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C reep strain rec overy of Fe–Ni–B amorphous m etallic ribbon

C reep strain rec overy of Fe–Ni–B amorphous m etallic ribbon. Juríková, K. Csach, J. Miškuf, V . O celík * Department of Metal Physics Institute of Experimental Physics Slovak Academy of Sciences

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C reep strain rec overy of Fe–Ni–B amorphous m etallic ribbon

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  1. Creep strain recovery of Fe–Ni–B amorphous metallic ribbon • Juríková, K. Csach, J. Miškuf,V. Ocelík * • Department of Metal Physics • Institute of Experimental Physics • Slovak Academy of Sciences • * University ofGroningen,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, Stará Lesná, 2006 published in: Central European Journal of Physics 5 (2) 2007, 177–187 OFK: ÚEF SAV Košice 12.12.2007

  2. OFK: ÚEF SAV Košice 12.12.2007

  3. 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 • Ahierarchy of internal stresses of different ranges : • macroscopic quenchingstresses (acting on scale of the wholesample) • submacroscopic quenchingstresses (several hundredths mm) • local stresses ofintercluster boundariesor atomic level stresses • At elevated temperatures these stresses disappear during SR – this process is influenced by applied mechanical stress. OFK: ÚEF SAV Košice 12.12.2007

  4. Introduction stress-annealingcreepstrain: 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 OFK: ÚEF SAV Košice 12.12.2007

  5. 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 Fe–Ni–Bmetallic glass after longtime loading derived from DSC and TMA studies • to demonstrate how the activation energy spectra model isapprociated for the description of creep strain recovery processin the material OFK: ÚEF SAV Košice 12.12.2007

  6. Experimental Material: amorphous metallic glass: Fe40Ni41B19 the thickness of ribbon : 17.3mm the width of samples : 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: ÚEF SAV Košice 12.12.2007

  7. Experimental • The thermal analysis measurements • (changes of enthalpy DH and length Dl) 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 Setaram TMA 92 (thermomechanical analyser) Perkin Elmer DSC 7 (diferential scanning calorimeter) OFK: ÚEF SAV Košice 12.12.2007

  8. Results – DSC DSC tracesfor the samplespreannealed at indicated temperaturesunder 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: ÚEF SAV Košice 12.12.2007

  9. 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 indicatedTa. OFK: ÚEF SAV Košice 12.12.2007

  10. Results – DSC • each of the measured DSC curves shows an exothermic effect (connected withlowering a free energy of the amorphous structure towards anequilibrium glassy state) for all annealing temperatures Ta: • the wide exothermic decreases • their starts tend to shift towards high temperatures as the stress-annealed Ta increases: T ~Ta + 100oC DSC tracesfor the samplesstress-annealed at indicated temperatures. OFK: ÚEF SAV Košice 12.12.2007

  11. Results – TMA • at temperatures below Ta  linear elongation of samples due to thermal expansion • at temperatures near Tacreep strain recovery shrinking is superposed The pure creep recovery curves were obtained by substracting the reference curves from curves measured on stress-annealed samples. The change of lengthmeasured during linear heating for samplesstress-annealed at differentTaand for a reference sample. OFK: ÚEF SAV Košice 12.12.2007

  12. Results – TMA – shear anelastic deformation Dl – the length change of a sample l0 – the effective length of a sample l0 =15 mm the totalanelastic strain:up to 5 x 10–3 Activation energy spectra – calculated from these non-isothermal experiments using a modern method based on Fourier techniques  The anelastic shear strainfor the samples stress-annealed at indicated temperatures. OFK: ÚEF SAV Košice 12.12.2007

  13. Model and method of calculation AES W.Primak 1955, M.R.J.Gibbs et al.1983 for non-isothermal experiment: T = To+b t , b – constant heating rate described by equation: DP(T)–total change in time of some measured physical property N(E) – spectrum of activation energies qa(E,T)– anisothermal characteristic annealing function: a, b – constants •  convolution integral • spectrum of activation energiescan be calculated by the method using Fourier transformations OFK: ÚEF SAV Košice 12.12.2007

  14. Results – AES • 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 deformationdefectsin the amorphous structure • It is evident: the creep strain recovery is determined by the temperature of stress-annealing • The heightof peaks in calculated AES tends to increasewith the increasing activation energy for a given stress-annealingtemperature. The positions of two most significant peaks in depending on thestress-annealing temperature Creep recovery spectracalculated from the linear heating experiments. OFK: ÚEF SAV Košice 12.12.2007

  15. Results – AES • Twotendencies of peak position dependence on the annealingtemperature are evident: • for lower temperatures of annealing the characteristic energy of peaks decreases as the stress-annealingtemperature increases • for higher stress-annealedtemperatures the opposite tendency is observed. • connected with different structural states of the samples obtainedduring the stress-annealing at different temperatures Peak positionsdepending on the annealing temperature. OFK: ÚEF SAV Košice 12.12.2007

  16. Discussion DirectionalStructural Relaxation (DSR) model by Khoniket al.(1995): homogeneousplastic flow of MGs as a result of SR oriented by the external stress the non-isothermal strainrecovery  a set of localatomic rearrangements, with distributed AES, in spatially separated regions of the structure – 'relaxation centers'– oriented favourably or unfavourably to theexternal stress. In the samples stress-annealed atlower temperatures both relaxation centers, the parallel and theantiparallel in sign to the external stress, rearrange during thestrain recovery process. As the annealing temperature increasesthe influence of antiparallely oriented relaxation centersdecreases, thus for higher temperatures only parallely orientedrelaxation centers contribute to the creep strain recoveryprocess. OFK: ÚEF SAV Košice 12.12.2007

  17. 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 influencesthe structural relaxation and creep strain recovery processes in themetallic glass. • Structural relaxation is qualitatively the same for samplespreannealed 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 samplesstress-annealed at lower temperatures. In the samplesstress-annealed at higher temperatures only the relaxation centersfavourably oriented to the external stress contribute to the creepstrain recovery process in the Fe-based amorphous ribbon. OFK: ÚEF SAV Košice 12.12.2007

  18. Thank for your attention OFK: ÚEF SAV Košice 12.12.2007

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