Substructure Evolution Research Articles

On the substructure evolution and its distinct role on dynamic strain aging in a multi-component cobalt base alloy at intermediate temperatures (300 to 700 °C)

Manifestations of dynamic strain aging (DSA) in a solid-solution multi-component cobalt-base alloy was evaluated over wide temperature [25–1000 °C] and strain rate range [10−4–10−1 s−1]. Trends in serration onset strain with temperature together with kinetic analysis revealed the features of Portevin-Le Chatelier effect (PLC) and inverse-PLC. Besides, distinct dependence of strain rate sensitivity on the strain rate jump direction is observed between 10−4 and 10−3 s−1 and >550 °C. Detailed post-deformation characterization was done using transmission electron microscopy. In the initial stages (∼0.05 strain), multipoles were found to be the predominant local obstacles responsible for DSA, and strain accumulation occurs through their unzipping. The substantial presence of widely separated (∼23 nm) extended dislocations indicates the strong dominance of Suzuki segregation. Besides, a continuous evolution of heterogeneous to homogeneous slip vis-à-vis multipole density and their arrangement with temperature is observed. This study provides new insights into substructural evolution and its role on DSA mechanisms in multi-component alloys.

Dynamic recrystallization mechanisms of as-forged Al–Zn–Mg-(Cu) aluminum alloy during hot compression deformation

The thermal deformation behavior of as-forged Al–Zn–Mg-(Cu) aluminum alloy at strain rate 0.001–1 s−1, temperature 663–783 K and deformation degree 0–60% was studied by isothermal compression test, and an accurate strain-compensated Arrhenius constitutive equation was established. Based on the microstructure characteristics of compressed specimens, the effects of strain rate, temperature and strain on dynamic recrystallization (DRX) behavior and substructure evolution were determined. The results show that the alloy undergoes discontinuous dynamic recrystallization (DDRX), continuous dynamic recrystallization (CDRX) and geometric dynamic recrystallization (GDRX) simultaneously at high temperature and low strain rate (743 K, 0.001–0.01 s−1), where CDRX includes microshear band assist (MSBA), homogeneous increase of misorientation (HIM) and progressive lattice rotation near grain boundaries (LRGB). However, with decreasing temperature and increasing strain rate, GDRX basically disappears, and DDRX becomes the main DRX mechanism. At medium to high strain rate (0.1∼1 s-1), high-density dislocations accumulate at the grain boundaries and numerous microshear bands form inside grains during the early deformation stage. The misorientation angles of these pre-existing geometrically necessary boundaries gradually increase with temperature and strain, ultimately transforming into subgrains and DRX grains.

Microstructure evolution, mechanical properties, and strengthening mechanisms of 6061 aluminum alloy processed via corrugated constrained groove pressing

In this study, a corrugated constrained groove pressing (CCGP) technology based on a semicircle groove was proposed, improving the uniformity of strain accumulation in deformed metal sheets through staggered groove pressing. First, 6061-O aluminum alloy was processed at room temperature via conventional CCGP (Conv-CCGP) and 90° cross CCGP (Cross-CCGP). The effects of strain route and deformation pass on the microstructure evolution of the CCGPed alloy were investigated and correlated with mechanical properties. Changes in the mechanical properties of the CCGPed alloy at room temperature measured via tensile in different directions and hardness tests were studied. Results showed that the strength and hardness of the CCGPed sheet in both routes were considerably improved, but ductility was decreased. Cross-CCGP can effectively alleviate the anisotropy of the deformed sheet and achieve optimal strength in roll direction (RD) and transverse direction (TD) at two passes. Yield strength (YS) and ultimate tensile strength (UTS) along RD were increased by 190.2% and 15.5% relative to the initial alloy, reaching 119 MPa and 134 MPa, respectively. Along TD, YS and UTS were increased by 255.0% and 36.0%, reaching 142 MPa and 151 MPa, respectively. Characterization techniques, including electron backscatter diffraction and high-resolution transmission electron microscopy, were used to establish comprehensive models of dynamic recovery-based substructure evolution and dynamic recrystallization-dominated grain refinement. Meanwhile, the integrated microstructural changes and theoretical calculations identified dislocation strengthening and local grain refinement as the major contributors to the strengthening of the CCGPed alloy. In addition, tensile fracture surface analysis showed that the CCGPed alloy still underwent plastic fracture, but the fracture mode shifted from tension fracture into shear fracture.

Substructure development, martensitic transformation and rapid work-hardening in an as-cast interstitial substituted N atom in FeMnCoCr high-entropy alloy

The microstructural evolution and room-temperature mechanical properties of nitrogen-added Fe50Mn30Co10Cr10 as-cast high-entropy alloy were investigated. Interestingly, an excellent strength-ductility trade-off with a rapid hardening stage was achieved in the as-cast structure (tensile strength: 740 MPa, ductility: 42 %) that was comparable with those counterparts holding wrought structure. These excellent mechanical properties were attributed to the capability of metastable FCC microstructure enabling deformation-induced martensitic transformation even at low imposed strain at room temperature. Simultaneously, the occurrence of substructure development independent of martensitic transformation, significantly contribute to increase the capability of the material for strain accommodation.

Development of Zeise's Salt Derivatives Bearing Substituted Acetylsalicylic Acid Substructures as Cytotoxic COX Inhibitors.

Zeise's salt derivatives of the potassium trichlorido[η2-((prop-2-en/but-3-en)-1-yl)-2-acetoxybenzoate]platinate(II) type (ASA-Prop-PtCl3/ASA-But-PtCl3 derivatives) were synthesized and characterized regarding their structure, stability, and biological activity. It is proposed that the leads ASA-Prop-PtCl3 and ASA-But-PtCl3 interfere with the arachidonic acid cascade as part of their mode of action to reduce the growth of COX-1/2-expressing tumor cells. With the aim to increase the antiproliferative activity by strengthening the inhibitory potency against COX-2, F, Cl, or CH3 substituents were introduced into the acetylsalicylic acid (ASA) moiety. Each structural modification improved COX-2 inhibition. Especially compounds with F substituents at ASA-But-PtCl3 reached the maximum achievable inhibition of about 70% already at 1 µM. The PGE2 formation in COX-1/2-positive HT-29 cells was suppressed by all F/Cl/CH3 derivatives, indicating COX inhibitory potency in cellular systems. The CH3-bearing complexes showed the highest cytotoxicity in COX-1/2-positive HT-29 cells with IC50 values of 16-27 µM. In COX-negative MCF-7 cells, they were 2-3-fold less active. These data clearly demonstrate that it is possible to increase the cytotoxicity of ASA-Prop-PtCl3 and ASA-But-PtCl3 derivatives by enhancing COX-2 inhibition.

Relationship of Internal Stress Fields with Self-Organization Processes in Hadfield Steel under Tensile Load

The effect of tensile strains on the microstructure of Hadfield steel was studied by transmission electron microscopy (TEM). Stages of the obtained stress–strain curves were observed, and correlated well with the evolution of the dislocation substructure. Based on an analysis of TEM images, quantitative parameters were determined, such as the material volume fractions, in which slip and twinning occurred, as well as twinning, which developed in one, two and three systems. Some transformation mechanisms were reported that caused great hardening of Hadfield steel. In particular, a complex defect substructure formed in a self-organized manner due to the formation of cells, the dislocations retarded by their walls, as well as the deceleration of dislocations on twins and, vice versa, of twins on dislocations. These factors affected both the average and excess local density of dislocations. Additionally, they resulted in elastic stress fields, which manifested themselves in the curvature–torsion gradient of the crystal lattice. A high level of stresses caused by solid-solution strengthening prevented the relaxation of elastic ones, contributing to the strain hardening of the Hadfield steel.

Microstructure evolution and constitutive modeling of as-cast A356 aluminum alloy in semi-solid deformation regime

The present work deals with the semi-solid deformation behavior of A356 aluminum alloys with an emphasis on microstructure evolution and constitutive modeling. The main idea of this research is based on investigating the semi-solid deformation behavior starting with primary dendritic cast structure, instead the conventional approach in which a feedstock is prepared through the various spheroidization process. Toward this end, hot compression tests in the semi-solid temperature range (580, 590, and 600 °C) under different strain rates (0.001, 0.01, 0.1 s−1) and different initial holding times (1, 7, and 14 min) have been conducted. Compared with the thixotropic deformation in which the solid globules slide within the liquid matrix through lubricated flow mechanisms, in the present case, the strain compensation through the deformation of solid grains was increased. The higher portion of the solid-grains from the applied strain provided a proper condition for substructure development and dynamic recrystallization in the semisolid temperature range. The penetration of the liquid phase along the newly developed boundaries was introduced as another factor influencing on the softening behavior of the material. Under these conditions, the specimens were deformed up to the high strain level (>0.5) without creating cracks or tearing under very low stresses less than 2 MPa. A phenomenological-physical constitutive equation was also developed considering deformation parameters (temperature, strain rate, and strain) and material's parameters (grain size, strain rate sensitivity, viscosity) which was capable of predicting the semisolid flow stress behavior of the material.

On the capability of grain refinement during selective laser melting of AlSi10Mg alloy

This work focuses on the origin and capability of grain refinement in the microstructure of selective laser-melted AlSi10Mg alloy. The grains in printed microstructure have been oriented in a random manner and there is no sign of preferred micro-texture. In the molten pool, columnar grains have been formed aligned with the building direction, and stretched toward the center of the molten pool. The fraction of refined equiaxed grains by the size of fewer than 10 μm is considerable (∼40%) which have been formed (i) during solidification owing to high solidification rate to thermal gradient ratio, or due to (ii) the occurrence of dynamic recrystallization during the manufacturing process after completion of the solidification. These refined equiaxed grains have been differentiated through plotting Grain Orientation Spread maps. The numerous thermal cycles experienced by each layer cause considerable tensile or compressive micro-plastic strains and give rise to dynamic recrystallization. The presence of incomplete grains can be clearly traced which are ready to be evolved into the recrystallized grains. This, besides the extensive substructure development and sub-grain formation, are main proof for governing of continuous mechanisms of recrystallization mechanisms. Finally, selective laser melting has been approached as a thermomechanical processing route, and the calculated micro-plastic strain has been compared with a critical strain of dynamic recrystallization obtained from the hot compression testing method.

Formula omitted]-CP: Open source dislocation density based crystal plasticity framework for simulating temperature- and strain rate-dependent deformation

This work presents an open source, dislocation density based crystal plasticity modeling framework, ρ-CP. A Kocks-type thermally activated flow is used for accounting for the temperature and strain rate effects on the crystallographic shearing rate. Slip system-level mobile and immobile dislocation densities, as well slip system-level backstress, are used as internal state variables for representing the substructure evolution during plastic deformation. A fully implicit numerical integration scheme is presented for the time integration of the finite deformation plasticity model. The framework is implemented and integrated with the open source finite element solver, Multiphysics Object-Oriented Simulation Environment (MOOSE). Example applications of the model are demonstrated for predicting the anisotropic mechanical response of single and polycrystalline hcp magnesium, strain rate effects and cyclic deformation of polycrystalline fcc OFHC copper, and temperature and strain rate effects on the deformation of polycrystalline bcc tantalum. Simulations of realistic Voronoi-tessellated microstructures as well as Electron Back Scatter Diffraction (EBSD) microstructures are demonstrated to highlight the model’s ability to predict large deformation and misorientation development during plastic deformation.

In Situ Study of the Microstructural Evolution of Nickel-Based Alloy with High Proportional Twin Boundaries Obtained by High-Temperature Annealing.

In this paper, we report an in situ study regarding the microstructural evolution of a nickel-based alloy with high proportional twin boundaries by using electron backscatter diffraction techniques combined with the uniaxial tensile test. The study mainly focuses on the evolution of substructure, geometrically necessary dislocation, multiple types of grain boundaries (especially twin boundaries), and grain orientation. The results show that the Cr20Ni80 alloy can be obtained with up to 73% twin boundaries by annealing at 1100 °C for 30 min. During this deformation, dislocations preferentially accumulate near the twin boundary, and the strain also localizes at the twin boundary. With the increasing strain, dislocation interaction with grain boundaries leads to a decreasing trend of twin boundaries. However, when the strain is 0.024, the twin boundary is found to increase slightly. Meanwhile, the grain orientation gradually rotates to a stable direction and forms a Copper, S texture, and α-fiber <110>. Above all, during this deformation process, the alloy is deformed mainly by two deformation mechanisms: mechanical twinning and dislocation slip.

High Pressure Torsion of Copper; Effect of Processing Temperature on Structural Features, Microhardness and Electric Conductivity.

By optimizing the fabrication method, copper components featuring (typically contradicting) advantageous electric conductivity and favorable mechanical properties can be acquired. In this study, we subjected conventional electroconductive copper to a single revolution of high pressure torsion (HPT) at room temperature (RT), searched for the conditions which would yield comparable structure characteristics (grain size) when deformed at a cryogenic temperature, and finally compared the mechanical and electric behaviors to assess specific differences and correlate them with the (sub)structural development. 180° revolution of cryo-HPT imparted structure refinement comparable to 360° revolution of room temperature HPT, i.e., the average grain size at the periphery of both the specimens was ~7 µm. The 360° RT HPT specimen exhibited preferential (111)||SD (shear direction) texture fiber in all the examined regions, whereas the 180° cryo-HPT specimen exhibited more or less randomly oriented grains of equiaxed shapes featuring substantial substructure development of a relatively homogeneous character and massive occurrence of (nano)twins. These structural features resulted in the increase in microhardness to the average value of 118.2 HV0.2 and the increase in the electric conductivity to 59.66 MS·m-1 (compared to 105 HV0.2 and 59.14 MS·m-1 acquired for the 360° RT HPT specimen). The deformation under the cryogenic conditions also imparted higher homogeneity of microhardness distribution when compared to RT processing.

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.

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.

Static Globularization Behavior and Artificial Neural Network Modeling during Post-Annealing of Wedge-Shaped Hot-Rolled Ti-55511 Alloy.

The globularization of the lamellar α phase by thermomechanical processing and subsequent annealing contributes to achieving the well-balanced strength and plasticity of titanium alloys. A high-throughput experimental method, wedge-shaped hot-rolling, was designed to obtain samples with gradient true strain distribution of 0~1.10. The samples with gradient strain distribution were annealed to obtain the gradient distribution of globularized α phase, which could rapidly assess the globularization fraction of α phase under different conditions. The static globularization behavior under various parameters was systematically studied. The applied prestrain provided the necessary driving force for static globularization during annealing. The substructure evolution and the boundary splitting occurred mainly at the early stage of annealing. The termination migration and the Ostwald ripening were dominant in the prolonged annealing. A backpropagation artificial neural network (BP-ANN) model for static globularization was developed, which coupled the factors of prestrain, annealing temperature, and annealing time. The average absolute relative errors (AARE) for the training and validation set are 3.17% and 3.22%, respectively. Further sensitivity analysis of the factors shows that the order of relative importance for static globularization is annealing temperature, prestrain and annealing time. The developed BP-ANN can precisely predict the static globularization kinetic curves without overfitting.

Grain boundary and grain interior strengthening in nano-micron grain sized Cu-1wt.%Al2O3 composite

A bulk Cu-1 wt%Al2O3 composite was synthesised to 97–99% theoretical density by mechanical alloying and spark plasma sintering to get varying grain sizes in the range 70 nm–2 µm. Tensile tests were done at room temperature, and the substructure evolution was examined as a function of the grain size and strain. The flow property revealed bilinear Hall–Petch relationship with a transition at 0.55 µm grain size. The stress–strain curves were found to follow the Hollomon-type relationship exhibiting an increase in strength coefficient and a decrease in strain hardening exponent with decreasing grain size. The variations in flow stress with grain size and strain delineate equivalent effects. The inter-dependence between the grain size and strain is explained by the substructure evolution and its contribution to the partitioning of work hardening between the grain interior and grain boundaries.

Insights into the Impact Behavior and Deformation Substructure Evolution of the N‐Bearing QN1803 and 304 Stainless Steels

The impact behaviors of N‐bearing QN1803 and 304 stainless steels are studied using instrumented Charpy impact test in the range of −192–100 °C. The fracture and deformation substructures are systematically investigated by scanning electron microscopy and transmission electron microscopy. The results show that QN1803 presents ductile–brittle transition temperature (DBTT) and 304 shows ductile fracture. With increasing temperature, the stacking fault energy (SFE) of QN1803 and 304 increases, but the former is always lower than the latter. QN1803 presents low impact toughness with low‐density dislocation at −192 °C. At high temperature, QN1803 and 304 show good impact toughness and the deformation substructures are dominated by high‐density dislocations and deformation twinning.

Dislocation density and twinning-related constitutive modeling of substructural evolution in Sn-5Sb-0.5(Ag/Cu) alloys under monotonic deformation

The development of a numerical model and experimental evaluation of microstructural evolution during the deformation of such materials are of pronounced significance. In the current work, evolutions of the substructure in three Sn alloys, namely Sn-5Sb, Sn-5Sb-0.5Ag, and Sn-5Sb-0.5Cu, were investigated using a dislocation density constitutive equation. The mechanical behavior of such alloys has been assessed, taking advantage of Ag and Cu addition and substructure-based factors like dislocation density, size, and misorientation. A comprehensive constitutive equation was developed for modeling the flow behavior of Sn-based alloy through the substructure-based tensile response of the material. It has been perceived that Cu is more in effect to improve the strength of Sn than Ag and Sn-5Sb-0.5Cu alloy is with enhanced work hardening rate during the tensile state of deformation. Dislocation density formulation results have direct implications on the capability to explicitly model the superimposed effect of dislocation densities and subgrain boundaries on the macroscopic mechanical properties of polycrystals. A numerical description of twinning stress in studied BCT alloys is described by the dislocation production kinetic model considering dislocation pile-up as the dominant mechanism. The developed slip-twinning transition model is incorporated into the deformation maps to identify the twinning domain.

The History of Development of Inland Transport Infrastructure: Technology and Economic Aspects. Part 2

The second part of the article (the first of which was published in the last issue of the journal) analyses the history of emergence of the rail track, as well as evolution of the railway-track substructure, including a review of the materials used for this.A separate section is devoted to evolution of railways in Russia.Brief conclusions are made regarding the importance of the historical and economic aspects of development of inland transport infrastructure.The importance of considering this issue is quite high when conducting comprehensive research intended to get economic assessment and forecast the strategic development of transport infrastructure.

Comparison of surface integrity of GH4169 superalloy after high-energy, low-energy, and femtosecond laser shock peening

In this study, different laser shock peening (LSP) techniques including high-energy laser shock peening (HE-LSP), low-energy laser shock peening (LE-LSP), and femtosecond laser shock peening (FS-LSP) were performed on GH4169 superalloy. The evolution of surface characteristics and microstructure was compared. Results show that the dominant deformation mechanism of GH4169 superalloy after various LSP treatments is dislocation slip, which leads to irregular atomic arrangement and high-density dislocations. The δ phase in the γ matrix restricts the movement of the dislocations and causes massive pile-ups at its boundaries. Thus, a special δ-phase/dislocation composite structure will promote the development of substructure in the deformed layer, although there is no obvious grain refinement. The deformed layer depth and damage mechanism are related to the pulse duration. The order of surface roughness, deformed depth, and arc height ranked according to LSP treatments is: HE-LSP > LE-LSP > FS-LSP. LE-LSP and FS-LSP could replace HE-LSP for thin-walled parts.

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.