PBS), and a dislocation glide process accommodating the incompatibilities due to IS. For this purpose, a new constitutive framework for inelastic deformation is first developed by a simple consideration of dislocation kinematics to reveal the existence of “internal strain” and “internal spin” tensors. The evolution relationship of internal strain tensor is then shown to lead into a kinematic relationship between the observable deformation variables. To complete the constitutive structures, the constitutive relationships for each deformation variable including the strain rate due to IS are also prescribed by kinetics consideration of each process. The theoretical results are then applied to the experimental results on a fine-grained 7475 Al and a Pb–Sn eutectic alloy obtained from load relaxation and tensile tests.

Rate Sensitivities for Low Temperature Deformation in Ruthenium Aluminide Alloys

Because of the need for new high temperature structural materials, a number of binary and multicomponent B2 aluminides have been investigated in recent years. Some alloys based on FeAl and Nb-Ti-Al are relatively ductile at low temperatures, but suffer from environmental embrittlement and/or relatively low melting temperatures. One apparent exception to the brittle behavior of the higher temperature B2 aluminides is ruthenium aluminide, RuAl, which has a melting point of approximately 2,060 C. Fleischer et al. have reported a high room temperature toughness and high compressive ductilities for a number of alloys based on RuAl, compared to a variety of other intermetallic compounds. The objective of the experiments reported here was to measure room temperature rate sensitivities for a number of the same RuAl-based alloys, to determine if the phenomenological flow parameters that relate to dislocation glide processes are also unusual, compared to other higher temperature B2 compounds.

Creep behavior of a reinforced Al-7005 alloy: Implications for the creep processes in metal matrix composites

Creep tests were conducted on an Al-7005 matrix alloy reinforced with 20 vol.% of irregularly shaped Al2O3 particulates. The results, which cover almost six orders of magnitude of strain rate, give high values for the apparent stress exponent and the apparent activation energy for creep. The introduction of a threshold stress into the analysis leads to a stress exponent of ∼4.4 and data which are consistent with creep controlled by a dislocation climb process and class M behavior in the Al-7005 matrix alloy. These results are compared with an earlier report of creep behavior in an Al-6061 matrix composite, also reinforced with 20 vol.% of irregularly shaped Al2O3 particulates, where the stress exponent was close to three and the data were consistent with control by a dislocation glide process and class A behavior in the Al-6061 matrix alloy. It is shown that the creep of metal matrix composites is controlled by creep of the matrix alloys and there is a consequent division of the creep behavior of the composites into two types, class M and class A, as in the creep of solid solution alloys.

Dislocation glide on non-compact crystal planes in copper

A single-shear creep technique was applied to copper single crystals in order to generate a dislocation glide process on non-compact crystal planes (100), (110) and (131). Optical metallography and interference microscopy of slip lines on the surface have shown that the shear strain was realized mostly by slip on the usual 111 planes and a massive non-compact glide probably did not appear. However, examples of non-compact glide from strongly stressed regions in the specimen are given.

An internal variable approach to deformation behavior of a Pb-Sn eutectic alloy

A new approach for inelastic deformation has been proposed by utilizing internal variables derived directly from a simple consideration of dislocation dynamics. An extension of the theory to a structural superplasticity has also been made recently by taking the dislocation glide process as the major accommodation mechanism for the boundary sliding process instead of the generally accepted diffusional accommodation. The internal variable theory of structural superplasticity has been successfully applied to the cases of quasi-single phase Al alloys. In this paper, the inelastic deformation behavior of a Pb-Sn eutectic alloy as well as the constituent pure metals has been examined specifically in connection with the internal variable theory. A special attention was focused on the deformation characteristics of the superplastic eutectic alloy, well known to reveal a pronounced {alpha}/{beta} phase boundary sliding. For this purpose, a series of load relaxation tests has been conducted to obtain the flow curves under the condition of a constant structure and the results have consequently been analyzed based on the internal variable theory of structural superplasticity. A method to determine the optimum strain rate for a superplastic forming process has been proposed to this end by the quantitative consideration of PBS and its accommodationmore » process. Additional tensile tests were also performed to verify the results of analysis.« less

Creep behaviour and dislocation substructure evolution in the KBr-Kl system

Creep behaviour and dislocation substructure as a function of strain was investigated for two solid solution alloys and the pure components in the KBr-Kl system. The creep characteristics for the KBr-Kl alloys are in good agreement with creep behaviour observed in other ionic and class I metallic solid solution alloys, where the creep rate is controlled by a viscous dislocation glide process. The creep resistance of the KBr-KI alloys is higher than that for the pure components at the same value of homologous temperature. The dislocation substructure of the KBr-Kl alloys and pure components at large strains consists of well defined subgrains. Subgrain formation is shifted to larger strains in the alloys compared to the pure components as a result of solute drag forces on dislocations during glide.

Creep characteristics of single crystalline Ni3Al(Ta,B)

The creep characteristics, including the nature of the creep transient after a stress reduction and activation energy for creep of single crystalline Ni3Al(Ta,B) in the temperature range 1083 to 1388 K, were investigated. An inverse type of creep transient is exhibited during stress reduction tests in the creep regime where the stress exponent is equal to 3.2. The activation energy for creep in this regime is equal to 340 kJ mol−1. A normal type of creep transient is observed during stress reduction tests in the regime where the stress exponent is equal to 4.3. The activation energy for creep in this regime is equal to 530 kJ mol−1. The different transient creep behavior and activation energies for creep observed in this investigation are consistent with the previous suggestion that then = 4.3 regime is associated with creep controlled by dislocation climb, whereas then = 3.2 regime is associated with a viscous dislocation glide process for Ni3Al at high temperatures.

High temperature creep behavior and substructure in KClKBr solid solution alloys

The creep behavior and dislocation substructure of pure KCl, pure KBr and two KClKBr solid solution alloys was investigated at 700°C. The reduced primary stage compared to that for the pure materials, stress exponent close to 3, nature of the creep transient after a stress reduction for the KClKBr alloys is in good agreement with creep behavior observed in class I metallic, KClNaCl and KClRbCl solid solution alloys, where the creep rate is controlled by a viscous dislocation glide process. The creep behavior of the KClKBr solid solution alloys is in agreement with the prediction of the glide-climb criterion for solid solution alloys developed by Mohamed and Langdon. The dislocation substructure of the KClKBr solid solution alloys consisted of well-developed subgrains whose size was larger than that for the pure materials at an equivalent value of normalized stress and varied inversely with applied stress.

In Situ Deformation in the T.E.M. on the two Phase Alloy Ti-24A1-9Nb First Results at Room Temperature

AbstractIn situ deformation experiments have been performed on a two-phase Ti-23.7Al-9.4Nb (%at) alloy at room temperature. Dislocation glide processes in each phase are studied and discussed. An exemple of dislocation transmission through an interface is analysed in detail.

Thermally Activated Processes and Dislocation Mobilities in Two-Phase γ-Titanium Aluminides

AbstractThe processes controlling the dislocation mobility in two-phase γ-titanium aluminides have been investigated over a wide temperature range by determining the activation volumes and activation energies of thermally activated dislocation glide processes. The deformation tests are supplemented by electron microscope observations. Accordingly, at room temperature the mobility of ordinary dislocations is determined by a combination of localized pinning and lattice friction. Additional glide resistance arises from dislocation dipoles and debris defects, which are trailed and terminated at jogs in 1/2 &lt;110] screw dislocations. Dislocation climb processes start above 900 K and seem to initiate the transition from brittle to ductile material behaviour.

Creep Behavior of Potassium Chloride–Rubidium Chloride Solid Solution Alloys

The creep behavior of three KCl–RbCl solid solution alloys and pure KCl and pure RbCl single crystals compressed along the 〈100〉 direction was investigated at 600°C at stresses between 0.5 and 15 MPa. The reduced primary creep stage, a value of the stress exponent of about 3 for the KCl–20 mol% RbCl and KCl–30 mol% RbCl alloys versus about 5 for pure KCl and pure RbCl and the results of the stress reduction tests for these alloys are in good agreement with creep behavior observed in class I metallic solid solution alloys, where the creep rate is controlled by a dislocation glide process. The dislocation substructure developed in the KCl–RbCl alloys consisted of well‐developed subgrains.

Generalized nonlinear inversion of kinetics data: application to the calcite ↔ aragonite transformation

This paper aims to describe an algorithm which enables one to obtain nonlinear kinetics laws from phase transformation kinetics experiments with good accuracy and reliability. This algorithm is an application of the generalized nonlinear inversion theory. After a brief description of the usual inversion methods, the algorithm and its advantages are described. It takes into account the experimental errors, it gives the most probable set of parameters with its relevant covariance matrix and it enables one to make comparisons between different kinetics laws and to test the quality of the fit. Furthermore, its use is more convenient and versatile than sophisticated least-squares regressions which would give the same information. As an example, this algorithm is used to invert data of the calcite ↔ aragonite transformation. They are well described by a first order kinetics law with a reaction exponent equal to one. It shows that the mechanism of the reaction is thermally activated but it does not permit one to distinguish between a nucleation and growth process and a dislocation glide process: the data give a good fit for both mechanisms. Transformation isochrons are then draw on the P− T plane for both equations. The dislocation glide mechanism enables one to take into account the deviatoric stress and the isochrons can be drawn in P− T− σ space. At high deviatoric stresses and room temperature and pressure, it is predicted that aragonite can be formed in a few hours, and this has been experimentally observed.

Flow mechanisms in sulphide minerals

The sulphide minerals as a group display a diverse range in strength and mechanical behaviour in crustal environments, deforming by a range of different flow mechanisms which can lead to the development of characteristic microfabrics. The dependence of creep rate on imposed parameters, such as stress difference, temperature, grainsize, effective confining pressure, and the activities of various components, varies for each type of flow mechanism. For particular phases, changes in the dominant operative flow mechanism may occur in response to changes in the imposed environmental variables. At high applied stress differences, low effective confining pressures, and low temperatures, brittle failure and cataclastic flow are significant deformation mechanisms in some sulphide minerals. At elevated confining pressures, brittle failure can be suppressed and low-temperature plasticity processes involving dislocation glide and/or deformation twinning may operate. With decreasing applied stress difference, and at elevated temperatures, even though strain may be accommodated largely by dislocation glide, deformation may be rate-controlled by recovery processes, and dynamic recrystallization may be significant in influencing microfabric development. The operation of dislocation glide processes leads to the development of crystallographic preferred orientations. At very low applied stress difference, various grainsize-dependent atomic-transfer deformation mechanisms may be important. In many fluid-present, low- to medium-grade metamorphic environments, solution-precipitation creep processes are active in the deformation of some sulphide phases; in fluid-absent situations, grain-boundary diffusion creep may be significant. At higher temperatures, where intragranular diffusion is relatively rapid, lattice-diffusion creep processes are expected to be a major deformation mechanism. In fine-grained sulphide assemblages at elevated temperatures, grain-boundary sliding may be an important deformation mechanism.

Further comments on the appropriateness of stacking fault energy - to - mechanical property correlations

Since most of the work of constricting an extended dislocation is done near the final separation, changes in the required separation (or core width) for reaction will cause large changes in the total work of constriction. This effect may distort any correlation between mechanical properties and stacking fault energy. Atomistic modeling of the appropriate processes is probably the most effective method for the determination of the work of constriction, which is the parameter of fundamental importance for dislocation glide processes. Stacking fault energy or partial separation distance should be regarded as merely qualitative parameters in this regard.

Nature of dark defects revealed in InGaAsP/InP double heterostructure light emitting diodes aged at room temperature

The nature of the dark defects which have appeared in the InGaAsP/InP double heterostructure light emitting diodes aged at room temperature were studied by transmission electron microscopy and other related methods. According to the observation of many diodes by electroluminescence topography, the dark defects were classified into five types: cross-hatched 〈110〉 dark line defects, regular tetragonal 〈110〉 dark line defects, regularly distributed dark spot defects, dark band defects, and dark spot defects whose circumference was bright. These dark defects corresponded to misfit dislocations, stacking faults which originated from the interface between the epitaxial layer and the substrate, precipitate-like spherical defects, mechanical damages induced during epitaxial growth, and alloyed regions produced by the penetration of the electrode metals, respectively. It was found that both the dislocation glide process and the dislocation climb motion due to the nonradiative recombination of the minority carriers which have been often observed in the GaAlAs/GaAs double heterostructure materials occurred only with difficulty in the InGaAsP/InP double heterostructure materials. These phenomena could be well explained by the difference between these materials in term of interaction between impurity atoms and dislocations and/or the electronic state of defects and nonradiative recombination rate at defects.

Slip dislocation formation during cw laser annealing of silicon

High-voltage electron microscopy (HVEM) has been used for the investigation of the defect structure in cw laser-annealed silicon. We report for the first time a (HVEM) analysis of the formation processes involved in the nucleation and glide of slip dislocations during epitaxial regrowth by cw laser annealing of ion-implantation damaged silicon layers. Based on the combined optical and HVEM observations a model of the dislocation generation and glide processes is presented.

Diffusion-controlled dislocation creep: a defense

A defense is made to the criticisms of Poirier against diffusion-controlled dislocation creep. The criticism that values of the activation volumes of creep offer no strong support to diffusion-controlled dislocation creep is turned around and is shown to be strong evidence against non-diffusion-controlled dislocation creep. It is shown that, just as in very fine grain material Coble creep takes over from Nabarro-Herring creep as the temperature is lowered, pipe diffusion will take over from bulk diffusion as the process-controlling dislocation climb. Thus the existence of low values of the creep activation energy in the low temperature range of high temperature creep is readily explained. It is shown that dislocation glide processes cannot account for these low activation energies unless one is willing to accept that for 21 metals it is only a remarkable coincidence that at relatively high temperatures the activation energy of creep is equal to the activation energy for self-diffusion.