Investigation on Additional Hardening Mechanisms in Off-Stoichiometric Al-Rich Ni 3 Al Alloys in Terms of Anti-Site Defe
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INVESTIGATION ON ADDITIONAL HARDENING MECHANISMS IN OFF-STOICHIOMETRIC Al-RICH Ni3Al ALLOYS IN TERMS OF ANTI-SITE DEFECT CONFIGURATION Seiji Miura, Satoshi Takizawa, Yoshinao Mishima* and Tetsuo Mohri Division of Materials Science and Engineering, Graduate School of Engineering, Hokkaido University, Kita-13, Nishi-8, Kita-ku, Sapporo 060-8628, Japan. * Department of Materials Science and Engineering, Interdisciplinary Graduate School of Science and Engineering, Tokyo Institute of Technology, 4259 Nagatsuta, Midori-ku, Yokohama 226-8502, Japan. ABSTRACT One of the unsettled issues of the mechanical properties of L12-Ni3Al compound is the effect of off-stoichiometry on the 0.2% flow stress at low temperature region. 0.2% flow stress at 77 K increases as deviating from the stoichiometric composition to both Ni-rich and Al-rich sides, but in Al-rich side the increasing rate is higher. According to the XRD results for the binary Ni3Al alloys, however, the composition dependence of the lattice constant is the same in both sides of the stoichiometric composition, which implies that the magnitude of lattice strain induced at off-stoichiometric composition is symmetrical. And it is not rational to associate the strengthening mechanism to a sole effect of isotropic strain due to an atomic size difference. In this study, attempts are made to explain the additional strengthening in Al-rich alloys in terms of the tetragonal distortion hardening mechanism, which arises from anisotropic atomic configuration around an anti-site Al atom. INTRODUCTION Mechanical properties of L12-Ni3Al intermetallic compound have been attracting broad interests and various studies have been reported [1, 2]. It is well known that the strength of this compound increases with increasing temperature at intermediate temperature range, namely positive temperature dependence of strength, which is attributed to cross-slip motion of screw dislocations into a cube plane controlled by a thermal activation process. At low temperature range, the anomalous mechanical behavior is less evident. However, there is also an unsettled issue of the mechanical properties of this compound. It is known as the effect of off-stoichiometry on the 0.2% flow stress. 0.2% flow stress at 77 K increases as deviating from the stoichiometric composition to both Ni-rich and Al-rich sides [3]. Figure 1 shows the effect of off-stoichiometry on the stress at 77 K and room temperature of Ni3Al and Ni3Ge [3]. Noteworthy is the fact that in Al-rich side the increasing rate is higher. Golberg et al. concluded that the off-stoichiometric effect on the stress at low temperature is due to NiAl
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Fig. 1
The effect of off-stoichiometry on the low temperature stresses for L12-Ni3Al and Ni3Ga alloys reported by Noguchi et al. [3].
Fig. 2
The composition dependence of average atomic volume VA in Ni-Al binary system [11].
dispersion formed during solidification [4]. However, by using well-homogenized materials a similar tendency has been reported by Aoki and Izumi [5], Rawlings a
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