Abstract
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.
Highlights
Grain refinement via semi-solid deformation is desired to obtain superior mechanical properties of cast components
Unlike any prior studies on semi-solids, these results capture the first instance of transgranular fracture of ductile grains (Fig. 1d, region of interest marked with a yellow circle)
(a–d) 2D schematic of the four stages: (a) motion and pinning of grains resulting in compressive and shear loads; (b) straining and dislocation movement under a stress state; (c) possible remelting and interface perturbation; and (d) crack growth from the surface of the grain accelerated by liquid entrainment. (e–h) Region of interest from Fig. 1b–e where transgranular fracture of single-crystal grains is occurring
Summary
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. Semi-solid microstructural response to the imposed deformation is important in advanced alloys because they are subject to deformation due to shrinkage forces and thermal contraction during conventional processing or when shear forces are applied[5,22,23,24] In these cases, a temperature window is encountered where solid grains and intergranular liquid coexist; that is, the material is semi-solid but capable of transmitting load[25,26]. During shearing of semi-solid melts containing equiaxed dendrites, secondary arms detach from the primaries, and subsequently coarsen to form globular microstructures This fragmentation process has been hypothesized to occur by formation of high-angle grain boundaries (large plastic deformations), and eventual detachment[30], in addition to remelting and pinch-off[25,32]. Indentation is typically employed on solids, the purpose of using an indenter in this study was to obtain a localized deformation, preferably by pushing a single grain to perturb and stress the granular system
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