Image analysis for grain shape characterization in lamp filaments
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I.
INTRODUCTION
L A M P filament wire must have enormous resistance to high-temperature creep. As with other metals, performance in this respect depends directly on grain structure. While grain structure depends on processing, creep at the stresses and temperatures of lamp filaments can be predicted directly from microstructure. The objectives of our work have been to define those features which determine creep resistance and to learn to measure them. The relationship between microstructure and filament creep has been appreciated qualitatively for a long time. v,2] Creep resistance has been associated with very long grains having interlocking boundaries which make large angles with the wire cross section. This grain structure results from manipulation of the alloying element during processing. Rows of tiny potassium bubbles are produced in the lamp wire product. These bubbles constrain grain growth during recrystallization so that the unique long interlocking grains result. The long-grain interlocking structure is critical, because the predominant deformation mechanism in lamp filaments is diffusional flow. Frost and Ashby have demonstrated this on deformation maps.[3l The diffusional flow mechanism most frequently proposed for lamp filament wire is grain boundary sliding. [1,4,5] Pink and Gaal have discussed these mechanisms for lamp filaments in considerable detail. [61 They correctly point out that the success of lamp fdament alloys derives from the suppression of grain boundary sliding. It is also true that when a filament fails because of excessive creep, it is because the grain structure which prevents sliding is deficient. What has been lacking is a quantitative relationship between those features recognized as important and the dependent performance. This has been true because it has not been possible to make the necessary measurements. This paper describes a remedy.
JOHN W. PUGH, Senior Consulting Metallurgist, and W A Y N E A. LASCH, Research Materials Scientist, are with General Electric Lighting Materials Technology, Cleveland, OH 44112. Manuscript submitted August 3, 1989. METALLURGICAL TRANSACTIONS A
II.
EXPERIMENTAL DETAILS
While the techniques in this paper can be applied to most lamp filament coil geometries, those used for the experiments were coiled coils. They were 12-mm long and made from potassium-doped tungsten wire having a 0.06-mm diameter. They were caused to recrystallize by passing current through them at successively higher voltages until temperatures well over the recrystallization temperature were reached. Creep of filaments was measured in filament coiled coils mounted horizontally in an atmosphere of forming gas at 2970 K. A force of 5 g's was imposed to cause the filament to sag significantly in the relatively short time of 1000 seconds. Deflection was measured after the filament had cooled by means of a Wild MM 235 digital measuring eyepiece to a precision of 10/xm. In this way, the relative creep performance could be judged quickly and reproducibly. Coiled coils were viewed
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