THE EFFECT OF FREQUENCY AND MICROSTRUCTURE ON CORROSION-FATIGUE CRACK-PROPAGATION IN HIGH-STRENGTH ALUMINUM-ALLOYS

Fatigue crack growth in high strength aluminium alloy 7150 commercial plate material has been studied in both laboratory air and acidified a queous salt solution. The aggressive aqueous environment enhanced fati gue crack growth rates by up to an order in magnitude compared to labo ratory air. The enhancement in fatigue crack growth rate was accompani ed by evidence of embrittlement in the crack path, involving both brit tle intergranular and transgranular failure modes. Both the enhancemen t of fatigue crack growth rates and the extent of intergranular growth modes are dependent on cyclic frequency which, along with the absence of a similar frequency effect in a spray-formed version of the materi al with a significantly different grain structure, supports a mechanis m of grain boundary hydrogen diffusion for at high Delta K in both spr ay formed and conventional plate materials coincides with the operatio n of identical transgranular corrosion fatigue modes dependent on stra in-controlled hydrogen diffusion ahead of the crack tip. (C) 1997 Acta Metallurgica Inc.