Transcript Creep strain recovery of Fe–Ni–B amorphous metallic ribbon
Creep strain recovery of Fe –Ni–B amorphous metallic ribbon
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 2005 15-th Conference of slovak physicists, Star á Lesná, 2006
published in:
Central European Journal of Physics 5 (2) 2007, 177 –187
OFK: ÚEF SAV Košice 12.12.2007
OFK: ÚEF SAV Košice 12.12.2007
Introduction
Metallic glasses (MGs) – metastable, highly non-equilibrium structures annealing below
T g
structural relaxation (SR)
atomic structure to a more stable state – subtle rearrangements of the 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 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.
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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 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
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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–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
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Experimental
Material:
amorphous metallic glass : Fe 40 Ni 41 B 19 the thickness of ribbon : 17.3
m m the width of samples : 4.0 mm
Annealing:
at temperatures
T a
= 150 – 300 time of the annealing: 20 hours o C 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
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Experimental
The thermal analysis measurements (changes of enthalpy D
H
and length D
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
(thermomechanical analyser)
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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 temperatures above
T x
= 415 o C much more extensive release of energy
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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
.
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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
+ 100 o C
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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.
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Results – TMA
3 D
l
/
l
0 – shear anelastic deformation D
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
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Model and method of calculation AES
W. Primak 1955, M. R. J. Gibbs et al.1983
for non-isothermal experiment:
T = T o +
b
t ,
b described by equation: D
P
(
T
) 0
N
(
E
) q
a
– constant heating rate (
E
,
T
) d
E
D
P(T) N
(
E
) q
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
E o
(
T
)
E E *
f corr E o
(
T
) 1
T
2
T
2
x
ln
o T
b
E kT
ln
E kT
2 1
a, b
convolution integral
spectrum of activation energies
can be calculated by the method using Fourier transformations
k b
a
1 – constants ln
o T
b 2
k b T
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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
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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
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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.
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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.
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Thank for your attention
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