Materials Science and Technology

The complexity of adopting new or using existing advanced alloy systems in demanding environments where safety is paramount is highlighted. Of particular importance is the ability to define operative deformation and degradation mechanisms that may limit part life or functionality. With respect to the underlying fundamental material science there is a high degree of commonality between the aero and power generation sectors. The vast array of new experimental and modelling techniques that can be brought to bear on long standing technical challenges offers the potential for a renaissance in materials science. Of particular interest from an industrial context would be the ability to ‘design’ material microstructure and texture optimised for service use manufactured via the most energy and time efficient route. The requirements and potential for progress in this area is summarised with an emphasis on gas turbine and nuclear reactor applications for hcp metals.

The influences of elevated Co and Ti levels on the mechanical properties of the Ni-base superalloy RR1000 have been investigated. Following heat treatment, the modified alloys had the typical γ–γ′ microstructure, with γ′ precipitate sizes comparable to similarly heat treated RR1000, but with a slightly higher volume fraction. The modified alloys exhibited a higher proof stress than RR1000 across the entire 20 to 800°C temperature range investigated. Superior creep rupture lives, when compared to RR1000, were observed in the modified alloys at 700°C, but not at 750°C, where extensive precipitation of topologically close packed σ phase occurred on the grain surfaces. The formation of this deleterious phase was linked to Cr and Mo enrichment of the γ matrix, caused by the elevated Co and Ti additions.

A non-invasive temperature measurement, control and profiling technique has been investigated for use with thermomechanical fatigue loading. The technique utilises an infrared thermography camera and Rolls–Royce developed thermal paint to control and monitor cyclic temperature. Thermal paint is used to maintain a stable surface emissivity upon the test piece. The accuracy of the technique is compared against type N thermocouples and a pyrometer for both temperature control and monitoring purposes. Diverse test specimen geometries and alloy compositions are used over a 100–700°C temperature range. Effects on temperature measurement accuracy such as thermocouple shadowing are highlighted and quantified. The non-invasive technique has proved accurate to within ±2°C of the reference thermocouples when in combination with the thermal paint coating.

There is considerable interest in reducing the volume of gas escaping around the blade tips in a gas turbine by fixing an abrasive, such as c-BN, onto the blade tip with an alloy, such as MCrAlY. Abrasion testing at 1100°C showed that the MCrAlY deformed and the abrasive particles oxidised. The shear stress in the MCrAlY was estimated by finite element methods to be in the range of 5–30 MPa, while measurements of the frictional heating suggest the local temperature is between 1100 and 1200°C. Creep testing showed that conventional MCrAlYs were too weak under these conditions. However, higher strengths were obtained using a continuous reinforcing phase together with NiAl. An Al₂O₃/ZrO₂ abrasive was investigated as a more oxidation resistant abrasive. In this case, reaction with the MCrAlY caused Ni to diffuse into the abrasive forming layers of NiO, weakening the particles and causing premature failure.

Isothermal oxidation of the Ni based superalloy, RR1000, has been performed at 700, 750 and 800°C for exposure periods up to 2000 h. A chromia external oxide scale with an outer layer of rutile was formed together with extensive internal oxidation of aluminium, both intragranularly and intergranularly. The internal oxidation was associated with a deeper γ′ particle free zone. Extensive metallographic measurements of both the depth of internal oxidation and of the γ′ particle free zone were made. The kinetics in each case approximated well to parabolic. Possible rate controlling processes are discussed.

Whereas the effect of contaminants in causing hot corrosion has been extensively studied at temperatures up to 1000°C, this paper describes a systematic study of the effects of various salt deposits on the cyclic oxidation of CMSX-4** in air at temperatures above 1000°C. The alloy was tested both in the bare and in the platinum aluminised (Pt-Al) coated conditions and variables included temperature, salt concentration, and sulphur level of the original alloy. It was found that salt deposition at very moderate levels, re-applied at regular intervals, significantly increased the initial weight gain and decreased the time to first spallation despite the rapid loss of the salt through evaporation in the first heating up cycle subsequent to salt deposition. The degraded microstructure of the specimens tested with salt resembled closely that of blades having experienced engine service conditions. Hence, cyclic oxidation with the addition of salt offers an accurate, flexible, reproducible and above all realistic method of assessing the life of turbine components.

The creep behaviour of the nickel superalloy RR1000 is studied through a number of constant-load creep tests. It is often assumed that creep data generated by constant-load testing are unsuitable for building a generalised creep model due to the non-constant stresses incurred. Analysis of existing models shows that significant errors may occur in many approaches, which attempt to recreate the strain evolution with time. A model is presented which is not reliant on time as a parameter and is therefore able to utilise constant-load creep data without enforcing the assumption of a constant stress. This model is demonstrated through numerical analyses to replicate the creep behaviour across a large range of stresses accurately. The proposed model is then adapted as an Abaqus user-subroutine to demonstrate capability within finite element analysis.

Cast Ti46Al8Nb with coarse lamellar microstructures was subject to fatigue crack propagation test at 650°C with R = 0·1. The notch region of each testpiece contains one or one dominant lamellar colony. A strong dependence of fatigue crack propagation threshold on lamellar orientation was found and the lamellar colonies with their interfaces parallel to the stress axis have the highest threshold and those 40–65° off the stress axis have the lowest threshold. It is elucidated that slip or twinning parallel to lamellar interface may interact with cross-lamella slip or twinning to help open up microcracks in front of the main crack.

The microstructure of aircraft gas turbine engine bearing steel has been characterised after service in Rolls-Royce Trent™ engines, with the focus on surface condition and the consequences of sliding contact. Carbide populations at the surfaces of rolling elements are found depleted by 30% after 30 000 h engine service. A single ball failure occurred after this period, leading to fatigue spalling that initiated below the contact surface. Comparisons between unused bearing raceways and those that have experienced service revealed that the microstructures resulting from secondary hardening remain remarkably stable. Plastic flow along the direction of rolling is confined to a shallow zone <2 μm beneath the contact surfaces. Transmission electron microscopy has revealed a new deformation mechanism in these bearing steels, in the form of mechanical twinning at interfaces and prior austenite grain boundaries. It is demonstrated that workhardening occurs to a depth of 1 mm in the raceway that has experienced 30000 h service.

Recent research relating to two α+β processed titanium alloys, designed for specific employment in aeroengine fan disc and aerofoil components, is described. Timetal 575 (Ti575) and Timetal 639 (Ti639) are both novel compositions that provide relatively high strength compared to Ti–6Al–4V. Static and cyclic behaviour of the two developmental alloys were evaluated at room temperature. A dwell fatigue assessment of Ti575 is reported, while the effects of unidirectional rolling and associated evolution of microtexture were studied for Ti639.

Increasing global demands for energy conservation and environmental protection have prompted automotive manufactures to develop lightweight automobiles. As the lightest of structural alloys, Mg alloys offer significant potential for weight reduction, but have yet to see significant application in automobiles, particularly in sheet form. The current technical issues preventing the widespread application of Mg sheet alloys in automobiles (cost, mechanical properties and formability, joining, corrosion resistance) are assessed and future research needs to develop viable Mg sheet alloys for these applications are identified.

Iron chalcogenides are of great interest because they have the simplest structure in the Fe based superconductor family, and show low anisotropies, high upper critical fields and high critical current densities. These compounds can, therefore, be considered as reference materials to study the behaviour of all layered Fe based superconductors, both to advance fundamental physical understanding and to explore potential high field applications. Research carried out since the discovery of superconductivity in these iron chalcogenides in 2008 is reviewed to explore the effects of key parameters such as structure, stoichiometry, pressure, elemental substitutions and strain on superconductivity. The effect of these parameters on the superconducting transition temperature of Fe_1+ySe and Fe_1+yTe_1−xSe_x materials is critically assessed and the published results compared in an attempt to elucidate the relationship between structural parameters and superconducting properties in these compounds. Challenges for future work are identified.

Stress corrosion cracking (SCC) behaviours of Z3CN20-09M stainless steel in high temperature water containing Cl⁻ were studied. The results indicated that SCC sensitivity was inconsistent with test temperature. The minimum and maximum of SCC sensitivity occurred at 320 and 290°C respectively, and SCC sensitivity at 250°C fell between them. SCC crack initiated preferentially at bottom of corrosion pit or along phase boundary between austenite and ferrite, and its propagation depended on relative orientation to the phase boundary. SCC crack parallel to the phase boundary propagated along the phase boundary, while that perpendicular to the phase boundary was hindered to propagate.

The effect of aluminium content (0·023, 0·038 and 0·070 wt-%) on microstructure and impact toughness of simulated coarse grained heat affected zone (CGHAZ) of high strength low alloy steels with different heat inputs (20, 100 and 200 kJ cm⁻¹) was investigated. The microstructure of simulated CGHAZ consisted of predominantly granular bainite. The martensite–austenite constituents became finer and its volume fraction decreased with increasing aluminium content, irrespective of heat input level. The impact toughness of the simulated CGHAZ improved remarkably with increasing aluminium content even at high heat input of 200 kJ cm⁻¹. It was attributed to the reduction in volume fraction of martensite–austenite constituents and the refined martensite–austenite constituents by addition of appropriate aluminium.

Within a wide project aiming at the production of fully bioabsorbable endovascular stents, cylindrical preforms of the AZ91 Mg alloy produced by SPS were submitted to hot extrusion tests. It has been found that hot extrusion produced a consolidation of the material accompanied by the occurrence of dynamic recrystallisation and a corresponding noticeable grain refinement. The microstructural characterisation revealed that the optimal extrusion conditions were 380°C and 20 mm min⁻¹. Hot compression tests were also carried out to help in the explanation of the extrusion results and, in particular, to evaluate the friction coefficient during extrusion. It has been found that the lubrication should be improved to further optimise the process and obtain a product with superior quality.

Shear band formation and evolution in Zr₄₀Ti₆₀ alloy impacted by split Hopkinson pressure bar at different strain rates were investigated. The critical compression strain for the initiation of the shear band was about 15%, and almost no changes were observed with increasing strain rate. The microstructure of the shear band with a width of 13 μm was characterised with elongated and broken grains. Moreover, nanosized dynamic recrystallisation grains (50–100 nm) were observed at the centre of the shear band with widths of 41 and 68 μm. In addition, the microhardness at the centre of the shear band decreased from 560 to 410 HV when the shear band width increased from 13 to 68 μm.

In this study, the fracture behaviour of steel based composite and nanocomposite fabricated by accumulative roll bonding process was investigated using scanning electron microscopy fractography. The nanocomposite failed through decohesion and then void nucleation and linking in the matrix near the reinforcement/matrix interface and without particle fracture mechanism, whereas the composite failed through both decohesion and particle fracture mechanisms and then void nucleation, growth, and coalescence in the matrix near the reinforcement. The results indicated that the fracture mode after first cycle for pure interstitial steel, composite and nanocomposite is combination of ductile and shear ductile fracture. But, with increasing the number of cycles, the dimple size and the dimple depth decreased.