In the present contribution, we describe the successful development of two ternary Mo-Si-Ti alloys with two-phase eutectic and eutectoid microstructure, respectively. In the case of Mo-20.0Si-52.8Ti (at.%), a fully eutectic microstructure consisting of body-centered cubic (bcc) solid solution (Mo,Ti,Si) and hexagonal (Ti,Mo) 5 Si 3 can be obtained in very good agreement with thermodynamic calculations. A fully eutectoid decomposed microstructure is observed subsequent to heat-treatment at 1300 °C for 200 h in the case of Mo-21Si-34Ti (at.%). For this alloy, bcc (Mo,Ti,Si) and tetragonal (Mo,Ti) 5 Si 3 appears after decomposition from the A15-type (Mo,Ti) 3 Si. Besides that, a small amount of hexagonal (Ti,Mo) 5 Si 3 forms in the silicide lamellae, too, which is attributed to Ti segregations in the as-cast microstructure. In addition to the focus on microstructure, both oxidation and creep behavior were preliminarily investigated and compared to other state-of-the-art Mo-based alloys. In the case of the eutectic alloy, a promising and unexpected oxidation resistance at 800 °C is observed whereas the eutectoid alloy exhibits catastrophic oxidation; a behavior that is typically observed under these conditions in alloys containing Mo-rich solid solution. The eutectic alloy shows an approximately one order of magnitude higher creep rate within the investigated temperature and stress range as compared to the eutectoid decomposed counterpart. This is attributed to the rather low intrinsic creep resistance of the hexagonal (Ti,Mo) 5 Si 3 and generally lower melting point of the former alloy, whereas in the latter case, creep seems to be controlled by the deformation of the bcc solid solution (Mo,Ti,Si) and the tetragonal (Mo,Ti) 5 Si 3 .
Apart from the reported transition from the fibrous morphology in NiAl-34Cr to lamellae by adding 0.6 at.% Mo, further morphology transformations along the eutectic trough in the NiAl-(Cr,Mo) alloys were observed. Compositions with at least 10.3 at.% Cr have lamellar morphology while the first tendency to fiber formation was found at 9.6 at.% Cr. There is a compositional range, where both lamellae and fibers are present in the microstructure and a further decrease in Cr to 1.8at.% Cr results in fully fibrous morphology. Alongside these morphology changes of the (Cr,Mo)ss reinforcing phase, its volume fraction was found to be from 41 to 11 vol.% confirming the trend predicted by the CALPHAD approach. For mixed morphologies in-situ X-ray diffraction experiments performed between room and liquidus temperature accompanied by EDX measurements reveal the formation of a gradient in composition for the solid solution. A new Mo-rich NiAl-9.6Cr-10.3Mo alloy clearly shows this effect in the as-cast state. Moreover, crystallographic orientation examination yields two different types of colonies in this composition. In the first colony type, the orientation relationship between NiAl matrix and (Cr,Mo)ss reinforcing phase was ( 100 ) NiAl|| ( 100 ) Cr,Mo and ⟨ 100 ⟩ NiAl|| ⟨ 100 ⟩ Cr,Mo. An orientation relationship described by a rotation of almost 60° about ⟨ 111 ⟩ was found in the second colony type. In both cases, no distinct crystallographic plane as phase boundary was observed.
Abstract.As new high temperature structural materials, Mo-Si-B alloys satisfy several requirements such as oxidation and creep resistance. Recently, novel Ti-rich Mo-Si-B alloys have shown an increased creep resistance compared to Ti-free alloys. However, due to the formation of a duplex SiO 2 -TiO 2 oxide layer, which allows for fast ingress of oxygen, the oxidation resistance is poor. To improve the oxidation resistance a borosilicate based coating was applied to a Mo-12.5Si-8.5B-27.5Ti (in at.%) alloy. After co-deposition of Si and B by pack cementation at 1000 °C in Ar, a conditioning anneal at 1400 °C is used to develop an outer borosilicate layer followed by an inner MoSi 2 and Mo 5 Si 3 layer. During both isothermal and cyclic oxidation after an initial mass loss during the first hours of exposure, a steady state is reached for times up to 1000 h at temperatures ranging from 800 to 1200 °C demonstrating a significantly enhanced oxidation resistance.
scite is a Brooklyn-based organization that helps researchers better discover and understand research articles through Smart Citations–citations that display the context of the citation and describe whether the article provides supporting or contrasting evidence. scite is used by students and researchers from around the world and is funded in part by the National Science Foundation and the National Institute on Drug Abuse of the National Institutes of Health.
hi@scite.ai
10624 S. Eastern Ave., Ste. A-614
Henderson, NV 89052, USA
Copyright © 2024 scite LLC. All rights reserved.
Made with 💙 for researchers
Part of the Research Solutions Family.