Evaluation of piston dilatometry for studying the melting behaviour of metals and alloys

The theory and use of double-piston cells and conventional dilatometry for the determination of the melting and liquid thermal expansion characteristics of metals and alloys have been evaluated and are reviewed in detail.The technique has been successfully applied to aluminium alloys with a boron nitride cell, and to cast iron and nickel alloys with an alumina cell. The technique has permitted the density as a function of temperature to be determined through the so-called 'mushy' zone and into the liquid state. Partial success has been obtained with a machinable glass-ceramic cell. For successful operation of the technique a number of criteria have to be satisfied, and these are described. On heating, the melting behaviour is followed only over the temperature range above that at which the dendritic network has no strength. On cooling from a fully molten condition, once the temperature has dropped below this dendrite collapse point, the force typically applied to the cell to avoid leakage is then insufficient to continue to deform the developing crystalline network. Because there is no mechanism for 'feeding' additional liquid, residual liquid is drawn away from the surface of the sample, or the sample either cavitates or collapses inwards. The piston movement does not then reflect the true volume changes. Despite this, the technique has been shown to be useful for estimating densities through the upper temperature part of the 'mushy' zone and into the liquid phase, including determination of the liquid expansion coefficient.