Bradley P. Wynne

A mechanism of ferrite softening in a duplex stainless steel deformed in hot torsion

A mechanism of dynamic softening of ferrite was studied in a 21Cr-10Ni-3Mo austenite/ferrite duplex stainless steel subjected to torsion at a strain rate of 0.7 s−1 at 1200°C. Transmission electron microscopy together with convergent beam electron diffraction were used with major emphasis on the study of misorientations across ferrite/ferrite boundaries. No evidence of discontinuous dynamic recrystallisation involving nucleation and growth of new grains was found within ferrite contrary to some suggestions made in the literature for similar experimental conditions. The softening mechanism has been classified as extended dynamic recovery characterised by a gradual increase in misorientations between neighbouring subgrains that were created by dynamic recovery processes at the earlier stages of deformation. The resulting dislocation substructure was a complex network of subgrain boundaries composed of a mix of higher- and lower-angle walls characterised by misorientation angles not exceeding 20° at a maximum obtained strain of 1.3.

EBSD investigation of the microstructure and texture characteristics of hot deformed duplex stainless steel

The microstructure and crystallographic texture characteristics were studied in a 22Cr‐6Ni‐3Mo duplex stainless steel subjected to plastic deformation in torsion at a temperature of 1000 °C using a strain rate of 1 s−1. High‐resolution EBSD was successfully used for precise phase and substructural characterization of this steel. The austenite/ferrite ratio and phase morphology as well as the crystallographic texture, subgrain size, misorientation angles and misorientation gradients corresponding to each phase were determined over large sample areas. The deformation mechanisms in each phase and the interrelationship between the two are discussed.

The Use of Bobbin Tools for Friction Stir Welding of Aluminium Alloys

The use of a double sided friction stir welding tool (known as a bobbin tool) has the advantage of giving a processed zone in the workpiece which is more or less rectangular in cross section, as opposed the triangular zone which is more typically found when conventional friction stir welding tool designs are used. In addition, the net axial force on the workpiece is almost zero, which has significant beneficial implications in machine design and cost. However, the response of these tools in generating fine microstructures in the nugget area has not been established. The paper presents detailed metallographic analyses of microstructures produced in 25mm AA6082-T6 aluminium wrought alloy, and examines grain size, texture and mechanical properties as a function of processing parameters and tool design, and offers comparison with data from welds made with conventional tools.

Grain size measurement by EBSD in complex hot deformed metal alloy microstructures

The measurement of grain size by EBSD has been studied to enable representative quantification of the microstructure of hot deformed metal alloys with a wide grain size distributions. Variation in measured grain size as a function of EBSD step size and noise reduction techniques has been assessed. Increasing the EBSD step size from 5% to 20% of the approximate mean grain size results in a change in calculated arithmetic mean grain size of approximately 15% and standard noise reduction techniques can produce a further change in reported size of up to 20%. The distribution of measured grain size is found not to be log‐normal, with a long tail of very small sizes in agreement with a computer simulation of linear intercept and areal grain size measurements through randomly oriented grains. Comparison of EBSD with optical measurements of grain size on the same samples shows that, because of the ability of EBSD to distinguish twins and resolve much smaller grains a difference of up to 50% in measured grain size results.

Hot working and crystallographic texture analysis of magnesium AZ alloys

Abstract The hot working behaviour of magnesium AZ (e.g. AZ31; Al: 3%, Zn: 1%) alloys and their associated crystallographic texture evolution is reviewed. Under hot working conditions, the stress–strain curves show flow softening at all the temperatures and strain rates indicating dynamic recrystallisation (DRX) is predominant. The mean size of the recrystallised grains in all the alloys decreases as the value of Zener–Hollomon parameter Z increases. The hot working range of the alloys dwell between 200 and 500°C and the strain rates between 10−3 and 5 s−1. The hot working of AZ series alloy shows discontinuous DRX as the main mechanism. Equal channel angular processing shows continuous DRX. The constitutive equation development shows a linear relationship between the stress and the Z parameter. The activation energy for the alloys ranges from 112 to 169 kJ mol−1 and Z values range from 107 to 1014 s−1. Textural examinations show basal texture as the predominant orientation.

An analysis of microband orientation in a commercial purity aluminium alloy subjected to forward and reverse torsion using Electron Backscatter Diffraction (EBSD)

High‐resolution electron backscatter diffraction has been used to study the effects of strain reversal on the evolution of microbands in commercial purity aluminium alloy AA1200. Deformation was carried out using two equal steps of forward/forward or forward/reverse torsion at a temperature of 300 °C and strain rate of 1 s−1 to a total equivalent tensile strain of 0.5. In both cases, microbands were found in the majority of grains examined with many having microband walls with more than one orientation. For the forward/forward condition, the microband clusters were centred around −20° and +45° to the equivalent tensile stress axis, whereas for material subjected to a strain reversal, the clusters were at −65° and −45°. There was no evidence of microbands that were formed in the forward deformation step in the reversed material, which would suggest that a strain of 0.25 is sufficient to dissolve any microstructure history generated by the first step. Furthermore, the microbands within the strain‐reversed material had a reduction in misorientation compared with the lineally strained material, suggesting that these microbands only formed at the onset of the second deformation step. This confirms that microband formation is complex and sensitive to strain path; however, it is still unclear to what extent microband formation is dependent on strain path history compared with the instantaneous deformation mode.

Comparison of the deformation characteristics of a Ni-30wt% Fe alloy and plain carbon steel

A major barrier confronting researchers studying the hot deformation of plain and low carbon steels is the inability to directly observe the deformation microstructures of hot worked austenite due to the unavoidable transformation to martensite on quenching to room temperature. Various model materials, such as austenitic stainless steels have been used to overcome this difficulty. However, these materials have markedly different stacking fault energies from plain and low carbon steels and this will affect the evolving deformation structure. In this work, a model austenitic Ni-30wt%Fe alloy, calculated to have a stacking fault energy similar to that of low carbon steel, has been tested using hot compression. Stress-strain curves obtained during hot deformation show characteristics similar to those generated during identical tests on a 0.15wt%C steel. This suggests that the two materials behave similarly during deformation under similar experimental conditions. An application of the Ni-Fe alloy in the study of microstructural changes in austenite during hot deformation is demonstrated. A series of hot torsion experiments on a 0.11wt%C steel have been found to produce deformation-induced, intragranular nucleation of ferrite from austenite when a single deformation pulse is applied at 675°C. A similar set of experiments have also been performed on the Ni-Fe alloy at 750°C. Transmission electron microscopy carried out on the Ni-Fe alloy torsion specimens has revealed that likely preferred sites for intragranular ferrite nucleation appear to be microbands produced in the austenite during deformation.

Mapping microstructure inhomogeneity using electron backscatter diffraction in 316L stainless steel subjected to hot plane strain compression tests

Abstract The microstructure inhomogeneity in 316L stainless steel subjected to hot plane strain compression tests has been assessed using electron backscatter diffraction (EBSD). Two variables were investigated: the effect of strain rate and the effect of friction at the tool/specimen interface. Tests were performed isothermally at 950°C at nominal equivalent tensile strain rates of 0·01 and 1 s−1. Low and high friction conditions have been simulated by applying both a glass based lubricant and a boron nitride spray respectively. Results suggest that friction causes a variation in microstructure from the surface to the midplane of the deformed specimen. Several methods used to quantify and represent this inhomogeneity are presented in the present paper. Electron backscatter diffraction measurement issues are discussed. A grain size mapping method using a two-dimensional moving average has been developed to overcome the difficulties associated with the visualisation of measurement results over large areas on EBSD maps. It has proved to be a powerful tool for the spatial statistics of large quantity data obtained by EBSD.

The Use of Fe-30% Ni and Fe-30% Ni–Nb Alloys as Model Systems for Studying the Microstructural Evolution during the Hot Deformation of Austenite

The development of physically-based models of microstructural evolution during thermomechanical processing of metallic materials requires knowledge of the internal state variable data, such as microstructure, texture, and dislocation substructure characteristics, over a range of processing conditions. This is a particular problem for steels, where transformation of the austenite to a variety of transformation products eradicates the hot deformed microstructure. This article reports on a model Fe-30 wt% Ni-based alloy, which retains a stable austenitic structure at room temperature, and has, therefore, been used to model the development of austenite microstructure during hot deformation of conventional low carbon–manganese steels. It also provides an excellent model alloy system for microalloy additions. Evolution of the microstructure and crystallographic texture was characterized in detail using optical microscopy, X-ray diffraction (XRD), SEM, EBSD, and TEM. The dislocation substructure has been quantified as a function of crystallographic texture component for a variety of deformation conditions for the Fe-30% Ni-based alloy. An extension to this study, as the use of a microalloyed Fe-30% Ni–Nb alloy in which the strain induced precipitation mechanism was studied directly. The work has shown that precipitation can occur at a much finer scale and higher number density than hitherto considered, but that pipe diffusion leads to rapid coarsening. The implications of this for model development are discussed.

The behaviour of incidental dislocation boundaries and geometrically necessary boundaries during warm tensile deformation of aluminium

It has been well established experimentally that, during plastic deformation of a polycrystal, individual grains are gradually subdivided into mutually misoriented regions separated by dislocation rotation boundaries. At small strains, slightly misoriented ordinary dislocation cells are created that are characterized by tangled dislocation boundaries. According to the theory of low-energy dislocation structures (LEDS), these boundaries result from the statistical mutual trapping of glide dislocations into low-energy configurations and are, therefore, termed incidental dislocation boundaries (IDBs). With increasing strain, planar dislocation walls called geometrically necessary boundaries (GNBs) develop gradually on the background of ordinary dislocation cells and, thus, divide the grain into cell blocks (CBs) that contain from one to several cells.