Cyclic deformation behavior of high-purity titanium single crystals: Part II. Microstructure and mechanism

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Microstructure and mechanism Xiaoli Tan, Florida International University H. Guo H.

1 Iowa State University From the SelectedWorks of Xiaoli Tan February, 1998 Cyclic deformation behavior of high-purity titanium single crystals: Part II. Microstructure and mechanism Xiaoli Tan, Florida International University H. Guo H. Gu, Xi'an Jiaotong University C. Laird, University of Pennsylvania N. D. H. Monroe, Florida International University Available at:

2 Cyclic Deformation Behavior of High-Purity Titanium Single Crystals: Part II. Microstructure and Mechanism X. TAN, H. GUO, H. GU, C. LAIRD, and N.D.H. MUNROE Strain-controlled cyclic tests have been conducted on high-purity titanium single crystals with different orientations. The fatigue mechanisms of the titanium crystals were studied by means of a scanning electron microscope (SEM) and a transmission electron microscope (TEM). It was found that single slip lines, wavy slip lines, double slip lines, twins, and associated slip lines occurred in differently oriented single crystals. A new type of fractographic morphology, parallel traces, was observed. Dislocation patterns and cyclic twins, as well as the mechanical response, were analyzed. The dependence of the deformation mechanisms on the orientations of the single crystals is discussed. I. INTRODUCTION IT is widely recognized that mechanical twinning can become an important mode of deformation in hexagonal close-packed (hcp) metals. One major conclusion of a recent review is that, in hcp metals with reasonably high surface energies, the greater the number of operative twin modes, the larger the overall ductility will be. [1] Experimentally, it is observed that deformation twinning can effectively strengthen a material under some circumstances and weaken it under others. This complexity results mainly from the intricate inter-relationship between slip, twinning, and cracking of hcp metals. The presence of more than one twinning system operative in either compression or tension is the main reason that titanium exhibits extensive ductility. [2] Titanium is known to have a high stacking-fault energy [3,4] and, thus, wavy slip behavior, which might also be expected to enhance the ductility of a specimen deformed at room temperature. Fatigue cracks have been observed to follow slip planes or twin/matrix interfaces. [5,6] Mechanical twinning appears to play a distinct role in the fatigue behavior of titanium. Twins of the {1121} type, formed during cyclic loading, exhibited permanent fatigue damage at the twin/matrix interface. [7] The {1012} twins introduced by prior deformation have also been shown to provide sites for preferential damage upon subsequent cyclic loading. [5] Ward-Close and Beevers [8] studied the high-cycle fatigue crack growth characteristics of coarse-grained titanium. Three distinct types of fracture morphology were identified: cleavagelike facets on the basal planes (0002), striations on planes normal to (0002), and furrows in the [0001] direction. X. TAN, Postdoctoral Fellow, formerly with the Research Institute for Strength of Metals, Xi an Jiaotong University, is with the Hemispheric Center for Environmental Technology, Florida International University, Miami, FL H. GUO, Graduate Student, formerly with the Research Institute for Strength of Metals, Xi an Jiaotong University, is with the Department of Metallurgy and Materials Engineering, Ecole Polytechnique, Montreal, PQ, Canada. H. GU, Professor, is with the Research Institute for Strength of Metals, Xi an Jiaotong University, Xi an , People s Republic of China. C. LAIRD, Professor, is with the Department of Materials Science and Engineering, University of Pennsylvania, Philadelphia, PA N.D.H. MUNROE, Associate Professor, is with the Hemispheric Center for Environmental Technology, Florida International University. Manuscript submitted April 23, The dislocation structures produced after cyclically deforming commercial-purity titanium to large cycle numbers resemble those produced in other metals. The observation of dipolar walls suggests that screw dislocation mobilities are sufficient to permit cooperative glide across dipolar walls. In addition, edge dislocations are concluded to bow out of walls. [9] At high plastic strain amplitudes, cyclic deformation is controlled by the motion of screw dislocations, while at low amplitudes, it is controlled by that of edge dislocations. The purpose of the present work is to show how the cyclic deformation mechanisms of titanium single crystals vary with their orientations and to identify the relation of twins and dislocation structures produced by cyclic deformation with the orientations of the crystals. II. EXPERIMENTAL The fatigue test specimens, with a gage section of mm, were shaped from high-purity titanium singlecrystal rods with a programmable spark erosion machine. The surfaces of the specimens were electropolished to obtain reasonably smooth surfaces, to allow for observation of slip bands and twins after cyclic testing. The surface morphology of the fatigued specimens was examined with a JSM35C scanning electron microscope (SEM). The microstructures in fatigued single crystals were analyzed using a JEM200CX transmission electron microscope (TEM). Three sets of thin foils were prepared: section type 1, parallel to the slip plane (0110); section type 2, normal to the primary Burgers vector [2110]; section type 3, normal to the slip plane but containing the Burgers vector i.e., parallel to the (0001) plane (Figure 1). III. A. Surface Topography RESULTS Single crystals 5, 10, 11, 12, 14, and 16 (note Figure 1 in Part I of this article [10] for crystal orientations and Figure 2 in Part I for specimen histories) were oriented for single prismatic slip, and, therefore, only one set of slip lines was detected on their surfaces. Trace analysis of the slip lines confirmed that the primary slip occurred on the (0110) plane. Single slip lines on the surfaces of single crystals 14 METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 29A, FEBRUARY

Fig. 1 Sections cut for TEM observations. Fig. 4 Double slip lines on the surface of single crystal 15. Fig. 5 Cyclic twin bands and consequently activated c a slip bands on the surface of single crystal 18.

Trace analysis of the two operating slip systems in each crystal indicated that they were coincident with the two most highly stressed prismatic slip systems.

The axis of crystal 18, which was treated differently by cycling in the SEM, was oriented parallel to the c-axis, so a glide was suppressed.

and 16 are shown in Figure 2. Particularly, wavy slip lines were dominant in single crystals 2, 4, and 17. Figure 3 shows the wavy slip lines on the surface of crystal 4.

$Fractographic Morphology Some specimens were cycled to fracture, at which point the strain had been increased to the maximum amplitude of the step tests, so that the fracture surface could be$

3 Fig. 1 Sections cut for TEM observations. Fig. 4 Double slip lines on the surface of single crystal 15. Fig. 5 Cyclic twin bands and consequently activated c a slip bands on the surface of single crystal 18. Fig. 2 Single slip lines on the surface of (a) single crystal 14 and (b) single crystal 16. prismatic slip, and, on their surfaces, two sets of slip bands were observed. Trace analysis of the two operating slip systems in each crystal indicated that they were coincident with the two most highly stressed prismatic slip systems. Figure 4 shows the double slip lines on the surface of crystal 15, which indicates that the two slip systems seemed to operate in separate regions of the crystal. The axis of crystal 18, which was treated differently by cycling in the SEM, was oriented parallel to the c-axis, so a glide was suppressed. Figure 5 shows the twin bands produced in such an orientation, and, consequently, c a slip bands appeared on the surface of crystal 18. Fig. 3 Wavy slip lines on the surface of single crystal 4. and 16 are shown in Figure 2. Particularly, wavy slip lines were dominant in single crystals 2, 4, and 17. Figure 3 shows the wavy slip lines on the surface of crystal 4. All observations shown were recorded at the end of the cycling, indicated by Figure 2 of Part I of this research, [10] and correspond to the saturated condition. Single crystals 1, 6, 8, 9, and 15 were oriented for double B. Fractographic Morphology Some specimens were cycled to fracture, at which point the strain had been increased to the maximum amplitude of the step tests, so that the fracture surface could be examined with the SEM. The fracture surface appearance in the double slip oriented single crystal 9 is seen to consist of rather irregular striated markings (Figure 6), somewhat similar to the typical fissure striation type of fatigue fracture morphology in commercial-purity titanium reported by Ward- Close and Beevers. [8] In single crystal 13, another type of morphology, parallel regular traces, was observed on the fracture surface (Figure 7(a)). Close examination of these features revealed a complex, fine structure consisting of many parallel fine lines (Figure 7(b)). The traces appeared to match the direction of the microcracks along the persistent slip bands (PSBs) visible on the fracture surface (Figure 7(c)). The fracture surface area shown in Figure 7(c) is near the initiation site of the main crack, which developed from the stage I microcracks shown in this figure. The par- 514 VOLUME 29A, FEBRUARY 1998 METALLURGICAL AND MATERIALS TRANSACTIONS A

$Fig. 6 Fractographic morphology of single crystal 9: (a) general and (b) in detail. allel traces should not be confused with fracture surface striations.$ from those observed here. There are no previous reports, to the authors knowledge, of the parallel traces observed here. C.

from those observed here. There are no previous reports, to the authors knowledge, of the parallel traces observed here. C.

the following associated and different dislocation microstructures are observed. (1) Region A: The microstructure of crystal 5 is typical of this region.

(2) Regions B I and B II : The common feature of the dislocation configuration in crystals oriented for double prismatic slip is cellular structure.

In single crystal 9, two sets of narrow dislocation walls were dominant on the slice parallel to the (2110) plane (Figure 9(c)).

4 Fig. 6 Fractographic morphology of single crystal 9: (a) general and (b) in detail. allel traces should not be confused with fracture surface striations. There is a previous report [11] of fracture surface morphology in titanium, but an atomic-force microscope was used and the specimen was polycrystalline, so the morphological details are different from those observed here. There are no previous reports, to the authors knowledge, of the parallel traces observed here. C. Dislocation Substructure and Cyclic Twins According to Figure 6 of Part I of this study, [10] single crystals may be classified into groups corresponding to different cyclic deformation modes, and the following associated and different dislocation microstructures are observed. (1) Region A: The microstructure of crystal 5 is typical of this region. Figure 8(a) shows the dislocation substructure observed on a slice along the glide plane. Twins are also observed in the same specimen (Figure 8(b)). (2) Regions B I and B II : The common feature of the dislocation configuration in crystals oriented for double prismatic slip is cellular structure. Dislocation cells with relatively loosely organized walls, which seem to consist of dipoles and loops, were found in crystals 1 and 6 (Figures 9(a) and (b)). In single crystal 9, two sets of narrow dislocation walls were dominant on the slice parallel to the (2110) plane (Figure 9(c)). It appears that the horizontal set of walls, which are parallel to the (0110) plane trace, was formed first and then subsequently intersected by the others. (3) Region C: Parallel dislocation walls with some screw dislocations between them were commonly observed in crystal 13 on the (2110) section (Figure 10(a)). The walls are parallel to the (0110) plane trace. Dislocations Fig. 7 Fractographic morphology of single crystal 13: (a) traces, (b) fine lines, and (c) direction of traces matches that of microcracks along PSBs. walls and twins were observed in crystal 2 (Figure 10(b)). (4) Region D: Single crystal 10 was cycled at steps of very low strain amplitude in order to investigate the dislocation substructure induced during the incipient strain steps. In comparison to the specimens in region A, TEM examination indicated that few twins were detected on the slices. Dislocation substructures viewed on the (0110) and (2110) sections are shown in Figure 11. Figure 11(a) indicates that long screw dislocations were observed, and their presence in the thin foils is attributed to their low mobility. (5) Region E: The axis of crystal 18 was oriented just parallel to the c-axis, so all the a slip systems would not operate under uniaxial loading. A high density of twins with distinct, fine structures and c a dislocations were observed in this crystal (Figure 12). In fatigued single crystal 7, which was oriented close to [0001], twins and dislocations were also found, while the dislocation density was low. Cyclic twins of different types were extensively observed in most fatigued single crystals. These twins have been METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 29A, FEBRUARY

$Fig. 8 Substructure in fatigued single crystal 5: (a) (0110) section and (b) (0001) section. identified from their appearance as well as by electron diffraction pattern analysis.$ Second-order cyclic twins traversing a large host ( 1102) twin were also observed in single crystal 2 (Figures 13(b) and (c)).

Second-order cyclic twins traversing a large host ( 1102) twin were also observed in single crystal 2 (Figures 13(b) and (c)).

DISCUSSION Single crystals 5, 10, 11, 12, 14, and 16 were oriented for single prismatic slip; therefore, only one set of slip bands was detected.

Particularly, single crystals 2, 4, 13, and 17 behaved in wavy-slip mode.

Single crystals 1, 6, 8, 9, and 15 were oriented for double prismatic slip. Dislocation cell structures were, therefore, dominant (Figure 9).

5 Fig. 8 Substructure in fatigued single crystal 5: (a) (0110) section and (b) (0001) section. identified from their appearance as well as by electron diffraction pattern analysis. Intersecting cyclic twins, observed in single crystal 2 (Figure 13(a)), indicate that irregular twin interfaces occurred at the intersection. Second-order cyclic twins traversing a large host ( 1102) twin were also observed in single crystal 2 (Figures 13(b) and (c)). Again, the interfaces of the host twin became irregular in association with this behavior. IV. DISCUSSION Single crystals 5, 10, 11, 12, 14, and 16 were oriented for single prismatic slip; therefore, only one set of slip bands was detected. At low strain amplitudes, dislocation substructures consisted of dislocation dipoles, loops, and networks (Figures 8(a) and 11). At high strain amplitudes, dislocation walls were formed. Particularly, single crystals 2, 4, 13, and 17 behaved in wavy-slip mode. The high Schmid factors for the (0111)[2110] slip systems, coupled with the observation of wavy slip in these crystals, indicated the occurrence of cross-slip. Single crystals 1, 6, 8, 9, and 15 were oriented for double prismatic slip. Dislocation cell structures were, therefore, dominant (Figure 9). Single crystals 7 and 18 were oriented for difficult prismatic slip. Twins and c a dislocations could be observed. It has been confirmed that prismatic slip is the most active deformation mode in titanium. In region D, only single prismatic slip is operative, and repeated to-and-fro glide of dislocations is involved with the occurrence of a plateau in the cyclic stress-strain curve (CSSC). As the steady state is approached, we consider that the multiplication and annihilation of both screw and edge dislocations are balanced. Fig. 9 Dislocation structures in (a) single crystal 1, (b) single crystal 6, and (c) single crystal 9. This is similar to the behavior in fcc single crystals, although the well-known ladder structure has not been observed in our investigation. In region A, cyclic twinning plays an important role in addition to that of single prismatic slip. The slip-twin and twin-twin interactions and the structure of twin/matrix in titanium have been a research topic in recent years. [12] The observed twins in our study correspond to {1012}, {1121}, and {1122}. It is quite reasonable that the twins can provide barriers to dislocation motion and reduce the scale of the substructure. This may be the explanation as to why the height of the plateau of the CSSCs in this region is higher than that in region D. The appearance of cross-slip on the first-order pyramidal plane {1011} of a -type dislocations has been confirmed by several authors under monotonic loading. This kind of cross-slip was also found in region C. It is not surprising, because {1011} is considered to be one of the most favored 516 VOLUME 29A, FEBRUARY 1998 METALLURGICAL AND MATERIALS TRANSACTIONS A

Fig. 10 Dislocation walls in (a) single crystal 13 and (b) single crystal 2. Fig. 11 Dislocation structures in single crystal 10: (a) (0110) slice and (b) (2110) slice.

Dipoles and loops were observed and are concluded to be formed by a cross-slip mechanism, at least in part, and mutual trapping may also operate to form dipolar

It seems likely that the dislocation mechanism in single crystal 13, which was oriented for easy (0111) slip, involves both prismatic slip and cross-slip on (0111).

This is why the CSSC is similar to those in region A. The occurrence of double prismatic slip was confirmed in regions B I and B II.

The results of copper single crystals oriented for double slip showed that the cyclic responses have higher hardening rates and saturation stresses, compared to those in

It has been concluded that c a slip is essential for good ductility of titanium. [14] In region E, the activity of a slip is suppressed due to its low Schmid factor.

6 Fig. 10 Dislocation walls in (a) single crystal 13 and (b) single crystal 2. Fig. 11 Dislocation structures in single crystal 10: (a) (0110) slice and (b) (2110) slice. Fig. 12 Cyclic twins in single crystal 18: (a) 125 MPa and (b) 54 MPa. secondary slip planes. Dipoles and loops were observed and are concluded to be formed by a cross-slip mechanism, at least in part, and mutual trapping may also operate to form dipolar structures. It seems likely that the dislocation mechanism in single crystal 13, which was oriented for easy (0111) slip, involves both prismatic slip and cross-slip on (0111). These two slip systems did not strengthen the crystal effectively, however, because they have the same slip direction. This is why the CSSC is similar to those in region A. The occurrence of double prismatic slip was confirmed in regions B I and B II. Extensive studies on multislip have been done in fcc single crystals. The results of copper single crystals oriented for double slip showed that the cyclic responses have higher hardening rates and saturation stresses, compared to those in single slip crystals, and celllike structure forms. [13] This is also true in titanium single crystals. It has been concluded that c a slip is essential for good ductility of titanium. [14] In region E, the activity of a slip is suppressed due to its low Schmid factor. The cyclic deformation along the c-axis, therefore, has to be accommodated either by c a slip or by twinning. Evidence for c a slip was demonstrated from crystals 7 and 18 in this investigation. The role of twinning in the fatigue of titanium single crystals should be emphasized again. Actually, we found twins in most of the specimens. Stacking faults and fatigue crack initiation have been found to be associated with twins. [15,16] The impingement and intersection of cyclic twins, the occurrence of second-order twins in host twins, the irregularity of twin/matrix interfaces, and the interaction METALLURGICAL AND MATERIALS TRANSACTIONS A VOLUME 29A, FEBRUARY

V. CONCLUSIONS From experiments on high-purity titanium single crystals for a wide range of orientations within the standard triangle, the following conclusions are drawn about the microstructure and

The {1010} 1120 prismatic slip is still the most easily operative slip mode during cyclic loading, except in the region near 0001, where c a pyramidal slips prevail.

Cyclic twins are found in most of the fatigued single crystals. The majority of the twins observed are in the types of {1012}, {1121}, and {1122}.

7 V. CONCLUSIONS From experiments on high-purity titanium single crystals for a wide range of orientations within the standard triangle, the following conclusions are drawn about the microstructure and mechanism of fatigued specimens. The microstructures of fatigued crystals depend on their orientations, as follows. The {1010} 1120 prismatic slip is still the most easily operative slip mode during cyclic loading, except in the region near 0001, where c a pyramidal slips prevail. Double prismatic slip occurs for orientations near the edges of the standard triangle, and cross-slip on (0111) occurs for orientations in the middle of the standard triangle. Cyclic twins are found in most of the fatigued single crystals. The majority of the twins observed are in the types of {1012}, {1121}, and {1122}. We also have observed impingement and intersection of twins and dislocation lines and stacking faults within twins. The cyclic stress-strain response of crystals with different orientations can be explained tentatively, in terms of the dislocation microstructure and the extent of twinning. ACKNOWLEDGMENTS Most sincere thanks are due to Professor H. Mughrabi for stimulating discussions during preparation of the article. Financial support of this research by the National Natural Science Foundation of China is gratefully acknowledged, as is the support of one of us (CL) by the National Science Foundation of the United States. REFERENCES Fig. 13 Cyclic twins in single crystal 2: (a) intersecting twins, and (b) and (c) bright-field/dark-field micrographic pair of second-order twins. between slip and twinning all still remain poorly understood. This report represents one of the first steps in studying the cyclic deformation behavior of titanium single crystals, and has revealed a complex series of processes: dislocation interactions, complex interaction of cyclic response and microstructure, and slip and twinning interaction behavior, all of which require much further research effort. 1. M.H. Yoo and J.K. Lee: Phil. Mag. A, 1991, vol. 63, p M.H. Yoo: Metall. Trans. A, 1981, vol. 12A, pp A. Akhtar and E. Teghtsoonian: Metall. Trans. A, 1975, vol. 6A, pp A. De Crecy, A. Bourret, S. Naka, and A. Lasalmonie: Phil. Mag. A, 1983, vol. 47, p J.I. Dickson, J. Ducher, and A. Plumtree: Metall. Trans. A, 1976, vol. 7A, pp M. Sugano and C.M. Gilmore: Metall. Trans. A, 1980, vol. 11A, pp D.I. Golland and C.J. Beevers: Met. Sci. J., 1971, vol. 5, p C.M. Ward-Close and C.J. Beeverse: Metall. Trans. A, 1980, vol. 11A, pp L. Handfield and J.I. Dickson: in Defects, Fracture and Fatigue, G.C. Sih and J.W. Provan, eds., Martinus Nijhoff Publishers, Hingham, MA, 1983, p X. Tan, H. Gu, C. Laird, and N.D.H. Munroe: Metall. Mater. Trans. A, 1998, vol. 29A, pp S.E. Harvey, P.G. Marsh, and W.W. Gerberich: Acta Metall. Mater., 1994, vol. 42, p T. Kehagias et al.: Scripta Metall. Mater., 1994, vol. 30, p N.Y. Jin and A.T. Winter: Acta Metall., 1984, vol. 32, p H. Numakura,Y. Minonishi, and M. Koiwa: Scripta Metall., 1986, vol. 20, p X. Tan and H. Gu: Scripta Metall. Mater., 1995, vol. 33, p X. Tan and H. Gu: Int. J. Fatigue, 1996, vol. 18, p VOLUME 29A, FEBRUARY 1998 METALLURGICAL AND MATERIALS TRANSACTIONS A

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