An Experimental Study of Deformation and Failure of Magnesium Alloys under Multiaxial Stress State
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Authors
Carneiro, Luiz Alberto
Issue Date
2022
Type
Dissertation
Language
Keywords
Combined loading , Fatigue , Magnesium , Multiaxial deformation , Torsion , Twinning
Alternative Title
Abstract
Research on the mechanical behavior of magnesium (Mg) and its alloys has received substantial attention in the past three decades, mainly ascribed to properties of these materials such as high specific strength, high specific stiffness, recyclability, and nontoxicity. However, unlike other metals, the fundamental knowledge of the multiaxial deformation of the hexagonal close-packed Mg is limited. Structural components are unavoidably subjected to cyclic loading and the fatigue failure of engineering materials under complex stress states is a major design concern. Therefore, a fundamental understanding on the multiaxial fatigue damage mechanisms is critical to ensure reliability and durability of Mg components. The current research aims at a better understanding on the micro-mechanisms involved in the monotonic and cyclic deformation and fracture behavior of Mg alloys subjected to multiaxial stress states.The mechanical response and microstructure evolution in a rolled AZ31B Mg alloy are, for the first time, characterized in detail under free-end torsion and seven combined axial-torsion loading paths, including tension-torsion and compression-torsion paths using specimens manufactured along the normal direction (ND) of the rolled plate. Under free-end torsion and tension-torsion loading paths, tension twinning is active and the twin volume fraction (TVF) increases as tension becomes more dominant under combined loading. The collective hardening effects by twin boundary (TB), twin-twin boundary (TTB), and twinning texture-induced non-basal slip activities result in a unique rise of the strain hardening rate, resulting in sigmoidal equivalent stress-equivalent plastic strain curves with three distinct distinctive stages of strain hardening. The biaxial stress state under torsion and compression-torsion paths induce both tension twinning and compressing twinning depending on the specific grain orientation. The decreasing twin activity for the compression-torsion paths as compression becomes dominant results in concave-down stress-strain curves, ascribed to slip-dominated deformation. A change in specimen length during free-end torsion (Swift effect) is evidenced, which is predominantly ascribed to tension twinning at a larger plastic strain. The effect of the crystal orientation with respect to the orientation of the applied principal stresses on the twin variant selection, ductility, strength, and post-fracture texture are discussed in detail.The effect of the initial texture and pre-designed twins on the cyclic shear deformation and shear fatigue of AZ31B Mg alloy was experimentally investigated by conducting cyclic torsion experiments on specimens fabricated from extruded bars along the extrusion direction (ED), and from a rolled plate along the ND in the as-received and pre-strained conditions. Under cyclic shear, the extruded texture presents the highest strength, followed by the pre-strained and as-received rolled counterparts, respectively. The higher strength of the two former textures is ascribed to the relatively higher activation of non-basal prismatic slips and dynamic Hall-Petch hardening effects of twin boundaries, respectively. The Swift effect under cyclic torsion is strongly affected by the initial texture. The axial strain increases for both as-received rolled and extruded specimens, but decreases for the pre-strained rolled material with the increasing number of loading cycles. The fatigue behavior of the three materials under cyclic torsion is discussed in terms of the effects of the initial texture, twinning-detwinning, and twin boundaries on the fatigue crack initiation and early-propagation mechanisms.