Dislocation Substructure Evolution Research Articles

Stress sensitivity and its influence on high-temperature creep behavior of a novel 2nd-generation low-cost nickel based single crystal superalloy

To meet the application requirements under complex service conditions for aero engine blades, the influence of stress-state on creep behavior of a self-designed Re-low Ni-based single crystal superalloy at 1038 °C over a wide stress range of 137–207 MPa was investigated. With applied stress decreasing from 172 to 137 MPa, the creep life rapidly extends by a factor of 2.41, showing strong stress dependence. Further, the creep mechanisms under different stresses are discussed based on microstructural characterization and threshold stress calculation results. With the increasing applied stress, interfacial dislocation networks exhibit sparse and irregular shapes. Particularly, in necking zone of the fractured specimen at 1038 °C/137 MPa, due to the long-duration accumulation of deformation storage energy, dislocation arrays within γ′ rafts gradually evolve into dislocation walls through recombination and annihilation of dislocations, then develop into subgrain boundaries. Finally, some recrystallization grains with large misorientations (>10°) form by subgrain-rotation. Moreover, based on the calculated threshold stress, dislocations climbing can play a major role in creep deformation throughout the steady-state stage under a stress range of 137–207 MPa. Nevertheless, with the arrival of tertiary stage, the increasing actual stress resulted from necking deformation can also promote dislocations to cut or bypass γ′ rafts under lower stresses (137 and 172 MPa), which is consistent with the final evolution of dislocation substructures. Overall, the findings from this work are helpful to understand the high-temperature creep mechanism of low-cost single crystal superalloys, so as to improve the creep resistance of materials.

Microstructural origins of enhanced work hardening and ductility in laser powder-bed fusion 3D-printed AlCoCrFeNi2.1 eutectic high-entropy alloys

Limited tensile ductility usually restricts the practical applications of new classes of high-strength materials in many industrial fields. Therefore, in-depth understanding of the work hardening behavior and its underlying plastic deformation mechanism are critical for the newly developed high-entropy alloys (HEAs). In this work, a geometric atomistic model of face-centered cubic (FCC)/ordered body-centered cubic (BCC (B2)) interfaces and the evolution of dislocation substructures have been investigated to explore the microstructural origins of work hardening responses for two additively manufactured AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs) with the respective lamellar and cellular microstructures. Unlike the lamellar interphase interfaces with the most classical Kurdjumov-Sachs (KS) FCC-BCC relationship of {111}FCC∥{110}B2〈011〉FCC∥〈111〉B2, the Nishiyama-Wassermann (NW) relationship, namely {111}FCC∥{110}B2〈011〉FCC∥〈001〉B2, is observed to be dominant on the cellular interphase interfaces. Furthermore, our intermittent high-resolution transmission electron microscopy (HR-TEM) results directly show that the deformation of lamellar AlCoCrFeNi2.1 alloy first proceeds with massive stacking faults (SFs) and then dislocation walls developed across phases interfaces, due to the effective dislocation transfer capability of lamellar boundaries. The large uniform elongation of the cellular AlCoCrFeNi2.1 alloy is attributed to the stable and progressive strain-hardening mechanism that is stemmed from the activated multiple slip systems, deformation-induced SF networks, and the associated Lomer-Cottrell locks in the middle and later stages of plastic deformation. Moreover, the nano-bridging of FCC cells in the 3D-printed microstructure provides unique channels for dislocation movement, which offsets the “blocking effect” of cellular boundaries and thus suppresses the pre-mature fracture.

Microstructural evolution and spheroidization mechanism of powder metallurgy Ti-6Al-4 V alloys after high-temperature forging

After high-temperature forging and rapidly cooling, the microstructure evolution and spheroidization mechanism of powder metallurgy Ti-6Al-4 V alloy have been clarified. The spheroidization process is divided into two asynchronous stages. The initial stage is primarily characterized by the initial shearing and twisting of GBα grain boundaries, as well as the deformation of lamellar structure. Subsequently, the fragmentation of original GBα grain boundaries and the fracture of intra-granular α lamellae are observed. This process is accompanied by grain twisting and substructure formation, especially the division of lamellae through interface splitting and the segregation of local chemical elements. The differential accumulation of elements around the fractured and twisted grains suggests that element segregation and interfacial energy play a significant role in microstructure evolution. Therefore, the static spheroidization process largely depends on the formation and evolution of dislocation substructures, and the reduction of interfacial energy may be a key factor driving this process. These findings provide a theoretical basis for manufacturing metals with specific anisotropic properties by controlling microstructure evolution.

Strengthening mechanisms of rail steel under compression.

The evolution of structural-phase states and dislocation substructure of rail steel under uniaxial compression to the degree of 50% was studied by transmission electron microscopy. The obtained data formed the basis for a quantitative analysis of the mechanisms of rail steel strengthening at degrees of deformation by compression 15, 30, 50%. Contributions to the strengthening caused by the friction of matrix lattice, dislocation substructure, presence of carbide particles, internal stress fields, solid solution and substructural strengthening, pearlite component of the steel structure are estimated. Using the adaptivity principle, which assumes the independent action of each of the strengthening mechanisms, the dependence of rail steel strength on the degree of plastic deformation by compression is estimated. A comparative analysis of the stress-strain curves σ(ε) obtained experimentally and calculated theoretically is performed

Evolution of structural-phase state and properties of hypereutectoid steel rails at long-term operation

The methods of modern physical materials science were used to analyze the evolution of microhardness, tribological properties, dislocation substructure and phase composition of the rails with increased wear resistance and contact endurance of DT 400 IR category after missed tonnage of 187 million gross tons on the experimental ring of Russian Railways. It is shown that extremely long-term operation of the rails is accompanied by a decrease (3.1 times) in wear parameter of the rolling surface and an increase (1.4 times) in microhardness, scalar dislocation density (1.5 times) and Fe3C carbide content (1.24 times). Operation of the rails led to a decrease in the crystal lattice parameter, which correlates with an increase in the content of iron carbide. We made the assumptions about physical causes of the change in parameters.

Hot Deformation Analysis and Microstructure Evolution of a Novel Al–Cu–Li Alloy by Isothermal Compression

The hot deformation behavior of a novel Al–Cu–Li alloy is investigated by isothermal compression with a temperature range of 350–510 °C and a strain rate range of 0.001–10 s−1. Constitutive analysis and hot processing maps are used to analyze the influence of deformation parameters on hot deformation behavior, and the related microstructure evolution is characterized by scanning electron microscope (SEM), electron backscatter diffraction (EBSD), and transmission electron microscope (TEM) observations. The results indicate that the microstructure evolution of the alloy during deformation was dominated by dislocation movement and substructure evolution. With the increase in temperature or the decrease in strain rate, the dislocations introduced by deformation gradually transform into dislocation walls and substructures, showing distinct dynamic recovery (DRV) and slight dynamic recrystallization (DRX) characteristics. The degree of DRX is very low, while it is more sensitive to strain rate than temperature. According to the analysis of hot processing maps and microstructure, the optimum deformation temperature and strain rate of the alloy are determined as at least 430 °C and 0.008–0.15 s−1, respectively. These results can be referable for the industrial thermomechanical processing of this novel Al–Cu–Li alloy.

History and temperature dependent cyclic crystal plasticity model with material-invariant parameters

Cyclic deformation of metallic materials depends on the interaction of multiple mechanisms across different length scales. Solid solution atoms, vacancies, grain boundaries, and forest dislocations interfere with dislocation glide and increase the macroscopic strength. In single phase metallic materials under cyclic loading, the localization of dislocation densities in sessile substructures explains a significant fraction of the strain hardening. Upon cycling, these dislocation structures evolve across stable configurations, which depend on the strain accumulation.This work advances substructure-sensitive crystal plasticity models capable of quantifying the cyclic hardening history at various temperatures for single phase FCC materials. The framework predicts the cyclic evolution of dislocation substructure based on the activation of cross slip activation for Al, Cu, and Ni single- and poly-crystals up to 0.5 homologous temperature. The increase in cross slip with temperature and deformation induces a transformation in dislocation structures, which predicts secondary hardening without any additional provision. Moreover, the approach relies on material-invariant mesoscale parameters that are specific to dislocation substructures rather than a material system. Hence, we demonstrate that crystal plasticity predictive power can be augmented by parameterizing the model with single crystal experimental data from multiple materials with common substructures. As a result, the crystal plasticity model shares parameter information across materials without the need for additional single crystal experimental data for calibration.

On the microstructural evolution and failure mechanism in laser powder bed fusioned Ti–6Al–4V during low cycle fatigue at room and elevated temperatures

Microstructural features and their evolution during cyclic deformation directly impact the low cycle fatigue (LCF) life of additively manufactured Laser Powder Bed Fusion (LPBF) Ti–6Al–4V. Tensile and strain controlled LCF tests were performed at room (RT) and elevated temperature (ET, @ 400 °C) to study the cyclic softening behaviour and failure mechanism of LPBF Ti–6Al–4V. The evolution of α′ grains and free dislocation density were studied using Electron Backscatter Diffraction (EBSD). LPBF Ti–6Al–4V has greater tensile strength than conventionally manufactured wrought Ti–6Al–4V due to its microstructure, with fine α′ needles which provide small slip lengths. For cyclic loading at ET, the interaction between the dislocations increases which in-turn increases the ability of material to overcome the obstacles to dislocation motion, resulting in higher cyclic softening compared to the RT test. During cyclic deformation, evolution of dislocation substructures takes place to subsequently produce Low Angle Boundaries (LABs) inside the prior α’ grains. The LABs progressively lead to nucleation and coalescence of voids with fatigue cycles, eventually leading to fracture. An increase in strain range (i.e. plasticity level) causes more significant dislocation pile-up, contributing to a greater amount of cyclic softening. The lack of fusion voids or pores, present at or near the surface, and microcracks, present at the rough surface, act as the crack initiation locations which propagate to cause fracture of the LPBF material under LCF loading, where the primary mode of fatigue fracture observed is intergranular.

Multi-type dislocation substructure evolution in a high-strength and ductile duplex high-entropy nanocomposites

Developing energy-efficient and scalable microstructural solutions that enable both an intrinsically high strength and high ductility has always been a pursuit in materials science. We have used a spark plasma sintering synthesis strategy to efficiently fabricate a duplex high-entropy nanocomposite composed of the high-entropy nano-intermetallic precipitates and high-entropy nanoscale solid-solution domains. The high-entropy nanocomposite with spontaneous phase separation accompanied by the reorganization of high-entropy components achieves the minimal lattice misfit (∼0.07%) integration of the high-entropy nano-intermetallic precipitates with the high-entropy nanoscale solid-solution domains. The resulting high-entropy nanocomposite exhibited a high tensile strength (i.e., a fracture strength close to 1.7 GPa) and large plasticity (i.e., a uniform elongation close to 20%). The duplex nanostructure in our high-entropy nanocomposite described herein had multi-type dislocation substructure evolution during deformation, including dislocation planar slip, coplanar dislocation arrays, dislocation walls, microbands, and stacking faults in solid-solution domains. Because of the dominance of multiple hardening modes, our high-entropy nanocomposite could ensure that the plasticity was large enough to maintain a high strength. Our findings break through the challenge of the tradeoff between efficiency and superb performance in preparing nanostructured materials, thereby significantly facilitating the development of high-performance nanocomposites and advanced nanostructured materials.

Contribution of dislocation substructures developed during cold rolling to the formation of rolling textures in Al–Mg alloys

Abstract In order to investigate the mechanism of the development of rolling textures in Al –Mg alloys, hot bands of high-purity Al, Al-3 wt. %Mg and Al-5 wt. %Mg alloys were solution-treated at 450 °C and cold-rolled by varying rolling reductions up to 97%. Rolling textures of these specimens were investigated with the orientation distribution function analysis. Comparisons of the results of the texture analysis with those of the hardness measurement and transmission electron microscopy observations revealed that, in all specimens, the texture development occurred mainly at the later half of work hardening, i. e., at rolling reductions above 80 %, where dislocation substructures were well developed. In pure Al, in which well defined, elongated cell structures were developed, {112}<111> and {123}<634> components developed remarkably, whereas in the Al-5% Mg alloys, in which only dense dislocation tangles and ill defined small cells were developed, the development of {112}<111> and {123}<634> components was strongly suppressed. All these observations thus suggested that the development of rolling textures was strongly affected by the development of dislocation substructures, which occurred concurrently during rolling, and that the formation of elongated cell structures was more favorable for the development of rolling textures. In cold rolling, elongated cell structures may be the final form of dislocation substructures, providing both mechanical stability and orientation stability.

Deformation mechanisms that explain the combination of high plasticity and strength in steels wires with extreme carbon content

The alloy design concepts of high performance steels, involve the knowledge on the stability of carbides associated with nanoscale phenomena present in the structure as result of the deformation suffered during the thermomechanical processes. The understanding of the nanoscale phenomena open new frontiers to understand mechanical behaviours in the steel, not totally clarified until now. The paper discuss the structure evolution during wire drawing of pearlitic steels and the impact of cementite stability on different nanoscale structure phenomena, which explain the specific mechanical behaviour to achieve ultra-high strength. Results on the kinetic and stability of carbides predicted on the base of thermodynamic simulation are correlated with thermal analysis tests results (dilatometry and differential scanning calorimetry) carried out by different authors in order to understand the cementite (Fe3C) dissolution during plastic deformation. In addition, information obtained by traditional and no traditional microscopy techniques and X ray diffraction complete the study and allows to understand more deeply the structure evolution, including nanoscale phenomena that justify the mechanical behaviour during wiredrawing and the final strength level. The dislocation substructure evolution together with the cementite dissolution during sever plastic deformation which leads to a steadily increase of the strain hardening is clarified. Copyright © 2018 VBRI Press.

Microstructure and deformation behaviour of austenitic low-density steels: The defining role of B2 intermetallic phase

The feasibility of significant weight reduction in conjunction with the superior strength-ductility combination makes Fe-Mn-Al-C-based steels the candidate material for automotive and structural applications. The alloy chemistry and processing conditions influence the microstructure in low-density steels (LDS); however, their role in deformation behaviour is not fully understood. In the present study, three different austenitic LDS grades viz. Fe-28Mn-9Al-0.9C, Fe-28Mn-9Al-5Ni-0.9C and Fe-15Mn-9Al-5Ni-0.9C alloys in hot-rolled condition were used for evaluating the role of secondary phases, especially B2 ordered Fe(Ni)Al phase on deformation characteristics. Ni-free LDS microstructure consisted of the γ-matrix with fine κ-carbides, whereas Ni containing low Mn alloy possessed coarser κ-carbides along with B2 in both bamboo-like stringer and polygonal particle morphology in the γ-matrix. The B2 containing alloy exhibited higher strength with reduced tensile ductility than the B2-free alloy. Hardness of the γ-matrix and B2 phase were similar with B2 grains exhibiting distinct pop-in events in the nanoindentation curves indicating incipient plasticity. The γ-matrix and B2 co-deform near yield, and a favourable orientation relationship (OR) between the γ-matrix and B2 facilitated easy slip transfer, while the non-favourable OR controlled ductility by strain accumulation and B2-cracking. The differences in the strain hardening behaviour of B2-free and B2 containing alloys were elucidated based on the changes in dislocation substructure evolution examined by an automated crystal orientation mapping in electron microscopy.

Micromechanical crystal plasticity back stress evolution within FCC dislocation substructure

Experiments and theories have suggested the evolution of intragranular back stress is primarily dependent on the characteristics and development of dislocation substructure in face-centered cubic (FCC) metals and alloys. Despite this, continuum modeling of complex cyclic loading phenomena such as inelastic ratchet strain accumulation typically appeals to one of several phenomenological back stress forms with weak connections to physical mechanisms. In the current work, a micromechanically-based back stress evolution law is derived and implemented in a crystal plasticity framework for FCC metals that directly considers the evolution of dislocation substructure. The model is used in a case study of stainless steel 316L (SS316L), and good agreement is found with experiments on single- and poly-crystalline specimens subjected to monotonic, fully-reversed cyclic loading, and stress-controlled cyclic loading with mean stress. Ratcheting is attributed to dislocation substructure formation, dissolution, and stabilization within this physically-based framework.

Microstructural effects and mechanical response of microalloyed ferrite and austenite subjected to metal forming at extremely different rates of deformation

The mechanical behaviors and microstructures of severely deformed microalloyed austenitic M_A and ferritic M_F steels were analyzed in wires drawn from wire rods (6.5 mm in diameter) down to the final diameter of 0.75 mm. The deformation induced microstructures in the cold drawn wires under “static” (low speed wire drawing) and “dynamic” (high speed wire drawing) conditions were compared and analyzed. The analysis has been focused on a detailed characterization of the effects of microalloying on the dislocation substructure development and the in-situ recrystallization. The correlations between mechanical behavior and dislocation substructure development with increasing deformation reveals that precipitation strengthening and microstructure refinement are the dominating mechanisms that control the mechanical behaviors of wires, especially in those subjected to deformation under dynamic loading conditions. An approach has been presented to model the evolution of work hardening process during complex deformation. Additional advantage of the presented approach is its capability of tracking evolution of dislocation density during the deformation process. The changes of dislocation density calculated for both materials show similar behaviour, but higher values of the dislocation densities were observed in the specimens of M_F than in the case of M_A.

Inconsistent creep between dendrite core and interdendritic region under different degrees of elemental inhomogeneity in nickel-based single crystal superalloys

The creep inconsistency between dendrite core and interdendritic region is investigated in a nickel-based single crystal superalloy under 1373 K and 137 MPa. Two specimens with higher and lower degree of elemental inhomogeneity on dendritic structures are compared. For specimen with higher inhomogeneity, stronger segregation of refractory elements reinforces the local strength in dendrite core, but damages the strength in interdendritic region. Creep strain is accumulated faster in interdendritic region giving rise to promoted dislocation shearing in γ′ phase, faster degradation of dislocation networks and facilitated topological inversion of rated structures. Although the segregation of refractory elements produces a high density of topologically close-packed (TCP) phase in dendrite core, faster accumulation of creep strain forms microcracks prior in interdendritic region that gives rise to final rupture of the specimen. In another specimen, increased solid solution time gives rise to overall reduced inhomogeneity. Creep inconsistency is relieved to show more uniform evolution of dislocation substructures and rafting between dendrite core and interdendritic region. The second specimen is ruptured by formation and extension of microcracks along TCP phase although the precipitation of TCP phase is relatively restricted under reduced inhomogeneity. Importantly, the balance of local strength between dendrite core and interdendritic region results in over 40% increase of creep rupture life of the second specimen.

Evolution of Dislocation Substructure in Deformed Ordered and Disordered Pd3Fe Alloys in Low-Stability State

Evolution of Dislocation Substructure in Deformed Ordered and Disordered Pd3Fe Alloys in Low-Stability State

Microstructure and Deformation Behaviour of Austenitic Low-Density Steels: The Defining Role of B2 Intermetallic Phase

The feasibility of significant weight reduction in conjunction with the superior strength-ductility combination makes Fe-Mn-Al-C-based steels the candidate material for automotive and structural applications. The alloy chemistry and processing conditions influence the microstructure in low-density steels (LDS); however, their role in deformation behaviour is not fully understood. In the present study, three different austenitic LDS grades viz. Fe-28Mn-9Al-0.9C, Fe-28Mn-9Al-5Ni-0.9C and Fe-15Mn-9Al-5Ni-0.9C alloys in hot-rolled conditions were used for evaluating the role of secondary phases, especially B2 ordered Fe(Ni)Al phase on deformation characteristics. Ni-free LDS microstructure consisted of the γ-matrix with fine κ-carbides, whereas Ni containing low Mn alloy possessed coarser κ-carbides along with B2 in both bamboo-like stringer and polygonal particle morphology in the γ-matrix. The B2 containing alloy exhibited higher strength with reduced tensile ductility than the B2-free alloy. Hardness of the γ-matrix and B2 phase were similar with B2 grains exhibiting distinct pop-in events in the nanoindentation curves indicating incipient plasticity. The γ-matrix and B2 co-deform near yield, and a favourable orientation relationship (OR) between the γ-matrix and B2 facilitated easy slip transfer, while the non-favourable OR controlled ductility by strain accumulation and B2-cracking. The differences in the strain hardening behaviour of B2-free and B2 containing alloys were elucidated based on the changes in dislocation substructure evolution examined by an automated crystal orientation mapping in electron microscopy.

Influence of Composition on the Evolution of Dislocation Substructure in the Low-Stability State Polycrystalline Cu-Al Alloys under Plastic Deformation

New concepts of dislocation physics of plasticity and strength are considered using quantitative methods of transmission diffraction electron microscopy. New concepts of dislocation physics of plasticity and strength are considered using quantitative methods of transmission diffraction electron microscopy. The analysis of changes in the parameters of the dislocation substructure (DSS) is given on the example of alloys Cu-0.5 and 14 аt. % Al and the influence of these parameters on the change in the substructure of the material at a temperature T=293 K is considered. It is shown that at each stage of deformation, there are usually two substructures ("old" and "new"). The blurring of the transition from stage to stage is associated with the presence of weakly stable pre-transition structural-phase States at certain degrees of deformation of several types of substructures simultaneously, i.e., a weakly stable structural-phase state of the system. Against the background of the "old" substructure, a "new" one is born, which in the process of deformation becomes the main one, and then the "old" one, in the depths of which another substructure is formed. Experimental evidence of this regularity is obtained for FCC alloys. The presence of grain boundaries complicates the diagrams: a third substructure is formed near the grain boundaries, which corresponds to the following substructures (later) in the sequence of DSS transformations.

Orientation and deformation conditions dependence of dislocation substructures in cold deformed pure tantalum

The evolution of dislocation arrangement during deformation can lead to the development of dislocation substructures which have direct consequences on the subsequent microstructural evolutions. Dislocation densities and substructures of various deformed microstructures of pure tantalum are analyzed in order to study the influence of deformation conditions. Electron BackScatter Diffraction (EBSD) and Electron Channeling Contrast Imaging (ECCI) are used in order to characterize the development of dislocation substructures. Investigation at grain scale shows the orientation dependence of the deformed state with the highest dislocation content for γ-fiber grains. This fiber appears to be quite stable in compression but less in rolling. Investigation at dislocation substructure scale highlights the influence of orientation on dislocation substructure forming. Developed substructures are present in the interior of γ-fiber grains. The evolution of the disorientation and size of substructures with strain depends greatly on the initial fraction of γ-fiber in the microstructure. Models describing these evolutions are proposed for γ-fiber grains. On the basis of the present analysis it is concluded that dislocation density parameter does not allow transcribing the differences of substructural development.

Influences of stress-aging on the precipitation behavior of δ phase (Ni3Nb) in a nickel-based superalloy

The precipitation behaviors of δ phases (Ni3Nb) in a nickel-based superalloy are investigated by stress-free and stress aging tests. The effects of aging parameters on the distribution features of δ phases are discussed. Also, the evolution of dislocation substructures is analyzed. The results show that the extra stress can accelerate the uniform precipitation of δ phase, and increase the dislocation substructures during stress aging treatment. But, in the stress-free aging process, δ phases easily concentrate around grain boundaries and randomly distribute within grains. During the stress aging, with the increase of extra stress or aging time, δ phase regularly and densely precipitates within grains, because the lattice distortion is affected by the extra stress. In addition, the basket-weave δ phases densely precipitate and mutually intersect to form enclosure spaces. The limited dislocations in the enclosure spaces can enhance the formation of subgrains and make the uniform nucleation of dynamic recrystallization grains. Meanwhile, the basket-weave δ phases caused by the extra stress significantly refine the deformed grains, and the uniform microstructures are obtained.