EFFECTS OF DISPERSOIDS UPON THE MICRO-MECHANISMS OF CRACK-PROPAGATION IN AL-MG-SI ALLOYS

The effect of dispersoids containing Mn or Cr. in a series of peak-aged AlMgSi alloys, upon the crack-tip plastic zone size has been studied by the use of Selected Area Channelling Patterns (SACPs). In the absence of such dispersoids. the alloy fractures in a brittle intergranular fashion; when the particles are present, the alloys are ductile but still fracture in a predominantly intergranular manner but with an increase in the size of the plastic zone associated with the crack tip. It is suggested that the toughening effect of the dispersoid phase is indirect through its effect upon the matrix grain size and upon the distribution of slip. Fracture occurs because slip bands impinge on grain boundaries, causing a local strain concentration which at some critical value nucleates a void, thereby causing an increment of crack extension. © 1979.