Elsevier

Intermetallics

Volume 124, September 2020, 106863
Intermetallics

Orientation relationship between bcc precipitates and Ti5Si3 matrix in Mo–Si–B–Ti–C quinary alloys

https://doi.org/10.1016/j.intermet.2020.106863Get rights and content

Highlights

  • Precipitation of bcc phase in D88-Ti5Si3 was observed in Mo–Si–B–Ti–C alloys.

  • Orientation relationship between bcc precipitates and Ti5Si3 matrix was established.

  • Cracking was found to occur preferentially on the (0001) plane in Ti5Si3.

Abstract

In this study, the crystallographic orientation relationship between bcc precipitates and a Ti5Si3 matrix in Mo–50Ti–14Si–6C–6B (at.%) alloy was established. The alloy microstructure consisted of the bcc solid solution, Ti5Si3, Mo5SiB2, and TiC phases. The metal-rich Ti5Si3 phase was formed during a solidification stage, followed by the precipitation of the bcc phase within the Ti5Si3 matrix in the subsequent annealing process. The orientation relationship between the two phases was described by the expression: 112bcc//0110Ti5Si3 and 311bcc//0001Ti5Si3. The formation of a low-energy interface between the two phases was explained by the edge-to-edge matching of their corresponding lattice planes at the faceted interface.

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//[0110]D88 and (311)A2//(0001)D88. The A2 precipitates exhibited plate-like shapes with faceted interfaces parallel to the (152)A2 and (4

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).

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