Phase-transformation kinetics of TiCr2 laves phases

Abstract

The thesis is divided into three parts: The first part deals with experimental progress in the field of differential thermal analysis DTA (see Chapter 2). This progress was necessary to enable the quantitative investigation of the phase transformation in TiCr2 Laves phases. For quantitative analysis of reaction/transformation kinetics using DTA a calibration of the DTA apparatus in terms of temperature and heat capacity is inevitable. Moreover the measurement signal has to be desmeared, since the measurement signal is smeared due to thermal lag [49], which affects the peak position and the peak shape. In the second part layer-stacking irregularities in C36-type Nb-Cr and Ti-Cr Laves were systematically studied by XRPD and HRTEM. The layer-stacking irregu-larities in both substances can be associated with a preceding C14 (2H)C36 (4H) phase transformation driven by synchro-Shockley dislocations dipoles, i.e. the syn-chro-Shockley dislocations dipoles leave behind a fingerprint in terms of certain layer stacking irregularities (see Chapter 3). The third part of this thesis is about phase transformation kinetics (Chapter 4, 5 and 6). During the investigation of the equilibrium phase transformation in TiCr2 occurring in the vicinity of the equilibrium phase-transformation temperature , it was found that application of the so-called Kissinger method [50] in the case of such an equilibrium transformation will lead to erroneous activation energies, since the original assumptions made for derivation of the Kissinger method are not valid. This was shown by theory and experiment in Chapter 4. Chapter 5 deals with the phase-transformation kinetics of the equilibrium phase transformations in TiCr2 Laves phases, i.e. the modular phase-transformation model has been successfully applied to DTA data of both transformations (i.e. for the phase transformation occurring upon heating and upon cooling). The model reveals the mechanism and the kinetics of the phase transformations and furthermore explains the differences of the phase transformations as a function of Laves-phase composition. The modular model presented in Chapter 5 was used in Chapter 6 to describe the influence of the plastic deformation on the phase-transformation kinetics in TiCr2 Laves phases. It is shown that plastic deformation considerably affects the phase-transformation kinetics. This is ascribed to the presence of dislocations formed upon plastic deformation during pressing.

Das Ziel dieser Arbeit ist die Untersuchung der Phasenumwandlungen in Laves-Phasen, dazu wird insbesondere die TiCr2 Laves-Phase als Modellsystem benutzt. Das Ti-Cr System eignet sich sehr gut als Modelsystem, da in diesem System die drei bekanntesten Laves-Phasen Polytypen als Funktion der Temperatur auftreten. Des Weiteren sind die Phasenumwandlungstemperaturen im Vergleich zu anderen Laves-Phasen vergleichsweise niedrig, wodurch ein relativ guter experimenteller Zugang gegeben ist.