ON THE DEVELOPMENT OF THE BRASS-TYPE TEXTURE IN AUSTENITIC STAINLESS STEEL *

FCC materials possess different rolling textures i.e. the copper-type and the brass-type and these textures are described in terms of two limited fibres (i.e. a fibre axis (ll0)//ND extending from {011}(100) to {011}(112) and the other fibre (ll0)60ND running from 112 (111) through 123 (634) to 011 (112) with (110) axis inclined about 60 from ND towards RD). While the brass-type texture in low stacking fault energy (SFE) materials is characterised by (ll0)//ND fibre with |011 }(112) as a major component, the copper-type (high and medium SFE materials) is described mainly by (ll0)60ND fibre with {112}(111) or {123}(634) as major orientations. It is well established and documented in the literatures (Hirsch and Liicke, 1988a; Hirsch, Lticke and Hatherly, 1988b) that the textures at low degrees of rolling (i.e.--20% reduction) are identical in all FCC materials i.e. high SFE (A1), medium SFE(Cu) and low SFE(t-brasses) and consist of orientations concentrated homogeneously along two fibres ((110)//ND and (110)60ND). At medium degrees of rolling (i.e. -60% reduction) only the (110)60ND fibre for high and medium SFE materials and the (110)//ND fibre for low SFE materials remain. The (110)60ND fibre deteriorates to one or more peaks in {112}(111)/{4 4 11}(11 11 8), {123}(634) and {011}(112) and the (ll0)//ND fibre degenerates to only {011}(112) with very little {011}(100) at high degree of rolling (i.e.--95% reduction). The structure of these experimentally observed rolling textures (both early and final stages) for high and medium SFE materials agrees fairy well with that of the simulated textures calculated on the basis of Taylor-type models [i.e. (FC) Full Constraints (Taylor, 1938) and Relaxed Constraints (RC)] assuming multiple slip during rolling deformation (Hirsch and Lticke, 1988c).

This paper is a reply to the comment by T. Leffers, pp. 53-58of this issue of Textures & Microstructures.
For low SFE materials (m-brass) after initial development of (ll0)//ND and (110)60ND fibres, the texture transition from copper-type to brass-type has been found to occur according to Wassermann's mechanism (Wassermann, 1963) where mechanical twinning along with deformation inhomogeneities play a dominant role in the formation of final brass-type texture (Hirsch, Lticke and Hatherly, 1988b).Here, mechanical twinning transforms the orientations in the vicinity of 112 }(111) to orientations near {552}(115) and also twins the orientations near {123}(634) to its own symmetrical positions and thus causes an abrupt decrease in the orientation density of 112 }(111) and also simultaneous increase in the density of the twin component 552 }(115).This decrease in the density of 112}(111) along with simultaneous increase in {552}(115) marks the beginning of texture transition and occurs at critical degree of rolling which decreases with decreasing SFE.In case of low Zn brass, furthey rolling causes normal slip rotation of {552}(115) towards {011 }(100) and then to {011 }(112) and finally leads to the formation of brass-type texture, i.e. (110)//ND fibre.On the other hand, for high Zn brass, the abnormal slip rotation of twin 552 (115) parent 112 (111) lamellae towards i.e. 111 }(112) followed by the formation of non-crystallographic shear band formation and subsequently the resumption of normal slip in the fine grains of {011}(100) towards {011}(112) lead to the formation of brass-type texture (Duggan  et al., 1978).Recently, Singh, Ramaswamy and Suryanarayana (1992), have supported the view point of Duggan et al. (1978) on the basis of the statistical appearance of the structure of ODF obtained in an austenitic stainless steel cold rolled at 473 K and also using the existing informations on microscopical observations of deformation inhomogeneities available in the literatures (Blicharski and Gorczyca, 1978; Hutchinson, Duggan and Hatherly, 1979).
On the contrary, Leffers and coworkers (Leffers and Grum Jensen, 1968; Leffers, 1969; Pedersen and Leffers, 1987; Leffers and Juul Jensen, 1988; Leffers and Blide- Sorensen, 1990; Leffers and Juul Jensen, 1991; Leffers and Hansen, 1992) have disputed and rejected Wassermann's twinning hypothesis and also suggested that the texture transition from copper-type to brass-type can be accounted for by using classical lower- bound Sachs model (Sachs, 1928) or modified Sachs model.Furthermore, they also claim that from the very beginning of rolling deformation, the development of textures in low SFE materials is totaly different from that in high and medium SFE materials.
In their simulation, they assume the occurrence of single slip in all grains.This arguments has been negated by a considerable body of research works.It is worth to mention that the Sachs type models do not simulate the formation and simultaneous destruction of the texture component near 111}(112) which occurs as a result of mechanical twinning and abnormal slip rotation on one hand and the shear bands formation on the other.Once again, Leffers (1993) has doubted the validity of Wassermann's twinning concept as a mechanism responsible for the transition of texture from copper-type to brass-type during early stage of deformation (upto about 50% cold reduction) and also tried to reinterpret the results obtained by Singh, Ramaswamy and Suryanarayana (1992) in terms of the above mentioned suggestions.
The aim of the present work is to clarify and to demonstrate that the statements [i.e.a) the initial development of brass-type texture is similar to that of copper-type and b) this development follows the FC Taylor model at least upto 30% cold reduction and finally c) mechanical twinning has direct effect on the texture transition during further rolling (<50%CR] made by Singh, Ramaswamy and Suryanarayana (1992) are correct and also that Leffers's reinterpretations do not hold good in this case.These clarifications are based on development of textures and on the evolution of microstructures as determined by orientation distribution functions (ODF) technique and transmission electron-microscopy (TEM) respectively.The details of the ODF technique used to analyse the textures have been given in the previous work (Singh, Ramaswamy and Suryanarayana (1992) whereas TEM has been used to examine the thin foils parallel to rolling plane and to provide further evidences on microstructural evolution during rolling.
in this section.In addition, this section also shows the position and density of many orientation fibres such as (110)//ND, (110)//TD and (111)//ND.This figure clearly reveals the depletion of cube orientation {001 }(100) with increasing deformation and finally this orientation disappears at 50% reduction.Simultaneously with the decrease in the density of {001}(100), there is increase in the orientation density of the components 225 }(554) and --{ 123 }(634).
On the other hand, Figure 5 shows the plot of average sharpness of texture (texture index J) with increasing degrees of rolling.This figure indicates a monotonous increase in the texture index upto 30% CR and then it remains constant for further rolling (i.e.>30% CR and <70% CR) and finally it again increases for higher degrees of rolling (>70%CR).The exact position of the (110)60ND fibres in Euler space are given in the form of their coordinates 1 and and plotted as a function 2 (Figure 6) for various degrees of rolling (i.e.0% CR, 30% CR and 50% CR).Furthermore, the extracted data of the position of these fibres (Hirsch and Liicke, 1988c) calculated on the basis of FC Taylor model based on the assumption of homogeneous multiple slip for various degrees of rolling (18% CR, 33% CR and 45% CR) have been superimposed on Figure 6.It is interesting to note that the experimentally observed (ll0)60ND fibre for HB(0%CR) runs very close to the and 1 lines of theoretically determined (110)60ND fibre calculated on the basis of FC Taylor model.The orientations of these experimental (ll0)60ND fibres are systemetically shifted towards higher and 1 values in the vicinity of the orientation {011 }(112).With increase in the degrees of rolling, these < 110>60 ND FIBRE (ORIENTATION FC Figure 6 EULER ANGLE Orientation of (110)60ND fibre in the Euler space given by , as function of 2- fibres further shift towards higher and lower 1 values in the region of orientations between ---{ 123 }(634) and {225}(554).This progressive shift towards higher values occurs in the deformation range where mechanical twinning predominates.

MICROSTRUCTURE EVOLUTION
The initial structure of HB has been described by the presence of mostly elongated grains as well as of some recrystallised grains (Singh, Ramaswamy and Suryanarayana, 1991).Furthermore, this initial structure also exhibits a full grown recovery twin along with a number of stacking faults within a grain [Figure 7(a)], while selected area diffraction (SAD) pattern of the faulted region is shown in Figure 7(b).The indexing of the SAD pattern is given in Figure 7(c).In Figure 7 the habit plane of the faults {111 }7 is oriented parallel to the (110)7 electron beam direction and from the relative rotation of the bright field (BF) and SAD pattern the faults are seen to lie along (112)7 direction.The extent of the intensity streaking in the SAD pattern is indicative of extremely thin faults.The SAD pattern taken from the region containing recovery twin oo.  was also of (110)q, orientation and the twin was found to orient itself along (l12)T direction.The density of dislocations in this recovered grain is relatively small as compared to other grains where cell structures are the essential features.At very low degree of rolling (10% reduction), plastic deformation is realized chiefly by multiple slip and besides, zones of uniform dislocations, there appear well shaped cell structures and a scattered mechanical twin band in one crystallographic plane (Figure 8).
At 30% reduction, profuse twinning in the form of planar bands can be found (Figure 9).The BF, dark field (DF) using (002)T reflection and SAD evidences shown in Figure , 9(a), (b) and (c) indicate that the planar bands shown in Figure 9(a) are mechanical twins (T) formed within suitably oriented grain of austenite (T).The indexing of the SAD pattern is shown in Figure 9(d).The habit plane of these twins is inclined to the electron beam (i.e.angle of specimen tilting 10) resulting in the composite pattern (114)7/(110)T.The measured volume fraction of the twins in suitably oriented grains is of the order of 35%.In contrast, other grains exhibit well developed cell structures.Further rolling (i.e.< 50% reduction) not only increases the amount of mechanical twins but also causes these twins to form in two intersecting crystallographic planes (Figure 10).

DISCUSSION
The examination of texture of HB exhibits that the HB is composed of grains whose orientations are concentrated along (110)60ND fibre with 011 (112) as, major and {225}(554) as minor components.In addition, recrystallised grains of cube orientation  {001 }(100) and other RD rotated cubes are also present.The fibre (110)60ND is one of the two fibres (i.e.(110)//ND and (110)60ND) which develope in materials of high to medium SFE at low degrees of rolling.The minor orientation {225 }(554) is in between {112}(111) and {4 4 11}(11 11 8) and differs approximately 5 from {112}(111) and 3 from {4 4 11}(11 11 8).Further, TEM features (Figure 7) indicate that the grain containing stacking faults and recovery twin is of {011 }(112) type-an indirect evidence and is fully recovered.At very low degrees of rolling (10% CR), the presence of well developed cells and a scattered mechanical twin band as shown in Figure 8 indicates that dislocation glide by multiple slip predominates with a weak participation of twinning in one crystallographic plane.As the rolling is continued (i.e. at 30%CR), the orientation density at {011}(112) and ~{123}(634) increases strongly and that at {225}(554) and {011 }(100) increases moderately and also there is a considerable decrease in the density of cube orientation 001 (100).This increase in orientation density at 011 (112) and ~{ 123 }(634) is partly due to starting texture and partly due to flow of cube {001 }(100) and other RD rotated cubes along (110)//ND fibre and then along (110)60ND fibre.Furthermore, orientations concentrate along (110)//ND and (110)60ND fibres (Figures 1,2 and 3).It is evident from Figure 4 that a maxima in orientation density at {225 }(554) occurs along with a scattering of {011 }(100) towards {144}(811) and {332}(113).On the other hand, the TEM evidence (Figure 9) clearly illustrate that the twinning is intensified and that profuse twinning occurs on one twinning system in suitably oriented grain (i.e.{225}(554)-an indirect evidence) but still multiple slip predominates which is menifested by the presence of well developed cells in other grains.The predominance of dislocation glide by slip in contrast to twinning is also corroborated by the plot of texture index with increasing degrees of rolling (Figure 5).It is worth to mention, here, that the rate of texture development (i.e.slope=0.61) is very close to that of copper- type (slope=0.79)calculated on the basis of relaxed Taylor model as extracted from Figure 1 (Leffers, 1993).On the contrary, the rate of texture development of brass- type calculated on the basis of modified Sachs model (Leffers, 1993) is 0.27 and even less.This small deviation of measured rate of texture development from the calculated copper-type is mainly due to the onset of twinning in suitably oriented grains.In addition, the very close agreement of the experimentally determined positions of (110)60ND fibre with the theoretically calculated positions of (110)60ND fibre (Figure 6) further substantiates the dominant role of multiple slip.However, the shifting of measured positions of (ll0)60ND fibre in the region of orientations between-{ 123}(634) and {225}(554) from the calculated ones towards higher and lower 1 values at 30% reduction again shows the onset of mechanical twinning.All these evidences {i.e.i) a moderate to high increase in the orientation density of all texture components of the two fibres ((ll0)//ND and (ll0)60ND), ii) a very close agreement of the measured (110)60ND fibre with the theoretically determined (110)60ND fibre on the basis of FC Taylor model and, iii) near matching of the measured rate of texture development with the calculated rate of development of copper-type texture suggest that the initial development of rolling textures (at 30% CR) is similar to that of copper-type and that this development agrees fairly well with the predictions of FC Taylor model (Hirsch  and Liicke, 1988c).Further, twinning is intensified with increasing degrees of rolling (30% CR) and this marks the beginning of transition from copper-type texture to brass- type and is also responsible for the modest increase in the orientation density of {225}(554).It follows from the observations (i.e. a modest increase in the orientation density of {225}(554) and an apparant scattering of {011}(100) towards {144}(811) and |111 }(112)) that at low degrees of reduction (30%CR) the increase of {225}(554) peak due to multiple slip is in completion with simultaneous twinning and abnormal rotation.
At 50% rolling reduction, the first detectable decrease in the, otherwise, stable {225}(554) orientation accompanied by an increase in orientation density of twin component {144}(811) and also simultaneous formation of peak at {332}(113) (Figure 4) show the dominant role of mechanical twinning (Wassermann, 1963) in intensifying the texture transition whereby the orientation {225}(554) undergoes mechanical twinning to form {144}(811) at 1, , 2 90, 80 and 45 and also the simultaneous operation of abnormal rotation of the twin-parent bands towards 111 }(112).Furthermore, TEM evidence (Figure 10) in close agreement with the textural feature (Figure 4) reveal clearly that mechanical twinning in suitably oriented grains predominates.The twin orientation 144}(811), instead of rotating towards.{011 }(100) and then to {011 }(112) (Wassermann, 1963), rotates in the opposite direction towards {111}(112) at =55 .This abnormal rotation has also been observed by other investigators (Asbeck and Mecking, 1978; Duggan et al., 1978) and indicates that slip occurs preferentially on 111 plane common to the twin plane and finally stops at {332}(113) at =65 .The formation of peak at {332}(113) illustrates that the shear stress for abnormal slip has decreased considerably and approached zero value once the twin-parent lamellae (planar bands) become oriented with the rolling plane.It follows from texture evidence (Figure 4) and TEM feature (Figure 10) that mechanical twinning plays a vital role in texture transition from copper-type to brass-type as manifested by nearly equal density of the peaks at {332}(113) and at {225}(554) and also by the presence of equal volume fraction of twin and matrix bands in suitably oriented grains (i.e.--{ 112}(111)) respectively.Further rolling (at 70% CR) causes the appearance of orientation {334}(110) differing by 8 from {111}(110) by the onset of twinning of 1123}(634) followed by abnormal rotation of {123 }(634) and its twin {123}(634) on 111 plane parallel to the twin plane.Simultaneously with the operation of twinning and abnormal rotation, the formation of shear bands in the rotated twin-matrix regions leads to rotation of {332}(113) and {111}(110) to near {011}(100) and {011}(112)   respectively as well as the re-establishment of multiple slip in the fine grains of shear bands at 90% CR causes the sharpening of final brass-type texture i.e. (110)//ND fibre (Singh, Ramaswamy and Suryanarayana, 1992).

CONCLUSIONS
The statements made by Singh, Ramaswamy and Suryanarayana (1992) are correct and the Leffers's reinterpretations do not hold good.The above demonstrated facts pertain and lead to the following statements: i) The initial texture development of brass-type is similar to that of copper-type at least upto 30% cold reduction and the texture consists of two limited fibres (i.e.,110)//ND and (110)60ND).
ii) This texture development follows Taylor model (Full constraints).iii) Mechanical twinning plays a dominant role in the transition of texture from copper- type to brass-type and twins {225}(554) component to {144}(811) in the form of twin-matrix bands (lamellae).

i il 002 Figure 7
Figure 7 Transmission electronmicrographs of HB(0% CR) a) bright field (BF) image b) selected area diffraction (SAD) pattern c) key to SAD pattern.

Figure 9
Figure 9  Transmission electron micrographs of 30% cold rolled (CR) HB at 473K: a) bright field (BF) image b) dark field (DF) image c) selected area diffraction (SAD) pattern d) key to SAD pattern.

Figure 10
Figure 10  Transmission electron micrograph of 50% cold rolled HB at 473K bright field image.