Dynamic Recovery Process Research Articles

The evolutions of mechanical properties and microstructures of Al-Mg-Mn-Sc-Zr alloy during dynamic stretching deformation

Metal materials usually show better mechanical properties during dynamic deformation. Al-Mg-Mn-Sc-Zr alloy is taken as the object in this research and the dynamic stretching behaviors are studied at a wide range of strain rate (0.001 s−1–800 s−1). This research finds that the peak stress and elongation of Al-Mg-Mn-Sc-Zr alloy exhibit significant increasing tends as the strain rates increase. The alloy’s work hardening ability is also improved. During dynamic stretching processes, the deformation time is short and there is not enough time for dislocations to undergo annihilation and rearrangement processes. The dynamic recovery process is severely inhibited, which not only retains the higher density of dislocations, but also improves the uniformity of dislocation distributions. As the strain rate increases, the movement velocity of the dislocations also increase, resulting in a decrease in the width of the dislocations. The increase on the dislocation density and the decrease on the width of the dislocations increase the resistance to the movement of the dislocations. As a result, the peak strength enhances significantly. Dislocation with high movement velocity can cut through Al3(Sc,Zr) particles when the strain rate exceed 500 s−1. Consequently, the formation of dislocation tangles is inhibited, leading to the reduction of flow localization areas and improvement of the uniformity of dislocation distributions. The improvement of dislocation distribution uniformity improves the plasticity of the alloy during dynamic deformation significantly.

Use of Sentinel-2 Satellite Data for Windthrows Monitoring and Delimiting: The Case of “Vaia” Storm in Friuli Venezia Giulia Region (North-Eastern Italy)

On the 29th of October 2018, a storm named “Vaia” hit North-Eastern Italy, causing the loss of 8 million m3 of standing trees and creating serious damage to the forested areas, with many economic and ecological implications. This event brought up the necessity of a standard procedure for windthrow detection and monitoring based on satellite data as an alternative to foresters’ fieldwork. The proposed methodology was applied in Carnic Alps (Friuli Venezia Giulia, NE Italy) in natural stands dominated by Picea abies and Abies alba. We used images from the Sentinel-2 mission: 1) to test vegetation indices performance in monitoring the vegetation dynamics in the short period after the storm, and 2) to create a windthrow map for the whole Friuli Venezia Giulia region. Results showed that windthrows in forests have a significant influence on visible and short-wave infrared (SWIR) spectral bands of Sentinel-2, both in the short and the long-term timeframes. NDWI8A and NDWI were the best indices for windthrow detection (R2 = 0.80 and 0.77, respectively) and NDVI, PSRI, SAVI and GNDVI had an overall good performance in spotting wind-damaged areas (R2 = 0.60–0.76). Moreover, these indices allowed to monitor post-Vaia forest die-off and showed a dynamic recovery process in cleaned sites. The NDWI8A index, employed in the vegetation index differencing (VID) change detection technique, delimited damaged areas comparable to the estimations provided by Regional Forest System (2545 ha and 3183 ha, respectively). Damaged forests detected by NDWI8A VID ranged from 500 m to 1500 m a.s.l., mainly covering steep slopes in the south and east aspects (42% and 25%, respectively). Our results suggested that the NDWI8A VID method may be a cost-effective and accurate way to produce windthrow maps, which could limit the risks associated with fieldwork and may provide a valuable tool to plan tree removal interventions in a more efficient way.

Maladaptation, fragmentation, and other secondary effects of centralized post-disaster urban planning: The case of the 2011 “cascading” disaster in Japan

Previous studies have documented the negative impacts and unexpected secondary effects of post-disaster housing development. Here, we build on this tradition to explore how post-disaster urban planning and risk mitigation measures affect internal migrations after a major disaster. In the aftermath of the 2011 Great East Japan Earthquake and Tsunami, the Japanese government put forward significant efforts to provide safe housing and land in more than 20 cities. We used GIS spatial analysis to identify urban footprint changes, which proved to be reliable indicators of internal migration. Our results reveal the secondary effects of planning interventions, and more specifically, how the maladaptation measures triggered rapid urban sprawl and increased risks of landslides and vulnerabilities in mountainous areas. We also find increased urban fragmentation, both socially and spatially. Maladaptation, urban fragmentation, and rapid changes in urban footprints emerged as the consequences of centralized government-mandated planning and housing development. We conclude that the uncertainty surrounding dynamic recovery processes requires incremental adaptive action. Planners and local authorities must recognize and remain attentive to the cascading effects of centralized planning decisions.

Solid solution strengthening mechanism in high pressure die casting Al–Ce–Mg alloys

A series of Al-xCe (x = 2, 4, 6, 8, 10 wt%) and Al–8Ce-yMg (y = 0, 0.10, 0.25, 0.50, 0.75 wt%) alloys were prepared by high-pressure die casting. The introduced cerium element promoted the nucleation of α-Al grains. Al11Ce3 phase served as the heterogeneous nucleation substrate of α-Al due to the small lattice mismatch of 6.72%. According to the results calculated by Nelson-Riley extrapolation function, the lattice constant of α-Al increased from 4.0511 Å to 4.0540 Å with the increasing of Mg content from 0 wt% to 0.75 wt%. Due to the solid solution strengthening effect of Mg atoms, the yield strength of Al–8Ce-yMg alloys and the hardness of α-Al matrix in the Al–8Ce-yMg alloys showed a parabolically increasing tendency, from 92 MPa to 115 MPa and 0.502 GPa–0.575 GPa, respectively. The work hardening capacity of Al–8Ce-yMg alloys was improved by the solid solution of Mg with the work hardening exponent increasing from 0.21 to 0.27. The solid solution of Mg atoms reduced the stacking fault energy of Al–8Ce-yMg alloys and suppressed the dynamic recovery process of the alloys during deformation process, which promoted the formation of dislocation tangles and dislocation networks in the α-Al matrix.

Multiscale constitutive modeling of additively manufactured Al–Si–Mg alloys based on measured phase stresses and dislocation density

To better understand and predict the mechanical properties of additive manufacturing (AM) Al–Si–Mg alloys, developing a physically-based constitutive model is crucial. Among different models, the dislocation-density-based Kocks-Mecking (K-M) constitutive model has been widely used. Unfortunately, two challenges arise when the K-M model is used for the multiphase Al alloys. Firstly, an accurate K-M model demands the separation of phase stresses. Secondly, a thorough analysis of the K-M model involves the measurement of dislocation density during deformation. In-situ neutron diffraction, a powerful method to measure the phase stress and dislocation density in bulk polycrystalline materials under loading, is employed to investigate the AM AlSi3.5Mg1.5 and AlSi3.5Mg2.5 (wt.%) alloys. Based on the present results and reported data of AlSi10Mg, a multiscale constitutive model is developed for different AM Al–Si–Mg alloys. At the microscale, the evolution of dislocation density in the Al matrix with plastic strain can be well predicted by the K-M model. Meanwhile, the developments of the average stresses in different phases with plastic strain can be well captured by the Voce model. The measured microscopic k2/nc values agree with the theoretical value two of the K-M model quite well. Here, nc is the characteristic factor of the microscale Voce model, while k2 is the coefficient associated with the dynamic recovery process in the microscale K-M model. At the macroscale, the mechanical behavior can also be well reproduced by the K-M model and the Voce model. However, the macroscopic K2/Nc ratio is far away from two, where Nc and K2 are the characteristic factor and coefficient of the macroscale Voce model and K-M model, respectively.

Thermal deformation behavior of Stellite 6B alloy

Stellite 6B is carbide-hardened Co-based superalloy. It finds extensive application in the harsh industrial environment, because of excellent wear characteristics, high hardness, good corrosion resistance, and superior mechanical properties. Hot compression tests were performed on Stellite 6B alloy to study high temperature dynamic recrystallization and microstructure evolution during thermal deformation. The tests were performed in the temperature 1000 °C, 1050 °C, 1100 °C, 1150 °C and 1200 °C and at the strain rates of 0.01 s−1, 0.1 s−1, 1 s −1 and 10 s−1. Stress-strain curves, and the microstructural evolution of the Stellite 6B alloy were investigated. The results showed that the recrystallization was nucleated at the carbides, original grain boundaries and twin grain boundaries, the dynamic recovery process is inhibited, and the softening effect by dynamic recrystallization is more significant.

Feedback Control for the Precise Shape Morphing of 4D-Printed Shape Memory Polymer

Four-dimensional printing (4DP) is a newly emerged technology that uses smart materials for additive manufacturing and thus enables shape and/or property change upon stimulus after the printing process. Present study on 4DP has been focused on open-loop stimulus, which can hardly ensure high shape precision and predictable final states. In this article, a new closed-loop 4DP (CL4DP) process supplementing 4D printed actuation with closed loop control methods is proposed. Image feedback is used for enhancing the conventional open loop 4DP morphing process and a controller is implemented to regulate the intensity of the stimulus accordingly in real-time. To achieve precise control, a nonlinear affine system model is built by model identification with measurement data to describe the dynamic shape recovery process of the 4D-printed shape memory polymer (SMP). Precise shape control is achieved and the effects of controller parameters on the precision of CL4DP are studied. Traditionally, SMP has a discrete number of selected steady states. With CL4DP, such steady states can be continuous and arbitrary.

Single-cell individualized electroporation with real-time impedance monitoring using a microelectrode array chip

The ability to precisely deliver molecules into single cells while maintaining good cell viability is of great importance to applications in therapeutics, diagnostics, and drug delivery as it is an advancement toward the promise of personalized medicine. This paper reports a single-cell individualized electroporation method with real-time impedance monitoring to improve cell perforation efficiency and cell viability using a microelectrode array chip. The microchip contains a plurality of sextupole-electrode units patterned in an array, which are used to perform in situ electroporation and real-time impedance monitoring on single cells. The dynamic recovery processes of single cells under electroporation are tracked in real time via impedance measurement, which provide detailed transient cell states and facilitate understanding the whole recovery process at the level of single cells. We define single-cell impedance indicators to characterize cell perforation efficiency and cell viability, which are used to optimize electroporation. By applying the proposed electroporation method to different cell lines, including human cancer cell lines and normal human cell lines individually, optimum stimuli are determined for these cells, by which high transfection levels of enhanced green fluorescent protein (EGFP) plasmid into cells are achieved. The results validate the effectiveness of the proposed single-cell individualized electroporation/transfection method and demonstrate promising potential in applications of cell reprogramming, induced pluripotent stem cells, adoptive cell therapy, and intracellular drug delivery technology.

Effect of Processing Conditions on the Microstructure, Mechanical Properties, and Corrosion Behavior of Two Austenitic Stainless Steels for Bioimplant Applications

Hot torsion tests were carried out to simulate the industrial thermomechanical processing of two austenitic steels for bioimplant applications, namely ISO 5832-9 and ASTM F138. The former has Ti, Nb, and V in the composition, being N-rich. However, the latter is Ni-richer and without extra alloying element additions. Special attention was paid to the effect of interpass times, particularly to the soaking temperature, which was reduced to decrease processing times and costs. Optical and electron microscopy, corrosion tests, and hardness measurements were used to characterize the effect of the above processing parameters on both alloys. No significant increase in processing loads was noticed after the reduction of the reheating temperature. This was explained in terms of the balance between partial particles dissolution and the increment in the solute drag effect provided by the elements put into solution. Such an increment in solid solution favored the dynamic recovery process, delaying the dynamic recrystallization one. However, strain-induced precipitation took place at lower temperatures, by using the extra N and Cr delivered to the matrix, and limiting the recrystallization softening. The rolling schedule promoted abundant grain refinement. The final grain size ranged from 2.5 to 11 µm, depending on reheating temperature, interpass time, presence of alloying elements, and N. In general terms, the corrosion resistance of the ISO steel soaked at the lowest temperature (1200 °C) was better than when reheated to the highest one (1250 °C). On the contrary, the F138 steel had worse corrosion behavior.

Mechanical response and microstructure evolution of commercially pure titanium subjected to repetitive bending under tension

The aim of this work was to study the cold formability of commercially pure Titanium alloy (Grade 2) by a testing methodology known as repetitive bending under tension (R-BUT). A dedicated test rig to perform the test was designed and fabricated to analyse the room temperature deformation behaviour of Ti-50A alloy sheet of 1.2 mm thickness. Samples from three different orientations were tested to investigate the effect of mechanical anisotropy on deformation behaviour. The results confirmed a significant increase in elongation to failure in samples subjected to R-BUT as compared to those subjected to standard tensile tests under similar conditions. This may be due to a delay in localised necking during R-BUT. Finite element analysis (FEA) suggested that a decrease in stress triaxiality in R-BUT could be the reason for enhanced formability of the materials compared to the conventional tension. Furthermore, electron backscatter diffraction (EBSD) study of the microstructure confirmed the development of highly strained regions that eventually lead to the formation of fine grains in the samples subjected to R-BUT. This could be due to a strain induced dynamic recovery process occurring during the test.

Modeling Uncertain and Dynamic Interdependencies of Infrastructure Systems Using Stochastic Block Models

Abstract Modeling the resilience of interdependent critical infrastructure (ICI) requires a careful assessment of interdependencies as these systems are becoming increasingly interconnected. The interdependent connections across ICIs are often subject to uncertainty due to the lack of relevant data. Yet, this uncertainty has not been properly characterized. This paper develops an approach to model the resilience of ICIs founded in probabilistic graphical models. The uncertainty of interdependency links between ICIs is modeled using stochastic block models (SBMs). Specifically, the approach estimates the probability of links between individual systems considered as blocks in the SBM. The proposed model employs several attributes as predictors. Two recovery strategies based on static and dynamic component importance ranking are developed and compared. The proposed approach is illustrated with a case study of the interdependent water and power networks in Shelby County, TN. Results show that the probability of interdependency links varies depending on the predictors considered in the estimation. Accounting for the uncertainty in interdependency links allows for a dynamic recovery process. A recovery strategy based on dynamically updated component importance ranking accelerates recovery, thereby improving the resilience of ICIs.

Influence of Strain Amplitude on the Microstructural Evolution and Flow Properties of Copper Processed by Multidirectional Forging

Commercial‐purity copper (99.8%) is processed by multidirectional forging (MDF) using strain amplitudes per compression (Δε) of 0.15 and 0.30, leading to accumulated strains (ε) of up to ≈10.8. It is shown that increasing the accumulated strain causes continuous reactions at the substructural level, involving the progressive evolution of dislocation arrangements toward structures having high misorientation angles. This evolution depends upon the strain amplitude in MDF processing and is characterized by the fragmentation of the original grains due to formation and intersection of micro shear bands (MSBs) assisted by dynamic recovery processes. Higher Δε enhances the “monotonic” character of processing, resulting in a higher fraction of MSBs and band intersections, increased work hardening, flow stresses, and dislocation density, lower cell/subgrain and grain sizes, and faster grain refinement kinetics, compared with MDF under lower Δε. The yield strength of copper, measured along a direction orthogonal to that of the previous compression step, is lower than the flow stress at the end of this compression step, and this behavior becomes more prominent with increasing ε and Δε.

Phase transformation and evolution of dislocation structure in the β phase of Ti-17 alloy during hot deformation

The hot deformation behavior and microstructural evolution of Ti–5Al–2Sn–2Zr–4Mo–4Cr (Ti-17) alloy in the β-phase field were examined. Hot compression tests were conducted at temperatures ranging from 700 and 1100 °C and strain rates ranging from 10-3 to 1 s-1. The obtained true stress–true strain curves exhibited steady-state flow behavior at temperatures above the β-transus temperature (Tβ ~ 890 °C). The microstructural observations suggested that the dynamic recovery process represents a dominant mechanism for the β-phase of the alloy. In contrast, the flow softening was identified at 700 °C at a strain rate of 10–3 s–1. The formation of fine acicular α microstructures was noticed in such a deformation condition, indicating that dynamic phase transformation can occur in the metastable β-phase during hot deformation. Such a dynamic phase transformation also occurred preferentially at the initial β-grain boundaries as well as sub-boundaries that created within the βmatrix via dynamic recovery. The β-processing conditions were also examined by constructing processing maps based on the dynamic materials model (DMM).

Influence of Cold Plastic Deformation on the Structure and Physicomechanical Properties of the Biocompatible Low-Modulus Zr51Ti31Nb18 Alloy

The influence of the degree of compression in the range of 46–84% upon cold rolling of rods of a biocompatible low-modulus Zr51Ti31Nb18 (IMP BAZALM) alloy preliminarily quenched from the β field on the formation of its structure, phase composition, and physicomechanical properties (hardness, elastic modulus) has been studied by the methods of optical microscopy, transmission electron microscopy, X-ray diffraction analysis, back-scattering electron diffraction, and microindentation. It has been established that with an increase in the degree of compression, a reorientation and elongation of the initially equilibrium β grains occur along the direction of rolling with the formation of a perfect filamentary structure in the rods with a minimum cross-section. The values of microhardness are stabilized in the range of 320–325 HV due to the development of dynamic recovery processes in the deformed structure, and the elastic modulus decreases from 68 to 55 GPa in the rolling plane of rods owing to the improvement of the {001}RP〈110〉RD-type texture, which leads to the appearance of the predominantly “low-modulus” 〈110〉 orientation in the direction of measurement.

Dynamic self-recovery of injured Escherichia coli O157:H7 induced by high pressure processing

This study was aimed to investigate the dynamic recovery process of high pressure processing (HPP)-injured Escherichia coli O157:H7. E. coli O157:H7 was treated with 100–500 MPa for 5 min, and the pressure with highest injury rate was selected to determine the optimal temperature and duration for repairing HPP-injured cells in PBS. The morphological and physiological changes during the repair process under the above optimal conditions were further investigated. The results showed a stronger inhibition effect with increased pressure, while the lethal effect was not consistent with the injury, since the highest injury rate was observed at 400 MPa for 5 min. Next, HPP-treated cells (400 MPa, 5 min) were repaired in PBS at different temperatures (20, 25, 30, 37 °C) for 108 h, and the optimal recovery condition of the injured cells was found to be 25 °C for 24 h. During the repair process, SEM and TEM morphology, leakage of ions, activities of Na+K+-ATPase and Ca2+Mg2+-ATPase, and the membrane fatty acid composition of HPP-treated cells gradually return to normal levels found in non-HPP cells, which confirmed the dynamic recovery process of HPP-injured E. coli O157:H7, as well as that HPP-injured E. coli O157:H7 could be repaired under appropriate conditions.

The effect of TiO2 particles on microstructural evolutions of HSLA steels subjected to friction stir welding

A composite of High Strength Low Alloy X70 steel/TiO2 was achieved through the Friction Stir Welding process. A homogeneous mixture of primary TiO2 particles (∼300 μm in size) and X70 steel particles (∼200 μm in size) were inserted into the groove of X70 steel. The TiO2 particles were fragmented to micron and sub-micron size particles and were dispersed in the nugget zone resulting in different microstructural properties in the nugget zone. The development of microstructure was characterized using optical and scanning electron microscopy as well as the electron backscatter diffraction technique. A very fine acicular ferrite microstructure due to the dynamic recovery process develops in the structure at the presence of sub-micron particles having 370 HV hardness. This increase in hardness is much greater than 195 and 165 HV of FSW layer with no TiO2 inserted and the as-received substrate. At the presence of micron-sized particles, the polygonal ferrite microstructure develops due to the dynamic recrystallization process having a hardness of 180 HV. The effect of particles on improving the hardness in the nugget zone is about 80 HV accompanying a very fine ferrite microstructure.

Dynamic recrystallization-dependent damage modeling during hot forming

Hot forming processes are extensively used to produce semi-finished and finished components. At elevated temperatures, dynamic recovery and recrystallization processes occur that enable large shape changes at low forming forces. In steel, non-metallic inclusions cannot be avoided during metallurgical processes. They may induce damage in the same way as at room temperature, but the softening of the matrix due to dynamic recrystallization may be used to control the initiation and progression of damage. Damage models do not take into account that dynamic recrystallization reduces the local stresses at the interface of matrix–inclusion. The purpose of the presented study is to analyze the interaction between dynamic recrystallization and damage initiation. Using representative volume elements, the influence of dynamic recrystallization on damage initiation is studied at different temperatures, strain rates, and stress states. Based on this study, a damage model is devised that couples dynamic recrystallization and damage on the macro level and hence can be used in macroscopic process simulations.

Similar yet unique: the victim's journey after acute sexual assault and the importance of continuity of care.

To examen the short-term and long-term responses of sexual assault victims who attended a sexual assault centre. Semi-structured interviews were held with twelve victims of sexual assault who received help from a sexual assault centre. Analyses were done in Atlas.ti. via a process of open, axial and selective coding. Shortly after assault, the victims' response was to strike a balance between denial and acknowledgement that the violence was real and not their fault. In the ling term most victims experienced a dynamic recovery process with fluctuating responses. Their social support network played a crucial role in reaching out for professional care. Shortly after assault sexual violence victims need the violence to be acknowledged by skilful, empathic care providers. In the long term, victims experience vitims experience a dynamic recovery process with fluctuating responses in which continuity of care is of the utmost importance.

Formation and Structure Features of the Weld Joints Made by Friction Stir Welding

The microstructure of the weld core and the heat-affected zone, as well as the zone affected by thermal strain located between them, formed during friction stir welding (FSW) of aluminum alloys, copper alloys, 12Kh18N10T steel, and OT4-1, VT-1 alloys is investigated. The core of the welded joints at FSW is formed in the structural superplasticity mode, and its structure has a shear-banded structure of the “onion pattern” type containing a large number of lamellae forming the shear bands under the influence of the movable welding tool. Attainment of the superplastic state during formation of the weld core is ensured by the gradual flow of various plastic deformation mechanisms in the mode of simple, collective, and anomalous dynamic recrystallization initiated by the dynamic recovery and polygonization processes with transition to post-dynamic recrystallization by the Bailey–Hirsch and Kahn–Burgers–Taylor mechanisms. The superplastic state is also maintained owing to twinning recrystallization (copper, steel, Ti and its alloys) and as consequence of α → γ or α → β phase transformations for a group of metals and alloys possessing multiple forms (high-alloy steel, titanium alloy).

Role of hierarchical microstructure of additively manufactured AlSi10Mg on dynamic loading behavior

Microstructure of an additively manufactured AlSi10Mg through direct metal laser sintering (DMLS) process is studied using multi-scale characterization techniques including scanning electron microscopy, electron backscatter diffraction, and transmission electron microscopy. The microstructure of DMLS-AlSi10Mg consists of hierarchical characteristics, spanning three order of magnitude, where nanometer sized to sub-millimeter scaled features exist in the structure. These characteristics included grain and cell structures, nanoscale Si precipitates and pre-existing dislocation networks. Dynamic mechanical behavior of the material is studied using a Split Hopkinson Pressure Bar apparatus over a range of strain rates varying between 800 s−1 and 3200 s−1. Investigation of the deformed microstructures reveals the role of hierarchical microstructure on the dynamic behavior of the material. The high strain-rate deformation is accommodated by dynamic recovery (DRV) process, where low angle grain boundaries evolve due to the generation of dislocations, evolution of dislocation networks, and annihilation of dislocations. Both cell walls and Si precipitates contribute to impeding the dislocation motion and development of dislocation networks. At high strain rates, dislocation networks evolve in the nanoscale DRVed subgrains.