Yield Stress in Semi-Solid Alloys – The Dependency on Time and Deformation History

An important aspect of the rheological characterization of semi-solid metals is the determination of the materials ability to exhibit yield stress. The yield stress is the stress level at which the material cease from purely elastic to plastic deformation. In semi-solid metal suspensions the yield stress is not a constant value, but depends on the transient state of the material. The investigation of yield stress in semi-solid metal alloys has been initiated in previous works, however, a profound understanding of the influence of the material history on the yield stress is so far lacking. In semi-solid metal suspensions, the physical cause of the yield stress is generally described by the formation of a particle "skeleton" structure which can hold a certain stress level without being subject to irreversible deformation. The yield stress in alloy suspensions can be described as a function of structural properties i.e. the solid fraction and the particle size, form and spatial distribution. It has been observed in previous works that the yield stress of semi-solid metals depends on the duration of resting time. It is generally assumed that this is due that as the material is resting, the particle network build up over time leading to an increase in the yield stress. In addition, we assume that the yield stress is also a function of the shear history as this affects the particle spatial distribution. The goal of the proposed work was to investigate the yield stress in semi-solid alloys including its dependency on the materials shear history and resting time. This was investigated both from a structural and a rheological point of view. The yield stress of the semi-solid alloys was measured with shear stress ramp and oscillation tests. Structural imaging measurements of quenched samples were performed to correlate structural parameters with the value of the yield stress. However, in semi-solid alloys, the experiments performed at a long time scale are influenced by aging of the material (Ostwald ripening). Therefore, rheological experiments were also performed with synthetic suspensions to investigate the long term yield stress dependencies without errors caused by the aging effects.