Contents lists available at ScienceDirect Intermetallics journal homepage: www.elsevier.com/locate/intermet Oxidation mechanism of W substituted Mo-Si-B alloys Tuba Karahan a,1 , Gaoyuan Ouyang b,c,1 , Pratik K. Ray b,c,* , M.J. Kramer b,c , Mufit Akinc b,c a Department of Metallurgical and Materials Engineering, Gedik University, Istanbul, Turkey b Department of Materials Science and Engineering, Iowa State University, Ames, IA, USA c Ames Laboratory, USDOE, Ames, IA, USA ARTICLE INFO Keywords: Silicide Transient oxidation Glass Silicate Mo-Si-B ABSTRACT The oxidation mechanism of a Mo 55 W 15 Si 15 B 15 alloy was established, and the effects of W content, oxidation temperature and microstructural length scale were determined. In addition to influencing the oxidation mechanism, the addition of W also destabilized the A15 phase which is consistent with our previous experiments in ternary Mo-W-Si alloys [1]. Microstructural investigation of the oxidized alloy revealed entrapped tungsten oxides at temperatures below 1300 °C, which volatilize above 1400–1500 °C. The presence of WO 3 in the oxide scale interrupts the surface coverage by the glassy borosilicate, thereby adversely affecting the oxidation behavior. In order to determine the effects of length scale, the microstructural evolution during the transient oxidation of cast and sintered alloys, with different microstructural length scales, was studied at 1100 and 1400 °C. Finer microstructure promoted faster borosilicate surface coverage at 1400 °C. 1. Introduction Designing tough oxidation resistant refractory metal silicides is a key technological challenge that needs to be overcomed in order to use these alloys for high temperature structural applications. High tem- perature oxidation behavior, mechanical properties and phase stability of Mo-Si-B based alloys have been extensively investigated over the last few decades [2–5]. However, no single alloy studied so far has demonstrated a combination of strength, toughness and oxidation resistance, which is essential for high temperature applications. This is a result of the conundrum posed by the Mo-Si-B ternary phase diagram. Alloys in the Mo-T2-A15 (Mo-Mo 5 SiB 2 -Mo 3 Si) phase field show excellent fracture toughness [6] while alloys in the intermetallic rich regions show good oxidation resistance. Ideally, an alloy compris- ing of a Mo rich phase, the T1 phase and the T2 phase will offer a combination of excellent fracture toughness due to the Mo rich solid solution [7,8], creep resistance due to the presence of the T1 phase [9], and oxidation resistance due to the presence of B doped T1 [10] as well as the T2 [11] phases. The A15 phase, which lies between the metal rich solid solution and the T1 phase neither affords significant oxidation resistance [11], nor does it impart fracture toughness, as a result of only four active slip systems at room temperature [12]. Therefore, it was anticipated that the removal of the deleterious A15 phase will help in forming the desired three-phase microstructure consisting of Mo rich solid solution, the T1 phase and T2 phase. The Mo 3 Si (A15) phase can be destabilized with controlled alloying additions of Nb and W, as demonstrated by Ray et al. [1,13]. The addition of elements such as Nb and W results in the Fermi surface moving out of the pseudo-gap in the density of states, thereby creating an electronic structure instability [1]. Furthermore, a multi-phase assembly containing Nb or W is more stable than the single phase A15 structure [13,14]. While the experiments and theoretical calcula- tions reported in the literature were carried out in ternary systems, without boron, it is expected that the addition of W will have a similar effect on the alloy phase assemblage even in the presence of boron due to its extremely low solubility in the A15 structure [15]. However, while the removal of the brittle A15 phase through W substitutions is expected to be beneficial for mechanical properties, the resulting effect on oxidation has not been established. W substitutions could increase the pesting range of the alloy at lower temperatures, resulting in poorer oxidation resistance at lower temperatures (T < 1300 °C). However, the removal of the A15 phase – which has poor oxidation resistance [11] – results in the formation of the T1 phase, which has better oxidation resistance [5]. This could conceivably improve the oxidation resistance above the pesting regime. Hence, it is important to investi- gate the oxidation behavior with W substitutions at different tempera- tures – in the pesting regime, as well as above the pesting regime. The oxidation behavior of Nb modified Mo-Si-B and Nb-Si-B alloys have been extensively studied by a number of researchers [16–19]. Unlike Mo, which oxidizes to form a volatile MoO 3 , the oxidation of Nb http://dx.doi.org/10.1016/j.intermet.2017.04.005 Received 9 November 2016; Received in revised form 16 February 2017; Accepted 7 April 2017 * Corresponding author. Department of Materials Science and Engineering, Iowa State University, Ames, IA, USA. 1 Equal first authors. E-mail address: prat@iastate.edu (P.K. Ray). Intermetallics 87 (2017) 38–44 0966-9795/ © 2017 Elsevier Ltd. All rights reserved. MARK