Effect of Alloy Purity on Fracture Behavior of Overaged 7000 Alloy Plates

Abstract

The article deals with the relations between microstructural parameters, microscopic fracture mechanisms, and resultant plane-strain fracture toughness, K_IC, of hot-forged 7000 alloy plates in overaged condition. Three industrially produced alloys with the (Zn + Mg + Cu) content of ∼11.5 mass% and different contents of (Fe + Si) impurities (from 0.23 to 0.37 mass%) were used. The variation in alloy purity generated differences in the geometric characteristics of coarse intermetallic phase particles, exerting significant effect on fracture behavior of these alloys. To evaluate the fracture mechanism, characterization of the broken K_IC test specimen surfaces was performed using scanning electron microscopy. Three micromechanisms governing fracture process were identified. The extent of primary voiding at coarse intermetallic phase particles increases systematically with the increase of (Fe + Si) level, whereas the extent of intergranular/transgranular fracture decreases. This observation was confirmed by a quantitative description of fractures by means of a profilometric method. It was found that morphological parameters of the fracture profiles clearly differentiate between different fracture modes and offer conclusive proof that coarse voiding at fractured intermetallic phase particles dominates fracture at low alloy purity. A higher fraction of coarse voiding is shown to result in an overall reduction of fracture toughness.

Keywords

7000 alloys microstructure failure modes fracture toughness fractography.

Get full access to this article

View all access options for this article.

References

Cvijović, Z. , Rakin, M. , Vratnica, M. and Cvijović, I. ( 2008). Microstructural Dependence of Fracture Toughness in High-strength 7000 Forging Alloys, Engineering Fracture Mechanics, 75: 2115-2129.

Cvijović, Z. , Vratnica, M. and Cvijović-Alagić I. (2009). The Influence of Multiscale-sized Second-phase Particles on Fracture Behaviour of Overaged 7000 Alloys, Procedia Engineering, 1: 35-38.

Cvijović, Z. , Vratnica, M. and Rakin, M. ( 2006). Micromechanical Modelling of Fracture Toughness in Overaged 7000 Alloy Forgings, Materials Science and Engineering A, 434: 339-346.

Deshpande, N.U. , Gokhale, A.M. , Denzer, D.K. and Liu, J. ( 1998). Relationship Between Fracture Toughness, Fracture Path, and Microstructure of 7050 Aluminum Alloy: Part I. Quantitative Characterization, Metallurgical and Materials Transactions A, 29A: 1191-1201.

Dorward, R.C. and Beerntsen, D.J. ( 1995). Grain Structure and Quench-rate Effects on Strength and Toughness of AA7050 Al-Zn-Mg-Cu-Zr Alloy Plate, Metallurgical and Materials Transactions A, 26A: 2481-2484.

Dumont, D. , Deschamps, A. and Brechet, Y. ( 2003). On the Relationship Between Microstructure, Strength and Toughness in AA7050 Aluminum Alloy, Materials Science and Engineering A, 356: 326-336.

Gokhale, A.M. , Deshpande, N.U. , Denzer, D.K. and Liu, J. ( 1998). Relationship Between Fracture Toughness, Fracture Path, and Microstructure of 7050 Aluminum Alloy: Part II. Multiple Micromechanisms-based Fracture Toughness Model, Metallurgical and Materials Transactions A, 29A: 1203-1210.

Kamp, N. , Sinclair, I. and Starink, M.J. ( 2002). Toughness-strength Relations in the Overaged 7449 Al-based Alloy, Metallurgical and Materials Transactions A, 33A: 1125-1136.

Li, X.M. and Starink, M.J. ( 2001). Effect of Compositional Variations on Characteristics of Coarse Intermetallic Particles in Overaged 7000 Aluminium Alloys, Journal of Materials Science and Technology, 17: 1324-1328.

10.

Morere, B. , Ehrstrom, J.C. , Gregson, P.J. and Sinclair, I. ( 2000). Microstructural Effects on Fracture Toughness in AA7010 Plate, Metallurgical and Materials Transactions A, 31A: 2503-2515.

11.

Pardoen, T. , Dumont, D. , Deschamps, A. and Brechet, Y. ( 2003). Grain Boundary Versus Transgranular Ductile Failure, Journal of the Mechanics and Physics of Solids, 51: 637-665.

12.

Roskosz, S. and Cwajna, J. ( 2003). Application of a Profilometry Method for the Quantitative Evaluation of Fractures, Secondary Cracks and Palmqvist’s Cracks in Sintered Carbides, Materials Characterization, 51: 69-77.

13.

Underwood, E.E. and Banerji, K. ( 1999). ASM Handbook - Fractography, 9th edn, Metals Park, OH, ASM International , Vol.12, pp. 193-210.