Journal article

High Apparent Creep Activation Energies in Mushy Zone Microstructures

Modelling represents an important tool in modern material processing which no longer follows the traditional trial and error route but rather represents what may be termed a right first time technology [1]. To successfully model technological solidification processes, thermodynamic and kinetic data are required. But mechanical aspects are important as well [2]: during solidification, temperature gradients or mechanical constraints imposed by the mold result in solidification stresses. These stresses must be considered for at least the following two reasons: first, they can lead to local air gap formation between metal and mold thus changing heat extraction, cooling rate and finally the cast microstructure [3]; second, at a larger scale they may influence the final product shape [4]. Moreover, they can assist in cavity formation and can produce cracking. Such stresses become important as soon as a significant amount of solid phase has formed during solidification. In principle, these stresses can be calculated using viscoelastic finite element stress analysis [5]. But, finite element calculations require as an input the constitutive law which governs the mechanical behavior. Therefore, there is an interest in mechanical data of solidifying alloys with mushy zone microstructures: Ackermann and Kurz [6] investigated the mechanical properties of a solidifying AIMg alloy perpendicular to the growth axis of the columnar crystals. The tensile behavior of solidifying AI-Cu alloys was studied by Wisniewski [7] and recently, Branswyck [8] proposed a modified indentation test which, in combination with FEM analysis, yields quantitative flow rules. Nevertheless, there is still a need for more mechanical data of solidifying alloys, especially creep data - where strain accumulates at a constant stress - only rarely exist for processing conditions.

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