Thermal desorption spectroscopy (TDS) was utilized to identify several metallurgical states in an Al -2Li - 2Cu-0.1Zr (wt pct) alloy, which trap absorbed hydrogen. Six distinct metallurgical desorption states for hydrogen were observed for tempers varying from the T3 to peakaged condition. Lower energy thermal desorption states were correlated with interstitial sites, lithium in solid solution, and δ′ (Al3Li) precipitates. These states have trap-binding energies ≤25.2 kJ/mol. Under the charging conditions utilized, approximately 4 pct of the total (e.g., trapped and lattice) hydrogen content was associated with interstitial sites, consistent with the view that the intrinsic lattice solubility of hydrogen in aluminum is very low. In contrast, dislocations, grain boundaries, and T1 (Al2CuLi) particles were found to be higher energy-trap states with trap-binding energies ≥31.7 kJ/mol. Approximately 78 pct of all absorbed hydrogen occupied these states. Moreover, greater than 13 pct of the available trap sites at grain boundaries were occupied. Such a high hydrogen coverage at grain-boundary sites supports the notion that hydrogen contributes to grain-boundary environmental cracking in Al-Li-Cu-Zr alloys. Also, it points out the error in assuming that hydrogen cannot play a major role in cracking of Al-based alloys due to the low lattice solubility.

The identification of hydrogen trapping states in an Al-Li-Cu-Zr alloy using thermal desorption spectroscopy

Lithium bearing aluminium alloys constitute a relatively new generation of high performance, lightweight aviation alloys that are being considered for a variety of applications requiring welded con

http://www.dunand.northwestern.edu/refs/files/p217.pdf

Weldability of Li-bearing aluminium alloys

This chapter provides a review of hydrogen interactions, gas phase hydrogen embrittlement (HE) and hydrogen environment embrittlement (HEE) in high purity aluminum and selected aluminum alloys. Hydrogen uptake occurs in Al alloys upon exposure to aqueous environments, water vapor saturated air and water vapor mixed with other gases such as nitrogen, argon or dihydrogen. Uptake does not readily occur in dry dihydrogen gas in the absence of water or water vapor. Uptake is favored when atomic hydrogen is produced either chemically (from a source such as dihydrogen, water dissociation, or other hydrogen containing molecules) or electrochemically (from water and proton reduction). The latter occurs spontaneously when water or protons are reduced on Al surfaces. This occurs more readily on transiently bare Al but is impeded by thick oxide films. Uptake and hydrogen accumulation may be limited by either water vapor transport to Al surfaces, surface reaction rates to produce atomic hydrogen from water, and diffusion through surface oxides and the metal. Solid state diffusion rates in aluminum when transport is dominated by perfect lattice rates of diffusion are typical of other face centered cubic materials such as nickel. However, hydrogen interacts with a variety of microstructural defects in Al and can become trapped at a variety of metallurgical sites. There are significant hydrogen-vacancy interactions and dissolved hydrogen may aid in vacancy formation. Embrittlement mechanisms remain controversial but hydriding, hydrogen induced local plasticity and hydrogen induced local decohesion are all possible mechanisms. These processes can degrade engineering significant parameters under both static and cyclic loading, such as fracture toughness, breaking stress and elongation to failure at a critical hydrogen level. Mitigation strategies can involve barrier-type coatings, ionic inhibitors to reduce corrosion and coupled hydrogen production as well as microstructural manipulation. The latter area is less well developed in the case of Al alloys given the difficulty of performing hydrogen analysis in this system. Future work should combine determination of engineering properties degraded by hydrogen with submicroscopic analysis of hydrogen metal interactions on Al alloys of commercial interest.

Hydrogen embrittlement of aluminum and aluminum-based alloys

Microscopic fatigue crack propagation (FCP) paths in peak-aged unrecrystallized alloy 2090 are identified as functions of intrinsicda/dN- δK kinetics and environment. The FCP rates in longitudinal-transverse (LT)-oriented 2090 are accelerated by hydrogen-producing environments (pure water vapor, moist air, and aqueous NaCl), as defined in Part I. Subgrain boundary cracking (SGC) dominates for δK values where the cyclic plastic zone is sufficient to envelop subgrains. At low δK, when this crack tip process zone is smaller than the subgrain size, environmental FCP progresses on or near 100 or 110 planes, based on etch-pit shape. For inert environments (vacuum and He) and pure O2 with crack surface oxidation, FCP produces large facets along 111 oriented slip bands. This mode does not change with δK, and T1 decorated subgrain boundaries do not affect an expectedda/dN- δK transition for the inert environments. Rather, the complex dependence ofda/dN on δK is controlled by the environmental contribution to process zone microstructure-plastic strain interactions. A hydrogen embrittlement mechanism for FCP in 2090 is supported by similar brittle crack paths for low pressure water vapor and the electrolyte, the SGC and 100/110 crystallographic cracking modes, the influence of cyclic plastic zone volume (δK), and the benignancy of O2. The SGC may be due to hydrogen production and trapping at T1 bearing sub-boundaries after process zone dislocation transport, while crystallographic cracking may be due to lattice decohesion or hydride cracking.

https://ntrs.nasa.gov/api/citations/19920021970/downloads/19920021970.pdf

Environmental fatigue of an Al-Li-Cu alloy: Part II. Microscopic hydrogen cracking processes

A short review of the stress corrosion cracking (SCC) behaviour of aluminium alloys is given. Mechanisms of environmentally assisted cracking are outlined. For aluminium alloys, in which stress corrosion cracks propagate predominantly along grain boundaries, anodic dissolution and hydrogen embrittlement have been proposed. Transgranular stress corrosion cracking occurring at severe loading conditions has found particular interest concerning localised corrosion-deformation interactions. Accelerated test methods for assessing the SCC behaviour are described, including the slow strain rate testing technique and the breaking load method. Results of recent studies on environmentally assisted cracking of aluminium alloys are summarised. Most of the work published in the last two decades has been on aluminium-lithium based alloys and improved high strength Al-Zn-Mg-Cu alloys in corrosion resistant retrogressed and re-aged tempers.

Environmentally assisted cracking of aluminium alloys

The stress corrosion cracking (SCC) susceptibility of two aluminum-lithium alloys, a binary AlLi and a ternary AlLiCu alloy, in 0.5 M NaCl solution was investigated using the constant elongation rate technique (CERT). Susceptibility increased with decreasing strain rate and with aging. The alloys were susceptible under both anodic and cathodic applied potentials. The susceptibility dependence of the alloys as a function of applied potential correlates well with published hydrogen permeability data. The susceptibility increased dramatically when hydrogen was charged into the specimen using a hydrogen re-combination “poison” during CERT testing. These experiments suggest that hydrogen plays a major role in the SCC of these alloys. A brittle hydride having the composition LiAlH4 forms in the AlLi system under conditions of severe SCC susceptibility. The brittleness of the hydride is explained. The formation of the hydride is a sufficient condition for SCC of AlLi alloys. A process of SCC in AlLi alloys is proposed wherein hydrogen causes damage by the formation of a hydride.

On stress corrosion cracking in aluminum-lithium alloys

A hydride phase, LiAlH 4 , has been identified in Al-2.0%Li-2.2%Cu alloy electrochemically charged with hydrogen. This is a hydride having the composition of LiAlH 4 . The orientation relationship of the hydride and the matrix has been determined and rationalized with the O lattice theory. The thermodynamic stability of the hydride is discussed and possible formation mechanisms explained. The hydride forms from the grain boundary phase, AlLi, as the maximum amount of available Li is already present in the AlLi (δ) phase.

D.J. Duquette

Papers

On stress corrosion cracking in aluminum-lithium alloys

Hydride formation in an AlLiCu alloy