OFK: ÚEF SAV Košice A. Juríková, K. Csach, J. Miškuf, V. Ocelík * Department of Metal Physics Institute of Experimental Physics Slovak Academy of Sciences * University of Groningen, Dept. of Applied Physics, Materials Science Centre, the Netherlands presented at: 5-th International Conference on Measurement, Smolenice, May th Conference of slovak physicists, Stará Lesná, 2006 published in: Central European Journal of Physics 5 (2) 2007, 177–187 Creep strain recovery of Fe–Ni–B amorphous metallic ribbon

OFK: ÚEF SAV Košice

Metallic glasses (MGs) – metastable, highly non-equilibrium structures annealing below T g  structural relaxation (SR) – subtle rearrangements of the atomic structure to a more stable state  topological and chemical short-range order  variations in many physical properties A hierarchy of internal stresses of different ranges :  macroscopic quenching stresses (acting on scale of the whole sample)  submacroscopic quenching stresses (several hundredths  m)  local stresses of intercluster boundaries or atomic level stresses At elevated temperatures these stresses disappear during SR – this process is influenced by applied mechanical stress. IntroductionIntroduction

OFK: ÚEF SAV Košice stress-annealing  creep strain:  a part of the deformation energy is released upon subsequent annealing under zero stress causing anelastic creep recovery  macroscopically reversible deformation but delayed in time: pre-deformed samples can partially restore their shape after stress removal  time-dependent anelastic strain recovery anelasticity in MGs – process distributed over a range of activation energies Taub and Spaepen (1984): the anelastic deformation response of MGs could not be described by a single relaxation process, a sum of exponential decays, spanning a spectrum of time constants, is required to describe the anelastic component of the homogeneous strain response of amorphous alloys to applied stress IntroductionIntroduction

OFK: ÚEF SAV Košice A study of the activation energy spectra (AES)  possible help in understanding the atomic processes which take place during the relaxation in metastable systems. Analysis of kinetics of anelastic deformation response  useful informations about the local short-range ordering and deformation defects in amorphous structure. The purpose of the presented work:  to report some results on creep strain recovery and SR processes in Fe–Ni–B metallic glass after longtime loading derived from DSC and TMA studies  to demonstrate how the activation energy spectra model is approciated for the description of creep strain recovery process in the material AimAim

OFK: ÚEF SAV Košice Material: amorphous metallic glass : Fe 40 Ni 41 B 19 the thickness of ribbon : 17.3  m the width of samples : 4.0 mm Annealing: at temperatures T a = 150 – 300 o C time of the annealing: 20 hours under an external tensile stress: 383 MPa (or without stress  reference specimens) (or without stress  reference specimens) inside a tube furnace in a flowing nitrogen atmosphere cooled down to room temperature (under the same stress) and unloaded ExperimentalExperimental

OFK: ÚEF SAV Košice The thermal analysis measurements (changes of enthalpy  H and length  l) carried out:  using: differential scanning calorimeter (DSC) and thermomechanical analyser (TMA) thermomechanical analyser (TMA)  during linear heating with the rate of 20 Kmin –1 and 10 Kmin –1  in a flowing nitrogen atmosphere Perkin Elmer DSC 7 (diferential scanning calorimeter) Setaram TMA 92 (thermomechanical analyser) ExperimentalExperimental

OFK: ÚEF SAV Košice Results – DSC DSC traces – similar shape for samples annealed under stress or without stress at a given annealing temperature  SR is qualitatively the same – start to have a different deviation at a temperature T ~ 200 o C at a given heating rate for all T a DSC traces for the samples preannealed at indicated temperatures under and without stress, and the differences between them. – the more significant changes associated with SR – at the temperatures T ~ T a o C  the energy accumulated during the creep starts to release – at temperatures above T x = 415 o C  much more extensive release of energy

OFK: ÚEF SAV Košice T a The differences of DSC data between the reference sample and the sample stress-annealed at the indicated T a. there is no sequence with the temperature of annealing annealing under stress causes in general more intensive SR and so a closer structure arrangement Results – DSC

OFK: ÚEF SAV Košice Results – DSC DSC traces for the samples stress-annealed at indicated temperatures.  each of the measured DSC curves shows an exothermic effect (connected with lowering a free energy of the amorphous structure towards an equilibrium glassy state) for all annealing temperatures T a :  the wide exothermic decreases  their starts tend to shift towards high temperatures as the stress- annealed T a increases: T ~ T a o C

OFK: ÚEF SAV Košice Results – TMA The pure creep recovery curves were obtained by substracting the reference curves from curves measured on stress-annealed samples.  at temperatures below T a  linear elongation of samples due to thermal expansion  at temperatures near T a  creep strain recovery shrinking is superposed The change of length measured during linear heating for samples stress-annealed at different T a and for a reference sample.

OFK: ÚEF SAV Košice Results – TMA The anelastic shear strain for the samples stress-annealed at indicated temperatures. Activation energy spectra – calculated from these non-isothermal experiments using a modern method based on Fourier techniques calculated from these non-isothermal experiments using a modern method based on Fourier techniques  – shear anelastic deformation – shear anelastic deformation  l – the length change of a sample l 0 – the effective length of a sample l 0 = 15 mm the total anelastic strain: up to 5 x 10 –3

OFK: ÚEF SAV Košice Model and method of calculation AES for non-isothermal experiment: T = T o +  t,  – constant heating rate described by equation:  P(T) – total change in time of some measured physical property N(E) – spectrum of activation energies  a  (E,T) – anisothermal characteristic annealing function: W. Primak 1955, M. R. J. Gibbs et al.1983  convolution integral  spectrum of activation energies can be calculated by the method using Fourier transformations the method using Fourier transformations a, b  – constants

OFK: ÚEF SAV Košice Results – AES Creep recovery spectra calculated from the linear heating experiments. Creep recovery spectra: Creep recovery spectra: – a discrete character consisting of a finite number of peaks – well defined characteristic energies that probably correspond to the different type of deformation defects in the amorphous structure It is evident: the creep strain recovery is determined by the temperature of stress-annealing It is evident: the creep strain recovery is determined by the temperature of stress-annealing The height of peaks in calculated AES tends to increase with the increasing activation energy for a given stress- annealing temperature. The height of peaks in calculated AES tends to increase with the increasing activation energy for a given stress- annealing temperature. The positions of two most significant peaks in depending on the stress- annealing temperature The positions of two most significant peaks in depending on the stress- annealing temperature 

OFK: ÚEF SAV Košice Results – AES Peak positions depending on the annealing temperature. Two tendencies of peak position dependence on the annealing temperature are evident:  for lower temperatures of annealing the characteristic energy of peaks decreases as the stress- annealing temperature increases  for higher stress-annealed temperatures the opposite tendency is observed. connected with different structural states of the samples obtained during the stress-annealing at different temperatures  connected with different structural states of the samples obtained during the stress-annealing at different temperatures

OFK: ÚEF SAV Košice DiscussionDiscussion Directional Structural Relaxation (DSR) model by Khonik et al. (1995): homogeneous plastic flow of MGs as a result of SR oriented by the external stress the non-isothermal strain recovery a set of local atomic rearrangements, with distributed AES, in spatially separated regions of the structure – 'relaxation centers' – oriented favourably or unfavourably to the external stress. the non-isothermal strain recovery  a set of local atomic rearrangements, with distributed AES, in spatially separated regions of the structure – 'relaxation centers' – oriented favourably or unfavourably to the external stress. In the samples stress-annealed at lower temperatures both relaxation centers, the parallel and the antiparallel in sign to the external stress, rearrange during the strain recovery process. As the annealing temperature increases the influence of antiparallely oriented relaxation centers decreases, thus for higher temperatures only parallely oriented relaxation centers contribute to the creep strain recovery process.

OFK: ÚEF SAV Košice Short summary  Different creep strain accumulation is realized during stress-annealing of the amorphous ribbon Fe–Ni–B depending on the annealing temperature. This fact influences the structural relaxation and creep strain recovery processes in the metallic glass.  Structural relaxation is qualitatively the same for samples preannealed under or/and without stress.  Both relaxation centers, the parallel and the antiparallel in sign to the external stress, rearrange during the creep strain recovery in the samples stress- annealed at lower temperatures. In the samples stress-annealed at higher temperatures only the relaxation centers favourably oriented to the external stress contribute to the creep strain recovery process in the Fe-based amorphous ribbon.

OFK: ÚEF SAV Košice Thank for your attention