Mechanical Properties of Co Alloys Based on a E2 1 Type Co 3 AlC Intermetallic Compound
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MECHANICAL PROPERTIES OF Co ALLOYS BASED ON A E2 TYPE Co AIC INTERKETALLIC COMPOUND
HIDEKI HOSODA, MASARU TAKAHASHI,
TOMOO SUZUKI* AND YOSHINAO MISHIMA*
Graduate Student, Department of Materials Science and Engineering, Tokyo Institute of Technology. Nagatsuta, Midorl-ku, Yokohama 227, Japan. *Department of Metallurgical Engineering, Tokyo Institute of Technology, 0-okayama, Meguro-ku, Tokyo 152, Japan. **Precision and Intelligence Laboratory, Tokyo Institute of Technology, Nagatsuta, Midorl-ku, Yokohama 227, Japan.
ABSTRACT Alloy design of a new Co-base heat resisting alloy is being pursued. Similar to N in the Ni-base superalloys, the introduction of E21 type intermetallic compound Co AlC as a strengthening phase In a Co solid solution matrix is being attempted. Three experimental alloys are prepared which consist of either two or three phases. Two phase alloys consist of the Co terminal solid solution and the Co3 AlC, while a three phase alloy Temperature dependence of contains additional B2 phase based on CoAl. strength is measured for all the three alloys to reveal that the alloys could exhibit comparable Intermediate to high temperature strength with Nibase superalloys and that, most astonishingly, are found to be quite ductile over all the test temperature including 77 K. INTRODUCTION It is well known that the excellent creep resistance as well as high temperature strength in commercial Ni-base superalloys is achieved by the dispersion of a L12 type intermetallic compound based on NI3Al taking up 60 % or more in volume in a NI solid solution matrix. The dipersold NI3 Al may have been better known because It exhibits an anomalous positive temperature dependence of strength arising from its intrinsic ordered structure. Commercial Co-base superalloys, although they show some superior properties to Ni-base superalloys such as resistance to thermal fatigue and hot corrosion[i,21, they have been limited in usage because of insufficient high temperature strength mainly due to lack of effective dispersion strengthener as NI Al in Ni-base superalloys. There has been a series of work on the mechanical properties of L12 Co3 Ti[3], but practical Importance has not been further investigated since the compound is not stable over 1200 K. whose In the present work, the E21 type Co3 AlC intermetallic compound, mechanical properties have rarely been investigated, is chosen for the possible dispersold in the Co-base alloys. The Co-Al-C ternary phase diagram is shown in Fig. 1 In which the phase field for Co3 AlC as well as its relation to the Co terminal solid solution at 1173 K can be found[4]. The crystal structure of E2 Co3 AlC Is constructed by Co atoms at the face center site, Al atoms at the corner site, and C atoms at the body center site as shown In Fig. 2. It is therefore clear that Co AlC can be regarded basically to be Ll2 Co3 Al containing a certain amount of carbon atoms, analogous to the situation for the NaCl type transition metal monocarbides Mat. Res. Soc. Symp. Proc. Vol. 288. 01993 Materials Research Soc
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