Materials Chemistry and Physics 101 (2007) 441–447 Hydrogen embrittlement studies of aged and retrogressed-reaged Al–Zn–Mg alloys Ashish Thakur ∗ , R. Raman, S.N. Malhotra Department of Metallurgical Engineering and Material Science, IIT Bombay, Powai 400076, India Received 31 December 2005; received in revised form 4 August 2006; accepted 28 August 2006 Abstract The hydrogen embrittlement (HE) of Al–4.35Zn–1.4Mg–0.059Zr alloys in different artificial aging tempers and after retrogression and reaging (RRA) treatments has been investigated by tensile testing hydrogen precharged specimens. The influence of RRA and hydrogen charging on the dislocation structure was studied by TEM. The under-aged temper was the most susceptible while the over-aged temper was the most resistant to HE. The RRA treatment improved the HE resistance of all the tempers. This has been attributed to the reduction in dislocation density upon retrogression and reaging. Flat fractography features near the surface of the hydrogen charged specimen have been correlated to the depth of hydrogen penetration. The hydrogen dislocation interaction and hydride cracking mechanism of HE have been addressed. © 2006 Elsevier B.V. All rights reserved. Keywords: HE; EIC; RRA; Hydrogen charging 1. Introduction There has been a renewed interest in Al–Mg–Zn alloys as candidate materials in aircraft components due to their reduced density and increased modulus over conventional aluminum alloys. However, they are susceptible to environment-induced cracking (EIC) which is a critical factor to be considered in the selection of aerospace materials [1]. Several studies have shown that hydrogen embrittlement (HE) is the main operative mech- anism of EIC in aluminium–zinc–magnesium alloys [2]. The reversible nature of embrittlement, its strain rate dependence, the discontinuous nature of crack propagation, identification of brittle hydride under HE conditions and the effect of cathodic overpotentials on hydrogen permeability resulting in enhanced embrittlement are some of the evidences in support of HE as the probable mechanism of EIC compared to the anodic dissolu- tion mechanism [3]. The susceptibility to EIC is also a function of microstructure (i.e. heat treatment conditions). In conven- tional aluminum alloys, the over-aged (OA) T73 temper has been reported to be relatively less susceptible to EIC. How- ever, over-aging leads to a decrease in the strength of alloys. This problem was tackled, subsequently, through a novel heat ∗ Corresponding author. Fax: +91 22 25723480. E-mail address: ashish sthakur@iitb.ac.in (A. Thakur). treatment of peak-aged (PA) alloys called retrogression-reaging (RRA) [4]. RRA, a two stage heat treatment, combines the bene- ficial effects of both PA and OA tempers. During the first stage of heat treatment (i.e. retrogression), the peak-aged sample is main- tained at a high temperature for short duration (depending upon the alloy composition and sample thickness) to dissolve some of the coherent matrix precipitates while retaining the other precip- itates at the grain and sub-grain boundaries. Immediately after retrogression, samples are subjected to reaging (second stage) during which the alloy regains its strength due to the precipita- tion of age hardening phases in the matrix. The microstructures of grain and sub-grain boundary precipitates of RRA treated samples resemble approximately that of the over-aged T73 tem- per. Resistance to HE increases after the RRA treatment due to the following reasons. First, the grain boundary precipitates (size >25 nm) have been proposed to act as irreversible hydro- gen trapping sites which result in the improved HE resistance of RRA alloys [5]. However, it must be noted that these large grain boundary precipitates play a beneficial role in improving HE resistance only if they are large in number and homogeneously distributed. Secondly, the improvement of EIC resistance of RRA samples has been attributed to the decrease in disloca- tion density [5]. The relatively higher dislocation densities in the untreated samples result in additional stress fields leading to crack initiation. Moreover, dislocations also serve as rapid hydrogen diffusion paths, thereby enhancing the build up of 0254-0584/$ – see front matter © 2006 Elsevier B.V. All rights reserved. doi:10.1016/j.matchemphys.2006.08.004