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Ni-based single crystal superalloy turbine blades have excellent mechanical strength and resistance to corrosion and oxidation due to a uniformly distributed gamma prime phase in a gamma matrix. However, defect grains have been often found on the surface of turbine blades after manufacturing, which can be potential sites of crack initiation. In this study, several different types of surface defect grains formed in third generation Ni-based single crystal turbine blades, such as stray grains, freckle chain grains, equiax grains, and a new grain formed in surface scale, had been investigated. The grain boundary regions were observed by high resolution electron microscopy. Although the formation mechanism of each grain defect is different, secondary phases, such as rhenium-rich particles, have been always found in each grain boundary. In addition, depending on the existence of the secondary phases as well as the size of defect grains, different microstructures were observed even in the same defect grain boundary. Finally, the observed results suggest that if there is any boundary region in a turbine blade, secondary phases, such as Re-rich particles, can be found.

Yttria is the best face coat material in the shell mold for the investment casting of γ-TiAl alloys. However, yttria inclusions may occur in the cast products and there is a lack of knowledge about the influence of yttria inclusions on the mechanical properties of TiAl. In the present study, the effect of yttria inclusions on room temperature tensile properties of two investment cast TiAl alloys, Ti-45Al-2Mn-2Nb (at%) + 0.08 vol% TiB2 and Ti-46 Al-2 Nb-2Cr-0.15B (at%), were studied. The results show that tensile failure tends to initiate from yttria inclusions, resulting in reduced plastic strain and ultimate strength. Yttria inclusions differ in location (surface /internal), size, and morphology (agglomerate /whole piece), those with larger sizes or located at the sample surface being more harmful. The tensile properties of inclusion containing TiAl parts can be estimated using the size and location determined by X-ray inspection. The critical size at which inclusions become detrimental to the mechanical properties of TiAl was discussed.

Borides have been widely used in cast TiAl alloy for grain refinement and a variety of stoichiometry and crystal structure of borides were reported. Here the effects of alloying elements Nb, Ta, and Mn on the structural stability of fine boride precipitates in TiAl alloys have been studied combining transmission electron microscopy (TEM) and first-principles calculations. The results show that most boride particles have the TiB stoichiometry. In the alloy containing Nb and Mn, all the TiB particles have the B27 structure and are highly enriched with Nb but depleted with Mn. In the alloy containing Nb and Ta, however, the intergrowth of B f and B27 structure has been observed, and the TiB particles are enriched with both Nb and Ta. First-principles calculations reveal different effects of Nb, Ta, and Mn on the structural stability of TiB polymorphs. Nb stabilizes B27 but destabilizes B f . Ta strongly stabilizes both B27 and B f structures. Mn strongly destabilizes both B27 and B f structures.

Using X-ray diffraction, hardness evaluation and scanning election microscopy, the interaction between a Ti–46Al–8Nb–B alloy and an yttria facecoat was analysed after investment casting with different cast bar diameters. Results showed that the interaction between an yttria facecoat and alloy can cause the oxygen in the facecoat to diffuse into the alloy to form an interaction layer at the metal/shell interface. Aluminium was observed to evaporate at the alloy surface, and the remaining Al ions at the interaction layer interacted with oxygen to form Al2O3 whiskers. Meanwhile, some Nb rich precipitates were found at the metal/shell interface. By increasing the casting bar diameters, the interaction layer thicknesses were increased with the associated growth of Nb rich precipitates.

Casting of titanium based alloys presents considerable problems including the extensive interactions that occur between the metal and refractory. An investigation was undertaken to develop a zirconia facecoat suitable to replace current zircon/silica facecoat used in investment casting titanium aluminide alloys. The stability of the zirconia slurry was assessed using pH, viscosity and plate weight, and the mould properties such as friability, strength and permeability were measured. The interaction between the zirconia face coat and a Ti–46Al–8Nb–1B alloy was studied by centrifugal investment casting at three mould pre-heating temperatures. Computer simulation of metal cooling profiles during casting was also carried out to assist the analysis. In this work, a stable zirconia slurry was developed, and the moulds produced using a zirconia facecoat have comparable mechanical properties and better permeability than those made of the traditional zircon/silica facecoat. The friability of the zirconia facecoat was much improved in comparison to that made of yttria face coat. During casting, metal non-fill defects were presented at a low preheat mould temperature of 500 °C. The interaction products found between the metal and mould included a combined (Ti, Zr) 5 (Si, Al) 3 and ZrAl 2 phases, a re-precipitated ZrO 2 phase, and a Al 2 O 3 film at the interface. The interaction between mould and metal also caused a high hardness at the interface due to oxygen penetration, which varied with samples using different mould pre-heat temperatures. The suggestion has been made that the mould pre-heat temperature should be less than 1200 °C.

Investment casting is widely used to cast near-net shape components, reducing material waste and process cost. Due to the high reactivity of titanium and its alloys, in order to reduce the interaction between the mold and molten titanium, very costly materials are used in the mold face coat. In order to reduce the material cost while maintaining the chemical inertness of the face coat, ZrO2 was added into an yttria-based face coat in different concentrations in this study. The face coat properties of the different systems were analyzed using dilatometry and XRD. The chemical inertness of the different face coat systems were tested using a sessile drop test using a Ti-45Al-2Mn-2Nb-0.2TiB alloy, and the interactions between the face coat and the alloy were analyzed by the interfacial microstructures, contact angle, and hardness changes. The results showed that small amounts of ZrO2 can be added into yttria without changing the chemical inertness of the face coat. High amounts of ZrO2 in the face coat can interact with TiAl alloy to form different interaction products. Meanwhile, both Y2O3 and ZrO2 filler materials were observed to dissolve in molten TiAl during the sessile drop test.

A stray grain exposed on the surface of a Re-containing single crystal Ni-based superalloy turbine blade was examined by high-resolution analytic electron microscopy. An intermediate layer with the size range of 3 to 4 μm, composed of elongated γ′ phase, was clearly detected forming a boundary between a normal matrix grain and a stray grain. Beyond the intermediate layer, the {100} direction of the γ′ phase in the matrix changed slightly, and a stray grain was formed. In each γ′ grain in the intermediate boundary, a Re-rich region of the size range of 100 to 200 nm was detected, suggesting a role for rhenium in the formation of stray grains.

In order to reduce the interaction between the Ti alloys and ceramic shell during the casting, materials with high thermal and chemical inertness were used in investment casting. An investigation was undertaken to analyze the influence of the change of binder systems on the slurries, facecoats and the thermo-chemical properties of the facecoat systems using an Y2O3–ZrO2 filler material. The results showed that, using alumina-sol as the binder in the slurry gave the longest life of around three days followed by that using the commercially available zirconia-sol at around 6 h, and the yttria sol based slurry giving a shortest life of around 1.5 h. Meanwhile using the alumina-sol can also enhance the facecoat sintering properties. There was no obvious evidence observed that the change of the binder system influenced the facecoat chemical inertness.

Textures of α2 and γ phases in Ti–47Al–2Cr–2Nb–0.15B (at.%) rectangular billet, extruded below its α transus temperature, were evaluated and their relation to the mechanical properties of TMTL microstructure was investigated. Strong α2 phase texture {11 2 ¯ 0} 1 ¯ 0> was found after extrusion and it evolved into {10 1 ¯ 0} 2 ¯ 0> recrystallization texture via oriented growth during α solution treatment. The α/α2 phase texture changes α grain growth behavior at Tα, such that it is slow at first but becomes rapid when the solution time exceeds a critical value, creating a new method of obtaining fine-grained TMTL microstructure. Mechanical properties of the TMTL microstructure were shown to be closely related to the Kearns texture factor of the α2 phase along the loading direction. The center and edge of as-extruded billet have distinct γ phase textures, dominated by β fiber (center) and featuring multiple texture components (edge), respectively. In the highly strained edge region, the presence of C-twin, Y, and Z texture components suggests the occurrence of mechanical twinning and shear band deformation, while B/G auxiliary component was found to act as a transition band from deformation to cubic recrystallization texture, in support of the oriented nucleation hypothesis for the formation of cubic texture.

Alumina has been used as a mould face coat material for investment casting titanium alloys for many years; the interaction between the Al2O3 and the alloys are exemplified by a hardened layer formed at the alloy interface. In this research, the interaction between the Al2O3 face coat and a Ti–46Al–8Nb–1B alloy was investigated by modelling the cooling profile of the mould during casting. The interaction between the mould and metal was identified from the hardened layer thickness and metal/shell interface microstructural changes. Results showed that a high mould pre-heat temperature and large bar diameter will enhance the interaction between the shell and the TiAl alloy. During the casting process, not only was oxygen observed penetrating into the metal through the decomposition of the face coat materials, but also silicon from the backup coat was found to interact with the metal during the casting process.

Investment casting is uniquely suited to the manufacture of Ti alloys for the production of near net-shape components, reducing material waste, and machining costs. Because of the high reactivity of titanium and its based alloy, the molds which are used in the investment casting process require high chemical inertness, which results in them being very costly and non-recyclable. In order to reduce the cost of these molds, traditionally using yttria as the face coat, two alternative molds are developed in this study with face coat materials of Y2O3-Al2O3 and Y2O3-Al2O3-ZrO2. The slurry properties and chemical inertness of the face coats were evaluated for viscosity, thermal expansion, friability, and phase development. The chemical inertness of these two molds were determined using both the sessile drop test and investment casting to identify the levels of interaction with a Ti-45Al-2Mn-2Nb-0.2B alloy. The results illustrated that the molds using Y2O3-Al2O3 and Y2O3-Al2O3-ZrO2 as the face coats both showed excellent sintering properties and chemical inertness when compared to the yttria face coat. They can consequently be used as two alternative face coats for the investment casting of TiAl alloys.

An Al2O3–Y2O3 based face coat material was researched by adding Al2O3 into yttria in different concentrations to reduce the mould cost whilst retaining the face coat chemical inertness. The face coat compositions and sintering properties at different temperatures were investigated using X-ray diffraction, friability and dilatometers. Meanwhile, the chemical inertness of the face coats was identified using a sessile drop test method to interact with a Ti–45Al–2Mn–2Nb–0·2TiB alloy. The results show that at different firing temperatures, a series of phase transformations have taken place between Y2O3 and Al2O3, resulting in different face coat compositions and sintering properties. However, the chemical inertness of the face coat against the molten TiAl alloy was not influenced by the phase transformations, and it was dependent on the amount of Al2O3 added to the face coat. In this experiment, no interaction was observed between the Al2O3–Y2O3 face coat material and TiAl alloy when Al2O3 additions we...

Investment casting is an economical method to manufacture near net-shape metal components. Due to the very high thermal and chemical inertness, yttria has been widely used as the mould face coat material for the investment casting of titanium alloy for many years. An investigation was undertaken to study the oxidation behaviour of TiAl alloy during casting in a mould using pure yttria as the face coat. This research shows that the TiAl alloy was still oxidized in the mould during casting when using yttria as the face coat. During high temperature casting, the yttria in the face coat was dissolved by high temperature molten metal flow. The oxygen from the yttria face coat diffused into TiAl and interacts with TiAl to form different microstructure and phases (e.g. precipitates such as oxygen enriched Ti3Al and Al2O3 phases). Meanwhile, the dissolved yttrium was then re-precipitated at the metal interfacial area as yttria inclusions after metal cool down.

Components which best utilise the properties of high temperature titanium alloys are characterised by thin sections of a few millimetres thickness and hundreds of millimetres length. These alloys however are difficult to work with, being highly reactive in a molten state, necessitating a low superheat during processing. Centrifugal casting is therefore utilised as a candidate production method, as under the centrifugal force, metal can rapidly fill thicknesses substantially less than a millimetre before it solidifies. However, due to the high liquid metal velocity developed there is a high risk of turbulent flow and of the trapping of any gas present within the liquid metal. This challenging application involves a combination of complex rotating geometries, significant centrifugal forces and high velocity transient free surface flows, coupled with simultaneous heat transfer and solidification. Capturing these interacting physical phenomena, free surface flows, trapped air and associated defects is a complex modelling task. Building upon earlier work on computational modelling the authors have previously described to capture and validate the fluid dynamics behaviour of rotating systems, this contribution considers the modelling and validation of such systems to capture the coupled flow and thermal solidification behaviour and associated defect development. A bench-mark test case is employed to validate the effect of solidification on the fluidity of an aluminium alloy. Validation is also performed against a series of casting experiments to establish the models ability to capture the filling process and predict defects due to air entrapment within the solidified metal.

The sintering behaviour of conventional yttria powder was investigated, with emphasis on the effect of sintering additives such as B2O3, YF3, Al2O3, ZrO2, and TiO2, etc. at sintering temperatures from 1000 °C to 1600 °C. Powder shrinkage behaviour was analysed using a dilatometer. The powder sintering mechanisms were identified at different temperatures using powder isothermal shrinkage curves. This analysis showed that the sintering additives B2O3 and YF3 could improve yttria sintering by changing the diffusion/sintering mechanisms at certain temperatures, while sintering additives TiO2, Al2O3 and ZrO2 appeared to retard the powder densification at temperatures around 1000 °C and are more suitable when used at temperatures in excess of 1300 °C. The powder with La2O3 added had the slowest densification rate throughout the test temperatures in this experiment and was also found to be more suitable when used at temperatures higher than 1550 °C.

Yttria is widely used as a key material in the manufacture of the mould face coat for the investment casting of Ti or titanium aluminide alloys, as it has the required thermal and chemical inertness. Because of the high melting temperature of yttria, moulds manufactured using an yttria face coat need high sintering temperatures to achieve a good surface finish, which increases the production cost of the mould. In this research, small amounts of sintering aids, B2O3, YF3, Al2O3 and ZrO2, were added into a simple yttria face coat and sintered at temperatures from 1000 to 1400°C to investigate the influence on the sintering properties of yttria. These experiments found that the sintering additive B2O3 showed the best performance in enhancing yttria face coat sintering over the test temperatures. Sintering aids such as YF3 only slightly altered the sintering properties of yttria at temperatures around 1000–1200°C, and powders such as YAZ were seen to be more suitable for sintering at temperatures arou...

Post-heat treatment recrystallisation is a significant cause of scrap in single crystal component production. This defect could be prevented by a reduction in the mechanical strength of the shell after the metal has solidified. Addition of a zirconia layer, to produce large cracks around grain boundaries and among the large grains during phase transformations, was considered as a method of reducing sintered strength. In this study, two types of zirconia stucco, monoclinic and partially stabilised, were introduced into the shell. The effect of the incorporation of a zirconia layer on the microstructure and mechanical properties of the shell were investigated. Shell with a monoclinic zirconia additional layer showed no reduction in the green strength but a better performance in high temperature deformation tolerance, maintaining shell performance during wax removal and metal casting. The shell strength after alloy solidification is reduced, suggesting that a reduction in recrystallisation may be pos...

Yttria is an important primary coat material in investment shells for casting extremely reactive TiAl alloys and polymer is usually added to slurry to improve strength of the shell. In this investigation, systems that vary the polymer content through 0%, 6% and 30% were produced and the samples were sintered at 1000 °C, 1200 °C and 1400 °C. The results suggest that polymer content and sintering temperature appear to have little effect on the hardened alpha layer of the cast alloy, which is elevated by near-surface oxygen content. Silica from the backing coat is seen to travel through the primary coat and diffuse into the alloy regardless of the shell system. Firing temperatures above 1200 °C increase the shell strength undesirably but the friability of the primary coat decreases as firing temperature increases. Higher mould permeability was found in the mould containing higher polymer level and this would be advantageous for complete mould filling.

The investment casting process is an economic production method for engineering components in TiAl-based alloys and offers the benefits of a near net-shaped component with a good surface finish. An investigation was undertaken to develop three new face coat systems based on yttria, but with better sintering properties. These face coat systems were mainly based on an yttria-alumina-zirconia system (Y2O3-0.5 wt pct Al2O3-0.5 wt pct ZrO2), an yttria-fluoride system (Y2O3-0.15 wt pct YF3), and an yttria-boride system (Y2O3-0.15 wt pct B2O3). After sintering, the chemical inertness of the face coat was first tested and analyzed using a sessile drop test through the metal wetting behavioral change for each face coat surface. Then, the interactions between the shell and metal were studied by centrifugal investment casting TiAl bars. Although the sintering aids in yttria can decrease the chemical inertness of the face coat, the thickness of the interaction layer in the casting was less than 10 μm; therefore, these face coats still can be possible face coat materials for investment casting TiAl alloys.

The investment casting process offers great freedom of design with the economic advantage of near net shape production and has been widely used to process TiAl alloys. An investigation was undertaken to develop stable and cheap refractory slurry systems, suitable for use as a face coat in the investment casting of titanium aluminides. A TiO2–Al2O3 (TiAlO) based face coat slurry was investigated and compared with a traditional yttria based face coat slurry. After sintering, the chemical inertness of the manufactured shells in contact with molten TiAl alloys was tested by simulating the casting process using a flash re-melting test, in which sample shells were in contact with titanium aluminides at given temperatures for varying time durations and subsequently re-solidified. Compared with yttria, the TiAlO face coat had relatively lower chemical inertness against molten alloys, forming a thick hardened layer at the metal/shell interface with massive interaction products along grain boundaries. The reaction also changed the wetting behaviour of the TiAl on the TiAlO face coat with the contact angle decreasing as a function of interaction time.

Centrifugal casting offers one potential route through to high quality products in difficult to cast high temperature low superheat alloys. The coupling of free surface flows and complex rotating geometries, results in significant centrifugal forces; combined with rapid heat transfer and solidification this yields a significant computational modelling challenge. The ultimate objective of the work reported here is to develop a comprehensive computational model of centrifugal casting that can reliably predict the macro-defects that arise from the process. This contribution describes the development of the computational model, enabling capture of the free surface detail of the flow during the filling stage of centrifugal casting in what are inevitably complex three dimensional geometries. The work reported here concentrates on resolving the fluid film formation, air entrainment within the turbulent gas-fluid interface and transport of gas bubbles through the mould. Validation of the above phenomena is a key issue and a series of water model experiments has been performed and recorded using high-speed video image capture. The objective is to validate and refine the computational model predictions using repeatable high quality experimental data, before application of the model in analysing full-scale centrifugal casting process. A number of key observations arise from this work, such as accurate prediction of the initial pour dynamics, maintaining a sharp fluid–air interface on complex meshes in order to capture the fluid film and air bubble behaviour, and employing higher order schemes to capture the vortex formation. These phenomena are not trivial to capture within the computational modelling tools, however, for the models to be useful in the analysing the full scale casting process, such physics must be reflected within their predictive capability.

The boundary region formed on the surface of nickel-based single-crystal turbine blades was investigated by high-resolution microscopy observation. There was a distinguishable intermediate layer with the size of about 2 to 5 μm between the matrix and surface defect grains such as stray grains, multiple grains, freckle grains, and even low-angle grain boundaries which were formed during the solidification of turbine blades. The intermediate layer was composed of many elongated γ′ as well as γ phases. In addition, only one side of the intermediate layer was coherent to the matrix grain or defect grain due to good orientation match. At the coherent interface, the γ′ (as well as γ) phase started to extend from the parent grain and coincidently, rhenium-rich particles were detected. Furthermore, the particles existed within both elongated gamma prime and gamma phases, and even at their boundary. Based on experimental observations, the formation mechanism of this intermediate layer was discussed.