Dynamic adjustment of ferrite grains during dynamic strain induced transformation

The dynamic adjustment of ferrite grains formed during 'dynamic strain induced transformation (DSIT)' is an important feature of this mechanism that has not been addressed previously. A novel experimental method was applied to follow the effect of deformation at different stages on ferrite formed initially through DSIT. It is shown that while the continuous dynamic recrystallisation (CDRX) appears to be an acceptable mechanism for re-refinement of coarser grain size (i.e. dα>2dDSIT), it cannot explain the steady state grain size for finer ferrite grains (i.e. dα<2dDSIT). Other potential mechanisms involved in this phenomenon are examined.

Viscoplastic behaviour of porous bronzes and irons

An experimental investigation is presented for the viscoplastic behaviour of porous metals. The interest is in the influence of porosity on the deformation behaviour of such materials under loading at various strain rates. Material samples of bronze with 10% tin and pure iron were fabricated by powder metallurgy technology with porosity ranging from 10 to 40%. The samples were then subjected to a large uniaxial compression under both quasi-static and dynamic loading with the maximum strain rate at 10 s−1. The materials show behaviour in an approximately bi-linear nature for strain up to 0.4. The data will be used to develop simple phenomenological constitutive models, which incorporate the volume fraction as a control factor.

Effect of process variables on formation of dynamic strain induced ultrafine ferrite during hot torsion testing

Ultrafine grain sizes were produced using hot torsion testing of a 0.11C-1.68Mn-0.20Si (wt-%) steel, with ultrafine ferrite (<1 µm) nucleating intragranularly during testing by dynamic strain induced transformation. A systematic study was made of the effect of isothermal deformation temperature, strain level, strain rate, and accelerated cooling during deformation on the formation of ultrafine ferrite by this process. Decreasing the isothermal testing temperature below the Ae₃ temperature led to a greater driving force for ferrite nucleation and thus more extensive nucleation during testing; the formation of Widmanstätten ferrite prior to, or early during, deformation imposed a lower temperature limit. Increasing the strain above that where ferrite first began 0.8 at 675C and a strain rate of 3 s¯¹ increased the intragranular nucleation of ferrite. Strain rate appeared to have little effect on the amount of ferrite formed. However, slower strain rates led to extensive polygonisation of the ferrite formed because more time was available for ferrite recovery. Accelerated cooling during deformation followed by air cooling to room temperature led to a uniform microstructure consisting of very fine ferrite grains and fine spherical carbides located in the grain boundaries regions. Air cooling after isothermal testing led to carbide bands and a larger ferrite grain size.

Analytical model of pass-by-pass strain in rod (or bar) rolling and its applications to prediction of austenite grain size

Rod rolling is a process where the deformation state of the workpiece between the work rolls is quite different from the strip rolling process. However, in most microstructure evolution models, the simple area strains (natural logarithm of the area reduction ratio) multiplied by a constant have been used to compute pass-by-pass evolution of austenite grain size (AGS) in rod (or bar) rolling, without any verification. The strains at a given pass play a crucial role in determining the recrystallization behavior (static or dynamic). In this study, an analytical model that calculates the pass-by-pass strain and strain rate in rod rolling has been developed and verified by conducting four-pass (oval–round) bar and plate rolling experiments. Numerical simulations have then been carried out for the four-pass rolling sequence using the area strain model and the new analytical model, focusing on the effect of the method for calculating the strain on the recrystallization behavior and evolution of AGS. The AGS predicted was compared with those obtained from hot torsion tests. It is shown that the analytical model developed in this study is more appropriate in the analysis of bar (or rod) rolling. It was found that the recrystallization behavior and evolution of AGS during this process were influenced significantly by the calculation method for the deformation parameters (strain and strain rate). The pass-by-pass strain obtained from the simple area strain model is inadequate to be used as an input to the equations for recrystallization and AGS evolution under these rolling conditions.

Effect of composition and austenite deformation on the transformation characteristics of low-carbon and ultralow-carbon microalloyed steels

Deformation dilatometry has been used to simulate controlled hot rolling followed by controlled cooling of a group of low- and ultralow-carbon microalloyed steels containing additions of boron and/or molybdenum to enhance hardenability. Each alloy was subjected to simulated recrystallization and nonrecrystallization rolling schedules, followed by controlled cooling at rates from 0.1 °C/s to about 100 °C/s, and the corresponding continuous-cooling-transformation (CCT) diagrams were constructed. The resultant microstructures ranged from polygonal ferrite (PF) for combinations of slow cooling rates and low alloying element contents, through to bainitic ferrite accompanied by martensite for fast cooling rates and high concentrations of alloying elements. Combined additions of boron and molybdenum were found to be most effective in increasing steel hardenability, while boron was significantly more effective than molybdenum as a single addition, especially at the ultralow carbon content. Severe plastic deformation of the parent austenite (>0.45) markedly enhanced PF formation in those steels in which this microstructural constituent was formed, indicating a significant effective decrease in their hardenability. In contrast, in those steels in which only nonequilibrium ferrite microstructures were formed, the decreases in hardenability were relatively small, reflecting the lack of sensitivity to strain in the austenite of those microstructural constituents forming in the absence of PF.

The generation of new high angle boundaries during the hot torsional deformation of aluminium

The crystallographic rotation field for deformation in torsion is such that it is possible for orientations close to stable orientations to rotate away from the stable orientation. A Taylor type model was used to demonstrate that this phenomenon has the potential to transform randomly generated low-angle boundaries into high-angle boundaries. After imposing an equivalent strain of 1.2, up to 40% of the simulated boundaries displayed a disorientation in excess of 15°. These high-angle boundaries were characterised by a disorientation axis close to parallel with the sample radial direction. A series of hot torsion tests was carried out on 1050 aluminium to seek evidence for boundaries formed by this mechanism. A number of deformation-induced high-angle boundaries were identified. Many of these boundaries showed disorientation axes and rotation senses similar to those seen in the simulations. Between 10% and 25% of all the high-angle boundary present in samples twisted to equivalent strains between 2 and 7 could be attributed to the present mechanism.

Effect of composition on the texture and deformation behaviour of wrought Mg alloys

The microstructure and mechanical response of three extruded magnesium alloys, Mg-3Al-1Zn (AZ31), Mg-1.5Mn (Ml) and Mg-lMn-0.4RE (ME10) are examined. The tensile yield strength of ME10 was nearly half that of AZ31 and Ml. The tensile elongations were 6%, 11% and 19% for Ml, AZ31 and ME10, respectively. This range of properties is large and is attributed to the unique extrusion texture produced in ME10, and the high density of fine particles in Ml.

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.

Correlation between the deformation and post-deformation softening behaviours in hot worked Austenite

The relation between the deformation and post-deformation softening behaviours of austenite is examined in a 304 stainless steel. This correlation has been exploited in the modelling of hot rolling and it is argued here that the key to this understanding lies in the deformation structure. The latter is characterized in the present work by the fraction of dynamic recrystallization. The value of this fraction at the peak in the flow stress curve is found to decrease with increasing stress (i.e. with decreasing temperature and increasing strain rate). By contrast, the fraction of dynamic  recrystallization at the strain corresponding to the point where  post-deformation softening becomes strain independent is found to be constant. These observations are used to explain the nature of the important difference between the flow curve peak and the onset of strain independent post-deformation softening.

Investigation of deformation mechanisms involved in the plasticity of AZ31 Mg alloy: in situ neutron diffraction and EPSC modelling

In situ neutron diffraction and Elasto-Plastic Self-Consistent (EPSC) polycrystal modelling have been employed to investigate which deformation mechanisms are involved in the plasticity of extruded AZ31 Mg alloy during the tensile loading along the extrusion direction. On the basis of this study we were able to determine the relative activity of the slip and twinning deformation modes. By tuning the parameters of the EPSC model (i.e. the critical resolved shear strengths and hardening parameters), excellent agreement with the experimental data has been achieved. It is shown that the strain in the crystallographic ⟨c ⟩direction is accommodated mainly by ⟨c + a ⟩ dislocation slip on second-order pyramidal planes. The results further indicate that either slip of ⟨a ⟩dislocations occurs on {10.1} pyramidal planes or cross-slip from basal and prismatic planes takes place.

Laminated plate elements based on higher-order shear deformation theories

Efficient and accurate finite elements are crucial for finite element analysis to provide adequate prediction of the structural behavior. A large amount of laminated plate elements have been developed for finite element analysis of laminated composite plates based on the various lamination theories. A recent and complete review of the laminated finite elements based on the higher-order shear deformation theories, including the global higher-order theories, zig-zag theories and the global-local higher-order theories is presented in this paper. Finally some points on the development of the laminated plate elements are summarized.

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.