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It is widely recognised that for the foreseeable future coal will remain an important energy source for generation of electric power in many areas of the world. However, it will be essential to develop generation technologies that include the capture and storage of emissions such as CO2 to combat the harmful effects of climate change. The key to the successful implementation of advanced generation technologies, the ultimate aim of which is the zero emission power plant, depends heavily on the availability of suitable materials. In this paper, the requirements for materials with improved high temperature performance will be considered in the context of three types of power generation system being developed to operate with greatly reduced emissions and with high levels of efficiency. The limitations of existing materials will be outlined and the need for materials with higher temperature capabilities and good fabricability will be discussed.

Since the early empirically based work of Kachanov and Robotnov substantial progress has been made in developing the concept of Continuum Damage Mechanics (CDM) as a tool for predicting material and component behavior in the creep regime. The key element in this process has been the progress that has been made in understanding and quantifying the physics of deformation and fracture in engineering alloys for high temperature service. In this paper, the application of the CDM methodology has been demonstrated in the prediction of the creep behavior of alloy steels for boiler pressure parts and for predicting the life to failure of model components and tubular testpieces operating in creep conditions. A model incorporating two damage state variables formed the basis of the methodology for creep behavior and a multi-axial variant of the model was used for the component life prediction. The important development described in this work has been the use of a simplifying procedure in dealing with damaged elements in the finite-element model, so that the analysis for component life prediction can be carried out on a personal computer rather than the large mainframe computers used previously. This greatly increases the usefulness of the procedure for practical design applications. The results show that the CDM methodology can be applied successfully in these important applications and will give much more satisfactory agreement with experiment than existing less robust methods. In addition the failure profile of the tubular testpieces can be accurately represented and this is a feature unique to the CDM approach.

In this paper, results are presented of creep tests conducted using a reference material for the purpose of validating the performance of creep test equipment which was commissioned following relocation. The results show that the data obtained meet the criteria established during the certification of the reference material thereby confirming the satisfactory performance of the creep equipment. Statistical analysis showed that the results obtained in the present investigation were consistent with those of the original database used to certify the reference material. When the present results were added the enlarged database exhibited a somewhat tighter distribution. The work has confirmed that results from tests where base-metal thermocouples were used for monitoring temperature show good repeatability but allowance has to be made for drift in temperature associated with instability of the thermocouple material.

Most of the very limited tensile creep information on monolithic ceramics has been generated using the four point bend test. Where data on ceramics under uniaxial tension do exist, it is invariably the case that lifetimes are much longer under flexure when, as is usual, the maximum principal tensile stress is used as the correlating parameter. In this paper, creep lifetime data for hot pressed silicon nitride are used to demonstrate that a geometry dependent ‘reference stress’ is a much superior correlating parameter and that its successful usage leads to important deductions about the process of fracture in this class of materials, under the stress and temperature regime studied.MST/1668

In order to develop databases of creep information for design purposes for steels and other alloys, large-scale test programmes were undertaken involving several casts , product forms, etc. Little attempt was made to separate materials variables from those due to testing, and, even for simple measures of performance like life to rupture, considerable variability occurred and scatter-bands were large. Design information was obtained using statistical methods and curve fitting to predict long-term performance. The simple design rules incorporated safety factors which, along with the relatively inaccurate materials data , resulted in very conservative designs for power plant and pressure vessels.

The creep behaviour of a series of alloys based on IN 597 (Nimonic 101) and with varying volume fractions of γ′ precipitate has been studied at 800°C in the standard condition of heat treatment and for two grain sizes. In order to assess the long-term structural stability of alloys of this type, which have high contents of Cr, creep tests were also carried out on material overaged for 5000 and 10000 h at 850°C. Optical and electron metallography have been used to examine the microstructural characteristics of the materials. The results show a consistent pattern of increasing creep resistance with increasing volume fraction of γ′ precipitate, although the effect was more significant in the coarse grained material. After overaging, the creep resistance and the life to rupture of the alloys were reduced, and acicular particles, believed to be γ′ phase, were observed in the microstructure. Some limitations of current mechanisms describing dislocation–particle interactions are discussed with respect to...

The creep properties and the microstructure of two groups of austenitic steels containing manganese as a replacement for nickel have been studied at 700°C and for a range of stresses. In one group, nitrogen was added to stabilize the austenite; in the other, the content of manganese was higher and the steels were expected to be stably austenitic without nitrogen. The objective of the programme was to demonstrate that austenitic steels containing manganese could be produced having creep properties comparable with those of the more conventional austenitic steels. The work has shown that in terms of life to rupture the present steels compare favourably with equivalent steels containing nickel. The addition of carbide- and nitride-forming elements such as tungsten, vanadium, and molybdenum gave a significant improvement in creep strength relative to the base compositions, and X-ray analysis of extracted residues has been used in order to identify the precipitates formed.

The effects of microstructural variations arising from the casting conditions on the creep performance of alloy IN939 have been assessed in order to determine the structural features having the greatest influence on creep properties. The results of creep tests at 850°C show that while grain size has an effect, the dendrite spacing is a more significant parameter in influencing performance. In particular, the life-to-rupture was greatest in specimens with fine dendrites as a consequence of the improved ductility with this type of structure. Thus the optimum combination of creep properties was obtained for material with fairly coarse grain size (~1mm) and fine dendrites (~20 μm). The structural changes associated with the various stages of the commercial heat treatment have been determined by optical metallography and electron microscopy of thin foils, and evidence has been found of dislocation bowing and shearing of γ′ particles in fractured creep specimens. The creep results are discussed in the c...

In the high–strength superalloys used for ‘hot-end’ components in gas turbine engines, small amounts of elements with low melting points can have a seriously detrimental effect on high–temperature performance. Some of the more recent work carried out to define harmful levels of trace element content and to develop an understanding of the mechanisms involved is briefly reviewed in this paper. The emphasis is on results available for cast alloys; the problem of gaseous impurities in recycled material is also considered. The implications for the specification and control of trace element content are briefly discussed.MST/161

Creep tests have been carried out at 750°C on sheet specimens of Nimonic 90 having thicknesses between 0·15 and 1·7mm and grain sizes in the range 0·04−0·07 mm, but mostly ∼0·05 mm. The results show that there is a significant decrease in creep performance when the thickness is less than about 1mm and that failure occurs as a result of grain-boundary cavitation, irrespective of section size. For specimen thicknesses of 0·28 and 1·7mm the stress dependence of the minimum creep rate was similar to that expected for bulk material, suggesting that diffusion creep was not responsible for the decrease in creep resistance of the thinner specimens. A simple mechanism is proposed to explain the effect of section size on life to rupture and the implications of the interpretation are discussed from the point of view of the practical applications of thin sections in turbine components.

The effects of trace amounts of Pb, Sn, Se, Te, and Tl on the creep and stressrupture properties of Nimonic 105 at 815°C have been investigated. The work confirmed that with the exception of Sn, all the elements have a deleterious effect when present in a sufficient amount. The most harmful impurities were Te and Pb. A generally more sensitive indication of the presence of harmful amounts of impurity was obtained with notched specimens. Optical metallography established that the deleterious effects were associated with greatly increased intergranular cavitation which became evident at an early stage of creep life when impurities were present. Auger electron spectroscopy showed that about one monolayer of lead was present on a cavitated grain boundary in a creep-tested specimen containing 34 ppm of Pb.

The ductility of a single-phase Ni–20% Cr alloy and two precipitation-hardened Ni–20% Cr-base alloys, hardened by Ni3(Al, Ti) and Ni3(Al,Nb), respectively, has been studied over a range of temperature at various strain rates. The results show that the alloy hardened by Ni3(Al,Nb), while less ductile than the single-phase alloy, is superior to that hardened by Ni3(Al,Ti) with respect to both tensile strength and elongation at fracture. In the single-phase alloy, there are two ductility troughs for a given strain rate when the strain rate is slow but in the two-phase alloys and at high strain rate in the single-phase alloy a single trough is obtained.Metallographic examination has indicated that the ductility behaviour of the single-phase alloy can be correlated with structural changes, void formation leading to a low elongation at fracture but recovery and recrystallization increasing ductility, presumably by relieving internal stresses at the grain boundaries. It is suggested that an important con...

The creep behaviour of nickel under cyclic loading has been studied, the aim being to relate the creep strain to the metallurgical recovery that occurs during each load reduction. Square-wave loading between stresses σ1 (40 or 80 N/mm2) and σ2 (0–σ1) for times t 1 (0.01–0.1 h) and t 2 (0.1–20 h) was applied at 650 or 750° C, and a point of principle was to apply the full stress σ1 initially until the metal was fully work-hardened so that all subsequent changes could be ascribed to recovery. The strain per cycle consists of three components, which could be expressed in terms of experimental conditions: (1) ∆e1, the normal strain in time t 1 at a stress σ1; (2) ∆e2, the strain in time t 2 at stress σ2; e2/e1 = (σ2/σ1)m, where m = 10 at 650° C and 5.2 at 750° C; (3) ∆er, the extra strain on reloading from σ2 to σ1; ∆er = bt 2⅔ + c, where b and c have been determined. The factor c can be negative, probably because strain-ageing occurs. A consequence is that three functions are needed to describe accur...

Damping capacity measurements have been carried out on copper-silicon alloy specimens before and after internal oxidation and the results compared with those for oxygen free, high conductivity copper specimens given the same treatment. High damping values have been noted for the fully internally oxidized copper-silicon material and it is suggested that this is caused, to some extent, by the generation of relatively free dislocations around the silica particles during the growth and subsequent cooling of these inclusions.

Zones free of γ′ precipitate formed adjacent to grain boundaries in the gauge-lengths of specimens of Ni–20 % Cr-base alloys subjected to creep tests or slow-strain-rate tensile tests in the temperature range 750–850°C (1023–1123 K) have been studied. Electron-probe microanalysis did not reveal any significant change in composition between the precipitate-free zones and the grain interior. Consideration of the variation in the stress-dependence of minimum creep rate with temperature and grain size indicates that a diffusion creep mechanism may explain the formation of the precipitate-free zones.

This paper presents a review of the information available on the properties and performance of the strong highly corrosion resistant alloy IN939 which is being used increasingly for vanes and blades in land-based gas turbines. Particular emphasis has been placed on the large amount of new data now available from the European Collaborative Project COST50 and it will be seen that the behaviour of the alloy has been well characterised although there is insufficient information on the low cycle fatigue performance. It is shown that the alloy compares favourably with IN738LC and in service has superior corrosion resistance.

The rapid development of metallic materials for high temperature applications that occurred during the 1960fs and early ’70s has given way in the last decade or so to a consolidation phase with the emphasis on optimisation and detailed improvement in alloys. Innovative developments in materials processing technology (melting, casting, fabrication) has enabled the upward trend in temperature capability and loadcarrying capacity to be maintained and considerable scope for further developments in this area remains through the use of computer-aided techniques. Also there has been an increase in the effort directed towards a better understanding of the behaviour of alloys in service as a basis for more effective usage and more efficient design and this activity will undoubtedly develop and intensify in the decade ahead.

The need for a standardized approach to hot-salt. corrosion testing is identified and the aims of the VAMAS initiative to improve the basts of test and measurement methods are described.

The influence of certain trace elements, namely Pb, Sn, Te, Se and Tl, on the creep and stressrupture properties of Nimonic 105 was investigated at 815 °C. The work was supported by the Ministry of Defence, and was planned in close collaboration with Rolls-Royce, who advised, in particular, on the trace elements to be studied and the levels at which harmful effects on creep performance might be expected. The results obtained have confirmed that, with the exception of Sn, all the elements reduce the life to rupture and the ductility at failure when present in sufficient amount. The most deleterious impurities were Te and Pb. Stress-rupture testing with notched specimens gave a generally more sensitive indication of the presence of harmful amounts of impurity than did tests with plain specimens.

(1972). The Study of Metal Structures and Their Mechanical Properties. International Metallurgical Reviews: Vol. 17, No. 1, pp. 225-225.