Healing behavior of preexisting hydrogen micropores in aluminum alloys during plastic deformation

Abstract

Synchrotron X-ray microtomography was used to observe the shrinkage and annihilation behaviors of hydrogen micropores in three dimensions during hot and cold plastic deformation of an Al-Mg alloy. It was examined, by exposing the material to a high temperature after plastic deformation, whether complete healing of micropores is achieved after plastic deformation. Although micropores generally show a pattern of shrinking and closing, closer inspection of a single specimen revealed a variety of geometrically variable behaviors. It is noteworthy that some of the micropores are reinitiated in positions identical to those before their annihilation, even after an 8 ~ 22 % macroscopic strain has been further applied after annihilation. We attribute local variations such as these to significant local strain variation, which we measured in a series of tomographic volumes by tracking microstructural features.

Four-Dimensional Annihilation Behaviors of Micro Pores during Surface Cold Working

“…Actually it has been reported by the present authors with respect to a similar local strain distribution during uniform 60% compression that although a macroscopic applied load is compressive, some regions are locally exposed to tension and even in the compressive regions, compressive strain varies between 0 $ 130%. 9) Wide variation in the morphologically variable behaviors of micro pores during the compression has also been attributed to the observed significant scattering in local strain. …”

“…Subsequently, an additional heat treatment was employed at 813 K for 14.4 ks and the specimen was scanned again. Since our previous paper reported that if micro pores are not sufficiently healed, heat treatment causes them to grow again, 9) the additional heat treatment was used to confirm that the micro pores were completely healed. Even if the micro pore size is smaller than the finite spatial resolution of an imaging setup, micro pore closure can be confirmed with this additional heat treatment without failure.…”

“…The experimental setup and the microtomography conditions have already been reported elsewhere. 4,9,12) The high-resolution detector consisted of a 4000Â2624 pixel cooled-CCD camera (used in the 2Â2 binning mode), an Lu 2 SiO 5 :Ce scintillator, and a 20Â optical lens. The experiment employed 20 keV X-rays monochromatized by a double-crystal monochromator using the Si (111) surface.…”

“…Even uni-axial loading causes a highly non-uniform threedimensional strain distribution, making the annihilation behavior of micro pores very complex. 9) Fatigue strength can be significantly improved simply by annihilating near-surface micro pores. It is well known that fatigue properties can be improved by the effect of residual compressive stress from surface cold working such as shot peening.…”

Kinetics of Hydrogen Desorption from Rapidly Solidified Al-Cr Alloys

“…The vacancy/ hydrogen pairs was deduced to migrate sufficient distance to be released from the inside of the specimen at much lower temperature than the 3rd peak temperature. Furthermore, Toda et al 24,32) have elucidated presence of hydrogen micropores in pure aluminum and its alloys processed through casting as well as hot and cold rolling. It was pointed out for Al-Mg alloys that micropores filled and pressurized with molecular hydrogen are predominant hydrogen trap site, while the occupancy for dislocation traps is very high both in the alloys and pure aluminum.…”