Institute of Metals Division - Creep and Stress Rupture Behavior of Aluminum as a Function of Purity

Extensive data of minimum creep rates and rupture times for high purity and commercial aluminum confirm the existence of a transition range from the low temperature-type to the high temperature-type behavior. The data are analyzed in line with the suggested theories of deformation of metals. CONSIDERABLY more work has been done on the rupture and creep behavior of commercial alloys than on the less complex, high purity metals. As a result, certain fundamental behaviors are overlooked. One of the lesser known variables affecting the high temperature behavior of metals is the impurity content (or alloy content). Impurities are known to affect strongly the recrystallization temperature, electrical resistivity, and other properties of metals, but the effect on creep and other high temperature mechanical properties has been determined for very few materials. In view of these facts aluminum appears to be a fairly ideal metal, since it is available in a wide range of purity contents from 99.995+ down through the 2s (99.0 + Al) and 3s (1.2 Mn) grades in small steps. It is highly resistant to oxidation and other surface instabilities, leaving recrystallization and grain growth as virtually the only instabilities. Control of grain size is also well standardized. Limited data on the creep of aluminum have been obtained by Dushman et a1.l for high purity aluminum; by Sherby2 on 2s aluminum of variable grain size; and by Dorn and Tietz³ on cold-worked 3s aluminum. Experimental Procedure The apparatus consisted of a constant stress, creep testing unit similar to the type described by Hop-kin.' The specimens were held at temperature for 30 min before loading. The loading time was less than 2 min. Factors affecting the accuracy of the data were: 1-—the temperature gradient and control, ± 3°F; 2-the stress determinations, ± 1.5 pct; and 3—elongation measurements (1 in. gage), ? 0.001 in. Three grades of aluminum were tested: 1— high purity (99.995 pct Al), 2—2s (99.3 pct Al), and 3—3s (98.2 pct Al). The spectrographic analyses of these materials are reported in Table I. The specimens were annealed then electropolished in a perchloric-acetic mixture before testing. Data on heat treatments and grain size obtained appear in Table 11. The entire creep curve was determined for almost all of the tests. A few tests were interrupted after the beginning of the third stage of creep, and in these cases the rupture time was estimated. Experimental Results Constant stress creep-rupture testing gives a much longer, more accurately measurable second stage of creep than constant load testing. This behavior is in agreement with the results obtained by Andrade for lead (see Fig. 1). If the test is run to completion, the third stage of creep appears as soon as stress intensification occurs. In the case of specimens which fail in a ductile manner, the stress intensification begins when the sample starts "necking". This differs from the "high temperature brittle" specimens in which the stress intensification is caused by intercrystalline cracking. These specimens