Variation Of Dislocation Density Research Articles

Phase-field modeling of stored-energy-driven grain growth with intra-granular variation in dislocation density

We present a phase-field (PF) model to simulate the microstructure evolution occurring in polycrystalline materials with a variation in the intra-granular dislocation density. The model accounts for two mechanisms that lead to the grain boundary migration: the driving force due to capillarity and that due to the stored energy arising from a spatially varying dislocation density. In addition to the order parameters that distinguish regions occupied by different grains, we introduce dislocation density fields that describe spatial variation of the dislocation density. We assume that the dislocation density decays as a function of the distance the grain boundary has migrated. To demonstrate and parameterize the model, we simulate microstructure evolution in two dimensions, for which the initial microstructure is based on real-time experimental data. Additionally, we applied the model to study the effect of a cyclic heat treatment (CHT) on the microstructure evolution. Specifically, we simulated stored-energy-driven grain growth during three thermal cycles, as well as grain growth without stored energy that serves as a baseline for comparison. We showed that the microstructure evolution proceeded much faster when the stored energy was considered. A non-self-similar evolution was observed in this case, while a nearly self-similar evolution was found when the microstructure evolution is driven solely by capillarity. These results suggest a possible mechanism for the initiation of abnormal grain growth during CHT. Finally, we demonstrate an integrated experimental-computational workflow that utilizes the experimental measurements to inform the PF model and its parameterization, which provides a foundation for the development of future simulation tools capable of quantitative prediction of microstructure evolution during non-isothermal heat treatment.

Thermal Mechanical Behavior and Microstructure Characteristics of As‐Cast FeCrNiAl Steel

To find the suitable deformation temperature range for the as‐cast Fe–5.5Cr–5.5Ni–3.0Al–0.27C high‐strength lightweight steel, the thermal mechanical behavior and the dependent microstructure characteristics are studied to investigate the high‐temperature mechanical response of the steel. In the results, it is shown that the microstructure of the steel owns a dendrite framework consisting of the martensite dendrite and the interdendritic austenite, among which precipitations of NiAl, Ni3Al, and δ‐Fe are induced by high‐temperature deformation. The flow stress dependence of deformation temperature shows two different variation tendencies, corresponding to the varied deformation mechanisms of strain‐induced precipitation of second phases and δ‐Fe nucleation, and then resulting in different variation of dislocation density. It is found that a periodical fluctuation of flow stress appears as the alternative occurrence of first rise and then drop followed by drop and rise, and the phenomenon is inferred to be caused by local dynamic recrystallization in the dendrites and the neighboring interdendritic spacings.

Anisotropy of microstructure, mechanical properties and thermal expansion in Invar 36 alloy fabricated via laser powder bed fusion

Invar 36 is a renowned iron-nickel alloy with an ultra-low coefficient of thermal expansion (CTE), which is widely used in aerospace and precision instruments. Laser powder bed fusion (LPBF) as a prevalent metal additive manufacturing technique could break through the limitations of traditional processes, such as low material utilization and difficulties in forming complex structures. In this work, Invar 36 alloy specimens were fabricated via LPBF process in 0° (H0 specimen), 45° (D45 specimen), and 90° (V90 specimen) build orientations. The microstructure, tensile properties, thermal expansion, and magnetic properties of the three orientations specimens were comprehensively investigated, and the anisotropy of microstructure, yield strength and CTE were analyzed. The V90 specimen exhibited the highest elongation (62.70%), while the D45 specimen owned the highest yield strength (496 MPa). The anisotropic mechanical properties can be attributed to variations in dislocation density, grain size, and intrinsic yield strength. The CTE of LPBF-fabricated Invar 36 alloy specimens were all lower than that of traditional manufacturing process at all temperature ranges. Since CTE is more related to magnetic properties, not sensitive to microstructure, the anisotropy of CTE is not significant in three build orientations. This study has significant implications to produce Invar 36 alloy devices with exceptional properties and 3D dimensional stability via the LPBF process.

Fracture Mechanism of Nanocomposite of Metal and Graphene with Defect Pores.

Molecular dynamics simulations were performed to investigate the fracture mechanism and mechanical response of Ni/Graphene nanocomposites under nanoindentation. The effects of size and location of defect pores were explored by examining the pore structure transition, microstructure transition, variation of HCP atomic fraction and dislocation density with indentation depth, load-displacement relationship, and stress distribution. It was found that when the long edges of the pore are located along the longer dimension, the pores are fractured by indentation forces from the short edges. The closer the pore is to the indent, the smaller loading force is required for the pores to reach its fracture limit. For the long edges located along the transverse direction, the maximum indentation depth increases with the distance of the pore away from the indenter. The density of HCP atoms and dislocations in the composite gradually increases with the indentation depth. To understand the physical mechanism of the fracture behavior, we also evaluated the stress distribution in graphene at the fracture point.

Enhanced strength-ductility synergy in high-entropy alloys via architecting three-level gradient hierarchical nanostructure

Gradient microstructural design represents a new strategy for enhancing strength-ductility synergy of high/medium entropy alloys (H/MEAs). However, most of the reported gradient H/MEAs have only one type of gradient microstructure, which has limited ability to realize the full potential of gradient structure. Here, we report a three-level gradient hierarchical nanostructure (GHN) in a Al0.3CoCrFeNi HEA by ultrasonic surface rolling processing (USRP) followed by aging treatment that can result in a six-fold enhancement in yield strength with almost no loss in ductility compared to the coarse structured counterpart. The proposed three-level GHN is characterized with gradient variations in grain size, nano-twin and dislocation density, B2 and σ precipitates content. Systematic experiments indicate that strong hetero-deformation-induced (HDI) hardening associated with the activation of deformation twinning, are responsible for this exceptional strength-ductility synergy. This three-level GHN engineering strategy in this work can also be feasibly applied to many other H/MEAs, and offers an available pathway to develop high strong and ductile metallic materials.

Influence of Dislocation Substructure on Size-Dependent Strength of High-Purity Aluminum Single-Crystal Micropillars

In order to understand the influence of dislocation substructures on the size-dependent strength (smaller is stronger) of micron-sized metals, we have fabricated single-crystal cylindrical micropillars with various diameters approximately ranging from 1 to 10 μm, which were prepared on the surface of the fully annealed sample and the subsequently cold-rolled samples of high-purity aluminum (Al). The annealed micropillars exhibited a size dependence of the resolved shear stress required for slip. The shear stress (τi) normalized by shear modulus (G) and the specimen diameter (d) normalized by Burgers vector (b) followed the correlation of τi/G=0.33(d/b)−0.63. The size-dependent strength was reduced by cold-rolling, resulting in lower power-law exponents (0.26~0.31) for the correlation in the cold-rolled specimens. The fine dislocation substructures introduced by the cold-rolling could be associated with the reduced size-dependent strength, which can be rationalized using the stochastic model of the dislocation source length in an assumption of homogenously distributed dislocations existing in the experimental micropillars. The inhomogeneous dislocation substructure with various dislocation cell sizes would contribute to a variation in the measured strength depending on the location, likely due to the probability of exiting the dislocation cell walls (local variation in dislocation density) in the micropillars fabricated on the cold-rolled Al samples.

Interfacial investigations on the microstructure evolution and thermomechanical behavior of explosive welded CLAM/316L composite

In this paper, a comprehensive study combined multiple characterizations and SPH numerical simulation was conducted to investigate the microstructure evolution, extreme thermomechanical behavior and consequences of the explosive welded CLAM/316L interface. The interfaces of the two welding materials underwent remarkable temperature rise and rapid cooling under high-speed collision conditions, forming a tight bond that presents a wavy structure with vortices. The SPH simulation adequately reproduced the heating and cooling process of the interface during explosive welding, revealing the high heterogeneity of the thermodynamic behavior during the impact process. Furthermore, the EBSD analyses showed the perplexing evolution of crystal structure at the bonding interface with vortex, including ultra-fine grains, columnar grains, equiaxed grains, deformed grains and serious lattice distortion. By correlating the temperature histories and grain characteristics at different positions of the interface, the formation mechanism of diverse grains was revealed, referring to crystallization from liquid, recrystallization from the solid and plastic deformation. The recrystallization at the interface was closely related to the material properties, and the significant variations in dislocation density and plastic strain values at the interface, as well as the thermal softening of the material, could be explained by different degrees of dynamic recovery and recrystallization.

Effects of void shape and location on the fracture and plastic deformation of Cu (crystalline) /Cu64Zr36 (amorphous) composites

Voids are probably the most widely studied type of manufacturing defects since they are often formed during material processing. In this paper, the fracture and plastic deformation of Cu/Cu64Zr36 crystalline/amorphous composites with different void locations and shapes are investigated by molecular dynamics simulations. The results show that changes of the void location and shape in the composites significantly change the dislocation motion, shear transformation zones (STZs) activation, shear band propagation, and fracture strength of the composites. The largest shear strain is always found around the void, where it is most likely to activate dislocation motion, STZs and shear band formation. A stepwise increase of the shear localization is mainly related to the nucleation and emission of dislocations, while the very rapid increase is caused by shear band formation. Moreover, the variation of dislocation density and activity of shear bands are mainly responsible for the stress fluctuations in the composites. This paper provides important information for understanding and improving the plastic and fracture behaviors of composites by changing the location and shape of void from an atomistic perspective.

Dislocation density transients and saturation in irradiated zirconium

Zirconium alloys are widely used as the fuel cladding material in pressurized water reactors, accumulating a significant population of defects and dislocations from exposure to neutrons. We present and interpret synchrotron microbeam X-ray diffraction measurements of proton-irradiated Zircaloy-4, where we identify a transient peak and the subsequent saturation of dislocation density as a function of exposure. This is explained by direct atomistic simulations showing that the observed variation of dislocation density as a function of dose is a natural result of the evolution of the dense defect and dislocation microstructure driven by the concurrent generation of defects and their subsequent stress-driven relaxation. In the dynamic equilibrium state of the material developing in the high dose limit, the defect content distribution of the population of dislocation loops, coexisting with the dislocation network, follows a power law with exponent α≈2.2. This corresponds to the power law exponent of β≈3.4 for the distribution of loops as a function of their diameter that compares favourably with the experimentally measured values of β in the range 3≤β≤4.

Substructure evolution in two phases based constitutive model for hot deformation of TC18 in α + β phase region

The α + β dual phase titanium alloys are key structural materials in aviation and aerospace industries, and the complicated flow behavior of these titanium alloys during hot deformation requires to establish a constitutive model incorporating physical mechanism for optimizing processing parameters and designing forming tools. This work aims to establish a constitutive model incorporating physical mechanism for hot deformation of TC18 in α + β phase region. Firstly, the flow behavior and microstructure evolution for hot deformation of TC18 in α + β phase region are characterized. The TC18 shows significant strain hardening rate and negative strain hardening exponent around and after peak flow stress, respectively. After peak flow stress, Dynamic Recovery (DRV) mechanism dominates the evolution of α and β phases according to the results of substructure evolution. Then, the internal state variables method is applied to establish a constitutive model incorporating physical mechanism for hot deformation of dual phase titanium alloys. The variation of dislocation density during the hot deformation of titanium alloys is modeled by considering the accumulation of dislocation due to the impediment to dislocation movement by substructure obstacles and the annihilation of dislocation due to the dynamic restoration effect. The interaction between dislocations, the subgrain boundaries and the grain/phase boundaries obstruct the dislocation movement in the α phase, and the first two obstructs the dislocation movement in the β phase during the hot deformation of TC18. The dislocation annihilation process in the α and β phases during the hot deformation of TC18 is dominated by DRV. Finally, the substructure evolution in the two phases based constitutive model for hot deformation of TC18 in α + β phase region is presented. This model is well applied to predict the flow stress and quantitively analyze the role of DRV effect in the evolution of α and β phases during the hot deformation of TC18.

Mechanical anisotropy and microstructural heterogeneity of a free-forged ME20 alloy with a large cross section

In this work, the mechanical anisotropy and microstructural heterogeneity of the large-sized Mg-1.5Mn-0.3Ce (wt.%, ME20) forging samples are thoroughly investigated. As-forged ME20 sample presents inhomogeneous microstructure with gradient variations of grain size, dislocation density, and texture intensity from the edge to the center positions due to the gradient accumulation of strain and temperature during forging. Despite of that, nearly the same tensile yield strength (TYS) was obtained from the edge to the center positions, which can be mostly ascribed to the combined effects of fine grain, sub-grain, precipitate, texture, and residual dislocation. In addition, as-forged ME20 sample shows an enormous disparity of TYS between the forging direction and lengthening direction, with the TYS difference of ∼200 MPa, which is primarily associated with the strong texture that dominates the activation of twinning/slipping modes. Therefore, the high level of mechanical anisotropy appears to be only sensitive to strong basal texture, whereas the heterogeneous microstructure does not induce obvious mechanical properties variation along gradient positions. These results can offer fundamental insight into further improving mechanical properties of Mg forging components with large cross-section.

Advanced finite element model for predicting surface integrity in high-speed turning of AA7075-T6 under dry and cryogenic conditions

The overall quality of a component and its mechanical performance and reliability during all its life cycle depend on several characteristics. The choice of the material plays an important role in satisfying the requirements and usually it makes the difference between failed and not failed products; however, another significant impact in the material performances is characterised by the manufacturing process. Nowadays, the numerical simulation represents an important tool to quickly predict several scenarios depending on process parameters’ changes, leading to a more flexible manufacturing process and time–cost saving due to the reduced experimental tests. This work presents an innovative finite element model (FEM) of high-speed turning of one of the most used aluminium alloys in aerospace field, namely the AA7075-T6. The developed simulation tool allowed to better investigate the metallurgical phenomena triggered by the varying process parameters tested and the cooling conditions used during the machining tests. A physics-based model to simulate the material behaviour has been developed and implemented via subroutine within the FEM software. The predictive capability of the model has been validated through experimental and numerical comparison of cutting forces, maximum temperature and grain size changes. The developed 3D FE model is capable of including the effects of different cooling conditions during machining of the aluminium alloy AA7075-T6 on the surface integrity (e.g. the microstructure and the dislocation density variation), whilst evaluating further important manufacturing process variables as cutting forces and maximum temperature.

Constitutive modeling of cellular-structured metals produced by additive manufacturing

Computational modeling is essential in the development and application of metal additive manufacturing (MAM). Most materials processed by laser or electron beam MAM exhibit a characteristic cellular structure, where the cell walls contain a higher dislocation density than the cell interior. This kind of structure is believed to be responsible for the enhanced properties of structural members produced by MAM. In this study, a constitutive description of MAM materials was developed to accurately simulate their mechanical response to loading. The modeling frame was given by the dislocation density evolution, with two distinctly different dislocation densities being considered: those in cell walls and cell interiors considered as two separate ‘phases’ of the material. By employing the constitutive model developed, numerical analyses were conducted for a broad range of MAM materials - from pure metals to alloys. A comparison of the numerical simulations with experimental data for Cu and Cu-Sn from literature demonstrated that the model provides an adequate description of its uniaxial tensile properties and the dislocation density variation. In particular, the influence of the dimensions of the cellular structure and the applied strain rate is accounted for faithfully. It is suggested that the model is applicable to a broader range of MAM-processed materials as well.

Experimental Study at the Phase Interface of a Single-Crystal Ni-Based Superalloy Using TEM.

The single-crystal Ni-based superalloys, which have excellent mechanical properties at high temperatures, are commonly used for turbine blades in a variety of aero engines and industrial gas turbines. Focusing on the phase interface of a second-generation single-crystal Ni-based superalloy, in-situ TEM observation was conducted at room temperature and high temperatures. Intensity ratio analysis was conducted for the measurement of two-phase interface width. The improved geometric phase analysis method, where the adaptive mask selection method is introduced, was used for the measurement of the strain field near the phase interface. The strained irregular transition region is consistent with the calculated interface width using intensity ratio analysis. An intensity ratio analysis and strain measurement near the interface can corroborate and complement each other, contributing to the interface structure evaluation. Using TEM in-situ heating and Fourier transform, the change of dislocation density in the γ phase near the two-phase interface of the single-crystal Ni-based superalloy was analyzed. The dislocation density decreases first with the increase in temperature, consistent with the characteristics of metal quenching, and increases sharply at 450 °C. The correlation between the variation of dislocation density at high temperatures and the intermediate temperature brittleness was also investigated.

The work hardening and softening behaviors of Mg–6Zn-1Gd-0.12Y alloy influenced by the VR/VD ratio

The effects of the VR/VD ratio (the volume fraction ratio of recrystallized grains and coarse deformed grains) on the work hardening and softening behaviors of Mg–6Zn-1Gd-0.12Y alloy were studied in detail by detecting the variation of dislocation density. The alloys with different VR/VD ratios were successfully fabricated by rolling and subsequently different annealing processes. It was shown that the higher VR/VD ratio closely related to the nucleation and growth of recrystallized grains could be obtained by an increase in annealing temperature. An increase in the VR/VD ratio from 0.16 to 24 reduced the work hardening rate in stage Ⅲ, while increased the work hardening rate in stage Ⅳ, which was closely related to the initial dislocation density and dislocation evolution. Low VR/VD ratio in the alloy, with high initial dislocation density, was identified to be beneficial for improving the work hardening rate in stage Ⅲ. And an increased VR/VD ratio could promote dislocation multiplication by stimulating the yield phenomenon due to the lack of initial dislocation density, and improve the work hardening rate in stage Ⅳ. With the VR/VD ratio increased, the softening behavior of the alloys increased in stage Ⅲ, and first decreased and then increased in stage Ⅳ. This was because the alloy with the lower dislocation density was easy to deform due to few obstacles, which accelerated the softening in stage Ⅲ, and improved dislocation density after yielding provided the strong driving force for the softening in stage Ⅳ. Meanwhile, the cycle stress relaxation tests further manifested that the stress drop values (Δσp) of the alloys were in close contact with the initial dislocation density and dislocation evolution influenced by the VR/VD ratio. As a result, when the VR/VD ratio was 1.6, the Mg–6Zn-1Gd-0.12Y alloy had excellent strength and toughness match owing to a dynamic balance between the work hardening rate and softening rate.

Investigation of structural, optical and magnetic properties of Y3-xCexFe5-yEryO12 compound

Herein, we have studied the structural, optical and magnetic properties of Y3-xCexFe5-yEryO12 (0.00 ≤ x ≤ 0.02), (0.00 ≤ y ≤ 0.06) compound synthesized via sol gel process. Structural characterization by X-ray diffraction (XRD) confirmed the YIG single phase formation, belong to the Ia3‾d cubic centrosymmetric crystal structure. The lattice constant, lattice strain, dislocation density and average crystallite size variations were calculated and discussed in terms of the rare earth's dopant concentrations. Fourier Transform Infrared Spectroscopy (FTIR) analysis showed redshift for the Fe–O stretching modes as the dopant content increase, testifying the crystal lattice expansion. Raman spectra inspection corroborated the YIG single phase formation and shifts in the vibrational modes confirmed the Ce3+ and Er3+ rare earths inclusion in the sites occupied by Y3+ and Fe3+ cations, respectively. The coral-like of YIG nanoparticles were confirmed by scanning (SEM) and transmission (TEM) microscopies, whereas the existence of the elements Y, Fe, O, Ce and Er, were verified by Energy Dispersive Spectroscopy analysis (EDS). The optical band gap (Eg) decreases, while Urbach energy (Eu) increases varying the Er content. The result confirms the effect of Ce3+ and Er3+ addition on the optical properties of YIG, contributing to localized oxygen vacancy defects formation. Using a phenomenological model, it was shown that, the highest probability of Er3+ cations occupation corresponds to octahedral sites. For the Y2.98Ce0.02Fe5-yEryO12 (0.00 ≤ y ≤ 0.06) compound, the theoretical saturation magnetization (Ms) was calculated from the cation distribution obtained by the phenomenological model, resulting a slight increase with the Er content. The result agrees with the experimental Ms measurements, although for y = 0.06 a discrepancy was detected, which can be attributed to the breaking of collinear array of magnetic moments due to high lattice distortions.

Influence of γ-radiation on the physical characteristics of thermally evaporated nanostructured CdS: Cl films

Thermally evaporated CdS: Cl films were subjected to gamma irradiation with an equivalent dose intensity of 4.11 kG/hour for extended periods (60 to 240) minutes. The resultant films were examined by different approaches to acquire structural, morphological and optical information. Upon gamma irradiation doses, the samples show variation in grain size, dislocation density, microstrain and texture coefficient. The XRD patterns exhibit the hexagonal crystal structure formation of CdS: Cl. FESEM images disclosed an extensive spherical nature of nanocrystallites. They demonstrated an increase in their size, while optical study reveals a decrease in bandgap (Eg) and an increase in Urbach energy (EU) due to gamma doses. Electrical studies display a reduction in resistivity upon irradiation doses of gamma. These tailored films can find application as window layers in optoelectronic devices.

Deformation mechanism of WC single crystals under nanoindentation: Effects of surface defects and orientation on pop-in and hysteresis

Nanoindentation was carried out on pure tungsten carbide (WC) on the basal (0001) and prismatic (1010) planes, using Berkovich and spherical indenters, in both single load and multi-load testing. The work focuses on correlating the load-displacement curves, including elastic to plastic deformation, size effect and hysteresis with the deformation behaviour of WC. With different specimen preparation processes, the elastic to plastic deformation started at different threshold loads: This observation was found to be due to the variation in surface dislocation density. Staircase deformation was observed thought to be caused by dislocation motion and the formation of slip bands; sudden displacement discontinuities in the load-displacement response - associated with dislocation loop nucleation - occurred at, or near the theoretical shear strength. Furthermore, discontinuities in load-displacement curves were also used to confirm that hysteresis loops were a result of plastic deformation, as they were absent when the loading was purely elastic.

Effects of cold-rolling and subsequent annealing on the nano-mechanical and creep behaviors of CrCoNi medium-entropy alloy

The influences of thermomechanical treatment on the microstructural evolution and nano-mechanical behaviors of CrCoNi medium-entropy alloy (MEA) were systematically studied through a series of nanoindentation experiments. Cold-rolling induced substantial grain refinement and during subsequent annealing, the CrCoNi MEA exhibited a highly temperature-dependent recrystallized microstructure characterized by significant variations in grain size, dislocation density, and twin fraction with increasing annealing temperature. The hardness of the cold-rolled sample increased with annealing temperature, reaching its highest value at 600 °C and then decreasing gradually. Annealing was also found to influence the creep behaviors of the CrCoNi MEAs. A cold-rolled sample annealed at 700 °C exhibited the highest resistance to creep. The results of a detailed microstructure examination, calculations, and theoretical analysis revealed that annealing twins play a key role in increasing both the strength and the creep resistance of the CrCoNi MEAs. Short-range order clusters also contributed to the strength and creep resistance.

Deformation Mechanism of WC Single Crystals Under Nanoindentation: Effects of Surface Defects and Orientation on Pop-In and Hysteresis

Nanoindentation was carried out on pure tungsten carbide (WC) on the basal (0001) and prismatic (1010) planes, using Berkovich and spherical indenters, in both single load and multi-load testing. The work focuses on correlating the load-displacement curves, including elastic to plastic deformation, size effect and hysteresis with the deformation behaviour of WC. With different specimen preparation processes, the elastic to plastic deformation started at different threshold loads: This observation was found to be due to the variation in surface dislocation density. Staircase deformation was observed thought to be caused by dislocation motion and the formation of slip bands; sudden displacement discontinuities in the load-displacement response - associated with dislocation loop nucleation - occurred at, or near the theoretical shear strength. Furthermore, discontinuities in load-displacement curves were also used to confirm that hysteresis loops were a result of plastic deformation, as they when the loading was purely elastic.