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Application of fractographic techniques to beryllium
M E T A L L O G R A P H Y 4, 533-550 (1971)
533
Application of Fractographic Techniques to Beryllium S. J. B U R N S , J. G U R L A N D , A N D M. H. R I C H M A N
Division of Engineering, Brown University, Providence, Rhode Island
Metallography of deformed beryllium surfaces reveals considerable plastic flow on the microscale by both slip and twinning. Cracks are arrested or deflected by grain boundaries, and the interaction of twins with grain boundaries seems to be the main cause of microcrack formation. These microcracks are also associated with inclusions and are both intergranular and transgranular. T h e latter are directly related to the grain size and expand in a brittle manner. T h e fracture surfaces exhibit both ductile and brittle characteristics and both transgranular and intergranular fracture.
Anzoendung fraktographischer Verfahren auf Beryllium Die Metallographie verformter Berylliumoberfl~ichen l~iBt in Mikrobereichen betr~ichtliche Verformungen erkennen, und zwar sowohl durch Gleiten wie auch dutch Zwillingsbildung. Risse werden durch Korngrenzen gestoppt oder umgelenkt. Die Wechselwirkung zwischen Zwillingen und Korngrenzen scheint die Hauptursache fiir Mikrorissbildung zu sein. Die Mikrorisse gehen auch von Einschliissen aus. Dabei gibt es interkristalline und transkristalline Risse. Die transkristallinen h~ingen direkt mit der KorngrtBe zusammen, sie breiten sich sprSde aus. Die Bruchfl~ichen zeigen nebeneinander die Kennzeichen yon spr6den und Verformungsbriichen sowie inter- and transkristalline Bruchstellen.
Application des Techniques de Fractographie au Beryllium La m~tallographie des surfaces du b~ryllium d~form~ indique une d~formation plastique considerable par glissage, et m~clage h l'~chelle microscopique. Les fissures sont soit arret~es, soit d~vi~es par les joints de grains et l'interaction des mflcles avec les joints de grains semble ~tre la raison principale de la formation des microfissures. Ces microfissures sont aussi associ~es aux inclusions et sont ~ la lois intergranulaires et transgranulaires. Ces derni~res sont directement reli~es ~t la taille des grains et se propagent suivant u n mode caracttristique des fractures fragiles. Les surfaces de fracture poss~dent des caracttristiques de ductilit6 et de fragilit6 et laissent voir des fractures transgranulaires et intergranulaires.
Introduction B e r y l l i u m possesses c e r t a i n p r o p e r t i e s w h i c h r e n d e r it a p o t e n t i a l l y u s e f u l m a t e r i a l for a e r o s p a c e s t r u c t u r e s , a i r c r a f t gas t u r b i n e s , a n d a i r c r a f t brakes, z T h e t r o u b l e w i t h b e r y l l i u m is t h a t it is relatively b r i t t l e at a m b i e n t t e m p e r a t u r e s - - a fact w h i c h p r e v e n t s full r e a l i z a t i o n of its p o t e n t i a l . T h e q u e s t i o n , t h e n , is Copyright © 1971 by American Elsevier Publishing Company, Inc.
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whether this brittleness can be overcome or whether it will prevent the engineering application of the metal. Unfortunately, our knowledge of the mechanism of embrittlement on the microstructural scale is still very incomplete. A recent study by Thevenow et al. 2 showed that the fracture of thin polycrystalline wrought beryllium sheets originates below the surface and that the initial mode of failure in bending is primarily by transgranular cleavage. Among the potentially embrittling features present in polycrystalline beryllium sheet are cold work, grain boundaries, pores, inclusions, chemical segregation, preferred orientation, and, of course, the hexagonal crystal structure of the metal. The object of the present investigation is to study some of the embrittling mechanisms in beryllium by microscopic observations. Crack initiation at inclusions has been well documented as, for instance, in the ductile rupture of aluminum alloyss and in the brittle fracture of iron at low temperatures. 4 The presence of inclusions within a polycrystalline aggregate may lower its resistance to brittle fracture because the inclusions are capable of acting as stress-raisers and sources of micro-cracks. Alternatively, the inclusions may produce dislocation pile-ups which, in turn, may generate Stroh cracks in neighboring parts of the microstructure, as has been suggested for microcracking in polycrystalline beryllium. 2 Both mechanisms give a lower value of toughness than would be expected for inclusion-free material. In the case of the former, the fracture surface should either contain a larger than random density of inclusions than exists in the matrix, or, in the event that a single inclusion per grain may produce a well-defined cleavage crack, the origin of fracture initiation of the cleavage surface should reveal the site of the inclusion. In the latter mechanism described above, inclusions would not obviously be identified in a metaUographic study of fracture surfaces and transmission electron microscopy would be necessary. It should be noted that inclusions may influence fracture in other, less direct ways than described thus far. Inclusions and other heterogeneities may interact with the preferred orientation produced by rolling, for instance, and cause the strength anisotropy of mechanically fibered microstructures. 5 However, even in steels, the origin of flaws responsible for fissures is not always clear. Procedure
Materials and Specimens Samples of hot-pressed block and hot-rolled sheets of commercial-purity beryllium were obtained from Nuclear Metals Division, Whittaker Corporation, West Concord, Massachusetts, for use as metallographie specimens, for microhardness testing, and for three-point bend tests. These samples were of two different width-to-thickness ratios. The dimensions of the two different samples are, respectively, 11 X ~1 X g1 inch and 141x ~ x ~6 inch.
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535
The samples of hot-pressed block were machined from material hot-pressed into a 1½X 1½ inch cylinder. The individual specimens for bend testing were cut so that the pressing direction was normal to the plane of the samples and would be parallel to the bending force to be used in the three-point bending experiments (as shown in Fig. 1). The hot-pressed samples have a random grain orientation as was indicated by the supplier and verified by x-ray analysis of our samples. The hot-rolled samples were cut from cross-rolled sheet. These samples have a strong preferred orientation, with the basal plane of the hexagonal cell of the beryllium aligned nearly parallel to the plane of the sample. The grain size of these samples is approximately 20 microns.
POLISHED~E Flc. 1. The three-point bending scheme used. The polished surface was observed at several stages of deformation.
The tensile surface of the bend specimens was metallographically polished by the supplier, and this prepared surface was used in the hardness tests, the metallographic studies, and the direct observations during bending. The surface was not etched in any way, and the metaUography was carried out by using various types of polarizing and bright-field illumination to reveal the grain and twin structures of the material.
Hardness Testing Hardness testing of polycrystalline beryllium samples was performed in order to examine the extent of plastic flow adjacent to the hardness indentations or during subsequent bending tests. The microhardness testing was performed on a Kentron Micro Hardness Tester with a 500-gm load. This was used to introduce Knoop indentations into the surface of the specimens such that each indentation covered several grains. After these indentations were studied directly by optical metallography, replicas were made of the surface and then further microhardness indentations were placed very close to an original indentation in order to observe any interactions between the strain fields associated with the new and old indentations.
536
S. J. Bums, J. Gurland, and M. H. Richman
Straining Device To observe the effect of the microstructure on the deformation and fracture behavior of beryllium, a straining device was constructed which subjected the bend specimens to loads of known value while allowing the tensile surface to be observed in the microscope. The tensile surface in this bend test is also the surface of maximum stress--a factor which renders the bend test very important in this work. The straining device employs a load cell which was designed so that the effective spring constant of the device was much larger than that of the specimen. This made it a "hard" machine and allowed stable crack growth to be produced. In principle, the observed crack is then a stable crack and calculations can be made based on the load required to initiate such a crack. It should be pointed out, however, that the stress level on the surface of the specimen can be calculated only as long as the specimen remains elastic.
Metallography Metallographic studies of the surface of polished beryllium samples were performed with bright-field, polarizing, Nomarski interference contrast, and sensitive tint a microscopy. To study the various stages of bending or the effect of adding microhardness indentations to those already present, at various stages in the mechanical testing history, the metallographic surfaces were replicated by cellulose acetate shadowed with chromium at 45 ° . These replicas were fastened onto glass microscope slides and, under bright-field illumination, the various grains and twins as well as slip and other deformation markings could be seen quite clearly. The grain boundaries became more easily visible after some straining because of the rotation of the various grains around their boundaries. With this replication technique, the same areas on a specimen surface could be studied and compared with one another after it was determined where to look-that is, where the most interesting events had begun to occur. Such phenomena as the initiation of cracks or fracture could then be traced back through the various replicas to the time and site of their initiation.
Electron Microfractography The fracture surfaces of beryllium samples broken in three-point bending were examined by means of electron microfractography. Both the hot-pressed block and the hot-rolled sheet samples were studied by this technique to investigate the mode of fracture and the characteristics of the failure surface. Replicas were made of the fracture surfaces by a two-stage plastic carbon process. Bioden RFA plastic film was used for the first-stage replica which was then shadowed at 27 ° with chromium and a carbon second stage prepared. a Sensitivity tint microscopy is achieved by inserting a half-wave retardation plate into the plane-polarized light path, thereby obtaining polarizing effects with bright illumination'.
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The shadowed carbon replicas were separated from the plastic by dissolution of the plastic in a solution of methyl acetate. After successive rinsings in solvent and distilled water, the replicas were placed on 100-mesh nickel grids and examined in a JEM-7 electron microscope. Results
Optical Metallography Hardness Tests. The imposition of Knoop indentations into the metallographically polished surface of beryllium revealed that there is considerable plastic deformation occurring adjacent to the indentations. That plastic flow occurs both by slip and mechanical twinning can be seen in Fig. 2, which is a replica photo-
FIG. 2. Slip and twinning produced by Knoop impressions in hot-pressed beryllium rod. Replica, magnification 500 x. micrograph of hot-pressed beryllium rod in which several microhardness impressions were made. The central indentation shows evidence of slip and mechanical twinning on both sides of the hardness marking. The hardness tests and metallography consistently reveal plastic deformation adjacent to the indentations. This is true not only for the hot-pressed rod but also for the samples of hot-rolled sheet (Fig. 3). Here the presence of mechanical twins and slip lines around the single indentation clearly indicates that on the microscale polycrystalline beryllium must be considered to be rather ductile, confirming ReL 7. 38
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S. J. Burns, J. Gurland, and M. H. Richman
FIG. 3. Slip markings produced by a Knoop indentation in hot-rolled beryllium sheet. Replica, magnification 500 ×.
Bend Tests. W h e n samples of beryllium are deformed in three-point bending, the observed cracks are not always running cracks. T h e existence of stable, disconnected cracks can be seen in Fig. 4, which shows the microstructure after
FIG. 4. Transgranular and intergranular microcrack segments formed along a fracture path parallel to the main crack in a bent, fractured hot-pressed beryllium sample. Sensitive tint illumination, magnificationS875 ×.
Beryllium Fractographic Techniques
539
the sample fractured, in an area adjacent to the main crack. The applied tensile stresses on the surface due to the bending load are approximately parallel to the short sides of the photomicrograph. The length of crack segments, between changes in crack orientation, is approximately equal to the grain size of the metal, but the cracks are both intergranular and transgranular in nature (Fig. 5). That grain boundaries can, in fact, act as crack arresters is shown in the photomicrograph, where the crack which originates inside the grain is deflected and stopped at both ends by grain boundaries. The arrest of a transgranular crack is accompanied by heavy localized slip in the adjacent grains. In general, the plastic deformation is not uniformly distributed among the grains, and it appears that those grains which suffered transgranular cleavage do not exhibit pronounced plastic deformation. A sequential study of crack initiation and interaction is presented in Fig. 6. In Fig. 6a, a grain boundary crack is formed between two large inclusions after some permanent deformation of the sample (approximately 4~o surface strain). On raising the load, this sample broke, and in Fig. 6b this same field of view was traversed by larger cracks connected to the main fracture. It is significant that the last crack was apparently interrupted by the pre-existing grain boundary crack--a fact that suggests a possible role of the grain boundaries as "delamination" sites. The formation of transgranular cracking was most frequently observed as a twin-created process. Figure 7 shows this type of cracking. The large platelets of material which are lamella-shaped are fresh twins--that is, twins which were formed in the grains after the specimen had been metallographically polished. These fresh twins are observed in both the polarized light and bright-field micrographs. Old twins were polished away and do not appear in the bright-field photomicrograph, nor are they as bright as fresh twins in polarized light. The thin straight line, dark in bright field, is a crack, presumably on a basal cleavage plane. The stress axis is marked in this figure. The crack is partially transgranular and partly intergranular. It should be noted that the polishing scratches across the cleavage crack are slightly displaced, indicating that the cracking has relieved some of the stresses produced by the twinning operation. The ends of the twin lamella are on grain boundaries, and the twins have spread completely across the grains. The tips of the twins seem to produce either slip lines or cracks which are small relative to the grain size of the metal. Figure 8 shows a second example of twin-crack interaction; that is, it shows the creation of a transgranular crack in a neighboring grain produced by high local stresses due to a fresh twin. The cracked grain is particularly interesting because the symmetry of the twins introduced during hot pressing is quite consistent with (10T2) twinning and a basal plane cleavage crack in this grain. It should be mentioned that there were many fresh twins on the surface of the specimen which did not seem to produce cracking of neighboring grains. On the other hand,
540
S. J. Burns, J. Gurland, and M. H. Richman
(a)
(b) FIG. 5. Cracks which have been stopped by grain boundaries in bent and fractured hot-pressed beryllium can be seen. (a) Bright-field illumination; the cracks appear as dark narrow lines. (b) Polarized light illumination ; the cracks, the microstructure, and the grain boundary precipitates are shown for the same field as in (a). Magnification 500 × .
Beryllium Fractographic Techniques
541
(a)
(b) Fro. 6. Crack s e g m e n t s in a d e f o r m e d hot-rolled sheet. (a) Bright-field illumination; a grain b o u n d a r y crack b e t w e e n two large inclusions in a b e n t s p e c i m e n is shown. (b) S a m e field as (a) after t h e sample was fractured. Magnification 500 × .
S. J. Burns, J. Gurland, and M. H. Richman
542
POLARIZED
LIGHT
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GRAIN BOUNDARIES
SCHEMATIC
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7
CRACK
BRIGHT
FIELD
FIG. 7. Twin-nucleated microcracking in bent hot-pressed beryllium. In polarized light, the crack appears as a bright line being stopped by a twin and being deflected by a grain boundary. Magnification approximately 600 x .
Beryllium Fractographic Techniques
543
no transgranular cracks were observed without a neighboring grain being twinned, indicating that the twinning mechanism is essential in the creation of microcracks in beryllium.
POLARIZED LIGHT O-
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/ t " ~ ,~
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GRAIN BOUNDARIES
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Fro. 8. Twin-nucleated cracking in bent hot-pressed specimen. Magnification approximately 700 ×. Having established that the twinning mode is partly responsible for the creation of microcracks, the behavior of polycrystalline beryllium was observed in a triaxial stress condition such as one finds beneath a hardness indentor. In this way, it could be learned whether the individual grains either predominantly twinned or slipped. A small spherical indentor (slightly larger in contact area
544
S. J. Burns, J. Gurland, and M. 11. Richman
than the grain size) was loaded against the sample to create a permanent impression. T h e impression was not intended to be so severe as to destroy the observation of individual grains or the twins within these grains, since it was desired to learn if existing twins were just growing or if fresh twins were being created. Areas on the specimen face were, therefore, photographed before (Fig. 9) and after (Fig. 10) the surface of the crystal was indented. None of the existing twins
FIC. 9. Microstructure of hot-pressed beryllium before hardness indentation, as seen with polarized light. Magnification 200 ×. were seen to grow. T h e creation of twins takes place only to a very limited degree, as can be seen by comparing the areas under the round impressions of Fig. 10 with the same areas of Fig. 9 before the indentation. Even the dragging of the indentor across the surface (Fig. 10) did not result in the formation of many twins. T h e definite impression left in the surface of the sample is presumably due mostly to slip.
Electron Microfractography T h e fracture surfaces of both the hot-pressed rod and hot-rolled beryllium sheet were replicated, and the fraetographic replicas were examined in the
Beryllium Fractographic Techniques
545
electron microscope. The hot-pressed rod was studied only in the 1¼ × ½x ~-inch configuration. The fracture of the hot-pressed sheet was studied in both sizes. The microfractographs of the hot-pressed rod show typical regions of the fracture surface of quite distinctly different characteristics. A region of brittle
FIG. 10. Microstructure of same field as Fig. 9 after several hardness indentations, upper left and upper center. Polarized light. Magnification 200 x.
transgranular failure in the central grain of Fig. 11 contains evidence of inclusion-cleavage interaction in the fracture of that grain. The central grain of Fig. 12 is similar except that its fracture surface is relatively free of inclusions. The failure surface has changed in appearance from ductile (dimpled) to brittle halfway up the side of the central grain, the brittle failure has changed direction at the boundary and gone across the central grain, and the cleavage has continued into the grain at the right along a different direction. Hot-rolled sheet has been examined in two width-to-thickness ratios. The samples with the smaller value of width-to-thickness ratio (Fig. 13) have failed
FIo. 11. Electron microfractograph of hot-pressed rod showing inclusion-cleavage interaction. Magnification 2800 ×.
F1o. 12.
Microfractograph of hot-pressed rod showing transition from ductile (dim-
Beryllium Fractographic Techniques
547
in a brittle intergranular mode with only few cleavage markings or steps along the boundaries leading into or out of the surface from grain to grain. The larger width-to-thickness ratio material has a smaller grain size, but the fracture surface (Fig. 14) consists partly of intergranular failure (upper section) and partly of transgranular cleavage (lower section). There are also many inclusions present in this fracture surface, especially at the grain boundaries. The interaction of cracks and inclusions is more dramatically illustrated in Fig. 15, where an entire grain boundary is "loaded" with them.
FIG. 13. Microfractograph of hot-rolled sheet showing intergranular failure in a specimen of width-to-thickness ratio of 4. Magnification 2800 x.
In comparing the fracture surfaces of the various materials, the hot-pressed rod exhibits transgranular cleavage and some regions which are ductile in appearance, whereas the hot-rolled sheet shows intergranular failure and some regions of transgranular cleavage. They all show indications of inclusion-crack interaction. The difference in fracture modes cannot be solely attributed to the various width-to-thickness ratios of hot-rolled sheet because of the variation in grain size.
F1o. 14. Microfractograph of hot-rolled sheet showing intergranular failure and transgranular cleavage in a specimen of width-to-thickness ratio of 12. Magnification 2800 x .
FIo. 15. Microfractograph of hot-rolled sheet of a width-to-thickness ratio of 12 showing a central grain boundary "loaded" with inclusions. Magnification 2800 x .
Beryllium Fractographic Techniques
549
A very rough estimate of the effective fracture surface energy was made. The calculation of the surface energy was based on the plane strain Griffith criterion, using the observed crack lengths (Fig. 7) and the calculated surface stress levels in bending of approximately 2 x 10 s dynes/cm 2. This gives a surface energy of 2000 ergs/cm 2. The agreement with the surface energy of 2300 ergs/cm a reported by Kadmar 6 is undoubtedly fortuitous, but it is qualitatively significant in illustrating the apparent lack of an appreciable plastic work contribution to the cleavage initiation process on the microscale, whereas there is considerable plastic deformation in propagating cracks on the macroscale. Certain grains are easily defermed near the tip of a propagating macroscopic crack. Transgranular and intergranular cracks do not link up perfectly, and the fracture opening is bridged by deformation of grains which link the fracture surfaces.
Conclusions In spite of the observed macroscopic brittleness of beryllium, considerable plastic deformation does occur on the microscale. This occurs both by slip and by twinning. Little interaction has been noted between the strain fields of microhardness indentations placed close together. Cracks can be arrested or deflected by grain boundaries. The interaction of twinning with grain boundaries seems to be the main cause of microcrack creation. The microcracks formed are directly related to the grain size and seem to spread in a completely brittle manner. Inclusions are also associated with microcracks of both the intergranular and transgranular types. Under the triaxial compression imposed by an indentor, slip is a more active mode of deformation than twinning. The fracture surface of the hot-pressed rod exhibits transgranular cleavage and some regions of ductile appearance. The hot-rolled sheet shows intergranular failure and some regions of transgranular cleavage. The difference in fracture modes cannot be solely attributed to variations in the width-to-thickness ratio because of the variation in grain size.
Acknowledgments This work was supported by the Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio. Many stimulating discussions with Professor R. W. Armstrong are also acknowledged.
550
S. J. Burns, J. Gurland, and M. H. Richman
References 1. K. L. Kojola, Air Force Materials Laboratory Technical Report (AFML-TR-65-29); H. Conrad and I. Perlmutter, AFML-TR-65-310. 2. V. H. Thevenow, M. Herman and D. R. Betner, J. Mater., 5 (1970) 363. 3. J. Gurland and J. Plateau, Trans. ASM, 56 (1963) 442. 4. C. J. McMahon, Jr., Ship Structure Committee, NAS-NRC, Report SSC-161 (1964). 5. F. deKazinczy and W. A. Backofen, Trans. ASM, 53 (1961) 55. 6. M. H. Kamdar, personal communication from RIAS, Martin-Marietta Corporation, Baltimore, Maryland, on U.S. Air Force Contract F 33 615-67-C-1334. 7. R. F. Bunshah and R. W. Armstrong, Mat. Res. Bull., 4 (1969) 239.
Accepted April 10, 1971
533
Application of Fractographic Techniques to Beryllium S. J. B U R N S , J. G U R L A N D , A N D M. H. R I C H M A N
Division of Engineering, Brown University, Providence, Rhode Island
Metallography of deformed beryllium surfaces reveals considerable plastic flow on the microscale by both slip and twinning. Cracks are arrested or deflected by grain boundaries, and the interaction of twins with grain boundaries seems to be the main cause of microcrack formation. These microcracks are also associated with inclusions and are both intergranular and transgranular. T h e latter are directly related to the grain size and expand in a brittle manner. T h e fracture surfaces exhibit both ductile and brittle characteristics and both transgranular and intergranular fracture.
Anzoendung fraktographischer Verfahren auf Beryllium Die Metallographie verformter Berylliumoberfl~ichen l~iBt in Mikrobereichen betr~ichtliche Verformungen erkennen, und zwar sowohl durch Gleiten wie auch dutch Zwillingsbildung. Risse werden durch Korngrenzen gestoppt oder umgelenkt. Die Wechselwirkung zwischen Zwillingen und Korngrenzen scheint die Hauptursache fiir Mikrorissbildung zu sein. Die Mikrorisse gehen auch von Einschliissen aus. Dabei gibt es interkristalline und transkristalline Risse. Die transkristallinen h~ingen direkt mit der KorngrtBe zusammen, sie breiten sich sprSde aus. Die Bruchfl~ichen zeigen nebeneinander die Kennzeichen yon spr6den und Verformungsbriichen sowie inter- and transkristalline Bruchstellen.
Application des Techniques de Fractographie au Beryllium La m~tallographie des surfaces du b~ryllium d~form~ indique une d~formation plastique considerable par glissage, et m~clage h l'~chelle microscopique. Les fissures sont soit arret~es, soit d~vi~es par les joints de grains et l'interaction des mflcles avec les joints de grains semble ~tre la raison principale de la formation des microfissures. Ces microfissures sont aussi associ~es aux inclusions et sont ~ la lois intergranulaires et transgranulaires. Ces derni~res sont directement reli~es ~t la taille des grains et se propagent suivant u n mode caracttristique des fractures fragiles. Les surfaces de fracture poss~dent des caracttristiques de ductilit6 et de fragilit6 et laissent voir des fractures transgranulaires et intergranulaires.
Introduction B e r y l l i u m possesses c e r t a i n p r o p e r t i e s w h i c h r e n d e r it a p o t e n t i a l l y u s e f u l m a t e r i a l for a e r o s p a c e s t r u c t u r e s , a i r c r a f t gas t u r b i n e s , a n d a i r c r a f t brakes, z T h e t r o u b l e w i t h b e r y l l i u m is t h a t it is relatively b r i t t l e at a m b i e n t t e m p e r a t u r e s - - a fact w h i c h p r e v e n t s full r e a l i z a t i o n of its p o t e n t i a l . T h e q u e s t i o n , t h e n , is Copyright © 1971 by American Elsevier Publishing Company, Inc.
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whether this brittleness can be overcome or whether it will prevent the engineering application of the metal. Unfortunately, our knowledge of the mechanism of embrittlement on the microstructural scale is still very incomplete. A recent study by Thevenow et al. 2 showed that the fracture of thin polycrystalline wrought beryllium sheets originates below the surface and that the initial mode of failure in bending is primarily by transgranular cleavage. Among the potentially embrittling features present in polycrystalline beryllium sheet are cold work, grain boundaries, pores, inclusions, chemical segregation, preferred orientation, and, of course, the hexagonal crystal structure of the metal. The object of the present investigation is to study some of the embrittling mechanisms in beryllium by microscopic observations. Crack initiation at inclusions has been well documented as, for instance, in the ductile rupture of aluminum alloyss and in the brittle fracture of iron at low temperatures. 4 The presence of inclusions within a polycrystalline aggregate may lower its resistance to brittle fracture because the inclusions are capable of acting as stress-raisers and sources of micro-cracks. Alternatively, the inclusions may produce dislocation pile-ups which, in turn, may generate Stroh cracks in neighboring parts of the microstructure, as has been suggested for microcracking in polycrystalline beryllium. 2 Both mechanisms give a lower value of toughness than would be expected for inclusion-free material. In the case of the former, the fracture surface should either contain a larger than random density of inclusions than exists in the matrix, or, in the event that a single inclusion per grain may produce a well-defined cleavage crack, the origin of fracture initiation of the cleavage surface should reveal the site of the inclusion. In the latter mechanism described above, inclusions would not obviously be identified in a metaUographic study of fracture surfaces and transmission electron microscopy would be necessary. It should be noted that inclusions may influence fracture in other, less direct ways than described thus far. Inclusions and other heterogeneities may interact with the preferred orientation produced by rolling, for instance, and cause the strength anisotropy of mechanically fibered microstructures. 5 However, even in steels, the origin of flaws responsible for fissures is not always clear. Procedure
Materials and Specimens Samples of hot-pressed block and hot-rolled sheets of commercial-purity beryllium were obtained from Nuclear Metals Division, Whittaker Corporation, West Concord, Massachusetts, for use as metallographie specimens, for microhardness testing, and for three-point bend tests. These samples were of two different width-to-thickness ratios. The dimensions of the two different samples are, respectively, 11 X ~1 X g1 inch and 141x ~ x ~6 inch.
Beryllium Fractographic Techniques
535
The samples of hot-pressed block were machined from material hot-pressed into a 1½X 1½ inch cylinder. The individual specimens for bend testing were cut so that the pressing direction was normal to the plane of the samples and would be parallel to the bending force to be used in the three-point bending experiments (as shown in Fig. 1). The hot-pressed samples have a random grain orientation as was indicated by the supplier and verified by x-ray analysis of our samples. The hot-rolled samples were cut from cross-rolled sheet. These samples have a strong preferred orientation, with the basal plane of the hexagonal cell of the beryllium aligned nearly parallel to the plane of the sample. The grain size of these samples is approximately 20 microns.
POLISHED~E Flc. 1. The three-point bending scheme used. The polished surface was observed at several stages of deformation.
The tensile surface of the bend specimens was metallographically polished by the supplier, and this prepared surface was used in the hardness tests, the metallographic studies, and the direct observations during bending. The surface was not etched in any way, and the metaUography was carried out by using various types of polarizing and bright-field illumination to reveal the grain and twin structures of the material.
Hardness Testing Hardness testing of polycrystalline beryllium samples was performed in order to examine the extent of plastic flow adjacent to the hardness indentations or during subsequent bending tests. The microhardness testing was performed on a Kentron Micro Hardness Tester with a 500-gm load. This was used to introduce Knoop indentations into the surface of the specimens such that each indentation covered several grains. After these indentations were studied directly by optical metallography, replicas were made of the surface and then further microhardness indentations were placed very close to an original indentation in order to observe any interactions between the strain fields associated with the new and old indentations.
536
S. J. Bums, J. Gurland, and M. H. Richman
Straining Device To observe the effect of the microstructure on the deformation and fracture behavior of beryllium, a straining device was constructed which subjected the bend specimens to loads of known value while allowing the tensile surface to be observed in the microscope. The tensile surface in this bend test is also the surface of maximum stress--a factor which renders the bend test very important in this work. The straining device employs a load cell which was designed so that the effective spring constant of the device was much larger than that of the specimen. This made it a "hard" machine and allowed stable crack growth to be produced. In principle, the observed crack is then a stable crack and calculations can be made based on the load required to initiate such a crack. It should be pointed out, however, that the stress level on the surface of the specimen can be calculated only as long as the specimen remains elastic.
Metallography Metallographic studies of the surface of polished beryllium samples were performed with bright-field, polarizing, Nomarski interference contrast, and sensitive tint a microscopy. To study the various stages of bending or the effect of adding microhardness indentations to those already present, at various stages in the mechanical testing history, the metallographic surfaces were replicated by cellulose acetate shadowed with chromium at 45 ° . These replicas were fastened onto glass microscope slides and, under bright-field illumination, the various grains and twins as well as slip and other deformation markings could be seen quite clearly. The grain boundaries became more easily visible after some straining because of the rotation of the various grains around their boundaries. With this replication technique, the same areas on a specimen surface could be studied and compared with one another after it was determined where to look-that is, where the most interesting events had begun to occur. Such phenomena as the initiation of cracks or fracture could then be traced back through the various replicas to the time and site of their initiation.
Electron Microfractography The fracture surfaces of beryllium samples broken in three-point bending were examined by means of electron microfractography. Both the hot-pressed block and the hot-rolled sheet samples were studied by this technique to investigate the mode of fracture and the characteristics of the failure surface. Replicas were made of the fracture surfaces by a two-stage plastic carbon process. Bioden RFA plastic film was used for the first-stage replica which was then shadowed at 27 ° with chromium and a carbon second stage prepared. a Sensitivity tint microscopy is achieved by inserting a half-wave retardation plate into the plane-polarized light path, thereby obtaining polarizing effects with bright illumination'.
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The shadowed carbon replicas were separated from the plastic by dissolution of the plastic in a solution of methyl acetate. After successive rinsings in solvent and distilled water, the replicas were placed on 100-mesh nickel grids and examined in a JEM-7 electron microscope. Results
Optical Metallography Hardness Tests. The imposition of Knoop indentations into the metallographically polished surface of beryllium revealed that there is considerable plastic deformation occurring adjacent to the indentations. That plastic flow occurs both by slip and mechanical twinning can be seen in Fig. 2, which is a replica photo-
FIG. 2. Slip and twinning produced by Knoop impressions in hot-pressed beryllium rod. Replica, magnification 500 x. micrograph of hot-pressed beryllium rod in which several microhardness impressions were made. The central indentation shows evidence of slip and mechanical twinning on both sides of the hardness marking. The hardness tests and metallography consistently reveal plastic deformation adjacent to the indentations. This is true not only for the hot-pressed rod but also for the samples of hot-rolled sheet (Fig. 3). Here the presence of mechanical twins and slip lines around the single indentation clearly indicates that on the microscale polycrystalline beryllium must be considered to be rather ductile, confirming ReL 7. 38
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S. J. Burns, J. Gurland, and M. H. Richman
FIG. 3. Slip markings produced by a Knoop indentation in hot-rolled beryllium sheet. Replica, magnification 500 ×.
Bend Tests. W h e n samples of beryllium are deformed in three-point bending, the observed cracks are not always running cracks. T h e existence of stable, disconnected cracks can be seen in Fig. 4, which shows the microstructure after
FIG. 4. Transgranular and intergranular microcrack segments formed along a fracture path parallel to the main crack in a bent, fractured hot-pressed beryllium sample. Sensitive tint illumination, magnificationS875 ×.
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the sample fractured, in an area adjacent to the main crack. The applied tensile stresses on the surface due to the bending load are approximately parallel to the short sides of the photomicrograph. The length of crack segments, between changes in crack orientation, is approximately equal to the grain size of the metal, but the cracks are both intergranular and transgranular in nature (Fig. 5). That grain boundaries can, in fact, act as crack arresters is shown in the photomicrograph, where the crack which originates inside the grain is deflected and stopped at both ends by grain boundaries. The arrest of a transgranular crack is accompanied by heavy localized slip in the adjacent grains. In general, the plastic deformation is not uniformly distributed among the grains, and it appears that those grains which suffered transgranular cleavage do not exhibit pronounced plastic deformation. A sequential study of crack initiation and interaction is presented in Fig. 6. In Fig. 6a, a grain boundary crack is formed between two large inclusions after some permanent deformation of the sample (approximately 4~o surface strain). On raising the load, this sample broke, and in Fig. 6b this same field of view was traversed by larger cracks connected to the main fracture. It is significant that the last crack was apparently interrupted by the pre-existing grain boundary crack--a fact that suggests a possible role of the grain boundaries as "delamination" sites. The formation of transgranular cracking was most frequently observed as a twin-created process. Figure 7 shows this type of cracking. The large platelets of material which are lamella-shaped are fresh twins--that is, twins which were formed in the grains after the specimen had been metallographically polished. These fresh twins are observed in both the polarized light and bright-field micrographs. Old twins were polished away and do not appear in the bright-field photomicrograph, nor are they as bright as fresh twins in polarized light. The thin straight line, dark in bright field, is a crack, presumably on a basal cleavage plane. The stress axis is marked in this figure. The crack is partially transgranular and partly intergranular. It should be noted that the polishing scratches across the cleavage crack are slightly displaced, indicating that the cracking has relieved some of the stresses produced by the twinning operation. The ends of the twin lamella are on grain boundaries, and the twins have spread completely across the grains. The tips of the twins seem to produce either slip lines or cracks which are small relative to the grain size of the metal. Figure 8 shows a second example of twin-crack interaction; that is, it shows the creation of a transgranular crack in a neighboring grain produced by high local stresses due to a fresh twin. The cracked grain is particularly interesting because the symmetry of the twins introduced during hot pressing is quite consistent with (10T2) twinning and a basal plane cleavage crack in this grain. It should be mentioned that there were many fresh twins on the surface of the specimen which did not seem to produce cracking of neighboring grains. On the other hand,
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(a)
(b) FIG. 5. Cracks which have been stopped by grain boundaries in bent and fractured hot-pressed beryllium can be seen. (a) Bright-field illumination; the cracks appear as dark narrow lines. (b) Polarized light illumination ; the cracks, the microstructure, and the grain boundary precipitates are shown for the same field as in (a). Magnification 500 × .
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(a)
(b) Fro. 6. Crack s e g m e n t s in a d e f o r m e d hot-rolled sheet. (a) Bright-field illumination; a grain b o u n d a r y crack b e t w e e n two large inclusions in a b e n t s p e c i m e n is shown. (b) S a m e field as (a) after t h e sample was fractured. Magnification 500 × .
S. J. Burns, J. Gurland, and M. H. Richman
542
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FIG. 7. Twin-nucleated microcracking in bent hot-pressed beryllium. In polarized light, the crack appears as a bright line being stopped by a twin and being deflected by a grain boundary. Magnification approximately 600 x .
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543
no transgranular cracks were observed without a neighboring grain being twinned, indicating that the twinning mechanism is essential in the creation of microcracks in beryllium.
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Fro. 8. Twin-nucleated cracking in bent hot-pressed specimen. Magnification approximately 700 ×. Having established that the twinning mode is partly responsible for the creation of microcracks, the behavior of polycrystalline beryllium was observed in a triaxial stress condition such as one finds beneath a hardness indentor. In this way, it could be learned whether the individual grains either predominantly twinned or slipped. A small spherical indentor (slightly larger in contact area
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S. J. Burns, J. Gurland, and M. 11. Richman
than the grain size) was loaded against the sample to create a permanent impression. T h e impression was not intended to be so severe as to destroy the observation of individual grains or the twins within these grains, since it was desired to learn if existing twins were just growing or if fresh twins were being created. Areas on the specimen face were, therefore, photographed before (Fig. 9) and after (Fig. 10) the surface of the crystal was indented. None of the existing twins
FIC. 9. Microstructure of hot-pressed beryllium before hardness indentation, as seen with polarized light. Magnification 200 ×. were seen to grow. T h e creation of twins takes place only to a very limited degree, as can be seen by comparing the areas under the round impressions of Fig. 10 with the same areas of Fig. 9 before the indentation. Even the dragging of the indentor across the surface (Fig. 10) did not result in the formation of many twins. T h e definite impression left in the surface of the sample is presumably due mostly to slip.
Electron Microfractography T h e fracture surfaces of both the hot-pressed rod and hot-rolled beryllium sheet were replicated, and the fraetographic replicas were examined in the
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545
electron microscope. The hot-pressed rod was studied only in the 1¼ × ½x ~-inch configuration. The fracture of the hot-pressed sheet was studied in both sizes. The microfractographs of the hot-pressed rod show typical regions of the fracture surface of quite distinctly different characteristics. A region of brittle
FIG. 10. Microstructure of same field as Fig. 9 after several hardness indentations, upper left and upper center. Polarized light. Magnification 200 x.
transgranular failure in the central grain of Fig. 11 contains evidence of inclusion-cleavage interaction in the fracture of that grain. The central grain of Fig. 12 is similar except that its fracture surface is relatively free of inclusions. The failure surface has changed in appearance from ductile (dimpled) to brittle halfway up the side of the central grain, the brittle failure has changed direction at the boundary and gone across the central grain, and the cleavage has continued into the grain at the right along a different direction. Hot-rolled sheet has been examined in two width-to-thickness ratios. The samples with the smaller value of width-to-thickness ratio (Fig. 13) have failed
FIo. 11. Electron microfractograph of hot-pressed rod showing inclusion-cleavage interaction. Magnification 2800 ×.
F1o. 12.
Microfractograph of hot-pressed rod showing transition from ductile (dim-
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in a brittle intergranular mode with only few cleavage markings or steps along the boundaries leading into or out of the surface from grain to grain. The larger width-to-thickness ratio material has a smaller grain size, but the fracture surface (Fig. 14) consists partly of intergranular failure (upper section) and partly of transgranular cleavage (lower section). There are also many inclusions present in this fracture surface, especially at the grain boundaries. The interaction of cracks and inclusions is more dramatically illustrated in Fig. 15, where an entire grain boundary is "loaded" with them.
FIG. 13. Microfractograph of hot-rolled sheet showing intergranular failure in a specimen of width-to-thickness ratio of 4. Magnification 2800 x.
In comparing the fracture surfaces of the various materials, the hot-pressed rod exhibits transgranular cleavage and some regions which are ductile in appearance, whereas the hot-rolled sheet shows intergranular failure and some regions of transgranular cleavage. They all show indications of inclusion-crack interaction. The difference in fracture modes cannot be solely attributed to the various width-to-thickness ratios of hot-rolled sheet because of the variation in grain size.
F1o. 14. Microfractograph of hot-rolled sheet showing intergranular failure and transgranular cleavage in a specimen of width-to-thickness ratio of 12. Magnification 2800 x .
FIo. 15. Microfractograph of hot-rolled sheet of a width-to-thickness ratio of 12 showing a central grain boundary "loaded" with inclusions. Magnification 2800 x .
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A very rough estimate of the effective fracture surface energy was made. The calculation of the surface energy was based on the plane strain Griffith criterion, using the observed crack lengths (Fig. 7) and the calculated surface stress levels in bending of approximately 2 x 10 s dynes/cm 2. This gives a surface energy of 2000 ergs/cm 2. The agreement with the surface energy of 2300 ergs/cm a reported by Kadmar 6 is undoubtedly fortuitous, but it is qualitatively significant in illustrating the apparent lack of an appreciable plastic work contribution to the cleavage initiation process on the microscale, whereas there is considerable plastic deformation in propagating cracks on the macroscale. Certain grains are easily defermed near the tip of a propagating macroscopic crack. Transgranular and intergranular cracks do not link up perfectly, and the fracture opening is bridged by deformation of grains which link the fracture surfaces.
Conclusions In spite of the observed macroscopic brittleness of beryllium, considerable plastic deformation does occur on the microscale. This occurs both by slip and by twinning. Little interaction has been noted between the strain fields of microhardness indentations placed close together. Cracks can be arrested or deflected by grain boundaries. The interaction of twinning with grain boundaries seems to be the main cause of microcrack creation. The microcracks formed are directly related to the grain size and seem to spread in a completely brittle manner. Inclusions are also associated with microcracks of both the intergranular and transgranular types. Under the triaxial compression imposed by an indentor, slip is a more active mode of deformation than twinning. The fracture surface of the hot-pressed rod exhibits transgranular cleavage and some regions of ductile appearance. The hot-rolled sheet shows intergranular failure and some regions of transgranular cleavage. The difference in fracture modes cannot be solely attributed to variations in the width-to-thickness ratio because of the variation in grain size.
Acknowledgments This work was supported by the Air Force Materials Laboratory, Wright-Patterson Air Force Base, Ohio. Many stimulating discussions with Professor R. W. Armstrong are also acknowledged.
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References 1. K. L. Kojola, Air Force Materials Laboratory Technical Report (AFML-TR-65-29); H. Conrad and I. Perlmutter, AFML-TR-65-310. 2. V. H. Thevenow, M. Herman and D. R. Betner, J. Mater., 5 (1970) 363. 3. J. Gurland and J. Plateau, Trans. ASM, 56 (1963) 442. 4. C. J. McMahon, Jr., Ship Structure Committee, NAS-NRC, Report SSC-161 (1964). 5. F. deKazinczy and W. A. Backofen, Trans. ASM, 53 (1961) 55. 6. M. H. Kamdar, personal communication from RIAS, Martin-Marietta Corporation, Baltimore, Maryland, on U.S. Air Force Contract F 33 615-67-C-1334. 7. R. F. Bunshah and R. W. Armstrong, Mat. Res. Bull., 4 (1969) 239.
Accepted April 10, 1971