In-Situ TEM investigation of dynamics and relaxation in amorphous NiTi

Hilke Sven, Davani Farnaz Abdollahzadeh, Wilde Gerhard, Peterlechner Martin

Abstract

In transmission electron microscopy (TEM) the method of electron correlation microscopy (ECM) has been introduced by He et al. [1] to investigate dynamics and relaxation phenomena at the atomic scale. Recently, the great ability has been shown using tilted dark-field in TEM in the super-cooled liquid of a metallic glass by Zhang et al. [2]. In this work we extended this approach to a wide range of temperatures (room temperature to 400 °C) using repeated cold rolled (RCR) amorphized NiTi. At all temperatures we found a two-step decay process of the autocorrelation function g2(Δt) which can be fitted using a double Kohlrausch-Williams-Watt (KWW) expression. Additionally we an-alyzed locally for each temperature the two-time-correlation-function (TTCF) as mostly used in the X-ray equivalent technique of x-ray photon correlation spectroscopy (XPCS) and found aging phe-nomena as well as local temporal double KWW and areas which were in thermodynamic equilib-rium. In 2017 Luo et al. [3] presented a decoupling of α and β relaxation in metallic glasses by follow-ing the stress decay under a constant strain. Until the glass transition they saw decoupled relaxation phenomena with two different activation energies. For the first time ECM data show a double KWW dependence in non-equilibrium dynamics. This is supported by the fact, that no glass transition was observed neither in differential scanning calorimetry (DSC) nor in the crossover of the ECM relaxa-tion times before crystallization. This indicates that the glass transition temperature of amorphous RCR NiTi is to close or even above the crystallisation temperature. References [1] He, L., Zhang, P., Besser, M. F., Kramer, M. J., & Voyles, P. M.; Microscopy and Microanalysis, 21(4), 1026-1033, (2015). [2] Zhang, P., Maldonis, J. J., Liu, Z., Schroers, J., & Voyles, P. M..; Nature communications, 9(1), 1129, (2018). [3] Luo, P., Wen, P., Bai, H. Y., Ruta, B., & Wang, W. H.; Physical review letters, 118(22), 225901, (2017).