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THE drive to improve fuel economy and reduce CO2 emissions continues to incentivize the development of low-cost lightweight high-strength cast alloys for automotive and other transport applications.[1] These light alloys are required to possess excellent strength and fatigue properties, together with good weldability and machinability, all at a low cost.[1,2] These properties are heavily influenced by the presence of the microstructural features and solidification defects like hot tear,[3,4] segregation,[5–7] and porosity,[8,9] which exist in forms of (a) gas porosity,[10] (b) shrinkage porosity,[11] and shrinkage bands in twin-roll and High-pressure die castings (HPDC).[12] The nucleation of these S. BHAGAVATH is with the Department of Mechanical Engineering, Indian Institute of Technology Bombay, Mumbai 400076, India and also with the Research Complex at Harwell, Harwell Campus, OX11 0FA, UK. B. CAI is with the School of Metallurgy and Materials, University of Birmingham, Edgbaston, Birmingham B15 2TT, UK. R. ATWOOD is with the Diamond Light Source, Harwell Campus, OX11 0DE, UK. M. LI and B. GHAFFARI are with the Ford Research and Advanced Engineering, Dearborn. P.D. LEE is with the Research Complex at Harwell and also with the University College of London, WC1E 6BT, UK. Contact e-mail: [email protected] S. KARAGADDE is with the Department of Mechanical Engineering, Indian Institute of Technology Bombay. Contact e-mail: [email protected] Manuscript submitted April 2, 2019.

METALLURGICAL AND MATERIALS TRANSACTIONS A

solidification defects can be traced to the semi-solid state having relatively high solid fractions during the solidification. It is known that at these higher solid fractions, a network of solid is formed and as a consequence, the permeability of the mushy zone will decrease, resulting in difficulty in further feeding of the liquid. Based on the amount of fractions of solid, the solid network has been interpreted as a continuous solid skeleton[13,14] and cohesion-less granular solid.[15–17] The thermo-mechanical response of this network under deformation is understood to play a key role in the formation of defects. Laboratory- and synchrotron-based semi-solid deformation tests have been extensively carried out by researchers to understand the thermo-mechanical behavior of several aluminum alloys, particularly binary Al-Cu alloys.[18] While tensile tests have been conducted to determine the strength and ductility of the network,[19] shear[20] and compression tests[18] were used to study the rheology of the semi-solid. Tzimas et al.[18] reported semi-solid compression tests of Al-4 wt pct Cu alloys that cover the effect of solid fraction, strain rate, and grain morphology and identified different factors affecting the flow resistance.[18] Kim et al.,[21] Kang et al.,[22] and Kapranos et al.[23] conducted compression experiments to study the rheological behavior of various aluminum alloys at different solid fractions and strain rates and reported liquid segregation and cracks at the edge of the specimens. The