DEFORMATION TWINNING IN Cd SINGLE CRYSTALS

Ν. Ugar", G. Qankaya8,1. Karamanb, A.E. Ekincib and B. Düzgiinb

"Department of Physics, Faculty of Arts and Sciences, and bDepartment of Physics, K.K. Education Faculty, Atatürk University, Erzurum, Turkey

ABSTRACT

Deformation twinning in hexagonal-close-packet (Lc.p.) crystals has been investigated under tension at room temperature. It was found that deformation twinning is possible in Cd single crystals at room temperature. But twinning was observed only in a limited number of crystallographic orientations, and twinning stress changed according to these orientations.

1. INTRODUCTION

Slip and twinning are two distinct modes of deformation by which metals and alloys accommodate the imposed plastic strain. Slip takes place by the glide of dislocations along a well defined crystallographic direction and plane. However, the twins which play a significant role in the deformation and fracture of metals at low temperatures are formed by cooperative movement of dislocations on planes parallel to the twinning plane /1/. In earlier works, Mahajan and Williams 111 showed that deformation twinning of metals and alloys takes place whenever the imposed condition makes it difficult for the material to deform by slip. It was also found that twinning becomes a favorable deformation mode since for materials with low stacking fault energy dynamic recovery by cross-slip becomes difficult /3/. The latter observations are attributed to a more rapid rise of the flow stress for slip with decreasing temperature than for twinning. Considerable attention has recently been given to the effect of crystal orientation on the occurrence of twinning and twinning stress. Narita and Takamura 13/ showed that twinning stress is decreased as the specimen axis approaches <111> and also as the deformation temperature is lowered when twinning occurs in stage HI of the stress-strain curve of copper-alloy crystals. This orientation dependence of the twinning stress has been explained by means of the orientation dependence of back stresses and obstacles arising from difference in the amount of primary slip and co-planar slip /4/. Also, several authors have shown that the stacking fault energy is one of the most important factors which controls twinning stress /5-7/.

97 Vol. 9, No. 2, 1998 Deformation Twinning in Cd Single Crystals

Plastic deformation by slip is a very limited process in h.c.p. metals since Burgers vectors other than y[1123] and γ[1120] are rarely observed /8-10/. Therefore, the second fundamental process, the

deformation and mechanical twinning, serves as the main deformation mode in such metals at low temperatures. In order to contribute to the understanding of the mechanism of this deformation, many deformation twin modes have been examined for especially Zn single crystals /11/.

In particular, although a few data have been published on deformation twinning in Cu3Au crystals /12/, Cu-Al-Ni /13/ and Fe-Al /14/ alloys, deformation twinning is still much less understood than slip. The purpose of this study is to investigate the occurrence of twinning and the relationship between twinning stress and crystal orientation in Cd single crystals.

2. EXPERIMENTAL METHOD

Single crystals 5 mm in diameter were grown in a glass tube at a rate of 15 mm/h using a modified Bridgman method. The crystals were 80-100 mm in length and their orientations were determined by the Laue back reflection method. The crystal orientations, which are the tensile axes, were shown in the unit triangle of the standard Stereographic projection (Fig. 1). The crystals were pulled along the growth directions with the use of an Instron-type machine at a strain rate of 1.2 χ 10"6 s'1 at room temperature. In various stages of deformation, the surfaces of the crystals were examined for slip lines and twin bands with an optical microscope. Load and elongation curves were recorded during the tensile tests for all crystals. Further experimental details are described in /15/.

Fig. 1: The crystallographic orientations of tensile axis. Symbols · and ο represent twinned and untwinned crystals, respectively.

98 Ν. Ucaretal. Journal of the Mechanical Behavior of Materials

3. EXPERIMENTAL RESULTS

The stress-strain curves were calculated from load-elongation curves and plotted for all crystals. In the crystals, the onset stress of twinning can be determined by noting the load drops appearing in the stress-strain curves 13 /. Also, Peissker Γ1Ι has suggested that the bend point, rather than the load drop, appearing on the stress-strain curve, may correspond to the onset of twinning. In this work, slip in crystals deformed under tensile stress at room temperature occurred along the < 1120 > direction on the basal plane only, regardless of the orientation of the crystals, in the Cd single crystals, then twinning occurred with increasing tensile stress (Fig. 2). Twinning was observed for the <21 13 >, < 1011 > and <0001> orientations. For the other crystals orientations, deformation twinning was not observed in Cd single crystals at room temperature. The onset and end of twinning were shown on the stress-strain curves in the crystal numbered 3 (Fig. 2). As seen in Fig. 2, for this orientation, a deformation of about 10% will take place by normal slip. Then, the beginning of a new region, which is twinning, originates with a sharp release of stress. The tensile test was stopped when this occurred. After the initial twin formation, the stress-strain curve shows no evidence of work-hardening since the stress oscillates with strain. In the various stages of slip and twinning, the samples were removed from the tensile machine and observed with an optical microscope. Twins were found on the (1012), (1122), (0111) and (0111) planes when the slopes of the stress-strain curves changed. From the σ values at the onset of twinning, twinning stress values according to twinning planes were calculated in the Cd single crystals.

Fig. 2: The stress-strain curve of Cd single crystals numbered 3. The arrow indicates the onset of twinning.

4. RESULTS AND DISCUSSION

In the h.c.p. lattice, the close-packet planes are (0001) planes. These basal planes are also the most frequent glide planes in h.c.p. crystals, unlike f.c.c. crystals. However, in the case of Cd, (0001),

99 Vol. 9, No. 2, 1998 Deformation Twinning in Cd Single Crystals

(Ol 11), (1101), (1100), basal and non-basal glide planes have been observed by several authors /10,15/16/, but they do not correspond to twin planes. The interaction and crystallography of deformation twins in α-Ti solid solution have been discussed by Nourbakhsh and Crowther /17/. They found that a large number of deformation twins belong to the (1 121), (1012), and (1122) planes and (1012) type twins are lecticular and usually contain numerous dislocations and stacking faults. In this work, as seen in Fig. 2, slip was taken along the <112 0> direction on the basal plane. Then, deformation twinning took place after a slip on the basal plane at room temperature. It can be understood that twinning is closely connected with the internal stress arising from deformation by slip. Meanwhile, the appearance of the twinning at room temperature shows that the stresses necessary for the growth of twins are smaller than those needed for glide by dislocations. For <1011>, <0001> and <21 1 3> crystal orientations, (1012), (1122) primary and (0111), (0111) conjugate planes were observed as twin planes in Cd single crystals. These planes, (011 1) and (112 2), are most active in slip /10,16/. This would suggest that twinning is a relief mechanism of internal stress accumulated by the active slip. These results also suggested that some dislocation extensions occur on these planes, and stacking faults also occur. The σ values which were obtained at the onset of twinning showed that twinning stress is lower than that for the other tensile axes, as the tensile axis is closer to <1011> (Fig. 3). This reduction has been understood by assuming that deformation twinning is a stress-relief process complementary to that by cross-slip /18/. This reduction has also been explained by propagating twins that have escaped from obstacles /19/. In conclusion, it has been shown by tensile tests on the oriented Cd single crystals that deformation twinning is possible at room temperature, and twinning stress and the appearance of twinning depend on the crystal orientation. A similar orientation dependence of twin formation has been reported by Narita and Takamura in f.c.c. crystals /3/. Meanwhile, the relationshipbetwee n the twin formation and the crystal orientation has been explained by the formation of twins on the primary and secondary slip planes by Fujita and Mori 1201.

1011 ,1010

2110 Fig. 3: Twinning stresses obtained from bend point appearing on the stress-strain curves in Cd single crystals. Stress values are MPa

100 Ν. Ucar et al. Journal of the Mechanical Behavior of Materials

REFERENCES

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