The Experts below are selected from a list of 294 Experts worldwide ranked by ideXlab platform
Akihiko Chiba - One of the best experts on this subject based on the ideXlab platform.
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hot forging characteristic of ti 5al 5v 5mo 3cr alloy with single metastable β microstructure
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014Co-Authors: Hiroaki Matsumoto, Masami Kitamura, Yunping Li, Yuichiro Koizumi, Akihiko ChibaAbstract:Abstract Hot deFormation characteristic of Ti–5Al–5V–5Mo–3Cr alloy with a starting microstructure of an equiaxed single β microstructure is examined in relation to microstructural evolution and the result according to processing map technique. In testing at 700 °C, Subgrain Formation is dominant at higher strain rate, while superplasticity occurs at lower strain rate. Herein, dynamic α precipitation from supersaturated β phase occurs during deFormation and affects the flow behavior. In testing at and above 800 °C, dynamic recovery (DRV) is dominant and continuous dynamic recrystallization (CDRX) also occurs especially in the vicinity of boundaries of prior-β-grains. There are three domains having an optimized power dissipation efficiency (ranging from 40% to 50%) in processing map. These three domains are reasonably explained in relation to microstructural conversion of frequent activations of grain boundary sliding, dynamic recovery and simultaneous occurrence of dynamic recovery and continuous dynamic recrystallization.
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Hot forging characteristic of Ti–5Al–5V–5Mo–3Cr alloy with single metastable β microstructure
Materials Science and Engineering: A, 2014Co-Authors: Hiroaki Matsumoto, Masami Kitamura, Yuichiro Koizumi, Akihiko ChibaAbstract:Abstract Hot deFormation characteristic of Ti–5Al–5V–5Mo–3Cr alloy with a starting microstructure of an equiaxed single β microstructure is examined in relation to microstructural evolution and the result according to processing map technique. In testing at 700 °C, Subgrain Formation is dominant at higher strain rate, while superplasticity occurs at lower strain rate. Herein, dynamic α precipitation from supersaturated β phase occurs during deFormation and affects the flow behavior. In testing at and above 800 °C, dynamic recovery (DRV) is dominant and continuous dynamic recrystallization (CDRX) also occurs especially in the vicinity of boundaries of prior-β-grains. There are three domains having an optimized power dissipation efficiency (ranging from 40% to 50%) in processing map. These three domains are reasonably explained in relation to microstructural conversion of frequent activations of grain boundary sliding, dynamic recovery and simultaneous occurrence of dynamic recovery and continuous dynamic recrystallization.
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frequent occurrence of discontinuous dynamic recrystallization in ti 6al 4v alloy with α martensite starting microstructure
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2013Co-Authors: Hiroaki Matsumoto, Liu Bin, Sanghak Lee, Yoshiki Ono, Akihiko ChibaAbstract:The microstructural conversion mechanism in an α′ martensite starting microstructure during hot deFormation (at 973 K (700 °C)-10 s−1) of the Ti-6Al-4V alloy is studied through detailed microstructural observations, kinetic analysis of deFormation in the microstructure, and various theoretical models. After compressing the α′ starting microstructure at 973 K (700 °C)-10 s−1 and at a height strain of 0.8, it is observed that the α′ starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 μm and a high fraction of high-angle grain boundaries. At the initial stage of deFormation, Subgrain Formation in martensite variants and the Formation of new grains with high-angle boundaries at interfaces of martensite variants, and \( \{ 10\bar{1}1\} \) twins are dominant. On increasing the height strain to 0.8, discontinuous dynamic recrystallization (DDRX) along with heterogeneous nucleation and fragmentation of grains with high-angle boundaries becomes dominant. In contrast, in the case of an (α + β) starting microstructure, continuous dynamic recrystallization (CDRX) is dominant throughout the deFormation process. Thus, we found that DDRX becomes dominant by changing the starting microstructure from the conventional (α + β) to the acicular α′ martensite one. This behavior of the α′ martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and \( \{ 10\bar{1}1\} \) twins, and the high-speed grain fragmentation along with Subgrain Formation in the α′ starting microstructure during the initial stage of deFormation.
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Frequent Occurrence of Discontinuous Dynamic Recrystallization in Ti-6Al-4V Alloy with α ′ Martensite Starting Microstructure
Metallurgical and Materials Transactions A, 2013Co-Authors: Hiroaki Matsumoto, Liu Bin, Sanghak Lee, Yoshiki Ono, Akihiko ChibaAbstract:The microstructural conversion mechanism in an α′ martensite starting microstructure during hot deFormation (at 973 K (700 °C)-10 s−1) of the Ti-6Al-4V alloy is studied through detailed microstructural observations, kinetic analysis of deFormation in the microstructure, and various theoretical models. After compressing the α′ starting microstructure at 973 K (700 °C)-10 s−1 and at a height strain of 0.8, it is observed that the α′ starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 μm and a high fraction of high-angle grain boundaries. At the initial stage of deFormation, Subgrain Formation in martensite variants and the Formation of new grains with high-angle boundaries at interfaces of martensite variants, and $$ \{ 10\bar{1}1\} $$ twins are dominant. On increasing the height strain to 0.8, discontinuous dynamic recrystallization (DDRX) along with heterogeneous nucleation and fragmentation of grains with high-angle boundaries becomes dominant. In contrast, in the case of an (α + β) starting microstructure, continuous dynamic recrystallization (CDRX) is dominant throughout the deFormation process. Thus, we found that DDRX becomes dominant by changing the starting microstructure from the conventional (α + β) to the acicular α′ martensite one. This behavior of the α′ martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and $$ \{ 10\bar{1}1\} $$ twins, and the high-speed grain fragmentation along with Subgrain Formation in the α′ starting microstructure during the initial stage of deFormation.
Hiroaki Matsumoto - One of the best experts on this subject based on the ideXlab platform.
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hot forging characteristic of ti 5al 5v 5mo 3cr alloy with single metastable β microstructure
Materials Science and Engineering A-structural Materials Properties Microstructure and Processing, 2014Co-Authors: Hiroaki Matsumoto, Masami Kitamura, Yunping Li, Yuichiro Koizumi, Akihiko ChibaAbstract:Abstract Hot deFormation characteristic of Ti–5Al–5V–5Mo–3Cr alloy with a starting microstructure of an equiaxed single β microstructure is examined in relation to microstructural evolution and the result according to processing map technique. In testing at 700 °C, Subgrain Formation is dominant at higher strain rate, while superplasticity occurs at lower strain rate. Herein, dynamic α precipitation from supersaturated β phase occurs during deFormation and affects the flow behavior. In testing at and above 800 °C, dynamic recovery (DRV) is dominant and continuous dynamic recrystallization (CDRX) also occurs especially in the vicinity of boundaries of prior-β-grains. There are three domains having an optimized power dissipation efficiency (ranging from 40% to 50%) in processing map. These three domains are reasonably explained in relation to microstructural conversion of frequent activations of grain boundary sliding, dynamic recovery and simultaneous occurrence of dynamic recovery and continuous dynamic recrystallization.
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Hot forging characteristic of Ti–5Al–5V–5Mo–3Cr alloy with single metastable β microstructure
Materials Science and Engineering: A, 2014Co-Authors: Hiroaki Matsumoto, Masami Kitamura, Yuichiro Koizumi, Akihiko ChibaAbstract:Abstract Hot deFormation characteristic of Ti–5Al–5V–5Mo–3Cr alloy with a starting microstructure of an equiaxed single β microstructure is examined in relation to microstructural evolution and the result according to processing map technique. In testing at 700 °C, Subgrain Formation is dominant at higher strain rate, while superplasticity occurs at lower strain rate. Herein, dynamic α precipitation from supersaturated β phase occurs during deFormation and affects the flow behavior. In testing at and above 800 °C, dynamic recovery (DRV) is dominant and continuous dynamic recrystallization (CDRX) also occurs especially in the vicinity of boundaries of prior-β-grains. There are three domains having an optimized power dissipation efficiency (ranging from 40% to 50%) in processing map. These three domains are reasonably explained in relation to microstructural conversion of frequent activations of grain boundary sliding, dynamic recovery and simultaneous occurrence of dynamic recovery and continuous dynamic recrystallization.
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frequent occurrence of discontinuous dynamic recrystallization in ti 6al 4v alloy with α martensite starting microstructure
Metallurgical and Materials Transactions A-physical Metallurgy and Materials Science, 2013Co-Authors: Hiroaki Matsumoto, Liu Bin, Sanghak Lee, Yoshiki Ono, Akihiko ChibaAbstract:The microstructural conversion mechanism in an α′ martensite starting microstructure during hot deFormation (at 973 K (700 °C)-10 s−1) of the Ti-6Al-4V alloy is studied through detailed microstructural observations, kinetic analysis of deFormation in the microstructure, and various theoretical models. After compressing the α′ starting microstructure at 973 K (700 °C)-10 s−1 and at a height strain of 0.8, it is observed that the α′ starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 μm and a high fraction of high-angle grain boundaries. At the initial stage of deFormation, Subgrain Formation in martensite variants and the Formation of new grains with high-angle boundaries at interfaces of martensite variants, and \( \{ 10\bar{1}1\} \) twins are dominant. On increasing the height strain to 0.8, discontinuous dynamic recrystallization (DDRX) along with heterogeneous nucleation and fragmentation of grains with high-angle boundaries becomes dominant. In contrast, in the case of an (α + β) starting microstructure, continuous dynamic recrystallization (CDRX) is dominant throughout the deFormation process. Thus, we found that DDRX becomes dominant by changing the starting microstructure from the conventional (α + β) to the acicular α′ martensite one. This behavior of the α′ martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and \( \{ 10\bar{1}1\} \) twins, and the high-speed grain fragmentation along with Subgrain Formation in the α′ starting microstructure during the initial stage of deFormation.
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Frequent Occurrence of Discontinuous Dynamic Recrystallization in Ti-6Al-4V Alloy with α ′ Martensite Starting Microstructure
Metallurgical and Materials Transactions A, 2013Co-Authors: Hiroaki Matsumoto, Liu Bin, Sanghak Lee, Yoshiki Ono, Akihiko ChibaAbstract:The microstructural conversion mechanism in an α′ martensite starting microstructure during hot deFormation (at 973 K (700 °C)-10 s−1) of the Ti-6Al-4V alloy is studied through detailed microstructural observations, kinetic analysis of deFormation in the microstructure, and various theoretical models. After compressing the α′ starting microstructure at 973 K (700 °C)-10 s−1 and at a height strain of 0.8, it is observed that the α′ starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 μm and a high fraction of high-angle grain boundaries. At the initial stage of deFormation, Subgrain Formation in martensite variants and the Formation of new grains with high-angle boundaries at interfaces of martensite variants, and $$ \{ 10\bar{1}1\} $$ twins are dominant. On increasing the height strain to 0.8, discontinuous dynamic recrystallization (DDRX) along with heterogeneous nucleation and fragmentation of grains with high-angle boundaries becomes dominant. In contrast, in the case of an (α + β) starting microstructure, continuous dynamic recrystallization (CDRX) is dominant throughout the deFormation process. Thus, we found that DDRX becomes dominant by changing the starting microstructure from the conventional (α + β) to the acicular α′ martensite one. This behavior of the α′ martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and $$ \{ 10\bar{1}1\} $$ twins, and the high-speed grain fragmentation along with Subgrain Formation in the α′ starting microstructure during the initial stage of deFormation.
Kw Siu - One of the best experts on this subject based on the ideXlab platform.
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Oscillation-induced softening in copper and molybdenum from nano- to micro-length scales
Materials Science and Engineering: A, 2013Co-Authors: Kw Siu, Alfonso H.w. NganAbstract:The fact that the application of a simultaneous oscillatory stress can lead to significant reductions in the quasi-static stress required to sustain deFormation has found a wide range of industrial applications. Recently, we discovered that, in addition to the widely believed effects of stress superposition, the oscillation-induced softening in aluminium is an intrinsic effect associated with enhanced dislocation annihilation and Subgrain Formation arising from the simultaneous oscillatory stress. However, such intrinsic effects have not been proven as a general phenomenon for other metals. In this study, macroscopic and nano-indentation were performed on copper and molybdenum. The results show that the simultaneous application of oscillatory stresses can lower the hardness of these samples. EBSD and TEM observations show that Subgrain Formation and reduction in dislocation density generally occurred when stress oscillations were applied. These suggest that the intrinsic oscillation-induced effects of softening and dislocation annihilation are a rather general phenomenon occurring in metals with different stacking fault energies and crystal structures.
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new insight on acoustoplasticity ultrasonic irradiation enhances Subgrain Formation during deFormation
International Journal of Plasticity, 2011Co-Authors: Kw Siu, Alfonso H.w. Ngan, I.p. JonesAbstract:Many industrial applications make use of ultrasonic vibration to soften metals. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deFormation resistance or mechanism is assumed to be unaltered by the ultrasound. In this study, indentation experiments performed on aluminum samples simultaneously excited by ultrasound reveal that the latter intrinsically alters the deFormation characteristics of the metal. The deFormation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. The softening effect of the ultrasound is found to constitute recovery associated with extensive enhancement of Subgrain Formation during deFormation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deFormation, the enhanced Subgrain Formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the enhanced Subgrain Formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Such effects of the ultrasound are interpreted by its ability to enhance dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation.
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New insight on acoustoplasticity – Ultrasonic irradiation enhances Subgrain Formation during deFormation
International Journal of Plasticity, 2011Co-Authors: Kw Siu, Alfonso H.w. Ngan, I.p. JonesAbstract:Many industrial applications make use of ultrasonic vibration to soften metals. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deFormation resistance or mechanism is assumed to be unaltered by the ultrasound. In this study, indentation experiments performed on aluminum samples simultaneously excited by ultrasound reveal that the latter intrinsically alters the deFormation characteristics of the metal. The deFormation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. The softening effect of the ultrasound is found to constitute recovery associated with extensive enhancement of Subgrain Formation during deFormation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deFormation, the enhanced Subgrain Formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the enhanced Subgrain Formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Such effects of the ultrasound are interpreted by its ability to enhance dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation.
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Effects on plastic deFormation by high-frequency vibrations on metals
1Co-Authors: Kw SiuAbstract:The effect of softening due to vibrations induced on metals has been used in many industrial processes such as forming, machining and joining. These industrial applications utilize ultrasonic vibrations in addition to quasi-static stresses in order to deform metals more easily. The phenomenon of ultrasonic softening is also called the Blaha effect or acoustoplastic effect. Besides the macro-scale softening due to ultrasonic vibrations imposed on quasi-static deFormation stress, sub-micron level softening due to vibrations was also observed in nanoindentation experiments in recent years. These experiments made use of the oscillatory stresses of the vibrations provided by the continuous stiffness measurement (CSM) mode of nanoindentation. Lowering of loading and hardness data has been observed at shallow indent depths where the amplitude of vibration is relatively large. Despite the common industrial usages of acoustoplastic effect and the observation of softening in CSM mode nanoindentation, the physical principle underlying is still not well understood. For acoustoplastic effect the existing understanding is usually one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal. For the softening observed in CSM mode nanoindentation, the effect is either attributed to instrumental errors or enhancement of nucleation of dislocations which makes them move faster. Investigations on the link between microscopical changes and the softening have been rare. In this thesis, indentation experiments in both macro and micro scales were performed on aluminium, copper and molybdenum samples with and without the simultaneously application of oscillatory stresses. Significant softening was observed, and the amount of softening from macro to micro scale indentation of similar displacement/amplitude ratios is similar. The deFormation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. Electron microscopy analyses reveal Subgrain Formation under the vibrated indents, which implies intrinsic changes. To further give physical insight into the phenomenon, dislocation dynamics simulations were carried out to investigate the interactions of dislocations under the combined influence of quasi-static and oscillatory stresses. Under a combined stress state, dislocation annihilation is found to be enhanced leading to larger strains at the same load history. The simulated strain evolution under different stress schemes also resembles closely certain experimental observations previously obtained. The discovery here goes far beyond the simple picture that the effect of vibration is merely an added-stress one, since here, the intrinsic strain-hardening potency of the material is found to be reduced by the oscillatory stress, through its effect on enhancing dislocation annihilation. The experimental and simulation results collectively suggest that simultaneous application of oscillatory stress has the ability to enhance dipole annihilation and cause Subgrain Formation. The superimposed oscillatory stress causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation. In addition, microscopic observations showed that Subgrain Formation and reduction in dislocation density generally occurred in different metals when stress oscillations were applied. These suggest that the intrinsic oscillation-induced effects of softening and dislocation annihilation are a rather general phenomenon occurring in metals with different stacking fault energies and crystal structures.
I.p. Jones - One of the best experts on this subject based on the ideXlab platform.
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new insight on acoustoplasticity ultrasonic irradiation enhances Subgrain Formation during deFormation
International Journal of Plasticity, 2011Co-Authors: Kw Siu, Alfonso H.w. Ngan, I.p. JonesAbstract:Many industrial applications make use of ultrasonic vibration to soften metals. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deFormation resistance or mechanism is assumed to be unaltered by the ultrasound. In this study, indentation experiments performed on aluminum samples simultaneously excited by ultrasound reveal that the latter intrinsically alters the deFormation characteristics of the metal. The deFormation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. The softening effect of the ultrasound is found to constitute recovery associated with extensive enhancement of Subgrain Formation during deFormation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deFormation, the enhanced Subgrain Formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the enhanced Subgrain Formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Such effects of the ultrasound are interpreted by its ability to enhance dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation.
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New insight on acoustoplasticity – Ultrasonic irradiation enhances Subgrain Formation during deFormation
International Journal of Plasticity, 2011Co-Authors: Kw Siu, Alfonso H.w. Ngan, I.p. JonesAbstract:Many industrial applications make use of ultrasonic vibration to soften metals. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deFormation resistance or mechanism is assumed to be unaltered by the ultrasound. In this study, indentation experiments performed on aluminum samples simultaneously excited by ultrasound reveal that the latter intrinsically alters the deFormation characteristics of the metal. The deFormation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. The softening effect of the ultrasound is found to constitute recovery associated with extensive enhancement of Subgrain Formation during deFormation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deFormation, the enhanced Subgrain Formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the enhanced Subgrain Formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Such effects of the ultrasound are interpreted by its ability to enhance dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation.
Alfonso H.w. Ngan - One of the best experts on this subject based on the ideXlab platform.
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Strength of metals under vibrations – dislocation-density-function dynamics simulations
Philosophical Magazine, 2014Co-Authors: Bingqing Cheng, H.s. Leung, Alfonso H.w. NganAbstract:It is well known that ultrasonic vibration can soften metals, and this phenomenon has been widely exploited in industrial applications concerning metal forming and bonding. Recent experiments show that the simultaneous application of oscillatory stresses from audible to ultrasonic frequency ranges can lead to not only softening but also significant dislocation annihilation and Subgrain Formation in metal samples from the nano- to macro-size range. These findings indicate that the existing understanding of ultrasound softening – that the vibrations either impose additional stress waves to augment the quasi-static applied load, or cause heating of the metal, whereas the metal’s intrinsic deFormation resistance or mechanism remains unaltered – is far from complete. To understand the softening and the associated enhanced Subgrain Formation and dislocation annihilation, a new simulator based on the dynamics of dislocation-density functions is employed. This new simulator considers the flux, production and anni...
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Oscillation-induced softening in copper and molybdenum from nano- to micro-length scales
Materials Science and Engineering: A, 2013Co-Authors: Kw Siu, Alfonso H.w. NganAbstract:The fact that the application of a simultaneous oscillatory stress can lead to significant reductions in the quasi-static stress required to sustain deFormation has found a wide range of industrial applications. Recently, we discovered that, in addition to the widely believed effects of stress superposition, the oscillation-induced softening in aluminium is an intrinsic effect associated with enhanced dislocation annihilation and Subgrain Formation arising from the simultaneous oscillatory stress. However, such intrinsic effects have not been proven as a general phenomenon for other metals. In this study, macroscopic and nano-indentation were performed on copper and molybdenum. The results show that the simultaneous application of oscillatory stresses can lower the hardness of these samples. EBSD and TEM observations show that Subgrain Formation and reduction in dislocation density generally occurred when stress oscillations were applied. These suggest that the intrinsic oscillation-induced effects of softening and dislocation annihilation are a rather general phenomenon occurring in metals with different stacking fault energies and crystal structures.
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The continuous stiffness measurement technique in nanoindentation intrinsically modifies the strength of the sample
Philosophical Magazine, 2013Co-Authors: Kw Siu, Alfonso H.w. NganAbstract:The continuous stiffness measurement (CSM) method is a well-established technique for obtaining elastic modulus and hardness data continuously during a nanoindentation process. The applicability of this technique is based on the assumption that the material properties of the specimen being tested are not affected by the imposed oscillatory excitation of the indenter. In this study, nanoindentation experiments on aluminium with a Berkovich tip show that nanometric oscillations of the CSM can lead to significant softening even though the indents made are micron-sized. The amount of softening is similar to that observed from macroscopic indentation tests with simultaneous ultrasonic excitation of similar displacement/amplitude ratios. Electron microscopy analyses reveal Subgrain Formation under the CSM nanoindents, which is also a feature of ultrasonically deformed bulk aluminium. The Oliver–Pharr and CSM method of hardness measurement are found to be erroneous with the CSM mode switched on.
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new insight on acoustoplasticity ultrasonic irradiation enhances Subgrain Formation during deFormation
International Journal of Plasticity, 2011Co-Authors: Kw Siu, Alfonso H.w. Ngan, I.p. JonesAbstract:Many industrial applications make use of ultrasonic vibration to soften metals. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deFormation resistance or mechanism is assumed to be unaltered by the ultrasound. In this study, indentation experiments performed on aluminum samples simultaneously excited by ultrasound reveal that the latter intrinsically alters the deFormation characteristics of the metal. The deFormation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. The softening effect of the ultrasound is found to constitute recovery associated with extensive enhancement of Subgrain Formation during deFormation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deFormation, the enhanced Subgrain Formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the enhanced Subgrain Formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Such effects of the ultrasound are interpreted by its ability to enhance dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation.
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New insight on acoustoplasticity – Ultrasonic irradiation enhances Subgrain Formation during deFormation
International Journal of Plasticity, 2011Co-Authors: Kw Siu, Alfonso H.w. Ngan, I.p. JonesAbstract:Many industrial applications make use of ultrasonic vibration to soften metals. The existing understanding of such an acoustoplastic effect is one in which the ultrasonic irradiation either imposes additional stress waves to augment the quasi-static applied load, or causes heating of the metal, whereas the metal’s intrinsic deFormation resistance or mechanism is assumed to be unaltered by the ultrasound. In this study, indentation experiments performed on aluminum samples simultaneously excited by ultrasound reveal that the latter intrinsically alters the deFormation characteristics of the metal. The deFormation microstructures underneath the indents were investigated by a combination of cross-sectional microscopic techniques involving focused-ion-beam milling, transmission electron microscopy and crystal orientation mapping by electron backscattered diffraction. The softening effect of the ultrasound is found to constitute recovery associated with extensive enhancement of Subgrain Formation during deFormation. By comparing the microstructures of samples deformed with and without simultaneous application of ultrasound, and those subsequently excited by ultrasound after deFormation, the enhanced Subgrain Formation is proved to be one due to the combined application of the quasi-static loading and the ultrasound, but not a simple addition of the two. Similarly, by comparing with samples deformed while being simultaneously or subsequently heated up, the enhanced Subgrain Formation by the ultrasound is proved to be a lot greater than that due to the heat that it generates within the metal. Such effects of the ultrasound are interpreted by its ability to enhance dipole annihilation. The superimposed ultrasound causes dislocations to travel longer distances in a jerky manner, so that they can continuously explore until dipole annihilation.
