Experimental and theoretical confirmation of an orthorhombic phase transition in niobium at high pressure and temperature

Abstract

Compared to other body-centered cubic (bcc) transition metals, Nb has been the subject of fewer compression studies and there are still aspects of its phase diagram which are unclear. Here, we report a combined theoretical and experimental study of Nb under high pressure and temperature. We present the results of static laser-heated diamond anvil cell experiments up to 120 GPa using synchrotron-based fast x-ray diffraction combined with ab initio quantum molecular dynamics simulations. The melting curve of Nb is determined and evidence for a solid-solid phase transformation in Nb with increasing temperature is found. The high-temperature phase of Nb is orthorhombic Pnma. The bcc-Pnma transition is clearly seen in the experimental data on the Nb principal Hugoniot. The bcc-Pnma coexistence observed in our experiments is explained. Agreement between the measured and calculated melting curves is very good except at 40-60 GPa where three experimental points lie below the theoretical melting curve by 250 K (or 7%); a possible explanation is given.

Effects of Leading-Edge Modification in Damaged Rotor Blades on Aerodynamic Characteristics of High-Pressure Gas Turbine

“…In general, the turbine blade are fabricated with metals or alloys, and the TIT is higher than the melting temperature of metals and alloys, which usually is affected by pressure [3,4]. Hence, if the TIT is increased to enhance efficiency, the flow and heat transfer characteristics of the blade surface will be significantly affected.…”

Ab Initio Phase Diagram of Tungsten

“…It is interesting to note another case of the phase diagrams of Nb and Ta, Group 5B partners in the periodic table, which appear to be similar as well. In both cases, a bcc-Pnma solid-solid phase transition occurs [24,58] but, in contrast to dhcp in Mo and W, the orthorhombic Pnma phase exists at high T only. In Ta, the bcc-Pnma-liquid triple point is at a pressure which is an order of magnitude higher than that in Nb [24,58].…”

“…Fast recrystallization observed in HP-HT experiments on transition metals may imply either a microstructural transition (discussed above) at which the sample texture changes but its crystal structure remains the same, or a true solid-solid phase transition. The former was recently proven to be the case in Mo [17], and it is very likely the case in V [19] as well as in W. The latter is apparently the case in Nb [58] and likely in Re as well [25]. In either case, the corresponding fast recrystallization lines have been misinterpreted vs. experimental [37,44,46,47] and other theoretical [45] data.…”

Effect of Laser Irradiation on Emissivity of Flame-Generated Nanooxides

“…The results here reported can have a significant impact on pyrometry measurements, which is considered a powerful diagnostic tool for the characterization of the particulate-generating flames. The method proposed can be also applied for determining temperatures in high-pressure high-temperature studies [ 8 ]. We have seen that due to the occurrence of different mechanisms under laser irradiation with different excitation energies, different spectral behaviors of emissivity should be considered for a proper evaluation of the effective particle temperature.…”