Al-Mg合金の比較的低温度・長期時効による耐食性の変化について
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概要
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Al-Mg alloy belongs to the strong corrosion-resisting aluminium alloys. However, the solid solubility at the aluminium side is relatively large at high temperatures but decreases rapidly as the temperature drops. Therefore, with an ordinary cooling velocity, an equilibrium state is not easily attained. In other words, if ageing be carried out at the low temperatures (20-80℃), ageing proceeds very slowly but continues during exceedingly long period of time. In this research, the change of the corrosion-resisting property of the Al-Mg alloy due to such a low temperature ageing is studied. The samples are made of high purity Al and Mg both being 99.99%. The composition of the samples is Mg = 2.49, 3.47, 4.57, and 5.73%, balance being Al. The ageing is carried out at 20, 40, 65, 80℃ after quenching at 415℃ and has been continued until 66,700 hrs (now yet under ageing). The corrosion-resisting property is measured by means of the reaction number RN which is a dissolving velocity of a specimen into I-N HCL water solution, denoted by volume of hydrogen gas (cc) divided by both a surface area (cm^2) of the specimen and a dissolving time (hr). The RN will be varied as ageing proceeds, because the concentratiom cell related to Mg atoms will be formed in the space lattice of Al-Mg alloy. The accuracy of measuring RN is ensured under three conditions : 1) the constant surface area of the test piece (30×30×2mm), 2) a volume of HCl water solution (500cc) large enough to satisfy a practically constant concentration of the acid during reaction, and 3) measuring in the constant temperature chamber (20±1℃). The results of the study are : (1) At the first stage of ageing, RN increases. Although the accurate position of Mg atoms located in the space lattice of Al (GPZ) is not able to be determined, the CLUSTERING of Mg atoms, at least, can be easily deduced, since the increse of RN will be exactly associated with the concentration cell referred to Mg atoms. (2) As ageing proceeds, at the second stage, β is precipitated along grain boundaries. (3) At the LAST STAGE, β particles precipitated along grain boundaries grow into a continuous film along the grains, and also its thickness increases, as well as the precipitations occur inside the grains. If the specimen aged until the LAST STAGE is immersed in I-N HCl water solution, β along the grain boundaries is selectively dissolved by the acid aolution and the specimen is disintegrated into crystal grains. The grains disintegrated are the crystals mainly composed of the TERAKAIDECAHEDRON and RHOMBIC DODECAHEDRON which are proved by X-ray analyses. Conclusively, by ageing of 20℃×66,700 hrs (nearly S years), in Al-5.73% Mg alloy, is found the LAST STAGE of ageing (DISINTEGRATED IN HCl SOLUTION). And by ageing of 65℃×66,700 hrs, in Al-2.56% Mg alloy, also is found the LAST STAGE of ageing. The ageing up to the LAST STAGE should not be employed in the practical use of Al-Mg alloy.
- 明治大学の論文