FRACTOGRAPHIC AND MECHANICAL PROPERTIES OF ALLOY AA7075 SUBJECTED TO T6, T7 AND RRA HEAT TREATMENTS

Abstract

This research presents a fractographic study of the AA7075 alloy subjected to T6, T7 and RRA heat treatments and how these thermal procedures affect its mechanical properties. The samples were evaluated by tensile tests; their fractography, through scanning electron microscopy (SEM). All treatments generated an increase in mechanical strength, the T6 effect being largest due to the fine precipitation of h' particles in the matrix, although the ductility and toughness of the AA7075-T6 alloy decreased with respect to their equivalent values in the untreated sample. The highest toughness values reported for the T7 treatment concur with those published by other researchers. Properties during the RRA treatment cycles were intermediate to that obtained with the T6 and T7 treatments. The fracture surface fractography revealed differences predicated on the microstructural condition. For all the thermal treatments, the reported failure was of a transgranular character, typical of ductile materials. A wide and irregular distribution of micro dimples and precipitates was evinced depending on the heat treatment applied, the precipitates being distributed over the entire fracture surface. The mechanism responsible for this failure is the formation, growth, and coalescence of micro dimples, although this mechanism seemed to be more complex in the untreated samples due to the abundant presence of defects. Our results seem to indicate that the larger the number of micro dimples, the lower the ductility of the sample and the higher its mechanical strength.