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In-situ characterization of hydrogen-induced defects in palladium by positron annihilation and acoustic emission
i n t e r n a t i o n a l j o u r n a l o f h y d r o g e n e n e r g y x x x ( 2 0 1 7 ) 1 e8
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In-situ characterization of hydrogen-induced defects in palladium by positron annihilation and acoustic emission ´ b, P. Hruska a J. Cı zek a,*, O. Melikhova a, P. Dobron ch Department of Low-Temperature Physics, Faculty of Mathematics and Physics, Charles University, V Holesovicka 2, CZ-180 00 Prague 8, Czech Republic b Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, CZ-121 16 Prague 2, Czech Republic a
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abstract
Article history:
Positron annihilation spectroscopy was employed for characterization of hydrogen-
Received 12 January 2017
induced defects in Pd. Positron annihilation studies were performed in-situ during elec-
Received in revised form
trochemical hydrogen charging and were combined with measurement of acoustic emis-
26 April 2017
sion, which is a non-destructive technique capable of monitoring of collective dislocation
Accepted 27 April 2017
motion. It was found that hydrogen loading introduced defects into Pd lattice, namely
Available online xxx
vacancies and dislocations. At low concentrations (a-phase) hydrogen loading created vacancies associated with hydrogen. Stresses induced by growing a'-phase particles led to
Keywords:
plastic deformation and introduced dislocations into the sample. Moreover, additional
Palladium
vacancies were introduced into the sample by crossing dislocations. Vickers hardness
Hydrogen
testing revealed that hydrogen absorbed in interstitial sites causes solid solution hard-
Acoustic emission
ening. Further hardening was caused by dislocations when a'-phase particles are formed.
Positron annihilation spectroscopy
Pd sample completely transformed into the a'-phase was subsequently unloaded. Decomposition of a'-phase particles during unloading caused further increase of dislocation density and led to an additional hardening. Loading-unloading of Pd sample with hydrogen continuously generates dislocations and makes the sample harder. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen absorbed in host metal lattice is strongly attracted to open volume defects, e.g. vacancies and dislocations. As a consequence, formation energies of open volume defects can be drastically reduced in the presence of hydrogen [1]. From the thermodynamic point of view this can be understood in the frame of the theory of defactants as hydrogen segregation
at open volume defects [2,3]. Moreover, additional defects can be introduced by formation of hydride particles [4e6]. Hence, hydrogen loading typically leads to an increase in the concentration of open volume defects. Hydrogen forms complexes with open volume defects, e.g. vacancies, and concentration of these complexes can be controlled by variation of hydrogen concentration in the sample [7]. Configurations of hydrogen-defect complexes depend on the host lattice structure as well as on the local electron density in defects.
* Corresponding author. Fax: þ420 2 2191 2567. ´z ek). E-mail address: [email protected] (J. Cı http://dx.doi.org/10.1016/j.ijhydene.2017.04.275 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. ´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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The lowest energy configuration of vacancy-hydrogen complexes has been calculated by density functional theory for number of metals, e.g. Pd [8,9], Nb [9], Al [10], Fe [11]. But experimental studies of vacancy-hydrogen complexes are relatively rare [4e7,12,13]. Positron annihilation spectroscopy (PAS) [14] is a non-destructive technique, which enables characterization hydrogen-defect complexes on the atomic scale. In the present work, PAS was employed for characterization of hydrogen-induced defects in Pd which is widely used as a model system for investigation of hydrogen absorbed in solid lattice. Pd is able to absorb relatively large amount of hydrogen and can be charged with hydrogen easily [15]. Hydrogen absorbed in Pd forms interstitial solid solution in fcc Pd lattice, so called a-phase. Above the critical temperature of z295 C hydrogen is fully soluble in Pd up to the atomic ratio 1.0 H/Pd [15,16]. At lower temperatures, the hydrogen solubility in the a-phase decreases and in certain range of hydrogen concentration the Pd-H system becomes a mechanical mixture of two phases: the a-phase with lower hydrogen content and hydrogen-rich a'-phase [17]. Both the a-phase and the a'-phase exhibit fcc structure and differ by hydrogen content. The a'-phase exhibits significantly higher lattice parameter than the a-phase due to higher concentration of absorbed hydrogen. At room temperature, the maximum hydrogen solubility in the a-phase is 0.017 H/Pd [17] and it exhibits lattice parameter of 0.3895 nm [18]. The miscibility gap, i.e. the region of co-existence of the a-phase and a'-phase, extends at room temperature from the hydrogen concentration of 0.017 H/Pd up to 0.58 H/Pd [17]. Within the miscibility gap the volume fraction of the a'-phase increases with increasing hydrogen concentration at the expense of the aphase. When the concentration of absorbed hydrogen reaches 0.58 H/Pd the system is completely transformed into the a'phase with lattice parameter of 0.4034 nm [18]. Our previous PAS study [19] of Pd charged electrochemically with hydrogen revealed that hydrogen loading introduced vacancies in the a-phase region and dislocations in the two-phase field when a'-phase particles were formed. These previous PAS measurements were performed ex-situ, i.e. the sample was firstly charged with hydrogen in a loading cell, and then it was taken out of the cell for measurement of positron lifetime (LT) spectrum. It took approximately 30 h to acquire statistics high enough for decomposition of a LT spectrum into individual components. Some hydrogen was likely desorbed from the sample during PAS measurement at room temperature. Hence, it is interesting to examine the formation of hydrogen-induced defects in-situ during hydrogen loading. In the present work ex situ PAS studies were supplemented by in-situ PAS investigations performed during electrochemical charging of Pd samples with hydrogen. Moreover, PAS studies were combined with measurement of acoustic emission (AE) [20,21] which is a non-destructive technique capable of monitoring of collective dislocation motion. Only few investigations of AE caused by hydrogen absorption in bulk Pd has been performed so far [22e24]. These pioneering studies revealed that AE events are emitted during hydrogen loading. However, the development of AE signals has not been correlated with hydrogen concentration in the sample so far. In addition, the AE technique has been
successfully employed for monitoring of hydrogen-induced stress release in thin films [25,26] as well as buckling and detachment of films from the substrate [27].
Experimental details Polycrystalline Pd (99.95% purity) specimens with dimensions 10 10 0.25 mm3 were firstly annealed at 1000 C for 2 h in a vacuum to remove virtually all defects introduced during previous casting and shaping. Subsequently the specimens were electrochemically charged with hydrogen in a cell filled with electrolyte consisting of a mixture of H3PO4 and glycerine (volume ratio 1:2). Hydrogen loading was performed at ambient temperature using a galvanostat and applying constant current pulses between a Pt counter electrode (anode) and the Pd sample (cathode). Before first hydrogen loading the Pd sample was unloaded for 24 h in order to remove all residual hydrogen from the lattice. The hydrogen concentration introduced into the sample was calculated from the transported charge using the Faraday's law. Unloading was performed in the same electrolyte using a constant voltage of 0.8 V and opposite polarity. The unloading voltage chosen is below the anodic oxidation potential (evolution of oxygen gas) [22]. Hydrogen absorption in Pd was monitored by measuring the electromotoric force EMF [28]. To convert EMF values into hydrogen pressures, the Nernst equation was used with a reference potential of the Ag/AgClsat electrode of 0.197 V against a standard hydrogen electrode, at 298 K. For bulk metalehydrogen systems, the result of the EMF measurement is equivalent to a pressureecomposition isotherm [25]. A digital positron lifetime (LT) spectrometer [29] with excellent time resolution of 145 ps (FWHM of the resolution function) was employed for LT investigations of Pd samples loaded with hydrogen. At least 107 positron annihilation events were accumulated in each LT spectrum which was subsequently decomposed into individual components using a maximum likelihood procedure [30]. A 22Na positron source with activity of ~1 MBq deposited on a 2 mm thick Mylar foil was always forming a sandwich with two identically treated Pd specimens. For in-situ LT measurements positron source was hermetically sealed between two Pd samples which were immersed in the electrolyte. The source contribution in the LT spectra was determined using a well annealed Pd sample. It consisted of two components with the lifetimes of z368 ps and z1.5 ns, and the corresponding intensities of z8% and z1% which come from positrons annihilated in the 22Na source spot and the covering Mylar foil, respectively. The source components were always subtracted from LT spectra. The in-situ AE measurements were performed in similar configuration as in Ref. [22] using a computer-controlled PCI-2 device (Physical Acoustic Corporation) using a piezoelectric transducer MST8S (3 mm diameter) with flat response between 100 and 600 kHz and a preamplifier giving a gain of 40 dB. The signal sampling rate was 2 Hz. The sensor was attached to the backside of the hydrogen loaded (or unloaded) Pd sample. The threshold level for detection of AE counts was set above the peak noise level to 26 dB. A Struers Durascan 2 hardness tester was used for the Vickers hardness (HV) measurements. Hardness of hydrogen
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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loaded samples was measured using the load of 0.5 kg applied for 10 s. Each HV value reported in the paper was obtained as average of 10 measurements.
Results and discussion Fig. 1 shows the development of EMF with the concentration of hydrogen absorbed in a Pd sample electrochemically charged with hydrogen. The sample was step-by-step charged with hydrogen till it was fully converted into the a0 -phase. When the loading was completed the sample was unloaded for 24 h and the loading cycle was repeated. If the hydrogen concentration xH absorbed in the sample obeys the Sievert's law then EMF is proportional to the logarithm of hydrogen concentration [25]. EMF ¼ EMFref þ
RT ln xH ; F
(1)
where EMFref is a reference potential, T is the thermodynamic temperature, R is the universal gas constant and F is the Faraday's constant. Eq. (1) is plotted in Fig. 1 by a solid line. One can see in the figure that EMF behaves according to the Sievert's law in the a-phase region as expected. But a strong deviation from the Sievert's law was observed at very low hydrogen concentrations. This is likely due to hydrogen trapping at grain boundaries of polycrystalline Pd sample. Note that similar deviation of EMF from the Sievert's law was observed in nanocrystalline Pd loaded with hydrogen [31e33]. Since nanocrystalline material contains high density of grain boundaries the deviation from the Sievert's law in the nanocrystalline Pd is extended to higher hydrogen concentrations than in the polycrystalline sample studied in the present work. Hydrogen firstly fills grain boundaries till all traps at grain boundaries are filled. Subsequently it occupies octahedral interstitial sites inside grains and EMF behaves according to Eq. (1). Note that drop in EMF at low hydrogen concentrations occurs only when the virgin sample is electrochemically
Fig. 1 e EMF as a function of the hydrogen concentration xH absorbed in Pd sample. The solid line is a prediction by the Sievert's law expressed by Eq. (1). The dashed lines denote boundaries of the a-phase field, two-phase field (a þ a') and a'-phase region.
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unloaded prior to the first loading otherwise the traps are already filled with residual hydrogen contained in the sample. Hence, hydrogen trapped at grain boundaries can be released from these traps by electrochemical unloading. Interestingly, for second and third loading the deviation from the Sievert's law extended to higher hydrogen concentrations see Fig. 1. This indicates that previous hydrogen loading introduced defects into the sample. These defects trap hydrogen and their concentration increases with increasing number of loading-unloading cycles. As will be shown in the following text these traps created by loading-unloading cycling are dislocations. It has been demonstrated [28,34,35] that dislocations act as trapping sites for hydrogen. The annealed Pd samples exhibited single component LT spectrum with the lifetime tB ¼ (110.1 ± 0.5) ps which agrees well with the calculated bulk lifetime for Pd [36]. Hence, one can conclude that the annealed sample exhibits very low density of defects and virtually all positrons are annihilated in the free state (i.e. not trapped at defects). The ex-situ LT investigations of hydrogen loaded Pd were analyzed in Ref. [19] and it has been found that LT spectra contained up to three components. The shortest component with lifetime t1 < tB comes from free positrons, the component with lifetime t2 z 160 ps represents a contribution of positrons trapped at dislocations and the longest component with lifetime t3 z 200 ps can be attributed to positrons trapped at vacancies. At low hydrogen concentrations when the system is in the a-phase region hydrogen loading introduced only vacancies, i.e. no plastic deformation took place. There is an attractive interaction between vacancy and hydrogen atom absorbed in Pd. This is caused by the fact that hydrogen atom segregated at internal surface of vacancy is energetically more stable than interstitial hydrogen in Pd lattice [8,37]. Even multiple hydrogen atoms can be trapped in single vacancy [38]. As a consequence the vacancy formation energy is reduced by the binding energies released when hydrogen atoms are trapped at the vacancy [1e3] and the equilibrium concentration of vacancies increases by many order of magnitude [1,4,39]. The formation of a'-phase particles was accompanied by introduction of dislocations and additional vacancies which were created by non-conservative movement of dislocations. In a material containing a high dislocation density a dislocation moving in its slip plane will intersect other dislocations crossing the slip plane. This unavoidably leads to formation of jogs at dislocations. Motion of a screw dislocation with jogs is possible only by a non-conservative process, i.e. climb of jogs, which creates rows of vacancies or interstitials [40]. Moreover, vacancies can be created also as a result of relaxation of the misfit strain energy between a'-phase particles and the matrix as suggested by Sakaki et al. [4,5]. Another set of Pd samples was investigated by in-situ LT measurement during electrochemical charging. A low current density of 0.05 mA/cm2 was used to ensure that hydrogen concentration in the sample increased very slowly. In total the in-situ loading experiment took 75 days and LT spectrum was recorded every hour. Hydrogen is very mobile in Pd lattice even at room temperature. Using the room temperature hydrogen diffusion coefficient in Pd of 4 1011 m2s1 [41] one can calculate that hydrogen diffusion through the sample
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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thickness of 0.25 mm takes on average 26 min. Hence 1 h period can be considered as sufficient for equilibration of hydrogen concentration across the sample thickness. Since measurement for 1 h is not enough for decomposition of LT spectra into individual components the spectra were analyzed using the mean positron lifetime tmean, which is a weighted average of lifetimes of individual components tmean ¼
X
ti Ii ;
(2)
i
and can be obtained by single component fitting of LT spectra. The development of tmean with the hydrogen concentration is plotted in Fig. 2. The mean positron lifetimes calculated from ex-situ LT measurements by Eq. (2) are plotted in Fig. 2 as well. From inspection of the figure one can conclude that there is very reasonable agreement between ex-situ and in-situ LT measurements. It does not mean that no hydrogen was released from Pd sample during ex-situ measurements but it testifies that hydrogen-induced defects are stable enough to survive in the samples at room temperature. As a consequence, the concentration of hydrogen-induced defects is comparable in samples investigated by ex-situ and in-situ LT measurement. From inspection of Fig. 2 it becomes clear that in the aphase region tmean increases approximately linearly due to creation of vacancies. In the two-phase field tmean increases further because of dislocations introduced by a to a' phase transition. However, the increase in the two-phase field is less steep and seems to occur in certain distinct steps. The AE signal measured during first hydrogen loading of a Pd sample is plotted in Fig. 3a. The cumulative number of the AE counts (i.e the total number of AE counts recorded from the start of loading till certain hydrogen concentration xH was absorbed in the sample) is plotted in Fig. 4 as a function of hydrogen concentration. In the a-phase region the AE activity
Fig. 2 e The mean positron lifetime obtained by in-situ LT measurement (open circles) plotted as a function of hydrogen concentration in the sample. The mean lifetimes calculated by Eq. (2) from data for samples measured exsitu are plotted in the figure as well (full stars). The dashed line denotes boundary of the a-phase and the two-phase (a þ a') region.
was quite low since hydrogen-induced vacancies do not produce any measurable AE signals. In the two phase field the AE activity increased remarkably due to dislocations produced by a to a' transition. It has to be mentioned that AE detects surface displacements created by gliding dislocations. However, a signal produced by single dislocation is too low to be detected [42]. Hence, measurable AE counts are produced by collective motion of hundreds dislocations [42]. Interestingly, AE signal in Fig. 3a has a burst character, where individual events occurring in a material result in discrete AE signals. It should be mentioned that similar feature was observed also in previous AE measurement reported in Ref. [22]. Moreover, sudden steps can be observed also in the development of tmean recorded by in-situ LT measurement, see Fig. 2. This indicates that defects are introduced in bursts in the two-phase field. Gradually increasing concentration of absorbed hydrogen causes continuous increase of stress in the lattice. When the hydrogen-induced stress reaches certain critical level a portion of sample volume is suddenly transformed into the a' phase. This is accompanied by generation and motion of many dislocations detected by AE as bursts and by LT spectroscopy as step-like increase of tmean. The AE signal plotted in Fig. 3a was measured on a sample, which was loaded up to the two-phase field (xH ¼ 0.13 H/Pd), then loading was stopped at the sample was unloaded. The AE signal recorded during unloading is shown in Fig. 3b. The hydrogen concentration shown on the x-axis in Fig. 3b was estimated from EMF measurements of unloaded sample. Since unloading was performed using constant voltage one cannot use the Faraday's law for determination of hydrogen concentration. It is clear from the figure that the AE events were created not only during hydrogen loading but also in the course of unloading. This is caused by the fact that dislocations were introduced not only by a to a' transition but also by backward transition from a' to a. The a 4 a' transitions are connected with plastic deformation due to large volume mismatch between the a and a' phase [15]. It should be mentioned that formation of dislocations during the a 4 a' transitions in Pd was observed by LT spectroscopy also in Ref. [5]. Comparing Fig. 3a and b one can notice that contrary to loading where the AE bursts occurred occasionally, in the course of unloading, the AE events appeared almost continuously as long as a' was transforming into a-phase. When the phase transition into the a-phase was completed and AE signals almost disappeared. Different character of the AE signals during loading and unloading is likely caused by two reasons: (i) the a' to a transition during unloading is faster than that of a to a' during loading since unloading was performed using a constant voltage of 0.8 V and the electric potential of unloaded sample was higher than in case of loading using constant current; (ii) dislocations introduced into the sample by a to a' transition during first loading provide nucleation sites for a particles formed during subsequent a' to a transition in the course of unloading. As a consequence, the nucleation rate of a-phase particles during unloading is faster. The unloaded sample was subjected to second loading with hydrogen and observed AE signals are shown in Fig. 3c. The AE events occur again in bunches but the frequency of their appearance became higher. This can be clearly seen from comparison of cumulative AE counts plotted in Fig. 4. In the
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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Fig. 3 e AE signals recorded during (a) first hydrogen loading; (b) unloading; (c) second hydrogen loading. The dashed line denotes boundary of the a-phase and the two-phase (a þ a') region.
second loading the cumulative number of AE counts increases significantly faster than in the first loading procedure. Dislocations introduced by previous loading and unloading cycle provide nucleation sites for a' particles. This makes the a' to a transition easier in the second loading. Higher rate of a' phase precipitation in the second loading leads to a higher rate of AE counts seen as faster increase in the cumulative number of AE counts in Fig. 4. Moreover, the cumulative AE curve for the second loading cycle has less step-like character compared to that for the first one. This testifies that formation of hydride particles occurs more frequently since precipitation of a' phase is facilitated by dislocations introduced by the previous
loading-unloading cycle. Loading-unloading cycling increases the net dislocation density in the sample since dislocations introduced by the next loading were added to those created by the previous loading-unloading cycle. The development of HV during hydrogen loading is shown in Fig. 5. The sample was loaded within the a-phase region up to hydrogen concentration xH ¼ 0.011 H/Pd. Then it was unloaded and after unloading it was loaded second time. HV strongly increased during hydrogen loading in the a-phase field due to solid solution hardening by hydrogen occupying the octahedral interstitial sites in fcc Pd lattice. By unloading hydrogen was removed from the interstitial sites and HV
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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Fig. 4 e The cumulative number of AE counts, i.e. cumulative histogram of AE counts in Fig. 3a and b, detected during first and second loading of Pd sample plotted as a function of hydrogen concentration. The dashed line denotes boundary of the a-phase and the twophase (a þ a') region.
Fig. 6 e The yield strength s plotted as a function of c2/3 H . Solid line is linear regression of the data in the a-phase region. The dashed line denotes boundary of the a-phase and the two-phase (a þ a') region.
where cH is the atomic concentration of solute and the constant K depends on type of the solute. Fig. 6 shows the yield strength s estimated from HV using the well-known relation s z HV/3 plotted as a function of c2/3 H . Obviously, the relation
(3) holds within the a-phase region testifying to solid solution hardening by absorbed hydrogen. The solute strengthening constant K ¼ (2.4 ± 0.1) GPa was obtained from linear regression of data in Fig. 6. This value is comparable with the room temperature solute strengthening constant of z2 GPa caused by hydrogen absorbed in V-10%Ti alloy [45]. It has to be noted that the strengthening constant K decreases with increasing temperature [46]. Another Pd sample was loaded with hydrogen in the whole concentration range and the development of HV is plotted in Fig. 7. Obviously HV increases with hydrogen concentration not only in the a-phase region but also in the two-phase field where HV increases due to strain hardening by dislocations introduced by a to a' phase transition. When the phase transition to a'-phase was completed the sample was unloaded. One can see in Fig. 7 that unloading causes additional strain
Fig. 5 e The dependence of HV on hydrogen concentration for Pd sample loaded and unloaded within the a-phase region. The dashed line denotes boundary of the a-phase and the two-phase (a þ a') region.
Fig. 7 e The development of HV during loading-unloading cycling of a Pd sample. The dashed lines denote boundaries of the a-phase filed, two-phase field (a þ a') and a'-phase region.
decreased back to the initial value. The development of HV during second hydrogen loading was almost the same as in the first one. This testifies that solid solution hardening by absorbed hydrogen is fully reversible process. Note that hydrogen-induced vacancies detected by PAS have negligible influence on HV in the a-phase region. It is known [43,44] that for solid solution hardening mechanism the yield strength obeys the relation 2=3
s ¼ KcH
(3)
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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hardening caused by dislocations introduced by the backward transformation from a' to a-phase. After unloading the sample was loaded with hydrogen again. The second loadingunloading cycle caused further hardening and HV finally saturated at HV z 190, i.e. the strength of the material increased by z 300%. Hence, both a 4 a' transitions during loading-unloading cycling introduce dislocations, which cause strengthening of Pd. This effect is called in literature ‘hydrogen phase naklep (cold work)’ (HNP) [47e49]. Similar phenomenon, i.e. strengthening of material by dislocations introduced by phase transition is known in steels (a 4 g and g 4 ε transformations). In Pd the strengthening is caused by a 4 a' transition controlled by absorbed hydrogen. One can see in Fig. 7 that HV reaches maximum values for xH ¼ 0.04e0.10 H/Pd, i.e. in the two-phase region when a' particles precipitate in the a matrix and their appearance and growth introduce dislocations. Since the a'-phase exhibits lower hardness that the a-phase HV decreases when the volume fraction of a'-phase becomes dominant, see Fig. 7. From inspection of Fig. 7 one can realize that the hardening effect during unloading is significantly higher than that during the first loading. This is caused by interaction of dislocations. In the first loading dislocations were introduced into a dislocation-free material. But in the subsequent unloading dislocations were created in material containing already a high dislocation density. Dislocations exiting already in the sample act as obstacles for motion of new dislocations created during unloading. Gliding dislocations inevitably intersect dislocations existing already in the sample and a dense network of anchored dislocations is formed. Such theredimensional array of dislocations has significantly higher hardening effect than isolated dislocations [40].
Conclusions Hydrogen-induced defects in Pd electrochemically charged with hydrogen were studied by in-situ PAS combined with AE and HV investigations. Good agreement between ex-situ and in-situ PAS measurement testifies that hydrogen induced defects are stable at room temperature. In the a-phase region hydrogen loading introduced vacancy-hydrogen complexes. Hydrogen absorbed in octahedral interstitial sites causes solid solution hardening of Pd and this process is fully reversible. Both the a to a' transition during loading and the backward transition from a' to a in the course of unloading introduce dislocations. As a consequence, repeated a 4 a' transitions during loading-unloading cycles lead to a remarkable strengthening of Pd. Moreover a 4 a' transitions become easier in a sample subjected to loadingunloading hydrogen cycling since dislocations introduced by these transitions serve as nucleation sites for a or a' particles.
Acknowledgement Financial support by the Czech Science Agency (project P108/ 12/G043) is highly acknowledged.
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[23] Ramesh R, Mukhopadhyay CK, Jayakumar T, Raj B. Characterization of acoustic emission generated during electrochemical charging and discharging of hydrogen in palladium. Scr Mater 1998;38:661e5. [24] Harada S, Takanori O, Jiaxiang P. Acoustic emission measurements on palladium hydrogenation process by using new gas pressure cell. Mater Trans 2012;53:1726e31. ´z P, Cı ek J, Pundt A. Stress release and [25] Gemma R, Dobron defect occurrence in V1xFex films upon hydrogen loading: H-induced superabundant vacancies, movement and creation of dislocations. Acta Mater 2014;67:308e23. ´z P, Cı ek J, Pundt A. [26] Wagner S, Kramer T, Uchida H, Dobron Mechanical stress and stress release channels in 10e350 nm palladium hydrogen thin films with different microstructures. Acta Mater 2016;114:116. ´z zka I, Vlach M, Zaludov N, Dobron P, ek J, Procha [27] Cı a Chmelı´k F, et al. Multi-scale analysis of hydrogen-induced buckling in Pd films. Proc Eng 2009;1:99e103. [28] Kirchheim R. Interaction of hydrogen with dislocations in palladiumdI. Activity and diffusivity and their phenomenological interpretation. Acta Metall 1981;29:835e43. ´z va r F, Cı zka I, Janotova J. The asset of ultraek J, Procha [29] Bec fast digitizers for positron-lifetime spectroscopy. Nucl Instrum Methods A 2005;539:372e85. zka I, Novotny´ I, Bec va r F. Application of maximum[30] Procha likelihood method to decomposition of positron-lifetime spectra to finite number of components. Mater Sci Forum 1997;255e257:772e4. [31] Mu¨tschele T, Kirchheim R. Segregation and diffusion of hydrogen in grain boundaries of palladium. Scr Metall 1987;21:135e40. [32] Mu¨tschele T, Kirchheim R. Hydrogen as a probe for the average thickness of a grain boundary. Scr Metall 1987;21:1101e4. [33] Kirchheim R, Mu¨tschele T, Kieninger W, Gleiter H, TD. Hydrogen in amorphous and Birringer R, Koble nanocrystalline metals. Mater Sci Eng 1988;99:457e62. [34] Kirchheim R. Activity and diffusivity of hydrogen in deformed palladium. Scr Metall 1980;14:905e10.
[35] Kirchheim R. Interaction of hydrogen with dislocations in palladium e II. Interpretation of activity results by a FermiDirac distribution. Acta Metall 1981;29:845e53. [36] Campillo Robles JM, Ogando E, Plazaola F. Positron lifetime calculation for the elements of the periodic table. J Phys Condens Matter 2007;19:176222. [37] You YW, Kong X-S, Wu X-B, Xu Y, Fang QF, Chen JL, et al. Dissolving, trapping and detrapping mechanisms of hydrogen in bcc and fcc transition metals. AIP Adv 2013;3:0121118. [38] Xing W, Chen X-Q, Xie Q, Lu G, Dianzhong L, Li Y. Unified mechanism for hydrogen trapping at metal vacancies. Int J Hydrogen Energy 2014;39:11321e7. [39] McLellan RB, Yang L. Hydrogen-vacancy interactions in palladium. Acta Metall Matter 1995;43:2463e7. [40] Hull D, Bacon DJ. Introduction to dislocations. 5th ed. Amsterdam: Elsevier; 2011. € lkl J, Alefeld G. Diffusion of hydrogen in metals. In: [41] Vo € lkl J, editors. Hydrogen in metals I. Berlin: Alefeld G, Vo Springer-Verlag; 1978. p. 321e44. [42] Heiple CR, Carpenter SH. Acoustic emission produced by deformation of metals and alloys - a review: Part I. J Acoust Em 1987;6:177e204. € rtung. [43] Labusch R. Statistische theorien der mischkristallha Acta Metall 1972;20:917e27. [44] Shercliff HR, Ashby MF. A process model for age hardening of aluminium alloysdI. The model. Acta Metall Mater 1990;38:1789e802. [45] Spitzig WA, Owen CV, Buck O, Rowland TJ. Hydrogeninduced strengthening in V-Ti alloys. J Less Comm Met 1985;115:45e56. [46] Zong YY, Huang S, Wang Y, Shan DB. Strengthening versus softening mechanisms by hydrogen addition in b-Ti40 alloy. Int J Hydrogen Energy 2014;39:3498e504. [47] Goltsov VA. The phenomenon of controllable hydrogen phase naklep and the prospects for its use in metal science and engineering. Mater Sci Eng 1981;49:109e25. [48] Goltsova MV. Reverse hydride transformations in the PdeH system. Int J Hydrogen Energy 2006;31:223e9. [49] Zhirov GI, Goltsova MV, Shatalova GE. Hydrogen phase naklep influence on palladium grain and fine structures. Int J Hydrogen Energy 2006;31:231e6.
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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In-situ characterization of hydrogen-induced defects in palladium by positron annihilation and acoustic emission ´ b, P. Hruska a J. Cı zek a,*, O. Melikhova a, P. Dobron ch Department of Low-Temperature Physics, Faculty of Mathematics and Physics, Charles University, V Holesovicka 2, CZ-180 00 Prague 8, Czech Republic b Department of Physics of Materials, Faculty of Mathematics and Physics, Charles University, Ke Karlovu 5, CZ-121 16 Prague 2, Czech Republic a
article info
abstract
Article history:
Positron annihilation spectroscopy was employed for characterization of hydrogen-
Received 12 January 2017
induced defects in Pd. Positron annihilation studies were performed in-situ during elec-
Received in revised form
trochemical hydrogen charging and were combined with measurement of acoustic emis-
26 April 2017
sion, which is a non-destructive technique capable of monitoring of collective dislocation
Accepted 27 April 2017
motion. It was found that hydrogen loading introduced defects into Pd lattice, namely
Available online xxx
vacancies and dislocations. At low concentrations (a-phase) hydrogen loading created vacancies associated with hydrogen. Stresses induced by growing a'-phase particles led to
Keywords:
plastic deformation and introduced dislocations into the sample. Moreover, additional
Palladium
vacancies were introduced into the sample by crossing dislocations. Vickers hardness
Hydrogen
testing revealed that hydrogen absorbed in interstitial sites causes solid solution hard-
Acoustic emission
ening. Further hardening was caused by dislocations when a'-phase particles are formed.
Positron annihilation spectroscopy
Pd sample completely transformed into the a'-phase was subsequently unloaded. Decomposition of a'-phase particles during unloading caused further increase of dislocation density and led to an additional hardening. Loading-unloading of Pd sample with hydrogen continuously generates dislocations and makes the sample harder. © 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved.
Introduction Hydrogen absorbed in host metal lattice is strongly attracted to open volume defects, e.g. vacancies and dislocations. As a consequence, formation energies of open volume defects can be drastically reduced in the presence of hydrogen [1]. From the thermodynamic point of view this can be understood in the frame of the theory of defactants as hydrogen segregation
at open volume defects [2,3]. Moreover, additional defects can be introduced by formation of hydride particles [4e6]. Hence, hydrogen loading typically leads to an increase in the concentration of open volume defects. Hydrogen forms complexes with open volume defects, e.g. vacancies, and concentration of these complexes can be controlled by variation of hydrogen concentration in the sample [7]. Configurations of hydrogen-defect complexes depend on the host lattice structure as well as on the local electron density in defects.
* Corresponding author. Fax: þ420 2 2191 2567. ´z ek). E-mail address: [email protected] (J. Cı http://dx.doi.org/10.1016/j.ijhydene.2017.04.275 0360-3199/© 2017 Hydrogen Energy Publications LLC. Published by Elsevier Ltd. All rights reserved. ´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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The lowest energy configuration of vacancy-hydrogen complexes has been calculated by density functional theory for number of metals, e.g. Pd [8,9], Nb [9], Al [10], Fe [11]. But experimental studies of vacancy-hydrogen complexes are relatively rare [4e7,12,13]. Positron annihilation spectroscopy (PAS) [14] is a non-destructive technique, which enables characterization hydrogen-defect complexes on the atomic scale. In the present work, PAS was employed for characterization of hydrogen-induced defects in Pd which is widely used as a model system for investigation of hydrogen absorbed in solid lattice. Pd is able to absorb relatively large amount of hydrogen and can be charged with hydrogen easily [15]. Hydrogen absorbed in Pd forms interstitial solid solution in fcc Pd lattice, so called a-phase. Above the critical temperature of z295 C hydrogen is fully soluble in Pd up to the atomic ratio 1.0 H/Pd [15,16]. At lower temperatures, the hydrogen solubility in the a-phase decreases and in certain range of hydrogen concentration the Pd-H system becomes a mechanical mixture of two phases: the a-phase with lower hydrogen content and hydrogen-rich a'-phase [17]. Both the a-phase and the a'-phase exhibit fcc structure and differ by hydrogen content. The a'-phase exhibits significantly higher lattice parameter than the a-phase due to higher concentration of absorbed hydrogen. At room temperature, the maximum hydrogen solubility in the a-phase is 0.017 H/Pd [17] and it exhibits lattice parameter of 0.3895 nm [18]. The miscibility gap, i.e. the region of co-existence of the a-phase and a'-phase, extends at room temperature from the hydrogen concentration of 0.017 H/Pd up to 0.58 H/Pd [17]. Within the miscibility gap the volume fraction of the a'-phase increases with increasing hydrogen concentration at the expense of the aphase. When the concentration of absorbed hydrogen reaches 0.58 H/Pd the system is completely transformed into the a'phase with lattice parameter of 0.4034 nm [18]. Our previous PAS study [19] of Pd charged electrochemically with hydrogen revealed that hydrogen loading introduced vacancies in the a-phase region and dislocations in the two-phase field when a'-phase particles were formed. These previous PAS measurements were performed ex-situ, i.e. the sample was firstly charged with hydrogen in a loading cell, and then it was taken out of the cell for measurement of positron lifetime (LT) spectrum. It took approximately 30 h to acquire statistics high enough for decomposition of a LT spectrum into individual components. Some hydrogen was likely desorbed from the sample during PAS measurement at room temperature. Hence, it is interesting to examine the formation of hydrogen-induced defects in-situ during hydrogen loading. In the present work ex situ PAS studies were supplemented by in-situ PAS investigations performed during electrochemical charging of Pd samples with hydrogen. Moreover, PAS studies were combined with measurement of acoustic emission (AE) [20,21] which is a non-destructive technique capable of monitoring of collective dislocation motion. Only few investigations of AE caused by hydrogen absorption in bulk Pd has been performed so far [22e24]. These pioneering studies revealed that AE events are emitted during hydrogen loading. However, the development of AE signals has not been correlated with hydrogen concentration in the sample so far. In addition, the AE technique has been
successfully employed for monitoring of hydrogen-induced stress release in thin films [25,26] as well as buckling and detachment of films from the substrate [27].
Experimental details Polycrystalline Pd (99.95% purity) specimens with dimensions 10 10 0.25 mm3 were firstly annealed at 1000 C for 2 h in a vacuum to remove virtually all defects introduced during previous casting and shaping. Subsequently the specimens were electrochemically charged with hydrogen in a cell filled with electrolyte consisting of a mixture of H3PO4 and glycerine (volume ratio 1:2). Hydrogen loading was performed at ambient temperature using a galvanostat and applying constant current pulses between a Pt counter electrode (anode) and the Pd sample (cathode). Before first hydrogen loading the Pd sample was unloaded for 24 h in order to remove all residual hydrogen from the lattice. The hydrogen concentration introduced into the sample was calculated from the transported charge using the Faraday's law. Unloading was performed in the same electrolyte using a constant voltage of 0.8 V and opposite polarity. The unloading voltage chosen is below the anodic oxidation potential (evolution of oxygen gas) [22]. Hydrogen absorption in Pd was monitored by measuring the electromotoric force EMF [28]. To convert EMF values into hydrogen pressures, the Nernst equation was used with a reference potential of the Ag/AgClsat electrode of 0.197 V against a standard hydrogen electrode, at 298 K. For bulk metalehydrogen systems, the result of the EMF measurement is equivalent to a pressureecomposition isotherm [25]. A digital positron lifetime (LT) spectrometer [29] with excellent time resolution of 145 ps (FWHM of the resolution function) was employed for LT investigations of Pd samples loaded with hydrogen. At least 107 positron annihilation events were accumulated in each LT spectrum which was subsequently decomposed into individual components using a maximum likelihood procedure [30]. A 22Na positron source with activity of ~1 MBq deposited on a 2 mm thick Mylar foil was always forming a sandwich with two identically treated Pd specimens. For in-situ LT measurements positron source was hermetically sealed between two Pd samples which were immersed in the electrolyte. The source contribution in the LT spectra was determined using a well annealed Pd sample. It consisted of two components with the lifetimes of z368 ps and z1.5 ns, and the corresponding intensities of z8% and z1% which come from positrons annihilated in the 22Na source spot and the covering Mylar foil, respectively. The source components were always subtracted from LT spectra. The in-situ AE measurements were performed in similar configuration as in Ref. [22] using a computer-controlled PCI-2 device (Physical Acoustic Corporation) using a piezoelectric transducer MST8S (3 mm diameter) with flat response between 100 and 600 kHz and a preamplifier giving a gain of 40 dB. The signal sampling rate was 2 Hz. The sensor was attached to the backside of the hydrogen loaded (or unloaded) Pd sample. The threshold level for detection of AE counts was set above the peak noise level to 26 dB. A Struers Durascan 2 hardness tester was used for the Vickers hardness (HV) measurements. Hardness of hydrogen
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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loaded samples was measured using the load of 0.5 kg applied for 10 s. Each HV value reported in the paper was obtained as average of 10 measurements.
Results and discussion Fig. 1 shows the development of EMF with the concentration of hydrogen absorbed in a Pd sample electrochemically charged with hydrogen. The sample was step-by-step charged with hydrogen till it was fully converted into the a0 -phase. When the loading was completed the sample was unloaded for 24 h and the loading cycle was repeated. If the hydrogen concentration xH absorbed in the sample obeys the Sievert's law then EMF is proportional to the logarithm of hydrogen concentration [25]. EMF ¼ EMFref þ
RT ln xH ; F
(1)
where EMFref is a reference potential, T is the thermodynamic temperature, R is the universal gas constant and F is the Faraday's constant. Eq. (1) is plotted in Fig. 1 by a solid line. One can see in the figure that EMF behaves according to the Sievert's law in the a-phase region as expected. But a strong deviation from the Sievert's law was observed at very low hydrogen concentrations. This is likely due to hydrogen trapping at grain boundaries of polycrystalline Pd sample. Note that similar deviation of EMF from the Sievert's law was observed in nanocrystalline Pd loaded with hydrogen [31e33]. Since nanocrystalline material contains high density of grain boundaries the deviation from the Sievert's law in the nanocrystalline Pd is extended to higher hydrogen concentrations than in the polycrystalline sample studied in the present work. Hydrogen firstly fills grain boundaries till all traps at grain boundaries are filled. Subsequently it occupies octahedral interstitial sites inside grains and EMF behaves according to Eq. (1). Note that drop in EMF at low hydrogen concentrations occurs only when the virgin sample is electrochemically
Fig. 1 e EMF as a function of the hydrogen concentration xH absorbed in Pd sample. The solid line is a prediction by the Sievert's law expressed by Eq. (1). The dashed lines denote boundaries of the a-phase field, two-phase field (a þ a') and a'-phase region.
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unloaded prior to the first loading otherwise the traps are already filled with residual hydrogen contained in the sample. Hence, hydrogen trapped at grain boundaries can be released from these traps by electrochemical unloading. Interestingly, for second and third loading the deviation from the Sievert's law extended to higher hydrogen concentrations see Fig. 1. This indicates that previous hydrogen loading introduced defects into the sample. These defects trap hydrogen and their concentration increases with increasing number of loading-unloading cycles. As will be shown in the following text these traps created by loading-unloading cycling are dislocations. It has been demonstrated [28,34,35] that dislocations act as trapping sites for hydrogen. The annealed Pd samples exhibited single component LT spectrum with the lifetime tB ¼ (110.1 ± 0.5) ps which agrees well with the calculated bulk lifetime for Pd [36]. Hence, one can conclude that the annealed sample exhibits very low density of defects and virtually all positrons are annihilated in the free state (i.e. not trapped at defects). The ex-situ LT investigations of hydrogen loaded Pd were analyzed in Ref. [19] and it has been found that LT spectra contained up to three components. The shortest component with lifetime t1 < tB comes from free positrons, the component with lifetime t2 z 160 ps represents a contribution of positrons trapped at dislocations and the longest component with lifetime t3 z 200 ps can be attributed to positrons trapped at vacancies. At low hydrogen concentrations when the system is in the a-phase region hydrogen loading introduced only vacancies, i.e. no plastic deformation took place. There is an attractive interaction between vacancy and hydrogen atom absorbed in Pd. This is caused by the fact that hydrogen atom segregated at internal surface of vacancy is energetically more stable than interstitial hydrogen in Pd lattice [8,37]. Even multiple hydrogen atoms can be trapped in single vacancy [38]. As a consequence the vacancy formation energy is reduced by the binding energies released when hydrogen atoms are trapped at the vacancy [1e3] and the equilibrium concentration of vacancies increases by many order of magnitude [1,4,39]. The formation of a'-phase particles was accompanied by introduction of dislocations and additional vacancies which were created by non-conservative movement of dislocations. In a material containing a high dislocation density a dislocation moving in its slip plane will intersect other dislocations crossing the slip plane. This unavoidably leads to formation of jogs at dislocations. Motion of a screw dislocation with jogs is possible only by a non-conservative process, i.e. climb of jogs, which creates rows of vacancies or interstitials [40]. Moreover, vacancies can be created also as a result of relaxation of the misfit strain energy between a'-phase particles and the matrix as suggested by Sakaki et al. [4,5]. Another set of Pd samples was investigated by in-situ LT measurement during electrochemical charging. A low current density of 0.05 mA/cm2 was used to ensure that hydrogen concentration in the sample increased very slowly. In total the in-situ loading experiment took 75 days and LT spectrum was recorded every hour. Hydrogen is very mobile in Pd lattice even at room temperature. Using the room temperature hydrogen diffusion coefficient in Pd of 4 1011 m2s1 [41] one can calculate that hydrogen diffusion through the sample
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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thickness of 0.25 mm takes on average 26 min. Hence 1 h period can be considered as sufficient for equilibration of hydrogen concentration across the sample thickness. Since measurement for 1 h is not enough for decomposition of LT spectra into individual components the spectra were analyzed using the mean positron lifetime tmean, which is a weighted average of lifetimes of individual components tmean ¼
X
ti Ii ;
(2)
i
and can be obtained by single component fitting of LT spectra. The development of tmean with the hydrogen concentration is plotted in Fig. 2. The mean positron lifetimes calculated from ex-situ LT measurements by Eq. (2) are plotted in Fig. 2 as well. From inspection of the figure one can conclude that there is very reasonable agreement between ex-situ and in-situ LT measurements. It does not mean that no hydrogen was released from Pd sample during ex-situ measurements but it testifies that hydrogen-induced defects are stable enough to survive in the samples at room temperature. As a consequence, the concentration of hydrogen-induced defects is comparable in samples investigated by ex-situ and in-situ LT measurement. From inspection of Fig. 2 it becomes clear that in the aphase region tmean increases approximately linearly due to creation of vacancies. In the two-phase field tmean increases further because of dislocations introduced by a to a' phase transition. However, the increase in the two-phase field is less steep and seems to occur in certain distinct steps. The AE signal measured during first hydrogen loading of a Pd sample is plotted in Fig. 3a. The cumulative number of the AE counts (i.e the total number of AE counts recorded from the start of loading till certain hydrogen concentration xH was absorbed in the sample) is plotted in Fig. 4 as a function of hydrogen concentration. In the a-phase region the AE activity
Fig. 2 e The mean positron lifetime obtained by in-situ LT measurement (open circles) plotted as a function of hydrogen concentration in the sample. The mean lifetimes calculated by Eq. (2) from data for samples measured exsitu are plotted in the figure as well (full stars). The dashed line denotes boundary of the a-phase and the two-phase (a þ a') region.
was quite low since hydrogen-induced vacancies do not produce any measurable AE signals. In the two phase field the AE activity increased remarkably due to dislocations produced by a to a' transition. It has to be mentioned that AE detects surface displacements created by gliding dislocations. However, a signal produced by single dislocation is too low to be detected [42]. Hence, measurable AE counts are produced by collective motion of hundreds dislocations [42]. Interestingly, AE signal in Fig. 3a has a burst character, where individual events occurring in a material result in discrete AE signals. It should be mentioned that similar feature was observed also in previous AE measurement reported in Ref. [22]. Moreover, sudden steps can be observed also in the development of tmean recorded by in-situ LT measurement, see Fig. 2. This indicates that defects are introduced in bursts in the two-phase field. Gradually increasing concentration of absorbed hydrogen causes continuous increase of stress in the lattice. When the hydrogen-induced stress reaches certain critical level a portion of sample volume is suddenly transformed into the a' phase. This is accompanied by generation and motion of many dislocations detected by AE as bursts and by LT spectroscopy as step-like increase of tmean. The AE signal plotted in Fig. 3a was measured on a sample, which was loaded up to the two-phase field (xH ¼ 0.13 H/Pd), then loading was stopped at the sample was unloaded. The AE signal recorded during unloading is shown in Fig. 3b. The hydrogen concentration shown on the x-axis in Fig. 3b was estimated from EMF measurements of unloaded sample. Since unloading was performed using constant voltage one cannot use the Faraday's law for determination of hydrogen concentration. It is clear from the figure that the AE events were created not only during hydrogen loading but also in the course of unloading. This is caused by the fact that dislocations were introduced not only by a to a' transition but also by backward transition from a' to a. The a 4 a' transitions are connected with plastic deformation due to large volume mismatch between the a and a' phase [15]. It should be mentioned that formation of dislocations during the a 4 a' transitions in Pd was observed by LT spectroscopy also in Ref. [5]. Comparing Fig. 3a and b one can notice that contrary to loading where the AE bursts occurred occasionally, in the course of unloading, the AE events appeared almost continuously as long as a' was transforming into a-phase. When the phase transition into the a-phase was completed and AE signals almost disappeared. Different character of the AE signals during loading and unloading is likely caused by two reasons: (i) the a' to a transition during unloading is faster than that of a to a' during loading since unloading was performed using a constant voltage of 0.8 V and the electric potential of unloaded sample was higher than in case of loading using constant current; (ii) dislocations introduced into the sample by a to a' transition during first loading provide nucleation sites for a particles formed during subsequent a' to a transition in the course of unloading. As a consequence, the nucleation rate of a-phase particles during unloading is faster. The unloaded sample was subjected to second loading with hydrogen and observed AE signals are shown in Fig. 3c. The AE events occur again in bunches but the frequency of their appearance became higher. This can be clearly seen from comparison of cumulative AE counts plotted in Fig. 4. In the
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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Fig. 3 e AE signals recorded during (a) first hydrogen loading; (b) unloading; (c) second hydrogen loading. The dashed line denotes boundary of the a-phase and the two-phase (a þ a') region.
second loading the cumulative number of AE counts increases significantly faster than in the first loading procedure. Dislocations introduced by previous loading and unloading cycle provide nucleation sites for a' particles. This makes the a' to a transition easier in the second loading. Higher rate of a' phase precipitation in the second loading leads to a higher rate of AE counts seen as faster increase in the cumulative number of AE counts in Fig. 4. Moreover, the cumulative AE curve for the second loading cycle has less step-like character compared to that for the first one. This testifies that formation of hydride particles occurs more frequently since precipitation of a' phase is facilitated by dislocations introduced by the previous
loading-unloading cycle. Loading-unloading cycling increases the net dislocation density in the sample since dislocations introduced by the next loading were added to those created by the previous loading-unloading cycle. The development of HV during hydrogen loading is shown in Fig. 5. The sample was loaded within the a-phase region up to hydrogen concentration xH ¼ 0.011 H/Pd. Then it was unloaded and after unloading it was loaded second time. HV strongly increased during hydrogen loading in the a-phase field due to solid solution hardening by hydrogen occupying the octahedral interstitial sites in fcc Pd lattice. By unloading hydrogen was removed from the interstitial sites and HV
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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Fig. 4 e The cumulative number of AE counts, i.e. cumulative histogram of AE counts in Fig. 3a and b, detected during first and second loading of Pd sample plotted as a function of hydrogen concentration. The dashed line denotes boundary of the a-phase and the twophase (a þ a') region.
Fig. 6 e The yield strength s plotted as a function of c2/3 H . Solid line is linear regression of the data in the a-phase region. The dashed line denotes boundary of the a-phase and the two-phase (a þ a') region.
where cH is the atomic concentration of solute and the constant K depends on type of the solute. Fig. 6 shows the yield strength s estimated from HV using the well-known relation s z HV/3 plotted as a function of c2/3 H . Obviously, the relation
(3) holds within the a-phase region testifying to solid solution hardening by absorbed hydrogen. The solute strengthening constant K ¼ (2.4 ± 0.1) GPa was obtained from linear regression of data in Fig. 6. This value is comparable with the room temperature solute strengthening constant of z2 GPa caused by hydrogen absorbed in V-10%Ti alloy [45]. It has to be noted that the strengthening constant K decreases with increasing temperature [46]. Another Pd sample was loaded with hydrogen in the whole concentration range and the development of HV is plotted in Fig. 7. Obviously HV increases with hydrogen concentration not only in the a-phase region but also in the two-phase field where HV increases due to strain hardening by dislocations introduced by a to a' phase transition. When the phase transition to a'-phase was completed the sample was unloaded. One can see in Fig. 7 that unloading causes additional strain
Fig. 5 e The dependence of HV on hydrogen concentration for Pd sample loaded and unloaded within the a-phase region. The dashed line denotes boundary of the a-phase and the two-phase (a þ a') region.
Fig. 7 e The development of HV during loading-unloading cycling of a Pd sample. The dashed lines denote boundaries of the a-phase filed, two-phase field (a þ a') and a'-phase region.
decreased back to the initial value. The development of HV during second hydrogen loading was almost the same as in the first one. This testifies that solid solution hardening by absorbed hydrogen is fully reversible process. Note that hydrogen-induced vacancies detected by PAS have negligible influence on HV in the a-phase region. It is known [43,44] that for solid solution hardening mechanism the yield strength obeys the relation 2=3
s ¼ KcH
(3)
´z ek J, et al., In-situ characterization of hydrogen-induced defects in palladium by positron anniPlease cite this article in press as: Cı hilation and acoustic emission, International Journal of Hydrogen Energy (2017), http://dx.doi.org/10.1016/j.ijhydene.2017.04.275
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hardening caused by dislocations introduced by the backward transformation from a' to a-phase. After unloading the sample was loaded with hydrogen again. The second loadingunloading cycle caused further hardening and HV finally saturated at HV z 190, i.e. the strength of the material increased by z 300%. Hence, both a 4 a' transitions during loading-unloading cycling introduce dislocations, which cause strengthening of Pd. This effect is called in literature ‘hydrogen phase naklep (cold work)’ (HNP) [47e49]. Similar phenomenon, i.e. strengthening of material by dislocations introduced by phase transition is known in steels (a 4 g and g 4 ε transformations). In Pd the strengthening is caused by a 4 a' transition controlled by absorbed hydrogen. One can see in Fig. 7 that HV reaches maximum values for xH ¼ 0.04e0.10 H/Pd, i.e. in the two-phase region when a' particles precipitate in the a matrix and their appearance and growth introduce dislocations. Since the a'-phase exhibits lower hardness that the a-phase HV decreases when the volume fraction of a'-phase becomes dominant, see Fig. 7. From inspection of Fig. 7 one can realize that the hardening effect during unloading is significantly higher than that during the first loading. This is caused by interaction of dislocations. In the first loading dislocations were introduced into a dislocation-free material. But in the subsequent unloading dislocations were created in material containing already a high dislocation density. Dislocations exiting already in the sample act as obstacles for motion of new dislocations created during unloading. Gliding dislocations inevitably intersect dislocations existing already in the sample and a dense network of anchored dislocations is formed. Such theredimensional array of dislocations has significantly higher hardening effect than isolated dislocations [40].
Conclusions Hydrogen-induced defects in Pd electrochemically charged with hydrogen were studied by in-situ PAS combined with AE and HV investigations. Good agreement between ex-situ and in-situ PAS measurement testifies that hydrogen induced defects are stable at room temperature. In the a-phase region hydrogen loading introduced vacancy-hydrogen complexes. Hydrogen absorbed in octahedral interstitial sites causes solid solution hardening of Pd and this process is fully reversible. Both the a to a' transition during loading and the backward transition from a' to a in the course of unloading introduce dislocations. As a consequence, repeated a 4 a' transitions during loading-unloading cycles lead to a remarkable strengthening of Pd. Moreover a 4 a' transitions become easier in a sample subjected to loadingunloading hydrogen cycling since dislocations introduced by these transitions serve as nucleation sites for a or a' particles.
Acknowledgement Financial support by the Czech Science Agency (project P108/ 12/G043) is highly acknowledged.
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