Magnesium Technology 2020: Fundamentals, Mechanical Behavior, Twinning, Plasticity, and Texture I
Sponsored by: TMS Light Metals Division, TMS: Magnesium Committee
Program Organizers: J. Brian Jordon, Baylor University; Victoria Miller, University of Florida; Vineet Joshi, Pacific Northwest National Laboratory; Neale Neelameggham, IND LLC
Tuesday 2:00 PM
February 25, 2020
Room: 6C
Location: San Diego Convention Ctr
Session Chair: Sepideh Kavousi, Colorado School of Mines; Dmytro Orlov, Lund University
2:00 PM Invited
An Investigation into the Role of Dislocation Climb During Intermediate Temperature Flow of Mg Alloys: Michael Ritzo1; Jishnu Bhattacharyya1; Ricardo Lebensohn2; Sean Agnew1; 1University of Virginia; 2Los Alamos National Laboratory
Textured Mg alloy samples were tensile tested at Zener-Holloman parameter values ranging from Z~50 at room temperature and 10-3 s-1 down to Z~18 at 350°C and 10-5 s-1. At high Z, the samples exhibit strong texture evolution indicative of significant prismatic slip of dislocations with <a> Burgers vectors. Correspondingly, the plastic anisotropy is high, r ~ 4. At low Z, the texture evolution is minimal and the response is nearly isotropic, r ~ 1. Previously, it has been asserted that the high ductility and low plastic anisotropy observed at low Z conditions is due to enhanced activity of non-basal slip modes, including prismatic slip of <a> dislocations and pyramidal slip of <a> and <c+a> dislocations. The present results call this understanding into question and suggest that the enhanced ductility is more closely associated with the climb of <a> dislocations.
2:30 PM Invited
Deviations from Theoretical Orientation Relationship Along Tensile Twin Boundaries in Magnesium: Brandon Leu1; Arul Kumar Mariyappan2; Yue Liu3; Irene Beyerlein1; 1University of California Santa Barbara; 2Los Alamos National Laboratory; 3Shanghai Jiao Tong University
Deformation twinning is a prevalent mode of plastic deformation in hexagonal close packed (HCP) magnesium. Twin domains are associated with significant lattice reorientation and localized shear. The theoretical misorientation angle for the most common {101 ̅2} tensile twin in magnesium is 86.3°. Through Electron Back-Scatter Diffraction characterization of twinning microstructure, we show that the twin boundary misorientation at the twin tips is approximately 85°, and it is close to the theoretical value only along the central part of the twin. The variations in twin/matrix misorientation along the twin boundary control the twin thickening process by affecting the nucleation, glide of twinning partials, and migration of twinning facets. To understand this observation, we employ a 3D crystal plasticity model with explicit twinning. The model successfully captures the experimentally observed misorientation variation, and it reveals that the twin boundary misorientation variations are governed by the local plasticity that accommodates the characteristic twin shear.
2:50 PM Invited
The Role of Faceting in {1012} Twin Nucleation: Christopher Barrett1; 1Mississippi State University
{1012} twinning is the most profuse twin mode in Mg and plays a major role in its plasticity and deformation. Identification of the mechanisms and locations of twinning nucleation is crucial to characterize the ensuing microstructural evolution and failure. Herein, we provide a new theory of hexagonal close packed twin nucleation. In essence, the theory is that twins need a preexisting interface upon which to grow. In the earliest stages of nucleation, this requirement implies that the twin must be able to facet onto the same plane as the local interface, whether it be a free surface, stacking fault, or grain boundary, and that the action of twinning must reduce the defect energy of the preexisting structure in order to remain stable until it can grow large enough to emit disconnections. The theory is demonstrated on {1012} twin nucleation at grain boundaries and stacking faults in Mg via molecular dynamics.
3:10 PM Invited
In-situ TEM Investigation of <c+a> Dislocations in Magnesium: Boyu Liu1; Fei Liu1; Bin Li2; Jian-Feng Nie3; Zhi-Wei Shan1; 1Xi'an Jiaotong University; 2University of Nevada; 3Monash University
The ductility of magnesium is intimately related to c+a dislocation. Understanding the behaviors of c+a dislocation is of critical importance for rationalizing the mechanical behavior and for alloy design. By using in-situ TEM mechanical testing of pillars of pure Mg single crystal, we found that c+a dislocation can accommodate considerable plasticity through gliding on pyramidal I and II planes. We also observed cross-slip of c+a dislocation, formation of dislocation dipole, and reversible motion of the c+a dislocation. Our findings provide information on the mobility of c+a dislocation and its relationship with plasticity of pure Mg. Our experimental strategy can be extended to understanding the dislocation behaviors in other hexagonal metals.
3:30 PM Break
3:50 PM Invited
Full-field Crystal Plasticity Modeling of {10-12} Twin Nucleation: YubRaj Paudel1; Christopher Barrett1; Haitham El Kadiri1; 1Mississippi State University
Historically, the ability of crystal plasticity to incorporate the Schmid's law at each integration point has been a powerful tool to simulate and predict slip-induced localization at the single and polycrystal levels. Unfortunately, this remarkable capability has not been replicated for materials where twinning becomes a noticeable deformation mechanism. The challenge resides mainly in the biased regional lattice transformation associated with twin formation in defiance of its obedience to a threshold stress. Inspired by results from micromechanics, digital image correlation and molecular dynamics, we developed an explicit twinning nucleation criterion based on hydrostatic stress gradient and volume fraction of twin inside a grain. Characteristic twin spacing parameter is used as a function of twin height to determine site specific nucleation points in case of multiple twins. This approach offered a good reproduction of the microstructure evolution and autocatalysis phenomenon as affected by twinning in a tri-crystal system.
4:20 PM Invited
The Incorporation of Discrete Deformation Twins in a Crystal Plasticity Finite Element Framework: Matt Kasemer1; Paul Dawson2; 1University of Alabama; 2Cornell University
The most prevalent twinning models homogenize the local deformation response by considering twins as “pseudo-slip” systems, obscuring the physical differences between slip and twinning - namely the discrete nature of twinning. Presented is a computational approach designed to consider discrete deformation twinning in a crystal plasticity finite element framework. A polycrystalline domain is pre-discretized at the sub-grain scale into lamellar regions dependent on the geometry of the twin systems, which facilitates a finite element mesh that attendant to this geometry. A twin is activated in a lamellar region by applying essential velocities to its nodes and rapidly mapping their locations to their expected twinned positions. The rest of the body deforms by crystallographic slip to enforce mechanical equilibrium. Results indicate stress relaxation in the parent grain, and regions of large stress concentrations in neighboring grains. These trends are discussed in light of global and local energetic observations.
4:40 PM
Thixomolding Progress: Raymond Decker1; Carolyn Woldring1; Jacob Edick1; Steve LeBeau1; 1Thixomat/nanoMAG
The international progression of Thixomolding will be described, along with an up-date on applications. The advantages of a new custom-designed alloy for Thixomolding will be enumerated as will improvements in the machine. Experiments on laser surface treatment and Friction Stir Welding will be covered along with the benefits of Thermomechanical Processing on microstructure and properties.
5:00 PM Cancelled
On the Load Multiaxiality Effect on the Cyclic Behaviour of Magnesium Alloys: A. Gryguc1; A. Karparvar1; D. Toscano1; Ali A. Roostaei1; Sugrib Shaha1; Behzad Behravesh1; Hamid Jahed1; 1University of Waterloo
While the majority of fatigue-related studies on wrought magnesium alloys are under uniaxial push-pull loading condition, structural members are mostly under multiaxial stresses in real-life applications. This study addresses the effect of load multiaxiality on the cyclic behaviour of several wrought magnesium alloys (AZ31B, AZ80, and ZK60 extrusion and extrusion-forged, and AM30 in extrusion) under multiaxial tension/compression-torsion loading. In particular, the influence of the presence of shear on normal stress response and vice versa is studied. In addition, phase angle effects on the stress-strain response and fatigue life are discussed. First, cyclic response under pure shear strain-controlled tests was studied. Then shear-axial strain-controlled tests with different strain ratio combinations of low-low, low-high, high-low, and high-high were performed. Further, three load phase angles of 0, 45, and 90 were considered. Total strain energy density (SED) per stabilized cycle was employed to quantify damage associated with each multiaxial loading combinations.