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.

Application of radial return mapping algorithm for finite strain elastoplasticity in a hybrid finite element and particle-in-cell model

The radial return mapping algorithm within the computational context of a hybrid Finite Element and Particle-In-Cell (FE/PIC) method is constructed to allow a fluid flow FE/PIC code to be applied solid mechanic problems with large displacements and large deformations. The FE/PIC method retains the robustness of an Eulerian mesh and enables tracking of material deformation by a set of Lagrangian particles or material points. In the FE/PIC approach the particle velocities are interpolated from nodal velocities and then the particle position is updated using a suitable integration scheme, such as the 4th order Runge-Kutta scheme[1]. The strain increments are obtained from gradients of the nodal velocities at the material point positions, which are then used to evaluate the stress increment and update history variables. To obtain the stress increment from the strain increment, the nonlinear constitutive equations are solved in an incremental iterative integration scheme based on a radial return mapping algorithm[2]. A plane stress extension of a rectangular shape J2 elastoplastic material with isotropic, kinematic and combined hardening is performed as an example and for validation of the enhanced FE/PIC method. It is shown that the method is suitable for analysis of problems in crystal plasticity and metal forming. The method is specifically suitable for simulation of neighbouring microstructural phases with different constitutive equations in a multiscale material modelling framework.

Characteristics of the deformed and recrystallised grains obtained after hot plane strain compression of a model Fe-30wt%Ni alloy

The development of physically-based models of microstructural evolution during hot deformation of metallic materials requires knowledge of the grain/subgrain structure and crystallographic texture characteristics over a range of processing conditions. A Fe-30wt%Ni based alloy, retaining a stable austenitic structure at room temperature, was used for modelling the development of austenite microstructure during hot deformation of conventional carbon-manganese steels. A series of plane strain compression tests was carried out at a temperature of 950 °C and strain rates of 10 s-1 and 0.1 s-1 to several strain levels. Evolution of the grain/subgrain structure and crystallographic texture was characterised in detail using quantitative light microscopy and highresolution electron backscatter diffraction. Crystallographic texture characteristics were determined separately for the observed deformed and recrystallised grains. The subgrain geometry and dimensions together with the misorientation vectors across sub-boundaries were quantified in detail across large sample areas and the orientation dependence of these characteristics was determined. Formation mechanisms of the recrystallised grains were established in relation to the deformation microstructure.

Fine structure of shear bands formed during hot deformation of two austenitic steels

Shear bands formed during both cold and hot plastic deformation have been linked with several proposed mechanisms for the formation of ultrafine grains. The aim of the present work was to undertake a detailed investigation of the microstructural and crystallographic characteristics of the shear bands formed during hot deformation of a 22Cr-19Ni-3Mo (mass%) austenitic stainless steel and a Fe-30 mass%Ni based austenitic model alloy. These alloys were subjected to deformation in torsion and plane strain compression (PSC), respectively, at temperatures of 900°C and 950°C and strain rates of 0.7s_-1 and 10s_-1, respectively. Transmission electron microscopy and electron backscatter diffraction in conjunction with scanning electron microscopy were employed in the investigation. It has been observed that shear bands already started to form at moderate strains in a matrix of pre-existing microbands and were composed of fine, slightly elongated subgrains (fragments). These bands propagated along a similar macroscopic path and the subgrains, present within their substructure, were rotated relative to the surrounding matrix about axes approximately parallel to the sample radial and transverse directions for deformation in torsion and PSC, respectively. The subgrain boundaries were largely observed to be non-crystallographic, suggesting that the subgrains generally formed via multiple slip processes. Shear bands appeared to form through a co-operative nucleation of originally isolated subgrains that gradually interconnected with the others to form long, thin bands that subsequently thickened via the formation of new subgrains. The observed small dimensions of the subgrains present within shear bands and their large misorientations clearly indicate that these subgrains can serve as potent nucleation sites for the formation of ultrafine grain structures during both subsequent recrystallisation, as observed during the present PSC experiments, and phase transformation.

EBSD study of the orientation dependence of substructure characteristics in a model Fe-30wt%Ni alloy subjected to hot deformation

The aim of the present investigation was to determine the orientation dependence of substructure characteristics in an austenitic Fe−30wt%Ni model alloy subjected to hot plane strain compression. Deformation was carried out at a temperature of 950 °C using a strain rate of 10 s^₋₁ to equivalent strain levels of approximately 0.2, 0.4, 0.6 and 0.8. The specimens obtained were analysed using a fully automatic electron backscatter diffraction technique. The crystallographic texture was characterized for all the strain levels studied and the subgrain structure was quantified in detail at a strain of 0.4. The substructure characteristics displayed pronounced orientation dependence. The major texture components, namely the copper, S, brass, Goss and rotated Goss, generally contained one or two prominent families of parallel larger-angle extended subboundaries, the traces of which on the longitudinal viewing plane appeared systematically aligned along the {111} slip plane traces, bounding long microbands subdivided into slightly elongated subgrains by short lower-angle transverse subboundaries. Relatively rare cube-orientated grains displayed pronounced subdivision into coarse deformation bands containing large, low-misorientated subgrains. The misorientation vectors across subboundaries largely showed a tendency to cluster around the sample transverse direction. Apart from the rotated Goss texture component, the stored energy levels for the remaining components were principally consistent with the corresponding Taylor factor values.

The effect of strain rate on the deformation mechanisms and the strain rate sensitivity of an ultra-fine-grained Al alloy

An ultra-fine-grained Al alloy was subjected to compression tests at different strain rates. The strain rate sensitivity of the flow stress was estimated. A strong effect of the baseline strain rate on the mechanisms of plastic deformation was found. It is suggested that a decrease of strain rate results in activation of micro shear banding due to grain boundary sliding. A connection between the strain rate, strain rate sensitivity, and the prevalent deformation mechanism was established.

Mechanisms of dynamic strain induced transformation and post-deformation evolution

In this study, a novel experimental approach was applied to study the mechanism of the equiaxed shape retention in dynamic strain induced ferrite during deformation. The post-deformation ferrite evolution in both static and dynamic transformation was studied. The refinement potential and the origin of their differences in both mechanisms were analysed.

Multimodal nanostructured titanium using severe plastic deformation

In the present study, multimodal nanostructured titanium was engineered using severe plastic deformation. The multimodal structured titanium exhibits an ultrahigh strength of over 940 MPa and a large failure elongation of 24%. The ultrahigh strength is mainly derived from the nanostructured structures; whilst the exceptional ductility originates from the large fraction of high angle grain boundaries, micro-scale structures, and the non-equilibrium grain boundary configuration. It is worth noting that apart from dislocation slip processes, the formation of deformation twins reduced the effective slip distance and increased the strain hardening capacity via the Hall-Petch mechanism, leading to high ductility of the multimodal structured titanium.

A study of plastic deformation during axisymmetric forward spiral extrusion and its subsequent mechanical property changes

Axisymmetric forward spiral extrusion (AFSE) accumulates large strains in its sample while extruding it through a die with engraved spiral grooves. A three-dimensional finite element model of AFSE has been developed using ABAQUS to investigate the deformation mode in detail, including the effect of groove geometry and the heterogeneity of plastic deformation. The numerical results demonstrated that the strain distribution in the AFSE sample cross section is linear in the radial direction within a concentric core while the distribution, outside the core, in the vicinity of the grooves is non-linear and non-axisymmetric. Mechanical properties and grain structure changes of the deformed sample were investigated. Improvements of mechanical properties in the processed samples can be attributed to the domination of the shear deformation mode in a plane normal to the extrusion axis and consequently the elongation of grains in the tangential direction

Asymmetric rolling of interstitial-free steel using differential roll diameters. Part I : mechanical properties and deformation textures

IF steel sheets were processed by conventional symmetric and asymmetric rolling (ASR) at ambient temperature. The asymmetry was introduced in a geometric way using differential roll diameters with a number of different ratios. The material strength was measured by tensile testing and the microstructure was analyzed by optical and transmission electron microscopy as well as electron backscatter diffraction (EBSD) analysis. Texture was also successfully measured by EBSD using large surface areas. Finite element (FE) simulations were carried out for multiple passes to obtain the strain distribution after rolling. From the FE results, the velocity gradient along selected flow lines was extracted and the evolution of the texture was simulated using polycrystal plasticity modeling. The best mechanical properties were obtained after ASR using a roll diameter ratio of 2. The textures appeared to be tilted up to 12 deg around the transverse direction, which were simulated with the FE-combined polycrystal plasticity modeling in good agreement with measurements. The simulation work revealed that the shear component introduced by ASR was about the same magnitude as the normal component of the rolling strain tensor.

Recrystallization of 304SS during and after high strain rate deformation

During the hot working of austenitic stainless steels the shape of the flow curve is strongly influenced by the strain rate. Low strain rate deformation results in flow curves typical of dynamic recrystallization (DRX) but as the strain rate increases the shape changes to a ‘flat-top’ curve. This has traditionally been thought to indicate no DRX is taking place and that dynamic recovery (DRV) is the only operating softening mechanism. Examining the work-hardening behaviour and corresponding deformation microstructures showed this is not the case for austenitic stainless steel, as clear evidence of dynamic recrystallization process can be seen. The post-deformation recrystallization kinetics can be modelled using a standard Avrami equation with an Avrami exponent, n, of 1.15. With an increasing value of the Zener-Hollomon parameter it was found that the kinetics of recrystallization become less strain rate sensitive until at the highest values (highest strain rates/lowest temperatures) the recrystallization kinetics become strain rate insensitive.

Development of a laboratory-based transmission diffraction technique for in situ deformation studies of Mg alloys

A laboratory-based transmission X-ray diffraction technique was developed to measure elastic lattice strains parallel to the loading direction during in situ tensile deformation. High-quality transmission X-ray diffraction data were acquired in a time frame suitable for in situ loading experiments by application of a polycapillary X-ray optic with a conventional laboratory Cu X-ray source. Based on the measurement of two standard reference materials [lanthanum hexaboride (NIST SRM 660b) and silicon (NIST SRM 640c)], precise instrumental alignment and calibration of the transmission diffraction geometry were realized. These results were also confirmed by the equivalent data acquired using the standard Bragg-Brentano measurement geometry. An empirical Caglioti function was employed to describe the instrumental broadening, while an axis of rotation correction was used to measure and correct the specimen displacement from the centre of the goniometer axis. For precise Bragg peak position and hkil intensity information, a line profile fitting methodology was implemented, with Pawley refinement used to measure the sample reference lattice spacings (d o (hkil)). It is shown that the relatively large X-ray probe size available (7 × 714mm) provides a relatively straightforward approach for improving the grain statistics for the study of metal alloys, where grain sizes in excess of 114μm can become problematic for synchrotron-based measurements. This new laboratory-based capability was applied to study the lattice strain evolution during the elastic-plastic transition in extruded and rolled magnesium alloys. A strain resolution of 2 × 10-4 at relatively low 2θ angles (20-65° 2θ) was achieved for the in situ tensile deformation studies. In situ measurement of the elastic lattice strain accommodation with applied stress in the magnesium alloys indicated the activation of dislocation slip and twin deformation mechanisms. Furthermore, measurement of the relative change in the intensity of 0002 and 10 3 was used to quantify {10 2} 011 tensile twin onset and growth with applied load.