Effects of chemical composition and B2 order on phonons in bcc Fe–Co alloys

Abstract

The phonon density of states ͑DOS͒ gives insight into interatomic forces and provides the vibrational entropy, making it a key thermodynamic function for understanding alloy phase transformations. Nuclear resonant inelastic x-ray scattering and inelastic neutron scattering were used to measure the chemical dependence of the DOS of bcc Fe-Co alloys. For the equiatomic alloy, the A2 → B2 ͑chemically disordered→ chemically ordered͒ phase transformation caused measurable changes in the phonon spectrum. The measured change in vibrational entropy upon ordering was −0.02Ϯ 0.02 k B /atom, suggesting that vibrational entropy results in a reduction in the orderdisorder transition temperature by 60Ϯ 60 K. The Connolly-Williams cluster inversion method was used to obtain interaction DOS ͑IDOS͒ curves that show how point and pair variables altered the phonon DOS of disordered bcc Fe-Co alloys. These IDOS curves accurately captured the change in the phonon DOS and vibrational entropy of the B2 ordering transition.

Design and operation of the wide angular-range chopper spectrometer ARCS at the Spallation Neutron Source

“…28) and subsequent single crystal magnetism studies [29][30][31] of these materials. ARCS has also been used to study the PDOS of thermoelectric materials 32 and alloys, 33 and has been characterized as a diffractometer in anticipation of use for dynamic pair distribution function studies. 34 The ARCS instrument is in operations within the SNS user program and the ARCS IDT continues its involvement by allocating 20% of the user beam time to exciting science and development projects from among these core users.…”

Magnetic and vibrational properties of high-entropy alloys

“…Therefore, even though the vibrational entropy may be large in magnitude, what matters is the difference in vibrational entropy between phases. Differences in S ph between phases can be small, as is the case for the ordering transition in FeCo alloys, 4 or quite large, as is the case for alloying vanadium with platinum. 5 The magnitude of the configurational entropy S conf is small compared to S ph at moderate temperatures.…”

“…The experimental procedures were the same as those reported for a recent experiment on FeCo alloys. 4 All measurements were performed at room temperature. The monochromator resolution function was measured in situ using a single APD in the forward direction.…”

Miscibility gap and phonon thermodynamics of Fe-Au alloys studied by inelastic neutron scattering and nuclear-resonant inelastic x-ray scattering

“…Several results are shown in Table 1. The Wills-Harrison model predicts that the Co-Fe and V-Fe bonds are softer than the average of CoCo and Fe-Fe, and V-V and Fe-Fe bonds, respectively, which is accurate [28,29] and typically results in an ordering tendency [30]. The model predicts that the Au-Fe bond in the fcc structure is softer than the average of Au-Au and FeFe bonds, but Fe-Au is evidently a system with unmixing tendencies.…”