The phonon entropy of metals and alloys : the effects of thermal and chemical disorder

phonons, entropy, inelastic neutron scattering, metals, alloys

Bogdanoff, Peter David ; Fultz, Brent T.

University:California Institute of Technology

Department:Engineering and Applied Science

关键词: phonons, entropy, inelastic neutron scattering, metals, alloys;   

Others  :  https://thesis.library.caltech.edu/5145/1/Bogdanoff_pd_2002.pdf

美国|英语

来源: Caltech THESIS

【 摘 要 】

Vibrational entropy is important for the thermodynamics of alloying, alloy formation, phase transitions and phase stability at high temperature. Vibrational entropies of alloying and alloy formation were calculated for 32 binary alloys and intermetallic compounds using phonon DOS curves taken from the literature. The vibrational entropies of formation span a wide range from -0.64 to +0.55 kB/atom, and the vibrational entropies of alloying ranged from -0.39 to +1.0 kB/atom. This range exceeds the range of configurational entropy of a binary alloy, which reaches a maximum value of +0.69 kB/atom and a minimum value of 0 k_B/atom.

The vibrational entropy of the NiTi martensitic transition was measured using low-temperature calorimetry and inelastic neutron-scattering. The high-temperature B2 phase of NiTi has a vibrational entropy 0.5 k_B/atom larger than the low-temperature martensitic phase. The difference in vibrational entropy accounts for the total entropy of the austenitic-martensitic phase transition.

Inelastic neutron scattering was used to show that the phonon DOS of V is unchanged between 20 and 1000°C, inconsistent with the phonon softening expected from thermal expansion. It is found that the effects of volume expansion and rising temperature exert equal and opposite shifts on the phonon DOS. The pure temperature dependence of the phonon DOS is due to strong phonon-phonon scattering, which in turn leads to a large anharmonic vibrational entropy contribution at high temperature.

The vibrational entropy of eight chemically disordered Cu-Au alloys was measured using inelastic neutron scattering. The analysis of the phonon entropy of a disordered alloy was performed in a novel way by modeling the partial vibrational entropies of Cu and Au. The partial vibrational entropies of Cu and Au were shown to be slowly varying and smooth functions of composition. The vibrational entropy of disordering in Cu₃Au is calculated as 0.24 ± 0.02 k_B/atom, substantially larger than results predicted from recent theoretical work.