The development of protective borosilicate layers on a Mo-3Si-1B (weight percent) alloy
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I. INTRODUCTION
OXIDATION studies of molybdenum-based alloys containing various amounts of silicon (10 to 19 wt pct) and boron (1 to 3 wt pct) have been performed at temperatures from 600 °C to 1600 °C.[1–10] These studies were usually performed in air or pure oxygen and showed that the boron and silicon in such alloys can produce a passivating borosilicate layer under high-temperature oxidation conditions. The properties of such borosilicate layers depend upon the amounts of silicon and boron in these layers. As the amount of boron is increased, the viscosity of the borosilicate becomes lower. A low viscosity of the borosilicate is necessary in order for it to flow and completely cover the alloy surface. On the other hand, as the borosilicate viscosity decreases, transport of oxygen through the borosilicate increases and the borosilicate becomes a less-effective passivating layer. As mentioned previously, the viscosity of the borosilicate layer is determined by the amounts of boron and silicon in the layer, which is determined by the amount of boron and silicon in the alloy. The oxidation temperature is also an important factor, since loss of some components of the borosilicate can occur due to vaporization. Most of the investigations concerned with the oxidation of Mo-Si-B alloys have examined the oxidation kinetics and have shown that these kinetics were determined by the borosilicate layer, containing different amounts of boron. At temperatures as high as 1500 °C to 1600 °C, vaporization of boron is such that the outer portion of the borosilicate approaches pure silica.[7] The preD.A. HELMICK, Materials Engineer, GE Energy, Materials and Processing Engineering, Greenville, SC 29615 and G.H. MEIER and F.S. PETTIT, Professors, are with the Department of Materials Science and Engineering, University of Pittsburgh, Pittsburgh, PA 15261. Contact e-mail: [email protected] Manuscript submitted November 15, 2004. METALLURGICAL AND MATERIALS TRANSACTIONS A
sent article is concerned with the development of the borosilicate layer on Mo-Si-B alloys and the effects of gas flow rate and temperature on the continuity of such layers. Mo-Si-B alloys containing less than about 50 vol pct of the Mo (solid solution) (Mo(ss))[11] phase (Figure 1), have poor room-temperature ductility due to the brittle nature of the two intermetallic phases, Mo3Si and T2.[12,13,14] If the volume fraction of the Mo(ss) phase is allowed to approach or exceed 50 vol pct, the room-temperature toughness becomes high enough for the alloy to be considered for industrial applications. This favorable room-temperature toughness is due to a microstructure consisting of a continuous matrix of the more ductile Mo(ss) phase rather than a continuous brittle intermetallic phase. The alloy studied in this investigation is molybdenum-3 wt pct silicon-1 wt pct boron.[15] The microstructure of this alloy is shown in Figure 2. This alloy contains three phases. These three phases include a Mo solid solution phase, a binary intermetallic Mo3Si phase, and a ternary i
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