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Orientation relationships in a partially recrystallised polycrystalline AlSi-alloy
Pergamon
1359-6454(95)00413-O
Acta mafer. Vol. 44, No. 8, Pp. 3407-3419, 1996 Copyright 0 1996 Acta Metallurgica Inc. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 1359-6454/96
$15.00 + 0.00
ORIENTATION RELATIONSHIPS IN A PARTIALLY RECRYSTALLISED POLYCRYSTALLINE AlSi-ALLOY S. R.
SKJERVOLD’ and N. RYUM’
‘Hydro Aluminium a.s., R & D Centre 2The Norwegian Institute of Technology, Department (Received
16 January
Karmsy, N-4265 Hgvik and of Metallurgy, N-7034 Trondheim,
1995; in revised form
Norway
11 July 1995)
Abstract-A moderately deformed polycrystalline AlSi alloy of initially random texture whose deformation structure is discussed in an accompanying paper was annealed until partial recrystallisation. The nucleation of recrystallised grains occurred preferentially in regions of large misorientation gradients. Particularly potent regions were shown to be the grain boundary and triple line regions, and the regions close to individual second phase particles or clusters of such particles. It was found that a majority of the recrystallised grains were developed in regions where a cumulative rotation of the lattice about a (1 1 1)-axis occurred. The angle of rotation about a (11 I)-axis of grains relative to the surrounding matrix was in the range 3545” for about 50% of these cases. The preferred selection of recrystallised grains having the - 40” (111) orientation relationship is suggested to be due to the easy formation of such sites
in the deformed matrix and also to their preferential growth potential
1. INTRODUCTION
Recrystallisation of a plastically deformed material has for decades been characterised as a nucleation and growth process [l]. There has also, for many years, been objections to this view because the term “nucleation” is used in an unfamiliar way in this case. The major objection has been that the basic concept of thermal fluctuations is ill-defined in this case and, more specifically, that such fluctuations are unlikely to occur at all in plastically deformed lattice. This has the consequence that the recrystallised grain can be formed only by means of an irreversible growth process. During the years much evidence in support of such a view has accumulated: “sites” are claimed to be formed during plastic deformation and can either be considered to belong to the deformed state or can grow out of this state. Thus, the term nucleation is not well founded. However, the term has such a long tradition within this field that continued use seems appropriate [2]. The way in which such “sites” are formed depends, among many other parameters, on the magnitude of the plastic straining and the way the strain is distributed within the material. Embury et al. [3] postulate that two structural elements are created in the deformed state, namely an overall global level of stored energy and grain boundaries capable of thermally activated migration. The last of these two elements thus constitutes the nuclei or “site” for the recrystallised grain. There have been many attempts to characterise these “sites” in greater detail, particularly in recent years when new experimental techniques, capable of 3407
measuring the local texture or lattice curvature, have become available [4-61. In the present work the aim was to identify the sites at which recrystallisation originates, and to relate the orientation of the new grains to the deformed matrix. The deformed structure of the polycrystalline Al-1.2%Si alloy is characterised in an accompanying paper [7]. After the completion of the present investigation a similar investigation by Humphreys and Ardakani [8] has been published. Even though the two investigations address the same general problem, the material studied and the experimental technique used were different. While Humphreys and Ardakani made a very thorough characterisation of the plastic deformation and subsequent recrystallisation of several Al-Si single crystals containing hard and relatively large Si-particle and focused on the behaviour in the neighbourhood of the particles, the present investigation studied a polycrystalline Al-1.2Si alloy and the heterogeneities in the deformed structure in this alloy, not only close to the particles but also adjacent to grain boundaries and triple lines. The two investigations thus complement each other. 2. EXPERIMENTAL
AND RESULTS
The material studied in the present investigation is the same as that described in the accompanying paper [7]. Specimens deformed to t = 0.20 and 0.40 where subsequently annealed in a salt bath at 350°C + 1°C for 60 and 10 s, respectively and then water quenched. This heat treatment resulted in a recrystallised fraction less than 10% and the size of
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the recrystallised grains was typically in the range of 5-20 pm. A limited number of grains with a size of 100 pm were also included. Depending on the scale of the microstructure the magnification varied from 300 to 1000 x which correspond to a step size of 0.6-1.1 and 0.2-0.3 pm/pixel, respectively.
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In order to obtain information about the crystallography relationship between the recrystallised grains and the sites in which the grains have formed, the crystallographic orientations of the recrystallised grains were determined at one or a few locations within the grains while the orientation of the surrounding matrix was most often determined by
(a>
(b) Fig. 1. (a) ECC-micrograph showing two recrystallised grains separated from each other by a low angle boundary, and formed in a TL-region of material A deformed to t = 0.20. (b) Orientation relationships between the recrystallised grain (3) and the surrounding matrix (H, I, J), plotted in a standard 11 l-pole figure.
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(a>
Fig. 2. (a) ECC-micrograph showing recrystallised grains of approximately equal orientation formed in a grain boundary region of material A deformed to t = 0.20. (b) Orientation relationships between the recrystallised grain (7) and the surrounding matrix (0, P, Q), plotted in a standard 11 l-pole figure.
a number of discrete measurements at different locations. The EBSP-technique and the presentation of crystallographic results are, in general, the same as those used in Ref. [7]. However, of particular interest in this case is the angle of deviation about an exact (Ill)-axis y’,,, from a rotation between the recrystallised grain and the matrix. The standard notation used in the expression of
orientation structure
relationships is; R~M(B(uvw),
in
partially
Y,,,)
where
recrystallised
Rand Mrefer matrix grain,
to the recrystallised grain and respectively. In the following, a series of examples are presented which are considered to be typical and the crystallographic relationships between the grains and the nucleation sites are pointed out.
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(4
(b) Fig. 3. (a) ECC-micrograph showing a recrystallised grain formed in the grain interior at a cluster of particles of material A deformed to t = 0.20. (b) Orientation relationships between the recrystallised grain (8) and the surrounding matrix (R), plotted in a standard { 11 I}-pole figure.
2.1. Crystallographic aspects of recrystallisation material deformed to E = 0.20
in
Case 1 (Fig. 1). Figure l(a) shows two grains 3 and 3’ formed in a triple line region. These two grains are slightly misoriented with respect to each other; 3-3’(3.2”(221)). The orientation of the recrystallised grain 3 and the surrounding matrix grains labelled H,
Fig
4(a).
ECC-micrograph
showing
recrystallised deformed
I and J are shown in Fig. l(b). As can be seen, the orientation of the matrix grain J scatters strongly. These orientations, which were recorded by a continuous scan in a region just below the recrystallised grains, demonstrate clearly that a large misorientation gradient develops locally even after such low degree of deformations. The rotation of the lattice in this particular matrix
grains formed to t = 0.4.
in the grain
interior
of material
B
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A
Fig. 4(b). A schematic illustration of the microstructure in Fig. 4(a) delineating clearly the grain boundaries of the recrystallised grains and some of the positions where orientation measurements were made.
grain
is about
an axis 15” away
from
a ( 111 )-axis
and
20” at a distance of 11 pm. A misorientation gradient of 1.8”/pm thus exists in this area close to the recrystallised grain 3. This recrystallised grain has the following orientation relationship to matrix grains J and I: 33J(30”( 11 l), Y,,, = 15”) and 3%1(56”(111), ‘I’,,, = 4.1”). Case 2 (Fig. 2). In Fig. 2(a), a cluster of 5 or 6 grains has apparently formed along a grain boundary. The possibility that they can have formed along a triple line above or below the section studikd can, of course, not be excluded. These grains have grown to a much larger size than the grains discussed above (notice that a magnification of only 500 x is used in this case). Only very small orientation differences, in the range 14”, were found between these recrystallised grains. The boundaries between
reaches
Fig.
a
4(c).
maximum
standard 111-pole figure showing the axis relationship between the recrystallised grain (4) and surrounding matrix positions (Al, A3, A4) in Fig. 4(a).
(111)-rotation
A
of
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these recrystallised grains are thus low angle grain boundaries. The orientations of the recrystallised grains and the three neighbouring matrix grains P, Q and 0 are shown in Fig. 2(b). As can be seen. grain 7 has an orientation quite similar to one of the neighbouring matrix grains Q: 7-Q(4”( 123)). The orientation relationship to the two other matrix grains are as follows: 77P(42”( 11 l), ‘I’,,, = 4.8”) and 7-0(42”(310)). It appears from Fig. 2(a) as if the recrystallisation front is moving into matrix grain Q. If these grains have nucleated in a grain boundary region, the mechanism must be different from the classical strain induced boundary migration (SIBM), since bulging of an existing grain boundary into the deformed matrix of a neighbouring matrix grain is not observed in this case. Case 3 (Fig. 3). A possible example of particle stimulated nucleation (PSN) is shown in Fig. 3(a). The recrystallised grain 8 is formed at one particle in a cluster of particles located in the interior of matrix of grain 8 and of the grain R. The orientation surrounding matrix R are plotted in Fig. 3(b). As shown in this figure, there is a wide scatter of orientations in the matrix surrounding the recrystallised grain. As an average orientation relationship between the recrystallised grain and the matrix grain it was found that 8%R(51”( 110)). 2.2. Crystallographic aspects of recrystallisation material deformed to t = 0.40
in
Recrystallised grains appear to have originated from different nucleation sites, as shown in Figs 4(a) and 5(a). It can also be seen that the recrystallised grains often appear in colonies both in the grain boundary region and in the grain interior. The orientation relationships between 86 recrystallised grains and the surrounding matrix has been examined by the EBSP-technique in SEM. The examples presented in the following are considered to be typical of the observations made. It should, however, be noted that the study is restricted to the nucleation within only three different matrix grain orientations. part of the Case 4 (Fig. 4). In the central micrograph, a string of recrystallised grains of approximately equal size can be seen. In addition a number of apparently isolated grains are scattered around in the matrix grain A. While the clustered grains are nearly equiaxed the isolated grains have grown in a more anisotropic way. A schematic illustration of the microstructure and the positions where the orientations have been determined is given in Fig. 4(b). In the following, the orientation relationship between the recrystallised grains and the surrounding matrix will be presented. The orientation of the isolated grain 4 and of the surrounding matrix A is shown in Fig. 4(c). As can be seen, the orientations of the surrounding matrix do not scatter much except for position A3 located just
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(3) Fig. 4(d). Standard I 1 I-pole figures representing the orientation relationships between the recrystallised grains and surrounding matrix in Fig. 4(a). (1) Recrystallised grains having an orientation close to the matrix orientation, and (2, 3) a (1 11)-rotation axis relationship to the surrounding matrix.
below the recrystallised grain. The orientation of position A3 relative to positions A 1 and A4 is formed by rotation of the lattice about an axis lying close to a (11 I)-axis. Also, the recrystallised grain 4 is seen to have one of its (1 11)-axes nearly parallel to this particular (111 )-axis in the matrix. The orientation relationship can be written in the following way: 4-A(46”( 11 l), ‘I’,,, = 3.5”). Four recrystallised grains 5, 6, 7 and 8 are located in the lower left corner of the micrograph. The orientation of the recrystallised grains 6 and 7 and the orientation of the surrounding matrix are shown in Fig. 4(d), part (1). As can be seen, the recrystallised grains have orientations which correspond to the outer spread of matrix orientations. While the rotation axis between the recrystallised grains and the surrounding matrix is almost fixed, the wide
range of the misorientation is noticeable. The orientation relationships are: CA(9%33”(221)) and 7-A(8-29”(221)). The two other grains in this region, grains 5 and 8, have orientation relationships to the matrix similar to grain 4 above, as demonstrated in Fig. 4(d), parts (2) and (3). Notice that the common ( 111)-axis is not the same for the two grains and also different from the one in grain 4. The orientation relationships are: 55A(15”(111), Y,,, = 8.8”) and 8SA(25”(111),Y = 11.1”). The next example deals with the recrystallised grains 9 and 10 in Fig. 4(a). These two grains are close to each other but the orientation relationships between these two grains and the surrounding matrix were found to be very different, as shown in Fig. 4(e), parts (1) and (2). Grain 9 has a (1 11)-axis common
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with the surrounding matrix where the orientation relationship can be written in the following way: 9%A(40”(111), Y,,, = 8”). Grain 10 on the other hand has a very low misorientation to the matrix, and can in fact be considered to be a large subgrain. The orientations of the recrystallised grains, 11-18, and of the surrounding matrix are shown in Fig. 4(f), parts (l))(3). The orientations of the deformed matrix do not scatter much. In this case the recrystallised grains appear to have formed on a chain of particles. The orientation relationships between the recrystallised grains 11-18 and the surrounding matrix A are given in Table 1. As can be seen, the average misorientation between the recrystallised grains and the surrounding matrix is quite high (30-59”). Moreover, a majority of the rotation axes between the recrystallised grains and the matrix are only a few degrees away from a (Ill)-axis. Special attention should be given to the orientation of the recrystallised grains 11 and 13 plotted in Fig. 4(f), part (2). As can be seen, grain 13 has almost the same orientation relationship to the matrix as grains 12, 1.5 and 16 shown in Fig. 4(f), part (1). However, grain 13 has one of its (11 l)-axes in common with the neighbouring recrystallised grain 11. The orientation relationship between grain 11 and 13 is: ll-13(59’(111), Y,,, = O.S”). A common (111 )-axis between two recrystallised grains was also found between grains 16 and 17 and between grains 17 and 18, see Fig. 4(f), part (3) for the latter case. The orientation relationships in these cases are: 1617(54”(111), Y,,, = 3.7”) and 17-18(59”(111),Y,,, = 1.3”). A 60” rotation about a (11 I)-axis corresponds to a twin orientation relationship between the two grains.
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Case 5 (Fig. 5). A triple line region is shown in Fig. 5(a). In Fig. 5(b) the same microstructure is given in a schematic way, clearly delineating the grain boundaries, and the different positions where the orientation was measured. The recrystallised grains are clustered in the TL-region and along the grain boundaries, particularly between matrix grains A and B. Almost all new grains in the TL-region are located in the upper left matrix grain C. The microstructure of the matrix shows some alignment of the deformed structure in grain A and C both of which have the CA -axis nearly parallel to a (1 IO)-axis. On the other hand, the CA-axis of grain B is nearly parallel to the (lOO)-axis. The orientation of the recrystallised grains 40, 41 and 42 and of the surrounding matrix are shown in Fig. 5(c), parts (1) and (2). As shown in Fig. 5(c), part (l), the orientation of matrix grain B changes rapidly in regions close to the recrystallised grains. The orientation relationship between position B3 and B6 in matrix was found to be as follows B3-B6(23”( 11 l), Y,,, = 4.7”). The recrystallised grain 41 is close to having one of its (ill)-axes within the orientation spread of the (11 I)-axes of both matrix A and B as shown in the Ill-pole figure. The more accurate orientation relationships are: 41&4(31”(221)) and 41lB(17”(311)). Grains 40 and 42 have quite different orientation relationships with the surrounding matrix [Fig. 5(c), part (2)]: 40-A(56”( 11 l), Y,,, = 12.8”), 40P B(48”(21 l)), 422A(32’(211)) and 42-B(39”(441)). Moreover, a common (Ill)-axis is found between the grains 40 and 42: 4042(35”( 11 l), Y,,, = 4.2”). In this case the misorientation angle 0 is less than the
Fig. 4(e). Standard 11 l-pole figures representing the orientation relationships between the recrystallised grains and surrounding matrix in Fig. 4(a). (1) Recrystallised grain having a (11 l)-rotation axis relationship, and (2) a parent orientation relationship to the surrounding matrix.
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(3) Fig. 4(f). Standard 11 l-pole figures representing the orientation relationships between recrystallised grains formed at a cluster of particles and the surrounding matrix in Fig. 4(a). (1) Recrystallised grains having a (11 I)-rotation axis relationship to the surrounding matrix. (2, 3) Examples showing a twin orientation relationship between two recrystallised grains.
frequently observed value of approximately 60” for twin orientations. All the grains formed at the grain boundary have a high angle boundary with respect to both matrix grains. They also have orientations differing significantly from the average matrix orientation.
Table 1. Orientation
relationships
Grain
W)
(uvw)
11-A 12-A 13-A 14-A 15-A l&A 17-A 18-A
52 42 44 59 35 30 51 34
111 111 221 II1 111 111 111 331
The orientation relationships between the grains 4653, formed in the TL-region, and the surrounding matrix grains (A, B and C) are presented in Table 2. Except for grains 52 and 53 which have grown mostly into matrix B, the growth of the other grains have been into matrix C. As seen in Table 2, several axes
between some of the recrystallised matrix (M) in Fig. 4(a)
grains and the surrounding Comments
YIII(0) 12.6 9.1 6.9 11.8 14.1 0.5
M-PSN, M-PSN M-PSN M-PSN, M-PSN M-PSN, M-PSN M-PSN,
Common
< 11 I)-axis
to grain
13
Common
(11 I)-axis
to grain
15
Common
< 1 1 1)-z&
to grain
17
Common
(11 l)-axis
to grain
17
SKJERVOLD and RYUM:
Fig. 5(a). ECC-micrograph
ORIENTATION
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showing recrystallised grains formed in the grain boundary TL-region of material B deformed to t = 0.4.
of rotation between the recrystallised grains and the surrounding matrix were observed. Thus, no preferential rotation axes seem to exist in grains formed in the TL-region. There appears to be some evidence of a correlation between the size of the grains and the orientation relationships to the matrix in which they are embedded. Such correlations are evident for the large grains 49-50, which both have a rotation axis close to (111) relative to matrix C. Grain 49 is highly misoriented with respect to all three matrix grains meeting in the triple junction, but migration of the boundary has occurred preferentially into matrix grain C. This can be interpreted as an effect of preferred growth of specific orientation relationships. Similar behaviour is apparent for grain 50 which is equally misoriented with respect to both matrix grains (B and C). As can be seen, growth of grain 50
region and
occurs into matrix C with a (11 I)-axis rotations relationship. Smaller grains, such as grains 46, 47, 48 and 52, have rotation axes different from the (1 11)-type but the misorientation between these grains and the matrix is also large. It should be noted, however, that these grains have impinged upon each other and thus have limited each others growth. The orientation relationships between the recrystallised grain 53 and the matrix B are presented to two values in Table 2. The first value, 53-B, represents the average misorientation between the recrystallised grain and the surrounding matrix, whereas 53-1B is the relationship between the grain and a single position in the matrix close to the grain boundary between the grain and the matrix. This demonstrates that the e( 111)-rotation relationship between a recrystallised grain and the matrix may
Matrix C
Matrix
Fig. 5(b). A schematic illustration boundaries
of the microstructure
of the matrix
grains
3415
in Fig. 5(a) delineating clearly (bold line) and the recrystallised grains.
the grain
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Fig. 5(c). (1, 2) Standard 111-pole figures showing the orientation relationship grains (40,41,42) formed in the GB-region and the surrounding
exist only locally. In a deformed matrix with great turbulence, such as the TL-region, the orientation relationships may cover a wide range of misorientations and rotation axes as well.
3. DISCUSSION
3.1. Nucleation sites
It is convenient to distinguish between three types of nucleation sites in this material under the experimental condition described: within the matrix (M), in the grain boundary region (GB) and in the triple line region (TL). The efficiency of these different types of sites was different, and varied with the amount of overall uniform plastic deformation within the range studied (t = 0.2-0.4). The efficiency of the different sites for nucleation of recrystallisation are summarised in Table 3. At both strains a total area of approximately 1.2 mm* was investigated in the SEM by means of the EBSP-technique. Table 2. Orientation Grain
W)
33 43 37 53 43 44 26 33 36 55 43 47 3x 36 25 36 38
310 532 432 441 441 331 441 110 221 Ill 331 Ill 421 100 310 221 Ill
-B -c
5&B -C 51&C 52-B -C 53-B 53mlB
Y,,,(‘)
10.0 11.1
1.2
between the recrystallised matrix (A, B).
At the lower strain level a number of 14 grains out of a total of 16 formed within the triple line region. Nucleation in the matrix was observed only once and then the nucleation occurred at a cluster of 3 particles. This indicates that a strain oft = 0.2 is too low to induce nucleation at one single particle, but nucleation at a cluster of particles is a possibility, as also suggested by Brechet et al. [9]. In addition, 14 grains have grown to such sizes that it was impossible to locate exactly the sites at which they had formed. In the material deformed to t = 0.4 the nucleation pattern was different. All together 86 grains were characterised in detail. Approximately 60% of these grains formed in the matrix and could in most cases be associated with second phase particles. The rest of the grains formed in the grain boundary or triple line region. The triple line region seems to be a more effective nucleation site than the grain boundary region in this case. As regards the nucleation mechanisms operative at the grain boundaries they seem in most cases to be different from the strain
relationships between some of the recrystallised surrounding matrix in Fig. 5(a)
43-A 4&A -c 47-A -C 48-A -C 49-A
IN AN AlSi-ALLOY
grains
and the
Comments TL-PSN TL/GB, Small grain TL/GB, Small grain TL/GB, Growth into matrix A TL/GB, Growth into matrix C TL/GB, Growth into matrix A TL/GB, Growth into matrix C TL, No growth into matrix A TL, No growth into matrix B TL, Growth into matrix C TL/GB, No growth into matrix B TL/GB, Growth into matrix C TL-PSN TL-PSN, Small grain TL-PSN, Small grain TL-PSN TL-PSN
SKJERVOLD
Table 3. The number
of grains
and RYUM: in various
ORIENTATION sites after
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10%
recrystallisation Nucleation site
f = 0.2
Triple line Grain boundary
14
8
(AM < 0.5)
1
2
Grain
(AM < 0.5) 1
22 54
boundary
Matrix
t = 0.40
induced grain boundary migration (SIBM) mechanism first suggested by Bailey and Hirsch [lo]. Typical examples are shown in Figs 2(a) and 5(a). These observations are also in contrast with the various observations by Doherty and co-workers [1 l-131 who demonstrated that the SIBM mechanism could explain practically all nucleation events observed at grain boundaries in moderately deformed aluminium. This difference may be due to the difference in grain size of the material used by Doherty and co-workers and in the present investigation. Doherty and co-workers had to use material with a grain size of approximately 1000 pm in order to obtain acceptable spatial resolution by means of the Kossel line technique they used to characterise the deformed structure. Even Beck and Sperry [14, 151, who first observed the SIBM nucleation mechanism, used a very coarse-grained material. The grain size of the present material was in the range 30-100 pm and is more typical for commercial materials. In such fine-grained material the SIBM nucleation mechanism may be suppressed by other more rapid in such nucleation mechanisms. For instance, material the deformation in the grain boundary region may be more turbulent than in the coarse grained material, thus favouring nucleation by subgrain growth or subgrain rotation, as suggested of by the SIBM by Jones et al. [16] instead mechanism. The presence of second phase particles may also rule out the significance of SIBM compared to a single phase material. It is thus suggested that the importance of SIBM is suppressed by a combination of decreasing grain size and the presence of second phase particles. These observations of the distribution of nucleation sites can now be compared with the measurements of misorientation gradients c( reported in the accompanying paper [7]. From these measurements it appears that a misorientation gradient larger than approximately 0.8”/pm is necessary for a potent recrystallisation site to form. At t = 0.2 such a large gradient is formed nearly exclusively in adjacent to special grain the triple line region, boundaries (AM > 0.5) and at clusters of particles in the matrix.
Table 4. Orientation
relationships between recrystallised and the deformed matrix
Category (I) Parent orientation (II) (I I I)-rotation (III) Random
grains
f = 0.20 [%I
t = 0.40 [%I
40 25 40
17 54 29
0
0
5
10 15 20 25 30 35 40 45 50 55 60 Rotation about
[“I
Fig. 6. Frequency distribution of rotation angles about a (Ill)-axis relating each new grain with its surrounding matrix which has been deformed to t = 0.4 prior to annealing.
The most favoured nucleation sites in material deformed to 6 = 0.40 are still within the regions having a misorientation gradient larger than the critical a value stated for material deformed to t = 0.20. The sites can thus be arranged in the following sequence: the grain interior (M), the grain boundary regions between grains with an individual difference in the Taylor factors exceeding approximately 0.5 (GB, AM > 0.5), and finally regions adjacent to large second phase particles (M-P and GB-P). In material deformed to t = 0.40, the vast majority of the recrystallised grains was probably nucleated at particles (PSN). 3.2. Orientation relationships between recrystallised grains and the deformed matrix The orientation relationship classified into three categories:
observed
(I) low angle orientation relationship (II) a (111) rotation axis relationship; (III) a random orientation relationship.
can
be
(0 < 15”);
The frequency of these different types of relationships is shown in Table 4 for the two levels of straining. In the following a short discussion of each of these categories is given. Typical examples of group I are grain 6 and 7 in Figs 4(a) and (d). The orientations of the recrystallised grains are seen to be within the spread of the orientation of the surrounding matrix and approximately in the range 10-15”. Due to these low orientation differences these recrystallised grains may be considered to be large, strain-free subgrains. These grains resemble the “island grains” observed by Bellier and Doherty [17] in pure aluminium compressed by 20% and annealed at 328°C. The mechanism by which these grains are formed is unclear and Bellier and Doherty suggest that subgrain coalescence may have been operative in these cases. In the present work it appears to be a (11 l)-axis rotation of the deformed lattice which develops subgrain nuclei with sufficient misorientation to sustain continued growth. A rather frequent orientation relationship found was for the recrystallised grain to have one
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(11 I)-axis in common with the surrounding matrix, Table 4. Such a relationship was most easily discovered when continuous scans were run in the matrix. One typical example is grain 9 in Figs 4(a) and (b) with the corresponding orientation relationship given in Fig. 4(e). As can be seen there is a rotation in the deformed matrix around a (11 I)-axis and the orientation of the recrystallised grain is found by an extension of this matrix rotation. These results also support the early work by Belher and Doherty [ 171 where they reported that transition bands misoriented by rotation about a common (11 l)-axis form new grains most readily during annealing of moderately deformed aluminium. In a more recent study by Ardakani and Humphreys [18] it was reported that recrystallised grains related to the deformed matrix by rotations of 35545” about the (1 1 1)-axes were frequently observed upon annealing of particle containing aluminium single crystals. The origin of this particular orientation relationship between recrystallised grains and the surrounding matrix was, however, not clear for the above authors. The observations of this particular orientation relationship observed by the above authors and in the present investigation becomes of particular importance when analysed in connection with the results obtained in the accompanying paper [7]. In that paper it is shown that the rotation of the lattice around a (11 I)-axis occurrs particularly frequently during compression, and misorientation gradients of 0.8”/~m or higher are observed quite frequently in relation to these rotations. A very likely explanation for these observations is that the formation of recrystallised grains occurs preferentially in just these regions and the orientation of the recrystallised grains with respect at the surrounding matrix is obtained by in the deformed an extension of the (1 1 1)-rotation matrix. This observation thus gives support to the oriented nucleation hypothesis of texture formation [19]. It must be added, however, that the magnitude of the rotation about the (Ill)-axis was rarely observed to exceed 20” in the deformed matrix [7], while the corresponding rotation of the recrystallised grain with respect to the surrounding matrix varied from 20 to 60” with the highest frequency between 35 and 45”, see Fig. 6. It thus seems that the rotation of the lattice produced during plastic deformation is too low to account fully for the observed rotation of the recrystallised grains. However, the discrepancy between the observed magnitudes of these two rotations is likely to be due to the nature of the experimental technique used in these measurements. The as-deformed structure was examined by a selection of random scans in the SEM, while the examination of the partially recrystalhsed material represents a selection of scans where the lattice curvature was particularly strong and a nucleation event had consequently taken place. Regions with strong curvature may have such low densities that the
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probability of finding them by random scanning is very low. Alternatively, these regions may be so small that they are below the spatial resolution limit of the EBSP-technique used, which is typically - 1 pm. Such regions can, for instance, be the inner part of the deformation zone around large second phase particles. Humphreys [20] has reported misorientations of up to 40” at -2 pm diameter particles in aluminium after a straining of E = 0.40. In the material used in the present investigation the mean particle diameter was -4.5 pm and high angles of misorientation are thus expected to form around such particles. Finally it should be pointed out that after t = 0.40 the majority region within the deformed matrix with a cumulative lattice curvature had a rotation axis different from the (11 I )-axis, see Figs 16 and 17 in [7]. These regions seem not to be as active as nucleation sites as the regions with the (lll)rotation. This indicates that a cumulative rotation around a (111 )-axis occurs more frequently than expected on a statistical basis and that regions with such a rotation grow more easily into recrystallised grains than regions with rotation around other axes, i.e. the oriented growth theory. This theory simply assumes that particular coincidence orientation relationships, such as 38”( 11 l), has a preferential growth potential. If some relaxation from the exact coincidence relationship is allowed and considering some experimental scatter, there is some support to the oriented growth theory in the present results. These observations must, however, be treated with some caution due to the spreads of orientation within the matrix surrounding the recrystalhsed grains, see grain 53 in Table 2. In local regions at the interface between the recrystallised grain and the surrounding matrix there are obviously orientation relationships along the boundary which do not exactly fulfil the requirements for selective growth (- 40”( 111)). How this affects the mobility of the grain boundary is beyond the scope of this work. Thus, the preferred selection of recrystallised grain of the (111) type is due in part to the easy formation of these sites in the deformed matrix and also to their preferential growth into recrystallised grains. The requirements in the oriented growth and in the oriented nucleation theory are satisfied simultaneously by this, i.e. a compromise between the frequency of available nuclei and their respective growth velocities. In the material deformed to t = 0.20 recrystallised grains were surrounded by 24 matrix grains but with no obvious orientation relationships towards any of them, i.e. category III in Table 4. These observations can be explained by the nucleation having taken place in a section lying either below or above the examined surface. In the planar section there will always be a certain proportion of the recrystallised grains which reveal their crystallographic orientation, but not that of the matrix grain from which they have nucleated.
SKJERVOLD and RYUM:
ORIENTATION
The fraction of category III grains was less in the material deformed to t = 0.40. Moreover, some of these random orientations could be explained by various mechanisms such as annealing twins and PSN. A twin relationship between two recrystallised grains is exactly defined by a 60”(111) as shown in the 111-pole figures in Fig. 4(f), parts (2) and (3). From these examples it also appears to be an orientation relationship to the surrounding matrix for one of the new grains. The deformation zone surrounding large second phase particles in a polycrystalline aluminium deformed in compression has been described by Humphreys and Kalu [21]. They showed that although five slip systems may operate within one grain according to Taylor’s theory, the number of active slip systems in the deformation zone is frequently as low as two. Different slip systems can be active in the deformation zone, thus giving rise to different rotation axes in different parts of the zone. When annealing such material, there is a strong possibility of nucleating grains of different orientation in the deformation zone. This model of Humphreys and Kalu can explain the observation made repeatedly in this investigation of two grains being nucleated at the same second phase particle. Also, the observation that the two grains have a well defined orientation relationship to the matrix is in accordance with this model. 4. CONCLUSIONS (1) The recrystallisation process in moderately deformed material is found to be rather inhomogeneous. Large areas of untransformed matrix exist while in neighbouring areas complete recrystallisation has occurred. On annealing a material deformed to t = 0.2 in compression, the vast majority of the recrystallised grains were formed at GBs, and particularly at TLs. In material deformed to E = 0.4 prior to annealing, the nucleation of recrystallisation was also observed to occur in regions away from the original grain boundaries. Most of these grains were formed at large second phase particles or at clusters of such particles. The recrystallised grains were, in general, formed in regions with misorientation gradients larger than OS”/pm. (2) From detailed characterisation of orientation relationships in the as-deformed and partially recrystallised structures, the majority of the successful nuclei are seen to develop from orientations already present in the deformed material, i.e. oriented nucleation. A rough estimate indicates that close to 90% of the nucleation events in the present work occur by a subgrain coarsening mechanism, coalescence or growth, in local regions of deformation induced misorientation gradients. Nucleation by strain induced boundary migration (SIBM) was observed only occasionally.
RELATIONSHIPS
3419
IN AN AlSi-ALLOY
(3) From the present results it appears that regions misoriented by a rotation about a (11 I )-axis form new grains most readily. This indicates that oriented nucleation becomes the controlling factor with respect to the recrystallisation textures. However, since a majority of the recrystallised grains were close to having a 40”( 111) relationship to the surrounding matrix, a possible influence of oriented growth must also be considered. (4) In the cases where no simple orientation relationship between the recrystallised grains and the surrounding matrix was found, the formation of annealing twins occurred quite frequently (40%). This fraction corresponds to some 109/o of all new orientations formed during recrystallisation. The formation of annealing twins has not been studied in any detail but these orientations are most likely developed by a “growth accident”. REFERENCES 1. J. E. Burke and D. Turnbull, Prog. Met. Phq’s. 3, 220
(1952). 2. B. Hutchinson, 3. J. D. Embury,
Scripta
metall.
27, 1471 (1992).
W. J. Poole and E. Koken,
27, 1465 (1992). 4. D. J. Dingley, Scanning
Scripta
metall.
Electron
Microscopy
2, 569
(1984). 5. J. Hjeien,
PhD Thesis (in Norwegian), University of Trondheim, Norway (1990). 6. J. Hjelen, ICAA3 (edited by L. Arnberg et al.), p. 408. Trondheim, Norway (1992). 7. S. R. Skjervold and N. Ryum, Acta metall. mater. 43, 3159
(1995).
8. F. J. Humphreys
and M. G. Ardakani.
Acta metall.
mater. 42, 749 (1994). 9. Y. Brechet, E. Koken
and J. D. Embury, private communication. 10. J. E. Bailey and P. B. Hirsch. Proc. R. Sot. A267. 11 (1962). . of Metalic Materials 11. R. D. Doherty, Recrystallisation (edited by F. Haessner), Vol. 23, pp. 23-61. Rieder Verlag, Stuttgart (1978). 12. A. B. C. Dadson and R. D. Doherty, Acra metall. 39, 2589
(1991).
13. A. B. C. Dadson and R. D. Doherty, 2053 (1992). 14. P. A. Beck and P. R. Sperry, Trans.
Acfa metall. 40, AIME
180, 240
(1949).
15. P. A. Beck and P. R. (1950). 16. A. R. Jones, B. Ralph 368A, 345 (1979). 17. S. P. Belher and R. D. (1977). 18. M. G. Ardakani and F.
Sperry,
J. appl. Phys.
and N. Hansen, Doherty,
Proc.
Acta metall.
J. Humphreys,
21, 150 R. Sot.
25, 521
Acra metall. 42,
763 (1994).
19. R. D. Doherty, Scripta metall. 19, 927 (1985). 20. F. J. Humphreys, Acta metall. 27, 1801 (1979). 21. F. J. Humphreys and P. N. Kalu, Acta metall. 38, 917 (1990).
1359-6454(95)00413-O
Acta mafer. Vol. 44, No. 8, Pp. 3407-3419, 1996 Copyright 0 1996 Acta Metallurgica Inc. Published by Elsevier Science Ltd Printed in Great Britain. All rights reserved 1359-6454/96
$15.00 + 0.00
ORIENTATION RELATIONSHIPS IN A PARTIALLY RECRYSTALLISED POLYCRYSTALLINE AlSi-ALLOY S. R.
SKJERVOLD’ and N. RYUM’
‘Hydro Aluminium a.s., R & D Centre 2The Norwegian Institute of Technology, Department (Received
16 January
Karmsy, N-4265 Hgvik and of Metallurgy, N-7034 Trondheim,
1995; in revised form
Norway
11 July 1995)
Abstract-A moderately deformed polycrystalline AlSi alloy of initially random texture whose deformation structure is discussed in an accompanying paper was annealed until partial recrystallisation. The nucleation of recrystallised grains occurred preferentially in regions of large misorientation gradients. Particularly potent regions were shown to be the grain boundary and triple line regions, and the regions close to individual second phase particles or clusters of such particles. It was found that a majority of the recrystallised grains were developed in regions where a cumulative rotation of the lattice about a (1 1 1)-axis occurred. The angle of rotation about a (11 I)-axis of grains relative to the surrounding matrix was in the range 3545” for about 50% of these cases. The preferred selection of recrystallised grains having the - 40” (111) orientation relationship is suggested to be due to the easy formation of such sites
in the deformed matrix and also to their preferential growth potential
1. INTRODUCTION
Recrystallisation of a plastically deformed material has for decades been characterised as a nucleation and growth process [l]. There has also, for many years, been objections to this view because the term “nucleation” is used in an unfamiliar way in this case. The major objection has been that the basic concept of thermal fluctuations is ill-defined in this case and, more specifically, that such fluctuations are unlikely to occur at all in plastically deformed lattice. This has the consequence that the recrystallised grain can be formed only by means of an irreversible growth process. During the years much evidence in support of such a view has accumulated: “sites” are claimed to be formed during plastic deformation and can either be considered to belong to the deformed state or can grow out of this state. Thus, the term nucleation is not well founded. However, the term has such a long tradition within this field that continued use seems appropriate [2]. The way in which such “sites” are formed depends, among many other parameters, on the magnitude of the plastic straining and the way the strain is distributed within the material. Embury et al. [3] postulate that two structural elements are created in the deformed state, namely an overall global level of stored energy and grain boundaries capable of thermally activated migration. The last of these two elements thus constitutes the nuclei or “site” for the recrystallised grain. There have been many attempts to characterise these “sites” in greater detail, particularly in recent years when new experimental techniques, capable of 3407
measuring the local texture or lattice curvature, have become available [4-61. In the present work the aim was to identify the sites at which recrystallisation originates, and to relate the orientation of the new grains to the deformed matrix. The deformed structure of the polycrystalline Al-1.2%Si alloy is characterised in an accompanying paper [7]. After the completion of the present investigation a similar investigation by Humphreys and Ardakani [8] has been published. Even though the two investigations address the same general problem, the material studied and the experimental technique used were different. While Humphreys and Ardakani made a very thorough characterisation of the plastic deformation and subsequent recrystallisation of several Al-Si single crystals containing hard and relatively large Si-particle and focused on the behaviour in the neighbourhood of the particles, the present investigation studied a polycrystalline Al-1.2Si alloy and the heterogeneities in the deformed structure in this alloy, not only close to the particles but also adjacent to grain boundaries and triple lines. The two investigations thus complement each other. 2. EXPERIMENTAL
AND RESULTS
The material studied in the present investigation is the same as that described in the accompanying paper [7]. Specimens deformed to t = 0.20 and 0.40 where subsequently annealed in a salt bath at 350°C + 1°C for 60 and 10 s, respectively and then water quenched. This heat treatment resulted in a recrystallised fraction less than 10% and the size of
3408
SKJERVOLD
and RYUM:
ORIENTATION
the recrystallised grains was typically in the range of 5-20 pm. A limited number of grains with a size of 100 pm were also included. Depending on the scale of the microstructure the magnification varied from 300 to 1000 x which correspond to a step size of 0.6-1.1 and 0.2-0.3 pm/pixel, respectively.
RELATIONSHIPS
IN AN AISi-ALLOY
In order to obtain information about the crystallography relationship between the recrystallised grains and the sites in which the grains have formed, the crystallographic orientations of the recrystallised grains were determined at one or a few locations within the grains while the orientation of the surrounding matrix was most often determined by
(a>
(b) Fig. 1. (a) ECC-micrograph showing two recrystallised grains separated from each other by a low angle boundary, and formed in a TL-region of material A deformed to t = 0.20. (b) Orientation relationships between the recrystallised grain (3) and the surrounding matrix (H, I, J), plotted in a standard 11 l-pole figure.
SKJERVOLD
and RYUM:
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IN AN AISi-ALLOY
(a>
Fig. 2. (a) ECC-micrograph showing recrystallised grains of approximately equal orientation formed in a grain boundary region of material A deformed to t = 0.20. (b) Orientation relationships between the recrystallised grain (7) and the surrounding matrix (0, P, Q), plotted in a standard 11 l-pole figure.
a number of discrete measurements at different locations. The EBSP-technique and the presentation of crystallographic results are, in general, the same as those used in Ref. [7]. However, of particular interest in this case is the angle of deviation about an exact (Ill)-axis y’,,, from a rotation between the recrystallised grain and the matrix. The standard notation used in the expression of
orientation structure
relationships is; R~M(B(uvw),
in
partially
Y,,,)
where
recrystallised
Rand Mrefer matrix grain,
to the recrystallised grain and respectively. In the following, a series of examples are presented which are considered to be typical and the crystallographic relationships between the grains and the nucleation sites are pointed out.
3410
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(4
(b) Fig. 3. (a) ECC-micrograph showing a recrystallised grain formed in the grain interior at a cluster of particles of material A deformed to t = 0.20. (b) Orientation relationships between the recrystallised grain (8) and the surrounding matrix (R), plotted in a standard { 11 I}-pole figure.
2.1. Crystallographic aspects of recrystallisation material deformed to E = 0.20
in
Case 1 (Fig. 1). Figure l(a) shows two grains 3 and 3’ formed in a triple line region. These two grains are slightly misoriented with respect to each other; 3-3’(3.2”(221)). The orientation of the recrystallised grain 3 and the surrounding matrix grains labelled H,
Fig
4(a).
ECC-micrograph
showing
recrystallised deformed
I and J are shown in Fig. l(b). As can be seen, the orientation of the matrix grain J scatters strongly. These orientations, which were recorded by a continuous scan in a region just below the recrystallised grains, demonstrate clearly that a large misorientation gradient develops locally even after such low degree of deformations. The rotation of the lattice in this particular matrix
grains formed to t = 0.4.
in the grain
interior
of material
B
SKJERVOLD
and RYUM:
ORIENTATION
A
Fig. 4(b). A schematic illustration of the microstructure in Fig. 4(a) delineating clearly the grain boundaries of the recrystallised grains and some of the positions where orientation measurements were made.
grain
is about
an axis 15” away
from
a ( 111 )-axis
and
20” at a distance of 11 pm. A misorientation gradient of 1.8”/pm thus exists in this area close to the recrystallised grain 3. This recrystallised grain has the following orientation relationship to matrix grains J and I: 33J(30”( 11 l), Y,,, = 15”) and 3%1(56”(111), ‘I’,,, = 4.1”). Case 2 (Fig. 2). In Fig. 2(a), a cluster of 5 or 6 grains has apparently formed along a grain boundary. The possibility that they can have formed along a triple line above or below the section studikd can, of course, not be excluded. These grains have grown to a much larger size than the grains discussed above (notice that a magnification of only 500 x is used in this case). Only very small orientation differences, in the range 14”, were found between these recrystallised grains. The boundaries between
reaches
Fig.
a
4(c).
maximum
standard 111-pole figure showing the axis relationship between the recrystallised grain (4) and surrounding matrix positions (Al, A3, A4) in Fig. 4(a).
(111)-rotation
A
of
RELATIONSHIPS
IN AN AlSi-ALLOY
3411
these recrystallised grains are thus low angle grain boundaries. The orientations of the recrystallised grains and the three neighbouring matrix grains P, Q and 0 are shown in Fig. 2(b). As can be seen. grain 7 has an orientation quite similar to one of the neighbouring matrix grains Q: 7-Q(4”( 123)). The orientation relationship to the two other matrix grains are as follows: 77P(42”( 11 l), ‘I’,,, = 4.8”) and 7-0(42”(310)). It appears from Fig. 2(a) as if the recrystallisation front is moving into matrix grain Q. If these grains have nucleated in a grain boundary region, the mechanism must be different from the classical strain induced boundary migration (SIBM), since bulging of an existing grain boundary into the deformed matrix of a neighbouring matrix grain is not observed in this case. Case 3 (Fig. 3). A possible example of particle stimulated nucleation (PSN) is shown in Fig. 3(a). The recrystallised grain 8 is formed at one particle in a cluster of particles located in the interior of matrix of grain 8 and of the grain R. The orientation surrounding matrix R are plotted in Fig. 3(b). As shown in this figure, there is a wide scatter of orientations in the matrix surrounding the recrystallised grain. As an average orientation relationship between the recrystallised grain and the matrix grain it was found that 8%R(51”( 110)). 2.2. Crystallographic aspects of recrystallisation material deformed to t = 0.40
in
Recrystallised grains appear to have originated from different nucleation sites, as shown in Figs 4(a) and 5(a). It can also be seen that the recrystallised grains often appear in colonies both in the grain boundary region and in the grain interior. The orientation relationships between 86 recrystallised grains and the surrounding matrix has been examined by the EBSP-technique in SEM. The examples presented in the following are considered to be typical of the observations made. It should, however, be noted that the study is restricted to the nucleation within only three different matrix grain orientations. part of the Case 4 (Fig. 4). In the central micrograph, a string of recrystallised grains of approximately equal size can be seen. In addition a number of apparently isolated grains are scattered around in the matrix grain A. While the clustered grains are nearly equiaxed the isolated grains have grown in a more anisotropic way. A schematic illustration of the microstructure and the positions where the orientations have been determined is given in Fig. 4(b). In the following, the orientation relationship between the recrystallised grains and the surrounding matrix will be presented. The orientation of the isolated grain 4 and of the surrounding matrix A is shown in Fig. 4(c). As can be seen, the orientations of the surrounding matrix do not scatter much except for position A3 located just
3412
SKJERVOLD and RYUM:
ORIENTATION
RELATIONSHIPS
IN AN AlSi-ALLOY
(3) Fig. 4(d). Standard I 1 I-pole figures representing the orientation relationships between the recrystallised grains and surrounding matrix in Fig. 4(a). (1) Recrystallised grains having an orientation close to the matrix orientation, and (2, 3) a (1 11)-rotation axis relationship to the surrounding matrix.
below the recrystallised grain. The orientation of position A3 relative to positions A 1 and A4 is formed by rotation of the lattice about an axis lying close to a (11 I)-axis. Also, the recrystallised grain 4 is seen to have one of its (1 11)-axes nearly parallel to this particular (111 )-axis in the matrix. The orientation relationship can be written in the following way: 4-A(46”( 11 l), ‘I’,,, = 3.5”). Four recrystallised grains 5, 6, 7 and 8 are located in the lower left corner of the micrograph. The orientation of the recrystallised grains 6 and 7 and the orientation of the surrounding matrix are shown in Fig. 4(d), part (1). As can be seen, the recrystallised grains have orientations which correspond to the outer spread of matrix orientations. While the rotation axis between the recrystallised grains and the surrounding matrix is almost fixed, the wide
range of the misorientation is noticeable. The orientation relationships are: CA(9%33”(221)) and 7-A(8-29”(221)). The two other grains in this region, grains 5 and 8, have orientation relationships to the matrix similar to grain 4 above, as demonstrated in Fig. 4(d), parts (2) and (3). Notice that the common ( 111)-axis is not the same for the two grains and also different from the one in grain 4. The orientation relationships are: 55A(15”(111), Y,,, = 8.8”) and 8SA(25”(111),Y = 11.1”). The next example deals with the recrystallised grains 9 and 10 in Fig. 4(a). These two grains are close to each other but the orientation relationships between these two grains and the surrounding matrix were found to be very different, as shown in Fig. 4(e), parts (1) and (2). Grain 9 has a (1 11)-axis common
SKJERVOLD
and RYUM:
ORIENTATION
with the surrounding matrix where the orientation relationship can be written in the following way: 9%A(40”(111), Y,,, = 8”). Grain 10 on the other hand has a very low misorientation to the matrix, and can in fact be considered to be a large subgrain. The orientations of the recrystallised grains, 11-18, and of the surrounding matrix are shown in Fig. 4(f), parts (l))(3). The orientations of the deformed matrix do not scatter much. In this case the recrystallised grains appear to have formed on a chain of particles. The orientation relationships between the recrystallised grains 11-18 and the surrounding matrix A are given in Table 1. As can be seen, the average misorientation between the recrystallised grains and the surrounding matrix is quite high (30-59”). Moreover, a majority of the rotation axes between the recrystallised grains and the matrix are only a few degrees away from a (Ill)-axis. Special attention should be given to the orientation of the recrystallised grains 11 and 13 plotted in Fig. 4(f), part (2). As can be seen, grain 13 has almost the same orientation relationship to the matrix as grains 12, 1.5 and 16 shown in Fig. 4(f), part (1). However, grain 13 has one of its (11 l)-axes in common with the neighbouring recrystallised grain 11. The orientation relationship between grain 11 and 13 is: ll-13(59’(111), Y,,, = O.S”). A common (111 )-axis between two recrystallised grains was also found between grains 16 and 17 and between grains 17 and 18, see Fig. 4(f), part (3) for the latter case. The orientation relationships in these cases are: 1617(54”(111), Y,,, = 3.7”) and 17-18(59”(111),Y,,, = 1.3”). A 60” rotation about a (11 I)-axis corresponds to a twin orientation relationship between the two grains.
RELATIONSHIPS
IN AN AlSi-ALLOY
3413
Case 5 (Fig. 5). A triple line region is shown in Fig. 5(a). In Fig. 5(b) the same microstructure is given in a schematic way, clearly delineating the grain boundaries, and the different positions where the orientation was measured. The recrystallised grains are clustered in the TL-region and along the grain boundaries, particularly between matrix grains A and B. Almost all new grains in the TL-region are located in the upper left matrix grain C. The microstructure of the matrix shows some alignment of the deformed structure in grain A and C both of which have the CA -axis nearly parallel to a (1 IO)-axis. On the other hand, the CA-axis of grain B is nearly parallel to the (lOO)-axis. The orientation of the recrystallised grains 40, 41 and 42 and of the surrounding matrix are shown in Fig. 5(c), parts (1) and (2). As shown in Fig. 5(c), part (l), the orientation of matrix grain B changes rapidly in regions close to the recrystallised grains. The orientation relationship between position B3 and B6 in matrix was found to be as follows B3-B6(23”( 11 l), Y,,, = 4.7”). The recrystallised grain 41 is close to having one of its (ill)-axes within the orientation spread of the (11 I)-axes of both matrix A and B as shown in the Ill-pole figure. The more accurate orientation relationships are: 41&4(31”(221)) and 41lB(17”(311)). Grains 40 and 42 have quite different orientation relationships with the surrounding matrix [Fig. 5(c), part (2)]: 40-A(56”( 11 l), Y,,, = 12.8”), 40P B(48”(21 l)), 422A(32’(211)) and 42-B(39”(441)). Moreover, a common (Ill)-axis is found between the grains 40 and 42: 4042(35”( 11 l), Y,,, = 4.2”). In this case the misorientation angle 0 is less than the
Fig. 4(e). Standard 11 l-pole figures representing the orientation relationships between the recrystallised grains and surrounding matrix in Fig. 4(a). (1) Recrystallised grain having a (11 l)-rotation axis relationship, and (2) a parent orientation relationship to the surrounding matrix.
SKJERVOLD
and RYUM:
ORIENTATION
RELATIONSHIPS
IN AN AISi-ALLOY
(3) Fig. 4(f). Standard 11 l-pole figures representing the orientation relationships between recrystallised grains formed at a cluster of particles and the surrounding matrix in Fig. 4(a). (1) Recrystallised grains having a (11 I)-rotation axis relationship to the surrounding matrix. (2, 3) Examples showing a twin orientation relationship between two recrystallised grains.
frequently observed value of approximately 60” for twin orientations. All the grains formed at the grain boundary have a high angle boundary with respect to both matrix grains. They also have orientations differing significantly from the average matrix orientation.
Table 1. Orientation
relationships
Grain
W)
(uvw)
11-A 12-A 13-A 14-A 15-A l&A 17-A 18-A
52 42 44 59 35 30 51 34
111 111 221 II1 111 111 111 331
The orientation relationships between the grains 4653, formed in the TL-region, and the surrounding matrix grains (A, B and C) are presented in Table 2. Except for grains 52 and 53 which have grown mostly into matrix B, the growth of the other grains have been into matrix C. As seen in Table 2, several axes
between some of the recrystallised matrix (M) in Fig. 4(a)
grains and the surrounding Comments
YIII(0) 12.6 9.1 6.9 11.8 14.1 0.5
M-PSN, M-PSN M-PSN M-PSN, M-PSN M-PSN, M-PSN M-PSN,
Common
< 11 I)-axis
to grain
13
Common
(11 I)-axis
to grain
15
Common
< 1 1 1)-z&
to grain
17
Common
(11 l)-axis
to grain
17
SKJERVOLD and RYUM:
Fig. 5(a). ECC-micrograph
ORIENTATION
RELATIONSHIPS
IN AN AlSi-ALLOY
showing recrystallised grains formed in the grain boundary TL-region of material B deformed to t = 0.4.
of rotation between the recrystallised grains and the surrounding matrix were observed. Thus, no preferential rotation axes seem to exist in grains formed in the TL-region. There appears to be some evidence of a correlation between the size of the grains and the orientation relationships to the matrix in which they are embedded. Such correlations are evident for the large grains 49-50, which both have a rotation axis close to (111) relative to matrix C. Grain 49 is highly misoriented with respect to all three matrix grains meeting in the triple junction, but migration of the boundary has occurred preferentially into matrix grain C. This can be interpreted as an effect of preferred growth of specific orientation relationships. Similar behaviour is apparent for grain 50 which is equally misoriented with respect to both matrix grains (B and C). As can be seen, growth of grain 50
region and
occurs into matrix C with a (11 I)-axis rotations relationship. Smaller grains, such as grains 46, 47, 48 and 52, have rotation axes different from the (1 11)-type but the misorientation between these grains and the matrix is also large. It should be noted, however, that these grains have impinged upon each other and thus have limited each others growth. The orientation relationships between the recrystallised grain 53 and the matrix B are presented to two values in Table 2. The first value, 53-B, represents the average misorientation between the recrystallised grain and the surrounding matrix, whereas 53-1B is the relationship between the grain and a single position in the matrix close to the grain boundary between the grain and the matrix. This demonstrates that the e( 111)-rotation relationship between a recrystallised grain and the matrix may
Matrix C
Matrix
Fig. 5(b). A schematic illustration boundaries
of the microstructure
of the matrix
grains
3415
in Fig. 5(a) delineating clearly (bold line) and the recrystallised grains.
the grain
3416
SKJERVOLD
and RYUM:
ORIENTATION
RELATIONSHIPS
Fig. 5(c). (1, 2) Standard 111-pole figures showing the orientation relationship grains (40,41,42) formed in the GB-region and the surrounding
exist only locally. In a deformed matrix with great turbulence, such as the TL-region, the orientation relationships may cover a wide range of misorientations and rotation axes as well.
3. DISCUSSION
3.1. Nucleation sites
It is convenient to distinguish between three types of nucleation sites in this material under the experimental condition described: within the matrix (M), in the grain boundary region (GB) and in the triple line region (TL). The efficiency of these different types of sites was different, and varied with the amount of overall uniform plastic deformation within the range studied (t = 0.2-0.4). The efficiency of the different sites for nucleation of recrystallisation are summarised in Table 3. At both strains a total area of approximately 1.2 mm* was investigated in the SEM by means of the EBSP-technique. Table 2. Orientation Grain
W)
33 43 37 53 43 44 26 33 36 55 43 47 3x 36 25 36 38
310 532 432 441 441 331 441 110 221 Ill 331 Ill 421 100 310 221 Ill
-B -c
5&B -C 51&C 52-B -C 53-B 53mlB
Y,,,(‘)
10.0 11.1
1.2
between the recrystallised matrix (A, B).
At the lower strain level a number of 14 grains out of a total of 16 formed within the triple line region. Nucleation in the matrix was observed only once and then the nucleation occurred at a cluster of 3 particles. This indicates that a strain oft = 0.2 is too low to induce nucleation at one single particle, but nucleation at a cluster of particles is a possibility, as also suggested by Brechet et al. [9]. In addition, 14 grains have grown to such sizes that it was impossible to locate exactly the sites at which they had formed. In the material deformed to t = 0.4 the nucleation pattern was different. All together 86 grains were characterised in detail. Approximately 60% of these grains formed in the matrix and could in most cases be associated with second phase particles. The rest of the grains formed in the grain boundary or triple line region. The triple line region seems to be a more effective nucleation site than the grain boundary region in this case. As regards the nucleation mechanisms operative at the grain boundaries they seem in most cases to be different from the strain
relationships between some of the recrystallised surrounding matrix in Fig. 5(a)
43-A 4&A -c 47-A -C 48-A -C 49-A
IN AN AlSi-ALLOY
grains
and the
Comments TL-PSN TL/GB, Small grain TL/GB, Small grain TL/GB, Growth into matrix A TL/GB, Growth into matrix C TL/GB, Growth into matrix A TL/GB, Growth into matrix C TL, No growth into matrix A TL, No growth into matrix B TL, Growth into matrix C TL/GB, No growth into matrix B TL/GB, Growth into matrix C TL-PSN TL-PSN, Small grain TL-PSN, Small grain TL-PSN TL-PSN
SKJERVOLD
Table 3. The number
of grains
and RYUM: in various
ORIENTATION sites after
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10%
recrystallisation Nucleation site
f = 0.2
Triple line Grain boundary
14
8
(AM < 0.5)
1
2
Grain
(AM < 0.5) 1
22 54
boundary
Matrix
t = 0.40
induced grain boundary migration (SIBM) mechanism first suggested by Bailey and Hirsch [lo]. Typical examples are shown in Figs 2(a) and 5(a). These observations are also in contrast with the various observations by Doherty and co-workers [1 l-131 who demonstrated that the SIBM mechanism could explain practically all nucleation events observed at grain boundaries in moderately deformed aluminium. This difference may be due to the difference in grain size of the material used by Doherty and co-workers and in the present investigation. Doherty and co-workers had to use material with a grain size of approximately 1000 pm in order to obtain acceptable spatial resolution by means of the Kossel line technique they used to characterise the deformed structure. Even Beck and Sperry [14, 151, who first observed the SIBM nucleation mechanism, used a very coarse-grained material. The grain size of the present material was in the range 30-100 pm and is more typical for commercial materials. In such fine-grained material the SIBM nucleation mechanism may be suppressed by other more rapid in such nucleation mechanisms. For instance, material the deformation in the grain boundary region may be more turbulent than in the coarse grained material, thus favouring nucleation by subgrain growth or subgrain rotation, as suggested of by the SIBM by Jones et al. [16] instead mechanism. The presence of second phase particles may also rule out the significance of SIBM compared to a single phase material. It is thus suggested that the importance of SIBM is suppressed by a combination of decreasing grain size and the presence of second phase particles. These observations of the distribution of nucleation sites can now be compared with the measurements of misorientation gradients c( reported in the accompanying paper [7]. From these measurements it appears that a misorientation gradient larger than approximately 0.8”/pm is necessary for a potent recrystallisation site to form. At t = 0.2 such a large gradient is formed nearly exclusively in adjacent to special grain the triple line region, boundaries (AM > 0.5) and at clusters of particles in the matrix.
Table 4. Orientation
relationships between recrystallised and the deformed matrix
Category (I) Parent orientation (II) (I I I)-rotation (III) Random
grains
f = 0.20 [%I
t = 0.40 [%I
40 25 40
17 54 29
0
0
5
10 15 20 25 30 35 40 45 50 55 60 Rotation about
[“I
Fig. 6. Frequency distribution of rotation angles about a (Ill)-axis relating each new grain with its surrounding matrix which has been deformed to t = 0.4 prior to annealing.
The most favoured nucleation sites in material deformed to 6 = 0.40 are still within the regions having a misorientation gradient larger than the critical a value stated for material deformed to t = 0.20. The sites can thus be arranged in the following sequence: the grain interior (M), the grain boundary regions between grains with an individual difference in the Taylor factors exceeding approximately 0.5 (GB, AM > 0.5), and finally regions adjacent to large second phase particles (M-P and GB-P). In material deformed to t = 0.40, the vast majority of the recrystallised grains was probably nucleated at particles (PSN). 3.2. Orientation relationships between recrystallised grains and the deformed matrix The orientation relationship classified into three categories:
observed
(I) low angle orientation relationship (II) a (111) rotation axis relationship; (III) a random orientation relationship.
can
be
(0 < 15”);
The frequency of these different types of relationships is shown in Table 4 for the two levels of straining. In the following a short discussion of each of these categories is given. Typical examples of group I are grain 6 and 7 in Figs 4(a) and (d). The orientations of the recrystallised grains are seen to be within the spread of the orientation of the surrounding matrix and approximately in the range 10-15”. Due to these low orientation differences these recrystallised grains may be considered to be large, strain-free subgrains. These grains resemble the “island grains” observed by Bellier and Doherty [17] in pure aluminium compressed by 20% and annealed at 328°C. The mechanism by which these grains are formed is unclear and Bellier and Doherty suggest that subgrain coalescence may have been operative in these cases. In the present work it appears to be a (11 l)-axis rotation of the deformed lattice which develops subgrain nuclei with sufficient misorientation to sustain continued growth. A rather frequent orientation relationship found was for the recrystallised grain to have one
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SKJERVOLD
and RYUM:
ORIENTATION
(11 I)-axis in common with the surrounding matrix, Table 4. Such a relationship was most easily discovered when continuous scans were run in the matrix. One typical example is grain 9 in Figs 4(a) and (b) with the corresponding orientation relationship given in Fig. 4(e). As can be seen there is a rotation in the deformed matrix around a (11 I)-axis and the orientation of the recrystallised grain is found by an extension of this matrix rotation. These results also support the early work by Belher and Doherty [ 171 where they reported that transition bands misoriented by rotation about a common (11 l)-axis form new grains most readily during annealing of moderately deformed aluminium. In a more recent study by Ardakani and Humphreys [18] it was reported that recrystallised grains related to the deformed matrix by rotations of 35545” about the (1 1 1)-axes were frequently observed upon annealing of particle containing aluminium single crystals. The origin of this particular orientation relationship between recrystallised grains and the surrounding matrix was, however, not clear for the above authors. The observations of this particular orientation relationship observed by the above authors and in the present investigation becomes of particular importance when analysed in connection with the results obtained in the accompanying paper [7]. In that paper it is shown that the rotation of the lattice around a (11 I)-axis occurrs particularly frequently during compression, and misorientation gradients of 0.8”/~m or higher are observed quite frequently in relation to these rotations. A very likely explanation for these observations is that the formation of recrystallised grains occurs preferentially in just these regions and the orientation of the recrystallised grains with respect at the surrounding matrix is obtained by in the deformed an extension of the (1 1 1)-rotation matrix. This observation thus gives support to the oriented nucleation hypothesis of texture formation [19]. It must be added, however, that the magnitude of the rotation about the (Ill)-axis was rarely observed to exceed 20” in the deformed matrix [7], while the corresponding rotation of the recrystallised grain with respect to the surrounding matrix varied from 20 to 60” with the highest frequency between 35 and 45”, see Fig. 6. It thus seems that the rotation of the lattice produced during plastic deformation is too low to account fully for the observed rotation of the recrystallised grains. However, the discrepancy between the observed magnitudes of these two rotations is likely to be due to the nature of the experimental technique used in these measurements. The as-deformed structure was examined by a selection of random scans in the SEM, while the examination of the partially recrystalhsed material represents a selection of scans where the lattice curvature was particularly strong and a nucleation event had consequently taken place. Regions with strong curvature may have such low densities that the
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IN AN AlSi-ALLOY
probability of finding them by random scanning is very low. Alternatively, these regions may be so small that they are below the spatial resolution limit of the EBSP-technique used, which is typically - 1 pm. Such regions can, for instance, be the inner part of the deformation zone around large second phase particles. Humphreys [20] has reported misorientations of up to 40” at -2 pm diameter particles in aluminium after a straining of E = 0.40. In the material used in the present investigation the mean particle diameter was -4.5 pm and high angles of misorientation are thus expected to form around such particles. Finally it should be pointed out that after t = 0.40 the majority region within the deformed matrix with a cumulative lattice curvature had a rotation axis different from the (11 I )-axis, see Figs 16 and 17 in [7]. These regions seem not to be as active as nucleation sites as the regions with the (lll)rotation. This indicates that a cumulative rotation around a (111 )-axis occurs more frequently than expected on a statistical basis and that regions with such a rotation grow more easily into recrystallised grains than regions with rotation around other axes, i.e. the oriented growth theory. This theory simply assumes that particular coincidence orientation relationships, such as 38”( 11 l), has a preferential growth potential. If some relaxation from the exact coincidence relationship is allowed and considering some experimental scatter, there is some support to the oriented growth theory in the present results. These observations must, however, be treated with some caution due to the spreads of orientation within the matrix surrounding the recrystalhsed grains, see grain 53 in Table 2. In local regions at the interface between the recrystallised grain and the surrounding matrix there are obviously orientation relationships along the boundary which do not exactly fulfil the requirements for selective growth (- 40”( 111)). How this affects the mobility of the grain boundary is beyond the scope of this work. Thus, the preferred selection of recrystallised grain of the (111) type is due in part to the easy formation of these sites in the deformed matrix and also to their preferential growth into recrystallised grains. The requirements in the oriented growth and in the oriented nucleation theory are satisfied simultaneously by this, i.e. a compromise between the frequency of available nuclei and their respective growth velocities. In the material deformed to t = 0.20 recrystallised grains were surrounded by 24 matrix grains but with no obvious orientation relationships towards any of them, i.e. category III in Table 4. These observations can be explained by the nucleation having taken place in a section lying either below or above the examined surface. In the planar section there will always be a certain proportion of the recrystallised grains which reveal their crystallographic orientation, but not that of the matrix grain from which they have nucleated.
SKJERVOLD and RYUM:
ORIENTATION
The fraction of category III grains was less in the material deformed to t = 0.40. Moreover, some of these random orientations could be explained by various mechanisms such as annealing twins and PSN. A twin relationship between two recrystallised grains is exactly defined by a 60”(111) as shown in the 111-pole figures in Fig. 4(f), parts (2) and (3). From these examples it also appears to be an orientation relationship to the surrounding matrix for one of the new grains. The deformation zone surrounding large second phase particles in a polycrystalline aluminium deformed in compression has been described by Humphreys and Kalu [21]. They showed that although five slip systems may operate within one grain according to Taylor’s theory, the number of active slip systems in the deformation zone is frequently as low as two. Different slip systems can be active in the deformation zone, thus giving rise to different rotation axes in different parts of the zone. When annealing such material, there is a strong possibility of nucleating grains of different orientation in the deformation zone. This model of Humphreys and Kalu can explain the observation made repeatedly in this investigation of two grains being nucleated at the same second phase particle. Also, the observation that the two grains have a well defined orientation relationship to the matrix is in accordance with this model. 4. CONCLUSIONS (1) The recrystallisation process in moderately deformed material is found to be rather inhomogeneous. Large areas of untransformed matrix exist while in neighbouring areas complete recrystallisation has occurred. On annealing a material deformed to t = 0.2 in compression, the vast majority of the recrystallised grains were formed at GBs, and particularly at TLs. In material deformed to E = 0.4 prior to annealing, the nucleation of recrystallisation was also observed to occur in regions away from the original grain boundaries. Most of these grains were formed at large second phase particles or at clusters of such particles. The recrystallised grains were, in general, formed in regions with misorientation gradients larger than OS”/pm. (2) From detailed characterisation of orientation relationships in the as-deformed and partially recrystallised structures, the majority of the successful nuclei are seen to develop from orientations already present in the deformed material, i.e. oriented nucleation. A rough estimate indicates that close to 90% of the nucleation events in the present work occur by a subgrain coarsening mechanism, coalescence or growth, in local regions of deformation induced misorientation gradients. Nucleation by strain induced boundary migration (SIBM) was observed only occasionally.
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(3) From the present results it appears that regions misoriented by a rotation about a (11 I )-axis form new grains most readily. This indicates that oriented nucleation becomes the controlling factor with respect to the recrystallisation textures. However, since a majority of the recrystallised grains were close to having a 40”( 111) relationship to the surrounding matrix, a possible influence of oriented growth must also be considered. (4) In the cases where no simple orientation relationship between the recrystallised grains and the surrounding matrix was found, the formation of annealing twins occurred quite frequently (40%). This fraction corresponds to some 109/o of all new orientations formed during recrystallisation. The formation of annealing twins has not been studied in any detail but these orientations are most likely developed by a “growth accident”. REFERENCES 1. J. E. Burke and D. Turnbull, Prog. Met. Phq’s. 3, 220
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19. R. D. Doherty, Scripta metall. 19, 927 (1985). 20. F. J. Humphreys, Acta metall. 27, 1801 (1979). 21. F. J. Humphreys and P. N. Kalu, Acta metall. 38, 917 (1990).