Dispersion-Strengthened and Precipitation-Hardening Alloys

At half of the solidus temperature on the absolute temperature scale (i.e., 1850°C for tungsten), solid-solution alloys lose much of their strength, and dispersion-strengthened or precipitation-hardened alloys are significantly stronger and creep resistant. This is caused by the interaction between the dispersoids and dislocations, as well as subgrains and grain boundaries. 

The discovery of NS-W was a fluke. In the early days of wire fabrication (1910-1925), it was realized that the source of the tungsten oxide used for metal powder fabrication played an important role in determining the wire quality [6.7]. Tungsten filaments, originating fix m material which was treated in clay crucibles, preferably those manufactured by the Battersea Company near London, were shown to be much more stable in shape at incandescent temperatures. Subsequent chemical analysis indicated that a 

FIGURE 6.3. Coil of a 230V/60W lamp before use (a) sagging and deformation of the coil after several hundred hours of operation due to bad NS quality (b). 

A schematic representation of the non-sag tungsten wire fabrication process is shown in Fig. 6.4. 

FIGURE 6.4. From tungsten blue oxide to non-sag wire (schematic presentation). 

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