Creep strain recovery of of Fe Ni B amorphous metallic ribbon A. Juríkov ková,, K. K Csach,, J. J Miškuf kuf, 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 2005 15-th Conference of f slovak physicists, Stará Lesná,, 2006 published in: Central European Journal of Physics 5 (2) 2007, 177 187 187
Introduction 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 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 µm) 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.
Introduction stress-annealing creep strain: a part of the deformation energy is released upon subsequent annealing under zero stress causing anelastic creep recoveryr 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, d spanning a spectrum of time constants,, is required to describe the anelastic component of the homogeneous strain response of amorphous alloys to applied stress
Aim A study of the activation energy spectra (AES) possible help in understanding the atomic processes which take place during the relaxation in metastable table systems. Analysis of kinetics of anelastic deformation response useful informations about the local short-range 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 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
Experimental Material: amorphous metallic glass : Fe 40 Ni 41 B 19 the thickness of ribbon : 17.3 µm the width of samples : 4.0 mm Annealing: at temperatures T a = 150 300 o C time of the annealing: : 20 hours under an n external tensile stress: : 3833 MPa (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
Experimental The thermal analysis measurements (changes of enthalpy H and length l) carried out: using: differential scanning calorimeter (DSC) and 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 (thermomechanicalal analys yser)
Results DSC DSC traces for the samples preannealed at indicated temperatures under and without stress, and the differences between them. 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 the more significant changes associated with SR at the temperatures T ~ T a + 100 o C the energy accumulated during the creep starts to release at t temperatures above T x = 415 o C much more extensive release of energy
Results DSC there is no sequence with the temperature of annealing annealing under stress causes in general more intensive SR and so a closer structure arrangement The differences of DSC data between the reference sample and the sample stress-annealed at the indicated T a.
Results DSC 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: DSC traces for the samples stress-annealed at indicated temperatures. T ~ T a + 100 o C
Results TMA 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. The pure creep recovery curves were obtained by substracting the reference curves from curves measured on stress-annealed samples.
Results TMA γ Α = 3 l / l0 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 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
Model and method of of calculation AES W. Primak 1955, M. R. J. Gibbs et al.1983 for non-isothermal experiment: T = T o +β t, β constant heating rate described by equation: P( T ) = N( E) θ ( E, T )de P(T) total change in time of some measured physical property N(E) spectrum of activation energies θ a (E,T,T) anisothermal characteristic annealing function: + 0 a Eo ( T ) E Θa ( x) = 1 exp( exp x) = + fcorr E * o ( T ) = α 1T + α 2T E υ E E x = ln T k o ln + 2 α1 = a 1+ ln β kt kt b a, b constants convolution integral spectrum of activation energies can be calculated by the method using Fourier transformations 2 υot β α 2 = k bt
Results AES Creep recovery spectra calculated from the linear heating experiments. 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 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
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
Discussion 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. In n 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.
Short summary Different creep strain accumulation is realized during stress-annealing of the amorphous ribbon Fe Ni 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.
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