I.P. Jones

Effects of major alloying additions on the microstructure and mechanical properties of γ-TiAl

Abstract The microstructure and mechanical properties of eight γ-TiAl based alloys with compositions in the range Ti–44Al–8(Nb,Ta,Zr,Hf)–(0–0.2)Si–(0–1)B have been investigated to assess the possibility of improving the properties of γ-TiAl through heavy alloying. It has been shown that the microstructures of these alloys can be significantly different from those of the binary or 48–2–2 type alloys as a result of differences in the phase equilibria. As expected with large additions of beta stabilisers such as Nb, Zr and Ta, the beta phase was stabilised to much lower temperatures than that in the Ti–44Al binary alloy. In some of the alloys the ω phase, which is a transformed product of the beta phase, is stable at room temperature and up to >900°C. In alloys which contain both beta- and gamma- stabilisers, there is no single α phase field in the transformation sequence and instead there is a (α+β+γ) three phase regime. The mechanical data obtained from these alloys indicate that heavy alloying can be used to increase the strength and creep resistance of γ-TiAl significantly although ductility generally remains poor. The addition of boron appears to be beneficial in that both strength and ductility are improved, particularly for materials with the duplex microstructure.

Microbial synthesis of core/shell gold/palladium nanoparticles for applications in green chemistry

We report a novel biochemical method based on the sacrificial hydrogen strategy to synthesize bimetallic gold (Au)–palladium (Pd) nanoparticles (NPs) with a core/shell configuration. The ability of Escherichia coli cells supplied with H2 as electron donor to rapidly precipitate Pd(II) ions from solution is used to promote the reduction of soluble Au(III). Pre-coating cells with Pd(0) (bioPd) dramatically accelerated Au(III) reduction, with the Au(III) reduction rate being dependent upon the initial Pd loading by mass on the cells. Following Au(III) addition, the bioPd–Au(III) mixture rapidly turned purple, indicating the formation of colloidal gold. Mapping of bio-NPs by energy dispersive X-ray microanalysis suggested Au-dense core regions and peripheral Pd but only Au was detected by X-ray diffraction (XRD) analysis. However, surface analysis of cleaned NPs by cyclic voltammetry revealed large Pd surface sites, suggesting, since XRD shows no crystalline Pd component, that layers of Pd atoms surround Au NPs. Characterization of the bimetallic particles using X-ray absorption spectroscopy confirmed the existence of Au-rich core and Pd-rich shell type bimetallic biogenic NPs. These showed comparable catalytic activity to chemical counterparts with respect to the oxidation of benzyl alcohol, in air, and at a low temperature (90°C).

Direct evidence for Suzuki segregation and Cottrell pinning in MP159 superalloy obtained by FEG(S)TEM/EDX

The effects of Suzuki segregation on the plastic flow behaviour of MP159 alloy deformed at high temperature and on the resulting dislocation structure have been examined. Elemental concentration profiles across both stacking faults and slip bands have been measured in a FEG TEM in nano-probe using the line scanning mode and EDX. It was found that Suzuki segregation resulted in continuously serrated plastic flow for deformation at temperatures from 450–670 °C and at a slow strain rate such as 1.0×10−4/s. TEM examination showed an increased dissociation width for dislocations and larger and more stacking faults after deformation at high temperatures as compared with those after deformation at room temperature. This can be interpreted as being due to the reduction of stacking fault energy by Suzuki segregation and/or Cottrell pinning. The elemental concentration profiles across stacking faults and slip bands showed that Mo and Al were more often found than other solutes to segregate to stacking faults and slip bands. Occasionally, the segregation of Ti and Nb could also be detected at stacking faults and slip bands.

Effects of secondary phase and grain size on the corrosion of biodegradable Mg-Zn-Ca alloys.

The bio-corrosion behaviour of Mg-3Zn-0.3Ca (wt.%) alloy in simulated body fluid (SBF) at 37°C has been investigated using immersion testing and electrochemical measurements. Heat treatment has been used to alter the grain size and secondary phase volume fraction; the effects of these on the bio-corrosion behaviour of the alloy were then determined. The as-cast sample has the highest bio-corrosion rate due to micro-galvanic corrosion between the eutectic product (Mg+Ca2Mg6Zn3) and the surrounding magnesium matrix. The bio-corrosion resistance of the alloy can be improved by heat treatment. The volume fraction of secondary phases and grain size are both key factors controlling the bio-corrosion rate of the alloy. The bio-corrosion rate increases with volume fraction of secondary phase. When this is lower than 0.8%, the dependence of bio-corrosion rate becomes noticeable: large grains corrode more quickly.

The Oxidation of NdFeB Magnets

The oxidation kinetics in air of a commercial NdFeB magnet have been investigated over the temperature range 335–500°C. The oxide microstructure has been characterized by SEM, XRD and cross-sectional TEM. The results show that the external scale formed consists of an outer layer of Fe2O3 and an inner layer of Fe3O4 but that the principal degradation process is the formation of an extensive zone of internal oxidation. HREM has been used to show that this zone contains NdO particles embedded in an α-Fe matrix. These particles are discrete and very small, approximately 2 nm in diameter, and have an amorphous structure. The α-Fe matrix has a columnar grain structure with a grain width of approximately 100 nm. It is argued that the high rates of internal oxidation arise because the external-oxide layers are not protective at the oxidation temperature, and oxygen penetrates to the zone front by fast diffusion along the columnar α-Fe grain boundaries.

The microstructure of sigma 1140+ SiC fibres

Abstract Transmission electron microscopy (TEM) studies have been carried out on ‘as-received’ SM1140 + SiC fibres grown by CVD onto W wire cores. The transition between the W core and the SiC deposit is not abrupt in that there is a reaction zone present at the interface which actually consists of a silicide layer and a carbide layer. The SiC is subdivided into four concentric layers across the fibre diameter, each with a different morphology. The possible causes for the differences in the SiC morphology are discussed.

Dislocation processes during the plastic deformation of γ-TiAl

In situ straining experiments in a high-voltage electron microscope have been performed on coarse-grained g -Ti± 52 at.% Al at room temperature and elevated temperatures, in addition to macroscopic compression tests. At all temperatures examined, ordinary dislocations, superdislocations of h 101] Burgers vectors and microtwins carry the deformation, with ordinary dislocations dominating. The processes controlling the deformation diA er greatly for the temperature ranges below and above about 850 K. At low temperatures, ordinary dislocations as well as superdislocations move jerkily between positions where they are locally pinned, which can best be described by a precipitation-hardening mechanism. At high temperatures, the h 101] superdislocations show a shape typical of the locking± unlocking mechanisms. The ordinary dislocations are created and move in a very instantaneous event. Later, theyare smoothly curved and move in a viscous way. The nonplanar arrangement of these dislocations indicates the importance of diA usion processes. The dynamic behaviour and the results of macroscopic deformation tests are explained by the formation of intrinsic atmospheres around the dislocations.

Spatial resolution in x-ray microanalysis of thin foils in stem

The influence of electron scattering in the spatial resolution of X-ray microanalysis of thin foils of silicon has been measured as a function of specimen thickness and accelerating voltage. The experimental observations show that high angle elastic scattering controls the spatial resolution and that this is not influenced by the diffraction conditions. Monte Carlo calculations have also been carried out and the general form of the theoretical and experimental results are in good agreement. For foil thicknesses normally used in defect analysis in silicon, ~5000 A, the spatial resolution as measured in this experiment (taken as the region within which 90% of the X-ray signal is generated) is about 700 A at 100 kV and 1800 A at 40 kV.

The effect of lamella thickness on the creep behaviour of Ti-48Al-2Nb-2Mn

The work reported in this paper forms part of a larger investigation of the effects of microstructure on creep in a series of two phase {gamma}--TiAl based alloys. Here the authors examine a fully lamellar Ti-48Al-2Mn-2Nb alloy. They have attempted to quantify the microstructure in terms of a set of parameters, the most important of which are considered to be: colony size; perfection of the lamellar structure; lamella thickness; proportion of {alpha}{sub 2}; how serrated the colony boundaries are; and nature of the lamella interfaces.

On the abnormal negative temperature dependence of yield strength in the L12 compound Fe3Ge

Abstract Both the as-annealed and room temperature deformed microstructure of the L12 compound Fe3Ge have been studied using transmission electron microscopy. The as-annealed microstructure consisted of a dense array of stacking faults on {111} planes and a small concentration of thermal APBs on {100} planes. In the deformed microstructure, {001} cube slip has been identified. The asannealed stacking faults may also produce a strengthening effect on such slip. A combination of these two factors causes the large and negative temperature dependence of yield strength in Fe3Ge. The absence of octahedral slip is attributed to the fact that there is no dislocation configuration on {111} planes which is both glissile and allowed energetically.