L12 (Al,Cr)3Ti-based two-phase intermetallic compounds-I. Plastic deformation behavior

Scripta Materialia, Vol. 36, No. 7, pp. 79%800,1997 Ekevis Science Ltd Copyright 0 1997 Acta Metallurgica Inc. Printed in the USA. All rights reserved 1359-6462/97 $17.00 + .OO

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L12 (Al,Cr)sTi-BASED TWO-PHASE INTERMETALLIC COMPOUNDS-I. PLASTIC DEFORMATION BEHAVIOR 5.71.Park, M.H. Oh*, D.M. Wee, S. Miura** and Y. Mishima** Dept. of Matls. Sci. and Eng., KAIST, Taejon 305-701, Korea Jointly Appointed at Center for Advanced Aerospace Materials, Pohang Univ. of Sci. Tech., Korea *Dept. of Matls. Sci. and Eng., Kmnoh National Univ. of Tech., Kumi 730-701, Korea **Precision and Intelligence Laboratory, Tokyo Institute of Technology, Yokohama 227, Japan (Received October 2 1, 1996) Introduction A number of recent works on intermetallic compounds in the Ti-Al system have concentrated on gamma-based TiAl, but AlsTi is especially attractive because of its low density and good oxidation resistance (1). However, AlsTi, with a low-symmetry tetragonal DO22structure, exhibits less than 1% compressive strain in the temperature range of 298 K to 893 K, for which the primary deformation mode is ordered twinning (2). Therefore, its brittleness limits its practical application as a structural material. More recently, much work has been conducted on the transformation of crystal structure through the addition of ternary alloying elements to improve the ductility of AhTi. The low-symmetry tetragonal DO22structure of AI3Ti can be changed to the higher-symmetry cubic Liz strncture with the addition of a third element such as Cr, Mn, Fe, Co, Ni, Cu, Zn, Rh, Pd, Ag, Pt or Au (3-5). In fact, the L&type intermetallic compounds formed by alloying Al,Ti with Cr and Mn have a small amount of tensile ductility when measured by bend testing at room temperature (6). Especially, the alloys with Cr have better oxidation resistance and much better compressive ductility at room temperature than those alloyed with other ternary alloying elements (7,8). However, the ternary Liz compounds are reported to be still extremely brittle in tension and to show brittle transgranular cleavage type in the f?actnre mode although they exhibit appreciable compressive ductility at ambient temperature (9). In this study, the plastic deformation behavior of two-phase intermetallic compounds consisting of mainly Liz phase and the second phases in Al-Ti-Cr system was investigated. As in the case of a2/y two-phase TiAl -compounds, the possibility of ductilizing two-phase intermetallic compounds based on Ll&qpe (Al,Cr)sTi was examined. The influence of pore formation, ingot cast structure and second phase’morphology on ductility of the alloy was also examined.

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Experimental Procedures Button ingots, approximately 15 g, were prepared by arc melting under an argon atmosphere and remelted at least five times to promote homogeneity of the as-cast structure. The compositions of the button ingots used in this study were represented on the ternary phase diagram of the Al-Ti-Cr system at 1423 K in Fig. 1. For Al-25Ti-1OCr and Al-2 lTi-23Cr alloys, large ingots, approximately 150 g with a dimension of 120 x 25 x 15 mm3, were also prepared in order to investigate the characteristics of the as-cast structure of large ingots. The Al-21Ti-23Cr alloy was directionally solidified at a growth rate of 300 mm/hr in an arc-melting directional solidification apparatus to align the second phase parallel to the growth direction. The arcmelted ingots and the directionally solidified ingot were homogenized at 1423 K for 48 h in a vacuum of 10-l Pa and then &mace-cooled. The phase identification of homogenized specimens was done with X-ray diffraction (XRD) using a Cu-Ka characteristic X-ray. The compositions of the matrix and second phases were assessed by energy dispersive X-ray spectroscopy (EDS). Microstructures of the as-cast and homogenized specimens were examined using an optical microscope (OM) and a scanning electron microscope (SEM). Specimens for compression testing, with a dimension of 3 x 3 x 7 mm’, were cut from homogenized alloys using an electrodischarge machine, and then mechanically polished with up to 0.5 pm alumina on all four sides. Compression tests were conducted at room temperature using an In&on testing machine (Model 4206) at different strain rates of 1.2 x 1Oa/s and 1.2 x 1O-2/s. Results & Discussion Fig. 1 shows the Al-rich comer of the isothermal section in the Al-Ti-Cr ternary phase diagram at 1423 K. This isothermal section shows that the Liz phase field exists together with six different second phase fields in Al-Ti-Cr system. The L12 phase field was found to be 6 at.% wide in Cr and 4 at.% wide in Ti (lo), and the six second phases surrounding it are TiAl, A12Ti, A13Ti, A1r7Cr9,Cr*Al and TiCrAl. In this study, the plastic deformation behavior of an Lll single phase alloy and two-phase alloys consisting of mainly the LIZ phase and 20% second phases was investigated using a compression test. The six second phases of the two-phase alloys were identified using XRD and EDS, and their crystal structures are summarized in Fig. 1.

Alloy composition Second phase (structure) AI-25l-k1OCr (a) Al-3on-6cr (b) Al-29Ti-60 TWO-phase alloys

TiAl (Lb) AhTl (GazHI)

(c) AI-25Ti-6Cr

AbTi (Wa)

(d) AI-PlTi-150

Ah09

(e) AI-ZlTi-23Cr

CnAl (Ci la)

(I) AI-ZSTi-160

TiAlCr (C14)

( - )

Figure 1. Aluminum-rich comer of the isothermal section of Al-Ti-Cr system at 1423 K. Closed circles represent the alloy compositions consisting of 20% second phases and the open circle the alloy compositions of AI-25Ti-100.

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1200

.

1000

-

800

-

(@AI-21 Ti-230 (@Al-30Ti-8Cr Q

AI-25Ti-100

0

10

20

30

(%T

50

60

70

Strain

Figure 2. Compressive stress-strain curves of Liz single phase alloy and two-phase alloys consisting of 20% second phases.

Fig. 2 shows the stress-strain curves obtained through a compression test at R.T. for the alloys shown in Fig. 1. The two-phase alloys showed higher yield strength and lower fracture strain than the LIZ single phase alloy. The Al-30Ti-8Cr alloy, containing 20% TiAl as a second phase, showed the highest yield strength and a relatively low fracture strain. Both the Al-25Ti-6Cr alloy containing 20% ALTi and the Al-21Ti-23Cr alloy containing 20% CrzAl showed the highest fracture strain (-15%) among the two-phase alloys. The Al-29Ti-6Cr alloy containing 20% Al*Ti failed without any significant plastic deformation. This suggests that the AlzTi phase has a particularly deleterious effect on the ductility of Liz alloys. It was also found from fracture surface observations that all of the alloys tested were found to be failed by brittle transgranular cleavage. This is because the alloys have intrinsically poor cleavage energy and strength (9), and thus it seems to be one of the reasons why L 12 ternary Al-T&X alloys are extremely brittle in tension. Fig. 3 shows the typical microstrutitures of Al-25Ti-10Cr and Al-21Ti-23Cr alloys. As shown in Fig. 3, the as-cast alloys contain a substantial volume fraction of second phases in the interdendritic region. During homogenization heat treatment at 1423 K for 48 h, most of the second phases disappear and an extensive number of pores formed in solutionizing second phases in the Al-25Ti-1OCr alloy. However, no residual pores were observed in Al-2 1Ti-23Cr alloy containing 20% CrzAl after homogenization. It was reported that the formation of pores during homogenization treatment in L 1Z ternary Al-Ti-Cr alloys was closely associated with the nature of the second phases formed during solidification and homogenization treatment (6). The reason why second phases form during solidification in Ll z ternary Al-T&X alloys is that the extent of the Liz phase field decreases and the geometric center of the Ll 2 phase field changes with decreasing temperature (11). Thus, the composition corresponding to the geometric center of the L 1z phase field at high temperature actually lies in a two- or three-phase region at room temperature. For such a situation, it becomes much more difficult to produce a Ll 2 single phase structure at R.T. In the case oPa Liz single phase alloy with a lower Cr content, the as-cast alloy contains Al&-9 as a second phase and also has a small number of pores due to solidification shrinkage. During homogenizatio:n treatment, a large number of pores form in the place of solutionizing Al&!r9. This result could be explained by using the Kirkendall effect. In the case of higher Cr content alloys, however, the as-cast alloys contain CrzAl or TiAlCr as second phases and no pores due to solidification shrinkage. Therefore, it is suggested that the presence of such pores affects the mechanical properties of the alloys. Recently, in fact, Mabuchi et al. (6) reported that no pores are observed after homogenization treatment in higher Cr content alloys, although most of second phases disappear during homogenization treatment. They also reported that the higher Cr content alloy (Al-25Ti-14Cr),

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Figure 3. Optical micrographs of AI-25Ti-10Cr and Al-21Ti-23Cr alloys: (a), (c) as-cast and (b), (d) homogenized at 1423 K for 48 h.

no porosity was observed, showed 0.9% ductility in bend test, while the lower Cr content alloy (Al-25Ti-SCr), in which a large number of pores existed, showed only about 0.25% ductility (6). Among the two-phase alloys in this study, the best compressive ductility was shown in both Al-25Ti6Cr and Al-21Ti-23Cr alloys. However, no porosity was observed in the alloy containing 20% CrzAl as a second phase (Al-21Ti-23Cr), while a large number of pores formed in the alloy containing 20% A13Ti (Al-25Ti-6Cr). Thus, Al-21Ti-23Cr seems to have a more desirable microstructure. Recently, Klansky et al. showed in their study that the hardness of the Cr2Al phase is higher than that of the Liz phase (10). Thus, the CrzAl phase itself may not contribute directly to the improvement of ductility of in which

(@Al-25Ti-100

Figure 4. A schematic diagram of ingot cast structure: (a) Al-25Ti-10Cr and (b) AI-21Ti-23Cr.

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0 [

600

AI-ZlTi-233

,*--0-o

F E

COMPOUNDS-I

300

5 F b v)

AI-XTi-1tlCr 200

z $ 100

400

600

600

1000

1200

Temperature (K)

Figure 5. Variation in compressive yield strength of Al25Ti-10Cr and AI-21Ti-230 alloys with temperature.

Angle (Degree)

Figure 6. Variation in compressive yield strength and strain to failure of directionally solidified AI-21Ti-23Cr alloy as a function of angle 8. The second phases are aligned 0” (parallel), 45” and 90” (normal) to the compressive axis in specimenA, B and C, respectively.

the Liz phase. Nevertheless, as shown in Fig. 2, the Al-21Ti-23Cr alloy shows the best ductility among the two-phase alloys tested. This fact explains how the disappearance of pores results in the improving tensile ductility of the alloys. In this study, the characteristics of the as-cast structure of large ingots with a dimension of 120 x 25 x 15 mm3 was also investigated for Al-25Ti-10Cr and Al-21Ti-23Cr alloys. As shown in Fig. 4, on the upper surface of the ingot of Al-25Ti-10Cr alloy, relatively large solidification shrinkage, 45 mm long and 1Omm wide, formed and extensive cracking with transverse directions were also observed. Such cracking seem to be due to internal residual stress caused by the solidification shrinkage. On the contrary, in the case of the Al-21Ti-23Cr alloy, solidification shrinkage, 25 mm long and 7 mm wide, was smaller than that of Al-25Ti-10Cr and cracking was not observed. This means that Al-21Ti-23Cr shows a more desirable cast structure than Al-25Ti-10Cr. Moreover, as previously mentioned, no evidence of pore formation was observed after homogenization heat treatment in the Al-2 1Ti-23Cr alloy. The variation of compressive yield strength with temperature for Al-25Ti-1OCr and Al-21Ti-23Cr alloys is shown in Fig. 5. The temperature dependence in yield strength for both alloys is similar to that for many L l2 Al-Ti-X intermetallic compounds (12). However, the yield strength of Al-21Ti-23Cr is about two times higher than that of Al-25Ti-1OCr over the entire temperature range tested. In addition, compression tests were performed for the directionally solidified Al-21Ti-23Cr alloys with three different orientations in order to investigate the effect of second phase morphology on deformation behavior of the two-phase alloy. The obtained values of yield strength and strain to failure are plotted in Fig. 6 as a function of the angle 0 at which the second phase aligns to the compressive axis.’ The compressive yield strength and the strain to failure of the alloy depends on the angle 8. Specimen B (0 = 45’) showed the highest strain and the lowest yield strength, and specimen A (e = 0’) showed the highest yield strength and a relatively high strain which was similar to that of specimen B. The yield strength and the strain of the alloys tested increased with the strain rate, but the general trend of orientation dependence is the same for both strain rates. Therefore, based on the results obtained, it is suggested that the deformation behavior of two-phase alloys could be changed by the second phase alignment, which shows the possibility of improving the strength and ductility of Liz (A1,Cr)3Ti-based two-phase intermetallic compounds through controlling the second phase morphology.

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Conclusions The two-phase alloys in this study showed higher yield strength and lower strain to failure than the Liz single phase alloy, Al-25Ti- 1OCr. However, the Al-2 l Ti-23Cr alloy containing 20% CrzAl as a second phase showed the best ductility among the two-phase alloys, and also showed a more desirable ingot cast structure than that of the Al-25Ti-10Cr alloy. The yield strength of the Al-21Ti-23Cr alloy is about two times higher than that of the Al-25Ti-10Cr alloy over the entire temperature range tested. The yield strength and the strain to failure of the directionally solidified Al-2 l Ti-23Cr alloy show the second phase orientation dependence, which means the possibility of improving the strength and ductility of L 12 (Al,CrhTi-based two-phase intermetallic compounds through controlling the second phase morphology. Acknowledgments The authors would like to thank professors T. Suzuki at Hokkaido University, M. Yamaguchi at Koyto University, Y. Nakayama at University of Osaka Prefecture and M. Nemoto at Kyushu University for their useful advice and helpful discussions. The financial support of KOSEF (Contract No. 961-0801007-2) is gratefully acknowledged. References 1.

2. 3. 4. 5. 6. 7. 8. 9. 10. 11. 12.

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