The present article will describe the science and technology of titanium aluminide (TiAl) alloys and the engineering development of TiAl for commercial aircraft engine applications. The GEnxTM engine is the first commercial aircraft engine that is flying titanium aluminide (alloy 4822) blades and it represents a major advance in propulsion efficiency, realizing a 20% reduction in fuel consumption, a 50% reduction in noise, and an 80% reduction in NOx emissions compared with prior engines in its class. The GEnxTM uses the latest materials and design processes to reduce weight, improve performance, and reduce maintenance costs.

GE’s TiAl low-pressure turbine blade production status will be discussed along with the history of implementation. In 2006, GE began to explore near net shape casting as an alternative to the initial overstock conventional gravity casting plus machining approach. To date, more than 40,000 TiAl low-pressure turbine blades have been manufactured for the GEnxTM 1B (Boeing 787) and the GEnxTM 2B (Boeing 747-8) applications. The implementation of TiAl in other GE and non-GE engines will also be discussed.

The α, β, γ, and δ polymorphs of Y2Si2O7 were synthesized using sol-gel and solid-state methods. The structures of the α and γ polymorphs were determined by identification of isostructural rare-earth disilicates, and the structures were refined using Rietveld analysis of X-ray powder diffraction data. The α polymorph crystallizes in space group P1, with a=6.5872(6) Å, b=6.6387(7) Å, c=12.032(1) Å, α=94.501(7)°, β=90.984(8)°, γ=91.771(7)°, and volume=524.16(9) Å3. The γ form is described by space group P21/c, a=4.68824(5) Å, b=10.84072(9) Å, c=5.58219(6) Å, and γ=96.0325(3)°. The anisotropic thermal expansion of each phase was measured using high temperature diffraction up to 1200 or 1400 °C, depending on the stability of the polymorph. The thermal expansion is highly anisotropic for all polymorphs, with the low-expansion direction normal to the long axis of the corner-shared SiO4 tetrahedra.

Extended abstract of a paper presented at Microscopy and Microanalysis 2007 in Ft. Lauderdale, Florida, USA, August 5 – August 9, 2007

Extended abstract of a paper presented at Microscopy and Microanalysis 2006 in Chicago, Illinois, USA, July 30 – August 3, 2005

Nb-silicide based in-situ composites are promising materials for future high-temperature structural applications. Nb-silicide composites are typically alloyed with Hf, Ti, Cr, and Al to provide a balance of mechanical and environmental properties. A thermodynamic description of the Nb-Cr-Si system has been developed previously in literature based on reported isothermal sections. According to the previously calculated phase diagrams, selected alloys were directionally solidified. The as-solidified microstructures could not be interpreted using the liquidus projection calculated from the existing thermodynamic descriptions. Therefore, an improved thermodynamic description was developed by incorporating the new experimental data.

Extended abstract of a paper presented at Microscopy and Microanalysis 2005 in Honolulu, Hawaii, USA, July 31--August 4, 2005

Phase equilibria in ternary Hf-Ti-Si alloys were studied in the as-solidified and heat treated conditions using scanning electron microscopy, x-ray diffraction, and electron beam microprobe analysis. Selected solid-solid phase equilibria at 1350°C and a partial liquidus projection of the Hf-Ti-Si system at the metal rich end of the phase diagram were established. These data were then used to develop a thermodynamic description of the Hf-Ti-Si system using the CALPHAD (CALculation of PHAse Diagram) approach. The calculated isothermal section at 1350°C and the liquidus projection can satisfactorily account for the available experimental phase equilibria data and solidification paths. Both the calculations and the experimental data suggested that the metal-rich end of the ternary phase diagram possesses one transition reaction: L + (Hf, Ti)5Si3 → Hf(Ti)2Si + β(Hf, Ti, Si).

This article reviews the most recent progress in the development of Nb-silicide-based in situ composites for potential applications in turbine engines with service temperatures of up to 1350°C. These composites contain high-strength Nb silicides that are toughened by a ductile Nb solid solution. Preliminary composites were derived from binary Nb-Si alloys, while more recent systems are complex and are alloyed with Ti, Hf, W, B, Ge, Cr, and Al. Alloying schemes have been developed to achieve an excellent balance of room-temperature toughness, fatigue-crack-growth behavior, high-temperature creep performance, and oxidation resistance over a broad range of temperatures. Nb-silicide-based composites are described with emphasis on processing, microstructure, and performance. Nb silicide composites have been produced using a range of processing routes, including induction skull melting, investment casting, hot extrusion, and powder metallurgy methods. Nb silicide composite properties are also compared with those of Ni-based superalloys.

Nb-silicide based composites have excellent potential for future high-temperature structural applications. Nb-silicide composites possess Nb together with high-strength silicides, such as Nb5Si3 and Nb3Si. Alloying elements such as Ti and Hf, are added to obtain a balance of properties such as creep performance and oxidation resistance. In Nb-silicide composites generated from Nb-rich binary Nb-Si alloys, Nb3Si is unstable and experiences eutectoid decomposition to Nb and Nb5Si3. The present paper describes a low temperature eutectoid phase transformation during which (Nb)3Si decomposes into (Nb) and (Nb)5Si3, where the (Nb)5Si3 possesses the hP16 structure, as opposed to the tI32 structure observed in binary Nb5Si3.

Nb-silicide based in-situ composites combine a ductile Nb-based solid solution with high-strength silicides, and they show great promise for aircraft engine applications. The Nb-silicide controls the high-temperature creep behavior of the composite. Previous work has shown that the silicide composition has an important effect on the creep rate, with particular attention on the role of Ti and Hf additions. The aim of the present study is to understand the effects of the substitutional elements on the stability of the silicide phase, ordering in the crystal lattice, including the hP16-tI32 transition, and the creep behavior of the monolithic phases. To pursue this goal monolithic alloys with a range of compositions were prepared and the creep rates were measured at temperatures of 1100–1350°C. The stress sensitivities of the creep rates of the monolithic phases were also determined.

Composites based on Nb-Si are attractive candidates for use as structural materials at the very high temperatures required for future aircraft engines. The composites described were produced by directional solidification, which gives a microstructure consisting of Nb dendrites with an Nb3Si-Nb eutectic. The aim of this paper is to provide a detailed characterization of precipitates observed in the Nb dendrites in both binary and higher-order alloys. The precipitates possess the Nb3Si stoichiometry, but not the stable Nb3Si structure. The precipitates form a metastable orthorhombic crystal structure which is related to the Nb matrix via a simple orientation relationship.

In-situ composites based on Nb-Si alloys are potential candidates for application as structural materials in future aircraft engines. In order to achieve the balance of properties that is required, additional alloying species, such as Ti, Hf, Mo, Cr, Al, are employed. The present paper describes the effect of Hf additions on phase stability of Nb-Si alloys. Liquid-solid and solid-state phase equilibria have been investigated. The Nb-Hf-Si liquidus surface and the isothermal section at 1500°C will be described. A broad range of Nb and Hf compositions were investigated for Si concentrations up to 35.0%.

This paper will discuss the relationship between microstructure and creep behavior in hightemperature niobium-silicide based in-situ composites. The creep behavior of composites generated from binary Nb-Si alloys, and higher order alloys containing Mo, Hf and Ti additions, will be described. In-situ composites were tested in compression at temperatures up to 1200°C and stress levels in the range 70 to 280MPa. It was found that the Hf concentration can be increased to 7.5 with little increase in creep rate, over that for the binary Nb3Si-Nb composite, but at higher concentrations the creep rate is increased at stress levels higher than 21OMPa. At stresses less than 21OMPa the Ti concentration can be increased to 21 without a detrimental effect on creep performance, but at higher concentrations there is a substantial increase in the creep rate.

This paper examines microstructure-property relationships in high-temperature directionally solidified (DS) in-situ composites based on Nb silicides, such as Nb3Si and Nb5Si3. These in-situ composites are based on the Nb3Si-Nb binary eutectic, and are alloyed with Ti. They were prepared using cold crucible Czochralski crystal growth. Ternary Nb-Ti-Si alloys with Ti concentrations from 9 to 45%, and Si concentrations from 10 to 25%, were directionally solidified to generate aligned two- and three-phase composites containing a Nb solid solution with Nb3Si and Nb5Si3 silicides. Fracture toughness values generally greater than 10 MPa√m were measured in these composites. For a given Si concentration, the fracture toughness of the Ti-containing composites was increased ∼ 6 MPa√m over that of the binary alloy composites. The effects of Si concentration, and a range of Nb:Ti ratios, on microstructure, phase equilibria, and fracture toughness were examined.

The effects of Mo contamination on the microstructure of lamp-grade tungsten wire are investigated in this study. Mo is often used as the mandrel material in the coiling of lamp filaments, and when utilized during high temperature heat treatments significant amounts of Mo can diffuse into the W. In this study, W-Mo diffusion couples were prepared and heat treated at temperatures of 2500°C and above to intentionally contaminate the W with Mo. After dissolution of the Mo and additional high temperature heat treatments of the W, equiaxed grains were observed in the W and excessive porosity was also observed. Explanations for these microstructural changes are discussed. Microprobe data were also obtained to quantify interdiffusion between the W and Mo.

In-situ composites based on binary Nb-Si alloys and consisting of a Nb solid solution with Nb3Si or Nb5Si3 have shown a promising combination of low temperature and high temperature mechanical properties. The environmental resistance and room temperature fracture toughness of these composites can be further enhanced by additions such as Ti, Hf, Cr, and Al. In the present study, ternary Nb-Ti-Si alloys were prepared by directional solidification to generate aligned two and three phase composites containing a Nb solid solution with Nb3Si and/or Nb5Si3. The present paper will describe microstructures, phase equilibria and fracture toughness of these composites. The improvement in the room temperature fracture toughness over binary Nb-Nb5Si3 composites is discussed.

The present paper describes ductile-phase toughening in V-V3Si in-situ composites that were produced by conventional arc melting (AM), cold-crucible induction melting (IM), and coldcrucible directional solidification (DS). Notched three-point bending tests were performed to determine the effects of synthesis method on the room temperature fracture toughness of eutectic compositions, which contain nearly equal volume fractions of V3Si and the V(Si) solid solution phase. Fracture toughness values ranged from 10 MPa√m for the AM eutectic to over 20 MPa√m for the IM and DS eutectic alloys. SEM fractography, fracture surface profiling, and chemical analyses were performed to correlate the toughness values with the microstructures and interstitial concentrations produced by the three synthesis methods.

New materials with improved high temperature properties are required for higher efficiency gas turbines. One group of alloys which show promise in this area are the eutectic systems Cr-Cr3Si, Nb-Nb3Si, and V-V3Si. Crystals of these refractory metalsilicide eutectics were directionally solidified using Czochralski crystal growth in conjunction with cold crucible levitation melting. The crystals were examined using scanning electron microscopy and transmission electron microscopy. In the present paper, these systems are examined with respect to the directionally solidified eutectic microstructures, phase equilibria, crystallography and mechanical properties.

Gas-atomized spray deposition involves the creation of a spray of droplets by a gas atomizer and the consolidation and solidification of these droplets on a substrate. The present paper describes an investigation of the fundamental characteristics of heat transfer and solidification during spray deposition. Spray deposition was used to manufacture Sn-15 and 38 wt. % Pb preforms using atomizer-substrate distances of 180 and 360 mm, gas flow rates of 2.5 and 3.4 g/s, and melt flow rates of 61 and 35 g/s. Analytical and numerical models were developed to predict the thermal history of the spray deposit for a range of deposit-substrate heat transfer coefficients. A deposit-substrate heat transfer coefficient of ∼104 W m−2 K−1 was determined by comparing measured and calculated spray-deposit thermal histories both during and after spray deposition. Microstructural analysis of transverse sections of the spray deposits revealed maximum values of spray-deposit density and cell/grain size at specific distances from the deposit-substrate interface. The distance between the density and cell/grain-size maxima and the deposit-substrate interface increased from 0.9 to 10 mm for Sn–15 wt. % Pb and from 2.6 to 11.3 mm for Sn–38 wt. % Pb as the atomizer-substrate distance was increased from 180 to 360 mm and the melt to gas mass flow rate ratio was decreased from 24 to 10. The origin of these microstructural features is described in terms of heat transfer during spray deposition.