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FABRICATION, PLASTICITY AND THERMAL STABILITY OF NANOTWINNED AL ALLOYS
Applications of Aluminum (Al) alloys in harsh environments involving high stress and high temperatures are often hindered because of their inherently low strength and poor performance at high temperatures. The strongest commercial Al alloys reported up to date have a maximum strength less than 700 MPa. Although ultrafine grained Al alloys prepared by severe plastic deformation have higher strength, they encounter grain growth at moderate temperatures.
This thesis focuses on adopting transition metal solutes and non-equilibrium approach to fabricate high-strength, thermally stable nanotwinned Al alloys. To understand the underlying deformation mechanisms of nanotwinned Al alloys, in-situ micromechanical tests, high resolution and analytical transmission microscopy and atomistic simulations were used. Our studies show that nanotwinned supersaturated Al-Fe alloys have a maximum hardness and flow stress of ~ 5.5 GPa and 1.6 GPa, respectively. The apparent directionality of the vertical incoherent twin boundaries renders plastic anisotropy and compression-tension asymmetry in the nanotwinned Al-Fe alloys, revealed by systematic in-situ tensile and compressive micromechanical experiments conducted from both in-plane and out-of-plane directions. Moreover, the nanotwinned Al-Fe alloys experience no apparent softening when tested at 200 °C. When selectively incorporating with one additional solute as stabilizer, the ternary nanotwinned Al alloys can preserve an exceptionally high flow stress, exceeding 2 GPa, prior to precipitous softening at an annealing temperature of > 400 °C. The thesis offers a new perspective to the design of future strong, deformable and thermally stable nanostructured Al alloys.