Global ETD Search
Search theses and dissertations gathered from participating repositories worldwide. Every result links back to the library that holds it. No account is needed.
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Showing 1 to 20 of 20 for “"Twin Boundary"”.
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Pulsed magnetic field-induced twin boundary motion on Ni-Mn-Ga
… depends on the position and number of mobile twin-boundaries in the crystal. The delay is maximum for a single twin-boundary moving from the fixed to the free end of the crystal. For several twin-boundaries distributed uniformly throughout the crystal the delay is shorter. The peak …
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Magnetic Microstructure and Actuation Dynamics of NiMnGa Magnetic Shape Memory Materials
… microscopy is applied to investigate the twin boundary and magnetic domain wall motion in bulk NiMnGa single crystals. Surface magnetic domain patterns on adjacent sides of bulk crystals are revealed for the first time providing comprehensive information about the domain arrangement inside …
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A first principles study of defects in titanium: interaction of twin boundaries with dislocations and oxygen interstitials
Interaction between gliding dislocations and twin boundaries affects the plastic deformation of hcp metals such as titanium. In addition, oxygen greatly affects both strength and twinning in titanium. Predictive models of strength and twinning rely on understanding of the underlying atomic scale …
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Understanding and development of combined acoustic and magnetic actuation of Ni₂MnGa single crystals
… field of 2 to 4 kOe must be overcome to initiate twin boundary motion and a larger field is required to achieve full actuation. The operating window of the stress output from FSMA actuators is narrow and limited to the range between 0.5 and 2 MPa. Outside the operating range, the strain output …
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Fabrication and characterization of Ni-Mn-Ga ferromagnetic shape-memory alloy composites
… strain effect in Ni-Mn-Ga FSMAs is a result of twin boundary motion in the martensite phase, and can be induced by either field or stress. Most Ni-Mn-Ga FSMA research so far has focused on actuation in single crystals. However, the mechanical loss inherent to twin boundary motion also makes this …
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Investigating intergranular fracture in nickel via atomistic simulations
… mechanisms. For fracture along coherent twin boundary with the crack front along the [112] direction, no bond breaking is observed and crack advance is solely attributed to the slip of atoms at its tip due to the emission of dislocations. The dislocation process leads to the blunting of …
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Modeling of twinning as a deformation mechanism in iron (bcc) crystals and iron-based alloys
We develop an analytical expression for twin nucleation stress in bcc metals considering GPFE (generalized planar fault energy) and the dislocations bounding the twin nucleus. We minimize the total energy to predict the twinning stress relying only on parameters that are obtained through atomistic …
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Magneto-mechanical properties and applications of Ni-Mn-Ga ferromagnetic shape memory alloy
… crystal structure with a cla ratio of 1.3 3. Twinning can occur on the { 112} planes. A twin boundary in such a crystal should cause a change of direction in the crystal axes of 7 .1 degrees. This angle was measured at 6.5 degrees in a single crystal of Ni-Mn-Ga. Twin angles and strains in the …
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Uncovering the link between twin-twin interactions and damage nucleation in an (α+β) Ti alloy
… these damage incidents occurring along {10̅12} twin-twin boundaries with a ~60° misorientation. The {10̅12} twinning mode is only activated in the localized neck, and twin activation is strongly dependent on initial crystallographic texture. The twinned domains are rotated such that prismatic …
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Acoustic assisted actuation of Ni-Mn-Ga ferromagnetic shape memory alloys
… in the FSMA that resolves as a shear across the twin-boundaries and aids the twin boundary in overcoming defect-related obstacles. The FSMA shows increased strain and a reduction in threshold field. The effect is most pronounced for crystals showing large initial threshold fields which are …
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An advanced perspective on twin growth in nickel-titanium
… approach to investigate the attributes of twinning in Nickel-Titanium (NiTi) shape memory alloy. In particular, atomistic mechanisms of twin growth in two different phases of NiTi are characterized with barrier energy in this thesis. In addition, energetics of dislocation twin boundary …
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An analysis of ferromagnetic shape memory alloys produced by melt spinning, sputtering and pulsed laser deposition.
… of Ni2MnGa at room temperature. The mechanism of twin boundary motion is understood to be responsible for the strain in FSMA’s and theoretically allows for a full crystal lattice distortion strain, which is 10% in the case of orthorhombic martensites and 5-6% in the case of tetragonal martensites. …
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Design and characterization of acoustic pulse shape memory alloy actuators
… asymmetric acoustic pulses are used to drive twin boundary motion. Experimental actuators were developed using a combination of Ni-Mn-Ga FSMA single crystals and a piezoelectric stack actuator. In bi-directional actuation without load, strains of over 3% were achieved using repeated pulses (at …
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On the role of lattice defects interactions on strain hardening: A study from discrete dislocation dynamics to crystal plasticity modelling
… thesis focuses on the effects of slip-slip, slip-twin, and slip-precipitates interactions on strain hardening, with the intent of developing comprehensive modelling capabilities enabling to investigate unit processes and their collective effects up to the macroscopic response. To this end, the …
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ON DEFORMATION STABILITY AND DUCTILE DAMAGE IN NANOTWINNED METALS
… of this thesis is to understand the role of twin boundaries (TBs) in the deformation and failure characteristics of nanotwinned (NT) materials. To that end, we implement a crystal plasticity finite element framework for nanotwinned FCC metals, which accounts for slip gradients that are …
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A Novel Shape Memory Behavior of Single-crystalline Metal Nanowires
… Only FCC metals with higher tendency for twinning (such as Cu, Au, Ni) show the shape memory because twinning leads to the reversible lattice reorientation. On the other hand, FCC metals with low likelihood of twinning (such as Al) do not show shape memory because these wires deforms via …
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Investigating the Effect of Austenite Grain Size and Grain Boundary Character on Deformation Twinning Behavior in A High-Manganese TWIP Steel: A TEM In-Situ Deformation Study
… (UFG) high manganese austenitic twinning-induced plasticity (TWIP) steels have been found to overcome the strength-ductility trade-off but their underlying mechanism of discontinuous yielding behavior has not been well understood. In this study, our systematic TEM characterization …
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Constitutive modeling of Cu-Al-Ni shape memory alloys
… to the other as they cross the moving phase boundary. (A martensitic phase can exist in the form of many "variants", and an interface between co-existing variants is a twin boundary.) The constitutive modeling of such materials is made difficult by the inherent anisotropic nature of such …
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Modeling mechanical properties – linking atomistics to continuum
… energy Co-33%Ni alloys, (b) polycrystalline nano-twinned Ni-x%Co alloys (x = 1, 1.62, 2.9 and 5.6), and (c) equiatomic NiTi shape memory alloys. The reason these materials are chosen as candidates for study is that these materials have very distinct microstructure and mechanical properties. The …
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On the fracture and fatigue crack growth of thin sheets of nanocrystalline metal alloys
… grain structure and high density of nanotwins as shown with extensive transmission electron microscopy. The crack growth rate exponent is less than 1.5 and the growth rates are finite stress intensity ranges as high as 100. We relate the fatigue crack growth resistance in the Ni-Co to …