The oxidation of tantalum-tungsten polycrystals and single crystals in the temperature range 850°–1100°C

Abstract

Single crystals and polycrystals of a Ta-3.9 at. % W alloy have been oxidised in the temperature range 850°–1100°C in oxygen at approximately 740 Torr. Recrystallised polycrystalline coupons exhibit cracking at the metal grain boundaries after oxidation at 950°C; this is believed to occur on cooling, and to be due to stresses imposed by the oxide growth process and the differential thermal contraction, and to the increase in resistance to fracture of the oxide and of the metal/oxide interface. However, cold-worked coupons show no sign of this effect. At 1050°C, recrystallised specimens ignited on admission of oxygen but cold-worked specimens oxidised without burning to produce compact, adherent scales. Single-crystal cylinders show similarly anisotropic scale-fracture morphologies to those observed in pure tantalum, but the anisotropy in oxidation rate is much less marked. After oxidation at 1100°C the single crystals fractured along {100}FX planes on cooling; this appeared to be initiated again by the high stresses and to propagate along suboxide platelets. There are considerable intrusions of pentoxide along the suboxide platelets. However, unlike the intrusions observed in the oxidation of niobium below 600°C, they do not appear to lead to metal fragmentation and an increase in rate; instead they appear to improve the scale/metal adhesion. This amount of tungsten reduces the oxidation rate of tantalum throughout this temperature range.