Grain-shape parameters for high-temperature creep resistance in powder metallurgy tungsten fine wires
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I.
INTRODUCTION
THE doping additions of Al2O3, KCl, and SiO2 have been used in the powder metallurgical manufacturing process of tungsten ingots to generate a high-temperature creep resistance. The doping additions develop many arrays of small bubbles along the wire axis during fabrication.[1] Some of the bubbles exist inside a grain and may strengthen the matrix in several ways.[2] The most likely mechanism by which bubbles strengthen the matrix is the so-called modulus-defect interaction. The equilibrium shape of the bubbles is often polyhedral, because this gives the lowest value of the total interfacial free energy for a fixed volume of bubbles.[3] If the total interfacial energy is not at a minimum, the bubbles are nonequilibrium, their shape being irregular.[4] Dislocations then move into the misfit-stress fields around nonequilibrium bubbles. Accordingly, dislocations are attracted by both equilibrium and nonequilibrium bubbles, because they annihilate a part of the dislocation stress field. The modulus-defect interaction mechanism is considered to be most effective in blocking the climb of dislocations at high temperatures.[5,6] On the other hand, most of the bubbles exist on grain boundaries and play a role in controlling the morphology of secondary recrystallized grains.[1] Grain boundary bubbles produce an elongated and interlocked grain structure by impeding the transverse grain growth across the wire axis.[7,8] The elongated grain structure reduces creep strains resulting from grain boundary sliding and cavitation. The interlocking grain structure suppresses the rotational grain boundary sliding due to torsional stresses imposed on coiled wires under the influence of gravity.[9,10] Sliding at a nonplanar grain boundary presents an important problem in the case of a coiled wire under the influence of gravity.[11] However, the increase in the degree of interlocking does not necessarily guarantee an additional reduction in creep strain, because it is forced to accompany an increase in the K. TANOUE is with the Department of Materials Science and Engineering, Faculty of Engineering, Kyushu Institute of Technology, Kitakyushu 804, Japan. Manuscript submitted March 26, 1997. METALLURGICAL AND MATERIALS TRANSACTIONS A
area of grain boundary. It is, therefore, necessary to select a proper combination of coarsening and interlocking of grains after the properties of these two factors are qualitatively estimated. The purpose of the present work is to define some grain-shape parameters characteristic of the grain morphology, such as grain aspect ratio, grain boundary roughness, and interlocking, and to clarify their relation to creep resistance at high temperatures. II.
EXPERIMENTAL PROCEDURE
The material used was commercial-grade doped tungsten wire, 0.13 mm in diameter and greater than 99.96 pct purity, prepared at Nippon Tungsten Co. Ltd. (Fukuoka, Japan). The concentrations of residual doping elements of Al, K, and Si were ,0.001, 0.008, and 0.001 5 0.0003 in mass pct, respectively. All of the heating was d
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