Effect of Interstitial Carbon Atoms on Phase Stability and Mechanical Properties of E2 1 (L1 2 ) Ni 3 AlC 1-x Single Cry
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Effect of Interstitial Carbon Atoms on Phase Stability and Mechanical Properties of E21 (L12) Ni3AlC1-x Single Crystals Yoshisato Kimura, Masato Kawakita, Hiroyasu Yuyama and Yaw-Wang Chai Tokyo Institute of Technology, Interdisciplinary Graduate School of Science and Engineering, Department of Materials Science and Engineering, 4259-J3-19 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. ABSTRACT Single crystals of E21 (L12) Ni3AlC1-x were prepared by the unidirectional solidification using the optical floating zone melting method to determine their mechanical properties. Particularly the effects of interstitial carbon atoms on mechanical properties were evaluated by compression tests at room temperature. Operative slip system of E21 Ni3AlC is {111} type which is the same as that of L12 Ni3Al. Strength of Ni3AlC single crystals increases with carbon concentration due to the solid solution effect, though the stress relief of yielding behavior is enhanced at the intermediate carbon content at around 3at%. A large gap appears in the carbon concentration dependence of critical resolved shear stress (as well as yield stress) at almost the same carbon content. This discontinuity in strengthening is attributed to the interaction between multiple solute carbon atoms and mobile dislocations. INTRODUCTION The E21 type intermetallic compounds M3AlC1-x, where M is Co, Fe, Ti and so forth, have been investigated with expectation that they can be promising strengthener for a new class of heat resistant alloys [1-14]. It is because the E21 type ordered crystal structure quite resembles to that of the L12 type Ni3Al which is well-known strengthener of Ni-base superalloys. Phase stability of E21 type compounds has been investigated for a long time in their history. However their mechanical properties had not been examined until Hosoda et al. reported the mechanical properties of Co3AlC1-x based multi-phase alloys [2], except for the cases in which E21 type compounds were used as strengthening precipitates. The present authors, Kimura et al., fabricated E21 Co3AlC1-x single crystals and reported their mechanical properties [7]. Not only as the strengthener but also as the matrix, E21 type compounds M3AlC1-x, can be promising candidate materials for high temperature structural application. Two types of ordered structures, E21 and L12, can be differentiated only by an interstitial carbon atom occupying at the cell center of the E21 type as shown in Fig. 1. In the chemical formula of E21 type M3AlC1-x, x stands for the deficiency of carbon concentration as a vacancy at the C-site of E21. We suggest that the E21 type intermetallic compounds can be classified into two groups. One is the ternary compounds which are interstitially stabilized by a ternary element carbon atom occupying at its cell center. Typical examples of this group are Co3AlC1-x and Fe3AlC1-x. Note that neither L12 Co3Al nor L12 Fe3Al exists at least as a stable phase in the binary system. The other is the binary L12-based compounds such as Ni3AlC1-x. To minimize the elast
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