Design and Synthesis of Metals (Tungsten) with Structural Hierarchy for Very High Temperatures
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DESIGN AND SYNTHESIS OF METALS (TUNGSTEN) WITH STRUCTURAL HIERARCHY FOR VERY HIGH TEMPERATURES G. Welsch, Department of Materials Science and Engineering, 10900 Euclid Avenue, Case Western Reserve University, Cleveland, Ohio 44106, USA
ABSTRACT The strength of metals is limited by the weakest link in their microstructures. At high temperature, the weakest link is usually a grain boundary. Mobile dislocations represent another type of weak link. The strength-limitation by this two-level hierarchy of crystal defects can be minimized through microstructure design. Although the strengthening mechanisms differ for grain boundaries and dislocations, it is possible to design microstructural architectures that strengthen both. The design must take the stress state into consideration that the component will encounter during its later use. Processing strategies can then be devised for the synthesis of such components. It is imperative to stabilize the designed microstructure at high temperature because this will determine how well and how long the material will be able to perform. Design, synthesis and stabilization are discussed in the present paper. The lamp filament wire technology is the basis for a new method that enables the synthesis and stabilization of artificial microstructures in high-temperature materials.
1. INTRODUCTION Lamp wire is the quintessential high-temperature material. As a filament it must function for long periods of time at very high temperature while under stress. The lamp wire technology was developed during the early part of this century and has remained prominent to the present time. An empirically developed sequence through numerous processing steps, known as 2 the Coolidge/PaczI' technology, realizes ductility in drawn tungsten wire and attains high-temperature creep strength through overlapping longitudinal grains. It is a microstructure that provides resistance against failure of grain boundaries, which would otherwise be the weakest links. Amongst engineering applications there exists no comparable method that produces creep strength to temperatures as high (to over 3000K) as does the Coolidge/Pacz method. The mechanisms which enable the lamp filaments' hightemperature properties provide the foundation for a new method that enables the synthesis of stabilized artificial grain architectures in other 3 structural alloys.
2.
HIERARCHY OF WEAK LINKS
A number of microstructural constituents may qualify as weak links. Broadly, they are the ones that give way at certain stress levels and lead to failure either directly or indirectly through preceding plastic deformation. According to their strength-limiting effects, the crystal defects may be ranked in a hierarchy of weak-links. The two defect types of greatest concern are grain boundaries and dislocations. If their effects on Mat. Res. Soc. Symp. Proc. Vol. 255. @1992 Materials Research Society
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high-temperature strength are considered independent of each other, unfavorably oriented grain boundaries are the most severely strengthlimiti
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