Physical modeling of the deformation mechanisms of semisolid bodies and a mechanical criterion for hot tearing
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I. INTRODUCTION
THE mechanical properties of semisolid bodies have been investigated in some detail, due in part to the susceptibility of some wrought aluminum alloys for hot tearing during the transient phase of the direct-chill (DC) casting process.[1,2] Hot tearing can be characterized as a brittle fracture in the semisolid state and research work on this subject has been reviewed.[3] Previous studies[4,5] on hot tearing have revealed that the fracture process is intergranular or interdendritic in nature and that it occurs above the solidus, thus, in the semisolid state. A critical temperature range corresponding to the existence of an embrittled microstructure was defined,[6] and this embrittlement has been associated with constrained, liquid films that surround the solid grains.[3,4] The tensile properties of embrittled, semisolid bodies have been characterized,[7–16] and they have been found to have low fracture strengths ranging between 0.1 and 10 MPa. Further, it has been shown that the embrittlement occurs at temperatures that correspond to the last stages of solidification,[17] when the solid fraction is high. The effect of the microstructure has been widely reported, and the evidence suggests that equiaxed microstructures are less prone to hot tearing than columnar microstructures.[18,19] This effect can be related to the distribution of the liquid around the solid grains and its relationship to the development of mechanical coherency.[20] D.J. LAHAIE, formerly with the Alcan-UQAC Chair on the Solidification and Metallurgy of Aluminum, Department of Applied Sciences, University of Quebec at Chicoutimi, is with Research in Motion Limited, Waterloo, ON, Canada N2L 3W8. M. BOUCHARD is with the Alcan-UQAC Chair on the Solidification and Metallurgy of Aluminum, Department of Applied Sciences, University of Quebec at Chicoutimi, Chicoutimi, PQ, Canada G7H 2B1. Manuscript submitted May 2, 2000. METALLURGICAL AND MATERIALS TRANSACTIONS B
The physical models proposed for hot tearing depend on the assumptions made with regard to the surface tensions of the phases present in the microstructure. For systems such as Al-Sn, in which the liquid does not completely wet the solid grains, there is some level of coherency between the solid grains above the solidus, leading to significant plasticity.[21] In these incomplete wetting conditions, the deformation mechanisms reported include grain boundary sliding and slip within the solid grains. The fracture stress of the semisolid body was related to the dihedral angle of the intergranular liquid via a Griffith[22] type of relationship. Most constitutive laws reported for mechanically coherent semisolids are based on viscoplastic deformation of the continuous, solid network.[11–13,23] For wetting systems, fluidflow models[24,25,26] have determined the buildup of pressure gradients in semisolid networks. One study[25] found stresses in the semisolid network that ranged between 0 and 50 kPa for solid fractions of up to 0.9. Other investigators[27] have proposed a hot-tearing
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