Solute-dislocation Interactions Research Articles

The origins of room temperature hardening of Al–Cu–Mg alloys

The most commonly cited mechanisms for the rapid age hardening of Al–Cu–Mg alloys at about 100–200 °C are hardening by Guinier–Preston B zone formation, formation of Cu–Mg co-clusters and a dislocation–solute interaction mechanism. New experiments on ageing–deformation–ageing cycles at room temperature indicate that no substantial additional age hardening occurs with the addition of deformation to the cycle, and hence, a dislocation–solute interaction mechanism appears unlikely. Instead, strengthening due to modulus hardening generated by the difference in shear modulus of Cu–Mg co-clusters and matrix is proposed as the main strengthening mechanism for room temperature hardening.

Solute-dislocation interactions: modelling and experiments in hydrogenated nickel and nickel base alloys

Abstract The dynamic interaction between diffusing hydrogen and mobile dislocations in nickel and binary nickel–16 wt.% chromium is analysed in terms of the solute drag phenomenon. Static strain ageing experiments are used to measure the saturated dislocation pinning force as a function of the hydrogen concentration. Hydrogen-dislocation interactions cause a shielding of the pair interactions between edge dislocations. The influence of this screening effect is analytically evaluated on the line tension of curved dislocations, and we assess its consequences on the critical force for the expansion of a dislocation loop and on the dissociation mechanism. These results are used to interpret experimental results on the plastic flow of hydrogen-charged nickel single crystals oriented for easy glide. This study illustrates the mechanisms of H-dislocation interactions and their consequences on the different contributions of hydrogen to the flow stress of nickel.

Effect of Hydrogen on Toughening of a Low Alloy Steel

A low alloy steel containing 0.10C, 0.25Si, 0.87Mn, 0.56Cr, 0.47Ni, 0.21Mo, 0.023S and 0.01P (mass%) was cold rolled to 1.6 mm thick sheets and recrystallised at 700°C to get ferrite grains of 8, 21.5 and 32.5 μm with a random distribution of some spheroidal carbide particles and a few inclusions. Tensile specimens prepared from these sheets were cathodically charged in 1 N NaOH and 0.1 N H 2 SO 4 solutions for periods varying from 2 to 24 h, with a current density of 50 mA/cm 2 Tensile tests were carried out with a cross-head velocity of 1.2 mm/min, fracture surfaces were examined by SEM and the deformed structure was examined by TEM. The increase in hydrogen content, up to certain limit, has been effective to cause an increase in both ultimate tensile stress and % elongation resulting a toughening of the steel, while the work hardening is not remarkable. Increase in the ferrite grain size has been observed to enhance this effect. The tensile behaviour has been correlated with observed fracture characteristics and dislocation substructure in the ferrite matrix. The toughening effect has been explained in the light of dislocation solute interactions and the damage caused by hydrogen.

The local internal stress in ferromagnetic alloys. Dislocation–solute interaction model for local internal stress source

In order to elucidate the Raleigh and Left Shift phenomena encountered in the investigations into damping capacity (internal friction) of ferromagnetic alloys (Acta Metall Sin 34 (1998) 1039; Trans Jpn Inst Met 25 (1984) 891; Trans Jpn Inst Met 20 (1979) 409; Mater Sci Forum 119–121 (1993) 415; Metall Mater Trans A 25 (1994) 111), the solute atom model (Mater Design (in press)) for the local internal stress source is modified and replaced by the dislocation–solute interaction model proposed by the authors. The coercive force as well as crystal lattice parameter of FeCrAl and FeCrAlSi alloys was measured and the local internal stress of these alloys was discussed. It is pointed out that only when the alloy is fully re-crystallized, the potential of damping capacity could be fully developed, and at the pre-requisite of which do not lower the energy density of domain walls (Mater Design 21 (2000) 541), to choose those alloying element that distort crystal lattice slightly is benefit to improve the damping capacity of ferromagnetic alloys.

Coarse-grained descriptions of dislocation behaviour

Recent computational advances have permitted mesoscale simulations of systems containing of the order of 106 dislocations. While such simulations are beginning to provide a wealth of information about dislocation energetics and dynamics, it is worth noting that the macroscopic deformation response of well-worked materials with dislocation densities ranging between 1010 and can be accurately described by a smaller number of macrovariables and an appropriate relation between these variables. The large-scale reduction in the number of degrees of freedom required to characterize plastic deformation implies that homogenization, or coarse graining, of variables is appropriate over some range of length and time scales. Here, we describe recent work in which temporal and spatial coarse-graining strategies are identified, which link the mesoscale with the continuum. More specifically, three distinct examples are considered: the impact of solute–dislocation interactions on dislocation mobility (including pinning effects); thermally induced dislocation interactions; dislocation structure and properties at spatially coarse scales, primarily in two dimensions. In this latter example we formulate a continuum Hamiltonian and identify the corresponding macrovariable set (including the dislocation density and its gradients) that is relevant at each level of the coarse-graining hierarchy. Our goal is to show how these seemingly unrelated problems can be viewed as part of a unified picture of coarse-grained dislocation behaviour.

An investigation of serrated yielding in 5000 series aluminum alloys

The effect of different thermal treatment temperatures (from 472 to 783 K) on the characteristics of serrated yielding of three commercial aluminum alloys, AA5052, AA5754 and AA5182, was investigated. In the high temperature treatment range, the stress drop (Δ σ) decreases with increasing thermal treatment temperature. This is the result of an increase in grain size as the thermal treatment temperature increases and can be explained from the solute–dislocation interaction model. For these alloys without any thermal heat treatment after cold rolling and those with a 472 K heat treatment after cold rolling, there is a critical strain before the onset of serrated yielding. The critical strain is larger for those after 472K heat-treatment as compared to those without heat-treatment. This result can be explained by the precipitation of Mg atoms from the solid solution when heat-treated at 472 K.

The use of hydrogenated nickel as a model system for studying solute-dislocation interactions

The effect of hydrogen on tensile tests of nickel and binary nickel – 16 wt. % chromium alloy is analysed in terms of solute drag phenomenon. Static Strain Ageing experiments are used to measure the saturated dislocation pinning force as a function of the H concentration. First order hydrogen-dislocation interactions cause a shielding of the pair interactions between edge dislocations. The influence of this screening effect is analytically evaluated on the self-energy and line tension of curved dislocations, the critical force for the expansion of a dislocation loop and the dissociation mechanism. These results are used to interpret experimental results on the plastic flow of hydrogen-charged nickel single crystals oriented for easy glide.This study illustrates the mechanisms of H-dislocation interactions and their consequences on the different contributions of hydrogen to the flow stress of nickel.

On the Interaction Between Mg Solute Atoms and Dislocations in Al-Mg Binary Alloys

AbstractThe interaction between solute atoms and dislocations is known to lead to negative strain rate sensitivity and poor formability. The negative rate sensitivity causes inhomogeneous flow and the Portevin-LeChatelier (PLC) effect. These observations motivate the present study of the solute-dislocation interaction in binary Al-Mg alloys. We report here on three issues. First, we determine the size and shape of stable Mg clusters at stationary dislocations of edge, 60° and pure screw type. We then evaluate the accuracy with which clustering is predicted by the classical pressure field-solute interaction formula, i.e. within the assumption that solute do not interact and the pressure field of the dislocation is unperturbed by the solute. Second, we investigate to what extent the presence of the cluster perturbs the far field of the dislocation. Finally, the effect of the solute on the stacking fault energy is investigated.

On the yield anomaly in stoichiometric CoTi

Compression tests performed on stoichiometric CoTi polycrystals over a range of temperatures and strain rates from 5×10 −6 to 1 s −1 show a positive temperature dependence of the yield stress before a decline at elevated temperature. This yield strength peak shifts to higher temperatures with increasing strain rate. At temperatures close to that of the peak yield stress, serrated yielding and a negative strain-rate sensitivity of the yield stress occur. Static strain-aging is also observed. These observations are consistent with strong solute–dislocation interactions and suggests that the anomalous yield behavior of CoTi is a manifestation of dynamic strain-aging.

First-principles study of solute-dislocation interaction in aluminum-rich alloys

The strain dependence of the solution energy of substitutional species is derived by expanding the binary alloy energy with respect to concentration and strain to the second order. Within the approximation of small strain and concentration inhomogeneities, the energy change is found to be proportional to the solute-induced stress, and the strain dependence of the solute energy is equivalent to that from the misfit model for solute atoms. However, the solute induced stress is easily calculated from first principles, and it is computed for Ag, Cu, Fe, Li, Mg, Mn, Si, and Zn atoms in aluminum. The results are used to estimate the solute-dislocation interaction energy. The enhancement of the solute concentration near a dislocation core and the role of dislocations on precipitation of these solutes are discussed.

Role of vacancy–solute complex in the initial rapid age hardening in an Al–Cu–Mg alloy

We have investigated the origin of the initial rapid hardening of an Al–1.3 at.% Mg–1.7 at.% Cu alloy by coincidence Doppler broadening of positron annihilation radiation and positron lifetime spectroscopy. Quenched-in vacancies are bound to Mg atoms rather than Cu atoms initially and the vacancy–Mg complexes easily migrate to vacancy sinks at 150°C. Vacancy–Mg–Cu complexes form during the initial 1 min aging at vacancy sinks, meanwhile vacancy density decreases rapidly. These results support that the dislocation–solute interaction is the origin of the initial rapid hardening.

Internal friction of Fe/Cu laminated composite metal

Recently, the demand for higher damping materials with higher strength has been requested mostly from the fme machine industries. The objective of this study is to clarify the mechanism of higher damping as well as higher strength by using Fe/Cu laminated composites as the model alloy. Fe/Cu laminated composites were made by hot rolling pure iron and oxygen-free copper laminated plates. Internal friction measurements were performed for cold rolled and annealed Fe/Cu laminated samples as well as Fe and Cu samples, using a transverse vibration method with an electrostatic driving system and a capacitance detecting system. In Cu, a grain boundary peak was observed at 800K and 870K in the cold rolled and annealed specimen, respectively. In Fe, a grain boundary peak was observed at 1030K and 1050K in the cold rolled and annealed specimen, respectively. In the cold rolled Fe/Cu, a rise in internal friction was observed between 300K and 600K. This must be due to dislocation-solute atom interaction or the recovery process of dislocations. Also, two further peaks were observed at 830K and 1000K. In the annealed Fe/Cu, two peaks were also observed at 870K and 1050K. In both cases, these peaks are similar to the grain boundary peaks in Cd and Fe, respectively.

Influence of dynamic solute-dislocation interaction on high temperature ductility of Al-Mg alloys

High temperature ductility of Al-Mg alloys was examined as a function of stress and strain rate. The ductility was observed to depend strongly on the stress and/or the strain rate in the temperature range between 500 and 800 K. The elongation maximum experimentally observed at about 10 MPa in Al-0.94 Mg alloy at 643 K is due to the low stress exponent, which spreads the deformation out over the whole gauge length, resulting in an elongation maximum. This study also predicts the elongation minimum at about 55 MPa at 533 K. The decrease in elongation observed in the high stress region at 533 K is due to the high stress exponent, which concentrates the deformation in the necked region, resulting in a decrease in elongation. In this study, a solute strengthening map was constructed using the concept of dynamic solute-dislocation interaction. In region I of the map, the deformation is characterized by high ductility and random distribution of dislocations, while in region II, the deformation is characterized by low ductility and dislocation clusters. This map can be used to predict the deformation behavior of many alloys. The predicted strain rate range of high ductility associated with high strain rate sensitivity due to the solute-dislocation interaction coincides with the range of high temperature superplasticity of Al-Mg alloys, which suggests that the dynamic solute-dislocation interaction contributes to this superplasticity.

The Yield Anomaly in CoTi

ABSTRACTCompression tests performed on both stoichiometric and cobalt-rich CoTi over a range of temperatures show a positive temperature dependence of the yield stress with increasing temperature, before a decline occurs at high temperatures. In the region of the peak yield stress, serrated yielding and a negative rate sensitivity of the yield stress were observed. Static strain-aging also occurs. These observations are consistent with strong solute-dislocation interactions. Results from quenching experiments and strain rate change tests are presented, together with transmission electron microscope observations of the dislocation structures below, at, and slightly above the peak temperature. The results suggest that the yield anomaly in CoTi can be accounted for by a classical dynamic strain aging mechanism.

Theory of dislocation-solute atom interactions in solid solutions and related nonlinear anelasticity

A theory for dislocation-solute atom interactions in solid solutions has been developed which allows one to calculate the nonlinear dislocation strain-amplitude-dependent internal friction. The suggested model accounts for different modes of dislocation-solute atom interactions: (i) Solute atoms distributed in the dislocation glide plane interact with the dislocation core and represent short-range obstacles for the dislocation motion; (ii) Solute atoms situated away from the dislocation glide plane create relatively weak long-range elastic stress fields, also impeding dislocation motion. We assume that dislocations move in a two-component system of obstacles which differ with respect to the thermodynamics of dislocation--point-defect interactions. Namely, dislocations overcome short-range obstacles under the combined action of applied stress and thermal energy, whereas relatively weak long-range obstacles are surmounted athermally. The model predicts a complicated multistage behavior of the nonlinear internal friction in the strain amplitude--temperature--solute concentration domain, which is in excellent agreement with recent experimental data.

Origin of the initial rapid age hardening in an Al-1.7 at.% Mg-1.1 at.% Cu alloy

The origin of the rapid hardening which occurs during the initial ageing stage of an Al-1.7 at.% Mg-1.1 at.% Cu alloy has been investigated by the threedimensional atom-probe (3DAP) and transmission electron microscopy techniques. Although the rapid hardening reaction occurs within 1 min at 150 degrees C, no evidence for solute clusters or Guinier-Preston-Bagaryatsky (GPB) zones was detected using the 3DAP after this ageing time. When a short-term aged specimen is deformed and then aged at 150 degrees C, further rapid hardening occurs. This suggests that the rapid hardening is associated with a solutedislocation interaction. Uniform dispersions of Cu-Mg co-clusters do not occur until closer to the end of the hardness plateau and it is thought that these evolve into GPB zones during the second stage of the age-hardening process.

Influence of solute-dislocation interaction on the superplastic behavior and ductility of Al-Mg alloys

In the automotive industry, there is considerable interest in replacing steel with aluminum in autobody sheet-metal parts to improve fuel efficiency and environmental requirements. Some important issues associated with the use of aluminum in transportation include cost, formability, corrosion resistance and strength. The present investigator suggested that the dislocation substructure, ductility and activation parameters are strongly dependent on the solute-dislocation interaction at intermediate temperatures in many alloys including Al-Mg alloys. The basic mechanism for solute-dislocation interaction may not change with temperature in an alloy although its effect and significance may vary with temperature. In this study, the solute-dislocation interaction model of the present investigators was applied to the high temperature region where the superplasticity is observed to examine the possible effect of solution strengthening on the superplastic behavior of Al-Mg alloys.

Elevated temperature hardening of INCONEL 690

The strain hardening response of INCONEL 690 during tensile deformation between 200–1200°C has been examined. At low temperatures, ⩽600°C, two stages of hardening, associated with dislocation–dislocation and dislocation–solute interactions were observed. Increasing temperatures to 700°C resulted in the introduction of a third hardening stage, transmission electron microscopy suggesting that this additional hardening stage arises because of a transition in the dislocation arrangement from random tangles to a cellular substructure. Finally, from 750°C to 1200°C a single hardening stage was observed, transmission microscopy showing this to be associated with a cellular/sub-grain dislocation substructure. Analysis of these results indicates that the third stage of hardening in INCONEL 690 at temperatures between 600°C and 700°C and single stage hardening between 750°C and 1200°C can be described by the Bodner–Partom (B–P) and/or Kocks–Mecking (K–M) dislocation–dislocation interaction model. Furthermore, between 750°C and 950°C the hardening behavior can be described by the modified K–M dislocation–constant periodic barrier size interaction model. At higher temperatures, >950°C, the modified K–M model may be utilized through inclusion of an experimentally determined strain dependent barrier spacing.

Stress-Drop Rates in Serrated Flow of Aluminium Alloys

Al–Cu–Li alloys are known to exhibit serrated flow behavior, a typical characteristic feature of the dynamic strain aging and the Portevin–Le Chatelier (PLC) effect. In present study, the effect of microstructure and texture on serrated tensile flow behavior in a thick plate of Al–Cu–Li–Mg alloy was investigated. The microstructure consisted of coarse elongated grains with significant texture variation from a {100} <110> major component at surface to {110} <112> major component at the center layer. Samples from different locations across thickness and orientations were subjected to monotonic tensile loading at different strain rates in solutionized as well as under aged temper conditions. All specimen showed serrations in stress-strain curves, but a decreased serrated intensity from surface to center of the plate. Detailed microstructural examination and texture analysis suggested that the solute-dislocation interaction and shearing of the δ' precipitates could be the possible reasons for the PLC effect in this alloy. The through-thickness anisotropy in strength and serrated flow behavior of the alloy are attributed to the grain morphology and the texture variation in the material.

Dynamic strain aging and plastic instabilities

A constitutive model proposed by McCormick [(1988) Theory of flow localization due to dynamic strain ageing. Acta. Metall. 36, 3061–3067] based on dislocation-solute interaction and describing dynamic strain aging behavior, is analyzed for the simple loading case of uniaxial tension. The model is rate dependent and includes a time-varying state variable, representing the local concentration of the impurity atoms at dislocations. Stability of the system and its post-instability behavior are considered. The methods used include analytical and numerical stability and bifurcation analysis with a numerical continuation technique. Yield point behavior and serrated yielding are found to result for well defined intervals of temperature and strain rate. Serrated yielding emerges as a branch of periodic solutions of the relaxation oscillation type, similar to frictional stick-slip. The distinction between the temporal and spatial (loss of homogeneity of strain) instability is emphasized. It is found that a critical machine stiffness exists above which a purely temporal instability cannot occur. The results are compared to the available experimental data.