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内容記述 |
As is well known, tungsten has the highest melting point among the existing metals at present and is one of metals having the highest tensile strength at high temperatures. It is, however, one of the most oxidizing metals at high temperatures in the existence of air. The authors attempt in this study to investigate the influence of chromium added to tungsten upon the improvement of the properties of this material and the removal of the defect mentioned above. In this research, the preparation of the W-Cr alloys was the most difficult task. Many attempts for the preparation of the samples by sintering powdered tungsten and chromium were all in vain, but finally it is succeeded by the thermit process. In Table 2 in the following chapters, the compositions of the samples by chemical analyses are given. It will be seen that a certain amount of aluminium, say 5 percentage, is always present as a result of the preparation by the thermit process. In this paper, the W-Cr equilibrium diagram is first dealt with. In Sargent's paper, the existence of W_7Cr_2, WCr_3 and WCr_7 is proposed, but there is no other literatures on the W-Cr equilibrium diagram, so far as the authors are aware. In the present study on the diagram, the melting temperatures, microscopic structures, X ray analyses and hardness tests were carried out in order. The melting temperatures of the samples were too high to be determined by the ordinary laboratory method. As a result of the observation of the microscopic structures of the samples in the state as cast and annealed at 1200℃ for a long duration, the aggregation of the small particles are seen. These structures differ considerably from those which appear in the samples prepared by the ordinary melting process, but they are nearly the same structures in the samples containing chromium of from zero to 400/8 in atomic percentage. The X-ray analyses were made by the reflection method, and as is shown in Fig. 3, it was found that a solid solution of chromium exists in tungsten in a range from zero to 400/8 atomic percentage of Cr. The result of the hardness tests is shown in Fig. 4. This curve indicates that the hardness of the samples rapidly increases as the percentage of chromium in tungsten increases, but the maximum point of the hardness in the curve is somewhat shifted towards the lower chromium side from the limit of the solid solution of chromium in tungsten obtained by the X-ray analyses. In this paper, however, the result of the X-ray analyses is adopted for the solid solution range of chromium in tungsten. Anyhow, the existence of some solid solution range in the W-Cr binary alloys is certain notwithstanding Sargent's proposition referred above. The authors consider that Fig. 5 is a portion of the supposed W-Cr equilibrium diagram. From the above stated investigations, the fact of the existence of the solid solution of chromium in the W-Cr alloys is presumable, and also from the actual examples, the high resistance to oxidation at high temperatures in the existence of the air, of the solid solution containing chromium is presumable. Therefore, oxidizing tests at 900℃ were carried out for the prepared samples, the results being as shown in Fig. 6. From this curve, the oxidizing property of tungsten at high temperatures is found greatly improved by adding about 13 percent of chromium to tungsten. |
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