Strain localisation patterns under equal-channel angular pressing

Instabilities of plastic flow in the form of localised shear bands were experimentally observed to result from equal-channel angular pressing (ECAP) of magnesium alloy AZ31. The appearance of shear bands and their spacing were dependent on velocity of the pressing and applied back-pressure. A generic gradient plasticity theory involving second-order strain gradient terms in a constitutive model was applied to the case of AZ31 deformed by ECAP. Linear stability analysis was applied to the set of equations describing the deformation behaviour in the process zone idealised as a planar shear zone. A full analytical solution providing a dispersion relation between the rate of growth of a perturbation and the wave number was obtained. It was shown that the pattern of incipient localised shear bands exhibits a spectrum of characteristic lengths corresponding to admissible wave numbers. The interval of the spectrum of wave numbers of viable, i.e. growing, perturbations predicted by linear stability analysis was shown to be in good agreement with the experimentally observed spectrum. The effect of back-pressure applied during ECAP was also considered. The predicted displacement of the shear band spectrum towards lower wave numbers, shown to be a result of the decreased shear strain rate in the shear zone, was consistent with the experimentally observed increase of the band spacing with increased back-pressure. A good predictive capability of the general modelling frame used in conjunction with linear stability analysis was thus demonstrated in the instance of the particular alloy system and the specific processing conditions considered.

Numerical modeling of the dynamic tensile behavior of irregular fibers

Most fibers are irregular, and they are often subjected to rapid straining during mechanical processing and end-use applications. In this paper, the effect of fiber dimensional irregularities on the dynamic tensile behavior of irregular fibers is examined, using the finite element method (FEM). Fiber dimensional irregularities are simulated with sine waves of different magnitude (10%, 30% and 50% level of diameter variation). The tensile behavior of irregular fibers is examined at different strain rates (333%/sec, 3,333%/sec and 30,000%/sec). The breaking load and breaking extension of irregular fibers at different strain rates are then calculated from the finite element model. The results indicate that strain rate has a significant effect on the dynamic tensile behavior of an irregular fiber, and that the position of the thinnest segment along the fiber affects the simulation results markedly. Under dynamic conditions, an irregular fiber does not necessarily break at the thinnest segment, which is different from the quasi-static results.

Static and metadynamic recrystallization of interstitial free steels during hot deformation

A rapid method was used to identify kinetics of the recrystallization for two IF (Interstitial Free) steels which have different phosphorous and boron contents. The static and metadynamic softening behaviour of the materials for a range of strain rates and temperatures were quantified. The critical strain for initiation of strain independent softening was estimated for the IF steels in respect to the time for 50 percent softening after deformation. The results showed that the strain for the initiation of strain independent softening (often referred to as metadynamic recrystallization) varies with the Zener Hollomon parameter. Classic static recrystallization was observed at strains below the strain independent softening for all processing conditions and the strain rate had a strong effect on the time for strain independent softening. Results also revealed that static and metadynamic recrystallization was delayed owing to the phosphorous and boron alloying elements. Hence, the large strain at above no-recrystallization temperature may be required for the early stage of Finishing Stands Unit (FSU) in hot strip rolling mills to initiate austenite grain refinement of phosphorous and boron added IF steels.

Ultrafine grained structure formation in steels using dynamic strain induced transformation processing

The refinement of ferrite grain size is the most generally accepted approach to simultaneously improve the strength and toughness in steels. Historically, the level of ferrite refinement is limited to 5-10 μm using conventional industrial approaches. Nowadays, though, several thermomechanical processes have been developed to produce ferrite grain sizes of 1-3 μm or less, ranging from extreme thermal and deformation cycles to more typical thermomechanical processes. The present paper reviews the status of the production of ultrafine grained steels through relatively simple thermomechanical processing. This requires deformation within the Ae3 to Ar3 temperature range for a given alloy. Here, the formation of ultrafine ferrite (UFF) involves the dynamic transformation of a significant volume fraction of the austenite to ferrite. This dynamic strain induced transformation (DSIT) arises from the introduction of extensive intragranular nucleation sites that are not present in conventional controlled rolling. The DSIT route has the potential to be adjusted to suit current industrial infrastructure. However, there are a number of significant issues that have been raised, both as gaps in our understanding and as obstacles to industrial implementation. One of the critical issues is that it appears that very large strains are required. Combined with this concern is the issue of whether a combination of dynamic and static transformation can be used to achieve an adequate level of refinement. Another issue that has also become apparent is that grain sizes of 1 μm can lead to low levels of ductility and hence many workers are attempting to obtain 2-3 μm grains, or to introduce a second phase to provide the required ductility. There are also a number of areas of disagreement between authors including the role of dynamic recrystallisation of ferrite in the production of UFF by DSIT, the reasons for the low coarsening rate of UFF grains, the role of microalloying elements and the effects of austenite grain size and strain rate. The present review discusses these areas of controversy and highlights cases where experimental results do not agree.

Dynamic softening of ferrite during large strain warm deformation of a plain-carbon steel

The evolution of dynamic ferrite softening in a plain-carbon steel was investigated by torsion tests during warm deformation at 810 °C, in the two-phase (ferrite + austenite) region, and strain rate of 0.1 s−1 with different strains up to 50. The warm flow behaviour and ferrite microstructural parameters, such as grain size, misorientation angle across ferrite/ferrite boundaries, and the fraction of high-angle and low-angle grain/subgrain boundaries were quantified using electron back scatter diffraction. The results show that with increasing strain up to not, vert, similar2, the ferrite grain size and fraction of high-angle boundaries rapidly decrease and the fraction of low-angle boundaries increases. However, these parameters remain approximately unchanged with increasing strain from not, vert, similar2 to 50. The dynamic softening mechanism observed during large strain ferritic deformation is explained by dynamic recovery and continuous dynamic recrystallization.

Enhanced tensile ductility of an ultra-fine-grained aluminium alloy

Aluminum alloy 6082 was subjected to equal-channel angular pressing (ECAP), which resulted in an ultra-fine-grained (UFG) microstructure with an average grain size of 0.2–0.4 μm. There was a pronounced effect of the grain refinement on the strain-rate sensitivity and tensile ductility. The Hart criterion of tensile necking fails to explain the observed ductility of the UFG material at low strain rates. A correlation between the observed stronger-than-expected ductility and a tendency to microshear band formation at low strain rates was established.

Tensile deformation of a ultrafine-grained aluminium alloy: micro shear banding and grain boundary sliding

This work investigates the relationship between the strain rate and the ductility and the underlying deformation mechanisms in an ultrafine-grained Al6082 alloy. At room temperature the uniform elongation of the material exhibits a marked increase with decreasing strain rate. This effect is related to the activation of micro shear banding, which is controlled by grain boundary sliding. The contribution of these mechanisms to uniform elongation is estimated. It is proposed that the grain boundary sliding suppresses the transformation of micro shear bands into macro shear bands. The activity of other deformation mechanisms during plastic deformation of the ultrafine-grained material is also discussed.

Deformation behaviour of an ultra-fine grained Al6082 alloy

This work focuses on the effect of strain rate on the deformation behaviour of an ultrafine grained Al alloy 6082 produced by equal channel angular  pressing. The uniform tensile elongation was found to increase with  decreasing strain rate very substantially. This effect is discussed in terms of the mechanisms that control plastic deformation of the alloy.

Hot deformation and recrystallization of austenitic stainless steel: Part 11. post-deformation recrystallization

The postdeformation recrystallization behavior of a hot-deformed austenitic stainless steel was investigated based on the first part of this study, in which the microstructure development during hot deformation and, in particular, the evolution of dynamic recrystallization (DRX), was studied. The effect of different parameters such as strain, strain rate, and temperature were examined. The dependency of the time for 50 pct softening, t 50, changed from “strain dependent” to “strain independent” at a transition strain (ε*) that was in the steady-state area of the hot deformation flow curve. The fully recrystallized microstructure showed a similar transition in strain sensitivity. However, this occurred at stains greater than ε*. A mathematical model was developed to predict the transition strain under different deformation conditions. Microstructural measurements show that the transition strain corresponds to approximately 50 pct DRX in the deformed structure at the point of unloading.

Hot deformation and recrystallization of austenitic stainless steel: part 1. dynamic recrystallization

The hot deformation behavior of a 304 austenitic stainless steel was investigated to characterize the evolution of the dynamically recrystallized structure as a starting point for studies of the postdeformation  recrystallization behavior. The effect of different deformation parameters such as strain, strain rate, and temperature were investigated. The flow curves showed typical signs of dynamic recrystallization (DRX) over a wide range of temperatures and strain rates (i.e., different Zener–Hollomon (Z) values). However, under very high or very low Z values, the flow curves’ shapes changed toward those of the dynamic recovery and multiple peaks, respectively. The results showed that while DRX starts at a strain as low as 60 pct of the peak strain, a fully DRX microstructure needs a high strain of almost 4.5 times the initiation strain. The DRX average grain size showed power-law functions with both the Zener–Hollomon parameter and the peak stress, although power-law breakdown was observed at high Z values.

Deformation mechanisms in an ultra-fine grained Al alloy

This work focuses on the deformation behavior of an ultra-fine grained Al-Mg-Si alloy processed by equal channel angular pressing over a wide range of temperatures and strain rates. The effect of temperature and strain rate on the homogeneity of plastic deformation, the evolution of microstructure, the strain rate sensitivity and the underlying deformation mechanisms are investigated. It is demonstrated that the localization of plastic deformation at the micro scale is triggered by grain boundary sliding due to grain boundary sliding due to grain boundary diffusion. The contributions of different deformation mechanisms during the plastic deformation of the material are discussed.

Microstructure evolution modeling during and after deformation in 304 austenitic stainless steel through cellular automaton approach

A 2D cellular automation approach was used to simulate microstructure evolution during and after hot deformation. Initial properties of the microstructure and dislocation density were used as input data to the cellular automation model. The flow curve and final grain size were the output data for the dynamic recrystallization simulation, and softening kinetics curves were the output data of static and metadynamic recrystallization simulations. The model proposed in this work considered the effect of thermomechanical parameters (e.g., temperature and strain rate) on the nucleation and growth kinetics during dynamic recrystallization. The dynamic recrystallized microstructures at different strains, temperatures, and strain rates were used as input data for static and metadynamic recrystallization simulations. It was shown that the cellular automation approach can model the final microstructure and flow curve successfully in dynamic recrystallization conditions. The postdeformation simulation results showed that the time for 50% recrystallization decreases with increasing strain for a given initial grain size and that dynamic recrystallization slows the postdeformation recrystallization kinetics compared to a model without dynamic recrystallization.

Microstructure-property relationship of the high strength low alloy steels during tensile testing at high strain rates

Positive The influence of strain-rate on the room temperature mechanical properties of the Dual-Phase and Transformation Induced Plasticity (TRIP) steels was investigated.The results showed that both the plastic strain, and strength properties increased with increasing strain rates at high strain rates.At strain rates lower than approximateil 1s ~ (-1) the properties no longer have an advantageous proportionality to strain rate and remain strain rate neutral.Possible explanations are offered for trends exhibited, in terms of thermal and athermal considerations, in relation to the respective microstructures of the two steels

Effect of roller leveling on the bending behaviour of aged steel strip

The common grades of steel used in roll forming are: hot rolled carbon steel, high strength low alloy and recovery annealed cold rolled sheet. These steels are prone to ageing and are often skin passed and/or roller leveled to eliminate ageing as it can lead to problems in forming. In roll forming, shape defects such as bow, twist and camber are considered to be related to very small plastic strains in the longitudinal direction and hence knowledge of the material properties in the elastic plastic transition range is necessary if the process is to be modelled accurately. Previous studies with aluminium have indicated that skin pass rolling can lead to residual stresses in the strip. In this work, the study was extended to aged carbon steel and to the effect of roller leveling on both aged material and strip that had been given a light cold rolling to simulate a skin pass treatment. The results suggest that roller leveling reduced the magnitude of residual stresses resulting from skin pass rolling.

The significant differences observed between tensile and bending test results, at and near, the elastic plastic transition reinforces the need to consider bending properties when assessing the effect of prior processing on strip for roll forming.