Cyclic behavior and microstructural stability of ultrafine-grained AA6060 under strain-controlled fatigue

AbstractThe strain-controlled fatigue behavior of AA6060, a precipitation hardening aluminum alloy, was investigated in ultrafinegrained (UFG) conditions after severe plastic deformation (SPD) by equal-channel angular pressing (ECAP). Two as-processed conditions, representing different stages of strain hardening and grain refinement as well as a ductility-optimized condition, achieved by a combined ECAP and aging treatment were considered. Low-voltage scanning transmission electron microscopy on samples stopped at characteristic stages of the fatigue process was applied to investigate the microstructural development. The as-processed as well as the optimized condition showed cyclic softening, which was found to be dependent on the amount of prestrain induced by ECAP processing. This is linked to dynamic recovery processes in the microstructure, indicated by a clearer distinction of grain boundaries and a reduction of dislocations in the grain interior. For all applied plastic strain amplitudes, ranging from Δεpl/2=1×10−3 to 5×10−3, the fatigue life of the ductility-optimized condition did not reach that of the severely work-hardened counterpart. For explaining this unexpected result, the differing (size-dependent) effectiveness of precipitates for the pinning of dislocations during cyclic loading was considered

Ultrafine grained plates of Al-Mg-Si alloy obtained by Incremental Equal Channel Angular Pressing : microstructure and mechanical properties

In this study, an Al-Mg-Si alloy was processed using via Incremental Equal Channel Angular Pressing (I-ECAP) in order to obtain homogenous, ultrafine grained plates with low anisotropy of the mechanical properties. This was the first attempt to process an Al-Mg-Si alloy using this technique. Samples in the form of 3 mm-thick square plates were subjected to I-ECAP with the 90˚ rotation around the axis normal to the surface of the plate between passes. Samples were investigated first in their initial state, then after a single pass of I-ECAP and finally after four such passes. Analyses of the microstructure and mechanical properties demonstrated that the I-ECAP method can be successfully applied in Al-Mg-Si alloys. The average grain size decreased from 15 - 19 µm in the initial state to below 1 µm after four I-ECAP passes. The fraction of high angle grain boundaries in the sample subjected to four I-ECAP passes lay within 53-57 % depending on the examined plane. The mechanism of grain refinement in Al-Mg-Si alloy was found to be distinctly different from that in pure aluminium with the grain rotation being more prominent than the grain subdivision, which was attributed to lower stacking fault energy and the reduced mobility of dislocations in the alloy. The ultimate tensile strength increased more than twice, whereas the yield strength - more than threefold. Additionally, the plates processed by I-ECAP exhibited low anisotropy of mechanical properties (in plane and across the thickness) in comparison to other SPD processing methods, which makes them attractive for further processing and applications

Effect of heat treatment conditions on the dynamic strength and failure behavior of titanium alloy Ti-6Al-4V

The effect of heat treatment conditions on high rate mechanical properties of the ($\alpha +\beta$) titanium alloy Ti-6Al-4V was investigated. Dependent on the solution annealing temperature, cooling rate and further annealing the strength and failure properties can be varied in a wide range. Microstructures obtained by annealing below the $\beta$-transus followed by furnace cooling show balanced properties of high strength, deformability, and therefore the highest energy consumption under dynamic compressive loading, but the lowest hardness values. Instrumented impact tests on Charpy U-notch specimens reveal evidence for a less brittle behaviour combined with the highest absorbed energy for furnace cooled conditions, only

Effect of strain-rate on the deformation response of D0 3 -ordered Fe 3 Al

The mechanical response of centrifugally cast Fe(3)A1 with the composition Fe-27A1 (at.%) containing microalloying additions of Nb, Zr, C, and B was investigated over a wide range of strain rates between 10(-4) and 10(3) s(-1) at room temperature. Tests were carried out in compression using a (i) screw-driven load frame, (ii) drop impact tester, and (iii) split-Hopkinson pressure bar at quasi-static, intermediate and dynamic strain rates respectively. Post deformation analysis was carried out by DSC, SEM/EBSD, TEM and micropillar deformation. In all instances, the stress-strain curves show initial hardening (-first 5% plastic strain) followed by a plateau in stress. A loss in work-hardening occurs at the highest strain rates examined (>10(3) s(-1)) and is likely associated with shear localization; in addition, (2 11) [11 1]-type twinning was observed at these high strain rates at room temperature. This observation is in line with previous theoretical calculations of the antiphase boundary (APB) energy. The consequence of a continuously increasing yield stress with strain rate and a loss in work hardening at the highest strain rates together yields a maximum in flow stress at the intermediate strain rate. (c) 2015 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved