The Orientation Characteristics of Different Recrystallization Stages in Copper

INTRODUCTION The general features concerning the development of the recrystallization texture in a polycrystalline metal have been extracted from a large amount of experimental investigations. The main problem in this field is to quantify the relative significance of existing models or experimental data with respect to the global process of recrystallization texture formation. The reason for this must be seen in the fact that the phenomenon is a statistical one, taking place in a larger volume, whereas as a rule only information about a particular recrystallized state or about a locally closely confined region of the material is available.


INTRODUCTION
The general features concerning the development of the recrystallization texture in a polycrystalline metal have been extracted from a large amount of experimental investigations. The main problem in this field is to quantify the relative significance of existing models or experimental data with respect to the global process of recrystallization texture formation. The reason for this must be seen in the fact that the phenomenon is a statistical one, taking place in a larger volume, whereas as a rule only information about a particular recrystallized state or about a locally closely confined region of the material is available.
In the present investigation the microstructure of the recrystallized grains and their deformed surroundings, and local orientations of defined areas, are studied in the transmission electron microscope (TEM) for various stages of recrystallization of the same initial state. The results are then suitably statistically analysed and presented. The material parameters were chosen such that, in the deformed state, as broad a spectrum of orientations of the nucleation process as possible was obtained (high proportion of shear bands). The nucleation conditions so produced are particularly suitable for the investigation of the mechanisms of oriented growth.

EXPERIMENTAL DETAILS
Through special thermo-mechanical treatment soft copper samples (purity: 99,9998%) were produced which showed a fairly random texture for a mean grain size of 100 pm.
The material was 95% reversibly cold-rolled. The ratio Id/h [Id: compressed length, h: sample thickness before each rolling step] varied here between to 5. In this way homogeneous deformation conditions are ensured throughout the sample cross-section. By choosing different annealing periods (10 minutes and 1,5 hours respectively) for the rolled material at 140 C, two primary recrystallization stages were generated which from calorimetric measurements correspond to a degree of recrystallization of about 10% and 50% respectively. The choice of just these recrystallization stages is based on earlier investigations of texture in rolled copper /1/.
Using the cold-rolled material as well as the annealed samples, thin foils perpendicular to the normal direction ("N Section") as well as to the transverse direction ("T Section") were prepared. In the TEM, at an accelerating voltage of 120 kV, bright field images and Kikuchi-patterns (nanobeam electron diffraction) were made at defined sites. The orientations were calculated on-line from the Kikuchi-patterns /2,3/. The accuracy of the orientation determination is about in the reference system of the microscope. Since the determination of the sample coordinate system is rather more difficult the orientation specifications in the sample system are only accurate to within 5 The orientations were determined of both the recrystallized grains and the surrounding deformed areas. For both annealing states and both foil sections the following number of single orientations were studied: 10% Sedes: N-Section 273, T-Section 69; 50% Series: N-Section 386, T-Section 122.
In addition the areas of the recrystailized grains were measured on the bright field images. For the N-Sections the grains were divided into two groups: "small" and "large". Into the "small" group of grains belong all those that have an area smaller or equal to 10-2 of the area of the largest grain of a given series.
Using the orientation data for the individual sites, two types of distribution function  At the initial stage of recrystallization (-10% recrystallized) the ODF of the new grains in the N-section is fairly complicated and moreover different for small and large grains ( Fig.la,b). The misorientation distributions between the recrystallized grains and the surrounding matrix in no case exhibit a tendency to a random distribution. The orientation parameters of the position of the maxima in the MDF's often can be approximated by coincidence orientation relationships. This is shown by the MDF's represented in Figs. 3a and b for the N-section as an example. Here for the small grains the 60 < 111 > (I:=3) relationship occurs preferentially, as well as misorientations with small angles of rotation, whereas in the case of the large grains it is the -35 < 112> (}'=35a) relationship. The situation in the T-section shows that, besides small angles of rotation, there are rotations in the ranges 35-60 < 111 > or 15 30 < 110 >. The misorientation distribution between the large recrystallized grains and the surrounding matrix shows a fairly strong maximum at 40 < 111 >, but also misorientations at 60 <111>, 40 <101>, 50 <101>, 30 <111>, 25 <101>, 40 <113> and 30 <101 >. They all correspond to coincidence orientation relationships with " equal to 7, 3,9,11,13b,19a,23 and 27a respectively (Fig.3d). For the small grains the relationships 00 <111>, 40 <101>" and misorientations with small angles of rotation occur. In the interval of CO d 20-60 possible axes of rotation scatter strongly around <111>. The situation in the T-section shows, besides strong maxima at 00 < 111 > and 50 <210> ('=15) rotations in the ranges 10-20 and 30-40 <101>, 40 <111>, 40-45 between <211> and <221>, -55 <1>.  The different ODF's and MDF's for small and large grains, and for the two recrystallization stages, indicate that the oriented growth of the nuclei has considerable effect on the generation of the primary recrystallization texture. We shall look at this more closely for two very remarkable results:

RESULTS AND CONCLUSIONS
The ODF of the small grains at the start of the process (Fig.l) yields information on the distribution of the nuclei. Their orientations scatter around the components of the deformation texture or are related to it by twinning. It is remarkable that, of these nuclei in this stage only the {123} <634 > and {122} <212> oriented ones grow, though not for instance the {112}<111> oriented nuclei (Fig.lb). The reason for this may be that a~{112}<111> nucleus either shows a --60 <111> orientation relation with the deformed surroundings (for complementary components), or has a very small misorientation. In both cases the mobility of the boundary is very low. On the other hand, a -{123}<634> nucleus has a -40 <111>, 40 < 221 > or -50 < 221 > orientation relation (for complementary components) or a very small misorientation. At least the mobility of a boundary between 40 < 111 > misoriented regions is not low. This means that the results described can without difficulty be attributed to oriented growth.

(ii)
The recrystallization component -{122}<212> is usually interpreted as a twin of the first generation of the cube component. The relationships here, cert=inly for some of the -{122}<212> components, are the opposite /5/: The -{122}<212> component is represented much more strongly at the start of recrystallization than the cube component (Fig.lb). During further annealing, based on our morphological observations, it would appear that a considerable number of -{001} < 100> oriented small grains are created by twinning in or near the -{122}<212> oriented regions.
The cube orientation can grow quickly as a compromise position to the components of the deformation texture. In this way it determines the recrystallization texture at the advanced stages of the process (Fig.ld). In accordance with this interpretation are the strong 40 < 111 > maximum of the MDF (Fig.3d) and the different ODF's for the environment of the {001}<100> and {122}<212> grains (Fig.2a,b).