PSI - Issue 10

N. Siskou et al. / Procedia Structural Integrity 10 (2018) 79–84 N. Siskou et al. / Structural Integrity Procedia 00 (2018) 000 – 000

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and precipitation hardening system. Aluminium alloy (AA) 2024 is a high strength alloy with a heterogeneous microstructure that consists of the aluminium matrix as well as of different intermetallic particles and dispersoids. The most common intermetallic particles in the microstructure of AA2024 are the S -phase (Al 2 CuMg) particles. Additionally, dispersoid particles such as Al 6 (Cu,Fe,Mn), Al 7 Cu 2 Fe and (Al,Cu) 6 Mn are formed by the addition of other alloying elements, according to Buchheit et al. (1997). The precipitation hardening mechanism includes firstly the formation of the Guinier-Preston-Bagaryatsky (GPB) zones, e.g. Bagaryatsky et al. (1952) from the super saturated solid solution (SSS) matrix. The precipitation sequence involves precipitation of the semi-coherent with the matrix S ΄΄ -phase and finally the transition to the equilibrium incoherent S -phase with the following sequence: The GPB zones and other precipitate structures prior to the equilibrium S -phase formation are considered as the dominant precipitates at the strengthening regime of the ageing curve, while the S -phase appears in the softening regime (over-ageing condition) according to Ringer et al. (1996) and (1998). Nevertheless, recent studies have showed that such strengthening phases may influence the corrosion behaviour. It is generally accepted e.g. Davis (1999) and Boag et al. (2010), that the 2xxx aluminium alloys are susceptible to localized corrosion due to the formation of a galvanic cell between the intermetallic particles (IM) and the matrix or the adjacent particles. The copper depleted regions adjacent to the grain boundaries, coming from the S -phase particles precipitation at the grain boundaries, are anodic with respect to the copper rich boundaries leading to a micro-galvanic corrosion of the surrounding matrix, e.g. Ilevbare et al. (2004) and Hughes et al. (2010). This phenomenon explains the transition from general to localized corrosion induced by the intermetallic particles. The effect of the microstructure such as precipitate size and shape as well as the role of grain stored energy on the pitting potential in Al – Cu – Mg alloys are well documented in the open literature, e.g. Ralston et al. (2010), Luo et al. (2012) etc. In the present work, the effect of corrosion exposure on the tensile mechanical properties degradation of AA2024-T3 as well as in the Peak-Ageing (PA) condition will be investigated. Tensile specimens were artificially aged to tempers corresponding to PA condition, selected from Alexopoulos et al. (2016), and subsequently exposed to exfoliation corrosion environment for different exposure hours. This study aims to compare the corrosion evolution and residual mechanical properties of the T3 condition against PA as the latter condition is a high-strength temper that is widely used in aircraft applications. The material used was the aeronautical aluminium alloy 2024 in T3 condition and in sheet form. The chemical composition of the material is a mixture of the following chemical elements as a percentage by weight; 4.35 % Cu, 1.50 % Mg, 0.64 % Mn, 0,50% Si, 0.50 Fe, 0.25 % Zn, 0.15 % Ti, 0.10 % Cr, 92.01% Al. Tensile specimens were machined from the sheets at the longitudinal (L) rolling direction of the material according to the ASTM E8 speci fication. Their typical dimensions were 150 ± 0.02 mm in length, 20 ± 0.02 mm in width and 3.2 mm nominal thick ness. The selected area for corrosion exposure was 55 ± 0.01 mm lengthwise and 12.5 ± 0.01 mm widthwise with the side-surfaces (thickness) covered. Initially, the tensile specimens were machined from the sheets according to ASTM E8 specification, surface cleaned with alcohol according to ASTM G1 specification and then were isothermally artificially aged (heat treated) in an electric oven Elvem T101 (2600 W) with ± 0.1 o C temperature control. Artificial ageing heat treatment was performed for 8 h at 190 o C and according to Alexopoulos et al. (2016) that corresponded to PA condition. After the heat treatment, the specimens were left to return to room temperature and the corrosion process followed. The ex foliation corrosion solution (hereafter will be called EXCO) was prepared according to ASTM G34 and consisted of sodium chloride (4.0 M NaCl), potassium nitrate (0.5 KNO 3 ) and nitric acid (0.1 M HNO 3 ) diluted in 1 l distilled water. All the surfaces of the tensile specimens were cleaned according to ASTM G34. After cleaning, all the surfaces of the specimens were insulated with PVC waterproof tape except of the surfaces at the reduced cross-section (masking SSS → GPB zone → S ΄΄ → S ΄ → S . 2. Material and specimens 3. Experimental procedure