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Proceedings Paper

Dimensional stability of superinvar
Author(s): Steven R. Patterson
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Paper Abstract

The unloaded dimensional stability and thermal expansivity of a single furnace melt of superinvar have been measured at room temperature using interferometric techniques. Thermal expansivity has been determined with an uncertainty of several parts in 108 per degree centigrade while dimensional stability has been determined with an uncertainty of order one part in i09 per day. Samples subjected to plastic deformation in their processing history displayed a stability improvement from 20. 5 x 1O/day to 5. 5 x 109/day and a reduction in thermal expansivity from 0. 56 x 1O/C to 0. 23 x 106/C associated with the increased mechanical work in the material. 1. MATERIAL SELECTION AND PROCESSING Modern scientific instruments place increasingly stringent demands on the dimensional stability of materials used for the construction of precision structures. The dimensional change of a material due to a change in temperature is characterized by its coefficient of thermal expansion (CTE). Dimensional changes may also occur over time in a fixed environment. In considering the selection of a material for demanding applications both the CTE and temporal stability must be considered. Indeed with reasonable control of the ambient temperature pressure humidity and magnetic field temporal instability may be the factor limiting performance. Superinvar the material studied here is one alloy in a class of materials of practical interest for the construction of precision systems. The invars are relatively inexpensive materials (by comparison other low-CTE materials) and may be fabricated using conventional metalworking techniques. Superinvar is of particular interest because it exhibits a lower CTE than the other invars and because of an early report by Jacobs Bradford and Berthold indicating that it possessed a particularly high degree of temporal stability. 1 This paper reports a series of CTE and temporal stability measurements performed on superinvar samples taken from the same melt but subjected to different degrees of processing. One of the experimental aims of this study was to learn about the conditions leading to the single high stability result reported by Jacobs. Accordingly the chemistry of the material was specified as a high purity superinvar with 31 Ni and 5 Co. This alloy lies at the minimum CTE point of the Fe-Ni-Co system2 and is the nominal alloy used by Jacobs. Table I shows the composition which was specified the analysis performed at the mill at the time of melt and an independent analysis of the material after receipt at Livermore. For reference purposes the composition of the sample measured by Jacobs is also listed. 3 The material used in the samples was cast in an air induction furnace as part of a large order. Although the same steel mill produced both this material and Jacobs'' samples the furnaces used for the two melts were different: Jacobs'' sample came from a melt in

Paper Details

Date Published: 1 November 1990
PDF: 7 pages
Proc. SPIE 1335, Dimensional Stability, (1 November 1990); doi: 10.1117/12.22880
Show Author Affiliations
Steven R. Patterson, Lawrence Livermore National Lab. (United States)

Published in SPIE Proceedings Vol. 1335:
Dimensional Stability
Roger A. Paquin, Editor(s)

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