Materials Science and Engineering A 487 (2008) 383–387 The effect of structural integrity on the tensile deformation characteristics of A206-T71 alloy castings Murat Tiryakio˘ glu a,∗ , James T. Staley Jr. b , John Campbell c a Department of Engineering, Robert Morris University, Moon Township, PA 15108, USA b Bodycote Materials Testing, Chicago, IL, USA c Department of Metallurgy and Materials, University of Birmingham, Edgbaston B15 2TT, United Kingdom Received 17 September 2007; accepted 2 November 2007 Abstract The effect of structural integrity on the tensile deformation of A206-T71 castings was evaluated. To generate a wide range of structural integrity, some castings were subjected to hot isostatic pressing (HIP) at various conditions and the others received no remedial processing. The analysis of true stress–strain curves of all castings showed that A206-T71 follows the Kocks–Mecking Stage III work hardening model in both non-HIPed and HIPed conditions. Nevertheless distinct differences were found within and between the HIPed and non-HIPed conditions. Kocks–Mecking parameters were correlated to elongation and toughness (strain energy). Structural integrity or quality, as measured by relative toughness, was found to have two types of effects on the Kocks–Mecking parameters. These effects are discussed in the paper. © 2007 Elsevier B.V. All rights reserved. Keywords: Al–Cu; Porosity; Bifilms; Kocks–Mecking; Work hardening 1. Introduction Structural defects, namely porosity and oxide bifilms, are introduced into aluminum castings as a result of poor molten metal quality and/or filling system design. Bifilms are entrained by a folding-over action of the surface oxides and therefore constitute cracks in the structure [1]. These cracks open dur- ing solidification (i) by the diffusion of hydrogen to fill the gap between the two layers of the bifilm, (ii) due to negative pressure due to the contraction of the solidifying metal, and/or (iii) by a straightening action from the intermetallics that precipitate het- erogeneously on them in the liquid state [2]. These defects not only reduce tensile strength, elongation and fatigue resistance of aluminum castings but also increase their variability [1,3–5]. Cast Al–Si alloys exhibit Stage III work hardening behavior [6–8], where work hardening rate, Θ, is written as [9,10]: Θ = dσ dε p = Θ 0 - Kσ (1) ∗ Corresponding author. E-mail address: tiryakioglu@rmu.edu (M. Tiryakio˘ glu). where σ is the true stress (MPa), ε p the true plastic strain, Θ 0 the initial work hardening rate and K is a dimensionless parameter that is mainly dependent on Stage II work hardening rate and strain rate, and includes the effect of precipitation hardening in heat-treatable alloys. It should be noted that Stage III is reached and completed if premature fracture does not take place due to the presence of major structural defects [6,7]. Such defects were found [6] to reduce the observed work hardening rate in Al–7% Si–0.6% Mg alloy castings significantly. The tensile deformation characteristics of cast Al–Si alloys were investigated in several studies [6–8,11,12]. Poole and Dowdle [12] tested an Al–Si eutectic alloy in tension and com- pression. Upon plotting Θ versus ε p , these authors noticed that there was an anomalous drop in Θ in the specimen deformed in tension, just before the final fracture. The specimen deformed in compression did not show such an anomalous drop. They attributed this drop to the accumulation of damage to Si particles: when damage reached a critical value, cracks suddenly prop- agated to produce premature fracture. Tiryakio˘ glu et al. [6,7] investigated the tensile deformation of two Al–7 wt.%Si–%Mg alloys with different structural quality (as differentiated by X-ray inspection), and concluded that porosity and bifilms caused the sudden drop in the work hardening rate during tensile deforma- tion. The same authors [7] proposed that an increase in structural 0921-5093/$ – see front matter © 2007 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2007.11.005