Tungsten Alloys

Abstract

A large number of tungsten alloys and composites were investigated in the past, but only some of them achieved technical importance. However, these are of significant commercial interest and will be discussed in the following sections. Alloys derived from melting metallurgy, such as ferrotungsten, melting base, steels, superalloys, and stellites, as well as the large group of cemented carbides, are not discussed here but will be treated in Chapters 8 and 9. The aim of alloying tungsten is to improve its chemical, physical, and mechanical properties at both ambient conditions and elevated temperatures. Beyond that, it is possible to combine the useful properties of tungsten with those of the alloying additives. Low-temperature brittleness is the most crucial aspect in the manufacture of pure tungsten metal. Therefore, in the past, much effort has been directed at lowering the ductile-to-brittle transition temperature (DBTT) and hence improving the fabricability of the metal. In this regard, tungsten-rhenium alloys have gained outstanding importance. They exhibit a significantly lower DBTT and are even stronger than unalloyed tungsten at high temperatures. To fully realize the potential of tungsten in high-temperature applications, dispersion strengthening and precipitation hardening turned out to be the most effective way to increase the high-temperature strength and creep resistance. Non-sag tungsten alloys or thoriated tungsten, used for lamp filaments, are examples of this group of materials. Moreover, alloys based on W-Re-Th02 and W-Re-HfC are among the strongest alloys used today for construction parts in high-vacuum and high-temperature technology. These tungsten alloys are commonly produced by powder metallurgical techniques. Solid-state sintering, liquid-phase sintering, and infiltration techniques are employed. Fine particle size and high purity of the powder components are a prerequisite in order to achieve an even distribution during sintering and a high degree of material purity. Powder metallurgy offers a simple route to the desired ratio of components, simply by mixing them properly. Concentration(s) of the alloying element(s) in tungsten base alloys may vary from traces in the case of microalloys (non-sag tungsten contains less than 100 Ilg/ g K) to more than 20 wt% (W-Re). Depending on the additives, single-phase or two-phase alloys are produced. Applications of the alloys described here are presented in Chapter 7. More information on the metal-physical principles can be found elsewhere [6.1-6.4]. 255 E. Lassner et al., Tungsten