Recrystallization of molybdenum wire doped with potassium-silicate
- PDF / 10,269,536 Bytes
- 16 Pages / 613 x 788.28 pts Page_size
- 54 Downloads / 217 Views
I.
INTRODUCTION
THE control of grain structure in the brittle refractory metals such as tungsten and molybdenum is one of the essential factors to improve the high-temperature property and the low-temperature ductility, particularly nonsag and ductile-brittle transition characteristics, t~-Sj The chemical doping of bubble formation substance, e.g., potassium-silicate, has been applied to develop a grain structure of high aspect ratio (length/width (L/W)) just as in nonsag (AKS-doped) tungsten filament for incandescent lamp. 16-1~ The uniform coarsening of fibrous structure, ttl,12j the formation and growth of bubbles, t13,14'~Sjand the secondary recrystallizationl~6,171have been observed during recrystallization of nonsag tungsten wire. Many studies have been conducted to understand these phenomena, t~-~9j and the results are summarized as follows. First, the uniform coarsening of fibrous structure was attributed to recovery or primary recrystallization,tl~.12] Second, the shape of recrystallized grain was mainly affected by the spatial dispersion of bubbles. I2,191 Third, the aspect ratio of grain (L/W) was increased with increasing the bubble row density. I2~ However, in molybdenum wire, the correlation between recrystallization and bubble dispersion has never MYOUNG KI YOO, Research Engineer, and JU CHOI, Principal Research Engineer, are with the Division of Metals, Korea Institute of Science and Technology, Cheongryang, Seoul, Korea. YUTAKA HIRAOKA, Professor, is with the Department of Applied Physics, Okayama University of Science, Okayama 700, Japan. Manuscript submitted April 25, 1994.
METALLURGICAL AND MATERIALS TRANSACTIONS A
been investigated extensively. In this article, bubble dispersion parameters--the average bubble size, the bubble row density, the bubble row distance, and the bubbles spacing in a given row--were changed by adding various amounts of potassium and silicon. Other process parameters, such as powder processing, doping method, and thermomechanical processing, were unchanged, since the recrystallization phenomena were relatively sensitive to the powder metallurgical parameters for preparing the ingots and to the heat treatment during deformation processes, i.e., swaging, rolling, and drawing, t18,~91 This study was performed to clarify the recrystallization of molybdenum wire doped with potassiumsilicate. The early stage of recrystallization was investigated by means of micro-Vickers hardness and transmission electron microscopy. The relationship between bubble dispersion and grain structure was examined by the optical and scanning electron micrographs. II.
EXPERIMENTAL PROCEDURE
The starting materials were molybdic trioxide, M o O 3, doped with potassium-silicate (referred to as "dopants" in this article). Table I shows the nominal amount of dopants added with respect to molybdenum. The molybdic trioxide powder was reduced to doped molybdenum powder through two-step reactions in a hydrogen atmosphere as follows: MoO3 +
H2 --~ MoO2 + HeO
at 500 ~ to 700 ~
[1]
at I000 ~ to 1100 ~
Data Loading...
