Paul Withey

Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades

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.

The Influence of ZrO2 Concentration in an Yttria-Based Face Coat for Investment Casting a Ti-45Al-2Mn-2Nb-0.2TiB Alloy Using a Sessile Drop Method

Abstract 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.

The Study of the Oxidation of a Ti–45Al–2Nb–2Mn–0.2B Alloy in the Investment Casting Using an Yttria Face Coat Mould

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.

Response to Discussion of “Formation of an Intermediate Layer Between Grains in Nickel-Based Superalloy Turbine Blades”

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Interactions between high Nb containing TiAl alloy and yttria facecoat during investment casting

Abstract 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.

The analysis of SATS results as a measure of pupil progress across educational transitions

Within any Educational System the transition of pupils from one stage to the next, and often the associated transition from one educational establishment to another, is an area of interest for educational establishments, educationalists and educational authorities due to the effects of this movement on pupil progress, their academic achievement and performance measures for schools. The National Curriculum Assessments (NCA) are used in England as a nationally administered examination to evaluate pupil progress and academic achievement at the transition from one Key Stage to another. Also within England different schools can have differently aged cohorts, for example Primary Schools cover ages 4 to 11 years whereas Infant Schools cover ages 4 to 7 years and Junior Schools 7 to 11 years. This investigation has examined the significance of type of primary school (i.e., all-age primary, 4 to 11 years, versus junior, 7 to 11 years) for achievement at age 11 years. Using national statistics, it was shown that junior and primary schools perform equivalently in terms of academic achievement at the end of the pupils’ time in the schools but primary schools seem to outperform junior schools in terms of the improvement in the pupils’ ability (value added) during their time in Key Stage 2. This work has shown that on average a junior school will have a lower value added score at Key Stage 2 to equivalently performing primary schools and that this difference, whilst small, is both statistically, and in terms of league table position, significant. Also, the data were compared to the much smaller group of schools which provide education from Key Stage 1 through to Key Stage 4 and beyond. These schools showed the same rate of progress (value added) through Key Stages 1 and 2 as the general population of schools but with lower points scores per student.

A Comparison of the Chemical Inertness of Two Y2O3-Al2O3-ZrO2 and Y2O3-Al2O3 Face Coat Materials Through Sessile Drop and Investment Casting Methods

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.

Influence of Al2O3 concentration in yttria based face coats for investment casting Ti–45Al–2Mn–2Nb–0·2TiB alloy

Abstract 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 were lower than 33·33 mol.-%; however, increasing addition amount of Al2O3 will increase the chance of face coat and metal interaction.

Enhanced sintering properties of yttria face coat by addition of B2O3, YF3, Al2O3 and ZrO2

Abstract 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 around 1300–1400°C. It may be possible to use these sintering aids for the production of investment casting moulds.