Constitution, oxidation and creep of eutectic and eutectoid Mo-Si-Ti alloys

Abstract

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 .

Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys

“…As 800 °C is the critical temperature where pesting occurs, the corresponding isothermal oxidation tests of alloy A and B are included as well. The following results can be concluded from these experiments: (i) The previously reported results on the oxidation behaviour of the eutectic and eutectoid reference alloys by Schliephake et al [8] are very well reproduced (see Suppl. 3).…”

“…1) and eutectoid Mo-21Si-34Ti (blue symbol) reference alloys in Ref. [8]. Two novel alloys within the chemical composition range framed by these two reference alloys were examined: (a) the Ti-rich alloy A Mo-21Si-43.4Ti (red symbol) being closer to the composition of the eutectic alloy and the Ti-lean alloy B Mo-21Si-38.7Ti (black symbol) being near to the eutectoid alloy.…”

“…The binary eutectic Mo-20Si-52.8Ti alloy consisting of MoSS and (Ti,Mo)5Si3 lamellae possesses outstanding oxidation behaviour, namely insignificant and constant mass change subsequent to an initial parabolic oxidation behaviour at 1100 and 1200 °C as well as absence of any pesting-like behaviour at 800 °C. This unexpected oxidation resistance at 800 °C has been attributed to the formation of a mixed SiO2-TiO2 oxide scale [8]. In contrast, the eutectoid alloy Mo-21Si-34Ti comprising fine-structured MoSS and (Mo,Ti)5Si3 lamellae exhibits pesting behaviour at 800 °C, but reasonable creep resistance by means of one order of magnitude lower minimum creep rate compared to the eutectic alloy.…”

“…However, inadequate oxidation resistance at intermediate temperatures below 1000 °C is still present in these alloys [6]. The development of two-phase eutectic or eutectoid Mo-Si-Ti alloys in recent research work by Schliephake et al [8] shows great potential in this respect. The novel eutectic and eutectoid twophase, fine-scaled Mo-Si-Ti alloys with comparably low density of 6 to 7 g/cm 3 have shown to be either oxidation-resistant in the entire temperature range or sufficiently creep-resistant.…”

Kinetics of oxide scale growth on a (Ti, Mo)5Si3 based oxidation resistant Mo-Ti-Si alloy at 900-1300∘C

“…(Mo, Ti)Si 2 coatings are reported to improve the oxidation resistance further up to 1500 ∘ C [25]. Therefore, the development of multi-phase Mo-Ti-Si-X alloys possessing superior creep properties [26] and good oxidation resistance would be the focus of further studies.…”