R.L. Higginson

Precipitation of NBC in a model austenitic steel

A model Fe–30 wt% Ni, 0.1 C, 1.61 Mn, 0.1 Nb microalloyed steel, that simulates conventional microalloyed C–Mn steels, but does not transform from the austenite phase on cooling, is reported. Plane strain compression testing was undertaken at 950°C at a constant true strain rate of 10 s−1. Samples were deformed in a two stage process. An initial true strain of 0.25–0.45 was followed by unloading, a hold of 1–1000 s and a final deformation to a total true strain of 0.5–0.9. A single deformation was undertaken under identical conditions, but to the total true strain of the double deformation tests. Electron spectroscopic imaging (ESI) in the TEM was used to determine precipitate size and distribution. A 1 s hold time between equal strains of ϵ=0.25 was sufficient for appreciable strain induced precipitation, although 40% static recrystallisation occurred during the hold time. Precipitation occurred entirely on dislocations, present principally as microband walls but also as a rudimentary cell structure within the microbands. No evidence was found for NbC precipitation in the matrix, which therefore remains supersaturated with Nb. NbC particle diameter was in the range 2.5–15 nm, with a density of 3.8×1021 particles/m3 for a 100 s delay period between two strains of ϵ=0.45 at 950°C. Both the size and number density are consistent with those observed in conventional microalloyed C–Mn steels. The behaviour of the model microalloyed Fe–30 Ni steel is discussed in relation to the data on conventional microalloyed steels.

Strain path effects under hot working: an introduction

Abstract Material models are often required to facilitate the development of new products. This is particularly true of hot rolled products, especially shaped sections. Most material models assume that the material behaviour can be described with reference to strain rate, temperature and an equivalent plastic strain. The use of the last variable implies that the “strain path” does not significantly influence material behaviour. Evidence suggests that this assumption is invalid and that the accuracy of future material models will be enhanced if strain path effects are taken into consideration. The importance of strain path effects under hot working conditions, is being investigated. The review discusses previous investigations on strain path effects, mainly carried out under cold working conditions. The conclusions from the review of this work is intended to determine frame of reference within which future test data and microscopic analysis can be assessed. The review also discusses the elementary basis of continuum plasticity models and their potential for describing strain path effects.

An investigation into the use of electron back scattered diffraction to measure recrystallised fraction

The Electron Back-Scattered Diffraction (EBSD) technique is in its infancy and is a highly promising area of development. Use of EBSD has been predominantly for the determination of crystallographic textures. Other applications have also been considered, which include: crystal structure determination, phase determination, grain boundary studies and both elastic and plastic deformation measurement. Although it has been acknowledged that an important use of the EBSD could be in the measurement of recrystallization and its kinetics there are a number of inherent problems with such measurements using EBSD. These problems include the ability of the system to index deformed microstructures even those on a fine scale, the difficulties of analyzing patterns in the region of grain boundaries and the problems of sample preparation which is critical in the quality of the diffraction patterns obtained. The aim of the present study is to determine whether it is possible to measure the volume fraction recrystallized using EBSP of partially recrystallized stainless steel. This has been done by investigation of the quality of matching between the observed and calculated diffraction patterns, and the quality of the observed patterns measured in terms of their contrast. The material used was stainless steel 316L.

Texture development in oxide scales on steel substrates

Abstract Oxide scale growth and microstructures on steels are complex and depend on a large number of variables e.g. temperature, time, atmosphere and alloying elements. The current study has illustrated that there is a complex crystallographic relationship between the scale layers, which also depends on these parameters.

Microstructural and microtextural characterization of oxide scale on steel using electron backscatter diffraction

High‐temperature oxidation of steel has been extensively studied. The microstructure of iron oxides is, however, not well understood because of the difficulty in imaging it using conventional methods, such as optical or electron microscopy. A knowledge of the oxide microstructure and texture is critical in understanding how the oxide film behaves during high‐temperature deformation of steels and more importantly how it can be removed following processing. Recently, electron back‐scatter diffraction (EBSD) has proved to be a powerful technique for distinguishing the different phases in scales. This technique gives valuable information both on the microstructure and on the orientation relationships between the steel and the scale layers. In the current study EBSD has been used to investigate the microstructure and microtexture of iron oxide layers grown on interstitial free steel at different times and temperatures. Heat treatments have been carried out under normal oxidation conditions in order to relate the results to real steel manufacturing in industry. The composition, morphologies, microstructure and microtexture of selected conditions have been studied using EBSD.

The effect of strain path on material behaviour during hot rolling of FCC metals

Models representing material behaviour are now an essential component of the development process for rolled products. Although models based on physical parameters are being proposed, most current models employ empirical equations, which assume that the deformation can be characterized by the strain rate, temperature and the equivalent plastic strain. However, deformation in a flat product rolling pass involves a partial reversal of shear strain, and in long product and section rolling there are more complex changes in strain path in sequential passes. This paper briefly reviews the mapping of strain paths and their effects on the micromechanics of deformation and the resulting flow stress. The influence of in–grain heterogeneity of strain is discussed in relation to the development of dislocation structures and their effects on texture evolution and subsequent recrystallization behaviour. The effects on recrystallization kinetics and resulting grain size are sufficiently large to lead to significant errors in modelling the local behaviour in multipass rolling, if strain–path effects are not considered.

The effect of strain path reversal during hot rolling on austenitic stainless steel

During flat rolling the metal undergoes complex strain paths due to the interaction of the stock with the rolls. These changes in strain path affect the stored energy and hence the recrystallisation behaviour of the material on annealing. The current paper investigates the effect of the roll pass schedule on the recrystallisation kinetics of type 316L stainless steel. By changing the direction of rolling and the reduction on the second pass the effect of the deformation on the recrystallisation kinetics was determined following static annealing. The effect was found to be at a maximum after lower second pass deformation, with the maximum effect at 0.8 from the centre of the sample half thickness. A forward/forward pass gave an increase in the recrystallisation kinetics at this point with more homogeneous recrystallisation seen after a forward/reverse pass. The origins of these differences were investigated by the study of the sub-structure developed during deformation. The differences in the kinetics cannot be attributed to a single microstructural feature, but to a combination of all of them.

Interface study by dual‐beam FIB‐TEM in a pressureless infiltrated Al(Mg)–Al2O3 interpenetrating composite

This paper considers the microstructures of an Al(Mg)–Al2O3 interpenetrating composite produced by a pressureless infiltration technique. It is well known that the governing principle in pressureless infiltration in Al–Al2O3 system is the wettability between the molten metal and the ceramic phase; however, the infiltration mechanism is still not well understood. The objective of this research was to observe the metal–ceramic interface to understand the infiltration mechanism better. The composite was produced using an Al‐8 wt% Mg alloy and 15% dense alumina foams at 915°C in a flowing N2 atmosphere. After infiltration, the composite was characterized by a series of techniques. Thin‐film samples, specifically produced across the Al(Mg)–Al2O3 interface, were prepared using a dual‐beam focussed ion beam and subsequently observed using transmission electron microscopy. XRD scan analysis shows that Mg3N2 formed in the foam at the molten alloy–ceramic infiltration front, whereas transmission electron microscopy analysis revealed that fine AlN grains formed at the metal–ceramic interface and MgAl2O4 and MgSi2 grains formed at specific points. It is concluded that it is the reactions between Al, Mg and the N2 atmosphere that improve the wettability between molten Al and Al2O3 and induce spontaneous infiltration.

Phase determination and microstructure of oxide scales formed on steel at high temperature

Even in simple low‐alloy steels the oxide scales that form during hot working processes are often a complex mixture of three iron oxide phases: haematite, magnetite and wüstite. The mechanical properties, and hence descalability, are intimately linked with phase distribution and microstructure, which in turn are sensitive to both steel composition and oxidation conditions. In this study electron backscatter diffraction in the SEM has been used to characterize the microstructures of oxide scales formed on two compositions of low‐alloy steel. The technique can unambiguously differentiate between the candidate phases to provide the phase distribution within the scale. This is used to investigate grain orientation relationships both within and between phase layers. It has been found that the strength of the orientational relationship between the magnetite and wüstite layers is dependent on steel composition, and in particular Si content. In a low‐Si (0.01 wt%) alloy only a very weak relationship was found to exist for a range of oxidation temperatures (800–1000 °C), whereas for the higher Si (0.37 wt%) alloy a strong relationship was observed under the same oxidation conditions. These orientational relationships are particularly important because, in this temperature range, the majority of oxide scale growth occurs at the magnetite/wüstite interphase boundary.

Substructure drag effects and recrystallization textures in aluminium

Many important recrystallization texture components in metals such as aluminium originate from nuclei in which the mobile high-angle boundary exists prior to, or is formed in the early stages of, annealing. Nucleation can then occur by a process known as strain-induced boundary migration (SIBM). It is possible that this process will involve several growing subgrains, and the drag from that substructure can then have a significant effect. A simple model is used to demonstrate how changes in the overall driving force for recrystallization and Zener drag can affect recrystallization textures when SIBM is involved. This is discussed in relation to experimental observations and the evidence for this process is reviewed.