Growth behavior of hydrogen micropores in aluminum alloys during high-temperature exposure

Abstract

X-ray microtomography was used to observe hydrogen micropores and their growth behavior at high temperatures in several aluminum alloys. High-density micropores were observed in highpurity Al-Mg alloys, but their density and volume fraction were much lower in pure aluminum. Our results have revealed that the growth behavior of micropores is dominated by Ostwald ripening. About 53 % of hydrogen is trapped in micropores in Al-Mg alloy with low hydrogen content, making micropores the predominant hydrogen trap site. Although total hydrogen content is similar to that in the alloy, the ratio of hydrogen trapped in micropores is below 7 % in pure aluminum. This difference is attributable to the lack of hydrogen precipitation sites in pure aluminium.Although the overall amounts of hydrogen at dislocations and grain boundaries are small in all the materials, the occupancies for these trap sites were concluded to be very high.

Kinetics of Hydrogen Desorption from Rapidly Solidified Al-Cr Alloys

“…Whereas outward quantitative resemblance of peak positions in TDS profiles, it should be emphasized that quenched-in vacancies play a dominant role in H trapping in pure aluminum foils produced by means of RSP. Indeed, RS samples exhibit a significant increase of the amount of hydrogen trapped at vacancies instead of pronounced hydrogen trapping at dislocations specified for traditionally processed aluminum specimens, 7,18,24) see Figs. 2(a) and 3. Thus, we associate the 270 and 410 C trap states as well as the highest binding energy trap site at 470 C in RS Al-Cr alloys to the same microstructural features as in RS pure aluminum.…”

“…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.…”

“…2(a). The data of recognized peak temperatures T m and trap sites assigned from literature 7,17,18,24,25) as discussed later are summarized in Table 1. Note that TDS spectrum from RS foil of pure aluminum 17) shown also in Fig.…”

“…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. 24) Therefore it is considered that in RS pure aluminum foils hydrogen resides at trapping sites at vacancies, dislocations and pores as listed in Table 1. Whereas outward quantitative resemblance of peak positions in TDS profiles, it should be emphasized that quenched-in vacancies play a dominant role in H trapping in pure aluminum foils produced by means of RSP.…”

High-resolution Observation of Steel Using X-ray Tomography Technique

“…XMT performed on light materials has generally enabled the reconstruction of volumes with maximum spatial resolution close to the physical limit of the projectiontype set-up (approximately 1 μ m). 1,2) However, in the case of ferrous materials, difficulties arise with respect to X-ray transmission. Since photoelectric absorption is dramatically enhanced with increasing atomic number, high energy Xrays are needed to realize X-ray imaging for ferrous materials.…”