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Mo-SiB Alloy Development

Mo-Si-B silicides consisting of the phases α-Mo (Mo solid solution), Mo3Si, and Mo5SiB2 have melting points on the order of 2000°C and have potential as ultra-high temperature structural materials. Mo-Si-B alloys can be processed such that the α-Mo is present in the form of isolated particles in a silicide matrix, or as a continuous matrix “cementing” individual silicide particles together. The latter microstructure is similar to that of WC-Co hard metals. This paper focuses on the relationship between the topology as well as scale of the microstructure of Mo-Mo3Si-Mo5SiB2 alloys, and their creep strength and fracture toughness. For example, the creep strength of Mo-Si-B alloys is improved by reducing the α-Mo volume fraction and by making the α-Mo phase discontinuous. The fracture toughness is improved by increasing the α-Mo volume fraction and by making the α-Mo phase continuous. Room temperature stress intensity factors as high as 21 MPa m were obtained. The room temperature fracture toughness of Mo-Si-B alloys can also be improved by microalloying with Zr. The room temperature ductility of Mo itself can be improved by adding MgAl2O4 spinel particles suggesting yet another way to improve the ductile phase toughening of Mo-Si-B alloys. 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. Oxide dispersion strengthened materials, with solidus temperatures as high as 1480°C [1], can operate above 1000°C. However, their creep strengths are relatively low. For example, the nickel-base ODS alloy MA754 has a 1000 h rupture strength of 78 MPa at 1150°C and the ironbase ODS alloy MA956 has a 100 h rupture strength of 51 MPa at 1100°C [1]. Also, these ODS alloys have high stress exponents which makes them prone to catastrophic fracture in creep since a small increase in stress can result in a dramatic increase in the creep rate. In order to increase the thermodynamic efficiency of fossil energy systems, strong, tough and oxidation resistant materials capable of service temperatures much higher than 1000°C are needed. While refractory elements such as Nb, Mo, Ta, and W have very high melting points, they lack oxidation resistance. One way to improve their oxidation resistance is to form silicides. 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 is 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 their fracture toughness may be higher. As indicated in the schematic ternary phase diagram in Fig. 1, two main alloy systems have been examined to date. In the first one, which was pioneered by Akinc and collaborators [3-5], intermetallic alloys consisting of Mo5Si3, the T2 phase Mo5SiB2, and the A15 phase Mo3Si were investigated. 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]. The second system is the focus of this paper and was pioneered by Berczik et al. [7,8]. Berczik investigated alloys consisting of α-Mo, Mo3Si, and T2. 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 increases with increasing α-Mo volume fraction. For a given α-Mo volume fraction, the fracture toughness is expected to be higher if the α-Mo forms a continuous matrix instead of individual particles [9]. The fracture toughness is also expected to increase if the microstructural scale of the α-Mo increases [10]. The oxidation resistance, on the other hand, is increased by decreasing the α-Mo volume fraction and by distributing the α-Mo in the form of isolated particles, instead of a continuous matrix. Clearly, then, the optimization of Mo-Mo3Si-T2 alloys requires a trade-off between fracture toughness on the one hand, and oxidation resistance on the other.