Effects of Testing Temperature on the Serration Behavior of an Al–Zn–Mg–Cu Alloy with Natural and Artificial Aging in Sharp Indentation

Abstract

Serration phenomena, in which stress fluctuates in a saw-tooth shape, occur when a uniaxial test is performed on an aluminum alloy containing a solid solution of solute atoms. The appearance of the serrations is affected by the strain rate and temperature. Indentation tests enable the evaluation of a wide range of strain rates in a single test and are a convenient test method for evaluating serration phenomena. Previously, the serrations caused by indentation at room temperature were clarified using strain rate as an index. In this study, we considered ambient temperature as another possible influential factor. We clarify, through experimentation, the effect of temperature on the serration phenomenon caused by indentation. An Al–Zn–Mg–Cu alloy (7075 aluminum alloy) was used as the specimen. The aging phenomenon was controlled by varying the testing temperature of the solution-treated specimen. Furthermore, the material properties obtained by indentation were evaluated. By varying the testing temperature, the presence and amount of precipitation were controlled and the number of solute atoms was varied. Additionally, the diffusion of solute atoms was controlled by maintaining the displacement during indentations, and a favorable environment for the occurrence of serrations was induced. The obtained results reveal that the variations in the serrations formed in the loading curvature obtained via indentation are attributed to the extent of interaction between the solute atoms and the dislocations.

Phase-Transitions at High, Very High, and Very Low Temperatures upon Nano-Indentations: Onset Forces and Transition Energies

“…But we have to secure our so obtained conclusion. It is thus highly rewarding that such checking is now possible by comparison with the high temperature studies of a technical heavy-duty light metal alloy Al 7075, with the result, that even moderately high temperatures provide comparable results [20].…”

“…Figure 2 shows the F N -h 3/2 plot [9] at room temperature, as calculated from the "repaired" (from pop-in events) [14] published loading curve. As the room-temperature indentation and the ones at 70˚C, 170˚C, and −196˚C are nanoindentations in Figure 5 of [20], we replace the unfortunate "k" of impossible "kN" units by N at 77 K and mN at room temperature and above. The depth notations ("mm") are in either case by µm.…”

“…Fortunately, the calculations with the formulas that are repeated in Section 2, are independent from the dimension of the force notations at the axes. Their numbers do not depend of what is written at the axes for the drawn F N versus h loading curve in Figure 5 of [20]. The dimensions of the numbers at the loading curve axes are corrected and "repaired" [14].…”

“…But our linear F N versus h 3/2 plot at 2.35 N and 0.907 µm (0.863 µm 3/2 ) shows that it is an exothermic phase-transition unsteadiness (see Section 3.3 below). The authors for correspondence of [20] have been informed about these corrections. Their loading curves are of outstanding value for the nano-indentation and beyond.…”

“…We reported on it in [13]. We are thus enforced to use it also here for room temperature or above and also for 77 K. If we did not do so, these could not be displayed within one Figure 5 in [20]. Our corrections are therefore safe and very well founded.…”