Megan Canavan | Trinity College Dublin (original) (raw)

Papers by Megan Canavan

Research paper thumbnail of Mechanism of stress relaxation and phase transformation in additively manufactured Ti-6Al-4V via in situ high temperature XRD and TEM analyses

Acta Materialia, 2020

Additive manufacturing is being increasingly used in the fabrication of Ti-6Al-4V parts to combin... more Additive manufacturing is being increasingly used in the fabrication of Ti-6Al-4V parts to combine excellent mechanical properties and biocompatibility with high precision. Unfortunately, due to the build-up of thermal residual stresses and the formation of martensitic structure across a wide range of typical processing conditions, it is generally necessary to use a post-thermal treatment to achieve superior mechanical performance. This investigation aims to obtain a deeper understanding of the micro/nanostructural evolution (a 0 martensite phase decomposition), accounting for the kinetics of phase transformation during the heat treatment of 3D-printed Ti-6Al-4V alloy. As the mechanism of phase transformation and stress relaxation is still ambiguous, in this study the changes in crystal lattice, phase, composition and lattice strain were investigated up to 1000°C using both in situ high temperature X-ray diffraction (XRD) and transmission electron microscopy (TEM). Based on the result a mechanism of phase transformation is proposed, via the accommodation/substitution of Al, V and Ti atoms in the crystal lattice. The proposed mechanism is supported based on elemental concentration changes during heat treatment, in combination with changes in crystal structure observed using the high temperature XRD and TEM measurements. This study provides a deeper understanding on the mechanism of phase transformation through martensitic decomposition, as well as a deeper understanding of the influence of post-thermal treatment conditions on the alloy's crystal structure.

Research paper thumbnail of Influence of temperature on morphological and optical properties of MoS2 layers as grown based on solution processed precursor

Thin Solid Films, 2018

 Novel route for synthesis of solution processed MoS 2 thin films using Mo-precursor.  Characte... more  Novel route for synthesis of solution processed MoS 2 thin films using Mo-precursor.  Characterization and comparison of synthesized MoS 2 film to geological MoS 2 crystal.  Effect of temperature on the morphology and crystalline properties of the MoS 2 film.  Low-cost, non-vacuum and facile synthesis method apt for large area film growth.

Research paper thumbnail of In-situ TEM Analyses over FIB Lamellae - Investigating High Temperature Conversion of Solution Processed Mo-precursor to MoS2 Semiconductor Films

Microscopy and Microanalysis, 2017

Research paper thumbnail of Atomic scale dynamics of a solid state chemical reaction directly determined by annular dark-field electron microscopy

Scientific Reports, 2014

Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in ma... more Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in materials science and developing a capability to observe the mechanisms of such processes on the atomic scale can offer new insights across a wide range of materials systems. Aberration correction in scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging at significantly reduced beam energies and electron doses. It has also made possible the quantitative determination of the composition and occupancy of atomic columns using the atomic number (Z)-contrast annular dark-field (ADF) imaging available in STEM. Here we combine these benefits to record the motions and quantitative changes in the occupancy of individual atomic columns during a solid-state chemical reaction in manganese oxides. These oxides are of great interest for energy-storage applications such as for electrode materials in pseudocapacitors. We employ rapid scanning in STEM to both drive and dire...

Research paper thumbnail of Novel in-situ lamella fabrication technique for in-situ TEM

Ultramicroscopy, 2018

In-situ transmission electron microscopy is rapidly emerging as the premier technique for charact... more In-situ transmission electron microscopy is rapidly emerging as the premier technique for characterising materials in a dynamic state on the atomic scale. The most important aspect of in-situ studies is specimen preparation. Specimens must be electron transparent and representative of the material in its operational state, amongst others. Here, a novel fabrication technique for the facile preparation of lamellae for in-situ transmission electron microscopy experimentation using focused ion beam milling is developed. This method involves the use of rotating microgrippers during the lift-out procedure, as opposed to the traditional micromanipulator needle and platinum weld. Using rotating grippers, and a unique adhesive substance, lamellae are mounted onto a MEMS device for in-situ TEM annealing experiments. We demonstrate how this technique can be used to avoid platinum deposition as well as minimising damage to the MEMS device during the thinning process. Our technique is both a cost effective and readily implementable alternative to the current generation of preparation methods for in-situ liquid, electrical, mechanical and thermal experimentation within the TEM as well as traditional cross-sectional lamella preparation.

Research paper thumbnail of Atomic scale dynamics of a solid state chemical reaction directly determined by annular dark-field electron microscopy

Scientific Reports, 2014

Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in ma... more Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in materials science, and developing a capability to observe the mechanisms of such processes on the atomic scale can offer new insights across a wide range of materials systems. Aberration correction in scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging at significantly reduced beam energies and electron doses. It has also made possible the quantitative determination of the composition and occupancy of atomic columns using the atomic number (Z)-contrast annular dark-field (ADF) imaging available in STEM. Here we combine these benefits to record the motions and quantitative changes in the occupancy of individual atomic columns during a solid-state chemical reaction in manganese oxides. These oxides are of great interest for energy-storage applications such as for electrode materials in pseudocapacitors. We employ rapid scanning in STEM to both drive and directly observe the atomic scale dynamics behind the transformation of Mn 3 O 4 into MnO. The results demonstrate we now have the experimental capability to understand the complex atomic mechanisms involved in phase changes and solid state chemical reactions. C hanges in stoichiometry lie at the heart of many materials applications. For instance, batteries commonly rely on the exchange of charged ions between anode and cathode to change their stoichiometries to either store or output energy. Diffraction techniques provide useful information on changes to the average structure, but to resolve the mechanisms through which such changes occur requires local information. The majority of high-resolution in situ experiments are performed in conventional TEM instruments with bright-field imaging 1 . Single-shot dynamic TEM 2 (DTEM) provides phenomenal nanosecond temporal resolution, but changes seen in such phase contrast images often cannot be unambiguously interpreted because of the relatively complicated coherent nature of image formation.

Research paper thumbnail of Mechanism of stress relaxation and phase transformation in additively manufactured Ti-6Al-4V via in situ high temperature XRD and TEM analyses

Acta Materialia, 2020

Additive manufacturing is being increasingly used in the fabrication of Ti-6Al-4V parts to combin... more Additive manufacturing is being increasingly used in the fabrication of Ti-6Al-4V parts to combine excellent mechanical properties and biocompatibility with high precision. Unfortunately, due to the build-up of thermal residual stresses and the formation of martensitic structure across a wide range of typical processing conditions, it is generally necessary to use a post-thermal treatment to achieve superior mechanical performance. This investigation aims to obtain a deeper understanding of the micro/nanostructural evolution (a 0 martensite phase decomposition), accounting for the kinetics of phase transformation during the heat treatment of 3D-printed Ti-6Al-4V alloy. As the mechanism of phase transformation and stress relaxation is still ambiguous, in this study the changes in crystal lattice, phase, composition and lattice strain were investigated up to 1000°C using both in situ high temperature X-ray diffraction (XRD) and transmission electron microscopy (TEM). Based on the result a mechanism of phase transformation is proposed, via the accommodation/substitution of Al, V and Ti atoms in the crystal lattice. The proposed mechanism is supported based on elemental concentration changes during heat treatment, in combination with changes in crystal structure observed using the high temperature XRD and TEM measurements. This study provides a deeper understanding on the mechanism of phase transformation through martensitic decomposition, as well as a deeper understanding of the influence of post-thermal treatment conditions on the alloy's crystal structure.

Research paper thumbnail of Influence of temperature on morphological and optical properties of MoS2 layers as grown based on solution processed precursor

Thin Solid Films, 2018

 Novel route for synthesis of solution processed MoS 2 thin films using Mo-precursor.  Characte... more  Novel route for synthesis of solution processed MoS 2 thin films using Mo-precursor.  Characterization and comparison of synthesized MoS 2 film to geological MoS 2 crystal.  Effect of temperature on the morphology and crystalline properties of the MoS 2 film.  Low-cost, non-vacuum and facile synthesis method apt for large area film growth.

Research paper thumbnail of In-situ TEM Analyses over FIB Lamellae - Investigating High Temperature Conversion of Solution Processed Mo-precursor to MoS2 Semiconductor Films

Microscopy and Microanalysis, 2017

Research paper thumbnail of Atomic scale dynamics of a solid state chemical reaction directly determined by annular dark-field electron microscopy

Scientific Reports, 2014

Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in ma... more Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in materials science and developing a capability to observe the mechanisms of such processes on the atomic scale can offer new insights across a wide range of materials systems. Aberration correction in scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging at significantly reduced beam energies and electron doses. It has also made possible the quantitative determination of the composition and occupancy of atomic columns using the atomic number (Z)-contrast annular dark-field (ADF) imaging available in STEM. Here we combine these benefits to record the motions and quantitative changes in the occupancy of individual atomic columns during a solid-state chemical reaction in manganese oxides. These oxides are of great interest for energy-storage applications such as for electrode materials in pseudocapacitors. We employ rapid scanning in STEM to both drive and dire...

Research paper thumbnail of Novel in-situ lamella fabrication technique for in-situ TEM

Ultramicroscopy, 2018

In-situ transmission electron microscopy is rapidly emerging as the premier technique for charact... more In-situ transmission electron microscopy is rapidly emerging as the premier technique for characterising materials in a dynamic state on the atomic scale. The most important aspect of in-situ studies is specimen preparation. Specimens must be electron transparent and representative of the material in its operational state, amongst others. Here, a novel fabrication technique for the facile preparation of lamellae for in-situ transmission electron microscopy experimentation using focused ion beam milling is developed. This method involves the use of rotating microgrippers during the lift-out procedure, as opposed to the traditional micromanipulator needle and platinum weld. Using rotating grippers, and a unique adhesive substance, lamellae are mounted onto a MEMS device for in-situ TEM annealing experiments. We demonstrate how this technique can be used to avoid platinum deposition as well as minimising damage to the MEMS device during the thinning process. Our technique is both a cost effective and readily implementable alternative to the current generation of preparation methods for in-situ liquid, electrical, mechanical and thermal experimentation within the TEM as well as traditional cross-sectional lamella preparation.

Research paper thumbnail of Atomic scale dynamics of a solid state chemical reaction directly determined by annular dark-field electron microscopy

Scientific Reports, 2014

Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in ma... more Dynamic processes, such as solid-state chemical reactions and phase changes, are ubiquitous in materials science, and developing a capability to observe the mechanisms of such processes on the atomic scale can offer new insights across a wide range of materials systems. Aberration correction in scanning transmission electron microscopy (STEM) has enabled atomic resolution imaging at significantly reduced beam energies and electron doses. It has also made possible the quantitative determination of the composition and occupancy of atomic columns using the atomic number (Z)-contrast annular dark-field (ADF) imaging available in STEM. Here we combine these benefits to record the motions and quantitative changes in the occupancy of individual atomic columns during a solid-state chemical reaction in manganese oxides. These oxides are of great interest for energy-storage applications such as for electrode materials in pseudocapacitors. We employ rapid scanning in STEM to both drive and directly observe the atomic scale dynamics behind the transformation of Mn 3 O 4 into MnO. The results demonstrate we now have the experimental capability to understand the complex atomic mechanisms involved in phase changes and solid state chemical reactions. C hanges in stoichiometry lie at the heart of many materials applications. For instance, batteries commonly rely on the exchange of charged ions between anode and cathode to change their stoichiometries to either store or output energy. Diffraction techniques provide useful information on changes to the average structure, but to resolve the mechanisms through which such changes occur requires local information. The majority of high-resolution in situ experiments are performed in conventional TEM instruments with bright-field imaging 1 . Single-shot dynamic TEM 2 (DTEM) provides phenomenal nanosecond temporal resolution, but changes seen in such phase contrast images often cannot be unambiguously interpreted because of the relatively complicated coherent nature of image formation.