Time-resolved synchrotron tomographic quantification of deformation-induced flow in a semi-solid equiaxed dendritic Al–Cu alloy

Abstract

The rheology of semi-solid alloys has been studied by a novel in situ tomographic technique. Via extruding an equiaxed Al-15 wt.%Cu alloy, the inhomogeneous coherent compression of the a-Al grains was quantified, including the interdendritic channel closure and formation of a liquid extrudate. This investigation not only provides important insights into the microstructural changes occurring during semi-solid deformation, but also offers a validation benchmark for segregation and rheological models.Understanding liquid migration through a deforming semi-solid medium is critical for a wide range of processes from metal casting [1,2] to volcanology [3][4][5]: during casting, deformation of semi-solid alloys can influence liquid flow, resulting in macrosegregation, which can degrade the mechanical properties of the final product [2,6-10]; semi-solid magma is deformed by convection and/or tectonic plate spreading, inducing melt migration and segregation [3,4,11].In solidification processing, deformation in the semisolid can induce a range of defects, including extrusion segregation in squeeze-casting [1] and surface exudation in direct-chill casting [7]. Although several prior investigations have identified deformation-driven melt flow as a possible mechanism of such defects [1,7,12], the influence of stress on a semi-solid alloy and the melt flow through the equiaxed microstructure are not clearly understood. Many models have been developed to predict the formation of those defects, based on the proposition of the mushy zone as a sponge saturated with liquid [2,7,13,14]. However, currently there are no direct validation techniques that capture the kinetics incorporated in this hypothesis; in situ synchrotron tomography is one possible solution.Recently, high speed X-ray tomography has been utilized to perform four dimensional imaging (4D, i.e. 3D plus time) of the pore-scale fluid flow [15], solidification [16][17][18], and the influence of deformation on semi-sold alloys [19][20][21]. Tensile and uniaxial compression tests have been used previously with the help of 4D imaging to study semi-solid deformation; these were mainly focused on the formation of damage (hot tearing) as a result of the granular response of the mushy zone [19,21,22]. In this paper, we describe the application of an indirect extrusion cell to study the rheological behavior of the mushy zone and the mechanisms responsible for the liquid migration induced by deformation. Such an indirect extrusion cell can also be used to study how extrusion segregation and exudation form, since it mimics their forming conditions.The sample was semi-solid, equiaxed dendritic Al-15 wt.%Cu; a cylindrical specimen 2.9 mm in diameter by 2.9 mm long was prepared using wire electro-discharge machining, and then inserted in a boron nitride holder with an inner diameter (ID) of 3 mm and outer diameter (OD) of 5 mm. An alumina tube (1.5 mm ID and 3 mm OD) was placed on top of the specimen forming an indirect extrusion cell (Fig. 1). The entire extrusion set-up was enclose...

Transgranular liquation cracking of grains in the semi-solid state

“…A bespoke thermal-mechanical setup was used to perform the semi-solid indentation. The apparatus consisted of a mechanical testing rig (P2R) 1 2 46 and a loop-feedback controlled resistance furnace with an X-ray transparent window 1 2 47 . The samples were previously prepared by heat-treating the alloy in the semi-solid state for 23 days at 555 °C (7 °C above the eutectic temperature) 1 .…”

Operando Visualization and Multi-scale Tomography Studies of Dendrite Formation and Dissolution in Zinc Batteries

“…Although time-resolved tomography is an indispensable method to study the mechanisms of metal dendrite formation in very high-temperature systems during the metal solidification, it has only recently been extended to the operando study of electrochemical systems. [28][29][30] SXCT offers the opportunity for non-destructive analytical insight with high spatial and temporal resolution of 2D and 3D data of electrode microstructures with a relatively large field of view (typically 1 mm resolution for 5x5x5 mm 3 sample size).…”

Rheological Characterization of Semi-Solid Metals: A Review

“…To investigate the structure is difficult. There are only a few publications [20][21][22][23][24] that are dealing with structural experiments with X-ray tomography for SSM in rest and compression. However, up to now, no work has been published for investigations of SSM under pure shear.…”