Creep strain recovery of Fe-Ni-B amorphous metallic ribbon

Creep strain recovery of Fe-Ni-B amorphous metallic ribbon

CCreep reepstrain strainrec recovery overyof ofFeNiB FeNiB amorphous amorphousm metallic etallicribbon ribbon A. Jurkov, K. Csach, J. Mikuf, V. Ocelk * 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 slovak physicists, Star Lesn, 2006 published in: Central European Journal of Physics 5 (2) 2007, 177187

OFK: EF SAV Koice 12.12.2007 OFK: EF SAV Koice 12.12.2007 Introduction Introduction Metallic glasses (MGs) metastable, highly non-equilibrium structures annealing below Tg 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. OFK: EF SAV Koice

12.12.2007 Introduction Introduction 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 timedependent 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 OFK: EF SAV Koice 12.12.2007 Aim Aim 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 FeNiB 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 OFK: EF SAV Koice 12.12.2007 Experimental Experimental Material: amorphous metallic glass : Fe40Ni41B19 the thickness of ribbon : the width of samples : 17.3 m 4.0 mm Annealing:

at temperatures Ta = 150 300oC time of the annealing: 20 hours under an external tensile stress: 383 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 OFK: EF SAV Koice 12.12.2007 Experimental 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 Kmin1 and 10 Kmin1 in a flowing nitrogen atmosphere Perkin Elmer DSC 7 (diferential scanning calorimeter)

Setaram TMA 92 (thermomechanical analyser) OFK: EF SAV Koice 12.12.2007 Results ResultsDSC 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 ~ 200oC at a

given heating rate for all Ta the more significant changes associated with SR at the temperatures T ~ Ta + 100oC the energy accumulated during the creep starts to release at temperatures above Tx = 415oC much more extensive release of energy OFK: EF SAV Koice 12.12.2007 Results ResultsDSC 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 Ta . OFK: EF SAV Koice 12.12.2007 Results ResultsDSC 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 Ta: the wide exothermic decreases their starts tend to shift towards high temperatures as the stressannealed Ta increases: DSC traces for the samples stress-annealed at indicated temperatures.

T ~ Ta + 100oC OFK: EF SAV Koice 12.12.2007 Results ResultsTMA TMA at temperatures below Ta linear elongation of samples due to thermal expansion at temperatures near Ta creep strain recovery shrinking is superposed The change of length measured during linear heating for samples stress-annealed at different Ta and for a reference sample. The pure creep recovery curves were obtained by substracting the reference curves from curves

measured on stress-annealed samples. OFK: EF SAV Koice 12.12.2007 Results ResultsTMA TMA 3 l / l0 shear anelastic deformation l the length change of a sample l0 the effective length of a sample l0 = 15 mm the total anelastic strain: up to 5 x 103 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 OFK: EF SAV Koice 12.12.2007 Model Modeland andmethod methodof ofcalculation calculationAES AES W. Primak 1955, M. R. J. Gibbs et al.1983 for non-isothermal experiment: T = To+t , constant heating rate described by equation: P(T ) N ( E ) a ( E , T ) dE 0

P(T) N (E ) a(E,T) total change in time of some measured physical property spectrum of activation energies anisothermal characteristic annealing function: a ( x) 1 exp exp x Eo (T ) E fcorr E* T E E x ln o ln 2 kT kT

Eo (T ) 1T 2T 2 k T 1 a 1 ln o b a, b constants k 2 bT convolution integral spectrum of activation energies can be calculated by the method using Fourier transformations OFK: EF SAV Koice

12.12.2007 Results ResultsAES 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 stressannealing temperature OFK: EF SAV Koice 12.12.2007 Results ResultsAES AES 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 stressannealing temperature increases for higher stress-annealed temperatures the opposite tendency is observed. Peak positions depending on the annealing temperature.

connected with different structural states of the samples obtained during the stress-annealing at different temperatures OFK: EF SAV Koice 12.12.2007 Discussion 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 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: EF SAV Koice 12.12.2007 Short Shortsummary summary Different creep strain accumulation is realized during stress-annealing of the amorphous ribbon FeNiB 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: EF SAV Koice 12.12.2007

Thank for your attention OFK: EF SAV Koice 12.12.2007

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