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EFFECT OF INTERFACIAL STRUCTURE AND ENERGY ON CORROSION AND
STRESS-CORROSION-CRACKING IN α Cu-Al ALLOY BICRYSTALS
M. Yamashita, T. Mimaki, S. Hashimoto, S. Miura
To cite this version:
M. Yamashita, T. Mimaki, S. Hashimoto, S. Miura. EFFECT OF INTERFACIAL STRUC- TURE AND ENERGY ON CORROSION AND STRESS-CORROSION-CRACKING IN
αCu- Al ALLOY BICRYSTALS. Journal de Physique Colloques, 1990, 51 (C1), pp.C1-715-C1-720.
�10.1051/jphyscol:19901114�. �jpa-00230022�
COLLOQUE DE PHYSIQUE
Colloque Cl, suppl.&.ment au nol, Tome 51, janvier 1990
EFFECT OF INTERFACIAL STRUCTURE AND ENERGY ON CORROSION AND STRESS-CORROSION-CRACKING IN a Cu-A1 ALLOY BICRYSTALS
M. YAMASHITA, T. MIMAKI, S. HASHIMOTO*~ ( l ) and S. MIURA*
Department of Mechanical Engineering. Faculty of Engineering, Doshisha yniversity. Kyoto-602, Japan
Department of Enginqering Science, Faculty of Engineering, Kyoto University, Kyoto-606, Japan
Abstract - Stabilities of the symmetrical <110>- and <loo>-tilt and <loo>-twist boundaries of an a Cu-AI alloy against both dissolution and Stress-Corrosion-Cracking (SCC) are discussed. The susceptibilities of the boundaries to SCC are strong1 y dependent upon the misorientation. Cusps exist at some angles which are corresponding to relatively small C-values. These boundaries indicating great resistances to SCC also possess high stabilities against dissolution. It is considered that these high stabilities against both dissolution and SCC reflect the stable structures and the low interfacial energies of the boundaries. The susceptibility of a grain boundary to SCC is much enhanced by a stress field arising from piled-up dislocations in the immediate vicinity of the boundary. It becomes evident that the susceptibility to intergranular SCC is suddenly decreased with decreasing the misorientation from approximately 15'. This may result from the increase of passing dislocations through the boundary, by which a stress concentration is avoidable, with decreasing the misorientation from the angle.
1 - INTRODUCTION
The importance of a grain-boundary structure (energy) is well recognized for various phenomena in polycrystalline aggregates. However, because of some difficulty arising from their complexities, little is available to argue the effect of a grain-boundary structure on intergranular corrosion (dissolution) and on Stress-Corrosion-Cracking (SCC). Especially, SCC is an ultimately complicated phenomenon under the influences of many factors such as electrochemical reaction, dislocation behavior, interfacial structure and energy, and crystallographic orientation of'grains.
In order to evaluate the effect of a grain-boundary structure, attempts should be made by
employing a series of symmetrical tilt or twist boundaries since the structure of which can depend only on the misorientation angle. In the present investigation, our attention is focused on the effect of an interfacial structure on dissolution and on SCC in synunetrical <110>- and <loo>-tilt and <loo>-twist boundaries. The mi sorientat ion dependences of the penetration depths of intergranular dissolution and those of the susceptibilities to SCC are revealed.
(l) Present address: Laboratoire GPM2. I. N. ?.G., Z. N. S. P. G., Domaine Univers i tairc, BP4fi, 38402 St. Martin d' Heres cedex, France
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:19901114
COLLOQUE DE PHYSIQUE
2 - EXPERIMENTAL PROCEDURES
Bicrystals with symmetrical <110>- and <loo>-tilt and <loo>-twist boundaries of a Cu-Sat. %A1 alloy were grown from seed crystals using the Bridgman method in vacuum. By taking account of Brandon's criterion /l/ for deviation angles from the ideal Coincidence-Site Lattice (CSL) relations, A 8 , these boundaries were regarded as the CSL boundaries. The crystallographic characteristics of each bicrystal is shown in Table 1. The grain boundaries of specimens were normal to the tensile axis and were located at the center of the gauge portion. The specimens were cyclically annealed in vacuum and then were mechanically polished. Finally, a deformed layer of specimen surface was removed by
electro-polishing. The cross-section and the gauge length of the specimens with the tilt boundaries were controlled to be 4.0X1.0 mm2 and 6.0 mm, respectively. The twist type specimens have circular cross-section with diameter of 2.0 mm.
A1 l the tilt boundaries listed in Table 1 were dissolved in a modified Livingston's dislocation etchant /2/ for 50 minutes at 303k2K. The depths of grooves developed at the boundaries were measured from the level of the surface, i.e. the common (110) or (100) plane, using an optical-cut method. It should be noted that no measurable groove is developed by the environment used for the present SCC test mentioned below even after 800 hours.
SCC test were conducted in a solution (NH40~:200ml+NaOH:100g+H~0:800ml) at 303k2K under constant applied stresses, a,, which were chosen in the range from 0.75 times to 2.0 times the yield stress, a,.
It was found by judging from the slip line observations in air at room temperature that the Schmid law holds, and the critical resolved shear stress of the bicrystals specimens was estimated to be 8.2 MPa.
Table 1 Crystallographic characteristics of each bicrystal.
P
Hisorientation
P -
3.7"
8.4"
35.1"
41.1"
43.0"
<110> 44.6"
Tilt 65.5"
68.3"
93.4'
D~sorientation (Rotation) (Rotation)
angle ax~s
3.71' 0.788 0.608 0.100 8.58' 0.738 0.608 0.197 35.12" 0.710 0.702 0.055 41.05" 0.707 0.707 0.003 43.00" 0.716 0.697 0.042 44.62" 0.712 0.703 0.006 58.82" 0.647 0.543 0.535 59.31' 0.609 0.563 0.558 60.17" 0.681 0.655 0.326
k-value
Deviation angle
A
e
Grain bounday
plane (110) (110) (221) (221)
<loo> 40.7" 40.79" 0.999 0.039 0.010 5 4.21" (013)
Tilt 41.5" 41.57' 0.999 0.040 0.009 5 4.96" (013)
8.2" 8.84' 0.932 0.285 0.225 1 15.8" 15.87" 0.993 0.109 0.052 25a 21.0" 21.16" 0.994 0.092 0.066 13a 23.3" 23.38' 0.997 0.070 0.035 13a
<loo> 25.0" 25.00' 0.999 0.032 0.018 85b Twist 27.0" 26.99' 0.999 0.033 0.024 17a 30.3" 30.92" 0.980 0.193 0.057
*
34.1' 34.07" 1.000 0.021 0.003 5 39.9" 39.90' 0.999 0.031 0.012 5 43.9" 43.90" 1.000 0.003 0.001 29a
* The boundary is not related to CSLs with the values of C>l01.
3 - RESULTS AND DISCUSSION
(1 l 0)- t i l t boundary
A triangular cross-sectional groove was formed as a result of the precedent dissolution at the grain boundary in the acid solution. Since the dissolution rate of the reference surface, i.e. the common (110) plane, is considered to be independent of the misorientation, it may be possible to interpret the differences in the depth of the grooves entirely in terms of a grain-boundary dissolution rate. The misorientation dependence of the depth is shown in Fig. l. Three sharp minima, or cusps, exist at 38.9"/C9(221), 70.5"/C3(111) and 129.5"/E11(113).
R o t a t i o n angle, 0 (')
Fig. l- Misorientation dependence of the depth of a dissolution groove, D, developed at symmetrical
<llO>-tilt boundaries in (Brz:2ml+CH:3COOH:60m1+HC1 :20ml+HzO:100ml).
Stress corrosion crack was initiated and propagated along the grain boundary, as shown in Fig.2.
The misorientation dependence of the susceptibility to intergranular SCC is shown in Fig.3, where the susceptibility is defined by the inverse of time spent for initiation and propagation of the brittle crack, t,;-l /3,4/. The susceptibilities of high-angle boundaries are increased abruptly with
increasing the applied stress in a relatively low stress level (approximately the yield stress). This may be due to an increase of piled-up dislocations at the boundary. Namely, the susceptibility of a grain boundary to SCC is much enhanced by a stress concentration arising from pile-up of dislocations in the immediate vicinity of the boundary. On the one hand, the low-angle boundaries show the extremely low susceptibilities. In the bicrystals with low-angle boundaries, the continuity of slip lines across the boundaries was confirmed with surface observations of the specimens strained by several percent. The passing of dislocations through the low-angle boundaries in Cu-5wt. %A1 a1 loy bicrystals has been shown from in-situ observations of dislocations using high voltage transmission
Fig. 2- Intergranular SCC observed in the bicrystal with 43" /X9(221) boundary tested under 32.6MPa for 2.9 hours.
Cl-718 COLLOQUE 1)E PHYSIQUE
electron microscopy by Fujita et al. /5/. The observed low susceptibilities of the low-angle boundaries are most likely due to the ease of slip transmission across the boundaries. That is, a stress concentrat ion arising from pi led-up dislocations is avoidable at these boundaries. The presence of a large stress concentration is a necessary condition for manifestation of SCC. It should be noted that the susceptibility of the 8.4"-boundary is slightly higher than that of the 3.7"-boundary under a high stress level. This indicates that the possibility of passing of dislocations through a low-angle boundary may decrease with increasing the misorientat ion. The susceptibility to intergranular SCC is strongly dependent on the misorientation. The cusps indicating significantly great resistances to SCC are noted at 38. g", 70.5" and 129.5", at which cusps are observed in the misorientation dependence of the intergranular groove depth mentioned above. These angles are also identical with the
misorientation angles at which energy cusps are observed in copper /6/ and aluminum /7,8/.
It is strongly suggested that the stabilities against dissolution and against SCC are closely related to the structural stability or energy; otherwise the strong misorientation dependences, especially the deep cusps, cannot be explicated. Even in a high stress level, where a large stress concentration overcomes the inherent resistance to SCC, the above stable boundaries still exhibit greater resistances than the less stable boundaries located near them, as evidenced by the clear cusps in Fig. 3. It, however, can be predicted that, under still higher stress levels, the effect of slip may become predominant, as has been pointed out by Vehoff et al. /g/.
Rotation angle. 0 C )
Fig.3- Nisorientation dependence of the susceptibility of symmetrical <llO>-tilt boundaries to SCC.
<LOO>-t il t boundary
The misorientation dependence of the susceptibility of <loo>-tilt boundary to SCC is shown in Fig. 4. Cusps exist at 22.6"/113a(015), 36.9"/15(013), 43.6"/229a(025) and 53. Ia/C5(012). It is also confirmed that the dissolution rates at these angles show the minimum values in the misorientation dependence, as evidenced in Fig. 5.
It is very interesting that the 15.O'/125a(017) boundary shows an unpredictable low susceptibility to SCC. The works both of Davis et al. /10/ and of Miura and Saeki /l]/ on the slip behavior in aluminum bicrystals have shown that the slip can pass through the boundary with misorientation of approximately 15". It has been also pointed out that the possibility of passing of dislocation through a grain boundary increases with decreasing the stacking fault energy /5/. Accordingly, it is
considered that dislocations rather move easily through the boundary in the Cu-A1 alloy than that in aluminum. Thus, it may be probable that the 15.0" boundary is within the angular region of transition from the low-angle boundary, through which some dislocations can pass, to the high-angle one and
resultingly the susceptibility of the boundary decreases suddenly with decreasing the misorientation from approximately 15".
l0
9
30 60 9 0Rotation a n g l e , 6 ('1
Fig.4- Misorientation dependence of the susceptibility of symmetrical <loo>-tilt boundaries to SCC.
Rotation a n g l e , 6 c)
FIG. 5- Misorientation dependence of the depth of a dissolution groove, D, developed at symmetrical
<loo>-tilt boundaries in (Brz:lml+CH3COOH:60ml+HC1:20ml+Hz0:100ml).
<l 00)-twist boundary
The misorientation dependence of the susceptibility of the <loo>-twist boundary to SCC is shown in Fig.6. The susceptibility suddenly decreases with decreasing the rotation angle from approximately 15' and the low-angle boundary shows extremely low susceptibility. These agree well with the results obtained in the <110>- and <loo>-tilt boundaries. In the high-angle region, cusps exist at 22.6"/C13a, 28. l0/X17a and 36.g0/C5. Another minimum may exist at 43.6"/C29a. The misorientation dependence is very similar to that of relative energies of copper /12/ and aluminurn /8/. In particular, the rotation angles at which cusps exist are identical.
It is noteworthy that the <loo>-twist boundaries required higher stress level (more than 1.1~~) than the <110>- and <loo>-tilt boundaries for cracking. This result is not considered to be due to the
Cl-720 COLLOQUE DE PHYSIQUE
difference in the structural stability between the <loo>-twist boundaries and the tilt boundaries mentioned above but is most likely caused by the difference in the crystallographic orientation of the stress direction. All the stress axes of the bicrystals are identical <loo>. Therefore, the
bicrystals are deformed by quadruple slip at an early stage of deformation; accordingly the motion of dislocations is suppressed at barriers such as Lomer-Cottrell sessile dislocations in the grain
interiors. Thus, the amount of piled-up dislocations which enhance the susceptibility to intergranular SCC at the <loo>-twist boundary may be smaller than those at the tilt boundaries under relatively low stress levels. Consequently, this result also strongly indicates the importance of the amount of piled-up dislocations at a grain boundary.
Rotation angle, 6 (')
Fig. 6- Misorientation dependence of the susceptibility of <loo>-twist boundaries to SCC, t r - l , where tf is the time to fracture.
The authors would like to thank Sumitomo Light Metal Industries, Ltd. for supplying materials.
The financial supports of the Light Metal Educational Foundation of Japan and the Ministry of Education, Science and Culture of Japan, as a Grant-in-Aid, are also acknowledged.
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