Mo-Si-B Alloy Development

Alloys consisting of the phases α-Mo (Mo solid solution), Mo3Si, and Mo5SiB2 (“T2”) were fabricated by ingot and powder metallurgy. A novel powder-metallurgical processing technique was developed which allows the fabrication of silicide alloys with a continuous α-Mo matrix. In these alloys the relatively ductile and tough α-Mo matrix acts as a “binder” for brittle silicide particles. An alloy processed in this manner with the nominal composition Mo-15Si-10B exhibited a room temperature fracture toughness of 14 MPa m. By controlling the composition and microstructure, other properties such as fracture toughness and creep strength can be optimized. A powder-metallurgically processed alloy with the composition Mo16.8Si-8.4B (at. %) exhibited a weight loss of only 5 mg/cm after 1 day at 1300°C in air. Partial substitution of Mo with Nb increased the creep strength at temperatures ranging from 1200 to 1400 ̊C significantly. Similar to the oxidation and fracture toughness properties, the microstructural scale and morphology have a pronounced influence on the creep strength. INTRODUCTION Nickel-base superalloys have outstanding oxidation and mechanical properties at elevated temperatures, but their service temperatures are inherently limited to temperatures around 1000 ̊C. In order to increase the thermodynamic efficiency of Fossil Energy systems, materials capable of much higher temperatures are needed. One approach, pioneered in Japan, focuses on precious metal based superalloys [1]. While these superalloys have simple fcc-based crystal structures, which allow significant plastic deformation, a heavy price is paid in terms of mass density and cost. An alternative approach is based on oxidation resistant intermetallic compounds which have lower densities and are less costly, but which are inherently brittle. In particular, silicide intermetallics, which can have outstanding oxidation resistance, are being considered. A prime example is MoSi2 that is widely used in heating elements for resistance furnaces. Its good oxidation resistance is due to the formation of a protective silica glass scale. However, MoSi2 is very brittle with a room temperature fracture toughness on the order of 3 MPa m [2]. Also, it becomes very weak at high temperatures [2]. If the Si concentration is reduced below that of MoSi2, phases such as Mo5Si3, Mo3Si, and α-Mo (Mo solid solution) form. These phases will have a lower oxidation resistance, but they may potentially impart greater fracture toughness, particularly in the case of α-Mo. Two main alloy systems have been examined to date. In the first one, which was pioneered by Akinc and collaborators [3], intermetallic alloys consisting of Mo5Si3, the T2 phase Mo5SiB2, and the A15 phase Mo3Si were investigated. These types of alloys are indicated in the schematic ternary phase diagram in Fig. 1. They exhibit excellent oxidation resistance at elevated temperatures (e.g., 1300°C). The boron additions are crucial for providing the observed oxidation resistance [4,5], as already hinted at in an early study of the ternary Mo-Si-B phase diagram by Nowotny et al. [6]. In the second system, which was pioneered by Berczik et al. [7,8], alloys consisting of α-Mo, Mo3Si, and T2 were investigated. While these alloys are not as oxidation resistant as Mo5Si3-T2-Mo3Si alloys, they contain a ductile phase, α-Mo. Depending on its volume fraction and distribution, the α-Mo can improve the room and high temperature fracture toughness significantly. The fracture toughness will increase with increasing α-Mo volume fraction and, for a given α-Mo volume fraction, will be higher if the α-Mo forms as a continuous matrix instead of individual particles [9]. Clearly, then, the optimization of Mo-Si-B alloys requires a tradeoff between fracture toughness on the one hand, and oxidation resistance on the other. Another issue is the creep resistance of these types of alloys. Akinc et al. have already shown that Mo5Si3-based alloys exhibit excellent creep resistance [3]. Mo3Si, (Cr,Mo)3Si, and the T2 phase are all very strong at elevated temperatures [1012]. Since it can be safely assumed that the creep strength of α-Mo is lower than that of Mo3Si and T2, the creep strength of Mo-Mo3Si-T2 alloys is likely to depend on the α-Mo volume fraction. It will also depend sensitively on the microstructural morphology if the α-Mo is distributed as a continuous matrix or “binder” phase instead of isolated particles, the creep strength will be low. In addition, as commonly observed in creep, the grain or phase size will play an important factor – generally, the creep strength tends to increase with increasing grain or phase size. The competing requirements for optimum oxidation resistance, fracture toughness, and creep strength are schematically shown in Fig. 2. Using the Mo-Si-B system we will illustrate the issues raised in Fig. 2. It should also be noted that several aspects of Mo-Si-B alloys are also being examined within the Department of Energy/Basic Energy Sciences Synthesis&Processing Center on “Design and Synthesis of Ultrahigh-Temperature Intermetallics.” For example, the significant thermal expansion anisotropy of Mo5Si3, which gives rise to profuse microcracking during processing, has now been understood based on first-principles calculations [13]. Based on these calculations alloying additions for Mo5Si3 have been found which reduce the ratio of the thermal expansions in the cand a-directions from 2 to 1.2 [14].