REPUBLIC OF SERBIA

MINISTRY OF DEFENCE

www.mod.gov.rs

MINISTRY OF DEFENCE

Material Resources Sector

Defensive Technologies Department

Military Technical Institute

www.vti.mod.gov.rs

STRESS CORROSION CRACKING OF AN Al-Zn-Mg-Cu ALLOY AFTER DIFFERENT PRECIPITATION HARDENING TREATMENTS

ANA ALIL

Institute GOŠA, Belgrade, ana.alil@institutgosa.rs

BORE JEGDIĆ

Institute GOŠA, Belgrade, bore.jegdic@institutgosa.rs

BILJANA BOBIĆ

Institute GOŠA, Belgrade, biljana.bobic@institutgosa.rs

MIROSLAV RADOSAVLJEVIĆ

Institute GOŠA, Belgrade, miroslav.radovanovic@institutgosa.rs

BORIS KATAVIĆ

Institute GOŠA, Belgrade, boris.katavic@institutgosa.rs

Abstract: Stress corrosion cracking (SCC) was tested in a high strength 7000 series aluminium alloy applied for weapons and military equipment. The effect of one step- and two-step precipitation hardening (aging) on SCC was investigated. The slow strain rate test (SSRT) and the fracture mechanics (FM) method were used for SCC testing. The measurements of electrical resistivity allowed evaluating an aging degree in the tested alloy. Both testing methods have shown that the alloy after two-step precipitation hardening is significantly more resistant to SCC. The critical stress intensity factor for SCC, K_ISCC is significantly higher, and the crack growth rate on the plateau, v_pl is more than one order of magnitude lower for the alloy in this state. Processes that take place at the crack tip are proposed and the effect of copper was analyzed. The effect of the testing solution (NaCl) temperature on the v_pl was determined. Two values of the apparent activation energy, E_a, were obtained; one value (46.6 kJ mol^-1) refers to higher temperatures, while the other (70.4 kJ mol^-1) refers to lower test temperatures. These values of E_a correspond to the different processes that control the v_pl, at higher and lower temperatures.

Key words: aluminium alloys, stress corrosion cracking, fracture mechanics, slow strain rate test.