Kinetics of twinning in magnesium under dynamic loading

Twinning is an important mode of deformation in many HCP materials including magnesium (Mg) and its alloys. Twinning in this material leads to important effects such as mechanical anisotropy, texture evolution, tension–compression asymmetry, and sometimes non-Schmid effects. Dynamic loading can introduce further complexity in the deformation behavior. The growth of twins takes place by the motion of the twin boundary (TB). Tension twins in Mg can accommodate considerable amounts of plastic deformation as they grow, and this affects the overall rate of plastic deformation. Detailed understanding of the kinetics of TB motion will enable us to work towards achieving the overarching goal of microstructural design of materials for performance. We undertake an experimental approach to gain insight into the kinetics of TB migration under dynamic loading. To achieve this goal we performed normal plate impact recovery experiments with microsecond pulse durations on pure polycrystalline Mg specimens. Estimates of average TB velocity under the known impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities in our experiments are of the order of several meter per second. These velocities are several orders of magnitude higher than those measured in Mg under quasi-static loading conditions. Further, twin nucleation and growth processes are investigated by conducting experiments with different durations of the loading pulse. This is achieved by using Mg specimens of different thicknesses. Electron back scattered diffraction is used to characterize the nature of the twins, microstructure, and twin fraction evolution. Detailed crystallographic analysis of the twins enables us to correlate the TB velocities to the twin variants

Microstructure-sensitive investigation of plasticity and fatigue of magnesium alloys

This dissertation identifies and quantifies the correlation between strain localizations at different scales and both macro- as well as microplasticity of Magnesium (Mg) based alloys. The extension of the work in the case of cyclic mechanical loading further enabled the investigation of reversible and irreversible microstructural processes that are ultimately linked to progressive fatigue damage development. To accomplish these goals, this dissertation presents a systematic experimental mechanics methodology combining multi-scale mechanical testing, in situ nondestructive evaluation (NDE) and targeted microstructure quantification. The presented research benefited from the novel integration between mechanical testing and multimodal NDE comprising both full field deformation measurements by using the digital image correlation method and time-continuous recordings of acoustic. Specific contributions of this work include the direct identification of the dominant effect of twinning in early stages of plasticity which is demonstrated in this research to be responsible for macroscopic effects on the monotonic and cyclic plasticity, as well as for microscopic processes that include slip-twin interactions and fatigue crack incubations. Such observations both enabled and were validated by careful texture evolution and grain-scale effects including pronounced intrusions/extrusions on the surface which are demonstrated to be responsible for micro-level strain accumulations that eventually, under cyclic loading conditions, lead to the onset of cracking. Surface morphology changes were found to be attributed to an evolving twinning-detwinning-retwinning activity which operates from early stages of the low cycle fatigue life up until later stages, while it was found to be associated with progressive damage development. Furthermore, the role of twinning in plasticity and fatigue of Mg alloys was verified using a Continuum Dislocation Dynamics Viscoplastic self-consistent (CDD-VPSC) polycrystal model. The simulation results reveal that the detwinning mechanism is in fact responsible for the anisotropic hardening behavior for various imposed strain amplitudes. Experimental results were further used to modify strain-based modeling approaches of fatigue life estimation. A number of the insights enabled with this research were further verified by performing a mechanical behavior characterization investigation of Mg alloys with Strontium (Sr) additions, which are currently considered for industrial applications. The presented results demonstrate that the major research accomplishments described in this dissertation could improve current manufacturing processes, which further allow extensions and applications of this research in fundamental and applied aspects of plasticity and fatigue of polycrystalline metals.Ph.D., Mechanical Engineering -- Drexel University, 201

Identification of fatigue precursors via quantitative nondestructive evaluation

Understanding evolving fatigue microstructure–properties–behavior relations is both challenging and needed in view of the time and length scales involved at both coupon and component levels. To reliably formulate quantitative descriptions of remaining useful life for advanced materials recent developments reported in the Integrated Computational Materials Engineering framework including in situ/ex situ experimental observations and high--performance simulations, continuously provide valuable insights. This discussion focuses on describing the role quantitative nondestructive evaluation (NDE) can play in this multidisciplinary effort to identify fundamental mechanisms that dominate fatigue life by presenting results obtained from a range of materials including polycrystalline alloys and fiber-reinforced composites. Emphasis is given on using multimodal NDE data to first identify intervals in time and regions at several length scales in which fatigue damage precursors could be found. Results from targeted observations of the evolution of such precursors are then used to formulate hypotheses on the fatigue behavior which are implemented in analytical and computational models. Challenges and future opportunities created by adopting this type of approach are also discussed

Dynamic response of ECAE-AZ31 magnesium under pressure shear

Lightweight and energy mitigation are considered the core characteristics required of any material system -subjected to impact loading conditions, and magnesium alloys represent potential materials for such systems. ECAE-AZ31, an Mg alloy system processed through Equal Channel Angular Extrusion (ECAE), is a particularly interesting candidate given its specific strength. We seek here to develop an understanding of its constitutive response at very high rates of loading. We perform an experimental investigation of the behavior of this material, at strain rates on the order of 105 s–1, using the high-strain-rate pressure-shear plate impact technique. We also present a brief description of the experimental setup and describe the diagnostic techniques used. Using the measured behavior and examination of the initial and deformed microstructures, we examine the influence of anisotropy and micromechanisms (twinning, dislocations) on the overall constitutive response. The goal is to parameterize such influences to incorporate them into a constitutive framework

MICRO CHARACTERIZATION OF MG AND MG ALLOY FOR BIODEGRADABLE ORTHOPEDIC IMPLANTS APPLICATION

ABSTRACT Magnesium as a candidate metallic biomaterial for biodegradable orthopedic implants was evaluated in-vitro in terms of degradation behavior, biocompatibility and mechanical property both in macro-and micro-scale. Micro structure of pure Mg and AZ61 after degradation in both simulated body fluid (SBF) and cell culture environment were analyzed. Different from AZ61, pure Mg degraded at a higher rate and attracted large amount of salt precipitation which formed a layer covering the surface. Much less pitting degradation and salt deposition were observed on both pure Mg and AZ61 in cell culture environment compared to in SBF. After culturing for 7 days, EAhy926 cells growing on AZ61 showed significant higher proliferation rate as of cells growing on pure Mg. Higher proliferation rates indicated that cells grew better on slow-degrading AZ61 than on fast-degrading pure Mg. Cells growing on AZ61 proliferated much better and assembled together to form a consistent tissue-like micro-structure, while cells spread and reached out on the surface of pure Mg, possibly due to low cell density and lack of cellular communication. The elastic modulus and tensile yield strength of magnesium are closer to those of natural bone than other commonly used metallic biomaterials. It was shown that Mg was biodegradable, biocompatible and had appropriate mechanical strength, thus Mg and its alloys showed great potential for deployment in a new generation of biodegradable orthopedic implants

Twinning in magnesium under dynamic loading

Twinning is an important mode of deformation in magnesium (Mg) and its alloys at high strain rates. Twinning in this material leads to important effects such as mechanical anisotropy, texture evolution, tension-compression asymmetry, and sometimes non-Schmid effects. Extension twins in Mg can accommodate significant plastic deformation as they grow, and thus twinning affects the overall rate of plastic deformation. We use an experimental approach to study the deformation twinning mechanism under dynamic loading. We perform normal plate impact recovery experiments (with microsecond pulse durations) on pure polycrystalline Mg specimens. Estimates of average TB velocity under the known impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities in our experiments are of the order of several m s−1. These velocities are several orders of magnitude higher than those so far measured in Mg under quasi-static loading conditions. Electron back-scattered diffraction (EBSD) is then used to characterize the nature of the twins and the microstructural evolution. Detailed crystallographic analysis of the twins enables us to understand twin nucleation and growth of twin variants under dynamic loading

Twinning in magnesium under dynamic loading

Twinning is an important mode of deformation in magnesium (Mg) and its alloys at high strain rates. Twinning in this material leads to important effects such as mechanical anisotropy, texture evolution, tension-compression asymmetry, and sometimes non-Schmid effects. Extension twins in Mg can accommodate significant plastic deformation as they grow, and thus twinning affects the overall rate of plastic deformation. We use an experimental approach to study the deformation twinning mechanism under dynamic loading. We perform normal plate impact recovery experiments (with microsecond pulse durations) on pure polycrystalline Mg specimens. Estimates of average TB velocity under the known impact stress are obtained by characterization of twin sizes and aspect ratios developed within the target during the loading pulse. The measured average TB velocities in our experiments are of the order of several m s−1. These velocities are several orders of magnitude higher than those so far measured in Mg under quasi-static loading conditions. Electron back-scattered diffraction (EBSD) is then used to characterize the nature of the twins and the microstructural evolution. Detailed crystallographic analysis of the twins enables us to understand twin nucleation and growth of twin variants under dynamic loading

Mechanical behavior of additively manufactured GRCop-84 copper alloy lattice structures

This study investigates the interplay between microstructure, topology and their combined effect on the quasi-static and dynamic behavior of additively manufactured Copper–Chromium–Niobium alloy (GRCop-84) lattice structures. Lattice structures made of GRCop-84 alloys are beneficial for wide range of applications due to the combination of the high strength and thermal conductivity imparted by GRCop-84 while minimizing weight and increasing the energy absorption through the use of the lattice structure. X-ray computed tomography (XCT) and optical microscopy were used to characterize the porosity and grain structure, respectively. Quasi-static and dynamic testing was performed on the as-built (AB) samples at strain rates of 10−1s−1 and 103s−1, respectively. The observations indicated that reducing the unit cell size from 4mm to 2mm led to a 66% reduction in porosity. Depending on the topology of the tested sample, the reduced porosity within the 2mm unit cell samples resulted in a 35% to 60% increase in the compressive yield strength. To understand whether topology is the only driving mechanism that influence the mechanical properties e.g., yield strength, the microstructure was altered through hot isostatic pressing (HIP) heat treatment while the topology was kept constant. It was noted that the 4mm unit cell size was more responsive to HIPing with a 40% reduction in porosity, while the 2mm unit cell size only experienced a 28% reduction in porosity. It was also noticed that there was a 48% reduction in porosity by minimizing the unit cell size from 4mm to 2mm in the case of the HIPed samples. Using this data, a correlation was recognized between microstructure and topology. It was found that HIPed samples experienced more plastic deformation and exhibited stress plateau that is common in cellular solids, indicating improved energy absorbing abilities compared to AB. AB Samples demonstrated higher compressive strength and failed due to the brittle nature of the AB microstructure. Lattice Structures with unit cell sizes of 4mm and 2mm experienced different collapse mechanisms, with 2mm unit cell lattices being topology dependent and 4mm unit cell lattices dependent on microstructure.NSF24 month embargo; available online: 31 May 2022This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]

Synchronous involvement of topology and microstructure to design additively manufactured lattice structures

This article presents a methodical approach to optimize microstructure (e.g., the crystallographic texture) and topology (e.g., unit cell and struts) concurrently to improve the mechanical properties of additively manufactured metallic lattice structures (AMLS), i.e., yield strength and plastic flow stress. Full-field elasto-viscoplastic Fast Fourier Transform (EVP-FFT) crystal plasticity (CP) simulations are employed to determine the optimal microstructure. The CP model parameters were calibrated to measured macroscopic stress–strain response and microstructural data for polycrystalline samples of additively manufactured (AM) Inconel 718 with solution treated and aged (STA) microstructure. Since the crystallographic orientation of the constituent single-crystal grains with respect to the loading direction has a significant impact on the mechanical behavior of the material, stress projection factor analysis was used to determine four candidate textures to explore in for a given unit cell topology. Full-field crystal plasticity simulations were used to determine macroscale yield surface parameters for each of the considered textures, thereby enabling macroscale lattice unit cell simulations that account for the underlying microstructure. The calibrated microstructure-dependent yield surfaces are used to investigate the effect of different microstructures on the mechanical response of different LS topologies with the same relative density. The results show that in a texture with crystallographic direction, parallel to the loading direction, the tensile and compressive yield strength are 20% and 58% larger, respectively compared to the AM STA IN718 texture. Furthermore, when this texture is used in conjunction with the Rhoctan topology, the results demonstrate 50% improvement in both the yield strength and modulus of elasticity relative to previously optimized AMLS designs that did not directly account for microstructure. This simultaneous consideration of microstructure and topology during optimization, thus, significantly enhances the structural integrity of the AMLS.NSF24 month embargo; available online: 12 February 2022This item from the UA Faculty Publications collection is made available by the University of Arizona with support from the University of Arizona Libraries. If you have questions, please contact us at [email protected]