Materials Science and Technology

A Nordlys F, fast electron backscatter diffraction camera has been used on a CamScan MX2500 field emission gun SEM in conjunction with a hot stage to study annealing phenomena in metals. The performance of the camera under a wide variety of conditions has been evaluated, and comparison is made with a conventional electron backscatter diffraction camera operating under the same conditions and with the same samples. Examples are given of applications of the system to the annealing of aluminium alloys.

By using a low SEM accelerating voltage in the range of 5–7·5 kV with an electron backscatter diffraction system, the interaction volume of electrons with matter is markedly decreased, and the spatial resolution is improved. The authors present measurements which prove this relationship. Electron backscatter diffraction at low acceleration voltage, however, is not a straightforward technique with the current equipment and a standard set-up. The authors show that an optimised working distance and dedicated sample preparation are essential to obtain improved spatial resolution for detailed nanoscale microstructure investigations.

Automated backscatter Kikuchi diffraction in the SEM is about to become a tool for process and quality control. Mandatory requirements for these applications are a high speed and provisions to enable re-examination of the results at any time. Means and motivation for high speed electron backscattering diffraction are discussed. Separate acquisition and store of pattern sequences in an unprocessed data format, followed by offline calculation of the grain orientations, has many advantages over online orientation microscopy. With actually >1000 acquired patterns per second it is significantly faster than conventional electron backscattering diffraction. The data can be examined again using the original backscatter Kikuchi sequences, and the presence of a priori unknown phases can be checked for. Online pattern solving, as a dual task, is optional.

A new ‘elementary facet’ method has been developed to measure grain boundary (GB) plane orientations using three-dimensional electron backscattered diffraction on serial sectioned surfaces in a sample. The method defines different types of elementary facets in an orthogonal coordinate system in the sample. These elementary facets are used to pave GBs. For a flat GB, its projected areas on the orthogonal coordinate planes in the sample are obtained from the numbers of different facets paved in the GB. From the projected areas of the GB plane in the three coordinate planes, the GB plane normal is derived. For uneven GBs, a convolution method has been developed to obtain the GB plane orientations at different GB locations. In essence, this new method measures the GB orientations at every elementary facet that is used to pave the GBs in the sample. Together with the measured crystal orientations of different grains, the GB five parameters at all facets can be obtained readily. The distribution of GB five parameters for the whole GB population in a sample can be derived from the collection of properties of individual GB facets in the sample.

Three-dimensional electron backscatter diffraction data, obtained by serial sectioning a nickel–base superalloy, has been analysed to measure the geometric arrangement of grain boundary planes at triple junctions. This information has been used to calculate the grain boundary character distribution (GBCD) and the grain boundary energy distribution (GBED). The twin content from the three-dimensional GBCD calculation compares favourably with the twin content estimated by stereology. Important factors in the analysis are the alignment of the parallel layers, the ratio of the out-of-plane to in-plane spacing of the discrete orientation data and the discretisation of the domain of grain boundary types. The results show that grain boundaries comprised of (111) planes occur most frequently and that these grain boundaries have a relatively low energy. The GBCD and GBED are inversely correlated.

The distribution of grain boundaries of particular crystallographic character can provide descriptive information on the properties of engineering materials. For example, the fraction and connectivity of corrosion susceptible grain boundaries typically correlates with the extent of intergranular corrosion and stress corrosion cracking resistance in sensitised austenitic stainless steels. A parameter defining the cluster compactness is proposed to describe the breakup of the network of corrosion susceptible grain boundaries. It may therefore provide a measure of intergranular stress corrosion cracking resistance. The cluster compactness of the network of random grain boundaries (>Σ29) in electron backscatter diffraction assessments of microstructure is shown to decrease with increasing fraction of Σ3 boundaries. However, the cluster compactness of the network of corroded grain boundaries identified after electrochemical testing is less sensitive to changes in microstructure obtained by thermomechanical processing.

Electron backscatter diffraction techniques have been used to assess the range of textures that exist in a number of common titanium alloys. Crystal orientation maps and pole figures have been obtained over representative areas to study preferred textures and regions of common crystallography (macrozones). Electron backscatter diffraction analysis has established different relationships between optical microstructures and crystallographic texture for different titanium alloys. For β processed alloys, there is a correspondence between optical colonies of α platelets and regions of crystallographic alignment. Various texture intensities, macrozones and effective structural unit sizes can be observed depending on the alloy and product form. These variations impact on the mechanical behaviour of titanium alloys.

A focused ion beam and electron backscatter diffraction were used to investigate the three-dimensional microstructure of small dynamically recrystallised grains in a deformed Cu–Sn alloy and to determine their orientation relationships. It was found that many of the grains, which form as thin sheets at the prior boundaries, are twin related. It is concluded that dynamical recrystallisation usually starts in regions where plastic deformation has resulted in significant lattice rotations beside the grain boundaries. Once such a new grain has formed, dynamic recrystallisation is propagated by successive twinning from the new grain, along the prior grain boundary, resulting in families of several hundred twin related dynamically recrystallised grains.

The present paper studied the grain refinement and carbides precipitation in a Fe–Mn–Si–Cr–Ni–C alloy aged after equal channel angular pressing (ECAP). The results show that after ECAP and subsequent aging, the average size of the grain is ∼10 μm. Meantime, defects introduced by ECAP can decrease the activation energy for nucleation of the carbides and thus more carbides are produced inside the grains and the grain boundary. Grain refinement together with second phase hardening effectively improves the strength of matrix. Consequently, the shape memory ratio of the alloy aged after ECAP is up to 89·4% when prestrain is 4·33%, being 40% higher than those aged without ECAP.

In order to control the behaviour of the SiC reinforced particles during the brazing process, a new method of electromagnetic field aided brazing (EMFAB) has been developed to join SiCp/A356 Al metal matrix composites with Zn–Al filler. The microstructural characteristics of the joint interface have been analysed by scanning electron microscopy. Results show that the behaviour of the reinforced particles in the solidifying brazing seam is controlled effectively by applying the electromagnetic field when the magnetic intensity is 0·5 T, ac square current frequency is 100 Hz and peak current value is 70 A. The SiC particles distribute homogeneously throughout the brazing seam without segregation behaviour at the joint interface. Moreover, the joining mechanisms of the EMFAB method have been given in detail.

An engineering approach is presented to analyse the whole process of damage initiation and development for laminated composites under impact loading as well as compressive loading after impact using the three-dimensional progressive damage theory. The real impact damage status of laminated composites is employed to analyse the residual compressive strength instead of the artificial premises adopted by traditional methods. A comparison of impact damage area and residual compressive strength from numerical simulations and experimental tests is given, which shows a good agreement with each other. Moreover, large numbers of experiments for obtaining the impact damage status parameters can be avoided by this engineering approach. A parametric modelling programme package based on the analytical method has been developed.

Forming limit diagram of a promising high strength copper alloyed interstitial free steel has been determined by Nakazima tests under various processing conditions. Various parameters such as limiting strain, shear strain and strain ratio have been derived from the diagrams and analysed. The results have been compared with those of two traditional high strength interstitial free steels. It is observed that the formability parameters of copper alloyed interstitial free steel are lower than those of high strength interstitial free steels due to the effect of copper. Copper alloyed interstitial free steel in continuous annealed condition exhibits the highest limiting strain values among all the processing conditions and is comparable to traditional high strength interstitial free steels.

Hot deformation behaviour of hypereutectoid steels (0·83–0·92%C) alloyed with 0–2%Al is investigated. Compression tests were carried out in the temperature range 1000–1100°C at 0·1–10 s^–1 strain rates. The stress–strain curves are analysed using work hardening rate flow stress plots to determine the critical and peak stress and the corresponding strains associated with dynamic recrystallisation. The addition of Al increases the strain hardening rate and lowers the peak strain. There is an optimum content beyond which peak strain increases with Al addition. Phase diagram calculations show that the multiphase microstructure present during deformation is responsible for the non-linear effect of Al. The steady state stress is modelled using the sine hyperbolic constitutive equation, and the activation energy for deformation is determined.

The present study applies a compressive type split Hopkinson pressure bar to investigate the adiabatic shear behaviour of unweldable Al–Sc alloy under high strain rates ranging from 3·0 × 10⁵ to 6·3 × 10⁵ s^–1. The effects of the strain rate on the shear stress, adiabatic shear band characteristics, and fracture features of the unweldable Al–Sc alloy are systematically examined. The results show that both the shear stress and the strain rate sensitivity increase with increasing the strain rate. In addition, it is shown that an adiabatic shear band is formed within the deformed specimens for all values of the strain rate. As the strain rate is increased, the width of the shear band decreases, but the microhardness increases. Moreover, the distortion angle and the magnitude of the local shear strain near the shear band both increase with increasing the strain rate. At a strain rate of 3·0 × 10⁵ s^–1, the fracture surface is characterised by multiple transgranular clearage fractures. However, for strain rates greater than 4·5 × 10⁵ s^–1, the fracture surface has a transgranular dimple-like characteristic, and thus it is inferred that the ductility of the unweldable Al–Sc alloy improves with increasing the strain rate.

Mixed films of cerium oxide (CeO₂) and tungsten oxide (WO₃) were deposited on unheated substrates by co-evaporation. The films were annealed in air at 500°C for 2 h. The surface morphology of the films was characterised using atomic force microscopy. The chemical composition was determined using X-ray photoelectron spectroscopy. The optical properties were derived from normal incidence reflectance and transmittance measurements. The films were investigated for the detection of carbon monoxide. The effects of the operating temperature and gas concentration on the performance of the sensor were investigated. The sensor response and recovery times were also measured.

In this study, die profiles of the rod extrusion processes are optimised to produce products of desirable microstructure at maximum production speed and minimum left out material in the die. The design problem is formulated as a non-linear programming problem which is solved using genetic algorithm. Selection of the processing parameters is carried out using dynamic material modelling. Using this approach, two rod extrusion problems are successfully designed.

In this research, microstructure and overload failure behaviour of resistance spot welded DP980 were investigated. Microstructural characterisation, microhardness test and static tensile shear test were conducted. Fusion zone size proved to be the most important controlling factor of spot weld peak load and energy absorption. The results of this study demonstrated that the conventional weld size recommendation of d=4t^1/2 is not sufficient to ensure the pullout failure mode for DP980 steel resistance spot welds during the tensile shear test. In pullout mode, generally, failure was initiated at heat affected zone/base metal interface, where softening occurs due to the tempering of martensite. However, when heavy expulsion occurs, pullout failure tends to be initiated at fusion zone/heat affected zone interface. It was shown that heavy expulsion and associated large electrode indentation can reduce load carrying capacity and energy absorption capability of DP980 spot welds.

Effect of Sn addition on the self-propagating high temperature synthesis reaction has been investigated. The results show that Sn serves not only as a diluent but also as a reactant and reacts with Ti to form Ti₆Sn₅ phase. As the Sn content ranges from 6 to 15 wt-%, the combustion temperature of the system decreases and the porosity of the products is reduced, however, the size of TiC particulate decreases and the shape of TiC particulate has significantly changed in the SHS reaction products. Specially, when Sn content is over 12 wt-%, the shape of TiC particles gradually transforms from the multi-angular shape to the spherical shape. The in situ formed master alloy containing Al and Sn, as well as a large volume fraction of TiC particulates is successfully synthesised.

This present paper studied the solidification characteristics of a high Fe–Cr–Ni austenitic stainless steel weld pool using an EBSD system. The results revealed that the crystallography of the weld metal grains was affected by both the heat affected zone (HAZ) base metal grains at the fusion boundary and the thermal condition in weld pool. It exhibited an orientation relationship of <100> // <100> and {100} // {100} between the HAZ base metal and weld metal along the fusion boundary, which was resulted from the epitaxial growth in the weld pool. Additionally, a high fibre texture <100> toward the weld centreline was determined by the effect of the competitive mechanism at the beginning of the weld solidification.

Conventional rolling and asymmetric rolling (ASR) processes were applied to aluminium alloy AA5754 with different roll metal frictional conditions. The rolling textures were determined by X-ray diffraction technique and the formation of shear texture was studied. It has been demonstrated that the ASR and friction have different effects on the generation of shear texture. The ASR forces the deformation texture to rotate about the transverse direction from the fcc plane strain compression texture, while a high friction generates a so called ideal fcc shear texture consisting of the {001}〈110〉 and {111}〈uvw〉 components. The effect of ASR penetrates throughout the sheet thickness, but that of friction exists only from the sheet surface to one-quarter thickness.