Elastic properties, thermal stability, and thermodynamic parameters of MoAlB

Abstract

MoAlB is the first and, so far, the only transition-metal boride that forms alumina when heated in air and is thus potentially useful for high-temperature applications. Herein, the thermal stability in argon and vacuum atmospheres and the thermodynamic parameters of bulk polycrystalline MoAlB were investigated experimentally. At temperatures above 1708 K, in vacuum and inert atmospheres, this compound incongruently melts into the binary MoB and liquid aluminum metal as confirmed by differential thermal analysis, quenching experiments, x-ray diffraction, and scanning electron microscopy. Making use of that information together with heat-capacity measurements in the 4-1000-K temperature range-successfully modeled as the sum of lattice, electronic, and dilation contributions-the standard enthalpy, entropy, and free energy of formation are computed and reported for the full temperature range. The standard enthalpy of formation of MoAlB at 298 K was found to be −132 ± 3.2 kJ/mol. Lastly, the thermal conductivity values are computed and modeled using a variation of the Slack model in the 300-1600-K temperature range.

Isothermal and Cyclic Oxidation of MoAlB in Air from 1100°C to 1400°C

“…22 We note, however, that these calculations were relatively simple and did not consider the formation of other Mooxides such as MoO 2 or oligomerized Mo-oxide species, which were reported in spectroscopic studies of oxide volatility during Mo metal oxidation.…”

“…The samples were fully dense (6.4 g/cm 3 ) and contained predominantly MoAlB, with 6 ± 3 vol.% Al 3 Mo, Al 8 Mo 3 and other Al-rich intermetallics, as well as <2 vol.% Al 2 O 3 impurities. 22 MoAlB powders were also prepared for oxidation testing, which were synthesized by cold-pressing the aforementioned mixture and heating it under dynamic vacuum in the hot press at 1000…”

“…Note that the Mo content is greater than Al in the underlying MoAlB substrate due to the non-stoichiometry generally found in MoAlB. 19,22 Moreover, the good adhesion between the Al 2 O 3 scales and the substrate, even in areas where pre-existing Al-rich intermetallics are present (see red arrows in Figs. 8b and 8c), suggest that they also contribute to Al 2 O 3 formation.…”

Low Temperature Synthesis of Phase Pure MoAlB Powder in Molten NaCl

“…It also has been confirmed to be stable at up to 1400 • C in an inert atmosphere [3]. Differently from in MoB, the Al atoms in MoAlB are "sandwiched" between the MoB layers, which renders MoAlB very oxidation resistant (via the formation of a dense alumina layer from the initial oxidation on heating) [2,3,8,9].…”

Phase stability and weak metallic bonding within ternary‐layered borides CrAlB, Cr2AlB2, Cr3AlB4, and Cr4AlB6

“…In 2015, Ade and Hillebrecht synthesized single crystals of previously reported Cr 2 AlB 2 , Cr 3 AlB 4 , and Cr 4 AlB 6 , performing a detailed characterization of their structure and hardness . In 2016, Barsoum et al succeeded in the fabrication and characterization of dense polycrystalline MoAlB, and more importantly demonstrated its potential for high‐temperature structural applications because of its high phase stability, damage tolerance, and excellent oxidation resistance up to 1300°C, originating from the layered structure, and significantly weaker Al1–Al2 bonds . Very recently, the high fracture toughness (5.4 ± 0.2 MPa·m 1/2 ), high work of fracture (117 ± 13 J/m 2 ), damage tolerance and more interesting noncatastrophic failure behavior were indeed observed in a “212” type MAB phase (Fe 2 AlB 2 ) by Bai et al upon the successful fabrication of dense polycrystalline bulk.…”