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Evaluation of methods for characterizing the melting curves of a high temperature cobalt-carbon fixed-point to define and determine its melting temperature.

Lowe, D H B; Machin, G (2012) Evaluation of methods for characterizing the melting curves of a high temperature cobalt-carbon fixed-point to define and determine its melting temperature. Metrologia, 49. pp. 189-199.

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Abstract

The future mise-en-pratique for the realisation of the kelvin depends on proposal of the melting temperatures of particular metal-carbon eutectic alloys as thermodynamic temperature references. At the moment there is no consensus on what should be taken as a melting temperature. Within the ITS-90 there is a tacit assumption that an ideal melt or freeze curve should be a completely flat plateau at a specific temperature. Any departure from the ideal is due to shortcomings in the realisation and should be accommodated within the uncertainty budget. However for the proposed alloy based fixed-points, melting takes place over typically some hundreds of milli-kelvin. Including the entire melting range within the uncertainties would lead to an unnecessarily pessimistic view of the utility of these as reference standards. Therefore some analysis of the shape of the melting curve is needed to give a value associated with some identifiable aspect of the phase transition. A range of approaches are or could be used; some purely practical, determining the point-of-inflection of the melt curve, some attempting to extrapolate to the liquidus temperature just at the end of melting, and a method that claims to give the liquidus and an impurity correction based on the analytical Scheil model of solidification that has not previously been applied to eutectic melting. The different methods have been applied to cobalt-carbon melt curves that were obtained under conditions for which the Scheil model might be valid. In the light of the findings of this study it is recommended that the point-of-inflection continue to be used as pragmatic measure of temperature but where required a specified limits approach should be used to determine the melting temperature.

Item Type: Article
Subjects: Engineering Measurements
Engineering Measurements > Thermal
Identification number/DOI: 10.1088/0026-1394/49/3/189
Last Modified: 02 Feb 2018 13:14
URI: http://eprintspublications.npl.co.uk/id/eprint/5360

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