Publikationsserver der RWTH Aachen University
Zeitabhängige Phänomene in athermischen Formgedächtnislegierungen
Abstract
dc:descriptionThree different material systems (Ni63Al37, Au50.5Cd49.5 and Au52.5Cd47.5), each exhibiting a weakly first order martensitic phase transition with shape memory effect, have been studied. These alloys are model alloys for the observation of precursor effects and therefore are particularly suited for measurements of time dependent phenomena. Firstly optical measurements of Laser light reflectivity were conducted in order to measure incubation time as well as the stability of the phase transition temperatures Ms, As and the corresponding hysteresis width B. All measurements were conducted in custom made sample chambers allowing for a temperature stability of 2mK over a timescale of some days. In comparison to similar reported studies this improves the temperature stability available by more than one order of magnitude so that incidental temperature fluctuations can be ruled out as the origin of incubation time phenomena. In all data pronounced fluctuations of all studied values are observed. With a statistical analysis by so called detrended fluctuation analysis (DFA) hitherto unknown behavior of the fluctuations is revealed. Depending on the sample anti-correlated or correlated behavior is observed; in total the Ni-Al sample shows the weakest and the Au52.5Cd47.5 sample shows the strongest correlation in fluctuation behavior. These assertions are possible due to the high number of measurement points compared to previous studies. For the first time measurements of the current value of Ms are carried out in between the incubation time measurements. Therefore the incubation times can be related to these temperature values resulting in a considerable reduction of spread if correlations in the values of MS exist. This shows qualitatively the validity of the results of DFA analysis. To reveal temperature-dependence of suspected dynamic above the MS temperature x-ray photon correlation spectroscopy (XPCS) was applied for the first time to a Au50.5Cd49.5 sample. The data evaluation is done either by DFA or by calculating one- and two-time correlation functions. Surprisingly, a strong acceleration of the dynamics is observed on approaching the phase transition. Therefore these findings contradict the notion, that the weakly first order phase transition of shape memory alloys resemble second order phenomena. In that case "critical slowing down" would have been observable. Despite a temperature resolution of up to 0.05K the dynamic can only be observed at a few temperatures in close vicinity of the phase transition. In particular it has to be stated that despite the long waiting times between temperature changes and in contradiction with classical theory of athermal martensites, non-equilibrium behavior is observed. A thorough investigation of the non-equilibrium dynamics is, however, only possible for a single temperature at which the fastest dynamics is measured. This is because at all other temperatures statistical fluctuations dominate due to extreme time scales. The hence observed non-ergodic behavior shows that the model of Kakeshita to describe incubation time is not suitable for the Au50.5Cd49.5 material system. The observed komplex behavior and subtle mutual implications of the different observations show the necessity of further studies of the intricate transformation mechanism.
Degree
thesis:*- Grantor dc:publisher
- Publikationsserver der RWTH Aachen University
- Year dc:date
- 2009
Author and committee
dc:creator, dc:contributor.*- Author dc:creator
-
- Müller, Leonard
- Contributors dc:contributor
-
- Klemradt, Uwe
Subjects
dc:subject × 12Rights
dc:rights- Statement dc:rights
-
- info:eu-repo/semantics/openAccess
- Language dc:language
- ger