Tungsten sintering, in practice, is always performed in reducing atmosphere which removes the oxygen coating of the powder particle surfaces. High-purity dry hydrogen is commonly used. Under vacuum or in inert gas, sintering is retained by residual oxygen, and the desired density will not be achieved.
The high temperatures used for sintering also lead to a significant purification of the metal by volatilization of impurities. Interstitials, such as oxygen and carbon, cannot be removed completely and have to be held at a low level; otherwise, they significantly affect the workability of the parts. Metallic elements, such as Fe, Ni, Cr, Nb, etc., are evaporized but remain partly in the ingot, forming solid solutions. If Fe or Ni is present in the form of large, heterogeneous contaminations-(such as iron particles stemming from reduction boats), these can lead to local melt formation and subsequent formation of large voids, which do not close during sintering, Elements which have practically no solubility in tungsten, such as alkali or earth-alkali metals, are volatilized. Grain boundary diffusion and carrier distiIlation effects most likely play an important role in“cleaning” the grain boundaries.
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