The oxide dispersoids are stable at the high sintering temperatures and do not dissolve in the tungsten matrix. They pin the grain boundaries during the later stages of sintering, and in that way they significantly restrict grain coarsening. So for example, a rod containing 0.75% of thoria has a grain size of 5000 to 10,000 grains per square millimeter, as compared to1500 grains per square millimeter for a similar rod of pure tungsten.
NS-doped tungsten powder contains small inclusions of potassium aluminosilicates, which were incorporated during the reduction process. During sintering, the silicates dissociate thermally and submicron potassium bubbles form in the tungsten ingot. Similar to the oxides, these bubbles pin the-grain boundaries and inhibit grain coarsening during sintering. Since potassium is gaseous during sintering, the bubbles are under high pressure, which is balanced by the surface tension of’ the pore. They can be seen as small pores in the fracture surfaces of NS-doped tungsten besides the significantly coarser residual sintering pores, as characteristic for undoped tungsten. They constitute the starting point for subsequent formation of rows of bubbles during thermo-mechanical processing.
Up to the sixties, the only sintering method used in practice was direct sintering. Although still in use for the production of doped tungsten, it has lost its importance. From then on, mainly because of the increasing demand for Iarger parts and the higher capacity of the aggregates, indirect sintering furnaces were developed. This technique is used nowadays as the main route for producing pure tungsten.
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