Magnesium Technology 2017: Mechanical Behavior: Twinning, Plasticity, Texture, and Fatigue I
Sponsored by: TMS Light Metals Division, TMS: Magnesium Committee
Program Organizers: Kiran Solanki, Arizona State University; Dmytro Orlov, Lund University; Alok Singh, National Institute for Materials Science; Neale Neelameggham, Ind LLC

Wednesday 8:30 AM
March 1, 2017
Room: 5B
Location: San Diego Convention Ctr

Session Chair: Bin Li, University of Nevada, Reno; Christopher Barrett, Mississippi State University

8:30 AM  Keynote
Twinning Super Dislocations to Help Understand Strength: Matthew Barnett1; 1Deakin University
    The strength of magnesium alloys is examined for some cases where deformation twinning plays a large role. It is proposed that a twin can be usfully approximated by two concentric loops of twinning dislocations. The inner loop can push the outer loop through gaps and into stress fields where it would otherwise not be inclined to go. The approximation is used to help explain the behaviour of magnesium under indentation and the impact of precipitation hardening on compressive strength. Some practical limits for strengthening are examined.

9:10 AM  
Basal Dislocation Transmutation through {1012} Twin Boundaries: Christopher Barrett1; Fulin Wang2; Sean Agnew2; Haitham El Kadiri1; 1Mississippi State University; 2University of Virginia
    Basal dislocations in hexagonal close-packed materials remarkably can enhance the mobility of {1012} twin boundaries which absorb them. This behavior has been extensively studied and leads to complex faceting, disclination content, and mobile disconnections. However, we recently uncovered that under loading which suppresses {1012} twin mobility, certain basal dislocations can punch through the boundary, generating <c+a> dislocations inside the twin. The transmutation requires two mixed basal dislocations to move into the boundary and produces a mixed <c+a> dislocation on the prismatic plane of the twin along with a mobile twinning disconnection. This reaction is nearly identical to one predicted decades ago by Tomsett and Bevis. The reaction is both stress and temperature sensitive and depends heavily on complex faceting reactions at the boundary. By studying the dependence of transmutation versus absorption upon stress and faceting, we have uncovered general new insights showing how interfaces react with dislocations.

9:30 AM  
Contraction Twinning Dominated Tensile Deformation and Subsequent Fracture in Extruded Mg-1Mn (wt%) at Ambient Temperature: Ajith Chakkedath1; Philip Eisenlohr1; Tias Maiti1; Carl Boehlert1; Jan Bohlen2; Sangborg Yi2; Dietmar Letzig2; 1Michigan State University; 2Magnesium Innovation Centre MagIC, Helmholtz Centre
    To improve the cold formability of Mg alloys it is critical to understand the mechanisms that limit the elongation-to-failure. In this study, a contraction twinning based mechanism was responsible for failure at low tensile strains in extruded Mg-1Mn (wt%). Contraction twinning dominated the tensile deformation at 50C. The contractions twins evolved into double twins which accounted for the formation of shear bands in the twinned volume. The shear band formation was expected to be due to the enhanced activity of basal <a> slip in the twinned region. Cracks developed along the shear bands and led to shear failure. The contraction twinning activity decreased with increased temperature, and at 250C, no contraction twinning was observed. This was expected to be due to the lower CRSS for pyramidal <c+a> slip compared to contraction twinning at elevated temperatures. The improved elongation-to-failure at elevated temperatures was attributed to the limited activity of contraction twinning.

9:50 AM  
Ductility Enhancement in Mg Alloys by Anisotropy Engineering: Shamik Basu1; Ebubekir Dogan1; Babak Kondori1; Ibrahim Karaman1; Amine Benzerga1; 1Texas A&M University
    Anisotropic plasticity is often invoked to rationalize low formability in Magnesium alloys. A mean-field theory suggests however that certain forms of plastic anisotropy hinder ductile damage accumulation. Here, a proof-of-concept is demonstrated in the case of Mg-Al-Zn alloys. Two textures produced by severe plastic deformation are compared with the as-received rolling texture in terms of their anisotropy-ductility correlations at ambient temperature. The 3D plastic anisotropy is characterized in each material using compression and tension specimens. The ductility is characterized using smooth and round-notched tensile bars. A micromechanical model is introduced to rationalize the trends in terms of the anisotropy effect on ductility (AED) index, which in the present experiments is tuned via texture manipulations at fixed chemical composition and grain size. The main finding suggests that plastic anisotropy can be engineered to aid ductility, irrespective of loading orientation or stress state triaxiality.

10:10 AM Break

10:30 AM  
Modeling the Effect of Alloying Elements in Magnesium on Deformation Twin Characteristics: M. Arul Kumar1; Irene J Beyerlein1; Ricardo Lebensohn1; Carlos Tome1; 1Los Alamos National Laboratory
    HCP magnesium metals are widely used in different industries due to their low density and high specific strength. Their applicability is restricted due to poor formability and high anisotropy in deformation behavior. The formability of magnesium can be improved by alloying additions and this modification can affect macroscopic plastic anisotropy. Alloying addition can also significantly control the twinning process. In this work we use a crystal plasticity based Fast Fourier Transform model to characterize deformation twinning in different alloy types. In the model, the influence of alloying addition is represented through their effect on the critical resolved shear stress (CRSS) values for all slip and twinning modes. From this study, we build an understanding of the influence of alloying elements on twin growth and twin transmission behavior. A new plastic anisotropy measure is proposed to quantify the effects of alloying elements on some important twinning characteristics in magnesium alloys.

10:50 AM  
Simulating Discrete Twin Evolution in Magnesium Using a Novel Crystal Plasticity Finite Element Model: Jiahao Cheng1; Somnath Ghosh1; 1Johns Hopkins University
    An advanced, image-based crystal plasticity FE model is developed for predicting discrete twin formation and associated heterogeneous deformation in the single and polycrystalline microstructure of Magnesium. Twin formation is sensitive to the underlying microstructure and is responsible for the premature failure of Mg. The physics of nucleation, propagation, and growth of deformation-twins are considered in the CPFE formulation. The twin nucleation model is based on dissociation of sessile dislocations into stable twin loops, while propagation is assumed by layer-by-layer atoms shearing on twin planes and shuffling to reduce the energy barrier. A non-local FE-based computational framework is developed to implement the twin nucleation and propagation laws, which governs the explicit formation of each individual twin. The FE formulation is further enhanced by a multi-time-domain subcycling method to overcome low computational speed issue. The simulation matches satisfactorily with the experiments in the stress strain-response and predicts heterogeneous twin formation with strain localization.

11:10 AM  
The Effect of {10-12} Twin Boundary on the Evolution of Defect Substructure: Fulin Wang1; C.D. Barrett2; K. Hazeli3; K. Molodov4; T. Al-Samman4; A. Oppedal5; D. Molodov4; A. Kontsos6; K.T. Ramesh3; H. El Kadiri5; S.R. Agnew1; 1Department of Materials Science and Engineering, University of Virginia; 2Center for Advanced Vehicular Systems, Mississippi State University; 3Hopkins Extreme Materials Institute, The Johns Hopkins University; 4Institute of Physical Metallurgy and Metal Physics, RWTH Aachen University; 5Department of Mechanical Engineering, Mississippi State University; 6Department of Mechanical Engineering and Mechanics, Drexel University
    Pure Mg single crystals were deformed at room temperature along two orientations in sequence, in order to activate a specific dislocation slip modes followed by {101 ̅2} twinning. The defects in both the matrix and twin crystals were analyzed with a transmission electron microscope (TEM). This study reveals the collective evolution of the defect substructure when a dislocated crystal is “invaded” by a moving twin boundary. It was found that the incorporation of 〈a〉 dislocations by twin boundary results in the formation of 〈c+a〉 dislocations, which demonstrates the broader applicability of the dislocation transmutation mechanism recently confirmed in polycrystalline Mg alloy, AZ31. When primarily [c]-containing defects in the matrix were incorporated by a moving twin boundary, including 〈c+a〉, pure [c] dislocations and I_1 stacking faults, the twin contains homogeneously distributed I_1 stacking faults, which appear to be connected on twin boundary to the faults in the matrix.

11:30 AM  
Zinc Segregation on Interfaces Induced by Severe Plastic Deformation of an Mg-Zn-Y Alloy at Room Temperature: D. Althaf Basha1; Ryoji Sahara1; Hidetoshi Somekawa1; Julian Rosalie2; Alok Singh1; Koichi Tsuchiya1; 1National Institute for Materials Science; 2Erich Schmid Institute of Materials Science, Austrian Academy of Sciences, Austria
    Interfacial segregation has significant effect on the microstructural stability and mechanical properties of an alloy. Here we show interfacial segregation induced by severe plastic deformation (SPD) at room temperature (RT). A Mg-3Zn-0.5Y (at%) alloy in a rolled condition was subjected to high pressure torsion (HPT) by application of 5 GPa load and torsional strain at RT. The microstructure was studied by TEM and STEM. Segregation of zinc was observed on all newly formed interfaces - twins formed by compressive load alone, low angle boundaries formed under torsion, and grain boundaries formed by recrystallization. It is shown by First principles calculations that Zn atoms prefer to segregate to dislocations on low angle boundaries. It is estimated that Zn atoms are transported by SPD induced vacancy flux at RT.