Anisotropic Deformation and Failure in Magnesium Alloy
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Authors
Wang, Yuqian
Issue Date
2021
Type
Dissertation
Language
Keywords
Alternative Title
Abstract
Magnesium (Mg) alloys are the lightest metallic materials that could be widely applied to load-bearing structural components. They are typically manufactured via two different forms: casted and wrought (rolled, extruded and forged). Wrought Mg alloys without casting defects possess superior mechanical properties to these of the casted counterparts. Due to their hexagonal closed-packed (HCP) crystal structure, Mg alloys have limited slip systems at room temperature. Twinning can be activated as an additional deformation mode to accommodate plastic deformation. Dislocation slips and twinning as well as their interactions result in some unique mechanical responses of Mg alloys under monotonic and cyclic loading. Moreover, for wrought Mg alloys, a strong basal texture is often introduced during the rolling and extrusion processes, leading to the c-axis of most grains being aligned perpendicular to the rolled direction (RD) or extrusion direction (ED). The strong texture and the pronounced variation of critical resolved shear stress (CRSS) of different deformation modes result in significant mechanical anisotropy of Mg alloys. The underlying deformation mechanisms stimulate profuse interests in both academia and industry. However, current state-of-the-art research lacks a systematic investigation of the anisotropic deformation and failure in Mg alloys. This dissertation aims to understand the monotonic deformation and fracture, cyclic deformation and fatigue behavior, as well as the associated orientation-dependent principal microscopic mechanisms.In this study, monotonic tension, monotonic compression, and cyclic fully reversed strain-controlled tension-compression experiments were carried out on rolled AZ31B Mg alloy specimens taken from five different material orientations (θ = 0º, 22.5º, 45º, 67.5º and 90º) with respect to the rolled direction (RD). The deformation behavior was analyzed from the stress-strain response. Electron back-scattered diffraction (EBSD) analysis was performed to characterize the microstructural evolution and failure mechanisms.
The anisotropic fracture behavior of Mg alloys was explored for the first time by statistical slip trace analysis. Significant anisotropy is observed in the monotonic deformation and tensile fracture behavior. At the macroscopic scale, under tension at θ = 0º, 67.5º and 90º, a brittle-like fracture is shown where irregular-shaped surfaces composed of ridges and islands are observed. For tension at θ = 0º, a microstructural analysis in the vicinity of microcracks confirms that a crack forms at the tip and/or boundary of compression and compression-tension (C-T) double twins. For the cases of tension at θ = 67.5º and 90º, microcrack initiation is due to high-angle grain boundary cracking, which is likely caused by stress concentration due to impingements of none co-zone twin-twin boundaries and tertiary tension-compression-tension twins on the high-angle grain boundaries (HAGBs). In contrast, shear fracture displaying a relatively flat fracture surface is exhibited from tension at θ = 22.5º and 45º. A microstructural analysis reveals that fracture at these two material orientations is a result of localized shearing accommodated by basal slips from which both crack initiation and propagation are originated.
The detailed characteristic cyclic plastic deformation was investigated by performing fully reversed strain-controlled tension-compression cyclic experiments. Significant anisotropy is exhibited in cyclic plastic deformation. Cyclic hardening was observed for all five material orientations at a high strain amplitude. A particular focus is put on the elastic limit range, which represents the activation stress for basal slips or twinning/detwinning during cyclic loading. For the material orientation θ ≠ 45º, the cyclic deformation exhibits a pronounced tension-compression asymmetry. The different deformation mechanism during the tension and compression reversals is intrinsically reflected by the evolution behavior of elastic limit range on the number of loading cycles and applied strain magnitude. The elastic limit range on the loading reversal which is favorable for twinning signifies the activation stress of basal slips and being independent on the loading cycles and the strain amplitude. In contrast, the value of elastic limit range on the loading reversals which is favorable for detwinning is closely related to the twin volume fraction at the peak stress prior to the loading reversal. A strong material dependence can be observed in the elastic limit range during cyclic loading, which reveals a highest value at θ = 0º and lowest value at θ = 45º.
Unique deformation behavior can be captured during cyclic loading in the θ = 45º material orientation. The values of elastic limit range on the tension and compression reversals are approximately identical irrespective of the number of loading cycles and loading magnitude. Twinning occurs in one group of grains and simultaneously detwinning occurs in another group of grains during loading and reversed loading. Low percentage of grains are favorable for twinning for the 45º orientation comparing with the other material orientations.
Results obtained from fully reversed strain-controlled tension-compression fatigue experiments reveal the material orientation effect on the fatigue behavior. Fatigue lives range from few loading cycles to over 10^6 cycles. The strain-life curves of all the material orientations form a relatively narrow band. The strain-life curve obtained from each material orientation is characterized with two distinct kink points which are closely associated with the dominating plastic deformation mechanisms, fatigue properties, and fatigue cracking behavior. The RD specimens show the highest fatigue strength while the 45º specimens display the lowest fatigue strength among the five material orientations.
Based on the two kink points from the strain-life curves, three different strain amplitudes were prescribed for each material orientation. It is found that both the material orientation and the loading magnitude can affect the fracture features at the initiation and early-stage propagation region on the fracture surface. At the macroscopic scale, fatigue at θ = 0º (RD), 67.5º and 90º (ND) results in brittle-like fracture surfaces, whereas fatigue at θ = 22.5º and 45º displays shear cracking behavior at the local region. For θ = 0º (RD), 22.5º and 45º, both intergranular and transgranular crack growth modes exist irrespective of strain amplitudes. The mechanism of early-stage transgranular crack propagation is different at high and low strain amplitudes. When the strain amplitude is above the upper kink point, transgranular cracking is induced by both basal and non-basal slips. As for the strain amplitude below the upper kink point, transgranular cracking is only induced by basal slip. In contrast, for θ = 90º (ND) and 67.5º, grain boundary cracking is persistent under three different strain amplitudes. However, the early-stage microcrack propagation mode is dependent on the strain amplitude. Transgranular cracking induced by basal and non-basal slips only occurs when the strain amplitude is above the upper kink point.