Role of grain boundary on the deformation of micropillars

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Deformation of Ti-6Al-4V micro-pillars with different β phase contents

Compressions of Ti-6Al-4V micro-pillars with increasing number of α/β interfaces or β fillets, orientated for prismatic , prismatic, pyramidal , basal and basal slips have been carried out to study the role of α/β interface and its number in the plastic deformation of the alloy. Micro-compression has been used to quantify the effect of the number of interfaces on CRSS for different slip systems. Following that, SEM was used to observe the distribution of shear bands across the pillars while TEM has been used to examine the interaction between dislocations and interfaces assisted with FIB sample preparation. The strengthening effect of α/β interface has been considered in light of the lattice parameter mismatch $\tau_{misfit}$ Koehler stress $\tau_k$ due to shear modulus mismatch, interface stress $\tau_f$ related to the interface energy and interface strain tensor, and $w$ interaction $\tau_w$ that is slip system change from one phase to another. The total CRSS of micropillars was estimated by considering the contributions from α and β phases based on their volume fractions and α/β interface strengthening effect. The interface strengthening estimated for prismatic and slips are 46 MPa and 18 MPa respectively, close to the experimental determined values of 49 MPa and 20 MPa. There is a significant strengthening effect of α/β interface on the CRSS values. Slip band nucleation, formation and distribution are strongly affected by the number of α/β interfaces or β fillets. More shear bands nucleate and form, and their distribution becomes more homogeneous with increasing the number of interfaces. Likely caused by stress and strain localization as well as the dislocation pile-up at interfaces. For the micropillars containing 2 β fillets, the CRSS of prismatic slip is 7-9% higher than that of prismatic slip. For the pillars containing multiple (~10) β fillets, the CRSS value for basal slip, which reaches 699 MPa, is higher than that for basal and slips which are all much higher than the those reported for the micro-pillars without the α/β interface. Pillar size effect on the CRSS value is sensitive to the number of α/β interfaces. The percentage increment of CRSS values from 10 μm to 5 μm sized pillars drops significantly from 14.2% to 3.4% with increasing the number of β fillets in the current work

Modelling and Simulation of Faceted Boundary Structures and Dynamics in FCC Crystalline Materials

Ph.DDOCTOR OF PHILOSOPH

Intrinsic structural transitions of the pyramidal I 〈c+a〉 dislocation in magnesium

The stability of a mixed dislocation on the pyramidal I plane in magnesium is studied using molecular dynamics simulations. The dislocation is metastable and undergoes a thermally-activated transition to either a sessile, basal-dissociated or a sessile basal-dissociated dislocation plus an dislocation. The transition is intrinsic to pure magnesium and occurs with an energy barrier of similar to 0.3 eV. The transformed structure is also consistent with experimental evidence in Ti and Zr, where pyramidal I slip is more prevalent. Enhancing the ductility of magnesium by stabilizing slip on pyramidal I planes thus appears unlikely to be viable

The origins of high hardening and low ductility in magnesium

Magnesium is a lightweight structural metal but it exhibits low ductility-connected with unusual, mechanistically unexplained, dislocation and plasticity phenomena-which makes it difficult to form and use in energy-saving lightweight structures. We employ long-time molecular dynamics simulations utilizing a density-functional-theory-validated interatomic potential, and reveal the fundamental origins of the previously unexplained phenomena. Here we show that the key dislocation (where indicates the magnitude and direction of slip) is metastable on easy-glide pyramidal II planes; we find that it undergoes a thermally activated, stress-dependent transition to one of three lower-energy, basal-dissociated immobile dislocation structures, which cannot contribute to plastic straining and that serve as strong obstacles to the motion of all other dislocations. This transition is intrinsic to magnesium, driven by reduction in dislocation energy and predicted to occur at very high frequency at room temperature, thus eliminating all major dislocation slip systems able to contribute to c-axis strain and leading to the high hardening and low ductility of magnesium. Enhanced ductility can thus be achieved by increasing the time and temperature at which the transition from the easy-glide metastable dislocation to the immobile basal-dissociated structures occurs. Our results provide the underlying insights needed to guide the design of ductile magnesium alloys

Comprehensive first-principles study of stable stacking faults in hcp metals

The plastic deformation in hcp metals is complex, with the associated dislocation core structures and properties not well understood on many slip planes in most hcp metals. A first step in establishing the dislocation properties is to examine the stable stacking fault energy and its structure on relevant slip planes. However, this has been perplexing in the hcp structure due to additional in-plane displacements on both sides of the slip plane. Here, density functional theory guided by crystal symmetry analysis is used to study all relevant stable stacking faults in 6 hcp metals (Mg, Ti, Zr, Re, Zn, Cd). Specially, the stable stacking fault energy, position, and structure on the Basal, Prism I and II, Pyramidal I and II planes are determined using all-periodic supercells with full atomic relaxation. All metals show similar stacking fault position and structure as dictated by crystal symmetry, but the associated stacking fault energy, being governed by the atomic bonding, differs significantly among them. Stacking faults on all the slip planes except the Basal plane show substantial out-of-plane displacements while stacking faults on the Prism II, Pyramidal I and II planes show additional in-plane displacements, all extending to multiple atom layers. The in-plane displacements are not captured in the standard computational approach for stacking faults, and significant differences are shown in the energies of such stacking faults between the standard approach and fully-relaxed case. The existence of well-defined stable stacking fault on the Pyramidal planes suggests zonal dislocations are unlikely. Calculations on the equilibrium partial separation further suggests dissociation into three partials on the Pyramidal I plane is unlikely and (c) dissociation on Prism planes is unlikely to be stable against climb-dissociation onto the Basal planes in these metals. (C) 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Flaw-driven Failure in Nanostructures

Understanding failure in nanomaterials is critical for the design of reliable structural materials and small-scale devices that have components or microstructural elements at the nanometer length scale. No consensus exists on the effect of flaws on fracture in bulk nanostructured materials or in nanostructures. Proposed theories include nanoscale flaw tolerance and maintaining macroscopic fracture relationships at the nanoscale with virtually no experimental support. We explore fracture mechanisms in nanomaterials via nanomechanical experiments on nanostructures with pre-fabricated surface flaws in combination with molecular dynamics simulations. Nanocrystalline Pt cylinders with diameters of ~120 nm with intentionally introduced surface notches were created using a template-assisted electroplating method and tested in uniaxial tension in in-situ SEM. Experiments demonstrate that 8 out of 12 samples failed at the notches and that tensile failure strengths were ~1.8 GPa regardless of whether failure occurred at or away from the flaw. These findings suggest that failure location was sensitive to the presence of flaws, while strength was flaw-insensitive. Molecular dynamics simulations support these observations and show that incipient plastic deformation commences via nucleation and motion of dislocations in concert with grain boundary sliding. We postulate that such local plasticity reduces stress concentration ahead of the flaw to levels comparable with the strengths of intrinsic microstructural features like grain boundary triple junctions, a phenomenon unique to nano-scale solids that contain an internal microstructural energy landscape. This mechanism causes failure to occur at the weakest link, be it an internal inhomogeneity or a surface feature with a high local stress

Atomistic modelling of all dislocations and twins in HCP and BCC Ti

Ti exhibits complex plastic deformation controlled by active dislocation and twinning systems. Understandings on dislocation cores and twin interfaces are currently not complete or quantitative, despite extensive experimental and simulation studies. Here, we determine all the core and twin interface properties in both HCP and BCC Ti using a Deep Potential (DP) and DFT. We determine the core structures, critical resolved shear stresses and mobilities of , , dislocations in HCP and /2 dislocations in BCC Ti. The slip consists of slow core migration on pyramidal-I planes and fast migration on prism-planes, and is kinetically limited by cross-slips among them. This behaviour is consistent with "locking-unlocking" phenomena in TEM and is likely an intrinsic property. Large-scale DFT calculations provide a peek at the screw core and glide behaviour, which is further quantified using DP-Ti. The screw is unstable on pyramidal-II planes. The mixed is nearly sessile on pyramidal-I planes, consistent with observations of long dislocations in this orientation. The edge and mixed are unstable against a pyramidal-to-basal (PB) transition and become sessile at high temperatures, corroborate the difficulties in -axis compression of Ti. Finally, in BCC Ti, the /2 screw has a degenerate core with average glide on {112} planes; the /2 edge and mixed dislocations have non-dissociated cores on {110} planes. This work paints a self-consistent, complete picture on all dislocations in Ti, rationalises previous experimental observations and points to future HRTEM examinations of unusual dislocations such as the mixed and PB transformed cores