Does Shear Thickening Occur in Semisolid Metals?
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INTRODUCTION AND BACKGROUND
PROCESSING of metals in the semisolid state is a widely established technology (e.g.,[1] Thixoforming, thixocasting, thixoforging, thixomolding, rheocasting, and rheoforming are all semisolid processing methods. They rely on the thixotropic property of metallic alloys in the semisolid state which have a spheroidal rather than dendritic microstructure; when sheared the material thins but when allowed to stand, it thickens again.[2,3] This behavior is exploited to drive the material into a die and obtain a near net shape component in one shot. Although the technology is established, the modeling of such complex systems still needs further development. In particular, the entry into the die occurs in a fraction of a second so it is the transient rheological behavior which governs the initial flow. In the literature, this rheological behavior is probed experimentally through applying rapid transitions in shear rate under isothermal conditions. In this section, the scene will be set for the reanalysis by outlining the basic nature of thixotropy in semisolid metallic alloy systems; the distinction between steadystate behavior and that associated with a transient; the connection between discontinuous shear thickening as encountered for dense suspensions of cornstarch in water and the potential for shear thickening to be
HELEN V. ATKINSON, Head of Department, is with the Department of Engineering, University of Leicester, University Rd., Leicester LE1 7RH, U.K. Contact e-mail: [email protected] VERONIQUE FAVIER, Professor, is with the Arts et Me´tiers Paris Tech, PIMM, UMR, CNRS 8006, 151 Bd. De l’Hopital, 75013 Paris, France. Manuscript submitted January 27, 2015. METALLURGICAL AND MATERIALS TRANSACTIONS A
masked by shear stress. In addition, the interplays between yield stress and thixotropy and the distinction between the existence of a yielded zone and shear banding/shear localisation are described. All these issues are relevant to the reanalysis here of the experimental data in the literature to arrive at a rationalization of the contradictory evidence as to whether shear thickening does occur in semisolid metals. Finally, in this section, the concept of ‘isostructure’ with rapid transitions in shear rate is highlighted along with the issues around the timescale for rapid shear rate jump experiments, before the aim of the paper is summarized.
A. Basic Nature of Thixotropy in Semisolid Metallic Alloy Systems The particles here are spheroids of solid metal in a Newtonian liquid metal matrix and there is the potential for particle aggregation by the formation of minute ‘welds’ at the points where the spheroids contact by a mechanism very akin to sintering. The particles therefore show some cohesion in the terminology of soil mechanics. However, many of the mechanisms of interaction which apply in non-metallic suspension systems (e.g., electrostatic, steric, induced electric or magnetic dipoles) do not apply here. When the material is allowed to stand, or is sheared at a relatively low shear rate
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