Time-Temperature Dependent Short- and Long-Range Structural Transformation in Medium-Entropy Alloys

Abstract

Neutron scattering experiments were conducted to reveal the origin of diffuse scattering in a medium-entropy alloy (MnCoNi). Results showed that a time-temperature-dependent short-and long-range structural transformation from a face-centered cubic to a primitive tetragonal structure occurs. In the MnCoNi alloy, the long-range ordered phase with a relatively small c/a ratio of 3.7(1)% can only be grown in a limited temperature range, whereas short-range structural transformation with a short correlation length of 14 nm can be formed in a wide temperature range. On the contrary, no sign of short-/long-range staggered atomic rearrangement, such as L1 0 and L1 2 structures, was observed in the CrCoNi alloy at least at room temperature. The difference in the forming ability of short-/long-range ordering between the MnCoNi and CrCoNi alloys was discussed.

Local Atomic Displacements and Sign of the Structural Transformation in Medium-Entropy Alloys Observed in Extended X-ray Absorption Fine Structure Spectra

“…3 and 4, respectively, as functions of temperature. The temperature dependence of the bond length was reasonably consistent with that evaluated from the diffraction experiment 13) within the experimental accuracy. This result indicates that our simple analysis is suitable for investigating local structures.…”

“…Here, we used the following constraint: The bond length R EXAFS is larger than the crystallographic bond length R D evaluated from the diffraction data. 13) This constraint is based on the following relationship: The EXAFS bond length R EXAFS is approximated by R EXAFS ¿ R D + MSRD perp /2R D , where MSRD perp is the MSRD perpendicular to the bond direction and is a measure of the anisotropy of atomic displacement. Because this value is always positive, the bond length R EXAFS is larger than R D owing to the contribution of MSRD perp .…”

“…The results of the neutron diffraction experiments are described elsewhere. 13) 2. Experimental Procedure…”