Abstract
In order to investigate fundamental properties of high-entropy alloys, this study has selected 5 elements, such as Al, Co, Cr, Fe and Ni, to prepare various AlxCoCrFeNi alloys for 0 □ x □ 2, and to explore the effects of microstructure on the heat conductivity, thermal expansion and electrical conductivity of the alloys in the temperature ranges of 298 – 573 K, 298 – 1073 K, and 298 – 400 K, respectively. Besides, the relation between properties and atomic bonding among elements in the alloys is also explored in this study. Experimental results from XRD and SEM show that the microstructure of these alloys is single FCC, duplex FCC + BCC, and single BCC for as-cast alloys (C-alloys) in the composition ranges of 0 □ xFCC □ 0.375, 0.5 □ xduplex □ 0.75, and xBCC □ 0.875, respectively, while the duplex phase area xduplex extends from 0.375 to 1.25 and xBCC □ 1.25 for the homogenized and quenched alloys (H-alloys). DSC analyses show no evidences of phase transformation in the temperature range of 298 to 773 K, indicating that the high-entropy effect makes the alloys more soluble among components in the alloys. The XRD peak intensities for H-alloys are lower than those of the pure component elements indicates that FCC and BCC structures in H-alloys have larger scattering effect for x-ray diffraction and more electron-phonon scattering and hence H-alloys have lower thermal and electrical conductivity. The ratio of thermal conductivity to electrical conductivity shows the contribution of phonon is comparable to that of electron in thermal conductivity. The anharmonic oscillation for atoms due to lattice distortion is large. As temperature increases the thermal expansion coefficient increases accordingly. This study shows various aspects of microstructural influence on the properties of the alloys. Since the atomic radius of Al is approximately 14.4 % greater than the radii of Co, Cr, Fe and Ni, the increasing amount of Al addition to the alloys decreases the XRD intensities of the alloys. This in turn increases the x-ray scattering in the alloys and influences properties of both single phases and duplex phase of the alloys. The relation of both thermal conductivity and electrical conductivity as a function of the amount of Al addition is seen to divide in three regimes just as that in the case of microstructure, i.e., FCC, FCC + BCC, BCC regimes. In both single-phase regimes, both thermal conductivity and electrical conductivity decrease as the amount of Al, x, increases. In duplex FCC/BCC regime both thermal conductivity and electrical conductivity are smaller than those in single-phase regimes. This is because of the additional scattering effect of FCC-BCC phase boundaries. Hardness increases monotonically with x for both C- and H-alloys, indicating that the atomic bonding strength also increases with x. In both single-phase regimes although the hardness increases slightly with x, the hardness keeps roughly constant in single-phase regimes, while the hardness of alloys in the duplex regime increases linearly with x. In BCC regime the hardness for H-alloys is slightly higher than that for C-alloys. This is attributed to the spinodal decomposition during homogenization of H-alloys at 1100 oC. The thermal expansion coefficient of H-alloys decreases with x is also attributed to the increase in bond strength as x increases. There are two phase-transformation temperatures for H-alloys in both thermal expansion measurements and DTA analyses. One is for ferromagnetic-to-paramagnetic transition (i.e., Curie temperature,) the other is for □–NiCoCr precipitation that is characterized by HTXRD and DTA. Curie temperature for single-phase H-alloys increases slightly with x, while there is a lower point for Curie temperature for duplex H-alloys. Since Curie temperature is proportional to molecular field in the Weiss theory of magnetism, the molecular field is also closely related to bond strength and microstructure of the alloys.