HAL Id: jpa-00230021
https://hal.archives-ouvertes.fr/jpa-00230021
Submitted on 1 Jan 1990
HAL is a multi-disciplinary open access archive for the deposit and dissemination of sci- entific research documents, whether they are pub- lished or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers.
L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
INFLUENCE OF HYDROGEN ON THE INTERFACE STRUCTURE AND PROPERTIES OF FATIGUED
Al-Zn-Mg-BICRYSTALS
W. Wunderlich, A. Niegel, H. Gudladt
To cite this version:
W. Wunderlich, A. Niegel, H. Gudladt. INFLUENCE OF HYDROGEN ON THE INTERFACE STRUCTURE AND PROPERTIES OF FATIGUED Al-Zn-Mg-BICRYSTALS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-709-C1-714. �10.1051/jphyscol:19901113�. �jpa-00230021�
COLLOQUE DE PHYSIQUE
Colloque Cl, suppl6ment au n O l , Tome 51, janvier 1990
INFLUENCE OF HYDROGEN ON THE INTERFACE STRUCTURE AND PROPERTIES OF FATIGUED AI-Zn-Mg-BICRYSTALS
W. WUNDERLICH", A. NIEGEL and H.J. GUDLADT
Max-Planck-Institut fur Metallforschung, Seestrasse 71, 0-7000 Ftuttgart 1, F . R . G .
Max-Planck-Institut fur Eisenforschung G m b H , Max-Planck-Strasse 1, 0-4000 Diisseldorf 1, F.R.G.
Rksumk:
L' en$c,hissement en hydroghe aux joints de grains a u?e forte influence sur les proprletes mgcaniques des alliages Al-Zn-Mg. Sous atmosphere humide, la propagation des fissures au cours des essa& d e fatigue estconsid6rablement acce'Mr6e. Si fa r6gion du joint de grain est enrichie en hydrogsne, une fragilisation locale du joint de grain apparait, conduisant al'iniriationet la propagation de fissures intergranulaires. Comme le montrent les micrographies o b t e n u e s en HREM, les pre'cipitks hydrog6ngs pr6sentent des interfaces planes e t facettges, t a n d i s que les 6 c h a n t i l l o n s non trait6sprCsentent des particules arrondies. Le joint de grain lui mgme presentedes changements structuraux qui provoquent la fragilisation.
Abstract:
Hydrogen enrichment at grain boundaries has a strong influence on the mechanical properties of Al-Zn-Mg-alloys. Under moisture crack propagation in fatigue experi- ments is accelerated drastically. If hydrogen is enriched at the grain boundary region, local embrittlement of the grain boundary takes place leading to intergranular crack initiation and propagation. As can be seen by HREM-micrographs the hydrogen a f f e c t e d
precipitates have planar and facetted interfaces, while the as recieved specimens showed round shaped particles. The grain boundary shows structural changes as well which causes the embrittlement.
The mechanical behavior of grain boundaries in aluminium alloys during fatigue experiments is of great interest for practical applications. Several types of stress corrosion cracking and hydrogen embrittlement mechanisms were discovered in the last years, which explained enhanced crack propagation in wet atmospheres [ l -61. Especially under cyclic loading conditions hydrogen atoms can penetrate into the grain boundary region and influence the velocity of a propagat~ng crack.
The goal of this paper is to clearify how hydrogen is stored in fatigued bicrystals and whether it causes any structural changes at the grain boundary region.
1 Det&
Bicrystals of a high purity Al-Zn 4.51-Mg 1.25 wt-%-alloy were produced using a modified strain-annealing-technique. The cylindrical specimens were stretched and homogenized in a salt bath (590°C, 1 h) thereafter. During the ordered recrystallization process the bicrystals are formed.
After optimizing the strain, the sink velocity and the salt bath temperature cylindrical bicrystals were produced and thereafter machined by spark erosion into flat specimens with a rectangular cross-section of 1.5 X 6 mm2 and a length of 50 mm.
Usually, the specimens were oriented for single slip in one of the two grains with the broad surface parallel to the Burgers-vector of the primary slip system. In most cases the grain boundary plane was normal to the long axis of the specimen and, therefore, to the load axis. For some special fatigue experiments specimens with a grain boundary plane nearly 45" to the load axis were prepared. The grain boundaries prepared by the strain annealing technique had no special symmetry. First analysis, however, obtained by Laue pattern and by TEM showed that small angle grain boundaries with an orientation difference less than 10" are prefered.
The specimens were homogenized for 30 min at 480°C, water quenched and thereafter elec- trolytically polished during the ageing at room temperature for 100h. In order to initiate cracks into the grain boundary during the fatigue experiments notches with a depth of about 500 pm
were spark eroded into the small face of the specimen in the neighbourhood of the grain boundary.
Finally the specimens were annealed at 135°C for 100 h to the peak aged state.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901113
Cl-710 COLLOQUE DE PHYSIQUE
The fatigue experiments were performed using an electromagnetic testing machine with sine shaped tension-compression loading with a minimum to maximum load ratio of R = -1 at frequencies of 100 Hz and 1 Hz. The atmosphere in the specimen chamber could be changed from pure dry nitrogen to different mixtures of water vapor with nitro en. Crack propagation can be measured in situ with an optical travelling microscope (magnification 2OOX) in x-y- direction as well as with a dc-potential drop method. The stress intensity factor and the crack length can be studied during each loading cycle. Further details are explained elsewhere [1,2].
The fatigued specimens were finally observed by several techniques. For the TEM-observations a precise target preparation of the region near the crack tip is needed. The specimens were therefore spark cut perpendicular to the propagated crack into slices with a thickness of about 200 pm. Then 3 mm discs were carefully punched from the vicinity of the crack tip and transparent TEM-foils were thinned with the electrochemical jet-polishing technique. Additionally, high resolution electron microscopy (HREM) with a point-to-point resolution of about 0.21 nm could be performed.
2immlki
The crack propagation rates da/dN during each cycle were measured on peak
seed
AI-Zn-Mg Bicrystals at a frequency of 100 Hz in different atmospheres (fig. 1). The fatigue tests have shown that for water vapor pressures higher than 1.2 kPa intergranular cracks were formed, whereas for lower water vapor pressures cracks propagate predominantly in a transgranular manner perpendicular to the stress direction (stage I1 crack propagation) [l]. Cracks at grain boundaries (fig. 1) have a velocity which is about two orders of magnitude higher than trans- granular cracks for stress intensity factors between 2 and 10M P ~ G
(fig. 1). At high stress intensities (AK > I O M P ~ ~ ) the crack propagation rate for both types of cracks is the same, indicating that all cracks leave the grain boundary and propagate transgranularly. At lower stress rates the in-situ study of the crack extension allows to determine the threshold value A K,, for crack propagation. For intergranular cracks this value strongly depends on the water vapor pressure, indicating that the grain boundary must be embrittled by the presence of hydrogen atoms. Hydrogen is concentrated in the boundary region by boundary diffusion and weakens it.Some bicrystals had a curved grain boundary. Two types can be distinguished (fig. 2): Either (1) the notch is placed in one grain and the crack will meet the grain boundary after growing a certain distance transgranuarly or (2) the crack starts in the grain boundary and leaves ~ t , when the grain boundary is curved. The experiment was performed in ambient air with a water vapor pressure of 1.2 kPa. In case 1 the transgranular crack changes its propagation rate if it meets the grain boundary (fig. 2) and propagates intergranularly with the same (faster) rate as shown in fig. 1. The opposite experiment (case 2 in fig. 2) shows a similar result: If the crack leaves the grain boundary the value for crack propagation decreases to that of transgranular cracks.
In order to understand hydrogen embrittlement phenomena in more detail the water vapor pressure in the specimen chamber was changed during the fatigue test. An intergranular crack was initiated in wet nitrogen (3.1 kPa). Thereafter, the environment was changed to dry nitrogen with a water vapor pressure of less than 0.035 kPa. Surprisingly, the crack propagation remained always intergranular. For further understanding of this phenomenon, the same specimens were heat treated (10 h at 135°C) a4ain prior to another fatigue test in the dry nitrogen atmosphere.
Even then the crack propagation remained intergranular. Only after a full heat treatment (ho- mogenisation at 480°C and ageing to peak hardness) the crack leaves the grain boundary and propagates transgranularly in the dry atmosphere. This is an evidence that hydrogen is trapped during the fatigue experiments in the vicinity of the grain boundary.
In case of peak aged specimens coherent q'-particles (size 15 nm) are formed in the matrix, but incoherent MgZn2-particles (size 100 nm) are found at grain boundaries surrounded by a pre- cipitation free zone (size 50 nm). The fracture surfaces cracked at low stress intensity factors appeared smooth in SEM micrographs (fig. 3) without any striations, while at high nK values grain boundary precipitates are observed on the intergranular fracture surface. (fig 4). The precipitates in the left hand side of fig 4) show a bright contrast. In this reglon the crack propagated under wet atmosphere, while on the right hand side the atmosphere was changed to dry nitrogen. In this region the precipitates are dark. On the opposite fracture surface the same change In brrghtness was observed. The reason for this change is not yet clear, but the disappearance
of precipitates in fig. 3 is explained by the TEM micrograph fig. 5: In a (underaged) specimen fatigued under moisture a small stage I (preparation) crack propagated towards the grain boundary.
Surprisingly, it did not follow the grain boundary plane, but the crack propagate at the border between precipitation free zone and matrix. Since it was formed at rather low stress intensity this observation corresponds with the SEM micrographs of fracture surfaces, where no grain boundary precipitates were observed (fig. 3).
The grain boundary region within the plastic zone in front of the crack tip was studied on several
specimens in detail by TEM and HREM. For specimen fatigued in dry nitrogen only transgranular cracks were observed. The TEM micrographs (fig. 6) showed a wavy and curved grain boundary with round grain boundary precipitates of MgZn as in the case of the as recieved specimen.
The microstructure in the specimens, which were tatigued in wet atmosphere, however, showed a significant difference. In these cases intergranular cracks were formed. The grain boundary planes near the crack tip in all these specimens are absolutely straight, although in the as recieved state randomly curved boundaries are usually present. The interfaces between the grain boundary precipitates and the matrix are also planar. In the HREM image (fig. 7) this effect, which was checked in several specimens, can be seen in more detail. Sometimes small facets mostly parallel to the closed packed (I l l)-planes of the matrix were observed, whereas for specimens fatigued in dry nitrogen no facetting was found. There the precipitates have a round shape. Certainly this difference in the grain boundary structure as well as in the interfacial structure is affected by the segregation of hydrogen during cyclic deformation.
The influence of moisture on the fatigue crack behavior of precipitation hardened Al-Zn-Mg alloys must be explained by true hydrogen embrittlement. A combination of several effects lead to an enhanced fatigue crack propagation in Aluminium under wet atmospheres [5]. At first the oxid layer at the surface must be broken and water moleculs are adsorbed on the pure Aluminium surface and dissociate. The Hydrogen atoms mainly diffuse along grain boundaries but a transport into the bulk by moving dislocations as well as by lattice diffusion is also possible. In all cases Hydrogen weakens the bonding between metal atoms especially in the grain boundary region leading to the fast intergranular crack propagation. The conceivable mechanisms of Hydrogen storage are in the order of increasing binding energy: a) pure dissolution, b) segregation at grain boundaries or interfaces, c) Hydrogen stabilized gas bubbles [7], d ) Mg- or Zn-Hydrids [8].
The experimental results truely indicated that the hydrogen is trapped in the vicinity of the grain boundary during fatigue. After the wet atmosphere was changed to dry nitrogen the crack propagation still remains intergranular. This behaviour cannot be explained by dissolved Hydrogen, because it will move out under dry atmosphere. Also the hydrogen storage in sub- mlcroscoplc gas bubbles as suspected by Christodoulou et al. [7] is not practical in this case. In the present experiments no evidence for gas bubbles or microvoids was found. Round shaped contrasts, which were found in some specimens, result, however, from some surface contamination due to specimen preparation.
The present high resolution investigation showed a correspondence between hydrogen trapping on one hand and facetted precipitation interfaces and planar grain boundary structure on the other hand. Reconstruction of grain boundaries, e.g. faceting, was already observed in several cases [g], when impurity atoms segregated at grain boundaries. Examples are faceting in Zn 1101, Bi-Segregation in Cu-Bicrystals [ l 1,121, S-Segregation in Ag-Ni-Alloys [13], Au-Segregation at Fe-Twist boundaries [14]. Such drastic changes In atomic grain boundary structures are already characterised by Balluffi [9] as phase transitions of grain boundary structures. A curved boundary may become either facetted or planar. These phase transitions can be influenced by changes in temperature or pressure or by changes of the chemical composition in the grain boundary region due to segregation.
Structural changes due to hydrogen segregation in Aluminium during fatigue was not yet observed experimentally, probably because TEM preparation of crack tip reglons are very difficult.
Hydrogen segregation at grain boundaries usually can hardly be detected directly, but mainly by measuring the properties. The observed change in the microstructure, however, seems to be the effective mechanism for hydrogen trapping in this case. Hydrogen influences the atomic bonds at grain boundary atoms in such a way that a flat boundary plane has the lowest energy.
Hence, this change in atomic structure stabilizes the Hydrogen storage in the boundary region.
It is not yet clear, whether the energy gain is the driving force for the structural change or whether Hydrogen simply weakens Aluminium bonds. This weakening could accellerate movements of atoms to find their lowest energy position even with the same driving force than before, namely the surface tension of the grain boundary. The grain boundaries were produced at high temperatures where high kinetic entropy disturbs any ordering in atomic scale at the grain boundary region. At low temperatures, however, ordering is suppressed since diffusion is low and atomic bonds are rather strong. This is the reason that as recieved specimens have curved grain boundaries.
I'n contrast to the other examples the reconstruction of the aluminium grain boundaries occurs even at room temperature. Pure self diffusion of Aluminium with its activation energy of 1.27 eV is too slow at room temperature. Interdiffusion of Zn in A1 (activation energy 0.5 eV [15]), however, is fast enough. This mechanism or a combined chemical diffusion with Al, -Zn and H may be possible explanations for the reconstruction at room temperature. Dislocations and frozen
Cl-712 COLLOQUE DE PHYSIQUE
vacancies can partly accelerate diffusion as well. Therefore, Hydrogen affected weakening of atomic bonds can explain both the ordering of the grain boundary structure as well as the embrittlement of grain boundaries during fatigue. The flat grain boundary might even lead to a faster crack propagation by structural reasons, since the crack tip is not deflected as in a curved boundary.
The interface at the grain boundary precipitates in Al-Zn-Mg alloys shows a similar reconstruction as the grain boundaries if hydrogen affects the specimen. Phase boundaries behave in general similar to grain boundaries. Evidence for faceting was found in the rotation ball experiments [l61 at LiF-Au and Ag-Ni. In the case of hydrogen segregation round shaped MgZn2-precipitates transformed to those with planar, faceted interfaces. Some crystallographic planes are prefered in the case of faceting. Directed atomic bonds might be the reason which can be described by anisotropic boundary energies.quantitative measurements of such low hydrogen concentrations are impossible. If it is more than one monolayer probably a Hydrid is already formed at a very thin layer between precipitate and matrix [8] (similar as in [17]). This might explain the different SEM contrast of those precipitates at wet and dry fracture surfaces. However, the HREM- micrographs showed nosignificantchange at lattice constants near the phase boundary.
In the fatigue experiment at low stress intensity factors SEM- and TEM-Micrographs indicated that the crack propagates in the precipitation free zone close to but not in the grain boundary plane. EDS analysis In the STEM [2] have shown a very low Zn content inside a precipitation free zone, whereas the hightest content was found at the rim of this zone. Gruhl[18] considered that Hydrogen is concentrated in the region with a high Zn content. The crack propagates in this region for low stress intensities, because the amount of Hydrogen 1s presumably not hlgh enough to embrittle the grain boundary itself. Fatigue experiments wlth different frequencies are in progress to clearify which mechanism is the time controlling process.
1) Hydrogen segregation during fatigue affects a reconstruction of the atomic grain boundary structure which can be described as a phase transition.
2) The interface structure of the precipitates became planar and facetted due to hydrogen segregation durin fatigue while the as recieved specimens showed round shaped particles. Both reconstructions ofboundaries l ) and 2) seem to be the present mechanism for hydrogen trapping.
3) The fatigue property of a grain boundary in ~l-Zn-Mg-bicrystals changed drastically under moisture. If hydrogen a enriched at the grain boundary reeion, local embrittlement of the grain boundary takes place leading to intergranular crack initiat~on and propagation.
4) Intergranular crack behavior remains, even if the environment is changed from wet to dry atmosphere. This indicates the rather strong trapping of hydrogen in the grain boundary region.
-
Prof. V. Gerold is gratefully acknowledged for useful discussions. The authors wish to acknowledge the Deutsche Forschungsgemeinschaft for the financal support.
-
[I] A. Nlegel, H.J. Gudladt, V.Gerold, Proc. FATIGUE 1987, Charlottesville, Va. USA [2] A. Niegel, H.J. Gudladt, V. Gerold, Journal de Physique, C5, 10 (1988) pp.C5-659
[3 M.O. Speidel, The Theory of Stress Corrosion Cracking in Alloys, Publ. by NATO Scientific
1
A fairs pivision Brussels 197 1, pp. 289-344
[4] J. Rlchter, H. Kaesche, Werkstoffe und Korrosion 32 (1981) pp.174 [5] R.P. Wei, G.W. Simmons, int. J. Fract. 17, (1981) pp. 235
[6] R.J. Jacko, D.J. Duquette, Hydrogen Effects in Metals, Ed. I.M. Bernstein, A.W. Thompson (1 98 1) 00.477-484
[7] ~ . ' ~ h i s t o d o u l o u , H.M. Flower, ibid. pp. 493-501 [8] C.D.S. Tuck, Metall. Trans. A, 10A, (1985), pp.1503
[9] R. Balluffi, T.E. Hsieh, Journal de Physique, CS, 10(1988), p p . C5-337-349
[l01 G.H. Bishop, W.H. Hartt, G.A. Bruggeman, Acta Met., 19, (1971), pp. 37-47
[ l l ] M.Meynhard, B.Blum, C.J.McMahon, S.Chlkwambanl, J.Weertman, J. d e Phys. C5 10 (1988) DD.CS-457
r r - -
[l21 J.S. Wang, J. Mater. Res 3 (1988) pp. 16
1131 A. C h a r a ~ , D.Roux, A.Rolland, G.Saindrenan, B. Aufray, J. de Phys., C5, 10 (1988) pp.
CS-429
[14] K.E. Sickafus, S.L. Sass, Acta Met. 35, (1987) pp. 69-79
[l51 I. Kaur, W. Gust, Fundamentals of Gram Boundary diffusion, Ziegler Press Stuttgart, (1988) 1161 R. Maurer, H.F. Fischmeister, Acta Met., 37, (1989), pp. 1177-1189
1171 F. Cabane, J. Cabane, Journal de Physique,C5, 10, (1988),pp.C5-423 [l81 W. Gruhl, Z. f. Metallkde., 75, (1984), pp. 819-826
00
crack o intergranular o intergranular m transgranular
IQ ',
stage
m
11-crackstage 11-Crack
o intergranular
.
# tmnsgrmular0
0 m lranslwcl
AK- Stress Intensity Factor Range (MPavFT) AK- Stress Intensity Factor Range ( M P a f i )
FIG. 1) Crack propagation rate vs AK in FIG. 2) The crack propagation rate changes atmospheres with different water vapor pres- if a transgranular crack meets the grain
sures. boundary (case 1) and vica versa (case 2).
FIG. 3) Fracture surface at low stress intensity FIG. 4) Fracture surface at high stress factor ( X = 1 M p a G ) intensity factor (AK = 3 M p a G ) . The atmosphere was changed from wet (white precipitates to dry nitrogen (black precipi- tates).
, crack
crack flanks overlap
FIG. 5) Crack propagation in the precipitation free zone (PFZ) near the grain boundary (GB)
Cl-714 COLLOQUE DE PHYSIQUE
FIG. 6) a) Curved grain boundary (specimens fatigued in dry nitrogen) b) planar grain boundary with facetted precipitations (wet nitrogen)
10
nm
-
FIG. 7) HREM micrograph of grain boundary precipitate; fatigued in a) dry b) wet nitrogen
