Electronic Effect on Low-Temperature Athermality of the Omega Phase

Low‑temperature athermal behavior of the omega phase of B.C.C. Ti binary alloys and Zr binary alloys has been expected qualitatively in terms of first‑principle population analysis of the molecular orbitals. The population analysis was performed using the cluster models with the atomic concentrations all fixed but with different 3d or 4d transition elements for alloying involved. The results show that in each alloy system the valence DOS band with the positive overlap population tends to shift downward with respect to the Fermi level as the e/a ratio is increased. This tendency of the phase stability implies that the Zr‑Ti system having the mini‑ mum value of the ratio 4 is the most unstable among the modeled alloys. The expectation has been confirmed by low‑temperature electron and x‑ray diffraction. The electron diffraction was carried out on single crystal Zr‑50at%Ti and also Ti‑24mass%V whose e/a ratio is greater than 4. The superlattice reflections of the omega phase given by the B.C.C. reduced incommensu‑ rate wavevector q＝(2/3＋Δ 2/3＋Δ 2/3＋Δ in the {110}* diffraction pattern were observed at �100K and room temperature. The results indicate that temperature dependence of the intensity distribution of the Zr alloy is much less significant than that of the Ti alloy. The x‑ray diffrac‑ tion was done on single crystal Zr‑30at%Ti ‑35at%Ti ‑50at%Ti to double check the temperature dependence. Monitoring of the intensity distribution with increasing temperature from �30K provides a feature of the dependence which is quite similar to that obtained by the electron dif‑ fraction. All results of the experiments are consistent with those of the population analysis.