Abstract
Ten different alloys based on the 7075 composition were used to study the effect of purity level, dispersoid type, and heat treatment on fracture toughness. Five purity levels ranging from 0.03 to 0.30 wt pct Fe + Si and two dispersoid types were investigated. Each alloy was given two heat treatments: the standard T651 heat treatment or a special thermomechanical treatment (TMT). Fracture toughness was measured using notched round tensile specimens taken from both the longitudinal and long-transverse directions. The notched round tensile test was modified to give the “plastic energy per unit area”. This fracture toughness parameter gave the same ranking for corresponding alloy/heat treatment combinations as the total energy per unit area measured on precracked Charpy specimens. The fracture toughness ranking for the ten alloys was the same in the longitudinal and long-transverse directions. This suggests the elongated distribution of constituent particles in the rolling direction does not change the failure mechanism. Fractographic evidence showed a bimodal distribution of ductile dimple size in all ten alloys. The number of large ductile dimples decreased with increasing purity level while the number of small ductile dimples increased. This is interpreted to mean that the smaller dispersoid and hardening particles become increasingly important in controlling the fracture toughness as the large intermetallic particles are eliminated by increasing the purity of these aluminum alloys. Since thermomechanical processing controls the amount and type of these smaller particles, it is a useful means for increasing fracture toughness in high purity aluminum alloys.
Published Version
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