Powdered samples and sintered specimens of TaC-WC solid solutions containing up to 40 mol % WC as well as sintered alloys bonded with cobalt were heat-treated in vacuum at 1300-1500°C for 15-300 min. And the decomposition of solid solutions during heat treatment was studied. The results obtained are as follows.
1) The sintered specimens of TaC-WC solid solutions containing up to 20 mol % WC were considerably stable against heat treatment. But the discontinuous precipitation by grain boundary reaction occurred in specimens containing more than 30 mol % WC. It seems that decomposition proceeds gradually to the solubility limit after the rapid decomposition at the first stage.
2) Powdered samples of the solid solutions including those containing 10 mol % WC, however, decomposed very rapidly to the solubility limit.
3) In alloys bonded with cobalt, the solid solutions decomposed to the solubility limit during sintering, as in powdered samples. But it was found that the microstructure of these alloys differed from that of WC-TaC-Co alloys to which TaC was added in the form of monocarbide, especially in the form of WC phase.

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High strength WC-10%Co alloys having grain size of about 2.2μ and high bending strength as reached arithmetic mean values of 350-370kg/mm² according to ASTM-B406-64 were vacuum-sintered. Mechanical properties of the alloys, such as hardness, crack length around Vickers hardness indentation and impact strength were investigated mainly in relation to the bending strength and carbon content, and the results were compared with those obtained in conventional alloys having strength of about 290-340kg/mm².
Experimental results are summarized as follows: (1) Hardness of all the alloys used in this study appeared not to be affected by the bending strength, but it depends on the carbon content. (2) Increase in bending strength and carbon content decreased crack length around Vickers hardness indentation and improved impact strength. (3) It was concluded that the alloys which are expected to be excellent for practical use should be obtained at the medium carbon content.

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The reduction process of NiO with hydrogen is divided into four steps with respect to the electrical resistance: (1) an initial increase in the electrical resistance up to a maximum, (2) a gradual decrease from the maximum, (3) an abrupt decrease, and (4) a successive decrease to a definite electrical resistance. In the initial increase, NiO reaches a stoichiometric composition. After completion of the stoichiometric composition, NiO is partly reduced to form islands of reduced Ni. At the third step, the islands spread throughout the surface of NiO to complete the conductive layer. Upon completion of the conductive layer, the electrical resistance decreases abruptly. The thickness of the conductive layer increases with an increase in the reducing time, which is responsible for the gradual decrease in the electrical resistance. Thermobalance having a sensitivity of 0.5mg detected no weight loss due to the hydrogen reduction of NiO at the first step.

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High-density iron-base parts has been developed to meet the requirement for high strength sintered materials. The process consists of mixing, compacting, sintering and repressing (hot working temperature range), and is characterized by rapid heating and rapid forming under pressure. Effects of iron powders from different sources on the ductility of sintered products were studied. It is shown that the non metallic inclusions have deleterious effect.

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