Orientation relationship between bcc precipitates and Ti5Si3 matrix in Mo–Si–B–Ti–C quinary alloys
Introduction
Increasing the operating temperature of a heat engine generally increases its energy conversion efficiency and reduces the level of CO2 emissions. Currently, Ni-based superalloys are utilized in the hot sections of various jet and gas turbine engines. However, a further increase in the temperature capability of Ni-based superalloys seems highly challenging, because their temperature capability is almost equal to 90% of their melting temperatures [1]. Therefore, there have been growing calls for developing novel ultrahigh-temperature materials as potential alternatives to Ni-based superalloys.
Over the recent decades, Mo–Si–B alloys have been considered promising candidates for the next generation ultrahigh-temperature materials [2,3]. However, it is difficult to achieve a satisfactory balance between high-temperature strength and room temperature toughness by performing only microstructural optimization [4,5]. Recently, TiC-added Mo–Si–B alloys (called MoSiBTiC alloys) have been attracted much attention because of their excellent high-temperature strength (1000 h creep rupture life under 137 MPa is 1360 °C) and reasonable room-temperature fracture toughness (>15 MPa√m) [[6], [7], [8], [9], [10], [11]]. The major constituent phases of MoSiBTiC alloys are bcc solid solution, Mo5SiB2, and TiC phases. Increasing Si content aiming at improving the alloy oxidation resistance [12,13] often results in unsatisfactory material performance, owing to the stabilization of less oxidation-resistant Mo3Si and the destabilization of the ductile bcc phase. Instead, Ti-enrichment of MoSiBTiC alloys has been proposed as an effective approach for improving their oxidation resistances without sacrificing the room temperature toughness, since oxidation-resistant Ti5Si3 and ductile bcc phases are stabilized while Mo3Si is destabilized [[14], [15], [16], [17], [18]]. Although the alloy oxidation resistance at a temperature of approximately 800 °C (a typical temperature for catastrophic oxidation in some Mo–Si alloys) is still low [15,19,20], it can be improved by further increasing the Ti content [20,21].
Despite the improved oxidation resistance resulted from Ti enrichment and Ti5Si3 incorporation [13,18,20,21], the low toughness of Ti5Si3 can represent another problem, because of its high susceptibility to crack formation during the cooling stage after solidification or high-temperature annealing. Upon solidification of Mo–Si–B–Ti–C quinary alloys, Ti5Si3 solidifies to form an elongated grain structure due to its preferential growth along the c-axis [12]. Furthermore, microcrack formation often occurs within the elongated Ti5Si3 grains because their thermal expansion along the a-axis and c-axis are highly anisotropic [12,13,22,23]. Needless to say, such microcracking must be suppressed to increase the material reliability.
This study focuses on the precipitation behavior of the bcc phase in the Ti5Si3 matrix of a Mo–Si–B–Ti–C quinary alloy with a four-phase microstructure consisting of bcc, Mo5SiB2, Ti5Si3, and TiC. The bcc phase precipitation can enhance the toughness of the alloy by inhibiting the propagation of the microcracks formed within the brittle Ti5Si3 matrix. The orientation relationship between bcc precipitates and the Ti5Si3 matrix is established in this study.
Section snippets
Experimental procedure
A Mo–50Ti–14Si–6C–6B (at.%) alloy was prepared by arc-melting a mixture of elemental powders in an Ar atmosphere. The purities of the powders were 99.99% for Mo and Si, 99.9% for Ti, and 99% for TiC and MoB (mass%). The arc-melted ingots were annealed at 1500 °C for 24 h in an Ar atmosphere. The constituent phases and their lattice parameters were examined by X-ray diffractometry (XRD) using powdered alloy specimens. Lattice parameter refinement was performed by the Rietveld method with the
Results
Fig. 1 shows the SEM micrographs of the Mo–50Ti–14Si–6C–6B alloy. The as-cast microstructure shown in Fig. 1(a) consists of bcc solid solution (A2), TiC (B1), Mo5SiB2 (T2), and Ti5Si3 (D88). The B1 and T2 phases precipitated from the liquid separately, while some B1 dendrites were enclosed by the T2 grains, suggesting that the B1 phase was solidified before the T2 phase. Subsequently, rod-shaped D88 grains were solidified prior to the formation of A2/D88 mono-variant eutectic microstructures as
Discussion
It is generally accepted that, upon solid-state precipitation from a supersaturated solid solution, certain orientation relationships that produce low-energy interfaces between the precipitates and the matrix are stochastically favored, because the energetic barrier for the precipitation reaction is primarily the interfacial energy between the two. Zhang and Kelly [29] have proposed the following criterion for the formation of low-energy interfaces between two crystalline phases. When the
Conclusions
Upon the solidification of the Mo–50Ti–14Si–6C–6B alloy, a (Ti + Mo)-rich Ti5Si3 phase with the D88 structure was formed from the liquid, and the bcc solid solution (A2) phase precipitated within the D88 matrix during the subsequent annealing at 1500 °C. The orientation relationship between the A2 and D88 phases was described by the expressions: [112]A2//[]D88 and ()A2//(0001)D88. The A2 precipitates exhibited plate-like shapes with faceted interfaces parallel to the ()A2 and (
CRediT authorship contribution statement
Tomotaka Hatakeyama: Data curation, Formal analysis, Writing - original draft. Nobuaki Sekido: Writing - original draft, Data curation, Formal analysis, Supervision. Yuta Kimura: Data curation, Formal analysis. Kyosuke Yoshimi: Supervision, Funding acquisition.
Declaration of competing interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgement
This work was supported by the Advanced Low Carbon Technology Research and Development Program (ALCA) of the Japan Science and Technology Agency (JST) (No. JPMJAL1303).
References (39)
- et al.
Room-temperature fracture toughness of MoSiBTiC alloys
Intermetallics
(2017) - et al.
Microstructure, high-temperature deformability and oxidation resistance of a Ti5Si3-containing multiphase MoSiBTiC alloy
Intermetallics
(2017) - et al.
Oxidation behavior of Moss–Ti5Si3–T2 composites at intermediate and high temperatures
Intermetallics
(2020) - et al.
Effect of Cr addition on microstructure and oxidation resistance of a Ti5Si3-containing MoSiBTiC alloy
Corrosion Sci.
(2020) - et al.
Oxidation mechanisms in Mo-reinforced Mo5SiB2(T2)-Mo3Si alloys
Acta Mater.
(2002) - et al.
Constitution, oxidation and creep of eutectic and eutectoid Mo-Si-Ti alloys
Intermetallics
(2019) - et al.
Characterisation of the oxidation and creep behaviour of novel Mo-Si-Ti alloys
Acta Mater.
(2020) - et al.
Thermal expansion and elastic moduli of the silicide based intermetallic alloys Ti5Si3(X) and Nb5Si3
Scripta Mater.
(1997) - et al.
Thermal expansion anisotropy of ternary titanium silicides based on Ti5Si3
Acta Mater.
(2004) - et al.
Thermal expansion of the Ti5Si3 and Ti6Si2B phases investigated by high-temperature X-ray diffraction
Intermetallics
(2006)
Annealing response of point defects in off-stoichiometric Mo5SiB2 phase
Intermetallics
Edge-to-edge matching model for predicting orientation relationships and habit planes - the improvements
Scripta Mater.
Stability of the Nb5Si3 phase in the Nb-Mo-Si system
Intermetallics
Crystallographic characteristics of an integrally directionally solidified Nb-Ti-Si based in-situ composite
Scripta Mater.
Variation in morphology and crystallographic orientation of directionally solidified Nb–Si based alloys at high withdrawal rates
J. Alloys Compd.
Nickel-based superalloys for advanced turbine engines: chemistry, microstructure, and properties
J. Propul. Power
Mo-Si-B Alloys : developing a revolutionary material
MRS Bull.
The hotter the engine, the better
Science
Optimization of Mo-Si-B intermetallic alloys
Metall. Mater. Trans. A Phys. Metall. Mater. Sci.
Cited by (5)
Crystallographic orientation and interface characteristics between in-situ TiC reinforcement and matrix before and after thermal deformation
2023, Materials CharacterizationCitation Excerpt :Although the size and distribution of the reinforcement are important for the improvement in the mechanical properties of in-situ TiC-reinforced steel, the interface characteristics and orientation relationship between the reinforcement and matrix may change during thermal deformation. In the transition region between these two main constituents (reinforcement and matrix) [29], the change in the orientation relationship and interface characteristics is expected to have a significant effect on the properties of the reinforced steel [30–37]. However, studies on the variation in the orientation relationship and interface characteristics between the reinforcement and matrix are scarce.
Microstructure and oxidation mechanism of multiphase Mo–Ti–Si–B alloys at 800 °C
2021, Corrosion ScienceCitation Excerpt :Zhao et al. analyzed precipitates in a Moss–Mo3Si–Ti5Si3–Mo5SiB2–TiC composite after annealing at 1700 °C and identified them as Mo3Si sub-micro plates based on the TEM diffraction patterns [19]. Hatakeyama et al. found that bcc Moss precipitates in Ti5Si3 during annealing at 1500 °C and revealed the orientation relationship between the matrix and the precipitates for a Moss–Ti5Si3–Mo5SiB2–TiC alloy [23]. According to the above results both in literatures and the present study, one may predict the formation of Mo3Si-precipitates in Mo3Si-containing Mo–Ti–Si–B-based alloys and Moss-precipitates in Mo3Si-free ones during high-temperature annealing.
Review of Research Progress on Mo–Si–B Alloys
2023, MaterialsFrom Mo–Si–B to Mo–Ti–Si–B Alloys: A Short Review
2023, Materials