Impression creep testing is a technique in which the deformation resulting from load applied via a rectangular indenter can be converted relatively straightforwardly into a proxy for creep minimum strain rate. This offers a valuable route to assess the creep performance ranking of in-service high temperature plant materials for a number of reasons: the small specimen size makes extraction feasible without significantly affecting the structural integrity of plant; the possibility to test a single specimen at several stresses or temperatures enables multiple assessments; and, increasingly, the maturity of underlying technical understanding and quality of results increases confidence in the technique. However, the method is not without challenges, in particular the capital and running costs associated with servo-electric test rigs. Development of a bespoke deadweight loaded testing system at Wood (formerly Amec Foster Wheeler) has enabled commercially sustainable impression creep testing, which has been successfully applied to ex-plant Grade 91 steel.

Fatigue endurance testing of nuclear plant materials is typically carried out using uniaxial specimens tested under strain control using a triangular waveform and a defined strain rate. This leads to long test durations and high testing costs when testing in the High Cycle Fatigue (HCF) regime, meaning few results for fatigue lives above 106 cycles are available. A novel test method is proposed here, in which the traditional strain-controlled test method is used until 105 cycles have elapsed, before testing is switched to load control at a higher frequency. Testing of a Type 304LN austenitic stainless steel in room temperature air was performed at an R ratio of -1, strain rate of 0.4%/s and strain amplitude of 0.18%. The results show no statistically significant difference between the HCF lives gained from the traditional and new test methods. The proposed new fatigue endurance test method is considered to be validated for use in the testing of nuclear grades of unstabilised austenitic stainless steel in air at room temperature.

The need to drive down the operating cost of aero-engines has led to increased interest in failure prediction and life extension of components in gas turbines. The aim of this paper is to present a generic microstructure-based remnant life assessment method for nickel-based superalloys, built upon an understanding of the dominant damage micromechanisms. Mechanical testing has been carried out on two superalloys (conventionally cast IN738LC and the single crystal CMSX4) to asses the effects of strain and thermally-induced damage. The microstructural re-arrangements accompanying creep and fracture have been studied using FEGSEM and the results obtained have been used to develop a microstructure-explicit deformation model that links microstructural evolution to the macroscopic behaviour. The model predictions are in good agreement with experiment in terms of the strain accumulation and microstructural changes. Thus, the proposed approach provides a potential framework for developing a valuable tool in undertaking remnant life assessment.

As the need for the prediction of component life and maintenance interval schedules becomes more demanding, there is an increasing requirement for thermo-mechanical fatigue (TMF) test data including fatigue crack growth rates under such conditions. The test equipment requirements to meet this challenge are discussed and finally a conventional servo-electric load frame is utilised in combination with a radiant lamp furnace to generate the desired thermal cycles. The radiant lamp furnace enables reasonably consistent temperature gradients to be achieved with notched test pieces.The temperature calibration method to achieve the desired thermal cycle will be briefly described, along with some considerations for ensuring reproducibility in the thermal cycles applied during tests. The measurement of crack growth under TMF conditions will also be considered. Such measurements are challenging because of the changing thermal conditions and the effect on conventional potential difference (PD) electrical methods. These effects make it difficult to continuously monitor crack size during the thermal/load cycles. Thus, convenient dwells within the TMF cycles, where both the load and temperature are held constant for a brief period, have been utilised to make time-averaged crack size measurements using PD methods. The performance of these experimental methods has been demonstrated with some trials on an advanced nickel base superalloy, RR1000.

The superplastic forming (SPF) of titanium alloys is an established technology. A reduction in grain size from that of the typical sheet materials would lead to enhanced SPF properties and hence a reduction in production cycle times. This study describes the microstructural development and superplastic behaviour of fine-grained Ti-6%Al-4%V alloys. Ball-milling Ti-6%Al-4%V powder produces a nanocrystalline material; however on consolidation by hot isostatic pressing rapid grain growth occurs. Addition of boron powder during milling leads to boride precipitates in the matrix of the consolidated alloy. The precipitates are dispersed inhomogeneously, resulting in localized grain refinement. Superplastic testing revealed cavitation formation but in comparison to conventional sheet material, large elongations were achieved at relatively high strain rates.

Titanium aluminide alloys offer considerable promise for use in high temperature applications, such as gas turbines. In this study an extruded Ti–46Al–5Nb–1W alloy has been examined, in terms of its tensile and creep behaviour. A reasonably fine and uniform microstructure was found in this bar product. This gave excellent properties, with tensile strengths up to ∼950 MPa at room temperature, along with 1% elongation. These properties were accompanied by a very good creep behaviour, with low primary strains at the lower stresses and very low secondary creep rates. Comparison of the creep properties of this titanium aluminide alloy with other similar compositions and some typical nickel alloys shows that it is significantly superior to first generation titanium aluminides but also nickel alloys, such as IN718 and Udimet 720Li. However, the strain controlled fatigue performance of the titanium aluminide alloy was significantly poorer than these same wrought nickel alloys.

The increasing performance requirements of gas turbines are driving higher operating temperatures in critical components, leading to greater creep and fatigue interactions. There is thus a requirement for thermomechanical fatigue (TMF) test data, including TMF crack growth rates. The test equipment required to meet this challenge is briefly discussed, along with the development of the test methodology. Thus, the accurate measurement and maintenance of thermal performance during TMF are considered. The determination of fatigue crack growth rates by conventional electrical potential difference measurements has been employed, utilising convenient dwells within the TMF cycles, where both load and temperature are held constant for a brief period. These experimental methods have been demonstrated with trials on the advanced nickel base superalloy RR1000 used for turbine discs. This work has also highlighted significant microstructural effects on the crack growth rates, with the coarser grain size offering a reduced crack growth rate.

Paper on the evaluation of repair techniques for IN 738 and MarM002 nickel superalloys presented as part of RTO-MP-AVT-163-Additive technology for repair of military hardware.

Microstructure and mechanical properties of γ-TiAl alloy with the chemical composition Ti–48Al–2Cr–2Nb (at.%) have been investigated. The alloy was prepared in one step using a combustion synthesis + compaction process, where the synthesis and the shaping take place at the same time. Two different combustion routes were used: forced self-propagating high-temperature synthesis + compaction (FSHS + C) and thermal explosion + compaction (TE + C). Fully lamellar microstructure was obtained in both processing routes after the appropriate homogenisation thermal treatment. Nevertheless, TE + C route showed coarser lamellar colonies. A remarkable oxygen content reduction was achieved in samples synthesised by FSHS + C route. Tensile properties (especially UTS) were found to be in the same range as other alloys obtained by conventional processing routes, however, low ductility was achieved. Good creep and fatigue properties were obtained. All fracture surfaces showed a brittle fracture mechanism. Finally, it was found out that the studied processing routes showed promising results as an alternative manufacturing technology for γ-TiAl based alloys.

The mechanical behaviour of high performance Ni alloys is required for many applications and where experimental data is not readily available then a suitable predictive approach would be beneficial. There are numerous routes to achieve this, however, here the data driven neural network method has been adopted to produce models for the tensile, creep and fatigue performance of nickel base alloys. These models have been successfully developed and tested against a range of criteria. The tensile and creep models have displayed excellent fidelity to known nickel alloy behaviour, while good correspondence was also achieved for the fatigue properties (both strain and stress controlled). Potential routes to further improve the performance of these models have been discussed.

A number of blocks of the high-strength alpha-beta titanium alloy Ti-6Al-2Sn-4Zr-Wo (Ti-6246) were manufactured using direct laser fabrication. Two processing methods were investigated; one of which used a high-power (18 kW) CO2 laser, the other using a (4 kW) Nd-YAG laser. Following manufacture, the blocks were sectioned and mechanically tested to compare the properties of material produced using the two processes. The mechanical assessment involved a combination of room temperature tensile testing and fracture toughness measurements. The results revealed that the Ti-6246 produced using both processing routes exhibited a low ductility (<3% elongation). The fracture toughness figures were, however, acceptable. The low ductility was thought to be linked to the presence of a large transformed beta grain size together with the presence of grain boundary alpha. (c) 2005 Qinetiq. Published by Elsevier B.V. All rights reserved.

gamma TiAl sheet offers the potential to replace nickel alloy and steel in some gas turbine applications, with weight savings of up to 40%. Whilst it is recognised that the joining of 7 TiAl sheets is a key manufacturing technology for such applications, it is relatively poorly developed and characterised. For this reason, brazing has been studied as a potential route to produce high quality structural joints in 7 TiAl sheets. The work reported here has used a Ti-Cu-Ni braze alloy at the joint interface. High strength brazed joints have been produced, with shear properties similar to solid state diffusion bonds in 7 TiAl sheet. The range of bonding parameters used produced significant variations in the microstructure of the joints but despite this, the measured shear strengths remained similar. Exposure of the joints to a temperature of 700 degreesC for up to 1000 h, showed that there was a slight increase in the shear strength with exposure time. However, oxidation studies noted that the brazed joints exhibited significantly poorer oxidation resistance in comparison with the base sheet alloy. This poor oxidation performance was thought to be tolerable and capable of being overcome for aerospace applications. (C) 2003 Elsevier Ltd. All rights reserved.

The lightweight gamma titanium aluminide alloys offer good mechanical properties at temperatures up to 700degreesC for gas turbine applications. An important characteristic of these materials will be their mechanical performance after exposure to anticipated service temperatures. Thus cast Ti-47Al-2W and extruded Ti-46Al-5Nb-1W have been subjected to thermal exposure at temperatures between 500degreesC and 700degreesC for up to 1000 hours. This showed that the bulk microstructure was stable and resulted in little or no change in tensile properties after exposure. However, once surface effects were included in the tensile properties, then the values of both tensile strength and ductility reduced significantly. This is thought to be associated with cracking at the surface, due to either the thin (similar to1 mum thick) oxide formed at elevated temperatures or, more likely, along favourably oriented and embrittled alpha(2) laths. However, other factors also probably contribute to the reduced tensile performance.

The lightweight γ titanium aluminide alloys offer good mechanical properties for gas turbine applications at temperatures up to 700 °C. An important characteristic of these materials will be their mechanical performance after exposure to anticipated service temperatures. Thus cast Ti-46Al-2W and extruded Ti-46Al-5Nb-1W have been subjected to thermal exposure at temperatures between 500 and 700 °C for up to 1000 h. This showed that the bulk microstructure was stable and resulted in little or no change in tensile properties after exposure. However, once surface effects were included in the tensile properties, then the values of both room temperature tensile strength and ductility reduced significantly. The room temperature property reductions were thought to be associated with changes in the surface residual stress state. Other potential factors include premature surface cracking, due to either localised hydrogen or oxygen embrittlement.

Historically, the issues connected with the lifing of power generation gas turbine components have been very different from those associated with aero engines. Specifically, component lives in the power generation application have been dictated by creep and high cycle fatigue, whereas low cycle fatigue has been the driver for aero engines. However, developments in the design and usage of gas turbines within the respective industries have resulted in this distinction becoming increasingly blurred. This paper highlights recent advances in the materials technology, stress analysis and lifing of aero engine components, which are potentially relevant to industrial gas turbines. In particular, the development of complex constitutive equations for modelling plasticity and anisotropic creep are discussed, with particular reference to the behaviour of single crystal turbine blades. Moreover, developments in the methodologies used to estimate safe service lives for the components are considered. Specifically, a new lifing procedure, capable of accurately predicting component lives from plain specimen data alone, is discussed.

For gas turbine applications there remains uncertainty over the most appropriate manufacturing route for gamma-TiAl based alloys, in addition a range of high strength alloys, containing niobium have been developed. To assess the efficacy of the extrusion process and the relative performance of some of these high niobium gamma-TiAl alloys, extruded Ti-46Al-5Nb-1W, Ti-44AI-8Nb-1B and Ti-44Al-4Nb-4Zr have been produced and tested. The Ti-46-5-1 alloy exhibited the highest strength of any of the alloys, by similar to150 MPa but exhibited similar ductility (1%) to the other materials. Similar creep properties were found for all three alloys evaluated and these were superior to first generation gamma-TiAl compositions. Good microstructural thermal stability of the Ti-46-5-1, in terms of change of tensile behaviour, was found after exposure at 700degreesC for up to 1000 hours. The addition of environmental effects to the assessment of room temperature tensile properties after exposure at 700degreesC showed a large drop in tensile ductility even after 250 hours at 700degreesC. It is thought that this effect is associated with the formation of a brittle surface layer. Excepting the environmental susceptibility of the mechanical properties, the Ti-46-5-1 appears to be an attractive composition, especially in extruded form. The high mechanical properties and good repeatability produced by extrusion suggest that extrusion may be an attractive manufacturing route for component production.

To maximise the benefit of using high strength titanium alloys in future, whilst maintaining a cost effective manufucturing route, will require the use of metallurgical joining technologies to assemble large components from smaller details. In the work reported here four joining technologies have been assessed manual TIG, EBW, friction, welding and diffusion bonding - for use with the near beta Ti-10V-2Fe-3Al alloy. Measurements of the tensile properties after welding show that the fusion welding techniques impose a strength penalty, compared with the bulk parent material. However, good consistency in the tensile properties was demonstrated fur the EB welds. In contrast, parent metal tensile properties were achieved by the friction welding and the diffusion bonding techniques because of the favourable microstructure developed during joint formation. Low impact energies were measured after diffusion bonding and post bond licit treatment. however, post bond heat treatment of the friction welds gave: a large increase in the toughness, although not to the level of the parent metal in the same licit treatment condition.

Superplastic forming of conventional titanium alloy sheet is limited commercially by the relatively long cycle times imposed by the high temperatures and slow strain rates required for forming. In order to minimise cycle times material with a fine grain size is required to enable increases in the forming rate and/or reductions in the deformation temperature. This study details an investigation of the production of Ti-6Al-4V + 0.5%B with a nanocrystalline grain size which was produced by mechanical milling. The material was consolidated by hot isostatic pressing at a range of temperatures during which TiB was formed by an in sitar reaction between the titanium and the boron. The aim of (lie TiB was to pin the fine grain size of the titanium produced by mechanical milling. The consolidated material was hot tensile tested at a range of temperatures and strain rates. A superplastic elongation of 360% was achieved when testing at 900 C at a strain rate of 6 x 10(-2)s(-1) compared with 220% for conventional Ti-6Al-4V Sheet.

The reduction in grain size of a metal can lead to significant improvement in mechanical properties. Mechanical alloying (MA) with a second phase is a possible route to producing fine-grained, particulate reinforced material. This study describes the microstructural development of Ti-6%Al-4%V milled with increasing concentrations of boron. Mechanical milling of Ti-6%Al-4%V powder produces a nanocrystalline material. MA of Ti-6%Al-4%V with boron results in the alloying of the two to form either a boride or an amorphous phase when the local concentration of boron is ∼ 50 at.%. During milling, the boron tends to remain near to its original particle form and in these boron-rich regions TiB is formed. Beyond these regions small amounts of boron (a few at.%) mix with the titanium matrix and reduce further the grain size of the titanium. An increase in the global concentration of boron increases the volume fraction of boride produced.