J.H. Schneibel

Microstructure and mechanical properties of Mo–Mo3Si–Mo5SiB2 silicides

Abstract Molybdenum boron silicides consisting of 20–50 vol.% of α-Mo and different ratios of the intermetallic phases Mo5SiB2 (‘T2’) and Mo3Si were prepared by arc-melting followed by dropcasting into Cu chill molds. For α-Mo volume fractions of ≈50% sound castings were obtained. For lower α-Mo volume fractions such as 25% macroscopic cracks were often observed. Preliminary oxidation tests verified the expected increase in the oxidation resistance as the B-concentration increases and the α-Mo volume fraction decreases. Also, the formation of glass films was observed. Depending on composition and heat treatment, the room temperature flexure strengths varied usually between 300 and 600 MPa. To some extent, these variations could be rationalized by differences in the microstructures. Annealing for 1 day at 1873 K in vacuum caused distinct microstructural coarsening. Annealing was sometimes accompanied by microcracking in the Mo3Si and the T2 phases. Cooling curves suggest that the liquidus temperature of the T2 phase is above 2400 K. Depending on the composition, final freezing of Mo–Si–B alloys was found to occur at temperatures as low as 2200 K. Care is therefore required during thermomechanical processing to avoid the formation of liquid phases.

Tensile properties and fracture toughness of TiAl alloys with controlled microstructures

Abstract The objective of this study is to improve the mechanical properties by careful control of both microstructure and alloy additions in two-phase TiAl alloys based on Ti-47Al-2Cr-2Nb (at%). Hot extrusion at temperatures above Tα produces refined lamellar structures, whose microstructural features can be further controlled by subsequent heat-treatment at and above 900 °C. The mechanical properties of the alloys with lamellar structures depend on three factors: colony size, interlamellar spacing, and alloying additions. The tensile elongation at room temperature is strongly dependent on lamellar colony size, showing increasing ductility with decreased colony size. The strength at room and elevated temperatures is sensitive to interlamellar spacing, showing increasing strength with decreased colony spacing. The fracture toughness at room temperature can be substantially improved by heat-treatment at 1320 and 1350 °C. The tungsten addition at a level of 0.2% improves the tensile strength, whereas the silicon addition at a level of 0.3% reduces the castability of the TiAl alloys. The TiAl materials produced by hot extrusion are much superior to those produced by conventional thermomechanical treatments.

Recent advances in B2 iron aluminide alloys : deformation, fracture and alloy design

Abstract This paper reviews the deformation, fracture and alloy design of B2 iron aluminides based on FeAl. Moisture-induced environmental embrittlement is shown to be a leading cause of low tensile ductility and brittle cleavage fracture of Ferich FeAl alloys at ambient temperatures. With increasing Al concentration, two other factors, namely intrinsic grain-boundary weakness and quenched-in vacancies become important in limiting the tensile ductility of FeAl alloys. FeAl alloys show a yield-strength anomaly at intermediate temperatures. Recent work indicates that the anomaly is a result of hardening by thermal vacancies at elevated temperatures. The understanding of the deformation and fracture behavior has led to the development of FeAl-base alloys and composites with improved metallurgical and mechanical properties for structural applications. These FeAl-based alloys can be prepared by melting and casting or by powder processing. The unique combination of the excellent oxidation and carburization/sulfidation resistance coupled with relatively low material density and good mechanical properties at room and elevated temperatures has sparked industrial interest in FeAl alloys and composites for a number of applications.

A Mo-Si-B intermetallic alloy with a continuous α-Mo matrix

Abstract A novel technique for fabricating high-temperature molybdenum–silicon–boron intermetallic composites is described. Vacuum annealing of silicide particles resulted in a loss of Si, and consequently a Mo solid solution layer, at the surfaces. Subsequent consolidation produced a microstructure of silicide particles bonded by a continuous Mo matrix. The α-Mo imparted significant room temperature fracture toughness.

Processing and mechanical properties of a molybdenum silicide with the composition Mo–12Si–8.5B (at.%)

Abstract Alloys with the nominal composition Mo–12Si–8.5B (at.%) were prepared by arc-melting or powder-metallurgical processing. Cast and annealed alloys consisted of approximately 38 vol.% α-Mo in a brittle matrix of 32 vol.% Mo3Si and 30 vol.% Mo5SiB2. Their flexure strengths were approximately 500 MPa at room temperature, and 400–500 MPa at 1200°C in air. The fracture toughness values determined from the three-point fracture of chevron-notched specimens were about 10 MPa m1/2 at room temperature and 20 MPa m1/2 at 1200°C in air. The relatively high room temperature toughness is consistent with the deformation of the α-Mo particles observed on fracture surfaces. Three-point flexure tests at 1200°C in air and a tensile test at 1520°C in nitrogen indicated a small amount of high temperature plasticity. Extrusion experiments to modify the microstructure of cast alloys were unsuccessful due to extensive cracking. However, using powder-metallurgical (PM) techniques, microstructures consisting of Mo3Si and Mo5SiB2 particles in a continuous α-Mo matrix were fabricated. The room temperature fracture toughnesss of the PM materials was on the order of 15 MPa m1/2.

Ambient to high temperature fracture toughness and fatigue-crack propagation behavior in a Mo–12Si–8.5B (at.%) intermetallic

Boron-containing molybdenum silicides have been the focus of significant research of late due to their potentially superior lowtemperature ‘‘pest’’ resistance and high-temperature oxidation resistance comparable to that of MoSi2-based silicides; however, like many ordered intermetallics, they are plagued by poor ductility and toughness properties. Of the various multiphase Mo–Si–B intermetallic systems available, alloys with compositions of Mo–12Si–8.5B (at.%), which contain Mo, Mo3Si, and T2 phases, are anticipated to have higher toughnesses because of the presence of the relatively ductile Mo phase. In this study, we examine the ambient to high (1300 � C) temperature fracture toughness (R-curve) and fatigue-crack growth characteristics of Mo-12Si-8.5B, with the objective of discerning the salient mechanisms governing crack growth. It is found that this alloy displays a relatively high intrinsic (crack-initiation) toughness at 800 up to 1200 � C( � 10 MPa p m), but only limited extrinsic R-curve (crack-growth) toughness. Although the lack of extrinsic toughening mechanisms is not necessarily beneficial to quasi-static properties, it does imply in a brittle material that it should show only minimal susceptibility to premature failure by fatigue, as is indeed observed at temperatures from ambient to 1300 � C. Of particular significance is that both the fracture toughness and the threshold stress intensity for fatigue are increased with increasing temperature over this range. This remarkable property is related to a variety of toughening mechanisms that become active at elevated temperatures, specifically involving crack trapping by the a-Mo phase and extensive microcracking primarily in the Mo5SiB2 phase. Published by Elsevier Science Ltd.

Liquid-phase sintered iron aluminide-ceramic composites

Abstract Iron aluminide intermetallics, which exhibit excellent oxidation and sulfidation resistance, were found to be suitable as the matrix phase in metal matrix composites, or the binder in hard metals (or cermets). In particular, ceramics such as TiB2, ZrB2, TiC and WC could all be liquid-phase sintered with an Fe-40 at% Al iron aluminide. The processing and properties of such iron aluminide composites were assessed. Density, hardness, bend strength, and fracture toughness data are presented. Nearly complete densification was achieved for ceramic volume fractions up to 60%. For iron aluminide-tungsten carbide composites containing 60 vol% WC, room temperature three-point bend strengths and fracture toughnesses reached 1460 MPa and 20 MPa m 1 2 , respectively. Consistent with the high fracture toughness values, the fracture surfaces showed evidence for ductile deformation of the iron aluminide binder.

Stoichiometry and mechanical properties of Mo3Si

Abstract The A15 phase Mo 3 Si is an important constituent of a new class of silicides based on Mo–Si–B. In this research it will be shown that, contrary to published results, single-phase Mo 3 Si is slightly off-stoichiometric. In addition, it remains single phase in a small composition range. Its room temperature fracture toughness is on the order of 3 MPa m 1/2 . The compressive strength at 1400°C in argon decreases with decreasing strain rate and increasing Si concentration.

FeAl-TiC and FeAl-WC composites-melt infiltration processing, microstructure and mechanical properties

Abstract TiC-based and WC-based cermets were processed with iron aluminide, an intermetallic, as a binder by pressureless melt infiltration to near full density (>97% TD). Phase equilibria calculations in the quaternary Fe–Al–Ti–C and Fe–Al–W–C systems at 1450°C were performed to determine the solubility of the carbide phases in liquid iron aluminide. This was done by using Thermocalc™ and the results show that molten Fe–40 at.% Al in equilibrium with Ti 0.512 C 0.488 and graphite, dissolves 4.9 at.% carbon and 64 at. ppm titanium. In the Fe–Al–W–C system, liquid Fe–40 at.% Al in equilibrium with graphite dissolves ≈5 at.% carbon and 1 at.% tungsten. Due to the low values for the solubility of the carbide phases in liquid iron aluminide, liquid phase sintering of mixed powders does not yield a dense, homogenous microstructure for carbide volume fractions greater than 0.70. Melt infiltration of molten FeAl into TiC and WC preforms serves as a successful approach to process cermets with carbide contents ranging from 70 to 90 vol.%, to greater than 97% TD. Also, the microstructures of cermets prepared by melt infiltration were very homogenous. Typical properties such as hardness, bend strength and fracture toughness are reported. SEM observations of fracture surfaces suggest the improved fracture toughness to result from the ductility of the intermetallic phase. Preliminary experiments for the evaluation of the oxidation resistance of iron aluminide bonded cermets indicate that they are more resistant than WC–Co cermets.

Iron aluminide-titanium carbide composites by pressureless melt infiltration - microstructure and mechanical properties

In this investigation, processing of fully dense TiC-based cermets with iron aluminide (Fe-40 at. % Al) as a binder by pressureless melt infiltration has been clearly demonstrated. The carbide contents in these composites varied from 70 to 85 vol. %. Specimens with 30 vol. % intermetallic exhibited bend strengths of 1034 MPa, fracture toughness of 18 MPa{center_dot}m{sup 1/2} and a Rockwell (R{sub A}) hardness of 83.5. Further improvements in bend strengths may be possible by controlling the grain size and by modifications of the Fe40Al/TiC interface strengths.