- Email: [email protected]
Texture analysis of L12 Al66Mn3Ti25 intermetallic compound deformed at high temperatures
Scripta Metallurgica et Materialia, Vol. 30, No. 8, pp. 1071-1072, 1994 Copyright © 1994 Elsevier Science Ltd Printed in the USA. All rights reserved 0956-716X/94 $6.00 + .00
Pergamon
TEXTURE ANALYSIS OF Llz Al6eMmTi2s INTERMETALLIC C O M P O U N D DEFORMED A T HIGH T E M P E R A T U R E S T.Suzuki*, M.Dahms**, T.Takabayashi* and H.Fukutomi* Department of Mechanical Engineering and Materials Science, Faculty of Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Institute fur Werkstofforschung, GKSS-Forschungszentrum, Geesthacht GmbH Max-Planck-Strasse, D-21502 Geesthacht, Germany (Received (Revised
November 18, 1993) December 29, 1993) Introduction
The intermetallic compound AI3Ti with excellent oxidation resistance is known as a candidate material for the high temperature structural use. Its industrial application, however, has not been achieved because of limited ductility at ambient temperature. Recently, several researchers reported that the transformation of crystal structure from D0z2 to L12 by the substitution of third elements such as Cr, Mn and Ni for AI was effective in improving the deformability of the compound[I-3]. Since mechanical properties are affected by crystal structure as well as microstructure, it is important to comprehend the factors which control the microstructure of this kind of intermetallic compound. The authors investigated the deformation characteristics and microstructure change of Llz A166MngTi25 by high temperature uniaxial compression tests[4]. Three regimes appeared in the dynamic restoration process, depending on the flow stress level. When the flow stress is higher or lower, dynamic recovery is the dominant dynamic restoration process, while dynamic recrystallization is predominant at the intermediate stresses. Texture examination showed that three kinds of fiber texture developed corresponding to the difference in the dynamic restoration process. Evaluation of the pole figure suggested that the main components of the texture are close to {110}, {110}+{111} and {111} for higher, intermediate and lower stress regimes, respectively. In this paper, crystallographic characteristics of the textures formed in the lower and higher stress regimes are examined by calculating the orientation distribution function, in order to understand the mechanism of texture formation. Experimental Procedure Materials were produced by arc melting. The nominal composition is AI66MngTi25. X-ray diffraction showed that the material was almost a single phase of L12 structure. Since the material included numerous pores as in the cast state, isothermal forging was conducted at 1323K at a strain rate of 1.0xl0- 4 s- ~ up to the true strain of -0.6. After forging, the material was annealed at 1473K for 24h. Specimens with the approximate dimensions of 6x6xl0mm 3 were made by mechanical cutting. Constant true strain rate compression tests were conducted in vacuum. The temperatures and strain rates for the higher and the lower stress regimes are 1173K and 1.0x10-3s-~ and 1473K and 1 . 0 x l 0 - 4 s - ' , respectively. The specimens were deformed up to the strain of -1.6, followed by furnace cooling. The cooling rates from 1473K to 1273K and from 1273K to 1073K were 2Ks- ~ and 1Ks- ', respectively. About one-half in thickness of the deformed specimen was removed by mechanical and electrolytic polishing, in order to examine the texture and microstrueture on the mid-plane section. Texture measurements were made by the Schulz reflection method using nickel filtered Cu K~ radiation. Based on {111}, {110} and {100} pole figures, a crystallite orientation distribution function was calculated. "/'he pole density distributions were examined by inverse pole figures.
1071
1072
TEXTURE ANALYSIS
Vol.
30, No.
8
Results and Discussion Figure 1 is the inverse pole figure of the specimen deformed to a strain of -1.6 in the higher stress regime. The flow stress at -0.02 strain was 277MPa. The pole density distribution is expressed by using the mean pole density as a unit. Evidently, the pole density gradually increases toward {110} and the maximum pole density exists on {110}. Brown et al[5] reported that the active slip system of this compound in the temperature range between 77K and 1073K was {111}<110> which is the same as for FCC metals. The main component of the texture formed by uniaxial compression of FCC metals was reported as {110}[6], which is the same as in Fig. 1. Figure 2 shows the inverse pole figure for the deformation up to -1.6 strain in the lower stress regime. The flow stress at -0.02 was 19MPa. Different from Fig. 1, the maximum pole density is seen at {111}. Thus, the main components of the texture for the higher and the lower stress regimes are determined as {110} and {111}, respectively. II1
II1
001
001
011
Fig.1 Inverse pole figure of the specimen deformed in the higher stress regime. (1173K, 1.0xl0- as- 1, -1.6 strain)
011
Fig.2 Inverse pole figure of the specimen deformed in the lower stress regime. (1473K, 1.0xl0- 4s- 1, -1.6 strain)
The generation of new grains as well as the trace of grain boundary migration were not observed in the deformation at the higher stress regime[4]. The coincidence of the main component of the fiber textures between Als6MngTi25 and FCC metals together with the results of microstmctural observation indicates that the texture shown in Fig.1 is formed by the {111}<110> slip. As reported in the previous paper[4], no obvious difference was observed between the higher and lower stress regimes in the size of grains measured parallel and normal to the compression axis. The difference of microstructure between these two regimes was the shape of crystal grains. The grains observed in the longitudinal sections were lenticular in the higher stress regime, while they were rather rectangular in the lower stress regime[5]. This difference indicated that grain boundary migration occurred extensively in the lower stress regime. It was suggested that the accumulation of pole density toward {111} might occur similarly to the overshooting which was observed in the case of {111}<110> slip of FCC single crystals, if the grain boundary migration reduced the constraint between crystal grains[4]. However, the fact that the main component exists almost exactly at {111} indicates that the above mechanism is not plausible. Factors other than {111}<110> slip should be taken into account to understand the formation of {111} fiber texture. Since the activation of {100}<110> slip was found in the high temperature deformation of various L12 intermetallic compounds, the {111} fiber texture might be discussed in terms of {100}<110> slip. However, the effect of a simultaneous activation of several kinds of slip systems should be considered in this case, because the {100}<110> slip system has only three independent slip systems, which cannot satisfy the yon Mises criterion. The theoretical estimation of the deformation texture when several kinds of crystallographically nonequivalent slip systems are activated is not possible at present. Detailed investigation on the deformation characteristics, such as the active slip systems in the temperatures above 1073K and dislocation structure is necessary to understand the formation of {111} fiber texture. References 1)H.Mabuchi, K.Hirukawa, H.Tsuda and Y.Nakayama, Scripta Metall., 24, 505(1990). 2)H.Mabuchi, K.Hirukawa and Y.Nakayama, Scripta Metall., 23, 1761(1989). 3)C.D.Turner, W.O.Powers and J.A.Wert, Acta Metall., 37, 2635(1989). 4)H.Fukutomi, T.Suzuki, T.Inazumi and T.Kamijo, J. Japan Inst. Light Metals, in print. 5)S.A.Brown, D.P.Pope and K.S:Kumar, Materials Society Symposium Proceedings, 288, 723(1992). 6)C.S.Barrett and L.H.Levenson, Trans., AIME, 137, 112(1940).
Pergamon
TEXTURE ANALYSIS OF Llz Al6eMmTi2s INTERMETALLIC C O M P O U N D DEFORMED A T HIGH T E M P E R A T U R E S T.Suzuki*, M.Dahms**, T.Takabayashi* and H.Fukutomi* Department of Mechanical Engineering and Materials Science, Faculty of Engineering, Yokohama National University, 156 Tokiwadai, Hodogaya-ku, Yokohama 240, Japan Institute fur Werkstofforschung, GKSS-Forschungszentrum, Geesthacht GmbH Max-Planck-Strasse, D-21502 Geesthacht, Germany (Received (Revised
November 18, 1993) December 29, 1993) Introduction
The intermetallic compound AI3Ti with excellent oxidation resistance is known as a candidate material for the high temperature structural use. Its industrial application, however, has not been achieved because of limited ductility at ambient temperature. Recently, several researchers reported that the transformation of crystal structure from D0z2 to L12 by the substitution of third elements such as Cr, Mn and Ni for AI was effective in improving the deformability of the compound[I-3]. Since mechanical properties are affected by crystal structure as well as microstructure, it is important to comprehend the factors which control the microstructure of this kind of intermetallic compound. The authors investigated the deformation characteristics and microstructure change of Llz A166MngTi25 by high temperature uniaxial compression tests[4]. Three regimes appeared in the dynamic restoration process, depending on the flow stress level. When the flow stress is higher or lower, dynamic recovery is the dominant dynamic restoration process, while dynamic recrystallization is predominant at the intermediate stresses. Texture examination showed that three kinds of fiber texture developed corresponding to the difference in the dynamic restoration process. Evaluation of the pole figure suggested that the main components of the texture are close to {110}, {110}+{111} and {111} for higher, intermediate and lower stress regimes, respectively. In this paper, crystallographic characteristics of the textures formed in the lower and higher stress regimes are examined by calculating the orientation distribution function, in order to understand the mechanism of texture formation. Experimental Procedure Materials were produced by arc melting. The nominal composition is AI66MngTi25. X-ray diffraction showed that the material was almost a single phase of L12 structure. Since the material included numerous pores as in the cast state, isothermal forging was conducted at 1323K at a strain rate of 1.0xl0- 4 s- ~ up to the true strain of -0.6. After forging, the material was annealed at 1473K for 24h. Specimens with the approximate dimensions of 6x6xl0mm 3 were made by mechanical cutting. Constant true strain rate compression tests were conducted in vacuum. The temperatures and strain rates for the higher and the lower stress regimes are 1173K and 1.0x10-3s-~ and 1473K and 1 . 0 x l 0 - 4 s - ' , respectively. The specimens were deformed up to the strain of -1.6, followed by furnace cooling. The cooling rates from 1473K to 1273K and from 1273K to 1073K were 2Ks- ~ and 1Ks- ', respectively. About one-half in thickness of the deformed specimen was removed by mechanical and electrolytic polishing, in order to examine the texture and microstrueture on the mid-plane section. Texture measurements were made by the Schulz reflection method using nickel filtered Cu K~ radiation. Based on {111}, {110} and {100} pole figures, a crystallite orientation distribution function was calculated. "/'he pole density distributions were examined by inverse pole figures.
1071
1072
TEXTURE ANALYSIS
Vol.
30, No.
8
Results and Discussion Figure 1 is the inverse pole figure of the specimen deformed to a strain of -1.6 in the higher stress regime. The flow stress at -0.02 strain was 277MPa. The pole density distribution is expressed by using the mean pole density as a unit. Evidently, the pole density gradually increases toward {110} and the maximum pole density exists on {110}. Brown et al[5] reported that the active slip system of this compound in the temperature range between 77K and 1073K was {111}<110> which is the same as for FCC metals. The main component of the texture formed by uniaxial compression of FCC metals was reported as {110}[6], which is the same as in Fig. 1. Figure 2 shows the inverse pole figure for the deformation up to -1.6 strain in the lower stress regime. The flow stress at -0.02 was 19MPa. Different from Fig. 1, the maximum pole density is seen at {111}. Thus, the main components of the texture for the higher and the lower stress regimes are determined as {110} and {111}, respectively. II1
II1
001
001
011
Fig.1 Inverse pole figure of the specimen deformed in the higher stress regime. (1173K, 1.0xl0- as- 1, -1.6 strain)
011
Fig.2 Inverse pole figure of the specimen deformed in the lower stress regime. (1473K, 1.0xl0- 4s- 1, -1.6 strain)
The generation of new grains as well as the trace of grain boundary migration were not observed in the deformation at the higher stress regime[4]. The coincidence of the main component of the fiber textures between Als6MngTi25 and FCC metals together with the results of microstmctural observation indicates that the texture shown in Fig.1 is formed by the {111}<110> slip. As reported in the previous paper[4], no obvious difference was observed between the higher and lower stress regimes in the size of grains measured parallel and normal to the compression axis. The difference of microstructure between these two regimes was the shape of crystal grains. The grains observed in the longitudinal sections were lenticular in the higher stress regime, while they were rather rectangular in the lower stress regime[5]. This difference indicated that grain boundary migration occurred extensively in the lower stress regime. It was suggested that the accumulation of pole density toward {111} might occur similarly to the overshooting which was observed in the case of {111}<110> slip of FCC single crystals, if the grain boundary migration reduced the constraint between crystal grains[4]. However, the fact that the main component exists almost exactly at {111} indicates that the above mechanism is not plausible. Factors other than {111}<110> slip should be taken into account to understand the formation of {111} fiber texture. Since the activation of {100}<110> slip was found in the high temperature deformation of various L12 intermetallic compounds, the {111} fiber texture might be discussed in terms of {100}<110> slip. However, the effect of a simultaneous activation of several kinds of slip systems should be considered in this case, because the {100}<110> slip system has only three independent slip systems, which cannot satisfy the yon Mises criterion. The theoretical estimation of the deformation texture when several kinds of crystallographically nonequivalent slip systems are activated is not possible at present. Detailed investigation on the deformation characteristics, such as the active slip systems in the temperatures above 1073K and dislocation structure is necessary to understand the formation of {111} fiber texture. References 1)H.Mabuchi, K.Hirukawa, H.Tsuda and Y.Nakayama, Scripta Metall., 24, 505(1990). 2)H.Mabuchi, K.Hirukawa and Y.Nakayama, Scripta Metall., 23, 1761(1989). 3)C.D.Turner, W.O.Powers and J.A.Wert, Acta Metall., 37, 2635(1989). 4)H.Fukutomi, T.Suzuki, T.Inazumi and T.Kamijo, J. Japan Inst. Light Metals, in print. 5)S.A.Brown, D.P.Pope and K.S:Kumar, Materials Society Symposium Proceedings, 288, 723(1992). 6)C.S.Barrett and L.H.Levenson, Trans., AIME, 137, 112(1940).