Title

The effect of composition, temper, and crack orientation on the stress corrosion cracking behavior of Al-Mg alloys

Conference Dates

May 29-June 3, 2016

Abstract

Non-heat-treatable Al-Mg alloys are used in modern marine applications, however these alloys are susceptible to intergranular corrosion (IGC) and intergranular stress corrosion cracking (IGSCC) following the precipitation of an active β phase (Al3Mg2) along the grain boundaries. The precipitation of β (sensitization) can occur during in-service thermal exposure of marine components (at temperatures as low as 50°C); the degree of sensitization (DoS) is typically characterized by nitric acid mass loss testing (NAMLT). Recent efforts quantitatively established the IGSCC susceptibility of 5083-H131 in NaCl via fracture mechanics-based slow-rising displacement testing where crack extension is monitored via high fidelity direct current potential (dcPD) techniques [1,2]. The effect of DoS and applied potential on the threshold stress intensity for SCC (KISCC) and the stage II crack growth rates (da/dtII) were established and interpreted in the context of varying crack tip conditions influencing a H environment assisted cracking (HEAC) mechanism [1,2]. This research extends this methodology to quantitatively evaluate the influence of different engineering relevant material compositions (5083 and 5456) and tempers (H131, H116, and solution heat treated and quenched) on the KISCC and da/dtII; such data are interpreted in the context of proposed HEAC damage mechanism. Furthermore, the effect of continuity and tortuosity of the susceptible grain boundary path is evaluated by investigating the IGSCC behavior for different crack orientations (SL and TL) and degrees of grain size isotropy.

Testing for each material was conducted at two DoS (22 and 40) values. Consistent with the slight variation in composition, the 5083 was shown to be only slightly more susceptible than 5456 at a constant temper. However, for a constant composition the temper demonstrated a more pronounced influence with H131 being the most susceptible followed by H116 then SHTQ. These trends are analyzed based on (1) strength differences between the tempers, (2) Mg content changes, and (3) inaccuracies of DoS in capturing the true degree of and morphology of β precipitation on the grain boundaries. A strong influence of grain orientation is observed where SL oriented cracks are significantly more susceptible than TL samples. Fracture surface analysis of the TL samples show significant degree of fissures parallel to the loading axis. The beneficial effect of this delamination toughening on the HEAC mechanism is rationalized through decohesion-based micro-mechanical models where the loss of constraint decreases the local hydrostatic stress, thus (1) lowers the local H concentration in the process zone and (2) reduces the local tensile force that drives grain boundary decohesion.

The focus of this research effort on systematically quantifying the SCC behavior (KISCC and da/dtII) of engineering relevant Al-Mg alloy system and interpreting these data in the context of the microstructure and prominent failure mechanisms is directly aligned with the stated goals of the conference.

References:

[1] Crane CB, Gangloff RP, “Stress corrosion cracking of Al-Mg alloy 5083 sensitized at low temperature,” Corrosion (2016); In-press. DOI: http://dx.doi.org/10.5006/1766

[2] Crane CB, Kelly RG, Gangloff RP, “Crack chemistry control of intergranular SCC in sensitized Al-Mg,” Corrosion (2016); In-press. DOI: http://dx.doi.org/10.5006/1852

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