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GRAIN BOUNDARY DISTRIBUTION IN MATERIALS SUSCEPTIBLE TO ANNEALING
TWINNING
V. Gertsman, R. Valiev, V. Danilenko, O. Mishin
To cite this version:
V. Gertsman, R. Valiev, V. Danilenko, O. Mishin. GRAIN BOUNDARY DISTRIBUTION IN MA-
TERIALS SUSCEPTIBLE TO ANNEALING TWINNING. Journal de Physique Colloques, 1990, 51
(C1), pp.C1-151-C1-154. �10.1051/jphyscol:1990122�. �jpa-00230280�
GRAIN BOUNDARY DISTRIBUTION IN MATERIALS SUSCEPTIBLE TO ANNEALING TWINNING V.Yu. GERTSMAN, R.Z. VALIEV, V.N. DANILENKO and O.V. MISHIN
Institute of Metals Superplasticity Problems, URSS Academy of Sciences, ul.Khalturina 39, Ufa 450001, U.R.S.S.
Abstract
-
Grain boundary distributions in various materials with low and medium stalking fault energy subjected to different recrystallization annealings were investigated. It was established that grain boundary spectra in all these materials are similar.The main feature of these spectra is that most of the boundaries are 23" boundaries. A model grain boundary spectrum Has obtained by computer simulation based on the assumption that 23 boundaries formed as a result of multiple twinning played a dominant role in the grain boundary ensemble. The model boundary distribution agrees well with the experimental data. The conclusion is made that in materials susceptible to annealing twinning most (or almost all) of the grain boundaries are 23 boundaries and the di- stribution of these boundaries characterizes some stable state to- ward which the grain boundary ensemble tends during its evolution.
Tn recent years it has been experimentally shown that in recrystallized f.c.c.
metals and alloys with no high stalking fault energy a significant fraction of grain boundary spectrum is occuphed by boundaries described by reciprocal density of coincidence sites 2 3 Cl-53. Such boundaries can be formed as a result of multiple twinning, k.e. repeated interaction of U boundaries with each other and with other 23 boundaries within one initial grain.
Experimental data on statistics of grain boundaries in materials susceptible to annealing twinning available up to now are summarized in the Table.
Represented here are only studies on recrystallized materials and statistics resulting from detailed studies of boundary misorientation distribution obtained by various diffraction techniques. A detailed description of these techniques can be found in related papers.
Article published online by EDP Sciences and available at http://dx.doi.org/10.1051/jphyscol:1990122
Cl-152 COLLOQUE DE PHYSIQUE
Table of the fractions C % ) of different types of grain boundaries in some f.c.c. metals and alloys
*
Gertsman V.Yu:, Danilenko V.N., and Valiev R.Z., submitted to Metallofizika (19891, in Russian**
Gertsman V.Yu., Alyabyev V.M., Mishin O.V., and Ponomareva E.G., submitted to Metallofizika (19891, in RussianBoundary type
Z 3 29 2 2 7 2 8 1 other
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This paper presents detailed data on grain boundary misorientation distribution with the Ni-Cr alloy used as an example. Grain misorientations were measured using electron selected area diffraction with an accuracy of about 1°. Figs. 1, a, b show the distributions of misorientation angles and axes for specimens with average grain sizes- of 6 and 1 3 y m (annealed at 993 K and 1273 K respectively). It can be seen that for both materials grain boundary distributions are practically the same: hhe boundary spectrum has a discrete character with maxima associated with 2 3 boundaries.
To analyze. the experimental results of grain boundary statistics in such materials, we must know which grain boundary spectra are realized during the annealing twinning. To this end the grain boundary spectrum and its evolution during twinning were computer simulated C53.
A single step of the simulated process is the appearance of an area in one of the grains, whose lattice is twin misoriented against the grain lattice.
The formation of such twin area results not only in a new boundary 2 3 but also in the change of misorientations of all boundaries in this area. If after some time, part of this area will again undergo twin transformation, this particular part will be double twinned in relation to the initial grain orientation. Appearance of another twin here will mean triple twinning etc.
and this process is actually multiple twinning. During this process repeated interaction of primary twin boundaries L 3 and other 23" boundaries takes place. Between the steps of format,ion of new twin orientations in one grain the same process may occur in other grains too. This process is supposed to be random and formation of new twins in variuuo regions of the structure does not seem to be mutually correlated.
Simulated grain boundary
spectrum resulting from mu1 tiple twinning M a t e r i a l s
The simulation showed that grain boundary distribution has a quasi-stationary character, i.e. boundary spectrum practically does not depend on the number of simulation steps already after a small amount of steps (about 20).
A typical simulated spectrum is given in the last, column of the Table (see above). Fig. lc shows one of the simulated disorientation distributions.
Ni
Data presented in the Table show that grain boundary spectra are similar in materials with different chemical composition subjected to different
References Type 304
stainless steel
C21
41 9 6 2 4 2
Average grain size, y m Ni-Cr
alloy
C33
42 15 4
-
39 1000
Ni-Cr alloy
100 6
C41 34.5
7.5 5 3 50.0
Type 316 stainless
steel
13
*
34.6 7.4 3.1
-
54.9
60-70
kk
34.6 6.0 5.3 3.8 50.4
C53
42.8 17.8
7.4 5.0 27.1
3
Cl-154 COLLOQUE DE PHYSIQUE
treatments. Comparison of the experimental data with the results of computer simulation indicates a good agreement between experimental and simulated grain boundary spectra [see the Table and Fig.1).
All the materials presented in the Table are capable of annealing twinning and they have structures formed at different stages of recrystallization.
Thus in all cases the structure was formed under internal stimuli: strain energy stored in the material and tendency to decrease the grain boundary total energy. The set of data brings us to a conclusion that there is a certain stable grain boundary misorientation distribution in all materials susceptible to annealing twinning. Some remarks can be made on a number of quantitative differences between experimental and simulated xrain boundary spectra. In the simulated distribution the fraction of X 3 boundaries, particularly with n>l, was found to be slightly elevated. This could be due to various reasons. Firstly, experimental errors hinder absolutely adequate determination of all boundary types and the Table presents only the l o w e ~ limits of the ~ 3 " boundary fraction. Hence, every experimental class of U boundaries in the Table might be enlarged at the expense of the group called
"other".
Moreover, many "other" boundaries may also be ~7~ boundaries with n values exceeding those presented in the Table. The following speaks in favour of this assumption. Computer calculations of 23" misorientations C6,73 showed that they rather densely cover the entire space of misorientation vectors both on angles and axes alre d at n<9 which means that for any misorientation a proximate E3' Ymtsorientation may be found. Generally speaking, the greater L value was set in the investigations C2-511 the greater number of 23 boundary classes were found to exist.
Another cause of divergency between experimental and simulated boundary spectra may lie in the fact that grain misorientations differ from the ideal due to dislocations in grain boundaries and within the grains: there are neither ideal crystals nor "ideal polycrystals". Deliberate introduction og deviation from the ideal in simulation results in the decrease of S 3 boundary fractions.
There can be still another, may be the m ~ s t important cause of divergency.
In simulation all boundaries belong to I3 class but in real material other misorientations may also exist. During crystallization and recrystallization the mutual misorientation of nuclei can be random. Boundary misorientations produced when these nuclei meet will bg inherited and bring into grain boundary spectrum deviations from the 2 3 boundary spectrum even if these misorientations change in the course of further structural evolutions.
In general, comparison of exp~rimental data and computer simulation results shows the dominant role of 2 3 boundaries formed as a result of multiple twinning in grain boundary spectrum in materials susceptible to annealing twinning. In structural evolution the boundary distribution is changed in a self-similar manner, i.e. the boundary spectrum has a quasi-stationary character. Such a stable state of boundary spectrum may be provided, for example, by the balance of grain boundary coalescence and dissociation processes.
REFERENCES
1. Sukhomlin, G.D. and Andreeva, A.V., Phys. Stat. Sol. (a) 78 (1983) 333.
2. Lim, L.C. and Raj, R., Acta Metall.
32
(1984) 1177.3. Don, J. and Majumdar, S., Acta Metall.
2
(1986) 961.4. Gertsman, V.Yu., Danilenko, V.N., and Valiev, R.Z., Phys. Metal. Metallogr.
(USSR) 68 (1989) 348.
5 . Gertsman, V.Yu. and Mishin, O.V., Metallofizika (USSR)
11
N4 (1989) 26.6. Andreeva, A.V. and Firsova, A.A., Poverkhnost' (USSR) N6 (1987) 149.
7. Gertsman V.Yu., Pshenichnyuk A.I., and Valiev R.Z., to be published in STRUCTURE OF INTERNAL INTERFACES (in Russian), Nauka Publ., Moscow (1989).
