Morrell, R; Quested, P N (1998) Dilatometric measurement of melting behaviour of metal alloys. NPL Report. CMMT(A)106

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Abstract

The use of double piston cells and conventional dilatometry for the determination of the melting and liquid thermal expansion characteristics of metals and alloys has been evaluated. The technique has been successfully applied to aluminium alloys using a boron nitride cell, and has permitted the density as a function of temperature to be determined through the so-called "mushy" zone and as a liquid. Tests on low-melting alloys were less successful due to leakage from the dies, and tests on high-melting alloys have not been completed during the time-span of this project due to difficulties of cost-effective die fabrication in appropriate materials.
It has been established that for successful operation a number of criteria have to be satisfied. The metal should not interact with, and preferably should not wet the cell. The pistons in the cell must slide smoothly, but the cell wall/piston gap must be a small as possible consistent with smooth, substantially friction-free sliding in order to avoid leakage. The starting metal dimensions must permit a thermal expansion mismatch allowance to the melting point to avoid cell fracture, but must not be excessive so that there is a large apparent contraction on melting.
It should be noted that because the pressure that can be applied to the pistons is limited to avoid leakage, the technique does not give the full melting curve, but only over the temperature range above that at which a rigid dendritic network exists. Below this temperature, the minimal force applied to the cell to avoid leakage is insufficient to crush the network, and residual liquid is drawn away from the surface of the test-piece, or the test-piece cavitates or collapses, and piston movement does not reflect the true volume changes.

Item Type:	Report/Guide (NPL Report)
NPL Report No.:	CMMT(A)106
Subjects:	Advanced Materials Advanced Materials > Metals and Alloys
Last Modified:	10 Sep 2018 13:22
URI:	http://eprintspublications.npl.co.uk/id/eprint/995

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