Dislocation-dislocation Interactions Research Articles

Effect of strain amplitude on the low cycle fatigue behavior and deformation mechanisms in alloy SU-263 at elevated temperature

The past studies in the low cycle fatigue (LCF) behavior of a γ′ strengthened nickel-based superalloy SU-263 were restricted to a maximum temperature of 923 K, though their service conditions far exceed this limit. In the present work, the LCF behavior of SU-263 is investigated at 1023 K with strain amplitude varying from ± 0.3% to ± 0.8% at 1023 K. During fatigue, the alloy displayed initial cyclic hardening followed by softening. The fatigue life decreased drastically with increasing strain amplitude. The alloy depicted gradual initial hardening at low strain amplitudes and sharp initial hardening at higher strain amplitudes, along with extensive softening until fracture. Significant increases in slip band density, stacking faults, dislocation network formation, and dislocation-dislocation and dislocation-precipitate interactions were identified as the deformation mechanisms responsible for cyclic hardening. In contrast, dislocation annihilation and shearing of γ′ precipitates were found to control cyclic softening. Dislocation-precipitate interactions were associated with looping at low strain amplitudes, while precipitate shearing occurred at high strain amplitudes. The alloy exhibited a tendency bilinear strain-life behavior in the Coffin-Manson plot with the corresponding shift in the fracture mode at ± 0.5% strain amplitude. At strain amplitudes below ± 0.5%, the alloy showed a mixed mode of failure, predominantly transgranular in nature, while at high strain amplitudes, the failure becomes predominantly intergranular.

Mechanism of microstructure and texture evolution during hot rolling of pure Mg and Mg-0.2%Ce alloy

ABSTRACT The extruded samples of pure Mg and Mg-0.2%Ce alloy were subjected to a 90% reduction in thickness at 250°C during hot rolling. The microstructure, texture, and mechanical properties of the hot-rolled samples were evaluated. The microstructural evolution shows the presence of shear bands in Mg-0.2%Ce alloy at an angle of 30-35° concerning the normal direction and no such bands are observed in pure Mg. Upon annealing the samples at 400°C for one hour, it is observed that the grain size of Mg-0.2%Ce is smaller than pure Mg indicating that the precipitates of Ce arrest grain boundary mobility. The ex-situ EBSD microstructure analysis results show a higher change in the value of α33 components after annealing for pure Mg compared to Mg-0.2%Ce alloy suggesting the precipitates of Mg12Ce stabilises the pyr. <c + a > dislocations. The basal texture is observed in both samples after hot rolling. The experimental texture was simulated using the visco-plastic self-consistent (VPSC) modelling where it is observed that the initial extruded texture is favourable for the higher activity of pyr. <c + a > dislocation and extension twinning in Mg-0.2% e alloy. Further, the yield locus shows that Mg-0.2%Ce alloy shows relatively lower anisotropy compared to pure Mg. The yield strength and UTS of the pure Mg are higher than the Mg-0.2%Ce alloy which is attributed to the relatively more dislocation-dislocation interaction in pure Mg compared to Mg-0.2%Ce alloy resulting in the lower mean free path of the dislocations in pure Mg.

Weak strain-rate sensitivity of hardness in the VCoNi equi-atomic medium entropy alloy

In this study, high strain rate nanoindentation was utilized to elucidate the dynamic behavior and the underlying deformation mechanisms of the equi-atomic vanadium-cobalt-nickel (VCoNi) medium entropy alloy. The hardness at high strain rate (average indentation strain rate: ∼11,000 s−1) exceeded that at low strain rate (constant indentation strain rate: 0.01 s−1) by only ∼3.5 %. Planar slip was identified as the predominant deformation mechanism at both strain rates, with no evidence of deformation twinning or phase transformation. The low strain rate sensitivity of VCoNi can be attributed to its lower lattice friction relative to that of conventional BCC metals and limited dislocation-dislocation interaction compared to that in typical FCC metals. The low strain rate sensitivity observed in VCoNi may also extend to other intermediate stacking fault energy alloy systems in which planar slip is prevalent.

Achieving good ductility in 2.1 GPa grade maraging steel

Maraging steels with ultrahigh strength are important for structural applications, but their low ductility severely limits engineering applications. Here we demonstrate improved ductility can be achieved in 0.18C10Ni8Co3Cr1.6Al1Mo1W maraging steel. This steel exhibits a high yield strength of 1.75 GPa, an ultrahigh tensile strength of 2.09 GPa and a uniform elongation of 5 %. Such good strength-ductility synergy arises from a unique microstructure: a refined hierarchical martensite matrix with dense coherent B2-nanoprecipitates. Strengthening models indicate that the ultrahigh strength can be attributed to block boundary strengthening, dislocation strengthening and precipitation strengthening. Utilizing transmission electron microscope coupled with geometric phase analysis at interrupted deformation stages, we reveal dynamic dislocation-nanoprecipitate and dislocation-dislocation interactions. Notably, mobile dislocations not only cut through nanoprecipitates but also penetrate pre-existing dislocation substructures and lath boundaries. Such deformation behaviors promote dislocation multiplication and accumulation, thereby conferring good strain hardening and ductility.

Cost-effective synthesis and thermomechanical processing of AlxCoCrFeNiCuy (x&y=0.5,1) high-entropy alloys

This study investigates the cost-effective synthesis and thermomechanical processing of AlxCoCrFeNiCuy (x&y = 0.5,1) high-entropy alloys through cold (CR) and hot rolling (HR). The alloys were synthesized using a thermochemical modelling-assisted aluminothermic non-centrifugal, self-propagating high-temperature synthesis (SHS) method, which is reported for the first time in the literature. Remelting and suction casting were conducted via vacuum arc melting upon the compositional verification through x-ray fluorescence (XRF) and energy dispersive spectroscopy (EDS) analyses. The deformabilities and hardness values of the alloys were correlated with microstructural changes that were analysed through scanning and transmission electron microscopy (SEM, TEM). Among the master alloys, FCC-A1 based Al0.5CoCrFeNiCu0.5 (Al0.5Cu0.5) and Al0.5CoCrFeNiCu (Al0.5Cu1.0) alloys exhibited highest cold deformability (145% and 170%, respectively), accompanied by significant work hardening (114% and 108% increase in hardness, respectively), primarily attributed to dislocation-dislocation interactions. It has been demonstrated for the first time that equiaxed single phase (FCC-A1) Al0.5CoCrFeNiCu0.5 high-entropy alloy can be obtained through the CR + annealing. The alloys exhibited reduced hot formability (62% and 85%, respectively) partly due to the formation of Cr-rich sigma phase instead of the Al-Ni rich B2 phase, most notable in the Al0.5Cu0.5 alloy, contrary to the CALPHAD simulations. This led to increased hardness in the alloys through precipitation hardening, achieving similar hardness values as those obtained through cold rolling (CR) with less deformation. The findings support the conclusion that the presented integrated processing strategy offers a cost-efficient means of producing Al0.5CoCrFeNiCuy alloys with enhanced mechanical and chemical properties for various engineering applications by the selection of cold and hot rolling routes.

Confluent effects of initial dislocation microstructures on yield behavior in single-crystal tungsten at high temperature

Though it is well known that initial dislocation microstructures play an important role in the deformation behavior of materials, how dislocation microstructures affect the macroscopic properties is still subjected to intensive research due to their complexity. In this work, we use discrete dislocation dynamics (DDD) to understand the effect of initial dislocation microstructures on the dynamic yield behavior of single-crystal tungsten above the critical temperature (800 K). DDD results suggest that the dislocation source length and the dislocation density are responsible for the initial yield stress and the flow stress of tungsten under dynamic loading, respectively. As and increase, the plastic yield mechanism transforms from dislocation-source activation into dislocation-dislocation interactions, resulting in the initial yield stress decreasing and the flow stress increasing. The confluent effects of and on the steady-state flow stress can be unified by a linear relationship between and Our results could be a valuable piece that connects dynamic yield behavior to the initial dislocation microstructures.

Integrated experimental and modeling approach for hot deformation behavior of Co–Cr–Fe–Ni–V high entropy alloy

The study aims to investigate the hot deformation behavior of Co–Cr–Fe–Ni–V high entropy alloy (HEA) at temperatures ranging from 1073 K to 1373 K and strain rates of 0.001, 0.01, 1, and 10 s−1, and to generate processing maps using dynamic materials modeling (DMM) to identify the optimum processing domain for industrial applications. The material's hardening and softening characteristics are also explored under various hot working conditions. Deformation twinning is observed in materials deformed at 0.1 s−1 at 1273 K and 1373 K, contributing to their observed hardening. The mean free path of dislocation defines the material's strength, and the transition point from dynamic recovery to dislocation-dislocation or dislocation-solute interaction occurs when the mean free path of dislocation reaches its lowest value. The inhomogeneity in the deformed sample is correlated with the strain field distribution using an integrated approach using finite element method (FEM) modeling and electron backscattered diffraction (EBSD) results. EBSD characterization reveals the presence of deformation bands and annealing twins at low and high temperatures, respectively. Additionally, an artificial neural network (ANN) model is proposed to predict the hot deformation behavior of Co–Cr–Fe–Ni–V HEA, with promising results, as evidenced by a correlation coefficient (R) of 0.9983 and an average absolute relative error (AARE) of 2.71% on the test dataset.

Influence of phase morphology, static recrystallization, and crystallographic texture on room temperature tensile properties of Ti–6Al–4V alloy: Comparison between post-tested equiaxed, bimodal, and lamellar microstructures

The influence of phase fractions, morphology, recrystallization, and crystallographic texture on the room temperature uniaxial tensile properties were investigated in a Ti–6Al–4V (Ti-64) alloy. The hot rolled and annealed Ti-64 alloy was heat treated to generate three different microstructures (equiaxed, bimodal, and lamellar) with varying volume fractions and dimensions of constituent phases (equiaxed primary α, transformed α laths, and β laths). The tested tensile specimens were characterized using techniques such as electron backscattered diffraction (EBSD), transmission electron microscopy (TEM), and X-Ray diffraction-based bulk texture analysis to determine the deformation mechanism of the alloy. Among three different microstructures, the equiaxed microstructure (α/β-900/950-FC) exhibited a superior strength-toughness combination, while the lamellar microstructure (β-1050-AC) displayed poor tensile properties. The bimodal microstructure (α/β-900/950-AC) showed intermediate tensile properties. Recrystallized α grains and their connectivity promoted an extended steady-state flow during deformation and ductile fracture. The bimodal microstructure work hardened faster as compared to the other two microstructures. TEM investigation on the post-tested microstructures showed that events such as dislocation multiplication and dislocation-dislocation interactions, formation of slip bands across the α grains, multiple slip and dislocation pile-ups contribute to hardening mechanisms during deformation. Whereas, slip transfer across α/α boundary and breaking of β laths are the events of softening mechanisms. The slip transfer across two adjacent α grains happens when the misalignment between grains is less than 10°. Equiaxed microstructures exhibited maximum slip transfer due to interconnected primary α grains. Bimodal microstructures (α/β-900/950-AC) exhibited slip transfer when a higher fraction of interconnected primary α is present in the microstructure. In contrast, the lamellar microstructure showed no evidence of slip transfer. The prism slip is the dominant active slip system in the equiaxed and bimodal microstructures. However, the lamellar microstructure with no primary α showed only the prismatic <c+a> and pyramidal <a> slip activities.

A comparative study on the microstructural features and mechanical properties of an Mg–Zn alloy processed by ECAP and SSE

The influence of the equal channel angular pressing (ECAP) and simple shear extrusion (SSE) processes on the microstructural and textural developments, as well as mechanical properties of Mg–4Zn alloy was studied. Application of 1, 2, and 4 passes of ECAP and SSE resulted in remarkable grain refinement of the initial microstructure of extruded material. Due to the greater shear strain and finer second phase particles, acting as obstacles to grain growth, smaller grain sizes were developed in the SSEed samples compared to the ECAPed counterparts. The fiber texture component of the extruded alloy was gradually transformed to a shear texture component after 4 ECAP passes, while 1, 2, and 4 passes of SSE produced a semi-fiber texture, in which basal planes were parallel to the longitudinal axis of the processed sample. Due to the applied reversal shear strain to the sample inside the SSE die, the volume fraction of the dynamically recrystallized (DRXed) regions was lower than that in the ECAP process which exerted a one-step shear strain to the sample. Shear punch test (SPT) results indicated that both ECAP and SSE improved the strength of the extruded alloy due to the grain boundary strengthening, texture hardening, and dislocation-dislocation interaction mechanisms. The strength of the SSEed samples was greater than ECAPed specimens, due to the finer grain size, higher volume fraction of un-DRXed grains, and lower Schmid factor of the semi-fiber texture component of the former.

Nonlinear thermo-elastic analysis of edge dislocations with Internal Heat Generation in Semiconductor Materials

In this work, the nonlinear thermo-elastic analysis of edge dislocations with internal heat generation is performed by the extended finite element method (XFEM) in semiconductor materials. The nonlinearity due to the temperature dependent electrical conductivity, thermal conductivity and thermal expansion coefficient is examined. The presence of an electric field in the direction of the dislocation line results in internal heat generation due to the electrical resistivity. The continuous Volterra model of dislocation is used for XFEM analysis of dislocations. The influence of Joule heat on thermal and elastic fields is observed. Different temperatures and electric field intensity values are taken for understanding the dislocation behavior. The Peach-Koehler (P–K) force for different dislocation configurations such as dislocation-dislocation interaction, dislocation-free surface interaction and dislocation-material interface interaction is evaluated. The results are found quite different with the amount of heat generation. The P–K force values are found more than that of pure elastic case. With the knowledge of the movability function, the obtained results can be used to predict the velocity of the dislocations.

Effects of nanosized precipitates on the Portevin-Le Chatelier behavior: Model prediction and experimental verification

The influence of homogenously-distributed nanosized Al3(Sc, Zr) precipitates on the Portevin-Le Chatelier (PLC) behavior in a model AlMgScZr alloy was investigated by experimental characterization and theoretical modeling. A remarkable advantage of the selected alloy is that the solute Mg concentration remains unchanged during the precipitation of Al3(Sc, Zr) nanoparticles, which effectively rules out the influence of solute concentration change on PLC. Transmission electron microscopy (TEM) and small angle neutron scattering (SANS) were used for quantitative characterization of precipitates and evaluation of key modeling parameters. By comparing the results for AlMg (without precipitates) and AlMgScZr (with precipitates) the effects of precipitates on the PLC phenomenon were clarified. It was found that the precipitates decrease the critical strain for the occurrence of PLC. Further, by evaluating dislocation densities of both alloys via synchrotron X-ray diffraction (XRD), the influence of precipitates on PLC was clarified by revealing the different roles of solute atoms and precipitates played in the dislocation-dislocation interaction. A model was proposed to account for these phenomena.

Static and dynamic Hall‒Petch relations in {332} TWIP Ti–15Mo alloy

This study examined the effect of grain size on the tensile properties in {332}<113> twinning-induced plasticity (TWIP) Ti–15Mo alloy and Ti–15Mo–1Fe alloy deformed by dislocation slip for comparison. At the yield point, a large Hall-Petch coefficient was observed for {332}<113> twinning, which was not only due to the increased stress concentration from the interaction between the activated twins before the macroscopic yield and the grain boundary, but also a larger twin volume for macroscopic yield in the coarse-grained Ti–15Mo alloy. It gradually decreased with increasing strain, and negative values were obtained at strains above 0.23. The dynamic Hall-Petch relation was established by the effective grain size, which was similar at strains of up to 0.20 in Ti–15Mo alloy with static grain sizes of 29 μm and 108 μm. The decreasingly modified Hall-Petch coefficient was resulted from the successive occurrence of twinning activation, twinning suppression, and the twin-dislocation and dislocation-dislocation interactions.

Mechanism of microstructure and texture evolution during shear loading of AA6063 alloys

The cyclic shear test of 6063 aluminum alloys under T4 and T6 conditions was carried out at room temperature. The microstructure and texture evolution were investigated using electron backscattered diffraction (EBSD) at several instances during cyclic shear deformation. Hardly any change in grain size was observed during the cyclic shear test of T4 and T6 samples. The local residual strain distribution measured in the form of average kernel misorientations (KAM) values increases continuously with an increase in shear strain for both samples. The increase in KAM value was higher for the T6 sample compared to the T4 sample during forward (F) shearing. On reversing the loading path, the increase in KAM value was higher for forward + reverse (FR) and forward + reverse + forward (FRF) straining of the T4 sample compared to the T6 sample. The cube texture was found to be unstable during shear deformation. A significant rotation of texture was observed in both T4 and T6 samples. However, the extent of rotation was slightly greater for the T6 sample as compared to the T4 sample. It was observed that the latent hardening of (1-11)<101>was more than (111)<1-01>in the T4 sample. Similarly, the latent hardening of (1-1-1)<011>was more than the (111)<1-01>in the T6 sample. The origin of latent hardening was attributed to the formation of sessile jogs during dislocation-dislocation interactions on different planes of T4 samples and the difference in volume fraction of precipitates on different planes of T6 samples. The mechanical properties show that the 0.2% YS increases with subsequent reverse and forward shearing for the T4 sample. However, the 0.2% YS during subsequent reverse shearing is lower than the initial forward shearing for the T6 sample. This decrease in yield stress upon loading reversal is attributed to the presence of deformation-induced intragranular back stresses within the T6 material. It is observed that the rate of cross slip of dislocations depends on the athermal dislocation content, presence of dislocation debris in the unfavorable grains, and the difference in the latent hardening values of different slip systems for the T4 and T6 materials.

(Invited) Recent Advances in the Modeling of ZnSySe1-y / GaAs (001) Heterostructures with Application to Dislocation Sidewall Gettering

In this talk we will describe state-of-the-art approaches to the modeling of strain relaxation and dislocations in ZnSySe1-y/GaAs (001) heterostructures, with applications to dislocation sidewall gettering (DSG) in devices. Current modeling approaches are based on the extension of the original Dodson and Tsao plastic flow model [B. W. Dodson and J. Y. Tsao, Appl. Phys. Lett., 51, 1325 (1987); Appl. Phys. Lett., 52, 852 (1988)] to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important recent breakthroughs have greatly enhanced the utility of these modeling approaches in three respects: i) pinning interactions have been included in graded and multilayered structures, providing a better description of the rate of strain relaxation as well as the limiting strain; ii) a refined model describing the interaction length for dislocation-dislocation interactions was formulated to include jogging in compositionally-graded and step-graded layers; and iii) inclusion of back-and-forth weaving of dislocations provides a more accurate description of heterostructures containing strain reversals, such as strained-layer superlattices or overshoot graded layers. We will describe these three key advances and use reasonable estimates of the kinetic parameters for ZnSySe1-y to explain and illustrate practical features of dislocation sidewall gettering in II-VI heterostructures.

(Invited) Recent Advances in the Modeling of ZnSySe1-y / GaAs (001) Heterostructures with Application to Dislocation Sidewall Gettering

In this paper we describe state-of-the-art approaches to the modeling of strain relaxation and dislocations in ZnSSe/GaAs (001) heterostructures, with applications to dislocation sidewall gettering (DSG) in devices. Current modeling approaches are based on the extension of the original Dodson and Tsao plastic flow model [B. W. Dodson and J. Y. Tsao, Appl. Phys. Lett., 51, 1325 (1987); Appl. Phys. Lett., 52, 852 (1988)] to include compositional grading and multilayers, dislocation interactions, and differential thermal expansion. Important recent breakthroughs have greatly enhanced the utility of these modeling approaches in three respects: i) pinning interactions have been included in graded and multilayered structures, providing a better description of the rate of strain relaxation as well as the limiting value; ii) a refined model describing the interaction length for dislocation-dislocation interactions was formulated to include jogging in compositionally-graded and step-graded layers; and iii) inclusion of back-and-forth weaving of dislocations provides a more accurate description of heterostructures containing strain reversals, such as strained-layer superlattices or overshoot graded layers. We will describe these three key advances and use reasonable estimates of the kinetic parameters for ZnSSe to explain and illustrate practical features of dislocation sidewall gettering in II-VI heterostructures.

On the three-dimensional spatial correlations of curved dislocation systems

Coarse-grained descriptions of dislocation motion in crystalline metals inherently represent a loss of information regarding dislocation-dislocation interactions. In the present work, we consider a coarse-graining framework capable of re-capturing these interactions by means of the dislocation-dislocation correlation functions. The framework depends on a convolution length to define slip-system-specific dislocation densities. Following a statistical definition of this coarse-graining process, we define a spatial correlation function which will allow the arrangement of the discrete line system at two points—and thus the strength of their interactions at short range—to be recaptured into a mean field description of dislocation dynamics. Through a statistical homogeneity argument, we present a method of evaluating this correlation function from discrete dislocation dynamics simulations. Finally, results of this evaluation are shown in the form of the correlation of dislocation densities on the same slip-system. These correlation functions are seen to depend weakly on plastic strain, and in turn, the dislocation density, but are seen to depend strongly on the convolution length. Implications of these correlation functions in regard to continuum dislocation dynamics as well as future directions of investigation are also discussed.

Mechanisms of creep deformation in a rapidly solidified Al–Fe–V–Si alloy

Creep experiments on a rapidly solidified Al-8.5 wt% Fe-1.3 wt% V-1.7 wt% Si alloy (FVS0812) suggest that it exhibits similar or superior deformation resistance compared to other high temperature Al alloys. However, aging at 798 K for 200 h resulted in severe loss of creep strength. Stress reduction results show that FVS0812 exhibits similar creep characteristics to those of other dispersion-strengthened alloys under constant structure conditions following stress reductions, even though the creep mechanism is determined to be significantly different. The operational activation area analysis reveals that the rate-controlling obstacles to dislocation glide are dislocation-dislocation interactions. The role of the particles is to carry a portion of the applied load rather than causing dislocation detachment, local climb, or general climb. This result contradicts the validity of modeling the material using the Rösler-Arzt approach or relying on a threshold stress that is based on the increase in dislocation line length during local or general climb.

How the interface type manipulates the thermomechanical response of nanostructured metals: A case study on nickel

The presence of interfaces with nanoscale spacing significantly enhances the strength of materials, but also the rate controlling processes of plastic flow are subject to change. Due to the confined grain volumes, intragranular dislocation-dislocation interactions, the predominant processes at the micrometer scale, are replaced by emission of dislocations from and their subsequent accommodation at the interfaces. Both processes not only depend on the interfacial spacing, but also on the atomistic structure of the interface. Hence, a thorough understanding how these processes are affected by the interface structure is required to predict and improve the behavior of nanomaterials. The present study attempts to rationalize this effect by investigating the thermomechanical behavior of samples consisting of three different interfaces. Pure nickel samples with predominant fractions of low- and high-angle as well as twin boundaries with a similar average spacing around 150 nm are investigated using high temperature nanoindentation strain rate jump tests. Depending on the interface structure, hardness, strain rate sensitivity and apparent activation volumes evolve distinctively different with testing temperature. While in case of high-angle boundaries for all quantities a pronounced thermal dependence is found, the other two interface types behave almost athermal in the same temperature range. These differences can be rationalized based on the different interfacial diffusivity, affecting the predominant process of interfacial stress relaxation.

Additive manufacturing of isotropic-grained, high-strength and high-ductility copper alloys

This work reports additive manufacturing of nickel-aluminum-bronze alloys by electron beam powder bed fusion (EB-PBF), with a relatively homogeneous microstructure at the micro- to meso-scale and exhibiting an exceptional combination of tensile strength and ductility — well surpassing that of conventionally wrought counterparts. The attainment of both high yield and ultimate tensile strength are attributed to collective effects of uniformly distributed precipitates, near-equi-axed grains, and a high fraction of high-angle grain boundaries. High tensile ductility originates from a sustainable work-hardening ability via dislocation-dislocation and dislocation-precipitate interactions, enhanced by moderate twinning, stacking faults, and back-stress hardening. Significantly, the EB-PBF alloys exhibit near-isotropic tensile properties in directions parallel and vertical to the build direction due to high-density precipitates enabled by equi-axed grains.

Tensile creep properties of a CrMnFeCoNi high-entropy alloy

Tensile creep tests were performed on a CrMnFeCoNi high-entropy alloy at temperatures from 1023 K to 1173 K. A uniform stress exponent 3.7 ± 0.1 was found across all temperatures. The apparent activation energies of creep under various applied stresses were determined to be around 230 kJ/mol and decrease with increasing stress, indicating a stress-assisted, thermally activated behavior. Steady-state creep microstructures feature no subgrain formation and high dislocation density within grains. Based on our results, the creep rate of CrMnFeCoNi is believed to be controlled by both dislocation-dislocation interactions and dislocation-lattice interactions.