Confirmation of a thin sheet toughening mechanism and anisotropic fracture in Al-Fe-X alloys
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I.
INTRODUCTION
IN a
recent study, Chan m observed that some AI-Fe-X alloys have a tendency to delaminate into thin sheet ligaments when tested by conventional Km and Jm methods. The thin sheet ligaments were arranged in a crackdivider configuration 121 with internal free surfaces between individual ligaments. Chan suggested that triaxial stresses within the process zone should be relaxed due to the presence of these internal stress-free surfaces. Because of reduction of mean stress, the critical strain at fracture and toughness should be enhanced. IL2.3IThis kind of toughness enhancement has been referred to either as "thin sheet toughening "tll or "crack-divider delamination toughening. "t41 Applying a semiempirical analysis to AI-Fe-X alloys, Chart tll provided indirect evidence which showed implicitly the presence of thin sheet toughening in AI-8Fe-I.4V-1.7Si (A1-Fe-V-Si) at both 25 ~ and 316 ~ and in A1-8Fe-2Mo-IV (A1-Fe-Mo-V) at 316 ~ The A1-Fe-Mo-V alloy did not exhibit internal delamination at 25 ~ as a result, thin sheet toughening should not be present in this alloy at ambient temperature. The premise of the thin sheet toughening mechanism is that the fracture toughness of a thick component or a plane strain specimen should be controlled by the fracture properties of individual ligaments after it has delaminated. Iu Therefore, the Kc value of an alloy which exhibits thin sheet toughening is expected to be independent of specimen thickness. In contrast, an alloy which does not exhibit crack-divider delamination is expected to show an increase of Kc with a reduction of specimen thickness. A rigorous means of confirming the presence of the thin sheet toughening in AI-8Fe-I.4V-I.7Si is to measure the Kc value as a function of specimen thickness. The objective of this paper is to report results of Kc vs specimen thickness which confirm the presence of thin sheet toughening in the A1-Fe-V-Si alloy at 25 ~ In addition, an anisotropy in fracture resistance created by thin sheet toughening is demonstrated and delineated from
K W A I S. CHAN, Principal Engineer, is with Southwest Research Institute. 6220 Culebra Road, San Antonio, TX 78284. Manuscript submitted December 13, 1988. METALLURGICAL TRANSACTIONS A
that induced by differences in tearing modulus. In studying the fracture toughenesses of five AI-Fe-X alloys, including AI-Fe-Ce, A1-Fe-Mo-V, A1-Fe-V, A1-Fe-V-Si, and AI-Cr-Zr alloys, an unusual fracture pattern, shown in Figure 1, was sometimes observed. The peculiar anisotropic fracture behavior arose from the tendency for cracks in some of these AI-Fe-X alloys to bifurcate and propagate parallel to the loading direction, resulting in these specimens fracturing into three pieces, as shown in Figure 1. The origin of this peculiar fracture behavior will also be identified. II.
EXPERIMENTAL PROCEDURES
The fracture resistances of five AI-Fe-X alloys, including A1-8Fe-I.4V-I.7Si, AI-8Fe-2Mo-IV, A1-8Fe7Ce, AI-10.5Fe-2.5V, and AI-5Cr-2Zr, were studied. Most of the works reported in this paper, however, were f
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