K.M. Kareh

Revealing the micromechanisms behind semi-solid metal deformation with time-resolved X-ray tomography

The behaviour of granular solid–liquid mixtures is key when deforming a wide range of materials from cornstarch slurries to soils, rock and magma flows. Here we demonstrate that treating semi-solid alloys as a granular fluid is critical to understanding flow behaviour and defect formation during casting. Using synchrotron X-ray tomography, we directly measure the discrete grain response during uniaxial compression. We show that the stress–strain response at 64–93% solid is due to the shear-induced dilation of discrete rearranging grains. This leads to the counter-intuitive result that, in unfed samples, compression can open internal pores and draw the free surface into the liquid, resulting in cracking. A soil mechanics approach shows that, irrespective of initial solid fraction, the solid packing density moves towards a constant value during deformation, consistent with the existence of a critical state in mushy alloys analogous to soils.

Transgranular liquation cracking of grains in the semi-solid state

Grain refinement via semi-solid deformation is desired to obtain superior mechanical properties of cast components. Using quantitative in situ synchrotron X-ray tomographic microscopy, we show an additional mechanism for the reduction of grain size, via liquation assisted transgranular cracking of semi-solid globular microstructures. Here we perform localized indentation of Al-15wt.%Cu globular microstructures, with an average grain size of ∼480 μm, at 555 °C (74% solid fraction). Although transgranular fracture has been observed in brittle materials, our results show transgranular fracture can also occur in metallic alloys in semi-solid state. This transgranular liquation cracking (TLC) occurs at very low contact stresses (between 1.1 and 38 MPa). With increasing strain, TLC continues to refine the size of the microstructure until the grain distribution reaches log-normal packing. The results demonstrate that this refinement, previously attributed to fragmentation of secondary arms by melt-shearing, is also controlled by an additional TLC mechanism.

In situ, time-resolved tomography for validating models of deformation in semi-solid alloys

The development and validation of physically-based models of semi-solid alloy deformation requires experimental data on the microstructural response to load. In this work we present three-dimensional in situ synchrotron results of the indirect extrusion of semi-solid aluminium-copper alloys. Globular Al-15wt.% Cu specimens were deformed at solid fractions of approximately 0.7 during time-resolved x-ray tomography. Key phenomena of the response to load were observed and quantified, with a focus on particle translations and rotations, and the resulting displacement field. This example shows that crystal displacements can be quantified during semi-solid deformation of bulk specimens, allowing for datasets to be produced suitable for validating models of complex phenomena during semi-solid flow.

Globule-Globule Interactions during Deformation in Semi-Solid Al-Cu Using Time-Resolved X-Ray Tomography

Optimising semi-solid processing and accurately modelling semi-solid deformation requires a fundamental understanding of the globule-scale mechanisms that cause the macroscopic rheological response. In this work, apparatus and analysis techniques are being developed for the time-resolved, three-dimensional imaging of semi-solid alloy deformation. This paper overviews synchrotron X-ray tomography results on globular Al-15wt%Cu deformed at 0.7 solid fraction using extrusion. The globule-globule interactions in response to load were quantified in terms of the response of individual globules with respect to globule translation, rotation, and deformation. The potential of time-resolved X-ray tomography in the study of semi-solid alloy deformation is then discussed.