Oxidation behavior and mechanical properties of Ti-enriched MoSiBTiC alloy

Mo-Si-B alloys are one of leading candidates for ultra-high-temperature applications. Macroalloying of Ti to the Mo-Si-B systems improves both strength/density ratio and high-temperature oxidation resistance. However, the study for Ti-added Mo-Si-B alloys have been still limited very much. In this study, Ti-enriched MoSiBTiC alloy with the composition of 38Mo-17Si-5B-20Ti-10TiC (at.%) was addressed from the viewpoint of oxidation and high temperature deformation. Alloy ingots of the 38Mo-17Si-5B-20Ti-10TiC alloy were prepared by conventional Ar arc-melting. Heat treatment was carried out in vacuum at 1600 or 1700 °C for 24 h. It was found that both the as-cast and heat-treated samples are composed of five phases, i.e., Mo solid solution, Mo3Si, Mo5SiB2, Ti5Si3 and TiC. Micro-cracks were often observed across Ti5Si3 phase, which were generated by thermal stress caused by the strong thermal expansion anisotropy of Ti5Si3. Oxidation behavior was investigated through the specific weight change against time at 1100 and 1300 °C in the atmosphere of pO2/pAr=0.25. The alloy displayed relatively good oxidation resistance. The oxidation rate coefficient obtained from the oxidation curves was below 10-2 g2m-4s-1 even at 1300 °C. This value is comparable to that of the TMS173 nickel-based SX superalloy. Mechanical property was examined by high-temperature compression tests. At 1400 °C, the peak stress reached over 700 MPa, which is at the same level as that of 1st-generation MoSiBTiC alloys [1]. Mechanical properties would be improved by microstructure controlling because the micro-cracking in Ti5Si3 degrades the strength and toughness of the alloy. Hot-working should be effective to destroy the inhomogeneous cast microstructure and facilitate microstructure refinement for the Ti-enriched MoSiBTiC alloy. [1] S. Miyamoto et al., Metall. Mater. Trans. A, 45 (2014) 1112

Effect of ZrC phase on high-temperature strength and room-temperature fracture toughness of ZrC-added Mo-Si-B alloys

Mo-Si-B-based alloys are one of leading candidate materials as ultra-high temperature structure materials. However, their high density and poor room-temperature fracture toughness have to be improved for the structural applications. Recently, we found that these problems can be solved by adding carbides such as TiC and ZrC. In this study, the high-temperature strength and room-temperature fracture toughness of ZrC-added Mo-Si-B alloys were investigated, and the effect of ZrC phase on the material properties were discussed. ZrC-added Mo-Si-B alloys (Mo-(3.2-7.0)Si-(6.5-14.0)B-(4.7-12.9)ZrC (at.%)) were prepared by arc-melting and heat-treated at 1800 °C for 24 h for homogenization. After heat-treatment, the microstructure was observed to investigate phase equilibria. Moreover, high-temperature compression tests at 1400 and 1600 °C and four-point bending tests with a Chevron notch at room temperature were conducted to investigate their mechanical properties. The constituent phases of the ZrC-added alloys were molybdenum solid solution (Moss), Mo5SiB2, ZrC and a small amount of Mo2B in a few cases. The density of the alloys ranged from 8.9 to 9.3 g/cm3, comparable to that of nickel-based superalloys. The alloys exhibited better high-temperature strength with relatively good deformability, for example, 1260 MPa at 1400 °C and 830 MPa at 1600 °C. The room-temperature fracture toughness of the alloys ranged from 12.4 to 20.3 MPa(m)1/2 depending on the volume fraction of Moss and ZrC. River patterns were observed on fracture surfaces of not only Moss but also ZrC phase, suggesting that ZrC also work for toughening by plastic deformation during crack propagation. Therefore ZrC plays a significant role in improving the high-temperature strength and room-temperature fracture toughness in the Mo-Si-B system

Microstructure and mechanical behavior of TiC-reinforced Ti-Mo-Al alloys

Ti-based alloys have gained extensive attractions in high-temperature engineering applications over the past several decades because of their low density, impressive strength and wear resistance. The continuing demands for advanced structural materials in aerospace and automobile sectors encourage further exploits of Ti-based alloys. Solid-solution hardening has been confirmed as an effective way to improve the mechanical performance of Ti-based alloys. Recent studies suggest that the incorporation of fibrous or particulate reinforcements, such as SiC, TiB and TiC, is necessary to maintain their high specific strength at elevated temperatures. In this study, Ti-Mo-Al (Ti50Mo35Al15, at.%) alloys with various TiC additions (1, 5, 10 at.%) were prepared by arc melting technique. We examined the microstructure of these as-cast alloys by X-ray diffractometry (XRD), scanning electron microscopy (SEM), transmission electron microscopy (TEM) and high-resolution TEM (HRTEM). Their mechanical properties were systematically evaluated via compression experiments at various temperatures (T=298, 1073 and 1273 K), Vickers hardness as well as four-point bending tests. According to the experimental observations, all the alloys prepared in this work were composed of two phases, Ti-Mo-Al solid solution ( phase) matrix and TiC particles. Most of the TiC particles precipitated along grain boundaries, following the N-W crystallographic relationship with the matrix. Moreover, the effect of TiC addition on the microstructure of Ti-Mo-Al alloys was mainly manifested in the reduction of average grain size, which is ~80 m in the alloy without TiC but ~30 m in the 10 at.% TiC-added one. The addition of TiC leads to an obvious enhancement of strength at both room and high temperatures, without impairing the ductility. It is worth noting that the maximum flow stress achieved in the TiC-reinforced Ti-Mo-Al alloys at 1273 K is ~400 MPa. Therefore, the reinforcement by TiC is an effective way in improving the mechanical performance of Ti-Mo-Al alloys

High-temperature creep strength and room-temperature fracture toughness of MoSiBTiC alloy

Quite recently, the author and his coworkers have developed a new high-temperature material based on Mo-Si-B alloys with TiC addition for ultrahigh temperature applications. The alloys are produced not by powder sintering but by casting, and the constituent phases are of Mo solid solution, Mo5SiB2 (T2), (Ti, Mo)C and (Mo, Ti)2C. The density is reduced to less than 9.0 g/cm3, which is comparable to that of Ni-base superalloys. The high-temperature compressive strength is much stronger than that of commercial heat-resistant molybdenum alloys such as TZM and MHC in a wide high-temperature range. In this paper, the recent progress of our research and development of the MoSiBTiC alloys is reviewed focusing on high-temperature creep strength and room temperature fracture toughness. The alloy having a primary phase during solidification of (Ti, Mo)C and thus a higher (Ti, Mo)C volume fraction was examined for tensile creep properties, and it was found that the alloy showed typical tensile creep curves accompanying transient, steady-state and acceleration creep stages in all the test conditions. The creep strength was relatively good, for example, the rupture time at 1350 °C under 170 MPa was about 750 h. The stress exponents, n, in the temperature range of 1400 – 1600 °C and the stress range of 100 – 300 MPa were ≈ 3 while it was 5 – 6 at 1350 °C, suggesting that the rate-controlling process of creep deformation is different between at and below 1350 °C and at and above 1400 °C in the stress range. Room-temperature fracture toughness of the MoSiBTiC alloys was measured by three-point or four-point bending tests using Chevron-notched specimens. The alloy having the primary phase of (Ti, Mo)C showed the fracture toughness value of better than 15 MPa(m)1/2 at room temperature. The value was better than that of the alloy having a primary phase of Moss and thus a higher Moss volume fraction. The obtained results indicated that (Ti, Mo)C phase works for improving not only high-temperature strength but also room-temperature fracture toughness

Synthesis of Mo–Si–B in situ composites by mechanical alloying

In this study, the synthesis of Mo-Si-B multi-phase alloys, so-called in-situ composites, was attempted with the combination of mechanical alloying (MA) and spark plasma sintering (SPS) processes. MA was conducted with mixed powders of Mo, Si and B using a planetary ball mill under various milling conditions. MAed powders were characterized by X-ray diffractometry (XRD) and scanning electron microscopy (SEM). The results obtained by XRD indicated that Mo-Si-B alloyed powders were successfully produced when elemental powders were milled at a higher milling energy. Vacuum heat treatments after the MA process promoted the formation of Mo-Si-B intermetallic phase in MAed powders. On the other hand, the MAed powders were successfully consolidated by a SPS technique, and as a result, sound compacts of Mo5SiB2-based composites were synthesized

Influence of vacuum annealing conditions on the surface oxidation and vacancy condensation in the surface of an FeAl single crystal

The influence of annealing atmosphere, temperature and time on the surface oxidation and vacancy condensation behavior of {111}-oriented single crystals of B2-type FeAl was investigated. AFM observation showed that as-annealed surfaces under a high vacuum were rugged and covered with a thin oxide film. The results obtained by TEM indicated that the thin oxide film was {001}-oriented κ-Al2O3 epitaxially grown on the {111}-oriented FeAl surface. AES measurements showed that the thickness of the oxide film was almost twice as thick as that of the passive Al2O3 film formed on the FeAl surface in an ambient atmosphere. It was found that the growth of surface mesopores is attributable to both the condensation of supersaturated vacancies in FeAl substrate and the Kirkendall effect by the surface oxidation during the vacuum annealing