Roger Grimes

Development of a high strain rate superplastic Al–Mg–Zr alloy

Abstract For superplastic forming of aluminium to break out of the niche market that it currently occupies, alloys will be required to possess a higher strain rate capability, appropriate in service properties, and a significantly lower price and to be capable of volume production. This paper describes an approach that has been developed in an attempt to address these fundamental requirements. A series of Al–Mg–Zr alloys with increasing levels of zirconium (0–1 wt-%)has been prepared via extrusion consolidation of cast particulate (solidification rate ∼103 K s-1). The superplastic properties of the resultant cold rolled sheet have been evaluated as a function of thermomechanical treatment and zirconium addition. It has been found that increasing the level of zirconium has the twofold effect of improving the superplastic properties of the alloy while significantly decreasing the concomitant flow stress. At present the optimum superplastic behaviour has been obtained at strain rates of 10-2 s-1, with the 1%Zr material exhibiting ductilities in excess of 600%. The manufacturing route produces a bimodal distribution of Al3Zr comprising >1 µm primary particles in combination with nanoscale solid state precipitates. The current postulation is that this high strain rate superplasticity is conferred by a combination of particle stimulated and strain induced recrystallisation.

Vacancies and defect levels in III–V semiconductors

Using electronic structure calculations, we systematically investigate the formation of vacancies in III-V semiconductors (III = Al, Ga, and In and V = P, As, and Sb), for a range of charges (−3≤q≤3) as a function of the Fermi level and under different growth conditions. The formation energies were corrected using the scheme due to Freysoldt et al. [Phys. Rev. Lett. 102, 016402 (2009)] to account for finite size effects. Vacancy formation energies were found to decrease as the size of the group V atom increased. This trend was maintained for Al-V, Ga-V, and In-V compounds. The negative-U effect was only observed for the arsenic vacancy in GaAs, which makes a charge state transition from +1 to –1. It is also found that even under group III rich conditions, group III vacancies dominate in AlSb and GaSb. For InSb, group V vacancies are favoured even under group V rich conditions.

Morphology and structure of ZnCr2O4 spinel crystallites

Computer simulation techniques have been used to predict the crystal morphology of the spinel ZnCr2O4. In agreement with experiment, crystallites are predicted to be essentially octahedral with the {111} surface dominating the structure. However, surfaces for materials with the spinel structure are highly complex and stabilized only by the formation of surface defects. This leads to a large number of different possible surface structures.

Antisites and anisotropic diffusion in GaAs and GaSb

The significant diffusion of Ga under Ga-rich conditions in GaAs and GaSb is counter intuitive as the concentration of Ga vacancies should be depressed although Ga vacancies are necessary to interpret the experimental evidence for Ga transport. To reconcile the existence of Ga vacancies under Ga-rich conditions, transformation reactions have been proposed. Here, density functional theory is employed to calculate the formation energies of vacancies on both sublattices and the migration energy barriers to overcome the formation of the vacancy-antisite defect. Transformation reactions enhance the vacancy concentration in both materials and migration energy barriers indicate that Ga vacancies will dominate.

The Development of a High Strain Rate Superplastic Al-Mg-Zr Alloy

In order for superplastic forming of aluminium to break out of the niche market low cost alloys are required that exhibit higher strain rate capability that are capable of volume production. This paper describes an investigation into the feasibility of producing such an alloy. A series of Al-4Mg alloys with 0, 0.25, 0.5, 0.75 & 1 % Zr additions was prepared using a cheap particulate casting route, in an attempt to achieve higher levels of Zr supersaturation than are possible with conventional casting. The particulate was processed into a sheet product via hot extrusion followed by cold rolling and the effect of a number of process variables on the SPF performance of the sheet was investigated. It was found that increasing the Zr content, and manipulation of the thermomechanical processing conditions improved the SPF performance. Ductilities in excess of 600% have been achieved at a strain rate of 0.01 s -1 , together with flow stresses less than 15MPa.

Co-doping with antimony to control phosphorous diffusion in germanium

In germanium, phosphorous and antimony diffuse quickly and as such their transport must be controlled in order to design efficient n-typed doped regions. Here, density functional theory based calculations are used to predict the influence of double donor co-doping on the migration activation energies of vacancy-mediated diffusion processes. The migration energy barriers for phosphorous and antimony were found to be increased significantly when larger clusters involving two donor atoms and a vacancy were formed. These clusters are energetically stable and can lead to the formation of even larger clusters involving a number of donor atoms around a vacancy, thereby affecting the properties of devices.

Mechanisms of nonstoichiometry in HfN1−x

Density functional theory is used to calculate defect structures that can accommodate nonstoichiometry in hafnium nitride: HfN1−x, 0≤×≤0.25. It is predicted that a mechanism assuming simple distributions of nitrogen vacancies can accurately describe the variation in the experimentally observed lattice parameter with respect to the nitrogen nonstoichiometry. Although the lattice parameter changes are remarkably small across the whole nonstoichiometry range, the variations in the bulk modulus are much greater.

Hot-pressed phosphate glass–ceramic matrix composites containing calcium phosphate particles for nuclear waste encapsulation

Sodium aluminium phosphate (NaAlP) glass–ceramic composites were produced as potential wasteforms for the immobilization of special categories of halide-containing radioactive waste. Sintering conditions for encapsulating a simulated waste (a calcinated mixture of calcium phosphate host and various oxides) in the cold-pressed NaAlP glass–ceramic were first determined and the results were compared with similar samples prepared by hot pressing. In both cases, the conditions aimed to provide a very high-density material, via as low production temperatures as possible, in conjunction with a high waste loading (75 wt.% simulated waste to 25 wt.% glass). It was found that by hot pressing and using a NaAlP glass–ceramic containing 2 mol% B2O3, significantly lower temperatures could be employed compared to the cold pressing and sintering route. The lowest temperature at which a sufficiently dense hot-pressed product was achieved (86% theoretical density), that exhibited mechanical properties similar to those of borosilicate glass (e.g. Young’s modulus 67 ± 2 GPa), was 550 °C. This processing temperature is considerably lower than values reported in the literature for similar systems. As such, hot pressing can be considered as a convenient technique for the fabrication of this type of composite for waste encapsulation.

Antisites in III-V semiconductors: Density functional theory calculations

Density functional based simulation, corrected for finite size effects, is used to investigate systematically the formation of antisite defects in III-V semiconductors (III = Al, Ga, and In and V = P, As, and Sb). Different charge states are modelled as a function of the Fermi level and under different growth conditions. The formation energies of group III antisites ( IIIVq) decrease with increasing covalent radius of the group V atom though not group III radius, whereas group V antisites ( VIIIq) show a consistent decrease in formation energies with increase in group III and group V covalent radii. In general, IIIVq defects dominate under III-rich conditions and VIIIq under V-rich conditions. Comparison with equivalent vacancy formation energy simulations shows that while antisite concentrations are always dominant under stoichiometric conditions, modest variation in growth or doping conditions can lead to a significantly higher concentration of vacancies.

Predicted calcium titanate solution mechanisms in calcium aluminoferrite and related phases

Computer simulation, using an ionic, Born-like model, is used to investigate the accommodation of titanium impurities in Ca-Al-Fe-O phases. Specifically, calcium titanate solution in Al2O3, Fe2O3, CF, C2F, CA, C2A, C3A, and C4AF (where C denotes CaO) is considered. The simulations predict that titanium impurities are found preferentially in the ferrite phases. At sufficiently high concentrations, the solution involves the formation of clusters containing 3 to 6 ions that strongly resemble the structure of the calcium titanate phase. The calculations reveal that the compensation mechanism varies appreciably over the range of compounds considered.