THE TEXTURE AND CREEP ANISOTROPY OF FUEL ELEMENT SHELLS MANUFACTURED FROM Crl6Nil5Mo3Ti TYPE STEELS

A study was undertaken into the structure, the texture, the strength, and the creep of the Crl6Nil5Mo3Ti type steel tubes cold-deformed to 20% strain that are used as the fuel element shells for fast-neutron reactors. The tubes were found to have fibrous (111) and (100) textures along their axis. The preferred orientations (011) normal to the radial plane and (011) and (112) normal to the plane tangential to the tube surface were determined. The strength, the creep strength, and the rupture strength proved to be higher along the tube axis. Qualitatively, this is attributed to a specific texture and the grain shape of the material.


INTRODUCTION
The austenitic stainless steels of the Crl6Nil5Mo3Ti type have been widely used as a material of fuel element shells for fast-neutron reactors (Chuev et al., 1994) and continue to be very promising for this application (Sagaradze et al., 1993). The tubes experience both tensile stresses along their axis and the internal pressure due to the swelling fuel and the fuel decay gas products (Samoilov et al., 1982). It is therefore of special interest to examine the anisotropy of the mechanical properties * Corresponding author. of the tubes. Probably, different mechanical properties along and across the tube axis are determined by specific features of the texture, which are caused in part by cold deformation to 20% strain. The texture characteristics (intensity, scattering, and orientation ratio) generally vary over the cross section of tubes, sheets, or wire (Wassermann and Graeven, 1962) as a result of a nonuniform rolling deformation of the material. This paper is concerned with the study of the texture, its distribution over the height of the samples, and the effect of the texture on the strength, the creep strength, and the rupture strength of fuel element shells made of the Crl 6Nil 5Mo3Ti type steels.

EXPERIMENTAL
The subject of study was thin-walled shells (6.9 mm in diameter, with walls 0.4mm thick) made from the Crl6Nil5Mo3Ti type steels colddeformed to 20 4-3 %. The chemical composition of these steels is given in Table I. The microstructure and the texture of the steels were examined in three mutually perpendicular planes: the surface and the transverse and longitudinal section planes (Fig. 1). The texture in the last two sections was studied on stacked samples comprising 20 plates each such that the surface of interest always measured at least 8 mm by 20 mm. Microsections for microscopic investigations were prepared by electropolishing at a d.c. voltage of 25 V in a solution containing 570 ml of orthophosphoric acid and 170 g of chromic anhydride. Electrolytic etching was done at a voltage of 10V in a solution containing 10ml of HNO3 and 30ml of HC1. The microstructure was examined in a Neophot-21 light microscope.
The texture was studied with a DRON-2.0 X-ray diffractometer using Mo Ks radiation and inverse pole figures. The integral intensity of the diffraction lines necessary for calculating the inverse pole figures was determined automatically from the number of pulses counted when  every line was scanned at a constant rate over the entire angle interval of the diffraction maximum, minus the background count. In ascertaining the homogeneity of the texture over the cross section of a tube, inverse pole figures were constructed, starting from the outer surface each time a layer 50 lam thick was removed from the tube.
The anisotropy of the mechanical properties was assessed on small nonstandard samples measuring 0.4 x 3 x 20 mm, which were prepared from short tubes (see Fig. 1). The tube was slit, spread by hand and straightened between flat rolls without any change in thickness accurate to + 0.01 mm. The uneven edges of the plates were ground so that the test samples had a rectangular cross section. The yield stress or02, the tensile strength aB, the specific elongation 6, and the reduction of area were measured on a ZD-10/90 tensile testing machine at 20C, making three tests for each orientation of the samples.
The anisotropy of the creep strength and that of the rupture strength were studied on flat samples prepared in accordance with the procedure described above. The difference was that reinforcing bars having pin holes, whose centers were aligned (accurate to within +0.1 ) with the sample's axis, were spot-welded to the grip pads of these samples. The creep tests were made in air at a temperature of 650C on an AIMA-5-2 machine with the samples loaded uniaxially. The test temperature and the test load were maintained constant to within +3 and +1% respectively.

RESULTS AND DISCUSSION
The microstructure of the steel 2 in three mutually perpendicular sections is shown in Fig. 2 are equiaxial and measure about 6 tm (see Fig. 2(a)). In the longitudinal section and in the plane parallel to the tube surface the grains are extended along the tube axis and measure about 9 pm in this direction (see Fig. 2(b) and (c)). Thus, in the unit volume the grain boundaries are nearly 1.5 times longer in the direction of the tube axis than they are in the perpendicular direction. The microstructure of the steel in three mutually perpendicular directions is analogous to that of the steel 2.
The inverse pole figure in the transverse plane ( Fig. 3(a)) suggests the presence of two well-defined maxima (111) and (100) having the density of 4.45 and 3.76 respectively. This means that two groups of fcc crystallites exist: in one group the body diagonal and in the other a cube edge is parallel to the tube axis. For the steel the inverse pole figures are similar (see Fig. 3(c) and (d)). This kind of texture is typical of drawn fcc metals and alloys having a medium stacking fault energy (SFE) (Honeycombe, 1968  (70 mJ/m2 (Zolotarevskii, 1983)) is close to that of the Cr16Ni15Mo3Ti type steel (calculated 70 mJ/m 2 and experimental 55 mJ/m 2 (Konobeev and Rudnev, 1983)), only 10% of the crystallites deviate from the (111) or (100) orientation when the copper wire is drawn from 3 mm to 0.3 mm in diameter (Wassermann and Graeven 1962). A fibrous texture is also typical of tubes when their diameter and wall thickness are reduced to the same extent during rolling (Wassermann and Graeven, 1962). This situation persists as long as the reduction ratio between the wall thickness and the diameter is less than 2. When this reduction ratio exceeds 2.5, the sheet texture appears in the material. The initial pierced tube billet had a diameter of 102 mm with walls 12 mm thick. The fabrication of the fuel element cladding tubes to the prefinished condition involved seven operations. The ratio in question was not greater than 1.48 in each of the operations. Every operation was followed by recrystallization annealing. In certain instances annealing does not cause fundamental changes in the texture produced by rolling (Wassermann and Graeven, 1962). Therefore a prefinished tube should have a fibrous texture of the (111) and (100) types. The finishing operation involved cold working to 20% of a billet 7.6 mm in diameter with walls 0.46 mm thick. This amount of straining could not incur radical changes in the existing texture, because the wall thickness/ diameter reduction ratio was close to unity. The same reduction in the wall thickness and the diameter produced almost equiaxial grains in the cross section of the tube wall, which were extended along the tube axis.
As opposed to wire, the tubes do not develop a texture characterized by a full rotation of the crystallographic directions about the fiber axis.
For example, in rolled copper tubes (Wassermann and Graeven, 1962) a (011) orientation normal to the plane tangential to the tube surface was observed in addition to the (111) and (100) orientations parallel to the tube axis. In the steel 2, also, the preferred (011) and (112) orientations with a density of 2.94 and 1.44 respectively were observed normal to the tangential plane ( Fig. 4(a)). Moreover, a (011) direction with a density of 3.62 is realized normal to the radial plane (see Fig. 3(b)). The density of this direction in the steel is slightly less (see Fig. 3(c)). The presence of the preferred (112) orientation (in addition to (011)) in a thin layer near the outer surface of the tube is probably due to a greater strain ofthe material at the point ofits contact with the tool. As a result, the reduction ratio between the wall thickness and the diameter exceeds 2.5 and the sheet texture is realized (Wassermann and Graeven, 1962).
Thus, the Crl 6Nil5Mo3Ti steel tubes 6.9 mm in diameter with walls 0.4 mm thick cold-deformed to 20% strain have a fibrous texture with the (111) and (100) directions along the rolling axis. Also, a preferred (011) orientation normal to the radial plane and preferred (011) and (112) orientations in the plane tangential to the tube surface exist. The last orientation disappears at a depth of 100 lam from the outer surface.
The mechanical test results for the steel 2 at 20C are summarized in Table II. Note that the samples cut out along the tube axis have a higher strength (c0.2 and ab) and a lower ductility ( and ) than the transverse samples. By way of example, Fig. 5 presents creep curves for the steel 2 samples at a-200 MPa and T-650C. One may see that the time to rupture of the samples oriented along the tube axis is longer than that of the samples oriented across the tube axis. This difference persists over the entire range of loads (170 _< _< 300 MPa) (Fig. 6). At cr 170 MPa the steadystate creep rate g of the samples oriented across the tube axis is 1.5 higher than g measured for the samples oriented along the tube axis. Given this value of or, the difference in the time to ruptureamounts to 37% (-ii >-+/-). Figure 6(c) shows a graph relating the long-term ductility er to the stress. One may see that er of the samples oriented along the tube axis is always higher than its counterpart for the transverse-oriented samples.
At the lowest stress a-170 MPa, the dependence reverses. Samples of the steel were tested at the same temperature T= 650C but in a wider  interval of stresses (127 < r < 367 MPa). The corresponding test results are presented in Fig. 7. It may be seen that they are qualitatively similar to those obtained for the steel 2. Quantitatively the values of and 7differ little but those of Cr differ considerably. The main difference with respect to Cr is that its values for the samples oriented along and across the tube axis become nearly equal at stresses much lower than those for the steel 2 samples. All these mechanical properties may be accounted for by specific features ofthe texture ofthe tubes and the grain shape ofthe steel. As was discussed in the foregoing, most crystallites along the tube axis and, consequently, in the longitudinal samples are oriented in the (111) and (100) directions. In the transverse samples the tensile axis goes across the tube axis and most crystallites along the sample's axis extend in the (110) direction (see Figs. 3 and 4). If a textured polycrystal is assumed in the first approximation to be an analog of a single crystal, then, according to Borodkina and Spektr (1968), at room temperature the orientations close to [111] are the "hardest" and those close to [110] are the "softest". Deformation of crystals with soft orientations begins and continues at minimum stresses thanks to a favorable orientation of the primary slip system. For this reason, they have a long first stage ofdeformation and a FIGURE 6 Stress dependence of (a) the steady-state creep rate k, (b) the time to rupture -, and (c) the long-term ductility er for the steel 2 samples cut out (1) along and (2) across the tube axis, as measured at 650C. low work-hardening rate. For example, during the first stage of tension the work-hardening rate of single crystals of copper (Honeycombe, 1968), whose SFE is close to that of the steel in question (Honeycombe, 1968;Konobeev and Rudnev, 1983) Most crystallites in the longitudinal samples are oriented in the (111) and (100) directions along the sample's axis. Therefore these samples should have a higher resistance to plastic deformation than the transverse samples whose axis coincides with the (110) direction. This explains why the samples oriented along and across the tube axis exhibit different mechanical properties.
Specific features of the texture formation account for different ductility of the Crl6Nil5Mo3Ti type steels subject to cold deformation along and across the tube axis. The specific elongation of the transverse samples is regularly higher than that of the longitudinal samples (Table II). This fact is probably due to a lower yield stress and, consequently, a greater specific elongation needed for the voids to coalesce (Knott, 1979).
During creep, the transverse samples have a lower ductility than the longitudinal samples. Most likely, this is because the contribution from grain-boundary defects (such as voids and cracks) is the higher, the longer the boundaries perpendicular to the tensile axis (Knott, 1979).
The examination of the microstructure shows (see Fig. 2) that these boundaries are nearly 1.5 times longer in the transverse samples and therefore they have a greater probability of fracture under a lower total strain. CONCLUSIONS 1. Tubes 6.9 mm in diameter with walls 0.4 mm thick fabricated from the Crl 6Nil 5Mo3Ti type steels and cold deformed to a strain of20% were found to have a fibrous texture, the preferred orientations along the rolling axis being (111) and (100). Other preferred orientations are (011) normal to the radial plane and (011) and (112) normal to the plane tangential to the tube surface. The last orientation disappears at a depth of 100 tm from the outer surface. 2. The strength, the creep strength, and the rupture strength proved to be higher along the tube axis. This fact may be qualitatively attributed to the predominance ofthe (111) and (100) texture components in the axial direction and the (011) component in the lateral direction, and also to the grain shape. 3. Fibrous texture of tubes fabricated of Crl6Nil5Mo3Ti type steel is not optimal from the viewpoint of use as fuel element shells in fastneutron reactor.