Toughness-strength relationships in high strength 7xxx aluminium alloys

Kamp, Nicolas (2002) Toughness-strength relationships in high strength 7xxx aluminium alloys. University of Southampton, Doctoral Thesis.

Record type: Thesis (Doctoral)

Abstract

Systematic material characterisation has been carried out on three high strength 7xxx aluminium alloys in peak-aged and overaged conditions: 7150, Zr-containing 7449 and Mn-containing 7449. Various microstructural features controlling the fracture process were considered: differential scanning calorimetry (DSC) has been used to characterise the content of η/η’ strengthening precipitates in the alloys, whilst the amount of coarse intermetallics, and particularly of the undissolved S-phase (Al₂CuMg), was studied via image analysis. The grain structure and grain boundary characteristics (precipitate free zone (PZF) and grain boundary precipitates) were also assessed. Standard K_Ic tests were performed in accordance with ASTM E399. Several failure mechanisms have been revealed by fractographic analysis: coarse voiding at intermetallics, fine tensile voiding and a combined intergranular/transgranular shear fracture mode. In the Zr containing alloys in particular, the increase in toughness with increased overageing is accompanied by a change in fracture mode from predominantly intergranular/transgranular shear failure to coarse voiding. A fracture toughness model was derived based on the microstructurally dependent work hardening factor, K_A, introduced in Ashby’s theory of work hardening, combined with the semi-empirical model for plane strain fracture toughness developed by Hahn and Rosenfield. This model predicts a linear relationship between K_Ic and K_A^0.85/σ_ys^0.35 which is shown to be consistent with the experimental data independent of the failure mechanism.