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A fractographic study on tensile fracture surface of duplex microstructure TiAl alloy
Scripta Materialia, Vol. 34, No. 12, pp. 1815-1818, 1996 Elsevier Science Ltd Copyright Q 1996 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/96 $12.00 + .OO
Pergamon PII S1359-6462(96)00003-6
A FRACTOGRAPHIC STUDY ON TENSILE FRACTURE SURFACE OF DUPLEX MICROSTRUCTURE TiAl ALLOY J. Zhang, J.D. Shi, D.X. Zou and Z.Y. Zhong Central Iron and Steel Research Institute, Beijing 10008 1, P.R. China (Received April 17, 1995) (Revised November 13, 1995) Introduction The mechanical properties of TiAl alloys depend significantly on the microstructure. Introducing duplex mixrostructure (DM) has improved ambient tensile plasticity and strength of TiAl alloys. Some aspects of the ductilizing mechanism have been established. The investigation of fracture behaviour on the side faces of compre,ssion test specimens indicated that a2 phase does not inhibit the initiation of crack but rather can retard the propagation of a crack [ 11. Observation of fracture surfaces demonstrated that the higher tensile ductility in DM is due to a small grain size and the fracture mechanisms are plasticityinduced grain boundary decohesion and cleavage [2]. To reveal the fracture micromechanism and the function of Lamellar colonies in the process of deformation, detailed fractography and cleavage crystallography are needed. Etch pitting i.s an efficient method to identify the crystalline features of fracture surfaces, since the shape of etch pits formed on the fracture surface depends on the orientation of the fracture facets and the orientation of the etch pit faces [3]. The relationship between the morphologies of the fracture surface and the orientation of’ fracture facets in the microstructure can be determined through SEM observation of the fracture surface being etch pitted. The objective of this article is to present the results of a Eractographic investigation on tensile fracture surfaces of a DM TiAl alloy through SEM observation and etch pitting technique. ExDerimental Procedures The material investigated was the Ti-46SAl-lCr-2.5V (at. pet) DM alloy with grain size around 40 urn. The tensile specimens were 93 x 18 mm screw connected cylinders. Their ambient elongations were approximately 3.2%. Etch pits were formed on y-TiAl phases by the constant voltage electrolysis method as described below. Etchant: 1% ~(Vol)Tetramethylin-ammonium 10% (Vol)Acetylacetone 89% (Vol) MLethanol.
1815
FRACTURE
1816
SURFACE OF TiAl
Vol. 34, No. 12
Figure 1. Fracture surface-in the equixed y grains. (a) unetched fractograph; (b) typical { 111)crack plane; (c) typical (011) crack plane.
Voltage: 13 Volts. Time: 15-30 seconds. The fracture surfaces were examined with KYKY AMRAY 1OOOBSEM. The crystalline indexes of fracture facets were identified according to the general relationship between shapes of etch-pits and orientations of FCC metal crystals [4]. Results and Discussion
Detailed observations with SEM revealed that the fracture facets of equiaxed y grains were flat and simple and the fracture crystalline features did not affect the morphologies (as shown in Fig. 1). There are no deflecting and branching mechanisms which appeared to have resisted the crack propagation in the equiaxed y grains. The fracture surface of the lamellar colonies exhibited three kinds of rough morphologies which have been related to the determined fracture features as listed below. (1) Terraced field morphology (as shown in Fig. 2) was frequently observed. From the shapes of the etch pits, the platforms appeared to be {1111 crystalline planes which are separated by (001) steps in
Figure 2. Terraced field fracture morphology. (a) ( 111) plane platforms; (b) (001) plane steps.
Vol. 34, No. 12
FRACTURESURFACEOF TiAl
1817
Figure 3. (a) Stepped fracture surface; (b) (011) y crack facets.
<112> direction, and the crack grew seemingly along ~011, direction. Similar to other published results [SJ, the crack followed a slip direction on a slip plane in FCC y TiAl. Uemori et al. reported that the dislocation could easily slip along ~110, on {111) in lamellar y [6]. It is reasonable to predict that the crack tip would be blunted often along the propagation direction. But Fig. 2 illustrates that a crack can grow through an entire lamellar colony along the direction without obvious deflection. There are however steps perpendicular to the propagation direction. At a small grain size, the lamellar y may not be long enough to lead to crack blunting with dislocation movements. This fracture morphology may be the result of the crack’s polyplanar initiating. (2) Some of the lamellar colonies exhibited stepped fracture surface (as shown in Fig. 3 (a)). Fine observations show that the platforms as well as steps are not a single plane but composed of a series of small facets. The shape of etch pits on these facets illustrated that the crack grew along <112> direction on (01 l}plane in lamellar y (see Fig. 3(b)). These steps could be inferred as being developed during the crack propagation. (3) There are a few lamellar colonies showing riverine morphology on the fracture surface as shown in Fig. 4. The shape of the etch pits indicates that the crack propagated along COOl> on {00 1 } in lamellar y and thus this morphology was formed as the cracks were blocked by the az lamellae.
Figure 4. Riverine facets with etch-pits on them showing (001) cracking crystallography.
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Vol. 34, No. 12
The crystalline indexes of fracture facets presently determined in the lamellar y phases coincide with the investigation on a y-TiAl single crystal [7], i.e. {Ill>, (011) and (001). There are only ( 111) and (0 1 1} crack planes observed in the equixed y grains. The reason why {001) y cracks are not observed in present collection is unknown. From this investigation, the rough fracture morphologies indicate that crack initiation and propagation in the lamellar colonies should be a high energy process, while the equiaxed y grains do not exhibit additional racture resistance. So, in addition to the grain size factor [2], there may be a structure factor to make a contribution to the higher tensile ductility of DM TiAl alloys. It is a lamellar structure that defers the failure of tensile specimens through introducing higher crack resistance and making more grains able to produce microdeformation and thus increasing the general plasticity. On the other hand, the equixed y can retard the grain coarsing in the CL~ + y phase region. Compared to the fully lamellar and near yamma microstructure, DM can get much finer grains through heat treatment. Therefore, it is possible to improve the ductility further by modifying the structure factor and grain size factor. Conclusions
1. The shapes of etch pits on the fracture facets indicate that the cracks appeared to follow the cl1 O> on (11 l}, <112> on {011} or ~001~ on {OOl} in lamellar y phases. 2. The cracks initiating and propagating through the lamellar colonies developed three rough morphologies which correspond to the determined fracture crystalline facets in lamellar y and can be inferred as a high energy processes. 3. The fracture surface of equixed y grains exhibited a flat appearance which indicates no additional crack resistance in these grains. AcknowledPment
The authors wish to thank Prof. Dr. S.Q. Li for her help and suggestions. References 1. K. Nonaka, K. Tanosaki, M. Fujita, A. Chiba, T. Kawabata, 0. Izumi, Mater.Trans. JIM, 33,802 (1992). 2. 3. 4. 5. 6. 7.
KS. Chan and Y-W. Kim, Metall. Trans. A, 23, 1663 (1992). E.I. Meletis and R.F. Hockman, J. Testing Eval., 12, 142 (1984). E.T. Betz and J. Prohaszka, J. Metallography, 7,91 (1974). KS. Chan and Y-W. Kim, Metall. Trans. A, 25, 1217 (1994). R. Uemori, T. Hanamura, H. Morikawa, Scripta Metall. Mater., 26,969 (1992). T. Kawabata, Y. Takezono, T. Kanai, 0. Izumi, Acta Metall., 36,963 (1986).
Pergamon PII S1359-6462(96)00003-6
A FRACTOGRAPHIC STUDY ON TENSILE FRACTURE SURFACE OF DUPLEX MICROSTRUCTURE TiAl ALLOY J. Zhang, J.D. Shi, D.X. Zou and Z.Y. Zhong Central Iron and Steel Research Institute, Beijing 10008 1, P.R. China (Received April 17, 1995) (Revised November 13, 1995) Introduction The mechanical properties of TiAl alloys depend significantly on the microstructure. Introducing duplex mixrostructure (DM) has improved ambient tensile plasticity and strength of TiAl alloys. Some aspects of the ductilizing mechanism have been established. The investigation of fracture behaviour on the side faces of compre,ssion test specimens indicated that a2 phase does not inhibit the initiation of crack but rather can retard the propagation of a crack [ 11. Observation of fracture surfaces demonstrated that the higher tensile ductility in DM is due to a small grain size and the fracture mechanisms are plasticityinduced grain boundary decohesion and cleavage [2]. To reveal the fracture micromechanism and the function of Lamellar colonies in the process of deformation, detailed fractography and cleavage crystallography are needed. Etch pitting i.s an efficient method to identify the crystalline features of fracture surfaces, since the shape of etch pits formed on the fracture surface depends on the orientation of the fracture facets and the orientation of the etch pit faces [3]. The relationship between the morphologies of the fracture surface and the orientation of’ fracture facets in the microstructure can be determined through SEM observation of the fracture surface being etch pitted. The objective of this article is to present the results of a Eractographic investigation on tensile fracture surfaces of a DM TiAl alloy through SEM observation and etch pitting technique. ExDerimental Procedures The material investigated was the Ti-46SAl-lCr-2.5V (at. pet) DM alloy with grain size around 40 urn. The tensile specimens were 93 x 18 mm screw connected cylinders. Their ambient elongations were approximately 3.2%. Etch pits were formed on y-TiAl phases by the constant voltage electrolysis method as described below. Etchant: 1% ~(Vol)Tetramethylin-ammonium 10% (Vol)Acetylacetone 89% (Vol) MLethanol.
1815
FRACTURE
1816
SURFACE OF TiAl
Vol. 34, No. 12
Figure 1. Fracture surface-in the equixed y grains. (a) unetched fractograph; (b) typical { 111)crack plane; (c) typical (011) crack plane.
Voltage: 13 Volts. Time: 15-30 seconds. The fracture surfaces were examined with KYKY AMRAY 1OOOBSEM. The crystalline indexes of fracture facets were identified according to the general relationship between shapes of etch-pits and orientations of FCC metal crystals [4]. Results and Discussion
Detailed observations with SEM revealed that the fracture facets of equiaxed y grains were flat and simple and the fracture crystalline features did not affect the morphologies (as shown in Fig. 1). There are no deflecting and branching mechanisms which appeared to have resisted the crack propagation in the equiaxed y grains. The fracture surface of the lamellar colonies exhibited three kinds of rough morphologies which have been related to the determined fracture features as listed below. (1) Terraced field morphology (as shown in Fig. 2) was frequently observed. From the shapes of the etch pits, the platforms appeared to be {1111 crystalline planes which are separated by (001) steps in
Figure 2. Terraced field fracture morphology. (a) ( 111) plane platforms; (b) (001) plane steps.
Vol. 34, No. 12
FRACTURESURFACEOF TiAl
1817
Figure 3. (a) Stepped fracture surface; (b) (011) y crack facets.
<112> direction, and the crack grew seemingly along ~011, direction. Similar to other published results [SJ, the crack followed a slip direction on a slip plane in FCC y TiAl. Uemori et al. reported that the dislocation could easily slip along ~110, on {111) in lamellar y [6]. It is reasonable to predict that the crack tip would be blunted often along the propagation direction. But Fig. 2 illustrates that a crack can grow through an entire lamellar colony along the direction
Figure 4. Riverine facets with etch-pits on them showing (001)
1818
FRACTURESURFACEOF TiAl
Vol. 34, No. 12
The crystalline indexes of fracture facets presently determined in the lamellar y phases coincide with the investigation on a y-TiAl single crystal [7], i.e. {Ill>, (011) and (001). There are only ( 111) and (0 1 1} crack planes observed in the equixed y grains. The reason why {001) y cracks are not observed in present collection is unknown. From this investigation, the rough fracture morphologies indicate that crack initiation and propagation in the lamellar colonies should be a high energy process, while the equiaxed y grains do not exhibit additional racture resistance. So, in addition to the grain size factor [2], there may be a structure factor to make a contribution to the higher tensile ductility of DM TiAl alloys. It is a lamellar structure that defers the failure of tensile specimens through introducing higher crack resistance and making more grains able to produce microdeformation and thus increasing the general plasticity. On the other hand, the equixed y can retard the grain coarsing in the CL~ + y phase region. Compared to the fully lamellar and near yamma microstructure, DM can get much finer grains through heat treatment. Therefore, it is possible to improve the ductility further by modifying the structure factor and grain size factor. Conclusions
1. The shapes of etch pits on the fracture facets indicate that the cracks appeared to follow the cl1 O> on (11 l}, <112> on {011} or ~001~ on {OOl} in lamellar y phases. 2. The cracks initiating and propagating through the lamellar colonies developed three rough morphologies which correspond to the determined fracture crystalline facets in lamellar y and can be inferred as a high energy processes. 3. The fracture surface of equixed y grains exhibited a flat appearance which indicates no additional crack resistance in these grains. AcknowledPment
The authors wish to thank Prof. Dr. S.Q. Li for her help and suggestions. References 1. K. Nonaka, K. Tanosaki, M. Fujita, A. Chiba, T. Kawabata, 0. Izumi, Mater.Trans. JIM, 33,802 (1992). 2. 3. 4. 5. 6. 7.
KS. Chan and Y-W. Kim, Metall. Trans. A, 23, 1663 (1992). E.I. Meletis and R.F. Hockman, J. Testing Eval., 12, 142 (1984). E.T. Betz and J. Prohaszka, J. Metallography, 7,91 (1974). KS. Chan and Y-W. Kim, Metall. Trans. A, 25, 1217 (1994). R. Uemori, T. Hanamura, H. Morikawa, Scripta Metall. Mater., 26,969 (1992). T. Kawabata, Y. Takezono, T. Kanai, 0. Izumi, Acta Metall., 36,963 (1986).