Simulation of dynamic recrystallization using random grid cellular automata

Computer simulation is a powerful tool to predict microstructure and its evolution in dynamic and post-dynamic recrystallization. CAFE proposed as an appropriate approach by combining finite element (FE) method and cellular automata (CA) for recrystallization simulation. In the current study, a random grid cellular automaton (CA), as micro-scale model, based on finite element (FE), as macro-scale method, has been used to study initial and evolving microstructural features; including nuclei densities, dislocation densities, grain size and grain boundary movement during dynamic recrystallization in a C-Mn steel. An optimized relation has been established between mechanical variables and evolving microstructure features during recrystallization and grain growth. In this model, the microstructure is defined as cells located within grains and grain boundaries while dislocations are randomly dispersed throughout microstructure. Changes of dislocation density during deformation are described considering hardening, recovery and recrystallization. Recrystallization is assumed to initiate near grain boundaries and nucleation rate was considered constant (site-saturated condition). The model produced a mathematical formulation which captured the initial and evolving microstructural entities and linked their effects to measurable macroscopic variables (e.g. stress).

Recrystallization in AISI 304 austenitic stainless steel during and after hot deformation

In order to improve the understanding of the dynamic and post-dynamic recrystallization behaviours of AISI 304 austenitic stainless steel, a series of hot torsion test have been performed under a range of deformation conditions. The mechanical and microstructural features of dynamic recrystallization (DRX) were characterized to compare and contrast them with those of the post-dynamic recrystallization. A necklace type of dynamically recrystallized microstructure was observed during hot deformation at 900 °C and at a strain rate of 0.01 s⁻¹. Following deformation, the dependency of time for 50% recrystallization, t₅₀, changed from “strain dependent” to “strain independent” at a transition strain (ε^*), which is significantly beyond the peak. This transition strain was clearly linked to the strain for 50% dynamic recrystallization during deformation. The interrelations between the fraction of dynamically recrystallized microstructure, the evolution of post-dynamically recrystallized microstructure and the final grain size have been established. The results also showed an important role of grain growth on softening of deformed austenite.

Microscale modelling of AISI 304 behaviour during hot deformation

This project aimed to model the microstructure evolution during and following hot deformation using a cellular automaton approach. The flow curves, softening kinetics and final microstructures were used as the input data for the post-deformation simulation to elucidate the effect of dynamic recrystallization on the post-deformation softening.

Hyaluronic acid–collagen network interactions during the dynamic compression and recovery of cartilage

A compression cell designed to fit inside an NMR spectrometer was used to investigate (i) the in situ dynamic strain response and structural changes of the internal pore network, and (ii) the diffusion and flow of interstitial water, in full thickness cartilage samples as they were mechanically deformed under a constant compressive load (pressure) and then allowed to recover (swell again) when the load was removed. Selective enzymatic digestion of the collagen fibril network and the glycopolysaccharide hyaluronic acid (HA) was performed to mimic some of the structural and compositional changes associated with osteoarthritis. Digestion of collagen gave rise to mechanical ‘dynamic softening’ and—perhaps more importantly—nearly complete loss in the ability to recover through swelling, both effects due to the disruption of the hierarchical structure and fibril interconnectivity in the collagen network which adversely affects its ability to deform reversibly and to properly regulate the pressurization and resulting rate and direction of interstitial fluid flow. In contrast, digestion of HA inside the collagen pore network caused the cartilage to ‘dynamically stiffen’ which is attributed to the decrease in the osmotic (entropic) pressure of the digested HA molecules confined in the cartilage pores that causes the network to contract and thereby become less permeable to flow. These digestioninduced changes in cartilage’s properties reveal a complex relationship between the molecular weight and concentration of the HA in the interstitial fluid, and the structure and properties of the collagen fibril pore network, and provide new insights into how changes in either could influence the onset and progression of osteoarthritis.

The microstructure evolution and softening processes during high-temperature deformation of a 21Cr-10Ni-3Mo duplex stainless steel

The austenite and ferrite microstructure evolution and softening mechanisms have been investigated in a 21Cr-10Ni-3Mo duplex stainless steel, containing about 60% austenite, deformed in torsion at 1200°C using a strain rate of 0.7 s-1. The above experimental conditions led to the formation of a small volume fraction of new austenite grains through discontinuous dynamic recrystallization (DDRX), which could not account for the observed large softening on the flow curve. DDRX grains mainly formed through the strain-induced migration of the pre-existing austenite grain boundaries, known to dominate in single-phase austenite, complemented by subgrain growth in the interface regions with ferrite. A significant portion of austenite dynamic softening has been attributed to the large-scale subgrain coalescence, the extent of which increased with strain, which seems to have contributed substantially to the observed flow stress decrease. The above process thus appears to represent an alternative mode of austenite dynamic softening to the classical DDRX in the duplex austenite/ferrite microstructure, characterised by limited availability of the pre-existing austenite/austenite high-angle boundaries, deformed at a high temperature. The softening mechanism within ferrite has been classified as "continuous DRX", characterised by a gradual increase in misorientations between neighbouring subgrains with strain and resulting in the progressive conversion of subgrains into "crystallites" bounded partly by low-angle and partly by large-angle boundaries.

Prediction of masonry compressive behaviour using a damage mechanics inspired modelling method

Masonry under compression is affected by the properties of its constituents and their interfaces. In spite of extensive investigations of the behaviour of masonry under compression, the information in the literature cannot be regarded as comprehensive due to ongoing inventions of new generation products – for example, polymer modified thin layer mortared masonry and drystack masonry. As comprehensive experimental studies are very expensive, an analytical model inspired by damage mechanics is developed and applied to the prediction of the compressive behaviour of masonry in this paper. The model incorporates a parabolic progressively softening stress-strain curve for the units and a progressively stiffening stress-strain curve until a threshold strain for the combined mortar and the unit-mortar interfaces is reached. The model simulates the mutual constraints imposed by each of these constituents through their respective tensile and compressive behaviour and volumetric changes. The advantage of the model is that it requires only the properties of the constituents and considers masonry as a continuum and computes the average properties of the composite masonry prisms/wallettes; it does not require discretisation of prism or wallette similar to the finite element methods. The capability of the model in capturing the phenomenological behaviour of masonry with appropriate elastic response, stiffness degradation and post peak softening is presented through numerical examples. The fitting of the experimental data to the model parameters is demonstrated through calibration of some selected test data on units and mortar from the literature; the calibrated model is shown to predict the responses of the experimentally determined masonry built using the corresponding units and mortar quite well. Through a series of sensitivity studies, the model is also shown to predict the masonry strength appropriately for changes to the properties of the units and mortar, the mortar joint thickness and the ratio of the height of unit to mortar joint thickness. The unit strength is shown to affect the masonry strength significantly. Although the mortar strength has only a marginal effect, reduction in mortar joint thickness is shown to have a profound effect on the masonry strength. The results obtained from the model are compared with the various provisions in the Australian Masonry Structures Standard AS3700 (2011) and Eurocode 6.

Ultrafine grained ferrite formed by interrupted hot torsion deformation of plain carbon steel

A plain carbon steel was deformed using a hot torsion deformation simulator. A schedule known to produce strain-induced ferrite was used with the strain interrupted for increasing intervals of time to determine the effect of an isothermal hold on the final microstructure. Microscopy and electron back-scattered diffraction (EBSD) were used to analyse the changes that occurred in the partially transformed microstructure during the hold and the subsequent applied strain. The strain-induced ferrite coarsened during the hold and this coarsened ferrite was refined during the second deformation. These results were compared to those obtained for a different plain carbon steel deformed in single pass rolling close to the Ar3 temperature.

Effect of carbon content on the recrystallization kinetics of Nb-steels

A rapid method was used to study the effect of carbon content on the kinetics of post-deformation softening, t50, in Nb-steels. The hot deformation behaviour of austenite was not affected by carbon. However, the t50 was influenced by the carbon with different effects in different temperature regimes. At deformation temperatures above the non-recrystallization temperature, Tnr, carbon produced a small change in the softening behaviour. However, the t50 was significantly retarded with increasing carbon content at deformation temperatures lower than Tnr, due to Nb(C,N) precipitates.

The recrystallization behaviour of Nb microalloyed steels

The deformation and recrystallization behaviour of a range of Nb microalloyed steels has been studied using hot torsion. This work focuses on the change from strain dependent to strain independent recrystallization behaviour as a function of the alloy content, initial microstructure and deformation conditions. It is found that there is a complex interaction between deformation, recrystallization and strain induced precipitation, which has significant implications for controlled rolling in hot strip and plate mills. The data also revealed that the pre-existing precipitates did not influence the behaviour of post deformation softening.

Grain size in Mg alloys: recrystallization and mechanical consequences

The present paper examines the development of grain size during the recrystallization of magnesium alloys and the influence the grain size has on the mechanical response. In magnesium alloys grain refinement improves the strength-ductility balance. This simultaneous increase in both strength and ductility is ascribed to the impact the grain size has on deformation twinning. The mechanisms by which the grain size is established during hot working are shown to be conventional dynamic recrystallization followed by post-dynamic recrystallization. The role of alloying additionon both of these reactions is briefly considered.

Characterization on ferrite microstructure evolution during large strain warm torsion testing of plain low carbon steel

Ferrite grain/subgrain structures evolution during the extended dynamic softening of a plain low carbon steel was investigated throughout the large strain warm deformation by hot torsion. Microstructural analysis with electron back-scattering diffraction (EBSD) scanning electron microscope (FEG/SEM) was carried out on the ferrite microstructural parameters. The results showed that the warm flow stress–strain curves are similar to those affected only by dynamic softening and an extended warm flow softening is seen during large strain deformation up to 30. Furthermore, with an increase in strain up to ~ vert, similar1 the grain size of ferrite, misorientation angle and fraction of high-angle boundaries gradually decrease and fraction of low-angle boundaries increases. With a further increase in the strain beyond ~, vert, similar2, these parameters remain approximately unchanged. No evidence of discontinuous dynamic recrystallisation involving nucleation and growth of new grains was found within ferrite. Therefore, the dynamic softening mechanism observed during large strain ferritic deformation is explained by continuous dynamic recrystallization (CDRX).

The use of high dynamic range luminance mapping in the assessment, understanding and defining of visual issues in post occupancy building assessment

The international focus on embracing daylighting for energy efficient lighting purposes and the corporate sector’s indulgence in the perception of workplace and work practice “transparency” has spurned an increase in highly glazed commercial buildings. This in turn has renewed issues of visual comfort and daylight-derived glare for occupants. In order to ascertain evidence, or predict risk, of these events; appraisals of these complex visual environments require detailed information on the luminances present in an occupant’s field of view. Conventional luminance meters are an expensive and time consuming method of achieving these results. To create a luminance map of an occupant’s visual field using such a meter requires too many individual measurements to be a practical measurement technique. The application of digital cameras as luminance measurement devices has solved this problem. With high dynamic range imaging, a single digital image can be created to provide luminances on a pixel-by-pixel level within the broad field of view afforded by a fish-eye lens: virtually replicating an occupant’s visual field and providing rapid yet detailed luminance information for the entire scene. With proper calibration, relatively inexpensive digital cameras can be successfully applied to the task of luminance measurements, placing them in the realm of tools that any lighting professional should own. This paper discusses how a digital camera can become a luminance measurement device and then presents an analysis of results obtained from post occupancy measurements from building assessments conducted by the Mobile Architecture Built Environment Laboratory (MABEL) project. This discussion leads to the important realisation that the placement of such tools in the hands of lighting professionals internationally will provide new opportunities for the lighting community in terms of research on critical issues in lighting such as daylight glare and visual quality and comfort.

Softening and dynamic recrystallization in magnesium single crystals during c-axis compression

Commercial purity (99.8%) magnesium single crystals were subjected to plane strain compression (PSC) along the c-axis at 200 and 370 degrees C and a constant strain rate of 10(-3) s(-1). Extension was confined to the < 1 1 (2) over bar 0 > direction and the specimens were strained up to a logarithmic true strain of -1. The initial rapid increase in flow stress was followed by significant work softening at different stresses and comparable strains of about -0.05 related to macroscopic twinning events. The microstructure of the specimen after PSC at 200 degrees C was characterized by a high density of {1 0 (1) over bar 1} and {1 0 (1) over bar 3} compression twins, some of which were recrystallized. After PSC at 370 degrees C, completely recrystallized twin bands were the major feature of the observed microstructure. All new grains in these bands retained the same c-axis orientation of their compression twin hosts. The basal plane in these grains was randomly rotated around the c-axis, forming a fiber texture component. The obtained results are discussed with respect to the mechanism of recrystallization, the specific character of the boundaries between new grains and the initial matrix, and the importance of the dynamically recrystallized bands for strain accommodation in these deformed magnesium single crystals. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.