Microstructural and Mechanical Studies of Interface-Dominant Materials Consisting of Nanoscale Hard/Soft Phases

Interface-dominant materials consisting of nanoscale hard/soft phases have demonstrated an increase in both strength and ductility. However, relevant research on the underlying mechanisms are dearth. In this work, using hypereutectic Al-20Si as model system an interface-dominant material was synthesized through Laser Surface Remelting(LRS). Then, the mechanical performance of these interface-dominant materials were extracted utilizing micromechanical testing. Finally, postmortem electron microscopy were performed to reveal the underlying strengthening and strain accommodation mechanisms. Morphological characterizations of LRS Al-Si alloy are performed to expand the understanding of far-from-equilibrium morphology enabled by ultrafast LRS technique. Three distinct microstructures were observed in LRS Al-Si alloy: (i) ultrafine-scale fully eutectic; (ii) heterogeneous structure with ultrafine-scale Al-Si eutectic embedded in coarse, primary Al matrix; (iii) the same structure as (ii) but with nanoscale faceted Si precipitates embedded in Al matrix. The Si fibers have diameters ranging from 45 to 65 nm, much finer than Si flake in the as-cast sample (1.3 ± 0.4 µm). In addition, a high density of nanoscale twins formed in the Si fibers at all scanning speeds in laser-refined eutectic, in contrast to the rare occurrence of twinned Si in as-cast alloy. Micromechanical pillar compression tests were utilized to extract mechanical performance from these LRS morphologies. In addition, the pillar sizes were design to exclude size effect and encapsulate enough Al colonies for extending the applicability. These micromechanical tests reveal a significant enhancement in mechanical performance for all LRS morphologies as compared with the as-cast counterpart. Furthermore, nanoscale fully eutectic shows the highest increase in flow stress while maintaining plastic co-deformation. LRS morphologies exhibit higher flow stress, exceeding 800 MPa and uniform plastic deformation above 20% compared to as-cast alloy that fractures at strains below 8% at flow strength of approximately 200 MPa. Lastly, indentation plastic zone of LRS alloy with nanoscale, fibrous, fully eutectic microstructure was characterized for a fundamental understanding of the underlying mechanisms. While cracking around the indents was observed in the as-cast microstructure, the LRS nano-fibrous eutectic microstructure demonstrated higher strength, strain-hardening, and uniformly distributed plasticity around indents without cracks. A transition from nano-fibrous Si to nearly-equiaxed Si nano-particles was observed immediately underneath the indent in LRS eutectic. The mechanism for this morphological evolution with increasing plastic strain is postulated to involve three stages of the deformation process: First, the formation of closely-spaced dislocation arrays in the nano-channels of Al matrix confined by Si fibers. Second, fiber segmentation when the stress field of the dislocation array exceeded the critical stress for fracture in Si nano-fibers. Third, recovery of dislocation substructures and sub-grain formation in Al leading to a rotation of fractured Si segments. The final microstructure consisted of Si nano-particles at triple points of the sub-grains in Al. A series of studies starting from morphological analysis, followed by mechanical testing and concluding with strain-dependent dislocation interaction observations reveals novel morphologies, properties, and mechanisms of interface-dominant materials with hard/ soft phases. These novel mechanisms permit more flexibility in customizing structural materials without altering the underlying composition. Furthermore, these findings provide a road map for designing interface-dominant materials with high strength-ductility synergy.PHDMaterials Science and EngineeringUniversity of Michigan, Horace H. Rackham School of Graduate Studieshttp://deepblue.lib.umich.edu/bitstream/2027.42/172635/1/hhlien_1.pd