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Fractographic aspects of cyclic cleavage
Materials Science and Engineering, 60 (1983) 231-240
Fractographic
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Aspects of Cyclic Cleavage
URIBE-PEREZ D~partement de Gdnie MStallurgique, Ecole Polytechnique, Montrdal, Qudbec H3C 3A 7 (Canada)
J. I. D I C K S O N
a nd I.
E. GE~KINLI Department of Metallurgy, lstanbul Technical University (Turkey) (Received February 14, 1983)
SUMMARY A n u m b e r of aspects of cleavage crack propagation occurring during fatigue were identified from a fractographic study o f low cycle fatigue samples o f a quenched renitrogenized mild steel, tested at 20 and 100 °C. The m o s t striking aspect was the frequent presence o f cyclic cleavage striations, whose visibility increased with an increase in the plasticity accompanying cleavage. Cyclic cleavage results in a locally accelerated crack growth rate and often in a local crack front that proceeds ahead o f the macroscopic front. Increasing hindrance to the continuation o f cyclic cleavage often results and is responsible for transitions from brittle to semibrittle to ductile striations. This observed continuity between the three types o f striation strongly indicates that the first t w o types o f striation correspond to temporary crack arrest sites.
1. INTRODUCTION Studies of fatigue crack propagation in iron, iron alloys and steels at relatively low temperatures have indicated that cleavage facets can be obtained within regions of ductile fatigue [1-7]. Recent studies [5-8] have indicated or suggested that under favourable conditions these facets can be produced by a cyclic cleavage mechanism in which a number of cycles are required for a cleavage crack to propagate across a grain. For certain testing conditions, i.e. for certain values of temperature, load ratio R etc., it has been observed [2, 3, 7] ,that the occurrence of cleavage facets together with ductile striations does not necessarily result in a measurable 0025-5416/83/$3.00
acceleration of the macroscopic fatigue crack propagation rate. M o o d y and Gerberich [4] have included the occurrence of cyclic cleavage in a model for fatigue crack propagation b e l o w the ductile-brittle transition temperature. The clearest evidence for cyclic cleavage has been presented by Neumann et al. [8] for Fe-3%Si single crystal~; they referred to this p h e n o m e n o n as fatigue crack propagation in the quasi-brittle mode. In polycrystalline material the evidence presented for cyclic cleavage is less clear. Few grains present identifiable fatigue striations [ 5, 7] and these striations generally are poorly defined. Jogs and discontinuous growth steps on river lines [5, 6] have been proposed as additional evidence for cyclic cleavage. The purpose of the present paper is to present and discuss some fractographic aspects of cyclic cleavage facets, including well
2. EXPERIMENTAL PROCEDURE The chemical composition of the hotrolled steel employed is given in Table 1. Cylindrical low cycle fatigue specimens having a gauge length of 20 mm and a diameter of 6.2 mm were machined with the long axis parallel to the rolling direction. The specimens were subjected to the following double heat treatment: a homogenization treatment © Elsevier Sequoia/Printed in The Netherlands
232 of 15 min at 915 °C followed by air cooling, and a second anneal of 1 h at 700 °C followed by a water quench. The microstructure obtained was equiaxed ferritic grains with an average grain size of approximately 28 pm and with a small a m o u n t of pearlite present at the grain boundaries. Fully reversed push-pull fatigue tests were performed in strain control using a sinusoidal waveform and an average strain rate of 4 × 10 -3 s-1 on a closed-loop servohydraulic MTS machine. Total strain amplitudes Aet of + 0.3% and + 0.5% and testing temperatures of 20 and 100 °C were employed. The fracture surfaces produced were studied in a JEOL T20 scanning electron microscope.
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3. RESULTS AND DISCUSSION
3.1. Distribution o f cleavage facets The usual appearance of the observed fracture surfaces was that close to the crack initiation site, which was often situated in the region of the knife edges of the extensometer; ductile striations which had been largely flattened by the occurrence of crack closure were present. Farther from the crack tip, evidence for crack closure was generally confined to localized regions. At some fairly small distance from the crack initiation site, individual cleaved grains and small clusters of cleaved grains appeared while the large majority of grains still presented ductile striations. The number of these clusters initially increased with increasing distance from the crack initiation site to reach a m a x i m u m near the centre of the cross section, where ductile striations became rare although thin regions of ductile tearing dimples delimiting the cleaved clusters could now be found; some of these dimples could also be found at some grain boundaries within clusters. Farther from the crack initiation site the size of the cleaved clusters decreased and ductile striations reappeared. The general observations indicated t h a t the crack propagation rate increased to reach a m a x i m u m close to the centre of the fracture surface and then decreased again in approximately the last third of this surface where, because the total strain was being controlled, the conditions became similar to those of crack propagation under a constant crack-mouth-opening displacement and the
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crack n o w propagated under a decreasing cyclic stress intensity factor AK.
3.2. Occurrence o f cyclic cleavage striations in the different regions For the tests at 20 °C which produced the more brittle cleavage facets, the clearest evidence for cyclic cleavage striations was found for the intermediate amounts of cleavage facets produced, generally on b o t h sides of the central portion of the fracture surface. In these regions, most cleaved grains presented cyclic cleavage striations, examples of which are shown in Figs. 1 and 2. The local or microscopic crack propagation direction which is taken to be perpendicular to the striations is indicated by arrows in the fractographs. The macroscopic crack propagation direction was usually within + 60 ° of the local direction, although a few grains could be found where the microscopic cleavage direction was almost directly opposite to the macroscopic direction, as is typical for cleavage fracture [10]. Close to the crack initiation site, where the size and number of cleaved Clusters were smallest, it was generally difficult to find cyclic cleavage striations. Figure 3 is a fractograph from such a region. The main feature of this figure is the presence of what Gerberich
et al. [5] referred to as "discontinuous growth steps" on the river lines, which they considered as evidence for cyclic cleavage. In Fig. 3, most of these steps can be seen to correspond to tongues and therefore to twins. In a number of sites, t w o or three of these steps are collinear, suggesting that they formed during the same cycle. Under close scrutiny a number of very fine barely discern-
Fig. 2. A n o t h e r e x a m p l e o f b r i t t l e cyclic cleavage s t r i a t i o n s is s h o w n (Aet = + 0.3%; T = 20 °C). T h e a r r o w in t h e c e n t r e i n d i c a t e s a fine t o n g u e or t w i n c o r r e s p o n d i n g t o a s h a r p - a n g l e d d i s c o n t i n u o u s step o n a fine river line. A t t h e s t r i a t i o n j u s t a h e a d o f t h e a r r o w o n t h e right, f e a t h e r m a r k s can be seen t o start.
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Fig. 1. A n e x a m p l e o f b r i t t l e cyclic cleavage s t r i a t i o n s is s h o w n , i n c l u d i n g s t r i a t i o n s crossing a grain b o u n d ary ( A e t = -+ 0.3%; T = 25 °C). I n t h e region a h e a d o f the arrow indicating the microscopic propagation d i r e c t i o n , fine river lines c a n be seen t o s t a r t or b e c o m e m o r e p r o n o u n c e d at s t r i a t i o n s a n d t w o striat i o n s are s e e n t o t e r m i n a t e a t t o n g u e s i n d i c a t i n g small twins.
Fig. 3. S h a r p - a n g l e d d i s c o n t i n u o u s steps o n river lines generally c o r r e s p o n d to t o n g u e s a n d t h e r e f o r e t o t w i n s ( A e t = +- 0.3%; T = 2 0 ° C ) . V e r y fine closely s p a c e d b a r e l y d i s c e r n i b l e s t r i a t i o n s parallel t o t h e d i s c o n t i n u o u s s t e p s are p r e s e n t o n t w o cleaved facets, e.g. j u s t a h e a d of t h e a r r o w s i n d i c a t i n g t h e local p r o p a g a t i o n direction.
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Fig. 4. A small cluster o f t w o cleaved grains is s h o w n near a crack initiation site ( A e t = +- 0.3%; T--- 2 0 °C). C y c l i c cleavage has b e e n initiated at the b o u n d a r y b e t w e e n the t w o grains. F i n e c y c l i c cleavage striations can be seen in each grain. A h e a d o f the l o w e r arrow a transition to ductile striations has occurred.
ible closely spaced striations can be detected (interstriation spacing, about 0.3 gm) parallel to the discontinuous steps on the river lines. On a few cleaved facets close to the crack initiation site, more discernible and often less brittle finely spaced striations (Fig. 4) could at times be f o u n d especially near grain boundaries and, in particular, near the boundaries at which the cyclic cleavage clusters terminated. In the middle section, where the a m o u n t of cleavage was highest, cyclic cleavage striations again became difficult to find. These, however, tended to be present on the smallest cleavage clusters, some of which extended only across a single grain. In the larger clusters a single well-formed brittle striation could occasionally be f o u n d on some facets. Again there was a t e n d e n c y for less brittle striations to occur near grain boundaries at which cleavage terminated. The results, however, indicated that for these larger clusters most of the cleavage occurred within a very small number of cycles. Only once was cleavage in a large cluster in which most facets did n o t contain striations observed to have been initiated in a facet t h a t contained striations. For samples cycled at 100 °C that presented cleavage facets, a larger a m o u n t of plasticity occurred, causing the cyclic cleavage striations to be more discernible but often to
Fig. 5. An example of semibrittle c y c l i c cleavage striations is s h o w n (Aet= +-0.3%; T = 20°C). At a number of sites, t w o semibrittle striations can be seen to merge.
have a less brittle appearance. A typical example of such semibrittle cyclic cleavage striations is presented in Fig. 5. For tests at 100 °C, cyclic cleavage striations were clearly discernible even on cleaved facets closest to the crack initiation site, which often occurred one or t w o grains ahead of the macroscopic crack front, with a portion of the cyclic cleavage propagating back towards this front. In this region, the cleavage facets showed either semibrittle striations or more widely spaced brittle striations, for which the interstriation spacing was even slightly larger than for the semibrittle striations observed close to the centre of the fracture surface, where the percentage of cleaved facets was a maximum. In this central portion, most grains still presented cyclic cleavage striations, but these were predominantly widely spaced brittle striations. Semibrittle striations again became the d o m i n a n t feature of the cleaved facets on the portion of the fracture surface that had been produced last and at a decreasing crack propagation rate. Some of the tests at 20 °C in which less dynamic strain aging occurred during cycling also produced mainly semibrittle striations on the cleavage facets as a result of the greater a m o u n t of plasticity accompanying crack propagation. Tests at 100 °C in which less dynamic strain aging occurred resulted in an absence of cleavage facets on the fracture surfaces.
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3.3. Detailed aspects of cyclic cleavage striations There was no ambiguity in identifying brittle striations since the only other lines that were generally present were river lines, usually almost perpendicular to the striations. As seen in Fig. 5, however, when the striations were of the semibrittle type, a multitude of lines including striations, river lines and slip traces were often present. Reliable identification o f the semibrittle striations, however, generally could be carried out by studying the regions near the grain boundaries at which cleavage initiated in a grain as well as near the boundaries at which cleavage terminated in a cluster. In many grains a region of well
Fig. 6. A transition f r o m brittle to semibrittle striations is observed as cyclic cleavage p r o c e e d s a w a y f r o m the site at which it has b e e n initiated on the cleavage facet (Aet = +- 0.3%; T = 20 °C). A c c o m panying this transition, a s e c o n d set o f river lines appears to f o r m rough triangular patterns on the facet, in the region ahead o f the arrow.
b e c o m e less brittle in appearance and slip traces as well as additional river lines appear. A double set o f river lines was at times observed for the semibrittle striations forming rough triangles on the cleavage facets (Fig. 6), with the apexes of the triangles generally corresponding to striations. The general features of these triangles resemble those that can be produced on cyclic cleavage facets during corrosion fatigue [ 1 1 - 1 3 ] , with the exception that the triangular patterns formed in corrosion fatigue are better defined and have a very regular appearance. Near the grain boundaries at which cyclic cleavage terminated, an increase in the ductility of the striations including often a transition from semibrittle striations to ductile striations (Fig. 8) before the crack arrived at this b o u n d a r y could often be observed. In a few cases a transition from brittle to semibrittle to ductile striations was observed within a single grain. In one case a transition from ductile to semibrittle striations was observed. This continuity between the brittle or semibrittle striations and the ductile striations constitutes particularly strong evidence that the brittle or semibrittle striations correspond to crack arrest markings. As was verified from stereofractographic observations b u t as can also be seen from Fig. 1, the brittle striations generally correspond to small steps of the same sign on the
Fig. 7. A transition f r o m semibrittle striations to m o r e brittle striations o n crossing a grain b o u n d a r y is s h o w n ( A e t = -+ 0.3%; T = 20 °C). One of the semibrittle striations in the l o w e r grain is indicated by an arrOW.
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Fig. 8. A transition from semibrittle to ductile striations is seen on a cleaved grain containing a pattern of lines that appears to be related to the dislocation substructures (see also Figs. 10 and 11) (Aet = + 0.3%; T = 100 °C).
cleavage facet. The semibrittle striations also generally correspond to small steps but these change signs fre~luently, which gives the cleaved surface a hill-and-vaUey appearance somewhat similar but less pronounced than t h a t associated with the ductile striations. The region between striations is crystallographic and essentially flat for the semibrittle striations while it is rounded for the ductile striations. Two semibrittle striations were also often seen to merge together (Fig. 5), suggesting that in some cycles only a portion of the crack front advances. Such merging of striations, which was rarely observed for brittle striations, was also a c o m m o n feature of the ductile striations. Brittle striations were generally straight while semibrittle striations were often curved. Many of the microscopic aspects of the semibrittle striations actually resemble more closely those of ductile rather than of brittle striations, although t h e y occur clearly on cleavage facets. These differences in microscopic aspects between the brittle and semibrittle striations at first led to errors in identifying the set of lines corresponding to the semibrittle striations, since the initial tendency was to choose the set which most strongly resembled the brittle striations and these were often slip lines or occasionally river lines. These errors became evident during systematic verification of the identification of the semibrittle striations by working forwards or backwards from grain boundaries
near which brittle or ductile striations respectively had formed on cleaved facets. The gradualness of the transition from brittle to semibrittle to ductile striations on the cleaved facets is striking and shows the influence of the a m o u n t of plasticity accompanying crack propagation on determining the exact nature of the striations. The increase in plasticity near the grain boundary at which cyclic cleavage terminated also explains w h y , for the tests at 20 °C which favoured the occurrence of brittle striations, near the crack initiation site the finely spaced cyclic cleavage striations were most often detected close to such grain boundaries. The question then arises whether cyclic cleavage is initiated at a rapid cyclic propagation rate and then decelerates as it propagates into the cluster or whether important deceleration only occurs very near the grain boundaries at the end of the cluster. Some deceleration generally was associated with transitions from brittle to semibrittle striations or from semibrittle to ductile striations. Even at 20 °C in tests where brittle striations were favoured, striations could be found on some of the facets in which cleavage had been initiated within a cluster and no strong deceleration appeared to occur within most of the cluster. In the regions in which the occurrence of cyclic cleavage striations was most favoured, however, such striations more frequently were absent in the grains closest to the cleavage initiation site, suggesting that the initial crack propagation rate in the first grains cleaved within the cluster may have been particularly high. An element of d o u b t remains since sufficient plasticity must occur at the site at which crack arrest occurs during the decreasing stress portion of the cycle in order for striations to be subsequently observable. Barely discernible brittle striations were observed for tests at 20 °C, in particular near the crack initiation site (Fig. 3), but also, with a much larger interstriation spacing, in the region where brittle cyclic cleavage striations were generally easily discernible. It is very probable t h a t some of the cleavage crack arrest sites have not been marked by striations. It is clear from the present observations as well as from comparisons made with observations obtained for cyclic cleavage at low temperatures [5-7] that, the greater the a m o u n t of plasticity accompanying the cyclic cleavage
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crack propagation, the easier it is to observe the striations. A similar conclusion has been arrived at for brittle striations produced in the ferritic phase of a duplex stainless steel during corrosion fatigue [12, 13] where, as the stress intensity factor and the a m o u n t of plasticity at the crack tip increased, the brittle striations became better delineated. As well as the observed continuity between brittle, semibrittle and ductile striations, a number of other observations suggested that the brittle and semibrittle striations correspond to crack arrest sites. Fine river lines or feather markings were often initiated or became more (Figs. 1 and 2) or occasionally less p r o n o u n c e d (Fig. 9) at striations. In a few cases, large river lines (Fig. 9) were also initiated at striations. A second set of river lines was occasionally observed; this resulted in rough triangular patterns (Fig. 6) which had also clearly been initiated at semibrittle striations. The interstriation spacing generally increased as the c~ack propagation rate, as determined from the average size of the cleavage clusters, increased. Also striations disappeared from most grains in the centre of specimens tested at 20 °C, where the tendency to cleavage often became sufficient that apparently a number of grains were cleaved in a single cycle.
Fig. 9. A h e a d o f the l o w e r arrow, fine river lines bec o m e less p r o n o u n c e d at c y c l i c cleavage striations (Aet -- + 0.3%; T - 20 °C). A h e a d o f t h e u p p e r arrow, large river lines can be seen to b e c o m e m o r e pron o u n c e d at striations, w h i c h t h e y f o l l o w for a s h o r t distance t o f o r m a jog.
From Figs. 1, 2 and 6 it can be seen that the cyclic cleavage interstriation spacing was o f t e n irregular, especially for brittle striations. Several contributing factors to this irregular interstriation spacing can be suggested. First of all, cyclic cleavage requiring cleavage crack acceleration, deceleration and arrest in each cycle can be expected to result in a less regular interstriation spacing than for ductile striations, where the amount of plasticity at the crack tip, a more predictable p h e n o m e n o n than cleavage, determines the interstriation spacing. Moreover, in b o t h Fig. 1 and Fig. 6 it can be seen that cyclic cleavage has occurred in a grain whose lateral neighbours were uncracked and which subsequently fractured by ductile fatigue. This aspect of many cyclic cleavage facets, as previously mentioned, often results in a decrease in the brittle character of the striations as cyclic cleavage proceeds in a grain. The striations were particularly easily discernible when cyclic cleavage propagated along a narrow length between t w o uncracked grains, which was the case in Fig. 1. The presence of uncracked neighbouring grains to the sides can also be expected to favour a variable interstriation spacing depending on the amount of hindrance to cyclic cleavage that occurs in each cycle. A similar strong variability in the spacing b e t w e e n brittle striations in the ferritic phase of a duplex stainless steel has been observed during corrosion fatigue [12, 13], where this variable interstriation spacing was clearly related to the effect of uncracked islands of austenite, bypassed b y propagation of the crack in the ferritic grains, on hindering locally the crack propagation in the ferrite. From Figs. 1 and 6 it can also be seen that a few brittle striations stop in the interior of grains. In Fig. 1, t w o striations can be seen to terminate at small tongues which indicate twins. The fact that cyclic cleavage striations c a n terminate in the interior of a grain confirms that the more brittle striations form with difficulty and further suggests that not all the crack arrest sites b e c o m e marked b y cyclic cleavage striations. Clearly, if only some of the crack arrest sites are marked by discernible striations, an additional tendency for the formation of an irregular interstriation spacing should follow. In Fig. 1, cyclic cleavage striations are seen to extend across a grain boundary. Although
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similar observations were made in a number of cases where the change in the orientation of the cleavage plane across the grain boundary was n o t pronounced, such observations were the exception rather than the rule.
3.4. Other details of cyclic cleavage The cleaning of the fracture surfaces with inhibited hydrochloric acid caused a small number of etch pits to form on some cleavage facets. The cubic and octahedral shape [12, 13] of these etch pits permitted verification of the fact that the cleavage facets corresponded to {100} planes, independently of whether brittle or semibrittle striations were present. In the regions where semibrittle striations were produced at both 20 and 100 °C, a number of grains presented patterns of lines which appeared to be related to the river line patterns but also resembled the dislocation substructure produced during cycling. Figures 8 and 10 present such patterns which resemble the dislocation substructures revealed by Pohl [14] by etching low carbon steel that had been cycled in stress control at 25 °C. Figure 11 is a transmission electron micrograph of the arrangement of dislocation walls separated by channels observed in a thin foil prepared from the same sample on whose fracture surface Fig. 10 was observed. The wall thick-
Fig. 10. P a t t e r n o f lines o f t e n o b s e r v e d o n cyclic cleavage f a c e t s p r e s e n t i n g s e m i b r i t t l e s t r i a t i o n s ( A e t = -+ 0.3%; T = 2 0 ° C ) . I n t h i s f r a c t o g r a p h t h e line pattern tends to make the striations difficult to observe. T h e t w o a r r o w s i n d i c a t e d t o u c h t h e s a m e s t r i a t i o n (see also Figs. 8 a n d 11).
ness (about 0.2 pm) and the widths of the channels between the walls (about 1 pm) are similar to those suggested by the pattern of lines in the fractograph. As already mentioned, the most pronounced discontinuous steps in the river lines in Fig. 3 correspond to tongues and therefore to twins. All these tongues appear to be on one side (that corresponding to the direction in which the crack is propagating) with respect to the line t h a t can be assumed to correspond to the cyclic cleavage striation or to the crack arrest site. Similar observations were also made in the presence of clear cyclic cleavage striations, including observations on samples which had been tested at 100 °C (Fig. 12). In this figure a number of fine tongues and three large tongues are seen to coincide with semibrittle striations. The initiation of a pronounced river line is associated with one of the larger tongues. These results therefore suggest t h a t twinning can occur at the arrested crack front and that, at least in some grains, the cyclic cleavage crack front tends to lie along a {110} or a (210} direction, the possible intersection directions for {112} twin planes with a {100} cleavage plane. The (110) orientation has been observed [12] for brittle striations in ferrite during corrosion fatigue and is also consistent with observations [15, 16] that (110 } is a preferential cleavage propagation direction. The more brittle cyclic cleavage striations were generally straight, consistent with a crystallographic crack front.
Fig. 11. T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s h o w i n g a d i s l o c a t i o n wall p a t t e r n o b s e r v e d for t h e same s a m p l e o n w h o s e f r a c t u r e surface Fig. 10 was photographed.
239 of cyclic cleavage. In contrast, jogs on river lines were o f t e n observed t o correspond to striations (Fig. 9) and appear to be a m ore reliable indication of cyclic cleavage. A bet t er example of good correspondence between p r o n o u n c e d jogs on a river line and cyclic cleavage striations is presented in Fig. 13. 4. CONCLUSIONS
Fig. 12. A number of very fine tongues and three larger tongues can be seen to correspond to semibrittle striations (A6t -+0.3%; T= 100 °C). A pronounced river line has been initiated at the striation at which the largest twin is present.
Fig. 13. Pronounced jogs on the large river line between the two arrows can be seen to correspond to cyclic cleavage striations (A6t - -+0.5%; T = 100 °C).
Gerberich e t al. [5] have previously suggested th at discontinuous growth steps and jogs in river lines could be considered as evidence for cyclic cleavage. Figures 3 and 12 where th e discontinuous steps are largely associated with twins parallel to the cyclic cleavage striations appear t o support this suggestion;however, observations in which twins n o t parallel to striations caused such steps were as or m or e frequent. An example can be seen near the centre of Fig. 2. Disc on tin u o u s growth steps in river lines therefore do n o t appear to be a reliable indication
From the present study the following conclusions can be drawn. (1) Under favourabte conditions of temperature and fatigue crack propagation rate, cyclic cleavage can occur w hereby a n u m b e r of cycles are required for cleavage to propagate across a grain. (2) When sufficient plasticity accompanies cyclic cleavage, well-formed brittle or semibrittle striations are form ed. (3) Cyclic cleavage results in a locally accelerated crack growth rate and tends to proceed ahead of the macroscopic crack front. This aspect results in an increasing a m o u n t of plasticity accompanying cyclic cleavage and is responsible for transitions from brittle to semibrittle striations or f r o m semibrittle to ductile striations. (4) The occurrence of Cyclic cleavage at the relatively high testing temperatures e m p l o y e d (22 and 100 °C) is related to marked strainaging effects in the present material. These relatively high temperatures also favoured the occurrence of sufficient plasticity to cause well-formed striations to occur. (5) Tongues just ahead of cyclic cleavage striations suggest t hat occasionally twins f o r m at the arrested crack front. (6) Sharp-angled discontinuous growth steps on river lines are generally associated with twins and are not always parallel to the cyclic cleavage striations. P r o n o u n c e d jogs on river lines, however, are generally collinear with cyclic cleavage striations and appear to be a reliable indicator of cyclic cleavage. ACKNOWLEDGMENTS Financial support from the Quebec Formation des Chercheurs et Actions Concert4es Program and from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
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The authors are grateful to Dr. J.-P. Bailon and to M. Ait Bassidi for helpful discussions and to Jean Claudinon and Jean-Philippe Bouchard for technical assistance. Part of this work was carried out while E. Ge~kinli was at the Ecole Polytechnique on sabbatical leave. The steel studied was donated by SidbecDosco Ltd. and obtained through the assistance of Dr. Paul Hastings.
REFERENCES 1 R. O. Ritchie and J. F. Knott, Mater. Sci. Eng., 14 (1974) 7. 2 L. H. Burck and J. Weertman, Metall. Trans. A, 7 (1976) 257. 3 L. H. Burck and J. Weertman, Scr. Metall., 12 (1978) 417. 4 N. R. Moody and W. W. Gerberich, Mater. Sci. Eng., 41 (1979) 271.
5 W. W. Gerberich, N. R. Moody and K. Jatavallabhula, Scr. MetaU., 14 (1980) 113. 6 J. P. Lucas and W. W. Gerberich, Mater. Sci. Eng., 51 (1981) 203. 7 J. I. Dickson, C. Lincourt, L. Haenny and J.-P. Bailon, to be published. 8 P. Neumann, H. Fuhlrott and H. Vehoff, in J. T. Fong (ed.), Fatigue Mechanisms, A S T M Spec. Tech. Publ. 675, 1979, p. 371. 9 I. Uribe-Perez, M.A.Sc. Thesis, Ecole Polytechnique de Montreal, 1981. 10 R. G. Hoagland, A. R. Rosenfield and G. T. Hahn, Acta Metall., 3 (1972) 123. 11 C. A. Subbington, Metallurgica, 68 (1963) 109. 12 M. Ait Bassidi, J. Masounave, J.-P. Ba~'lon and J. I. Dickson, in J. W. Provan and G. C. Sih (eds.), Proc. 2nd Int. Conf. on Defects, Fracture and Fatigue, Martinius Nijhoff, 1983, p. 359. 13 M. Ait Bassidi, J. Masounave, J. I. Dickson and J.-P. Bai]on, Can. Metall. Q., to be published. 14 K. Pohl, Doctoral Thesis, University of Karlsruhe, 1981. 15 J. F. McNeil and H. R. Limb, J. Inst. Met., 58 (1958) 79. 16 D. Hull, P. Beardmore and A. P. Valintine, Philos. Mag., 12 (1965) 1021.
Fractographic
231
Aspects of Cyclic Cleavage
URIBE-PEREZ D~partement de Gdnie MStallurgique, Ecole Polytechnique, Montrdal, Qudbec H3C 3A 7 (Canada)
J. I. D I C K S O N
a nd I.
E. GE~KINLI Department of Metallurgy, lstanbul Technical University (Turkey) (Received February 14, 1983)
SUMMARY A n u m b e r of aspects of cleavage crack propagation occurring during fatigue were identified from a fractographic study o f low cycle fatigue samples o f a quenched renitrogenized mild steel, tested at 20 and 100 °C. The m o s t striking aspect was the frequent presence o f cyclic cleavage striations, whose visibility increased with an increase in the plasticity accompanying cleavage. Cyclic cleavage results in a locally accelerated crack growth rate and often in a local crack front that proceeds ahead o f the macroscopic front. Increasing hindrance to the continuation o f cyclic cleavage often results and is responsible for transitions from brittle to semibrittle to ductile striations. This observed continuity between the three types o f striation strongly indicates that the first t w o types o f striation correspond to temporary crack arrest sites.
1. INTRODUCTION Studies of fatigue crack propagation in iron, iron alloys and steels at relatively low temperatures have indicated that cleavage facets can be obtained within regions of ductile fatigue [1-7]. Recent studies [5-8] have indicated or suggested that under favourable conditions these facets can be produced by a cyclic cleavage mechanism in which a number of cycles are required for a cleavage crack to propagate across a grain. For certain testing conditions, i.e. for certain values of temperature, load ratio R etc., it has been observed [2, 3, 7] ,that the occurrence of cleavage facets together with ductile striations does not necessarily result in a measurable 0025-5416/83/$3.00
acceleration of the macroscopic fatigue crack propagation rate. M o o d y and Gerberich [4] have included the occurrence of cyclic cleavage in a model for fatigue crack propagation b e l o w the ductile-brittle transition temperature. The clearest evidence for cyclic cleavage has been presented by Neumann et al. [8] for Fe-3%Si single crystal~; they referred to this p h e n o m e n o n as fatigue crack propagation in the quasi-brittle mode. In polycrystalline material the evidence presented for cyclic cleavage is less clear. Few grains present identifiable fatigue striations [ 5, 7] and these striations generally are poorly defined. Jogs and discontinuous growth steps on river lines [5, 6] have been proposed as additional evidence for cyclic cleavage. The purpose of the present paper is to present and discuss some fractographic aspects of cyclic cleavage facets, including well
2. EXPERIMENTAL PROCEDURE The chemical composition of the hotrolled steel employed is given in Table 1. Cylindrical low cycle fatigue specimens having a gauge length of 20 mm and a diameter of 6.2 mm were machined with the long axis parallel to the rolling direction. The specimens were subjected to the following double heat treatment: a homogenization treatment © Elsevier Sequoia/Printed in The Netherlands
232 of 15 min at 915 °C followed by air cooling, and a second anneal of 1 h at 700 °C followed by a water quench. The microstructure obtained was equiaxed ferritic grains with an average grain size of approximately 28 pm and with a small a m o u n t of pearlite present at the grain boundaries. Fully reversed push-pull fatigue tests were performed in strain control using a sinusoidal waveform and an average strain rate of 4 × 10 -3 s-1 on a closed-loop servohydraulic MTS machine. Total strain amplitudes Aet of + 0.3% and + 0.5% and testing temperatures of 20 and 100 °C were employed. The fracture surfaces produced were studied in a JEOL T20 scanning electron microscope.
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3. RESULTS AND DISCUSSION
3.1. Distribution o f cleavage facets The usual appearance of the observed fracture surfaces was that close to the crack initiation site, which was often situated in the region of the knife edges of the extensometer; ductile striations which had been largely flattened by the occurrence of crack closure were present. Farther from the crack tip, evidence for crack closure was generally confined to localized regions. At some fairly small distance from the crack initiation site, individual cleaved grains and small clusters of cleaved grains appeared while the large majority of grains still presented ductile striations. The number of these clusters initially increased with increasing distance from the crack initiation site to reach a m a x i m u m near the centre of the cross section, where ductile striations became rare although thin regions of ductile tearing dimples delimiting the cleaved clusters could now be found; some of these dimples could also be found at some grain boundaries within clusters. Farther from the crack initiation site the size of the cleaved clusters decreased and ductile striations reappeared. The general observations indicated t h a t the crack propagation rate increased to reach a m a x i m u m close to the centre of the fracture surface and then decreased again in approximately the last third of this surface where, because the total strain was being controlled, the conditions became similar to those of crack propagation under a constant crack-mouth-opening displacement and the
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crack n o w propagated under a decreasing cyclic stress intensity factor AK.
3.2. Occurrence o f cyclic cleavage striations in the different regions For the tests at 20 °C which produced the more brittle cleavage facets, the clearest evidence for cyclic cleavage striations was found for the intermediate amounts of cleavage facets produced, generally on b o t h sides of the central portion of the fracture surface. In these regions, most cleaved grains presented cyclic cleavage striations, examples of which are shown in Figs. 1 and 2. The local or microscopic crack propagation direction which is taken to be perpendicular to the striations is indicated by arrows in the fractographs. The macroscopic crack propagation direction was usually within + 60 ° of the local direction, although a few grains could be found where the microscopic cleavage direction was almost directly opposite to the macroscopic direction, as is typical for cleavage fracture [10]. Close to the crack initiation site, where the size and number of cleaved Clusters were smallest, it was generally difficult to find cyclic cleavage striations. Figure 3 is a fractograph from such a region. The main feature of this figure is the presence of what Gerberich
et al. [5] referred to as "discontinuous growth steps" on the river lines, which they considered as evidence for cyclic cleavage. In Fig. 3, most of these steps can be seen to correspond to tongues and therefore to twins. In a number of sites, t w o or three of these steps are collinear, suggesting that they formed during the same cycle. Under close scrutiny a number of very fine barely discern-
Fig. 2. A n o t h e r e x a m p l e o f b r i t t l e cyclic cleavage s t r i a t i o n s is s h o w n (Aet = + 0.3%; T = 20 °C). T h e a r r o w in t h e c e n t r e i n d i c a t e s a fine t o n g u e or t w i n c o r r e s p o n d i n g t o a s h a r p - a n g l e d d i s c o n t i n u o u s step o n a fine river line. A t t h e s t r i a t i o n j u s t a h e a d o f t h e a r r o w o n t h e right, f e a t h e r m a r k s can be seen t o start.
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Fig. 1. A n e x a m p l e o f b r i t t l e cyclic cleavage s t r i a t i o n s is s h o w n , i n c l u d i n g s t r i a t i o n s crossing a grain b o u n d ary ( A e t = -+ 0.3%; T = 25 °C). I n t h e region a h e a d o f the arrow indicating the microscopic propagation d i r e c t i o n , fine river lines c a n be seen t o s t a r t or b e c o m e m o r e p r o n o u n c e d at s t r i a t i o n s a n d t w o striat i o n s are s e e n t o t e r m i n a t e a t t o n g u e s i n d i c a t i n g small twins.
Fig. 3. S h a r p - a n g l e d d i s c o n t i n u o u s steps o n river lines generally c o r r e s p o n d to t o n g u e s a n d t h e r e f o r e t o t w i n s ( A e t = +- 0.3%; T = 2 0 ° C ) . V e r y fine closely s p a c e d b a r e l y d i s c e r n i b l e s t r i a t i o n s parallel t o t h e d i s c o n t i n u o u s s t e p s are p r e s e n t o n t w o cleaved facets, e.g. j u s t a h e a d of t h e a r r o w s i n d i c a t i n g t h e local p r o p a g a t i o n direction.
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Fig. 4. A small cluster o f t w o cleaved grains is s h o w n near a crack initiation site ( A e t = +- 0.3%; T--- 2 0 °C). C y c l i c cleavage has b e e n initiated at the b o u n d a r y b e t w e e n the t w o grains. F i n e c y c l i c cleavage striations can be seen in each grain. A h e a d o f the l o w e r arrow a transition to ductile striations has occurred.
ible closely spaced striations can be detected (interstriation spacing, about 0.3 gm) parallel to the discontinuous steps on the river lines. On a few cleaved facets close to the crack initiation site, more discernible and often less brittle finely spaced striations (Fig. 4) could at times be f o u n d especially near grain boundaries and, in particular, near the boundaries at which the cyclic cleavage clusters terminated. In the middle section, where the a m o u n t of cleavage was highest, cyclic cleavage striations again became difficult to find. These, however, tended to be present on the smallest cleavage clusters, some of which extended only across a single grain. In the larger clusters a single well-formed brittle striation could occasionally be f o u n d on some facets. Again there was a t e n d e n c y for less brittle striations to occur near grain boundaries at which cleavage terminated. The results, however, indicated that for these larger clusters most of the cleavage occurred within a very small number of cycles. Only once was cleavage in a large cluster in which most facets did n o t contain striations observed to have been initiated in a facet t h a t contained striations. For samples cycled at 100 °C that presented cleavage facets, a larger a m o u n t of plasticity occurred, causing the cyclic cleavage striations to be more discernible but often to
Fig. 5. An example of semibrittle c y c l i c cleavage striations is s h o w n (Aet= +-0.3%; T = 20°C). At a number of sites, t w o semibrittle striations can be seen to merge.
have a less brittle appearance. A typical example of such semibrittle cyclic cleavage striations is presented in Fig. 5. For tests at 100 °C, cyclic cleavage striations were clearly discernible even on cleaved facets closest to the crack initiation site, which often occurred one or t w o grains ahead of the macroscopic crack front, with a portion of the cyclic cleavage propagating back towards this front. In this region, the cleavage facets showed either semibrittle striations or more widely spaced brittle striations, for which the interstriation spacing was even slightly larger than for the semibrittle striations observed close to the centre of the fracture surface, where the percentage of cleaved facets was a maximum. In this central portion, most grains still presented cyclic cleavage striations, but these were predominantly widely spaced brittle striations. Semibrittle striations again became the d o m i n a n t feature of the cleaved facets on the portion of the fracture surface that had been produced last and at a decreasing crack propagation rate. Some of the tests at 20 °C in which less dynamic strain aging occurred during cycling also produced mainly semibrittle striations on the cleavage facets as a result of the greater a m o u n t of plasticity accompanying crack propagation. Tests at 100 °C in which less dynamic strain aging occurred resulted in an absence of cleavage facets on the fracture surfaces.
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3.3. Detailed aspects of cyclic cleavage striations There was no ambiguity in identifying brittle striations since the only other lines that were generally present were river lines, usually almost perpendicular to the striations. As seen in Fig. 5, however, when the striations were of the semibrittle type, a multitude of lines including striations, river lines and slip traces were often present. Reliable identification o f the semibrittle striations, however, generally could be carried out by studying the regions near the grain boundaries at which cleavage initiated in a grain as well as near the boundaries at which cleavage terminated in a cluster. In many grains a region of well
Fig. 6. A transition f r o m brittle to semibrittle striations is observed as cyclic cleavage p r o c e e d s a w a y f r o m the site at which it has b e e n initiated on the cleavage facet (Aet = +- 0.3%; T = 20 °C). A c c o m panying this transition, a s e c o n d set o f river lines appears to f o r m rough triangular patterns on the facet, in the region ahead o f the arrow.
b e c o m e less brittle in appearance and slip traces as well as additional river lines appear. A double set o f river lines was at times observed for the semibrittle striations forming rough triangles on the cleavage facets (Fig. 6), with the apexes of the triangles generally corresponding to striations. The general features of these triangles resemble those that can be produced on cyclic cleavage facets during corrosion fatigue [ 1 1 - 1 3 ] , with the exception that the triangular patterns formed in corrosion fatigue are better defined and have a very regular appearance. Near the grain boundaries at which cyclic cleavage terminated, an increase in the ductility of the striations including often a transition from semibrittle striations to ductile striations (Fig. 8) before the crack arrived at this b o u n d a r y could often be observed. In a few cases a transition from brittle to semibrittle to ductile striations was observed within a single grain. In one case a transition from ductile to semibrittle striations was observed. This continuity between the brittle or semibrittle striations and the ductile striations constitutes particularly strong evidence that the brittle or semibrittle striations correspond to crack arrest markings. As was verified from stereofractographic observations b u t as can also be seen from Fig. 1, the brittle striations generally correspond to small steps of the same sign on the
Fig. 7. A transition f r o m semibrittle striations to m o r e brittle striations o n crossing a grain b o u n d a r y is s h o w n ( A e t = -+ 0.3%; T = 20 °C). One of the semibrittle striations in the l o w e r grain is indicated by an arrOW.
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Fig. 8. A transition from semibrittle to ductile striations is seen on a cleaved grain containing a pattern of lines that appears to be related to the dislocation substructures (see also Figs. 10 and 11) (Aet = + 0.3%; T = 100 °C).
cleavage facet. The semibrittle striations also generally correspond to small steps but these change signs fre~luently, which gives the cleaved surface a hill-and-vaUey appearance somewhat similar but less pronounced than t h a t associated with the ductile striations. The region between striations is crystallographic and essentially flat for the semibrittle striations while it is rounded for the ductile striations. Two semibrittle striations were also often seen to merge together (Fig. 5), suggesting that in some cycles only a portion of the crack front advances. Such merging of striations, which was rarely observed for brittle striations, was also a c o m m o n feature of the ductile striations. Brittle striations were generally straight while semibrittle striations were often curved. Many of the microscopic aspects of the semibrittle striations actually resemble more closely those of ductile rather than of brittle striations, although t h e y occur clearly on cleavage facets. These differences in microscopic aspects between the brittle and semibrittle striations at first led to errors in identifying the set of lines corresponding to the semibrittle striations, since the initial tendency was to choose the set which most strongly resembled the brittle striations and these were often slip lines or occasionally river lines. These errors became evident during systematic verification of the identification of the semibrittle striations by working forwards or backwards from grain boundaries
near which brittle or ductile striations respectively had formed on cleaved facets. The gradualness of the transition from brittle to semibrittle to ductile striations on the cleaved facets is striking and shows the influence of the a m o u n t of plasticity accompanying crack propagation on determining the exact nature of the striations. The increase in plasticity near the grain boundary at which cyclic cleavage terminated also explains w h y , for the tests at 20 °C which favoured the occurrence of brittle striations, near the crack initiation site the finely spaced cyclic cleavage striations were most often detected close to such grain boundaries. The question then arises whether cyclic cleavage is initiated at a rapid cyclic propagation rate and then decelerates as it propagates into the cluster or whether important deceleration only occurs very near the grain boundaries at the end of the cluster. Some deceleration generally was associated with transitions from brittle to semibrittle striations or from semibrittle to ductile striations. Even at 20 °C in tests where brittle striations were favoured, striations could be found on some of the facets in which cleavage had been initiated within a cluster and no strong deceleration appeared to occur within most of the cluster. In the regions in which the occurrence of cyclic cleavage striations was most favoured, however, such striations more frequently were absent in the grains closest to the cleavage initiation site, suggesting that the initial crack propagation rate in the first grains cleaved within the cluster may have been particularly high. An element of d o u b t remains since sufficient plasticity must occur at the site at which crack arrest occurs during the decreasing stress portion of the cycle in order for striations to be subsequently observable. Barely discernible brittle striations were observed for tests at 20 °C, in particular near the crack initiation site (Fig. 3), but also, with a much larger interstriation spacing, in the region where brittle cyclic cleavage striations were generally easily discernible. It is very probable t h a t some of the cleavage crack arrest sites have not been marked by striations. It is clear from the present observations as well as from comparisons made with observations obtained for cyclic cleavage at low temperatures [5-7] that, the greater the a m o u n t of plasticity accompanying the cyclic cleavage
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crack propagation, the easier it is to observe the striations. A similar conclusion has been arrived at for brittle striations produced in the ferritic phase of a duplex stainless steel during corrosion fatigue [12, 13] where, as the stress intensity factor and the a m o u n t of plasticity at the crack tip increased, the brittle striations became better delineated. As well as the observed continuity between brittle, semibrittle and ductile striations, a number of other observations suggested that the brittle and semibrittle striations correspond to crack arrest sites. Fine river lines or feather markings were often initiated or became more (Figs. 1 and 2) or occasionally less p r o n o u n c e d (Fig. 9) at striations. In a few cases, large river lines (Fig. 9) were also initiated at striations. A second set of river lines was occasionally observed; this resulted in rough triangular patterns (Fig. 6) which had also clearly been initiated at semibrittle striations. The interstriation spacing generally increased as the c~ack propagation rate, as determined from the average size of the cleavage clusters, increased. Also striations disappeared from most grains in the centre of specimens tested at 20 °C, where the tendency to cleavage often became sufficient that apparently a number of grains were cleaved in a single cycle.
Fig. 9. A h e a d o f the l o w e r arrow, fine river lines bec o m e less p r o n o u n c e d at c y c l i c cleavage striations (Aet -- + 0.3%; T - 20 °C). A h e a d o f t h e u p p e r arrow, large river lines can be seen to b e c o m e m o r e pron o u n c e d at striations, w h i c h t h e y f o l l o w for a s h o r t distance t o f o r m a jog.
From Figs. 1, 2 and 6 it can be seen that the cyclic cleavage interstriation spacing was o f t e n irregular, especially for brittle striations. Several contributing factors to this irregular interstriation spacing can be suggested. First of all, cyclic cleavage requiring cleavage crack acceleration, deceleration and arrest in each cycle can be expected to result in a less regular interstriation spacing than for ductile striations, where the amount of plasticity at the crack tip, a more predictable p h e n o m e n o n than cleavage, determines the interstriation spacing. Moreover, in b o t h Fig. 1 and Fig. 6 it can be seen that cyclic cleavage has occurred in a grain whose lateral neighbours were uncracked and which subsequently fractured by ductile fatigue. This aspect of many cyclic cleavage facets, as previously mentioned, often results in a decrease in the brittle character of the striations as cyclic cleavage proceeds in a grain. The striations were particularly easily discernible when cyclic cleavage propagated along a narrow length between t w o uncracked grains, which was the case in Fig. 1. The presence of uncracked neighbouring grains to the sides can also be expected to favour a variable interstriation spacing depending on the amount of hindrance to cyclic cleavage that occurs in each cycle. A similar strong variability in the spacing b e t w e e n brittle striations in the ferritic phase of a duplex stainless steel has been observed during corrosion fatigue [12, 13], where this variable interstriation spacing was clearly related to the effect of uncracked islands of austenite, bypassed b y propagation of the crack in the ferritic grains, on hindering locally the crack propagation in the ferrite. From Figs. 1 and 6 it can also be seen that a few brittle striations stop in the interior of grains. In Fig. 1, t w o striations can be seen to terminate at small tongues which indicate twins. The fact that cyclic cleavage striations c a n terminate in the interior of a grain confirms that the more brittle striations form with difficulty and further suggests that not all the crack arrest sites b e c o m e marked b y cyclic cleavage striations. Clearly, if only some of the crack arrest sites are marked by discernible striations, an additional tendency for the formation of an irregular interstriation spacing should follow. In Fig. 1, cyclic cleavage striations are seen to extend across a grain boundary. Although
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similar observations were made in a number of cases where the change in the orientation of the cleavage plane across the grain boundary was n o t pronounced, such observations were the exception rather than the rule.
3.4. Other details of cyclic cleavage The cleaning of the fracture surfaces with inhibited hydrochloric acid caused a small number of etch pits to form on some cleavage facets. The cubic and octahedral shape [12, 13] of these etch pits permitted verification of the fact that the cleavage facets corresponded to {100} planes, independently of whether brittle or semibrittle striations were present. In the regions where semibrittle striations were produced at both 20 and 100 °C, a number of grains presented patterns of lines which appeared to be related to the river line patterns but also resembled the dislocation substructure produced during cycling. Figures 8 and 10 present such patterns which resemble the dislocation substructures revealed by Pohl [14] by etching low carbon steel that had been cycled in stress control at 25 °C. Figure 11 is a transmission electron micrograph of the arrangement of dislocation walls separated by channels observed in a thin foil prepared from the same sample on whose fracture surface Fig. 10 was observed. The wall thick-
Fig. 10. P a t t e r n o f lines o f t e n o b s e r v e d o n cyclic cleavage f a c e t s p r e s e n t i n g s e m i b r i t t l e s t r i a t i o n s ( A e t = -+ 0.3%; T = 2 0 ° C ) . I n t h i s f r a c t o g r a p h t h e line pattern tends to make the striations difficult to observe. T h e t w o a r r o w s i n d i c a t e d t o u c h t h e s a m e s t r i a t i o n (see also Figs. 8 a n d 11).
ness (about 0.2 pm) and the widths of the channels between the walls (about 1 pm) are similar to those suggested by the pattern of lines in the fractograph. As already mentioned, the most pronounced discontinuous steps in the river lines in Fig. 3 correspond to tongues and therefore to twins. All these tongues appear to be on one side (that corresponding to the direction in which the crack is propagating) with respect to the line t h a t can be assumed to correspond to the cyclic cleavage striation or to the crack arrest site. Similar observations were also made in the presence of clear cyclic cleavage striations, including observations on samples which had been tested at 100 °C (Fig. 12). In this figure a number of fine tongues and three large tongues are seen to coincide with semibrittle striations. The initiation of a pronounced river line is associated with one of the larger tongues. These results therefore suggest t h a t twinning can occur at the arrested crack front and that, at least in some grains, the cyclic cleavage crack front tends to lie along a {110} or a (210} direction, the possible intersection directions for {112} twin planes with a {100} cleavage plane. The (110) orientation has been observed [12] for brittle striations in ferrite during corrosion fatigue and is also consistent with observations [15, 16] that (110 } is a preferential cleavage propagation direction. The more brittle cyclic cleavage striations were generally straight, consistent with a crystallographic crack front.
Fig. 11. T r a n s m i s s i o n e l e c t r o n m i c r o g r a p h s h o w i n g a d i s l o c a t i o n wall p a t t e r n o b s e r v e d for t h e same s a m p l e o n w h o s e f r a c t u r e surface Fig. 10 was photographed.
239 of cyclic cleavage. In contrast, jogs on river lines were o f t e n observed t o correspond to striations (Fig. 9) and appear to be a m ore reliable indication of cyclic cleavage. A bet t er example of good correspondence between p r o n o u n c e d jogs on a river line and cyclic cleavage striations is presented in Fig. 13. 4. CONCLUSIONS
Fig. 12. A number of very fine tongues and three larger tongues can be seen to correspond to semibrittle striations (A6t -+0.3%; T= 100 °C). A pronounced river line has been initiated at the striation at which the largest twin is present.
Fig. 13. Pronounced jogs on the large river line between the two arrows can be seen to correspond to cyclic cleavage striations (A6t - -+0.5%; T = 100 °C).
Gerberich e t al. [5] have previously suggested th at discontinuous growth steps and jogs in river lines could be considered as evidence for cyclic cleavage. Figures 3 and 12 where th e discontinuous steps are largely associated with twins parallel to the cyclic cleavage striations appear t o support this suggestion;however, observations in which twins n o t parallel to striations caused such steps were as or m or e frequent. An example can be seen near the centre of Fig. 2. Disc on tin u o u s growth steps in river lines therefore do n o t appear to be a reliable indication
From the present study the following conclusions can be drawn. (1) Under favourabte conditions of temperature and fatigue crack propagation rate, cyclic cleavage can occur w hereby a n u m b e r of cycles are required for cleavage to propagate across a grain. (2) When sufficient plasticity accompanies cyclic cleavage, well-formed brittle or semibrittle striations are form ed. (3) Cyclic cleavage results in a locally accelerated crack growth rate and tends to proceed ahead of the macroscopic crack front. This aspect results in an increasing a m o u n t of plasticity accompanying cyclic cleavage and is responsible for transitions from brittle to semibrittle striations or f r o m semibrittle to ductile striations. (4) The occurrence of Cyclic cleavage at the relatively high testing temperatures e m p l o y e d (22 and 100 °C) is related to marked strainaging effects in the present material. These relatively high temperatures also favoured the occurrence of sufficient plasticity to cause well-formed striations to occur. (5) Tongues just ahead of cyclic cleavage striations suggest t hat occasionally twins f o r m at the arrested crack front. (6) Sharp-angled discontinuous growth steps on river lines are generally associated with twins and are not always parallel to the cyclic cleavage striations. P r o n o u n c e d jogs on river lines, however, are generally collinear with cyclic cleavage striations and appear to be a reliable indicator of cyclic cleavage. ACKNOWLEDGMENTS Financial support from the Quebec Formation des Chercheurs et Actions Concert4es Program and from the Natural Sciences and Engineering Research Council of Canada is gratefully acknowledged.
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The authors are grateful to Dr. J.-P. Bailon and to M. Ait Bassidi for helpful discussions and to Jean Claudinon and Jean-Philippe Bouchard for technical assistance. Part of this work was carried out while E. Ge~kinli was at the Ecole Polytechnique on sabbatical leave. The steel studied was donated by SidbecDosco Ltd. and obtained through the assistance of Dr. Paul Hastings.
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5 W. W. Gerberich, N. R. Moody and K. Jatavallabhula, Scr. MetaU., 14 (1980) 113. 6 J. P. Lucas and W. W. Gerberich, Mater. Sci. Eng., 51 (1981) 203. 7 J. I. Dickson, C. Lincourt, L. Haenny and J.-P. Bailon, to be published. 8 P. Neumann, H. Fuhlrott and H. Vehoff, in J. T. Fong (ed.), Fatigue Mechanisms, A S T M Spec. Tech. Publ. 675, 1979, p. 371. 9 I. Uribe-Perez, M.A.Sc. Thesis, Ecole Polytechnique de Montreal, 1981. 10 R. G. Hoagland, A. R. Rosenfield and G. T. Hahn, Acta Metall., 3 (1972) 123. 11 C. A. Subbington, Metallurgica, 68 (1963) 109. 12 M. Ait Bassidi, J. Masounave, J.-P. Ba~'lon and J. I. Dickson, in J. W. Provan and G. C. Sih (eds.), Proc. 2nd Int. Conf. on Defects, Fracture and Fatigue, Martinius Nijhoff, 1983, p. 359. 13 M. Ait Bassidi, J. Masounave, J. I. Dickson and J.-P. Bai]on, Can. Metall. Q., to be published. 14 K. Pohl, Doctoral Thesis, University of Karlsruhe, 1981. 15 J. F. McNeil and H. R. Limb, J. Inst. Met., 58 (1958) 79. 16 D. Hull, P. Beardmore and A. P. Valintine, Philos. Mag., 12 (1965) 1021.