On the fracture and fatigue properties of Mo-Mo3Si-Mo5SiB2 refractory intermetallic alloys at ambient to elevated temperatures (25 °C to 1300 °C)

Abstract

The need for structural materials with high-temperature strength and oxidation resistance coupled with adequate lower-temperature toughness for potential use at temperatures above ϳ1000 ЊC has remained a persistent challenge in materials science. In this work, one promising class of intermetallic alloys is examined, namely, boron-containing molybdenum silicides, with compositions in the range Mo (bal), 12 to 17 at. pct Si, 8.5 at. pct B, processed using both ingot (I/M) and powder (P/M) metallurgy methods. Specifically, the oxidation ("pesting"), fracture toughness, and fatigue-crack propagation resistance of four such alloys, which consisted of ϳ21 to 38 vol. pct ␣ -Mo phase in an intermetallic matrix of Mo 3 Si and Mo 5 SiB 2 (T 2 ), were characterized at temperatures between 25 ЊC and 1300 ЊC. The boron additions were found to confer improved "pest" resistance (at 400 ЊC to 900 ЊC) as compared to unmodified molybdenum silicides, such as Mo 5 Si 3 . Moreover, although the fracture and fatigue properties of the finer-scale P/M alloys were only marginally better than those of MoSi 2 , for the I/M processed microstructures with coarse distributions of the ␣ -Mo phase, fracture toughness properties were far superior, rising from values above 7 MPaΊm at ambient temperatures to almost 12 MPaΊm at 1300 ЊC. Similarly, the fatigue-crack propagation resistance was significantly better than that of MoSi 2 , with fatigue threshold values roughly 70 pct of the toughness, i.e., rising from over 5 MPaΊm at 25 ЊC to ϳ8 MPaΊm at 1300 ЊC. These results, in particular, that the toughness and cyclic crack-growth resistance actually increased with increasing temperature, are discussed in terms of the salient mechanisms of toughening in Mo-Si-B alloys and the specific role of microstructure.

Fracture and fatigue resistance of Mo–Si–B alloys for ultrahigh-temperature structural applications

“…In this regard, the increase in fracture resistance exhibited by the "medium" alloy at 1300°C, consistent with other Mo-Si-B alloys [8,9], is a highly desirable property. Observations of crack profiles (Fig.…”

“…MPa√m [8,9], i.e., they showed only marginal improvements in toughness over monolithic silicides. Additionally, the fracture toughness appeared to improve at higher temperatures; specifically the "medium" alloy displayed a 50% rise in peak toughness from 10 to 15 MPa√m on increasing the temperature from 25° to 1300°C.…”

“…In previous reports [8,9], we have described alloys containing (α-) molybdenum particles surrounded by the hard but brittle Mo 3 Si and Mo 5 SiB 2 (T2) intermetallic phases, which display definitive, but still small, improvements in toughness relative to monolithic silicides. Clearly, the key to achieving high fracture resistance in these materials is in making more effective use of the relatively ductile molybdenum phase, not unlike nickel-base (γ/γ′) superalloys, where high fracture toughnesses are obtained with a similarly high fraction of intermetallic (γ′) precipitates.…”

Fatigue of Brittle Materials

“…Accordingly, the use of laminate reinforcements in the arrester orientation, which require the crack to renucleate across the ductile layer, both increase the crackinitiation toughness and remain equally potent under cyclic loading. Such behavior is shown by coarser-scale Nb/Nb 3 Al laminated composites [73,83] and the newly developed boron-modified molybdenum silicide alloys [84,85] described below in section 4.12.5.2.1.…”

“…One excellent example of the use of both intrinsic and extrinsic toughening mechanisms acting in concert to promote some degree of resistance to fatigue-crack growth in intermetallics materials [84,85] is afforded by the recently developed boron-modified molybdenum silicide alloys [86]. Whereas MoSi 2 -based silicides have good oxidation resistance up to 1700°C and relatively easy processibility, they are plagued, like many ordered intermetallics, by poor ductility and fracture toughness properties; in addition, they are susceptible to severe low temperature oxidation ("pesting") problems [87].…”