Enhancement of the Stress Corrosion Sensitivity of AA5083 by Heat Treatment

Abstract

In this study, the stress corrosion cracking (SCC) resistance of AA5083 is intentionally degraded by a series of progressively longer annealing treatments at 448 K (175°C) that create a two-phase microstructure. Precipitation of strongly anodic Mg 2 Al 3 , known as b-phase, occurs heterogeneously with substantial precipitation along the grain boundaries, as observed by differential interference microscopy. Ultimate tensile strength, yield strength, and strain to failure of AA5083 alloy were found to be independent of the amount of b-phase precipitates, making AA5083 an ideal system to study the relative contributions of anodic dissolution and hydrogen embrittlement. Open circuit dropwise exposure SCC tests with precracked double cantilever beam (DCB) specimens made from the AA5083 alloy with different heat treatment conditions were conducted using 3.5 pct NaCl solution at an initial stress intensity factor (K I ) of 15 ksi ffiffiffiffi in p: 16:5 MPa ffiffiffiffi m p ð Þ : Two SCC characteristics, initial crack growth rate and incubation time, were found to be strongly dependent on the amount of b-phase precipitates. Initial crack growth rate increased sigmoidally as a function of heat treatment time with an inflection point between 120 and 240 hours of sensitization time, while the incubation time decreases monotonically with sensitization time. Additionally, fracture surfaces investigated by scanning electron microscopy demonstrated characteristics of intergranular cracking with multiple crack tips. Discussion centers on the evidence supporting anodic dissolution of b-phase grain boundary precipitates as a primary mechanism of SCC in severely sensitized AA5083 alloy and the potential contribution of hydrogen embrittlement in the failure of grain boundary ligaments between b-phase grain boundary precipitates in less severely sensitized conditions.

Investigation of fatigue and fracture behaviour of sensitized marine grade aluminium alloy AA 5754

“…Fatigue crack growth (FCG) tests are performed on thermally exposed alloy 5456 at three different temperatures 80°C, 100°C, and 175°C at times ranging from 1 to 10 000 hours. Gao and Quesnel experienced the degradation in resistance towards SCC behaviour due to long‐term ageing (sensitization at 175°C) of AA 5083 alloy. Experimental evidences showed that initial crack growth rate of sensitized AA5083 alloys increased with increase in ageing time.…”

Mechanical properties, intergranular corrosion behavior and microstructure of Zn modified Al–Mg alloys

“…The distribution and morphology of the b phase were reported to be strongly affected by sensitized conditions such as temperature and time [8][9][10][11]. Precipitation of b phase at the grain boundaries was increased with an increase in the sensitization time at a certain temperature.…”

Stress Corrosion Cracking of Aluminum Alloys

“…[38] Under slow straining in an aqueous NaCl solution, such a precipitate becomes anodic to the alloy matrix and dissolves, leading to the observed intergranular cracking and exemplifying the grain boundary anodic dissolution mechanism. [26][27][28][29][30][31] This is verified by the optical micrographs, Figures A-8 and A-10, in which fewer or no precipitates are observable in the vicinity of intergranular cracks of specimens, tested under slow straining in 20% NaCl solution.…”

“…There have been several mechanisms proposed, capable of explaining the SCC behavior of aluminum alloys. Those mechanisms include, but not limited to, anodic dissolution [26][27][28][29][30][31] and hydrogen embrittlement [32][33][34][35][36][37] of grain boundary.…”