Sakae Takeuchi1 and Tsuneo Homma1
1The Research Institute for Iron, Steel and Other Metals, Tohoku University, Sendai
In order to study the mechanism of fatigue at elevated temperatures, the process of fatigue failure under the reversed torsion and bending stresses was observed microscopically on single crystal or polycrystal specimens of pure metals, 99.99%Al and Pb, over the range of temperatures from 0.5 to 0.85 Tm°K in argon or air atmosphere. The results obtained are as follows:
(1) When a single crystal of pure Pb or Al was fatigued under the repeated stress at elevated temperatures, the initiation and growth of subgrains took place, accompanied by a migration of the subgrain boundaries in the maximum shear stress direction of the specimen axis. Their grain boundary faces were aligned nearly orthogonally in argon atmosphere, and along the boundaries slidings of the subgrains occurred with one another and formed cracks. In air atmosphere the alignment of the boundary face in the direction of the maximum shear stress was not complete, resulting in a poor alignment of cracks.
(2) In the case of polycrystals, it was observed that coarse grained specimens showed the same behavior as in the single crystal in the case of a large amplitude of the reversed stress, but under a lower stress the migration of the original grain boundary and the alignment of its face in the directions of the maximum shear stress took the main role in the crack formation as previously reported.
(3) The size of subgrains formed in the fatigue failured single crystal specimen depended appreciably on the testing temperature. Generally, the size became smaller with the decrease of temperature and the prolongation of endurance life.
(4) The relationship between the mechanisms of the fatigue failure at a low temperature and at a high temperature was discussed from the viewpoint of the role of the substructure formed during the fatigue.
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