Papers by Author: I. Pong

Abstract: The early view of superplasticity was that it was a phenomenon that could only be exhibited by fine grained, two phase alloys. This effectively ruled out most alloys that possessed attractive service properties. The first material to demonstrate good superplastic properties from a virtually single phase microstructure was the Al-6%Cu-0.5%Zr, AA 2004 but this was followed by superplastic versions of AA7475, AA8090 and AA5083. Superplasticity was also demonstrated in magnesium based alloys at an early stage. More recently different grain control additions, such as scandium or erbium have been investigated and it has also been demonstrated that, in certain circumstances, aluminium simply with the addition of a grain controlling element can exhibit good superplastic behaviour. While conventional wisdom teaches that large fabricating strains are required to confer good superplastic properties in the sheet product, recent results with both aluminium and magnesium alloys cast doubt on this belief. Although, for many years, strip casting has appeared to provide an attractive semi-fabricating route for superplastic sheet problems with centre line segregation in alloys with a wide freezing range have precluded its use. It has been demonstrated that recent developments in strip casting enable production of alloys with as wide a freezing range as AA5182 to be cast with a fine, equiaxed grain structure across the strip thickness. The paper will review the state of these various developments and their implications for the manufacture of superplastic sheet materials.

Abstract: The reactive diffusion and phase formation sequences in two types of ‘internal tin’ superconducting wires designed for the ITER project, which investigates the production of electricity by means of nuclear fusion, have been studied during heat treatments both in situ, using electrical resistometry [1] and ex situ, using optical and scanning electron microscopy, energy dispersive X-ray spectrometry (EDS) and X-ray Micro-Tomography (XMT). XMT reveals long pores in the longitudinal direction which may result in tin deficiency thereat and hence local off-stoichiometric Nb3Sn. Microscopy suggests there are incomplete conversion of elemental tin to copper-tin intermetallics before ramping above the tin melting temperature, nonuniform distribution of tin before formation of Nb3Sn, and filament movement and bridging, stacking cracks and unreacted niobium at the end of the heat treatment. FEGSEM shows a fine microstructure which nevertheless could still be improved.

Abstract: The bronze process is a mature technology for the production of Nb3Sn superconducting wires exploiting reaction diffusion behaviour in the Cu-Nb-Sn system. However, the superconducting properties depend strongly on the applied heat treatment, and optimisation of the heat treatment is still largely by trial and improvement. Modelling of the reaction-diffusion behaviour would allow improved heat treatments to be designed; combination of this with a nondestructive in situ characterisation technique would also permit improved superconducting wires to be produced. A finite difference reaction diffusion model has been designed to permit rapid calculation of the bronze matrix composition and Nb3Sn layer thickness profiles across the wire cross-section as a function of time for any applied heat treatment. The model has also been designed to calculate the electrical resistivity of the wire, which has previously been demonstrated as a suitable in situ characterisation technique. This model has been applied to isothermal and more complex heat treatments and compared with experimental results. Good qualitative agreement has been found, and plans for further improvement of the model are described in detail.