"Interface in High Strength Nanocrystalline and Nanotwinned Al alloys" 

Xuanyu Sheng, MSE PhD Candidate 

Advisor: Professor Xinghang Zhang

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ABSTRACT

A fundamental understanding of strengthening mechanisms has long underpinned the development of aluminum (Al) alloys for structural applications. In the present work, novel strengthening strategies, including solute-assisted twinning, grain boundary (GB) engineering, nanoscale phase modulation, and the introduction of grain boundary concourses (GBCs) are systematically explored to achieve the concurrent enhancement of strength and deformability in Al-based systems. Investigations on Al-Mg sputtered films reveal that Mg solute facilitates twin boundary formation, with twinned grain fractions increasing during film growth and reaching a maximum at intermediate thicknesses. By distinguishing between intergranular and intragranular twins, the competing mechanisms of twin survival during coalescence and twin elimination during grain growth are elucidated. Furthermore, experimental analyses of nanocrystalline Al-1Mg alloys demonstrate that Mg segregation is strongly dependent on GB character, with significant solute enrichment observed at high-angle and incoherent twin boundaries, while coherent twin boundaries exhibit minimal segregation. Spinodal decomposition induced strengthening was also explored for which vertically aligned nanolaminate structures produced via spinodal decomposition in Al-12.4Pd alloys are shown to enhance mechanical properties substantially, yielding a flow stress exceeding 2.2 GPa and promoting pronounced work-hardening behavior. In addition, a distinct strengthening approach for pure Al films is introduced by engineering dense GBC networks through deposition under substrate bias, wherein dislocation-rich interfaces generate heterogeneous stress fields that impede dislocation propagation, achieving a flow stress of 600 MPa without grain refinement or alloying. Collectively, these findings establish a comprehensive framework for tailoring the structure-property relationships in nanostructured Al alloys and provide new strategies for the design of lightweight, high-performance structural materials.