Brian L. Metcalfe
article posted 27 April 2016
Brian Metcalfe joined the Atomic Weapons Establishment in 1965 in a career which spanned forty six years. During his career he was engaged in the research and
devel opment of ceramic, glass and glass-ceramic materials for a diverse range of programmes including glass-ceramic-to-metal seals, chemically strengthened glasses, ceramic fibre
reinforced glass-ceramic matrix composites and ceramic hosts for the immobilization of high chloride content nuclear waste.
He is a Fellow of the Institute of Materials, Minerals and Mining, a member of the So ciety of Glass Technology and an Associate Member the Nuclear Institute. Since retiring he has
continued his interest in glass as a Visiting Lecturer at Imperial College and is the author or co-author of over sixty technical publications.
Effect of nucleation temperature on the hardness of a lithium zinc silicate glass
Brian L. Metcalfe*, Caroline Alglave & Doni J. Daniel
Dept. of Materials, Imperial College, London, UK
An investigation into the nucleation and crystallisation of a lithium zinc silicate glass having the composition 53.64 % Si02
, 17.84% Li2
0, 17.73% ZnO, 5.25% Na2
, 1.23% P2
included the measurement of micro-hardness. It was observed that the micro-hardness of glass specimens nucleated over a range of temperatures spanning the optimum nucleation temperature for the glass as determined by the method of
Marotto et. al. [1,2] increased from 580 VPN for as-quenched specimens to 650 VPN for the specimen nucleated at the optimum nucleation temperature . Investigations into the reason for this effect proved inconclusive. All samples were Xray amorphous but small angle neutron scattering experiments demonstrated the presence of
particles approximately 10-16nm in diameter in the specimens. High resolution transmission electron microscopy (TEM) performed on the specimens led to almost immediate
nucleation of the specimens when inserted into the electron beam thereby preventing identification of these particles.
This work has recently been revisited  as the advent of new and improved electron microscopy techniques offers a greater possibility of identifying the nature of these particles.
Results from both micro- and nano-indentation measurements show a similar trend to that observed in the original work although the peak hardness occurs at a slightly higher
temperature, 480°C compared to that previously reported, 465°C. Again all the samples were X-ray amorphous. TEM was performed on the specimen nucleated at
480°C using bright field, high-angle annular dark field (HAADF) and energy disperse spectroscopy (EDS) techniques. The bright field images show particles up to approximately lOnm and which have some periodicity, dispersed in an amorphous matrix. Contrast in
HAADF images show these particles to contain high atomic number elements and this is supported by EDS data which shows a much higher concentration of zinc in them.
TEM bright field images showing periodicity in the dispersed phase
1 Marotta, A., Buri, A. and Branda, F., Nucleation in glass and differential thermal analysis, J. Mater. Sci. 16(2) (1981) pp 341-344.
2 Brand, F., Marotta, A. and Buri, A., A new parameter to evaluate nucleation in glasses from DTA curves, Thermochimica Acta 128 (1988) pp 39-43.
3 Donald, I. W., Metcalfe, B. L. and Morris, A. E. P., Influence of transition metal
oxide additions on the crystallisation kinetics, microstructures and thermal expansion characteristics of a lithium zinc silicate glass. J. Mater. Sci. 27(11) (1992) pp
4 Alglave, C., Hardness increase of a lithium zinc silicate glass under heat-treatment, MSc thesis, Imperial College London, September 2014.