Dislocation Storage Rates Research Articles

Strain Hardening in Dilute Binary Al–Cu, Al–Zn, and Al–Mn Alloys: Experiment and Modeling

During thermo-mechanical processing, dissolved alloying elements have a huge impact on the microstructure evolution by influencing the overall dislocation storage rate. Especially, for non-heat treatable Al alloys, the effects of strain-hardening and solid solution strengthening are of significant practical interest. In the present work, a detailed study of the room temperature work-hardening behavior of binary Al–Cu, Al–Zn, and Al–Mn alloys with varying solute concentrations is carried out. Stress–strain curves at different strain rates are recorded and computationally analyzed by an advanced 3-Internal-Variables-Model (3IVM) approach for the dislocation density evolution. The initial strengthening rate is examined as a function of the solute concentration.

Effect of prefatigue deformation on the uniaxial tensile properties of a dilute Cu-Al alloy

To examine the influence of prefatigue on the uniaxial tensile properties of a Cu-3at%Al alloy with a high stacking fault energy (SFE), the prefatigued tests are carried out at different total strain amplitudes ranging from 1.0 × 10−3 to 5.0 × 10−3 up to the same accumulated plastic strain of 31, and uniaxially tensile tests are subsequently conducted on these prefatigued alloy specimens. With increasing strain amplitude, the induced fatigue dislocation structures change from elongated veins and planar stacking faults to dislocation walls and cells. For the Cu-3at%Al alloy specimen prefatigued at an intermediate Δεt/2 of 2.3 × 10−3, the induced microstructure consists of loose cells, walls and few ill-developed persistent slip bands, which allows for an increase in dislocation storage rate but nearly does not affect dislocation annihilation rate during tensile deformation, thus exhibiting a higher strain hardening capability; therefore, the tensile strength can be improved nearly without loss of uniform elongation compared to the unfatigued specimen. In a word, an appropriate low-cycle prefatigue treatment can cause, to some extent, a strengthening effect on the static mechanical properties of high SFE metals, but such a strengthening effect is not as significant as the case for low SFE metals.

Investigation of enhanced strength anisotropy in an extruded Mg–Al–Ca–Mn alloy at cryogenic temperature

This study systematically compared the microstructural evolution and mechanical behavior of the extruded AXM10304 alloy during tensile deformation along both the extrusion direction (ED) and transverse direction (TD) at cryogenic temperature (CT) and room temperature (RT). The results indicate that the variations in work hardening mechanisms contribute to the observed strength anisotropy in both the ED and TD samples. The yield strength discrepancy between different orientations is ∼88 MPa at RT and ∼179 MPa at CT. It is evident that the alloy exhibits more pronounced strength anisotropy during cryogenic deformation. It is noteworthy that in the CT-ED samples, the ratio of the critical resolved shear stress (CRSS) for pyramidal I and II to that of basal <a> dislocations is relatively minimal, facilitating the activation of more pyramidal dislocations. Cross-slip in CT-ED provides a relatively facile path for dislocation slip, resulting in material softening during straining. The CT-TD sample exhibits the highest work hardening capacity and a consistent dislocation storage rate. Twinning significantly contributes to the hardening observed in the TD samples. This study also illustrates that tailoring twinning morphology to a refined needle-like shape through temperature manipulation can further enhance the work hardening of Mg alloys.

Compressive properties of a severely deformed fine-grained Mg−6Gd−3Y alloy

An extruded Mg−6Gd−3Y alloy was processed by 6 passes of simple shear extrusion (SSE) at 553 K to refine the microstructure. The microstructural studies were performed through electron back-scattered diffraction (EBSD) and also transmission electron microscopy (TEM) to investigate the microstructural features which emerged after SSE. It was revealed the SSE-processed alloy had a duplex microstructure containing both fine dynamically recrystallized (DRX) grains and some large deformed grains. A relatively high fraction of low angle grain boundaries (LAGBs), induced within the un-DRXed deformed grains, was identified by EBSD results. TEM studies revealed the formation of round Mg5Gd-type nano-particles through dynamic precipitation only in the DRXed regions of microstructure in the SSE-processed alloy. Compressive mechanical properties and strain hardening behavior of the alloys were assessed by compression testing of both alloys at room temperature. SSE processing resulted in enhanced yield stress and reduced ductility. Both the extruded and SSE-processed alloys exhibited a three-stage strain hardening behavior with a much more pronounced stage II of hardening in the latter one. According to the mechanical properties results, there was a noticeable difference in the strain hardening rate and also dislocation storage rate of the extruded and SSE-processed alloys. This discernible strain hardening behavior of the two alloys was attributed to the formation of fine DRXed grains, dynamic precipitation of nano-sized Mg5Gd particles, and induction of LAGBs, which emerged after SSE.

Work hardening and softening behaviors of Mg-Zn-Gd-Ca alloy regulated by bimodal microstructure

The present work mainly focuses on the regulation of the bimodal microstructure (fine recrystallized grains and elongated un-recrystallized grains) on the work hardening and softening behaviors of Mg-6Zn-1Gd-0.3Ca alloy. Meanwhile, the grain refinement mechanism was discussed through EBSD. The results show the work hardening capacity of the alloy increases with improving the volume fraction of fine recrystallized grains, which is mainly due to the increase of dislocation storage rate caused by grain coarsening. The stress reduction (∆P1) value in the early stage of tensile deformation is the biggest for the alloy with fine recrystallized grains of ∼66.8 %, which is attributed to the greater stored energy can provide greater driving force for softeing effect. However, the stress reduction (∆Pi(i=2,3,4)) values of the alloys in the later stage of tensile deformation decrease monotonically with the increase of fine recrystallized grains amount. Moreover, the analysis shows that the fine recrystallized grains of Mg-6Zn-1Gd-0.3Ca alloy are mainly formed by dynamic recrystalliztion (DRX).

Kinematical barriers enhanced dislocation strengthening mechanisms in cold-worked austenitic steels

We experimentally investigated microstructure evolution and strengthening behaviors of three cold-worked austenitic steels that deform via dislocations, twinning and stacking faults. The development of stacking faults and twins acting as kinematical barriers for dislocation motion was found to be influenced by stacking fault energy and accumulated dislocations simultaneously, which in turn accelerate dislocation storage rate, strengthen the alloys, and develop high strain hardening rate. We thus proposed a nonadditive strengthening equation to rationalize experimental observation by combining the dislocation-based two-internal-variable model coupled with kinematical barriers.

Effect of heat treatment on strain hardening ability of selective laser melted precipitation-hardened GH4169 superalloy

The effect of heat treatment on microstructures and tensile properties of GH4169 superalloy fabricated by selective laser melting (SLM) was investigated. The results show that the strain hardening ability of specimens differs widely after the heat treatment. The solution/aging (SA) specimen exhibits superior strain hardening ability. Such excellent strain hardening ability of the SA specimen is mainly due to the evenly distributed shear-resistant needle-shaped δ phase within the specimen, which interacts with the dislocations to leave Orowan loops and leads to a significant increase in the dislocation storage rate during the plastic strain. The findings may provide a potential strategy to enhance the strain hardening ability and ductility of the SLM-fabricated metallic parts utilizing appropriate heat-treatment routines.

Effect of extrusion speed on microstructure, mechanical properties and work-hardening of multi-microalloying MgAlCoCrFeNi alloy

In this study, a novel multi-microalloying MgAlCoCrFeNi alloy was extruded at 300°C with different extrusion speeds (0.2, 1, and 2 mm/s). The microstructure evolution, tensile properties and work-hardening behavior of the alloy were studied. Results showed that increasing the extrusion speed could improve the homogeneity of the microstructure, transitioning from a bimodal microstructure (fine recrystallized grains and elongated un-recrystallized grains) at 0.2 and 1 mm/s to a fully recrystallized one at 2 mm/s. The lower extrusion speed could improve the YS of the alloy, which possesses a maximum YS of ~213 MPa at 0.2 mm/s, an increase of ~12% compared to the YS at 2 mm/s, which was mainly attributed to the fine grain strengthening and dislocations (residual dislocations) strengthening. Moreover, the work hardening capacity of the alloy increases with the extrusion speed. As the extrusion speed increases from 0.2 mm/s to 2 mm/s, the work hardening exponent n increased from 0.08 to 0.14, and the work hardening capability Hc increased from 0.31 to 0.52. The analysis results showed that the improvement of work hardening ability was mainly due to the increase of dislocation storage rate caused by grain coarsening.

Achieving superior combination of yield strength and ductility in Mg–Mn–Al alloys via ultrafine grain structure

Ternary magnesium-based alloys with an aluminum content of 0.5 wt.% and a varying manganese content of x = 1, 2, and 3 wt.% (Mg-xMn-0.5Al) were prepared and characterized. The study focused on the effect of Mn addition on the formation of ultrafine grains (UFGs, average grain size <1 μm) and the mechanical properties of the alloys. After extrusion at a relatively low temperature of 250 °C, the average grain sizes of Mg-2Mn-0.5Al and Mg-3Mn-0.5Al samples were successfully refined to values of 0.9 ± 0.39 μm and 0.8 ± 0.34 μm, respectively, of submicron scale. The Mg-3Mn-0.5Al sample simultaneously demonstrated superior yield stress (YS, 274 MPa) and fracture elongation (FE, 48.9%). The high YS was associated with the UFG structure, residual dislocations, and nanosized precipitates, while the high FE was attributed to the low dislocation storage rate and UFGs with random orientations. The formation of UFGs is mainly caused by a recrystallization process promoted by particle-stimulated nucleation. The grain growth is suppressed by dynamic precipitated α-Mn particles.

Strain hardening behavior and strengthening mechanism in Mg-rich Al–Mg binary alloys subjected to aging treatment

The aging hardening response and tensile strain hardening characteristics of cast Al–Mg alloys with high Mg contents (5–13 wt%) were investigated. Aging hardening became significant when the Mg content was greater than 10 wt%. As the Mg concentration increased in the solid solutionized matrix, the strain hardening rate increased. During aging heat treatments at 423 and 453 K, the strain hardening rate increased as the aging time increased and it increased more rapidly as the Mg content in the alloys increased. The effects of the Mg matrix concentration and the precipitates on the strain hardening characteristics were interpreted using the Kocks-Mecking model. For the solid solutionized alloys, as the Mg matrix concentration increased, the dislocation storage rate (k1) increased, whereas the recovery rate (k2) decreased. For the aged alloys, both k1 and k2 increased with increasing Mg content and aging time. The contributions of the Mg matrix concentration and precipitates to the k1 and k2 values in the aged alloys could be separated. The effect of the precipitates to the k1 and k2 values were more significant than the effect of Mg matrix concentration in the aged alloys. The increase in Mg matrix concentration promoted a transition in the Portevin–Le Chatelier (PLC) effect from type B to type C. After the aging treatment, the type B + C serrations occurred in the Al-10 wt%. Mg alloy due to significant decrease of Mg concentration in the matrix. The critical strain for the onset of PLC increased with increasing Mg matrix concentration and aging time. These results were explained in terms of the concentration of plastic-deformation-induced vacancies involving in the diffusion process of solutess, which may decrease as the Mg matrix concentration and the amount of precipitates increase. After aging, the contribution of solid solution strengthening to yield strength decreased, whereas the contribution of particle strengthening to yield strength greatly increased. The particle strengthening effect accounted for 56.8% of the yield strength in Al-13 wt% Mg alloy.

Effect of dislocation density-twin interactions on twin growth in AZ31 as revealed by explicit crystal plasticity finite element modeling

In this work, we employ the recently developed framework for the explicit modeling of discrete twin lamellae within a three-dimensional (3D) crystal plasticity finite element (CPFE) model to examine the effects of dislocation densities in the twin domain on twin thickening. Simulations are carried out for 1¯012101¯1 extension twins in a magnesium AZ31 alloy. The model for the twin lamellae accounts for the crystallographic twin-matrix orientation relationship and characteristic twin shear transformation strain. The calculations for the mechanical fields as a result of twinning consider that one of three types of twin-dislocation density interactions have occurred. One case assumes that the expanding twin retains in its domain the same dislocation density as the parent. The second one considers that twin expansion has lowered the dislocation density as the twin thickens, and the last one, the Basinski effect, assumes that when twin sweeps the region, the dislocation density incorporated in the twin domain is amplified. In the modeling approach, the twin is thickened according to a criterion that maintains the stress state in the vicinity of the grain at a pre-defined characteristic twin resistance. The calculations show that most of the averaged properties, such as the rate of dislocation storage in the entire twin grain, the twin growth rate, the stress field in the twinned grain and neighboring grains, and the slip activity in the parent matrix are not significantly altered by dislocation storage in the twin. The results indicate that, however, the slip activity in the twinned domain is affected. In particular, in the increased dislocation density case, the rate of dislocation density in the twin domain increases at low strains when the twin is first growing from 2% to 5% volume fraction. This initial boost in the dislocation density storage rate causes the newly expanded dislocation twin to contain more stored dislocations than the other cases for all strain levels. Another interesting difference concerns the preference for one or two twins for the same total twin volume fraction; for the increased dislocation twin or twin that retains the dislocation density as it grows, formation of two twins is favored. For a twin that removes dislocation density, only one twin is preferred. The results imply that in the case with reduced dislocation density leads to lower stored dislocations and dislocation storage rates, and lower pyramidal slip activity.

Mechanical properties optimization of a Cu–Be–Co–Ni alloy by precipitation design

The mechanical performance of precipitation hardening alloy depends much on the nature of precipitates. For the sake of properties optimization, various precipitation states were designed to clarify the correlation between the precipitation and mechanical properties of a Cu–Be–Co–Ni alloy. The characters of each precipitation state were quantitatively characterized using TEM. Then both tensile and tension–compression (Bauschinger) tests were conducted to investigate the isotropic and kinematic contributions to the work hardening. Based on the quantitative analysis results of the precipitates, a flow stress model with the integration of yield stress, isotropic and kinematic work hardening behavior was established. The model indicates that the strength and ductility are determined by two contradictory factors: the dislocation dynamic recovery rate and dislocation storage rate. Optimization of strength and ductility relies on the balance of these two contradictory factors. Large non-shearable precipitates will increase both the dynamic recovery and the storage rate while fine shearable precipitates will make them decrease simultaneously. It is suggested that the alloy containing the mixture of above two kinds of precipitates will achieve the optimization.

Influence of alloy composition, solidification rate and artificial aging on plastic deformation behaviour of Al–Si–Cu–Mg casting alloys

The plastic deformation behaviour of three Al–Si casting alloys was investigated using the Kocks–Mecking strain hardening theory. Three coarsenesses of the microstructure, two aging temperatures and a number of aging times were used. For Al–Si–Mg and Al–Si–Cu–Mg alloys, the dislocation storage rate decreases while the dislocation recovery rate increases with aging time during underaging, whereas the concentration of alloying elements in solid solution decreases. The storage rate reaches a minimum at the peak aged condition and increases at overaging. The storage and recovery rates of the Al–Si–Cu alloy increase with aging time in the underaged condition and start to decrease during overaging, which indicates that a mixture of shearable and non-shearable precipitates are present during underaging, whereas all precipitates become non-shearable on overaging.

Intrinsic size-controlled strain hardening behavior of nanolayered Cu/Zr micropillars

The strain hardening behavior of nanolayered Cu/Zr micropillars was studied using the microcompression technique, which revealed that both the strain hardening exponent and the strain hardening rate are controlled by the intrinsic rather than extrinsic size. The maximum strain hardening rate is observed at a layer thickness of ∼20 nm, which can be explained physically in terms of reduced dislocation storage rates. A dislocation model is modified to elucidate the dislocation storage in the nanolayered materials and predicts a critical intrinsic size below which the strain hardening rate drops off.

Modeling dislocation storage rates and mean free paths in face-centered cubic crystals

The first part of a dislocation-based constitutive formulation for strain hardening in face-centered cubic crystals is presented. This multiscale approach is based on the storage–recovery framework expanded at the scale of slip systems. A parameter-free formulation is established for the critical stress and the storage rate, taking advantage of recent results yielded by dislocation dynamics simulations. The storage rate of dislocations in the presence of forest obstacles is modeled for the first time at the level of dislocation intersections and reactions. The mean free path per slip system is found to be inversely proportional to the critical stress. It also depends on the number of active slip systems, which leads to an orientation dependence of stage II strain hardening in agreement with experimental data. The total storage rate is obtained by including three additional contributions, notably that of the self-interaction, which leads to a model for stage I hardening.