The Relationship between Crack Propagation and Microstructure in an Aluminium Casting Alloy

R.W.Coade, S.A. Nugent, D.S.Saunders, J.R.Griffiths and B.A. Parker
Department of Materials Engineering,
Monash University, Clayton 3168, Victoria

Introduction


The aim of the work is to understand the microstructural parameters that control toughness and crack propagation with a view to recommending improved microstructures with a higher toughness. Work to date has been centred on the Australian casting alloy CP601 and this paper reports further progress in the study of the fracture behaviour of this alloy. CP601 is a strontium modified A1-7Si-0.4Mg casting alloy - the microstructure is of aluminium rich dendrites in an A1-Si eutectic. On solution treating at 540 C the silicon rapidly spheroidises and the structure becomes essentially 'grains' of aluminium rich phase with silicon at the 'grain' boundaries. The 'grain' boundaries are of two kinds, those resulting from the spaces between the original dendrite arms and those resulting from the zones between discrete dendrites. The differing concentrations of silicon in these locations has an effect on the development of the microstructure during solution treatment. Relatively long solution treatments (12 to 1000 hrs) result in the formation of larger silicon particles along the interdendritic boundaries at the expense of these between the dendrite arms. The crack during fracture follows the surfaces containing the larger silicon particles (1).