- Email: [email protected]
Some TEM-observations of fatigued pseudoelastic Cu-Zn-Al alloys
METALLOGRAPHY 18:107-113 (1985)
107
Some TEM-Observations of Fatigued Pseudoelastic Cu-Zn-AI Alloys
M. ANDRADE,* J. JANSSEN,+ AND L. DELAEY
Department of Metallurgy and Materials Engineering, Katholieke Universiteit, Leuven. Belgium
The substructure of a fatigued pseudoelastic I3~-Cu-Zn-AI single crystal has been analyzed by transmission electron microscopy. Small regions containing a high density of dislocations were found in a 131 matrix relatively free from dislocations. The presence of such unexpected dislocation configurations is discussed, and a model accounting for their formation is proposed in connection with the formation o f ~ ' martensite during the pseudoelastic cycles.
Introduction The C u - Z n - A I alloys are known to exhibit pseudoelasticity in the appropriate range of temperature and stress, due to the occurrence of a rev e r s i b l e stress-induced martensitic transformation. Because of this reversible character of the pseudoelastic deformation, it has been assumed that the fatigue life of pseudoelastic cycled Cu-Zn-A1 alloys should be high. However, even at very low stress levels, specimens of the alloys are found to have very poor fatigue properties [1, 2, 3]. Moreover, the already reported [1, 3, 4] presence of dislocations and retained martensite in the [31 matrix after pseudoelastic cycling suggests that not only reversible processes take place. Different authors have reported the presence of some specific types of dislocations left in the [31 phase after thermal and stress-assisted cycles [5, 6]. The question whether these dislocations are due to the martensitic transformation itself, or whether they are just the result of the interaction * Present address: Fundagao Centro Tecnologico de Minas Gerais-CETEC, Belo Horizonte, Brazil. + Present address: Lamitref Koper N.V., Hemiksem, Belgium. ,~ Elsevier Science Publishing Co., Inc., 1985 52 Vanderbilt Ave., New York, NY 10017
0026-0800/85/$03.30
108
Andrade, Janssen, and Delaey
between preexisting defects in the [~1 matrix with the new forming phase, remains to be answered. So far, the fatigue behavior of Cu-Zn-A1 alloys has been investigated in polycrystalline specimens only, and, to the authors' knowledge, no literature is available about the fatigue properties of C u - Z n - A l single crystals. Clearly, in the case of polycrystals, the influence of grain boundaries should not be disregarded. In fact, according to the current view, it is the stress concentration at grain boundaries, due to the large, elastic anisotropy that is typical of these alloys close to the martensitic transformation temperature (Ms) [6] or to the limited number of shear systems of the martensite plate variants that form along the grain boundaries [7], that leads to failure. This idea is supported by the intergranular type of fracture generally observed to occur in polycrystalline Cu-Zn-A1 alloys. In the case of single-crystal Cu-Zn-A1 specimens, however, the fatigue behavior must be explained solely in terms of the substructure. This work concentrated on the characterization of the substructure of C u - Z n - A l single crystals, cycled in the pseudoelastic range until fracture. It is interesting to observe that, also in this case, the failure occurred unexpectedly early. In the present paper only a special feature of the substructure found in the ~3~ phase after pseudoelastic cycling will be reported. This does not mean that other features, perhaps less striking in appearance, have not been noticed. The results obtained in this investigation are not at all conclusive, but they serve to document some important aspects of the substructure of the [~1 phase that have not yet been reported.
Experimental Procedure Tensile specimens, with a gauge length of 32 mm and cross section of 5 mm x 6.1 mm, were machined from a single crystal alloy with composition 67.78 at.% Cu-18.98 at.% Zn-13.14 at.% AI. They were annealed for 15 min at 800°C and quenched in an oil bath at 60°C where they were left for two hours. The Ms temperature measured after these procedures was - 20°C. Pseudoelastic cycles (0 to 3% deformation) were performed at room temperature. Fracture occurred after 417 cycles. Longitudinal strips were cut from the fractured specimens and mechanically polished to 0.3 mm thickness. Disks, 3 mm in diameter, obtained from these strips by spark machining, were electrolytically thinned in a double jet electropolisher. The samples were examined in a transmission electron microscope operating at 200 KV.
Fatigued Pseudoelastic Cu-Zn-Al Alloys
109
Results
Transmission electron microscopy revealed the presence of small rectangular regions of localized plastic deformation in certain areas of the specimens. As can be seen in Fig. I, which shows a typical example of such regions, the surrounding matrix had a comparatively low dislocation density. The possibility that such dislocation-rich regions might be associated with the presence of a phase other than the 13~phase was checked through the analysis of the corresponding diffraction patterns. However, the patterns obtained did not contain any spot that could not be indexed as belonging to the 13~ phase, which proves that the dislocation-rich regions were indeed the matrix phase, heavily deformed. The absence of Kikuchi lines in these diffraction patterns, as shown in Fig. 2a, in contrast to the presence of these lines in the patterns corresponding to the surrounding matrix, shown in Fig. 2b, further supports this conclusion. It is also important to mention that photographs taken at a lower magnification, such as that in Fig. 3, have shown that, far from being randomly distributed, the deformed regions appear to be aligned in two different directions, denoted by A and B in this figure. It should be noted that the
l~m FIG. 1. Substructure of pseudoelastic cycled 131 single crystal showing localized deformation leading to matrix strain contrast.
Andrade, Janssen, and Delaey
110
(a)
(b) FIG. 2. Diffraction patterns taken from (a) the dislocation-rich regions; and (b) the surrounding 131 matrix.
Fatigued Pseudoelastic Cu-Zn-AI Alloys
111
0.25tJm
0,25~rn
\
FIG. 3.
The distribution of the heavily faulted regions.
112
Andrade, Janssen, and Delaey
small rectangular regions induce strain field contrast in the surrounding matrix phase. Discussion As the figures indicate, the substructure of the analyzed [31 phase consists of small regions showing signs of heavy and very localized deformation, aligned in a particular way in a matrix containing comparatively few and randomly distributed dislocations. The majority of the dislocations inside the areas of low dislocation density are typical of the quenched [31 phase and, therefore, no further comments will be made regarding them in this discussion. Most remarkable, however, are the characteristics of the deformed regions, and, although it is not the aim here to discuss in detail the possible influence of these regions on the fatigue behavior, they certainly must play an important role. For instance, it is clear that the matrix immediately around the deformed areas is a region of stress concentration, and is therefore a favorable location for crack nucleation. There is also the possibility that such regions act as barriers for the motion of dislocations, causing strain hardening of the material and subsequent failure. But the purpose of this paper is rather to suggest a mechanism by means of which such heavily deformed regions could have formed during the pseudoelastic cycles. It has already been reported [9] that when two favorably oriented 18R martensite variants cross each other, ~' martensite is formed in such a way that the number of slip systems is increased, and deformation may proceed more easily in this region. It is worth noting that the two possible slip systems in such a' variant are very close to the shear systems necessary to form the two original 18R variants. Taking into account that the formation of a' was possible for the particular alloy studied, it is suggested here that the observed heavily deformed regions should correspond to an area where cx' was formed through the intersection of two variants at a certain stage of the pseudoelastic cycle. If this hypothesis is true, plastic deformation could thus have occurred easily in this region. Upon releasing the stress, the two variants, and hence the c~' variant formed at their intersection, reverted back to the matrix, leaving behind the plastic deformation introduced in this and previous cycles. It was not possible, in fact, to prove that ~x' formed in the specimens. However, since the deformed regions were very small (3 ixm x 1 Fxm), the a' variants, if formed in the manner just described, would be very small and impossible to see during the pseudoelastic cycling. In Figs. 8a and 9 of [1] such a crossing of martensite variants has indeed
Fatigued Pseudoelastic Cu-Zn-Al Alloys
I I3
been shown to occur. Due to the plastic deformation that occurred in the a' regions, the original shape will not be restored upon retransformation. Because no plastic deformation has occurred in the surrounding matrix, elastic, possibly accompanied by some further plastic, deformation will be needed to accommodate the difference in shape. This elastic accommodation explains the observed strain field. Conclusion
Pseudoelastic fatigued [3-phase samples may contain, after cycling, small heavily plastic deformed regions. The shape and the distribution of these regions suggest that they originate from intersecting stress-induced martensite plates. The strain fields surrounding these areas may be a further cause for embrittlement, and result in premature failure of pseudoelastic cycled [3-phase samples.
Part of this work has been supported by the Belgian Ministry of Science (Dienst Wetenschapsbeleid). M.A. would like to thank the Fundagao Centro Tecnologico de Minas Gerais--CETEC and the Conselho Nacional de Desinvolvimento Cientifico e Tecnologico--CNPg of Brazil for the award of a scholarship. References 1. R. Oshima and N. Yoshida, Fatigue of thermoelastic C u - Z n - A l alloys, J. de Physique 43, C4-803 (1982). 2. L. Delaey, J. Janssen, D. Van de Mosselaer, G. Dullenkopf, and A. Deruyttere, Fatigue properties of pseudoelastic C u - Z n - A I alloys, Scripta Met. 12, 373 (1978). 3. K. N. Melton and O. Mercier, Fatigue life of Cu-Zn-A1 alloys, Scripta Met. 13, 73 (1979). 4. J. Janssen, M. Follon, and L. Delaey, The fatigue properties of superelastic C u - Z n - A I alloys, in Strength o f Metals and Alloys (P. Haasen, V. Gerold, G. Kostorz, Eds.), vol. 2, 1125 (1979). 5. S. Kajiwara and T. Kikuchi, Dislocation structures produced by reverse martensitic transformation in a C u - Z n alloy, Acta Met. 30, 589 (1982). 6. D. Rios Jara, M. Morin, C. Esnouf, and G. Gu6nin, Study of dislocations in cyclically transformed 13-phase in C u - Z n - A I alloys, J. de Physique 43, C4-735 (1982). 7. S. Miyazaki, T. Kawai, and S. Otsuka, On the origin of intergranular fracture in 13-phase shape memory alloys, Scripta Met. 16,431 (1982). 8. K. Takezawa, T. Izumi, H. Chiba, and S. Sato, Coherency of the transformation strain at the grain boundary and fracture in Cu-Zn-A1 alloy, J. de Physique 43, C4-819 (1982). 9. H. Sato, K. Takezawa, and S. Sato, Mechanical behavior associated with the B ---, B'. ---, cd, transformation in C u - Z n - A I single crystals, Joint U S - J a p a n Seminar (Troy, N.Y. U.S.A.), (1979) 92. Received June 1984; accepted October 1984.
107
Some TEM-Observations of Fatigued Pseudoelastic Cu-Zn-AI Alloys
M. ANDRADE,* J. JANSSEN,+ AND L. DELAEY
Department of Metallurgy and Materials Engineering, Katholieke Universiteit, Leuven. Belgium
The substructure of a fatigued pseudoelastic I3~-Cu-Zn-AI single crystal has been analyzed by transmission electron microscopy. Small regions containing a high density of dislocations were found in a 131 matrix relatively free from dislocations. The presence of such unexpected dislocation configurations is discussed, and a model accounting for their formation is proposed in connection with the formation o f ~ ' martensite during the pseudoelastic cycles.
Introduction The C u - Z n - A I alloys are known to exhibit pseudoelasticity in the appropriate range of temperature and stress, due to the occurrence of a rev e r s i b l e stress-induced martensitic transformation. Because of this reversible character of the pseudoelastic deformation, it has been assumed that the fatigue life of pseudoelastic cycled Cu-Zn-A1 alloys should be high. However, even at very low stress levels, specimens of the alloys are found to have very poor fatigue properties [1, 2, 3]. Moreover, the already reported [1, 3, 4] presence of dislocations and retained martensite in the [31 matrix after pseudoelastic cycling suggests that not only reversible processes take place. Different authors have reported the presence of some specific types of dislocations left in the [31 phase after thermal and stress-assisted cycles [5, 6]. The question whether these dislocations are due to the martensitic transformation itself, or whether they are just the result of the interaction * Present address: Fundagao Centro Tecnologico de Minas Gerais-CETEC, Belo Horizonte, Brazil. + Present address: Lamitref Koper N.V., Hemiksem, Belgium. ,~ Elsevier Science Publishing Co., Inc., 1985 52 Vanderbilt Ave., New York, NY 10017
0026-0800/85/$03.30
108
Andrade, Janssen, and Delaey
between preexisting defects in the [~1 matrix with the new forming phase, remains to be answered. So far, the fatigue behavior of Cu-Zn-A1 alloys has been investigated in polycrystalline specimens only, and, to the authors' knowledge, no literature is available about the fatigue properties of C u - Z n - A l single crystals. Clearly, in the case of polycrystals, the influence of grain boundaries should not be disregarded. In fact, according to the current view, it is the stress concentration at grain boundaries, due to the large, elastic anisotropy that is typical of these alloys close to the martensitic transformation temperature (Ms) [6] or to the limited number of shear systems of the martensite plate variants that form along the grain boundaries [7], that leads to failure. This idea is supported by the intergranular type of fracture generally observed to occur in polycrystalline Cu-Zn-A1 alloys. In the case of single-crystal Cu-Zn-A1 specimens, however, the fatigue behavior must be explained solely in terms of the substructure. This work concentrated on the characterization of the substructure of C u - Z n - A l single crystals, cycled in the pseudoelastic range until fracture. It is interesting to observe that, also in this case, the failure occurred unexpectedly early. In the present paper only a special feature of the substructure found in the ~3~ phase after pseudoelastic cycling will be reported. This does not mean that other features, perhaps less striking in appearance, have not been noticed. The results obtained in this investigation are not at all conclusive, but they serve to document some important aspects of the substructure of the [~1 phase that have not yet been reported.
Experimental Procedure Tensile specimens, with a gauge length of 32 mm and cross section of 5 mm x 6.1 mm, were machined from a single crystal alloy with composition 67.78 at.% Cu-18.98 at.% Zn-13.14 at.% AI. They were annealed for 15 min at 800°C and quenched in an oil bath at 60°C where they were left for two hours. The Ms temperature measured after these procedures was - 20°C. Pseudoelastic cycles (0 to 3% deformation) were performed at room temperature. Fracture occurred after 417 cycles. Longitudinal strips were cut from the fractured specimens and mechanically polished to 0.3 mm thickness. Disks, 3 mm in diameter, obtained from these strips by spark machining, were electrolytically thinned in a double jet electropolisher. The samples were examined in a transmission electron microscope operating at 200 KV.
Fatigued Pseudoelastic Cu-Zn-Al Alloys
109
Results
Transmission electron microscopy revealed the presence of small rectangular regions of localized plastic deformation in certain areas of the specimens. As can be seen in Fig. I, which shows a typical example of such regions, the surrounding matrix had a comparatively low dislocation density. The possibility that such dislocation-rich regions might be associated with the presence of a phase other than the 13~phase was checked through the analysis of the corresponding diffraction patterns. However, the patterns obtained did not contain any spot that could not be indexed as belonging to the 13~ phase, which proves that the dislocation-rich regions were indeed the matrix phase, heavily deformed. The absence of Kikuchi lines in these diffraction patterns, as shown in Fig. 2a, in contrast to the presence of these lines in the patterns corresponding to the surrounding matrix, shown in Fig. 2b, further supports this conclusion. It is also important to mention that photographs taken at a lower magnification, such as that in Fig. 3, have shown that, far from being randomly distributed, the deformed regions appear to be aligned in two different directions, denoted by A and B in this figure. It should be noted that the
l~m FIG. 1. Substructure of pseudoelastic cycled 131 single crystal showing localized deformation leading to matrix strain contrast.
Andrade, Janssen, and Delaey
110
(a)
(b) FIG. 2. Diffraction patterns taken from (a) the dislocation-rich regions; and (b) the surrounding 131 matrix.
Fatigued Pseudoelastic Cu-Zn-AI Alloys
111
0.25tJm
0,25~rn
\
FIG. 3.
The distribution of the heavily faulted regions.
112
Andrade, Janssen, and Delaey
small rectangular regions induce strain field contrast in the surrounding matrix phase. Discussion As the figures indicate, the substructure of the analyzed [31 phase consists of small regions showing signs of heavy and very localized deformation, aligned in a particular way in a matrix containing comparatively few and randomly distributed dislocations. The majority of the dislocations inside the areas of low dislocation density are typical of the quenched [31 phase and, therefore, no further comments will be made regarding them in this discussion. Most remarkable, however, are the characteristics of the deformed regions, and, although it is not the aim here to discuss in detail the possible influence of these regions on the fatigue behavior, they certainly must play an important role. For instance, it is clear that the matrix immediately around the deformed areas is a region of stress concentration, and is therefore a favorable location for crack nucleation. There is also the possibility that such regions act as barriers for the motion of dislocations, causing strain hardening of the material and subsequent failure. But the purpose of this paper is rather to suggest a mechanism by means of which such heavily deformed regions could have formed during the pseudoelastic cycles. It has already been reported [9] that when two favorably oriented 18R martensite variants cross each other, ~' martensite is formed in such a way that the number of slip systems is increased, and deformation may proceed more easily in this region. It is worth noting that the two possible slip systems in such a' variant are very close to the shear systems necessary to form the two original 18R variants. Taking into account that the formation of a' was possible for the particular alloy studied, it is suggested here that the observed heavily deformed regions should correspond to an area where cx' was formed through the intersection of two variants at a certain stage of the pseudoelastic cycle. If this hypothesis is true, plastic deformation could thus have occurred easily in this region. Upon releasing the stress, the two variants, and hence the c~' variant formed at their intersection, reverted back to the matrix, leaving behind the plastic deformation introduced in this and previous cycles. It was not possible, in fact, to prove that ~x' formed in the specimens. However, since the deformed regions were very small (3 ixm x 1 Fxm), the a' variants, if formed in the manner just described, would be very small and impossible to see during the pseudoelastic cycling. In Figs. 8a and 9 of [1] such a crossing of martensite variants has indeed
Fatigued Pseudoelastic Cu-Zn-Al Alloys
I I3
been shown to occur. Due to the plastic deformation that occurred in the a' regions, the original shape will not be restored upon retransformation. Because no plastic deformation has occurred in the surrounding matrix, elastic, possibly accompanied by some further plastic, deformation will be needed to accommodate the difference in shape. This elastic accommodation explains the observed strain field. Conclusion
Pseudoelastic fatigued [3-phase samples may contain, after cycling, small heavily plastic deformed regions. The shape and the distribution of these regions suggest that they originate from intersecting stress-induced martensite plates. The strain fields surrounding these areas may be a further cause for embrittlement, and result in premature failure of pseudoelastic cycled [3-phase samples.
Part of this work has been supported by the Belgian Ministry of Science (Dienst Wetenschapsbeleid). M.A. would like to thank the Fundagao Centro Tecnologico de Minas Gerais--CETEC and the Conselho Nacional de Desinvolvimento Cientifico e Tecnologico--CNPg of Brazil for the award of a scholarship. References 1. R. Oshima and N. Yoshida, Fatigue of thermoelastic C u - Z n - A l alloys, J. de Physique 43, C4-803 (1982). 2. L. Delaey, J. Janssen, D. Van de Mosselaer, G. Dullenkopf, and A. Deruyttere, Fatigue properties of pseudoelastic C u - Z n - A I alloys, Scripta Met. 12, 373 (1978). 3. K. N. Melton and O. Mercier, Fatigue life of Cu-Zn-A1 alloys, Scripta Met. 13, 73 (1979). 4. J. Janssen, M. Follon, and L. Delaey, The fatigue properties of superelastic C u - Z n - A I alloys, in Strength o f Metals and Alloys (P. Haasen, V. Gerold, G. Kostorz, Eds.), vol. 2, 1125 (1979). 5. S. Kajiwara and T. Kikuchi, Dislocation structures produced by reverse martensitic transformation in a C u - Z n alloy, Acta Met. 30, 589 (1982). 6. D. Rios Jara, M. Morin, C. Esnouf, and G. Gu6nin, Study of dislocations in cyclically transformed 13-phase in C u - Z n - A I alloys, J. de Physique 43, C4-735 (1982). 7. S. Miyazaki, T. Kawai, and S. Otsuka, On the origin of intergranular fracture in 13-phase shape memory alloys, Scripta Met. 16,431 (1982). 8. K. Takezawa, T. Izumi, H. Chiba, and S. Sato, Coherency of the transformation strain at the grain boundary and fracture in Cu-Zn-A1 alloy, J. de Physique 43, C4-819 (1982). 9. H. Sato, K. Takezawa, and S. Sato, Mechanical behavior associated with the B ---, B'. ---, cd, transformation in C u - Z n - A I single crystals, Joint U S - J a p a n Seminar (Troy, N.Y. U.S.A.), (1979) 92. Received June 1984; accepted October 1984.