Dynamic Restoration Processes Research Articles

Effect of Stacking Sequence on Mechanical Properties and Microstructural Features within Al/Cu Laminates.

The study presents a method to prepare Al/Cu laminated conductors featuring two different stacking sequences using rotary swaging, a method of intensive plastic deformation. The primary focus of the work was to perform detailed characterization of the effects of room temperature swaging on the development of microstructures, including the Al/Cu interfaces, and internal misorientations pointed to the presence of residual stress within the laminates. The results revealed that both the Al and Cu components of the final laminates with 5 mm in diameter featured fine, more or less equiaxed, grains with no dominating preferential texture orientations (the maximum observed texture intensity was 2.3 × random for the Cu components of both the laminates). This fact points to the development of dynamic restoration processes during swaging. The analyses of misorientations within the grains showed that residual stress was locally present primarily in the Cu components. The Al components did not feature a substantial presence of misorientations, which confirms the dynamic recrystallization. Tensile testing revealed that the laminates with both the designed stacking sequences exhibited comparable UTS (ultimate tensile strength) of almost 280 MPa. However, notable differences were observed with regard to the plasticity (~3.5% compared to less than 1%). The laminate consisting of Al sheath and Cu wires exhibited very low plasticity as a result of significant work hardening of Al; this hypothesis was also confirmed with microhardness measurements. Observations of the interfaces confirmed satisfactory bonding of both the metallic components.

A rule-based modeling approach for network application availability assessment under dynamic network restoration scheme

The efficient and accurate assessment method for application availability is essential for network operators to make the resilience-cost trade-off policy. However, the backup path of application is dynamically discovered upon both other application requests and global network resource distribution under network restoration scheme, makes the current analytical network availability models difficult for assessing application availability effectively in large-scale network scenarios. This paper proposed a rule-based modeling approach to capture application resilience behavior and measure the uncertainties during dynamic restoration process. Firstly, we propose an evolution object formalism for modeling the attributes of both logical application and network infrastructure. Secondly, the network evolution condition model is proposed to describe the failure modes of individual components. Finally, the network evolution rule is proposed, which utilizes general graph algorithm and resource match function as local rules to determine application restoration resource contention outcomes during the state transition. Through simulations, the proposed approach evaluates application availability in terms of recovery time during system state transition controlled by the rules. The effectiveness of the proposed approach is verified to an actual optical network case under two typical network restoration schemes and compare its accuracy against two analytical approaches based on RBD (Reliability Block Diagram) and Markov Chain. The sensitivity results reveal that link capacity has a more significant impact on application availability than link failures.

A strategy to enhance the distribution systems recoverability via the simultaneous coordination of planning actions and operational resources

This paper presents a planning and operational strategy to improve the recoverability of distribution systems (DSs) to deal with a set of possible line fault scenarios. The strategy simultaneously optimizes the allocation of dispatchable distributed generation (DG) units while coordinating a dynamic restoration process based on a radial topology reconfiguration, an islanding operation, a demand response program, and the pre-positioning and dispatch of mobile emergency storage units. The uncertainty and variability associated with solar irradiation and demand are captured via a multi-period formulation based on a stochastic mixed-integer linear programming model. The objective function of this model minimizes the investment cost of new dispatchable DG units and the amount of energy shedding within the system. Simulations are performed on adapted 33-node and 53-node test systems to validate the proposed strategy under four different test conditions, numerical results reveal the advantages of simultaneously solving the planning and operational stages to improve the recoverability of the system.

Shear strain induced recrystallization/recovery phenomena within rotary swaged Al/Cu composite conductors

The presented study aims to assess the effects of room temperature rotary swaging on structure, dynamic restoration processes in particular, within newly designed Al/Cu composite conductors. Swaging was processed from the original diameter of 50 mm (assembled billets) to the diameters of 20 mm, 15 mm, and 10 mm (final conductors). Cross-sectional samples of the swaged conductors were subjected to thorough structure analyses via electron microscopy, analyses of HV0.2 Vickers microhardness and specific electric resistivity were performed to supplement the study. Swaging to 15 mm primarily imparted structure recovery, while the final swaging to 10 mm introduced dynamic recrystallization within both the composite components (the fractions of high angle grain boundaries within Al and Cu were 78.7%, and 74.2%, respectively). The 10 mm composite Cu also featured increased fraction of Cube (Euler angles of φ 1 = 0°, ϕ = 0°, and φ 2 = 0°) ideal recrystallization texture orientation. As regards the specific electric resistivity, this parameter was influenced primarily by accumulated dislocations, grain size and arrangement (e.g. bimodal distribution), as well as possible presence of texture. The lowest specific electric resistivity of 20.589 Ωm·10 −9 was measured for the 15 mm conductor exhibiting dynamic restoration. The Cu component of this composite also featured the highest HV0.2 Vickers microhardness value. • Rotary swaging at room temperature successfully used to produce Al-Cu conductors. • Swaging to 15 mm diameter imparted dynamic recovery. • Swaging to 10 mm diameter introduced dynamic recrystallization. • Electric resistivity decreased within dynamically recovered structure.

Complex dynamic restoration processes leading to a high degree of deformability in a dual-phase Al0.5CoCrFeNi high entropy alloy

The current research provides scientific evidence based on experimental and modeling approaches that complex softening processes characterized by dynamic recovery (DRV) and dynamic recrystallization (DRX) combined with twinning contribute efficiently to the hot deformation processing of the Al 0.5 CoCrFeNi alloy. An as-cast face-centered-cubic (fcc) oriented Al 0.5 CoCrFeNi dual-phase high entropy alloy, HEA, was deformed in uniaxial compression to a true strain 0.8 at temperatures between 900 and 1100 ºC and strain rates from 0.0013 to 0.1 s −1 . A dynamic material model was applied to predict the processing windows, and the underlying deformation mechanisms were characterized using scanning electron microscopy and electron backscattered diffraction. The optimum processing window for the studied alloy is at 900–960 ºC/ 0.0013–0.002 s −1 , where the efficiency of power dissipation and strain rate sensitivity ranges from 45% to 50% and 0.28–0.33, respectively. The processing map also exhibits a domain of flow instability located in the lower temperature and higher strain rate regions, resulting mainly from flow localization. The studied alloy represents a dual-phase fcc + bcc/B2 microstructure in an as-cast (undeformed) state. The fcc matrix occupying 94 vol% forms dendrites with an average diameter of 210 µm, decorated by discrete networks of bcc/B2 phase of average grain size 3.5 µm residing in interdendritic regions. The microstructural analyses corroborate the coincidence of DRX and twinning in the fcc phase in all deformed specimens. However, only DRV takes place within the bcc phase. The current study reveals a well-suited parameter range to achieve a high degree of hot deformability in dual-phase HEAs by taking advantage of twinning combined DRX to refine the microstructure significantly. Future work will have to identify possible application cases. • The possibility of hot deformation processing of a duplex fcc+bcc Al 0.5 CoCrFeNi alloy. • Complex dynamic restoration processes lead to a high degree of hot workability. • The optimum processing window locates at 900–960 ºC/ 0.0013–0.002 s −1 . • Shear localization yields flow instability in lower temperature and higher strain rate regions.

Characteristics of dynamic softening during high temperature deformation of CoCrFeMnNi high-entropy alloy and its correlation with the evolving microstructure and micro-texture

The characteristics of dynamic recrystallization (DRX) of a CoCrFeMnNi high–entropy alloy (HEA) was investigated via hot compression testing in the temperature range 950–1100 °C and at true strain rates of 10−2 and 10−1 s−1. The discontinuous DRX was found to be the dominant mechanism corroborating the microstructural evolution. The progress of the initiation of DRX was investigated in terms of critical strain/stress required using the Poliak–Jonas analytical criterion. Consequently, a new kinetic model based on Avrami–type function was established for the HEA to predict the DRX fractional recrystallization. It was revealed that the volume fraction of DRX grains increased with increasing strain. In the case of 10−2 s−1, steady–state flow was achieved after the completion of one DRX process cycle resulting in further straining, leading to the occurrence of dynamic restoration processes involving formation of substructures and generation and annihilation of dislocations inside the DRX grains which effectively increased the fraction of partially deformed DRX (substructured) grains. A good agreement between the proposed DRX kinetics model and microstructure observation results validated the accuracy of DRX kinetics model for CoCrFeMnNi HEA. The preferred orientation of the non–recrystallized grains was towards the formation of <101> fiber texture, whereas a random micro–texture is revealed in the recrystallized grains.

Microstructure Evolution During Hot Deformation of REX734 Austenitic Stainless Steel

Mechanical properties of a REX734 austenitic stainless steel were examined through compression testing over a wide range of temperatures (1173 K to 1373 K (900 °C to 1100 °C)) and strain rates (0.1 to 40 s−1) that cover deformation conditions encountered in different metalworking processes. The evolution of microstructure was studied using electron microscopy combined with electron backscatter diffraction and energy-dispersive spectroscopy. Partially recrystallized microstructures were obtained after compression testing at 1173 K (900 °C), while after deformation at 1273 K and 1373 K (1000 °C and 1100 °C), the material was fully recrystallized almost in all examined cases. The role of dynamic and metadynamic restoration processes in the formation of final microstructure was investigated. Σ3 twin boundaries lost their twin character and transformed into general high-angle grain boundaries as a result of deformation, while during recrystallization new Σ3 twin boundaries formed. The evolution of precipitates during compression testing and their role in the recrystallization process was also discussed.

Physical Simulation and Processing Map of Aluminum 7068 Alloy

Elevated temperature deformation of as-cast aluminum 7068 alloy was done to optimize its workability by physical simulation using Gleeble 3800. The microstructural evolution was traced using electron microscopy and electron back-scattered diffraction studies. Hot deformation involving uniaxial isothermal compression was done in the 300°C–475°C temperature range and 10−3–100 s−1 strain rate up to a true strain of 0.69. From the true stress–true strain plot it is observed that flow stress initially increases sharply because of the multiplication of dislocations during the initial phase of deformation; i.e., only work hardening is predominant during this phase. Subsequently, after attaining the peak value the flow stress increases, decreases, or remains constant. Because of dynamic restoration processes such as dynamic recovery and recrystallization, the flow stress decreases or remains constant. During this phase, the competition between work hardening and dynamic softening governs the slope of true stress–strain curve. The decrease in the Zener-Hollomon parameter with increasing temperature and decreasing strain rate follows the same trend as flow stress. Activation energy for this alloy is calculated as 206 kJ mol−1. The deformed microstructure shows serrated grains, and also well-formed subgrains are observed mostly inside the grains. Moreover, substructural strengthening is observed in this alloy, due to the presence of the high density of precipitates and subgrains. Microstructural analysis confirms the high power dissipation of the stable region may be mainly due to dynamic recovery. In the unstable region, flow instabilities may be due to adiabatic shear band formation, particle cracking, and debonding. The optimized working region is determined from the developed processing map.

Understanding the Mechanism of Dynamic Recrystallization During High-Temperature Deformation in Nb-1Zr-0.1C Alloy

In the present investigation, a systematic study of the evolution of microstructure and crystallographic texture during hot deformation of Nb-1Zr-0.1C was carried out in the temperature range 1773-1973 K (1500-1700 °C) at different strain rates of 0.001, 0.01 and 0.1 s−1. The aim was to examine the mechanisms of dynamic recovery and recrystallization in a high-temperature range. A detailed microstructural analysis of the deformed samples was performed using the electron backscatter diffraction technique to study the occurrence and nature of various dynamic restoration processes; the different regimes of dynamic recovery and recrystallization were identified. The orientations of the dynamically recrystallized grains were found to be (001) .

Processing–Microstructure–Crystallographic Texture–Surface Property Relationships in Friction Stir Processing of Titanium

Friction stir processing (FSP) is a solid-state technique for microstructural modification of metallic materials. The aim of this work is to establish processing–microstructure–texture–surface properties relationship in commercially pure titanium (cp-Ti) processed by FSP under different processing conditions. The effect of processing conditions on the microstructural changes and the evolution of crystallographic texture is systematically studied. The changes in the chemical composition of the passive surface layer are characterized by x-ray photoelectron spectroscopy. The corrosion behavior of cp-Ti after FSP is evaluated in simulated body fluid and is related to the microstructure, texture and composition of passive layer. Substantial grain refinement was observed after FSP. Shear type deformation texture evolved during FSP with dynamic restoration processes weakening the overall shear texture. The corrosion resistance improved after processing at lower rotational speed due to the presence of basal planes at the surface and the incorporation of TiN in the passive layer. The results of this study suggest that surface properties of cp-Ti like passive layer and corrosion resistance are altered by FSP and can be controlled using appropriate processing parameters.

Dynamic Restoration Processes in a 23Cr-6Ni-3Mo Duplex Stainless Steel: Effect of Austenite Morphology and Interface Characteristics

The austenite and ferrite microstructure evolution and restoration mechanisms were studied during hot uniaxial compression of a 23Cr-6Ni-3Mo duplex stainless steel with two markedly different austenite morphologies (i.e., equiaxed and Widmanstatten). The deformation was performed at a temperature of 1273 K (1000 °C) at a strain rate of 0.1 s−1. The strain was preferentially partitioned in ferrite for both the microstructures studied. Both austenite morphologies displayed frequent splitting into complex-shaped deformation bands, containing dislocation cells and local stacking faults. Equiaxed austenite was favorable to the local development of microbands (MBs), while its Widmanstatten counterpart appeared to be completely resistant to their formation. This was attributed to the complexity of deformation inside the irregularly shaped Widmanstatten plates precluding the formation of self-screening MB arrays. The MB boundaries were typically aligned along highly stressed slip planes. The presence of discontinuous dynamic recrystallization (DDRX) within both the austenite morphologies was very limited. A slightly higher fraction of DDRX was detected in Widmanstatten austenite, compared to equiaxed austenite, which was ascribed to its higher contribution to the overall deformation and lower fraction of low-mobility coherent twin boundaries. Furthermore, it was demonstrated that continuous dynamic recrystallization (CDRX) was the main restoration mechanism within ferrite for both the microstructure types studied. The CDRX development within ferrite was accelerated in the microstructure with equiaxed austenite. This was related to the comparatively lower fraction of coherent interphases in this microstructure, which would hinder the slip transmission across the interphase and make the strain concentrate within ferrite.

Influence of Al3Ni crystallisation origin particles on hot deformation behaviour of aluminium based alloys

Binary Al–Ni, Al–Mg and ternary Al–Mg–Ni alloys containing various dispersions and volume fraction of second-phase particles of crystallisation origin were compressed in a temperature range of 200–500 °C and at strain rates of 0.1, 1, 10, 30 s−1 using the Gleeble 3800 thermomechanical simulator. Verification of axisymmetric compression tests was made by finite-element modelling. Constitutive models of hot deformation were constructed and effective activation energy of hot deformation was determined. It was found that the flow stress is lowered by decreasing the Al3Ni particle size in case of a low 0.03 volume fraction of particles in binary Al–Ni alloys. Intensive softening at large strains was achieved in the alloy with a 0.1 volume fraction of fine Al3Ni particles. Microstructure investigations confirmed that softening is a result of the dynamic restoration processes which were accelerated by fine particles. In contrast, the size of the particles had no influence on the flow stress of ternary Al–Mg–Ni alloy due to significant work hardening of the aluminium solid solution. Atoms of Mg in the aluminium solid solution significantly affect the deformation process and lead to the growth of the effective activation energy from 130–150 kJ/mol in the binary Al–Ni alloys to 170–190 kJ/mol in the ternary Al–Mg–Ni alloy.

Room temperature superelastic responses of NiTi alloy treated by two distinct thermomechanical processing schemes

The room temperature superelastic response of NiTi alloy was investigated through two distinct thermomechanical processing (TMP) approaches. In the first TMP route, the experimental material was primarily cold compressed (up to true strains of 0.2 and 0.5) and then annealed at different temperatures (400 and 500°C) for 10, 30, 60 and 120min. In the second TMP course however, the material was hot compressed at temperatures of 700, 800 and 900°C up to the true strains of 0.2 and 0.5. The results indicated that the cold pressing to true strain of 0.5 followed by annealing at 400°C for 60min on the one hand, and hot pressing at 800°C to true strain of 0.5 on the other hand, were ended to the higher superelasticity effects (2.1% and 3.5% residual strain, respectively) in the courses of applied TMP routes. The detailed analysis showed that the superelasticity was improved at room temperature in the former one through increasing the critical stress for dislocation slip due to the occurrence of static restoration processes and the presence of higher austenite fraction. In the latter case however, the improvement was related to the occurrence of dynamic restoration processes; these were justified by microstructural examinations, precise flow curves analyses and the high transformation enthalpy in the second approach. The overall results directed to the capability of hot deformation to improve the superelasticity effects, which were enhanced formerly by conventional cold processing followed by subsequent annealing methods.

Grain growth and stabilisation of nanostructured aluminium at high temperatures: review

Nanostructured (NS) materials have a large stored energy due to their large grain boundary area and thus tend to be unstable with respect to grain growth during high temperature annealing or deformation. This problem can limit the application of NS materials at high temperatures (>0·5Tm, absolute melting temperature), especially Al alloys owing to their low melting points. Restoration processes and grain growth in NS Al based materials are critically reviewed, with emphasis on nanostructure grain stabilisation at high temperatures. The mechanisms of normal and abnormal grain growth during isothermal annealing are presented, followed by consideration of thermal stabilisation by the addition of solute atoms/impurities and/or dispersion of second phase particles. Grain growth is significantly facilitated by applying deformation at elevated temperatures during preparation or further processing of semifinished NS materials. The dynamic restoration processes, dynamic grain growth and dynamic particle coarsening are addressed in NS Al. Finally, grain growth during consolidation of nanocrystalline powders (one of the principal methods to fabricate bulk NS Al) is presented, and the effects of processing parameters on grain size stabilisation are discussed.

The preservation of the chromatic image of historical cities as a cultural value. The old city of Valencia (Spain)

Abstract Understanding the physical and material characteristics of our historical cities is essential for the preservation of their hereditary and cultural values. Beyond the historical logic of the urban scene and its functional nature, the city is sensed through the spaces, which it creates and configures. The spaces of the historical city represent the soul of its history and the final result of its development. Through the architectural configuration of these spaces, we can visualize the historical and vital logic that underlies the city, the heritage of the people and cultures that make it up. The Study for the Recovery of Urban Spaces in Valencia Historical Center is an interdisciplinary project of intervention in patrimonial architecture, aiming to recover the original image of spaces in the historical city center. This is a project started in 1995 and ended in 2013, which is formed by a series of activities in which both, the Polytechnic University of Valencia as a research institution and the public administrations involved in the processes of regeneration and restoration of the historic center, take part. The aim of the project has been the start-up of a dynamic restoration process of the city's historic urban centre and its landscape value. By combining scientific studies, tasks of awareness and sensitization of population, it aims to determine the formal and chromatic characteristics of original spaces that create the architecture of the city.

An investigation into the homogeneity of microstructure, strain pattern and hardness of pure aluminum processed by accumulative back extrusion

The present work focuses on the evolution of microstructure and mechanical characteristics of commercially pure aluminum during accumulative back extrusion (ABE) processing. The electron backscatter diffraction (EBSD) analysis and microhardness measurements were utilized to study the effect of pass number and deformation route on the microstructure and hardness across the ABE-processed specimens. The distributions of effective strain and shear stress during ABE were studied using finite element analysis with DEFORMTM 3D software. The results demonstrated that an effective plastic strain of about 2 was introduced after each separate pass of ABE. This process produced a high-strength ultrafine-grained material where the grain refinement was attributed to the shearing and/or mechanical subdivision of the initial coarse grains along with dynamic restoration processes. The distributions of both the simulated strain and the measured microhardness were inhomogeneous after the first ABE pass, but thereafter outstanding hardness homogeneity was achieved when the consecutive ABE passes were applied. Moreover, the application of the reversing route was found to introduce a high level of hardness homogeneity after each applied ABE pass.

Hardness and microstructure homogeneity of pure copper processed by accumulative back extrusion

The present work deals with the microstructure evolution of a pure copper processed by a new severe plastic deformation method. A set of pure copper (99.99%) work-pieces with coarse-grained microstructures was processed by accumulative back extrusion (ABE) method at room temperature. The optical and scanning electron microscopy (SEM) and hardness measurements were utilized to study the microstructural evolution and hardness homogeneity. The results indicated that ABE is a capable process to provide a homogenous grain refined microstructure in pure copper. The observed grain refinement was discussed relying on the occurrence of dynamic restoration processes. The analysis of microstructure and hardness showed outstanding homogeneity improvement throughout the work-pieces as the consecutive ABE passes were applied. The homogeneity improvement was attributed to the propagation of the shear bands and also the heavily deformed regions. A reversing route was also applied in the ABE processing to investigate its effect on the development of microstructural homogeneity. Comparing to the conventional route, the application of the reversing route was found to yield better homogeneity after less passes of the process.

Effect of cold rolling on microstructure and mechanical properties of copper subjected to ECAP with various numbers of passes

Microstructure and mechanical properties of copper subjected to 1–10 ECAP passes via route Bc were compared with those for further cold rolled specimens. Microstructure was studied by means of TEM and EBSD analysis in three orthogonal sections. Rolling caused transformation of reasonably equiaxed structure after ECAP into lamellar one due to geometrical effect of deformation, but almost no additional grain refinement occurred during rolling. Significantly higher extent of HABs (about 20% irrespectively to passes number) was found after rolling. This was associated with more pronounced dynamic restoration process during rolling. Mechanical properties were estimated by tension testing at room temperature. Yield strength increased by ∼100MPa after rolling due to finer boundary spacing according to Hall–Petch relationship.

Processing of Ultrafine Grained Cu-30%Zn Alloy through Severe Plastic Deformation Using Accumulative Roll Bonding

In the present research, the microstructural features of ultrafine grained Cu-30 Zn alloy via ARB at room temperature were investigated by X-ray diffraction peak profile analysis. The character of dislocations was determined by analyzing the dislocation contrast factors. The average contrast factors for the different reflections obtained by determination of the type of dislocations and Burgers vectors in crystals. Also, using the modified Williamson–Hall and Warren–Averbach procedure size parameters, the effective outer cut-off radius and density of dislocations were determined. Assuming that the grain size distribution is log-normal, the median and the variance of the size distribution of sub grains were obtained. It was found that the crystallite size is reduced substantially, while the dislocation density increases up to 2 cycles of ARB. After 2nd cycle, dislocation density decreases. This is attributed to the occurrence of dynamic restoration process which takes place during next ARB cycles.

Hot Deformation Characteristics of 34CrMo4 Steel

Abstract The 34CrMo4 (AISI 4130) steel is extensively utilized in the compressed natural gas cylinders. Due to the importance of thermomechanical processing in the production of these cylinders, the dynamic recrystallization (DRX) characteristics of 34CrMo4 steel were investigated. The effect of hot deformation parameters such as temperature and strain rate on the dynamic restoration processes of a 34CrMo4 alloy was studied. Hot compression tests were performed in the temperature range of 900 to 1100 °C and the strain rate range of 0.001 to 0.1 s −1. Hot workability of this alloy has been analyzed by employing flow localization parameter. The flow curves observed are typical of occurrence of dynamic recrystallization and have exhibited single or multiple peaks in flow stress curve before reaching steady state, depending on the temperature and strain rates. The flow stress decreases with the increase of deformation temperature and the decrease of strain rate. Finally the activation energy obtained in this research is 490.749 kJ/mol.