Stacking faults in fatigued titanium single crystals

Stacking faults in fatigued titanium single crystals

Scripta Metall~a etIvfa&dia, Pergamon Vol. 33, No. 12, pp. 1977-19841995 ELwvierScicnceLtd ckpyli&,o 1995 Acta Mctalhngica Inc. PnntedmtheUSAAllh&t...

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Scripta Metall~a

etIvfa&dia,

Pergamon

Vol. 33, No. 12, pp. 1977-19841995 ELwvierScicnceLtd ckpyli&,o 1995 Acta Mctalhngica Inc. PnntedmtheUSAAllh&tstesemed 095~716xm $9.50+ .oo

0956-716X(95)00422-X

STACKING FAULTS IN FATIGUED TITANIUM SINGLE CRYSTALS Tan Xiao-Li and Gu Hai-Cheng Research Institute for Strength of Metals, Xi’an Jiaotong University, Xi’an, 7 10049, CHINA (Received March 20,1995) (Revised July 10,199S) Iutroduction

It is generally accepted that the me&anical behavior and interior dislocation substructure of metals and alloys are functions of the stacking fault energies (SFEs). The higher the SFE, the more difbcult it is to split a dislocation into two partial dislocations separated by a stacking fault. Whereas materials with high SFE allow easy cross-slip of screw dislocations. Titanium is known to have a high SFE. Akhtar and Teghtsoonian [l] estimated the SFE on the prismatic planes of titanium as O.l45J/m*. DE Crecy et al. [2] measured the core extension of the edge dislocation in the prismatic planes with high resolution electron microscopy and deduced the SFE to be of the order of O.l5OJ/m*. Even though stacking faults in titanium have been observed experimentally [3-51, there is still doubt on them due to the high values of SFE. The purpose of the present study is to obtain further tiormation on stacking faults in fatigued titanium single crystals and to discuss the relation between stacking fault and twinning. Exuerimental Procedure

The crystals were grown from the melt in a floating zone electron-beam furnace after refined twice in a vacuum electric arc furnace using low iron content titanium iodide as starting material. A vacuum of 6.7 x lOa Pa was maintained during growth. The total contents of impurities are within 43.66 wt. ppm (The 29 kinds of impurities analyzed are B, F, P, Cl, S, Mg, Al, Si, V, Cr, Mn, Fe, Co, Ni, Cu, Zn, As, Zr, Sn, Ag, MO,Bi, W, U, Th, Ca, Na, K and Li.). The interstitial element contents are (wt. ppm): 0 117, C 19, N 10, and H 0.1 respectively [5]. Strain-annealing method was used to prepare single crystals. Aher the reversed push-pull cyclic tests with total stain control, single crystals were sliced according to the predetermined orientationsfor transmiss’ion electron microscopy observation. Three sets of thin foils were prepared: section type 1, parallel to the slip plane (01 TO); section type 2, normal to the Burgers vector [2nO]; section type 3, normal to the slip plane but containing the Burgers vector, i.e. parallel to the basal plane (0002). Results aud Discussion

TEM examinations indicated that stacking faults were occasionally observed in section type 2, i.e., in (2TIO) slices. Fig. 1 shows typical stacking faults in a twin in the slice. The host twin is in type of ( 1121}. Close

1977

1978

Vol. 33, No. 12

STACKING FAULTS

Figure 1. Stacking faults in (11%)

twin, (2iiO) slice: (a) B = [2iiO],

g = 0002 m; (b) BF, (c) DF

examination under bright field(BF) and dark field(DF) are shown in Figs. 1b and 1c respectively. Two sets of stacking faults associated with partial dislocations were identified according to the fringe contrast. In the present study, all of the observed stacking faults were found to situate within cyclic twins. Fig. 2 shows a series of stacking faults traversing the cyclic twin and bounded by the twin interfaces. Electron diffraction analysis (Fig. 2c) indicates that the host is also in type of { 1121} This is coincident with the results of surface trace analysis. TRM observation reveals no dislocations around the cyclic twins (Fig. la, Fig. 2a) under the condition where g = 0002, was operating, which implies that neither dislocations nor dislocations presented. The formation of a ribbon of stacking fault on a ( 1TOO} plane has been considered earlier by Tyson [6]. On the basis of a hard sphere model, the following reaction has been deduced 5

[1120]

-

5

[4263]

+ +

[2463]

(1)

The energy of the stacking fault so formed is expected to be so high that it is not consistent with the experimentally obtained values. An alternative dissociation model has been proposed by Regnier and Dupouy [7] on the basis of hcp bee transformation: ;

[1120]

-

$

[1120]

+ F

[1120]

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STACKING FAULTS

Y

oil0 0000

oil2

1120:

0002

. 1

l

. .

.

l

.

3Elt

.2 201, . l

Figure 2. Stacking faults traverse a cyclic twin, (2110) slice: (a) BF, g = 0002,; @) DF, g = 1120,; (c) EDP

The experimentally obtained values of SFE for the prismatic planes of titanium are compatible with this dissociation model [ 11. Most of the observed stacking faults in titanium have been reported within twins [3-51. These results indicated that the energy barrier during the formation of stacking faults may be overcome by the twinning process. YCQ [S] analyzed possible interactions of the perfect dislocation with twins in hcp metals. Dislocation reactions at a twin interface can be described as shown in Fig. 3 and can be expressed in a general form as: qIlkf),

- q&r),

* r%

(3)

Where X is the Burgers vector of an original dislocation gliding in (hkil), slip plane, whereas X* is the Burgers vector of the resulting dislocation in the corresponding (hkil), plane, n is a positive integer and b, is the Burgers vector of a unit twin dislocation. As for { 11 Z 1> type twins, the reactions of
trpe dislocations with the twin interfaces may be written [S]

or

(5)

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Vol. 33, No. 12

FAULTS

Figure 3. Incorporation of a slip d&cation

into a twin [S]

It is suggested that the formation of stacking faults may be related to reaction (4) as the slip plane ( 1TOO)and the corresponding plane (r 100) are both prismatic planes. Combining the dissociation model formulated by equation (2), the formation of stacking faults during { 1121} type twinning process could be described as

(6)

Stacking fault were observed within { 11211 cyclic twins in (2TTO) slices of fatigued high purity titanium single crystals. The results can be interpreted that
type dislocations react with ( 1121> twin interfaces during twinning process, which facilitates the dislocations’ dissociation. AcknowledPements

Financial support from the National Natural Science Foundation of China is gratefully acknowledged. References 1. 2. 3. 4. 5. 6. 7. 8.

A. Akhtarand E. Teghtsoonian, Metall. Trans. A, 6& 2201(1975) A De Crecy, A. Bowret, S. N&a and A. Lasalmonie, Phil. Msg. A, 47,245 (1983) J.P. Poirtoier, J. Antolinand J.M. Dupouy, Can. J. Phys., 45,1221(1967) M. Blicberski, S. Nourbakbsh and J. Nutting, Metal Sci., 13,516 (1979) Gu Haicheng, Guo Huifiug, Chang Shufen and C. Laird, Mat. Sci. Eng., A188,23 (1994) W.R. Tyson, Acta Metall., 15,574 (1967) P. Regnier and J.M. Dupouy, Phys. Status Solidi, 39,79 (1970) M.H. Yoo, Metall. Trans. A, 12A, 409 (1981)