Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent e... more Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent experiments reveal striking differences in the plasticity of bcc nanopillars for tension and compression. Here we present results from molecular dynamics simulations of nanopillars of bcc Fe in tension and compression. We find that a totally different deformation mechanism applies in each cases: dislocation glide in compression and twinning in tension. This difference explains experimentally-observed asymmetry in the nanopillar morphology.
Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent e... more Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent experiments reveal striking differences in the plasticity of bcc nanopillars for tension and compression. Here we present results from molecular dynamics simulations of nanopillars of bcc Fe in tension and compression. We find that a totally different deformation mechanism applies in each cases: dislocation glide in compression and twinning in tension. This difference explains experimentally-observed asymmetry in the nanopillar morphology.
During high speed cutting processes, metals are subject to high strains and strain rates. The dyn... more During high speed cutting processes, metals are subject to high strains and strain rates. The dynamic nature of the deformation during high speed cutting makes it difficult to detect atomic scale deformation mechanisms experimentally. Atomic scale plasticity behaviour is often studied using various micromachining techniques such as micropillar compression testing, nanoindentation, and nanoscratching. However, strain rates in micromachining experiments are far lower than those seen during high speed cutting. Atomistic simulations can be used to study high strain rate plasticity at nanometre length scales. In this thesis, we present results from molecular dynamics simulations of plasticity in nanostructures. Results from simulations of uniaxial strain of both bcc and fcc nanopillars are presented. We find that the outcomes of these uniaxial strain simulations depend sensitively on the initial configurations of the systems. In particular, the choice of crystallographic surfaces on the ...
During high speed cutting processes, metals are subject to high strains and strain rates. The dyn... more During high speed cutting processes, metals are subject to high strains and strain rates. The dynamic nature of the deformation during high speed cutting makes it difficult to detect atomic scale deformation mechanisms experimentally. Atomic scale plasticity behaviour is often studied using various micromachining techniques such as micropillar compression testing, nanoindentation, and nanoscratching. However, strain rates in micromachining experiments are far lower than those seen during high speed cutting. Atomistic simulations can be used to study high strain rate plasticity at nanometre length scales. In this thesis, we present results from molecular dynamics simulations of plasticity in nanostructures. Results from simulations of uniaxial strain of both bcc and fcc nanopillars are presented. We find that the outcomes of these uniaxial strain simulations depend sensitively on the initial configurations of the systems. In particular, the choice of crystallographic surfaces on the ...
We show that alloying with rare earth metals (REMs) can dramatically improve the machineability o... more We show that alloying with rare earth metals (REMs) can dramatically improve the machineability of a range of titanium alloys, even though the REM is not incorporated in the alloy matrix. The mechanism for this is that under cutting, shear bands are formed within which the nano-precipitates of REM are shear mixed. This lowers the melting point such that the mechanism of deformation changes from dislocation mechanism to localised amorphisation and shear softening. The material then fractures along the thin, amorphous shear-band. Outside the shear band, the REM remains as precipitates. The new alloys have similar mechanical properties and biocompatibility to conventional materials.
We show that alloying with rare earth metals (REMs) can dramatically improve the machineability o... more We show that alloying with rare earth metals (REMs) can dramatically improve the machineability of a range of titanium alloys, even though the REM is not incorporated in the alloy matrix. The mechanism for this is that under cutting, shear bands are formed within which the nano-precipitates of REM are shear mixed. This lowers the melting point such that the mechanism of deformation changes from dislocation mechanism to localised amorphisation and shear softening. The material then fractures along the thin, amorphous shear-band. Outside the shear band, the REM remains as precipitates. The new alloys have similar mechanical properties and biocompatibility to conventional materials.
Submitted for the MAR12 Meeting of The American Physical Society High Strain Rate Deformation of ... more Submitted for the MAR12 Meeting of The American Physical Society High Strain Rate Deformation of BCC Materials: Molecular Dynamics Simulations CON HEALY, GRAEME ACKLAND, University of Edinburgh-To improve machining processes, a good understanding of plasticity of bulk metals and at high strain rates is required. During machining processes, it is thought that strain rates of ∼10 6 s −1 are reached. These strain rates are currently not achievable in experimental high strain rate testing techniques. Here we investigate high strain rate deformation of bcc Fe using Molecular Dynamics simulations. Simulations include those of nano-scale single crystal pillars and of a nano-crystalline sample. We show that bcc materials may exhibit different deformation behavior at high strain rates. Under compression at strain rates of ∼10 8 s −1 , nano-scale single crystal pillars may exhibit slip on atypical planes if the direction of maximum resolved shear stress points along that plane. The stress-strain behavior of these pillars is characterized by sudden strain bursts due to slip events. The yield stress required for these slip events can be as high as 2 GPa, much higher than the Peierls stress which is around 25 MPa for an edge dislocation in Fe. The nano-crystalline sample showed stress-strain behavior closer to that of a bulk metal. At high temperatures, pillars exhibit surface premelting. A large portion of the pillar turns amorphous when strained.
Submitted for the MAR12 Meeting of The American Physical Society High Strain Rate Deformation of ... more Submitted for the MAR12 Meeting of The American Physical Society High Strain Rate Deformation of BCC Materials: Molecular Dynamics Simulations CON HEALY, GRAEME ACKLAND, University of Edinburgh-To improve machining processes, a good understanding of plasticity of bulk metals and at high strain rates is required. During machining processes, it is thought that strain rates of ∼10 6 s −1 are reached. These strain rates are currently not achievable in experimental high strain rate testing techniques. Here we investigate high strain rate deformation of bcc Fe using Molecular Dynamics simulations. Simulations include those of nano-scale single crystal pillars and of a nano-crystalline sample. We show that bcc materials may exhibit different deformation behavior at high strain rates. Under compression at strain rates of ∼10 8 s −1 , nano-scale single crystal pillars may exhibit slip on atypical planes if the direction of maximum resolved shear stress points along that plane. The stress-strain behavior of these pillars is characterized by sudden strain bursts due to slip events. The yield stress required for these slip events can be as high as 2 GPa, much higher than the Peierls stress which is around 25 MPa for an edge dislocation in Fe. The nano-crystalline sample showed stress-strain behavior closer to that of a bulk metal. At high temperatures, pillars exhibit surface premelting. A large portion of the pillar turns amorphous when strained.
We describe a student project in electronic speckle pattern interferometry. The project includes ... more We describe a student project in electronic speckle pattern interferometry. The project includes holographic recording of diffraction gratings in thick self-processing photopolymer layers made from off-the-shelf chemicals. The gratings are employed in a simple electronic speckle pattern interferometer to measure in-plane rotation.
Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent e... more Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent experiments reveal striking differences in the plasticity of bcc nanopillars for tension and compression. Here we present results from molecular dynamics simulations of nanopillars of bcc Fe in tension and compression. We find that a totally different deformation mechanism applies in each cases: dislocation glide in compression and twinning in tension. This difference explains experimentally-observed asymmetry in the nanopillar morphology.
ABSTRACTIt is now possible to create perfect crystal nanowires of many metals. The deformation of... more ABSTRACTIt is now possible to create perfect crystal nanowires of many metals. The deformation of such objects requires a good understanding of the processes involved in plasticity at the nanoscale. Isotropic compression of such nanometre scale micropillars is a good model system to understand the plasticity. Here we investigate these phenomena using Molecular Dynamics (MD) simulations of nanometre scale single crystal BCC iron pillars in compression.We find that pillars with large length to width ratio may buckle under high strain rates. The type of buckling behaviour depends sensitively on the boundary conditions used: periodic boundary conditions allow for rotation at top and bottom of the pillar, and result in an S shaped buckle, by contrast fixed boundaries enforce a C shape. Pillars with a length to width ratio closer to that used in experimental micropillar compression studies show deformation behaviour dominated by slip, in agreement with the experiments. For micropillars or...
Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent e... more Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent experiments reveal striking differences in the plasticity of bcc nanopillars for tension and compression. Here we present results from molecular dynamics simulations of nanopillars of bcc Fe in tension and compression. We find that a totally different deformation mechanism applies in each cases: dislocation glide in compression and twinning in tension. This difference explains experimentally-observed asymmetry in the nanopillar morphology.
Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent e... more Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent experiments reveal striking differences in the plasticity of bcc nanopillars for tension and compression. Here we present results from molecular dynamics simulations of nanopillars of bcc Fe in tension and compression. We find that a totally different deformation mechanism applies in each cases: dislocation glide in compression and twinning in tension. This difference explains experimentally-observed asymmetry in the nanopillar morphology.
During high speed cutting processes, metals are subject to high strains and strain rates. The dyn... more During high speed cutting processes, metals are subject to high strains and strain rates. The dynamic nature of the deformation during high speed cutting makes it difficult to detect atomic scale deformation mechanisms experimentally. Atomic scale plasticity behaviour is often studied using various micromachining techniques such as micropillar compression testing, nanoindentation, and nanoscratching. However, strain rates in micromachining experiments are far lower than those seen during high speed cutting. Atomistic simulations can be used to study high strain rate plasticity at nanometre length scales. In this thesis, we present results from molecular dynamics simulations of plasticity in nanostructures. Results from simulations of uniaxial strain of both bcc and fcc nanopillars are presented. We find that the outcomes of these uniaxial strain simulations depend sensitively on the initial configurations of the systems. In particular, the choice of crystallographic surfaces on the ...
During high speed cutting processes, metals are subject to high strains and strain rates. The dyn... more During high speed cutting processes, metals are subject to high strains and strain rates. The dynamic nature of the deformation during high speed cutting makes it difficult to detect atomic scale deformation mechanisms experimentally. Atomic scale plasticity behaviour is often studied using various micromachining techniques such as micropillar compression testing, nanoindentation, and nanoscratching. However, strain rates in micromachining experiments are far lower than those seen during high speed cutting. Atomistic simulations can be used to study high strain rate plasticity at nanometre length scales. In this thesis, we present results from molecular dynamics simulations of plasticity in nanostructures. Results from simulations of uniaxial strain of both bcc and fcc nanopillars are presented. We find that the outcomes of these uniaxial strain simulations depend sensitively on the initial configurations of the systems. In particular, the choice of crystallographic surfaces on the ...
We show that alloying with rare earth metals (REMs) can dramatically improve the machineability o... more We show that alloying with rare earth metals (REMs) can dramatically improve the machineability of a range of titanium alloys, even though the REM is not incorporated in the alloy matrix. The mechanism for this is that under cutting, shear bands are formed within which the nano-precipitates of REM are shear mixed. This lowers the melting point such that the mechanism of deformation changes from dislocation mechanism to localised amorphisation and shear softening. The material then fractures along the thin, amorphous shear-band. Outside the shear band, the REM remains as precipitates. The new alloys have similar mechanical properties and biocompatibility to conventional materials.
We show that alloying with rare earth metals (REMs) can dramatically improve the machineability o... more We show that alloying with rare earth metals (REMs) can dramatically improve the machineability of a range of titanium alloys, even though the REM is not incorporated in the alloy matrix. The mechanism for this is that under cutting, shear bands are formed within which the nano-precipitates of REM are shear mixed. This lowers the melting point such that the mechanism of deformation changes from dislocation mechanism to localised amorphisation and shear softening. The material then fractures along the thin, amorphous shear-band. Outside the shear band, the REM remains as precipitates. The new alloys have similar mechanical properties and biocompatibility to conventional materials.
Submitted for the MAR12 Meeting of The American Physical Society High Strain Rate Deformation of ... more Submitted for the MAR12 Meeting of The American Physical Society High Strain Rate Deformation of BCC Materials: Molecular Dynamics Simulations CON HEALY, GRAEME ACKLAND, University of Edinburgh-To improve machining processes, a good understanding of plasticity of bulk metals and at high strain rates is required. During machining processes, it is thought that strain rates of ∼10 6 s −1 are reached. These strain rates are currently not achievable in experimental high strain rate testing techniques. Here we investigate high strain rate deformation of bcc Fe using Molecular Dynamics simulations. Simulations include those of nano-scale single crystal pillars and of a nano-crystalline sample. We show that bcc materials may exhibit different deformation behavior at high strain rates. Under compression at strain rates of ∼10 8 s −1 , nano-scale single crystal pillars may exhibit slip on atypical planes if the direction of maximum resolved shear stress points along that plane. The stress-strain behavior of these pillars is characterized by sudden strain bursts due to slip events. The yield stress required for these slip events can be as high as 2 GPa, much higher than the Peierls stress which is around 25 MPa for an edge dislocation in Fe. The nano-crystalline sample showed stress-strain behavior closer to that of a bulk metal. At high temperatures, pillars exhibit surface premelting. A large portion of the pillar turns amorphous when strained.
Submitted for the MAR12 Meeting of The American Physical Society High Strain Rate Deformation of ... more Submitted for the MAR12 Meeting of The American Physical Society High Strain Rate Deformation of BCC Materials: Molecular Dynamics Simulations CON HEALY, GRAEME ACKLAND, University of Edinburgh-To improve machining processes, a good understanding of plasticity of bulk metals and at high strain rates is required. During machining processes, it is thought that strain rates of ∼10 6 s −1 are reached. These strain rates are currently not achievable in experimental high strain rate testing techniques. Here we investigate high strain rate deformation of bcc Fe using Molecular Dynamics simulations. Simulations include those of nano-scale single crystal pillars and of a nano-crystalline sample. We show that bcc materials may exhibit different deformation behavior at high strain rates. Under compression at strain rates of ∼10 8 s −1 , nano-scale single crystal pillars may exhibit slip on atypical planes if the direction of maximum resolved shear stress points along that plane. The stress-strain behavior of these pillars is characterized by sudden strain bursts due to slip events. The yield stress required for these slip events can be as high as 2 GPa, much higher than the Peierls stress which is around 25 MPa for an edge dislocation in Fe. The nano-crystalline sample showed stress-strain behavior closer to that of a bulk metal. At high temperatures, pillars exhibit surface premelting. A large portion of the pillar turns amorphous when strained.
We describe a student project in electronic speckle pattern interferometry. The project includes ... more We describe a student project in electronic speckle pattern interferometry. The project includes holographic recording of diffraction gratings in thick self-processing photopolymer layers made from off-the-shelf chemicals. The gratings are employed in a simple electronic speckle pattern interferometer to measure in-plane rotation.
Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent e... more Tension-compression asymmetry is a notable feature of plasticity in bcc single crystals. Recent experiments reveal striking differences in the plasticity of bcc nanopillars for tension and compression. Here we present results from molecular dynamics simulations of nanopillars of bcc Fe in tension and compression. We find that a totally different deformation mechanism applies in each cases: dislocation glide in compression and twinning in tension. This difference explains experimentally-observed asymmetry in the nanopillar morphology.
ABSTRACTIt is now possible to create perfect crystal nanowires of many metals. The deformation of... more ABSTRACTIt is now possible to create perfect crystal nanowires of many metals. The deformation of such objects requires a good understanding of the processes involved in plasticity at the nanoscale. Isotropic compression of such nanometre scale micropillars is a good model system to understand the plasticity. Here we investigate these phenomena using Molecular Dynamics (MD) simulations of nanometre scale single crystal BCC iron pillars in compression.We find that pillars with large length to width ratio may buckle under high strain rates. The type of buckling behaviour depends sensitively on the boundary conditions used: periodic boundary conditions allow for rotation at top and bottom of the pillar, and result in an S shaped buckle, by contrast fixed boundaries enforce a C shape. Pillars with a length to width ratio closer to that used in experimental micropillar compression studies show deformation behaviour dominated by slip, in agreement with the experiments. For micropillars or...
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