Study on the Structure of Sintered and Re-treated Tungsten Bar

The Structure of tungsten bar, K2O- SiO2- and Al2O3-doped or undoped, sintered and/or re-treated, was studied by means of optical and electron microscopes. Distribution of crystal grain size indicates the restraining effect of Al2O3 doping. Crystal boundary, sintering pore and etching pit are very characteristic. The relationship between the number or size of the pore and Al2O3 doping quantity is made clear, and confirmed by the apparent specific gravity. When SiO2 and K2O are removed by hydrofluoric acid washing, the effect of Al2O3 doping becomes prominent. When the restraining effect of Al2O3 doping is very strong, mean crystal grain size is usually small but sometimes, coarsening takes place in a particular region of the crystals, and the crystal boundry moves through the obstacles such as sintering pores or non-metallic inclusions, making huge crystal grains. Such phenomena was still further studied by the observation on the structure of bars heated up to just below the melting point in which the pore had become larger. The grain size of the doped re-treated bar is generally larger than that of the undoped re-treated bar, and the characteristics of boundary etc. are very similar to that of the sintered bar. A brief additional explanation is given on the recrystallized structure and recrystallization temperature of the wire worked out from these sintered bars.