Role of Thermal Vacancies on Temperature Dependence of Lattice Parameter and Elastic Moduli in B2-type FeAl
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Role of Thermal Vacancies on Temperature Dependence of Lattice Parameter and Elastic Moduli in B2-type FeAl Mi Zhao1, Kyosuke Yoshimi1, Kouichi Maruyama1 and Kunio Yubuta2 1 Graduate School of Engineering, Tohoku University, Sendai, 980-8579, Japan. 2 Institute for Materials Research, Tohoku University, Sendai, 980-8577, Japan. ABSTRACT Temperature dependence of the lattice parameter and elastic moduli in Fe-40 and -43Al (at.%) was investigated by high temperature X-ray diffractometry (XRD) and the ElectroMagnetic Acoustic Resonance (EMAR) method. The thermal vacancy concentration was estimated from the activation enthalpy and entropy data of vacancy formation previously reported for FeAl. It was found that both the lattice parameter and the elastic moduli of FeAl have a linear relationship with temperature even in the temperature range where thermal vacancy concentration rapidly increases (above 400 °C), thus suggesting that newly generated thermal vacancies at elevated temperature do not make significant influence on the lattice parameter and the elastic properties of B2-type FeAl. INTRODUCTION Iron aluminides has been widely studied as structural materials because of their interesting performance at high temperature. For B2-type FeAl, thermal vacancy behavior is one of the major topics. B2-type FeAl contains a high concentration of thermal vacancies at high temperature and they are easy to be frozen into the material after cooling down due to the high vacancy concentration and low vacancy migration speed [1]. Excess vacancy hardening is well proved by the increasing sample hardness with increasing quenching temperature [2-3]. Chang et al. proved that in a large composition range with 40~51 at.% Al, the hardness increased with increasing aluminum content and the change in hardness with temperature is a good indication of relative vacancy concentration in B2 FeAl [4]. Moreover, the supersaturated vacancies can also result in an increase in the critical resolved shear stress (CRSS) and a decrease in the ductility of single crystals [5]. Xiao and Baker suggested that vacancies control the room temperature mechanical properties of FeAl by comparing the vacancy concentration with the yield strength as a function of Al content [6]. George and Baker proposed a vacancy hardening model to explain the positive temperature dependence of yield stress (strength anomaly) in FeAl at elevated temperature [7]. Supersaturated vacancies in FeAl can be eliminated by intermediate temperature annealing [2]. Nagpal and Baker reported that the hardness of furnace-cooled FeAl samples was apparently reduced after annealed at 400 °C for 118 hours [3]. Yoshimi et al. showed that after annealing at 425 °C for 120 hours, the hardness of Fe-48Al was lower than that of Fe-39Al (at.%) [8]. Since supersaturated vacancies influence some mechanical properties of FeAl as described above, this study focuses on the role of thermal vacancies on the lattice parameter and elastic moduli. The temperature dependence of the lattice parameter and elastic modu
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