Abstract
In this work the avalanche dynamics of five shape memory samples has been analyzed by acoustic emission spectroscopy. The acoustic emission spectroscopy is particularly suitable for this analysis as it couples with high sensitivity to small structural changes caused by nucleation processes, interface movements, or variant rearrangements [91]. Owing to its high time resolution it provides a statistical approach to describe the jerky and intermittent character of the avalanche dynamics [20]. Rate-dependent cooling and heating runs have been conducted in order to study time-dependent aspects of the transition dynamics of the single crystals Ni{sub 63}Al{sub 37}, Au{sub 50.5}Cd{sub 49.5}, and Fe{sub 68.8}Pd{sup single}{sub 31.2}, and the polycrystalline sample Fe{sub 68.8}Pd{sup poly}{sub 31.2}. Moreover, a ferromagnetic Ni{sub 52}Mn{sub 23}Ga{sub 25} single crystal has been studied by temperature cycles under an applied magnetic field and additionally by magnetic-field cycles at a constant temperature in the martensitic phase. All samples analyzed in this work show power law behavior in the acoustic emission features amplitude, energy, and duration, which indicates scale-free behavior. The access to these power law spectra allows an investigation of energy barriers separating the metastable states, which give rise to avalanche transition dynamics. By performing rate-dependent experiments the importance of thermal
More>>
Citation Formats
Ludwig, Benno.
Avalanche dynamics of structural phase transitions in shape memory alloys by acoustic emission spectroscopy.
Germany: N. p.,
2009.
Web.
Ludwig, Benno.
Avalanche dynamics of structural phase transitions in shape memory alloys by acoustic emission spectroscopy.
Germany.
Ludwig, Benno.
2009.
"Avalanche dynamics of structural phase transitions in shape memory alloys by acoustic emission spectroscopy."
Germany.
@misc{etde_21317685,
title = {Avalanche dynamics of structural phase transitions in shape memory alloys by acoustic emission spectroscopy}
author = {Ludwig, Benno}
abstractNote = {In this work the avalanche dynamics of five shape memory samples has been analyzed by acoustic emission spectroscopy. The acoustic emission spectroscopy is particularly suitable for this analysis as it couples with high sensitivity to small structural changes caused by nucleation processes, interface movements, or variant rearrangements [91]. Owing to its high time resolution it provides a statistical approach to describe the jerky and intermittent character of the avalanche dynamics [20]. Rate-dependent cooling and heating runs have been conducted in order to study time-dependent aspects of the transition dynamics of the single crystals Ni{sub 63}Al{sub 37}, Au{sub 50.5}Cd{sub 49.5}, and Fe{sub 68.8}Pd{sup single}{sub 31.2}, and the polycrystalline sample Fe{sub 68.8}Pd{sup poly}{sub 31.2}. Moreover, a ferromagnetic Ni{sub 52}Mn{sub 23}Ga{sub 25} single crystal has been studied by temperature cycles under an applied magnetic field and additionally by magnetic-field cycles at a constant temperature in the martensitic phase. All samples analyzed in this work show power law behavior in the acoustic emission features amplitude, energy, and duration, which indicates scale-free behavior. The access to these power law spectra allows an investigation of energy barriers separating the metastable states, which give rise to avalanche transition dynamics. By performing rate-dependent experiments the importance of thermal fluctuations and the impact of martensite respectively twin stabilization processes have been examined. In the case of the Ni{sub 52}Mn{sub 23}Ga{sub 25} sample, the magnetic-field-induced variant rearrangement at slow field cycles leads to stronger signals than the rearrangement at quick cycles. This behavior can be explained by twin stabilization processes, which are accompanied by a reduction of the twin boundary mobility. For Ni{sub 63}Al{sub 37}, the combination of relevant thermal fluctuations, different involved time scales, and a high degree of intrinsic disorder leads to a lower acoustic activity and weaker signals under decreasing cooling rates. In the case of Au{sub 50.5}Cd{sub 49.5}, by contrast, the low rates allow aging to become significant. This leads to higher energy barriers and, as a consequence, to stronger acoustic emission signals. The excellent agreement of this result with a model introduced by Otsuka et al. [127], suggests that aging should be included into the framework of driving rate effects in avalanche-mediated phase transitions [135]. In contrast to what has been stated earlier [20, 172], the author concludes that only the detection of rate-dependent acoustic activity or the detection of incubation times are necessary and sufficient conditions for time-dependent dynamics. Rate-dependent exponents are a sufficient but not necessary condition for time-dependent behavior. The determination of power law exponents has been proven to be a reliable tool for the characterization of the transition dynamics and associated energy barriers, because the exponents are robust and do not depend on experimental details. This observation reveals a close relation to the concept of self organized criticality [9]. In this work it has been shown for the first time by the evaluation of power law exponents how an applied magnetic field alters energy barriers during structural transitions of magnetic shape memory alloys. Depending on the symmetry of the product phase opposing results can be found: The symmetry-breaking process of a martensitic transition under an applied magnetic field leads to an increase of the activity due to a twofold process, including phase and twin boundary motion. Furthermore, the application of a magnetic field breaks the degeneracy, which leads to larger constraints reflected in stronger signals due to the satisfaction of the invariant habit plane condition. By contrast, less acoustic activity and weaker signals appear in association with the symmetry-conserving premartensitic transition under an applied field. The application of a field leads to an alignment of the magnetic moments. The lowered disorder reduces the energy barriers, which leads to less acoustic activity and weaker signals. At fields higher than 2.5 kOe the acoustic activity is completely suppressed, which suggests an overall weakening of the first order character of the premartensitic transition. The amplitude and energy exponents of the material systems NiAl, FePd, and Ni{sub 2}MnGa (field-dependent) have been for the first time experimentally determined in this work. The amplitude exponents of NiAl and CuZnAl (3.1 {+-} 0.2) and of Ni{sub 2}MnGa and FePd (2.4 {+-} 0.2), determined in this work and reported in literature (see [22, 94, 139]), can be grouped into different classes. This reinforces the universality hypothesis [52] and the assumption that the symmetry of the product phase determines the universality class of the exponents [22]. Moreover, the results of this work suggest that further criteria, such as the degree of disorder or rate-dependent aging effects, should be taken into consideration as well. (orig.)}
place = {Germany}
year = {2009}
month = {Sep}
}
title = {Avalanche dynamics of structural phase transitions in shape memory alloys by acoustic emission spectroscopy}
author = {Ludwig, Benno}
abstractNote = {In this work the avalanche dynamics of five shape memory samples has been analyzed by acoustic emission spectroscopy. The acoustic emission spectroscopy is particularly suitable for this analysis as it couples with high sensitivity to small structural changes caused by nucleation processes, interface movements, or variant rearrangements [91]. Owing to its high time resolution it provides a statistical approach to describe the jerky and intermittent character of the avalanche dynamics [20]. Rate-dependent cooling and heating runs have been conducted in order to study time-dependent aspects of the transition dynamics of the single crystals Ni{sub 63}Al{sub 37}, Au{sub 50.5}Cd{sub 49.5}, and Fe{sub 68.8}Pd{sup single}{sub 31.2}, and the polycrystalline sample Fe{sub 68.8}Pd{sup poly}{sub 31.2}. Moreover, a ferromagnetic Ni{sub 52}Mn{sub 23}Ga{sub 25} single crystal has been studied by temperature cycles under an applied magnetic field and additionally by magnetic-field cycles at a constant temperature in the martensitic phase. All samples analyzed in this work show power law behavior in the acoustic emission features amplitude, energy, and duration, which indicates scale-free behavior. The access to these power law spectra allows an investigation of energy barriers separating the metastable states, which give rise to avalanche transition dynamics. By performing rate-dependent experiments the importance of thermal fluctuations and the impact of martensite respectively twin stabilization processes have been examined. In the case of the Ni{sub 52}Mn{sub 23}Ga{sub 25} sample, the magnetic-field-induced variant rearrangement at slow field cycles leads to stronger signals than the rearrangement at quick cycles. This behavior can be explained by twin stabilization processes, which are accompanied by a reduction of the twin boundary mobility. For Ni{sub 63}Al{sub 37}, the combination of relevant thermal fluctuations, different involved time scales, and a high degree of intrinsic disorder leads to a lower acoustic activity and weaker signals under decreasing cooling rates. In the case of Au{sub 50.5}Cd{sub 49.5}, by contrast, the low rates allow aging to become significant. This leads to higher energy barriers and, as a consequence, to stronger acoustic emission signals. The excellent agreement of this result with a model introduced by Otsuka et al. [127], suggests that aging should be included into the framework of driving rate effects in avalanche-mediated phase transitions [135]. In contrast to what has been stated earlier [20, 172], the author concludes that only the detection of rate-dependent acoustic activity or the detection of incubation times are necessary and sufficient conditions for time-dependent dynamics. Rate-dependent exponents are a sufficient but not necessary condition for time-dependent behavior. The determination of power law exponents has been proven to be a reliable tool for the characterization of the transition dynamics and associated energy barriers, because the exponents are robust and do not depend on experimental details. This observation reveals a close relation to the concept of self organized criticality [9]. In this work it has been shown for the first time by the evaluation of power law exponents how an applied magnetic field alters energy barriers during structural transitions of magnetic shape memory alloys. Depending on the symmetry of the product phase opposing results can be found: The symmetry-breaking process of a martensitic transition under an applied magnetic field leads to an increase of the activity due to a twofold process, including phase and twin boundary motion. Furthermore, the application of a magnetic field breaks the degeneracy, which leads to larger constraints reflected in stronger signals due to the satisfaction of the invariant habit plane condition. By contrast, less acoustic activity and weaker signals appear in association with the symmetry-conserving premartensitic transition under an applied field. The application of a field leads to an alignment of the magnetic moments. The lowered disorder reduces the energy barriers, which leads to less acoustic activity and weaker signals. At fields higher than 2.5 kOe the acoustic activity is completely suppressed, which suggests an overall weakening of the first order character of the premartensitic transition. The amplitude and energy exponents of the material systems NiAl, FePd, and Ni{sub 2}MnGa (field-dependent) have been for the first time experimentally determined in this work. The amplitude exponents of NiAl and CuZnAl (3.1 {+-} 0.2) and of Ni{sub 2}MnGa and FePd (2.4 {+-} 0.2), determined in this work and reported in literature (see [22, 94, 139]), can be grouped into different classes. This reinforces the universality hypothesis [52] and the assumption that the symmetry of the product phase determines the universality class of the exponents [22]. Moreover, the results of this work suggest that further criteria, such as the degree of disorder or rate-dependent aging effects, should be taken into consideration as well. (orig.)}
place = {Germany}
year = {2009}
month = {Sep}
}