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Microstructural evolution of a nickel-base single-crystal superalloy during homogenisation at ultra-high temperature has been investigated using scanning electron microscope and electron probe micr...

A newly developed heavily alloyed polycrystalline nickel-base superalloy containing rhenium and intended as a structural material for turbine discs in gas turbine engines has been studied. Homogeni...

The castability and mechanical properties of thin-walled nickel-base superalloy castings fabricated by gravity casting and centrifugal casting were investigated. It is shown that, despite its microporosity has slight change, the casting fabricated by centrifugal casting presents fewer misruns and less Laves phase than that of gravity casting. The ultimate tensile strength, yield strength and stress rupture lives of centrifugal casting are improved by about 2.2, 7.4 and 41.1%, respectively. However, the elongation (EL) and reduction in area are sharply decreased by about 10.9 and 25.0%, respectively. Paradoxically, the stress ELs of both castings are similar. The seemingly contradictory results in mechanical properties stem from the opposite effects of centrifugal force on hardening elements macrosegregation and melt turbulence.

The formation of topologically close-packed (TCP) phases in nickel-base superalloys is an issue of increasing importance as alloys are designed with higher refractory element contents to meet the r...

The effects of temperature, dwell and environment on the fatigue crack growth behaviour of two nickel base turbine disc superalloys RR1000 and Udimet 720Li are presented in the present paper. Fatigue tests were carried out at room temperature in air using a 20 Hz sinusoidal loading waveform and at elevated temperatures of 650 and 725°C in both air and vacuum environments using a trapezoidal loading waveform with dwell times at a maximum load of 1 or 20 s. The fatigue crack propagation resistance of both materials is rationalised in terms of the influence of microstructure as well as the effects of alloy chemistry.

Simple compression and microscopy techniques were employed to characterise the microstructural origin of the deformation behaviour of nickel base superalloy IN625 during large strain testing. The alloy exhibited a four-stage strain hardening response similar to that previously reported for low stacking fault energy face centred cubic alloys. At strains lower than about −0·06 (stage A), a falling regime of the hardening rate was observed. This stage was followed by a second stage (stage B) of slow increasing hardening rate, which was found to be coincident with the formation of Lomer–Cottrell locks. The second falling regime of strain hardening (stage C) was seen in the strain range of −0·25 to −0·65. The occurrence of this stage was attributed to the increasing ease of dislocation cross-slip with increasing strain and consequently to the decreasing Lomer–Cottrell lock efficiency in hindering dislocation movement. Beyond a strain of −0·65, a final slightly constant hardening regime (stage D) was de...

Some open questions concerning the microstructural mechanisms which govern the high temperature strength of monocrystalline nickel base superalloys will be reviewed in the context of previous work. Emphasis will be laid on microstructural effects observed in microyielding, creep, isothermal and thermomechanical fatigue and on the role of the γ/γ′ lattice misfit with respect to γ/γ′ rafting. The conditions for cutting of the γ′ rafts will be stated in a simplified two-dimensional dislocation model. Effects of the connectivity of the coarsened γ/γ′ microstructure and of the orientation of γ/γ′ rafts on the strength properties will be discussed, and potentially beneficial effects of a positive lattice misfit will be addressed. Attention will be drawn to counteracting alloying effects which modify both the lattice mismatch and the diffusion kinetics. The main conclusion is that the optimisation of superalloy properties can only be sought in improving the most important properties without deterioration...

Two numerical models to simulate microsegregation and phase formation during directional solidification and subsequent solution heat treatment of an advanced experimental ruthenium containing single crystal nickel base superalloy are tested and compared. The first method is based on a one-dimensional front tracking for the primary solidification and a homogenisation method for the final stages of solidification as well as for the solution heat treatment. Calculations for this model are carried out in one-dimension using cylindrical coordinates. The second is based upon the phase field method, applied to solidification and subsequent solution heat treatment, where calculations are carried out in two-dimension. Both models are coupled to thermodynamic and kinetics databases modelled according to the CALPHAD method. A concept of computer based optimisation of solution heat treatments is proposed. The results show that both methods are capable of handling the complexity of contemporary superalloys, and realistic results are obtained from both models. © 2009 Institute of Materials, Minerals and Mining.

The microstructure evolution of a conventionally cast nickel base superalloy has been investigated during aging treatment at 900°C. It shows that two kinds of γ′ particles existed in the as heat specimens. The finer spherical γ′ ones became notably coarsening. The coarsening kinetics can be approximated by a linear volumetric growth as predicted by LSW theory. The larger cubic γ′ ones sphericised and grew at the expense of finer ones. The primary MC decomposition course was shown in detail, which occurred by the formation of γ′ films, Cr rich M23C6 and Mo rich M6C.

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.

An enthalpy based method was used to determine the solidification characteristics in the Ni base superalloy IN713LC with the emphasis on the late stages of solidification. Solidification co...

The effect of solidification rates on grain boundary (GB) cohesion in mushy zone during directional solidification was explored. The mushy zone structures were frozen by tin bath quenching. It was found that high solidification rate leads to an extended mushy zone thus vulnerable region, while there are larger areas where dendrites are bridged at higher solidification rate, i.e. there is a high fraction of bridging areas in the mushy zone as solidification rate is increased. The smaller hot tearing susceptibility at higher solidification rate cannot be explained by the greater mushy zone or vulnerable region, but it can be explained by stronger GB cohesion. The results suggest more attention should be paid to the GB cohesion rather than the range of mushy zone or vulnerable region in order to avoid hot tearing at least in some alloys.

Nickel base superalloy CMSX - 2 single crystal bars were shot peened to induce surface residual stresses. Partial solutionising at 1300 ° C revealed a relationship between dissolution of the γ ′ phase into the matrix and recrystallisation. Recrystallisation occurred with dissolution of γ ′ phase in the dendritic core at 1300 ° C. Recrystallisation had preferentially begun at the dendritic core exposed at the shot peened surface and then gradually proceeded to the interdendritic regions. Phases more resistant to dissolution such as the coarse γ ′ particles and the γ - γ ′ eutectic phase in the interdendritic region dissolved into the matrix with recrystallisation, even at 1300 ° C, which is lower than the solutionising temperature of the alloy. Residual stresses assisted dissolution of the existing phases. The recrystallised grains grew preferentially in the dendritic core where rapid dissolution of γ ′ phase occurred. The growth of the recrystallised grains was impeded by the coarse γ ′ particles ...

Models have been developed and used as tools to design a new 'made to measure' nickel base superalloy for power plant applications. In Part 1 Gaussian processes were used to model the mechanical pr...

The dislocation structure of a fine gamma precipitate strengthened nickel base superalloy during steady state creep has been studied. Extensive shearing of gamma particles by partial dislocations, creating stacking faults in both gamma and gamma phases, have been observed in specimens under stresses ranging from 120 to 250 MPa. Detailed analysis of the fault nature and dislocation dissociation revealed that a unit matrix dislocation of a2110 dissociated into two partials of a3112 and a6112. Extrinsic stacking faults were created in both gamma and gamma phases, presumably being created by the movement of the leading a3112 partial, while the movement of the a6112 partial eliminates the stacking fault in the matrix, but leaves a partial dislocation loop around the faulted gamma precipitates.

The capabilities of two unified constitutive models to predict the mechanical behaviour of nickel base superalloy IN738LC under uniaxial loading conditions have been investigated over the temperature range 450–850°C. The material parameters of each model have been identified from an experimental investigation and complemented by available data from the literature. Mechanical responses from isothermal tests performed at 450 and 850°C (such as creep, monotonic, and fully reversed isothermal cyclic tests) were used for the identification of model parameters, and model capabilities were examined by comparison with in phase and out of phase thermomechanical fatigue and stress relaxation tests. Both models were found to capture all features of the material responses under uniaxial loading, although with varying degrees of accuracy.

Combustion turbine components exposed to elevated temperatures and high stresses are subject to periodic replacement as a result of mechanical property degeneration to levels below that required for continued safe operation. In view of the very significant cost associated with replacement components there has been much interest in the rejuvenation of parts. This report reviews published literature on the rejuvenation of nickel base superalloys. The restoration of microstructure and properties to levels equivalent to the original material have been achieved with the use of reheat treatment alone or recovery cycles incorporating both hot isostatic pressing and reheat treatment. The majority of the literature considers the restoration of creep properties, although some success in the rejuvenation of fatigue properties has also been reported.

Mechanical alloying involves the severe deformation of mixtures of powders until they form the most intimate of atomic solutions. Inert oxides can also be introduced to form a uniform dispersion of fine particles which strengthen the consolidated product. Large quantities of iron and nickel base alloys with unusual properties are produced commercially using this process. The theory describing the way in which the powders evolve into a solution is reviewed. There are some fundamental constraints which dictate how the microstructure must change during mechanical alloying for the process to be at all viable. The strange recrystallisation behaviour of the alloys can be understood if it is assumed that unlike normal metals, the grains in the mechanically alloyed sample are not topologically independent. Another topic discussed is the mechanical blending of microstructures containing different phases, both with and without a net reduction in free energy.

The high temperature cyclic creep behaviour of a directionally solidified Ni base superalloy has been investigated and compared with constant load creep data. Irrespective of whether holding time is allowed during off loading periods or not, the creep rupture lives are shorter for the cyclic creep. The effect of cyclic loading is to cause further increase in strain rate acceleration. The frequency of load change does not have any significant effect on rupture time. The evolution of the particulate microstructure as well as other transient creep phenomena such as anelasticity have practically no effect on the observed strain rate acceleration. However, the presence of the shrinkage pores and presumably the variation in their size and population introduce some measure of scatter in the rupture lives.

An overview of electron backscatter diffraction is presented in which experimental procedures are reviewed together with a basic theoretical description of the mechanism of pattern generation. Manual and automated indexing procedures are described. The new technique of orientation imaging microscopy is presented, with examples from recrystallised AI-Mg alloy, deformed superplastic material, and fractured nickel base alloy.

Room temperature fatigue tests in the form of four point bending were performed at a frequency of 20 Hz and a load ratio R= +0·1 on electropolished Waspaloy specimens taken from a forged turbine di...

Microstructural inhomogeneities and variations in the extent of erosion of base metal in nickel base wide gap brazed joints produced by the preplacement technique with braze mixes of different gap filler contents were investigated, from which the flow behaviour of the braze mix constituents and the formation mechanisms of the various types of macrovoid were deduced. The results show that the formation of various types of macrovoid is closely related to the flow behaviour of the constituents of braze mixes during brazing, the latter in turn being strongly influenced by the braze mix ingredients, the brazing temperature, and the gap depth. For a wide gap brazed joint to be free from macrovoids, the braze mix must be sufficiently viscous to bridge the gap faying surfaces and must flow as a whole into the gap. Braze mixes with gap filler contents of 30–40% are ideal for such requirements. With too Iowa gapfiller content, the molten filler would flow preferentially ahead of the mass-of braze mix, leadi...

Vacuum brazing of wide gaps has been used increasingly to repair hot section components of second generation aeroengines over the years. It has also been extended to repair first generation components and those components too badly damaged for weld repair alone, and used in situations where welding will cause appreciable mechanical distortion. In the present investigation, the effects of material and process parameters on the formation of various types of constituent void, namely, interfacial, interstitial, and shrinkage voids, in nickel base wide gap brazes produced by prepacking with dry braze mix powders of Nicrobraz LC and Nicrogap 116 are reported. The results showed that the size and number of interfacial and interstitial voids increased with gap depth and width, and with gap filler content in the braze mix, but decreased with increasing brazing temperature. At intermediate and high gap filler contents, shrinkage voids were not readily observable as they were absorbed into other constituent ...

Macrovoids are voids resembling notches or cracks that can act as stress raisers in brazed joints. These are undesirable and should be eliminated if physical soundness of the joint is to be achieved. In the present work, nickel base wide gap brazed joints were produced by the preplacement technique, using Nicrobraz LC and Nicrogap 116 as afiller metal and gap filler, respectively. After brazing, the joints were sectioned depthwise, prepared metallographically, and examined by optical microscopy to detect the presence of macro voids, the latter being defined as voids of characteristic length greater than the mean diameter of gap filler particles in joints made with braze mixes, or larger than one tenth of the width of the gap in joints made with filler metals only. The results showed that three major types of macro void could be identified: irregularly shaped macrovoids throughout the longitudinal section of the joint (type 1); irregularly shaped macrovoids at the tail end of the joint (type 2); an...

The design and development of the future generation of nickel base superalloys for the demanding environments of rocket engines should be based on the metallurgical and structural characteristics of the γ′ phase. The distinctive features of this phase (e.g. strength, stability, size, shape, amount, distribution, uniformity, and order) can be controlled when a superalloy is appropriately designed and subsequently processed by an innovative solidification processing method. This concept has been successfully applied in the design and development of a superalloy. The resulting microstructure is almost perfect requiring no further treatment or processing.MST/1954

The various types of void formed in wide gap brazed joints of C1023 nickel base superalloy produced using prepacks of nickel base braze mixes have been investigated and systematically categorised. Of particular interest are interfacial and interstitial voids and unwetted pockets, which are features frequently found in joints brazed using such a technique. Examination of brazed joints produced under a wide range of conditions revealed that during heating to the brazing temperature, the braze mix partially sinters together, causing the prepack to shrink towards the centreline, leaving two channels next to the joint faying surfaces. At the same time, relatively large pockets of free space are created within the partially sintered mass of prepack. At the brazing temperature, the filler metal deposited at the gap mouth becomes molten and this molten filler is drawn into the gap preferentially through the fine capillary paths in the partially sintered mass of prepack. The relatively large pockets of fre...

The mechanical properties of a Ni base weld have been examined after long term aging in the temperature range 600–900°C. Impact testing revealed a substantial decrease in toughness after heat treatment at 600 and 700°C. In particular, after aging at 700°C there was a marked loss of ductility, which was associated with elongated particles appearing in the fracture surfaces. The concomitant microstructural changes occurring have been investigated using analytical electron microscopy. In material heat treated at 600°C for 10000 h, seven phases were identified: M23C6, MC, η, γ′, γ″, G, and δ. At 700°C, the following phases were distinguished after 3000 h: M23C6, MC, and η. After longer testing times G, γ′, and γ″, were formed. Whereas G, γ′, and γ″ formed intragranular needles, η formed intragranular laths or plates at 700°C. The minimum in ductility observed in material aged at 700°C can be explained in terms of copious intragranular precipitation of γ′, γ″, and η.MST/1131

The mechanical properties of structural materials can be strongly affected by environmental conditions. In particular, during long term exposure at elevated temperatures, i.e. under typical creep conditions, technical atmospheres must be expected, in general, to cause corrosive attack. Thus, to obtain reliable design data it is the interaction of corrosion and mechanical behaviour which must be considered as the lifetime determining factor. The present work is an overview of the influence of sulphidation and carburisation on the creep behaviour of high temperature alloys with emphasis on heat resistant steels. Sulphur is known to be a particularly aggressive species and severe attack on material due to sulphidation is encountered in gas turbines (nickel base alloys) as well as in coal gasification or fluidised bed combustion plants (heat resistant steels). Sulphidation is found to reduce the creep strength of high temperature alloys and the creep ductility in particular can be drastically reduced ...

Revue et classification des plus recents travaux experimentaux sur les interactions entre la corrosion et le fluage dans le cas des superalliages base nickel. Modelisation de ces phenomenes permettant d'obtenir une approche valable du comportement des composants en service

Three superheater transition joints, between 2·25Cr–1 Mo and 316 stainless steel, welded with nickel–base weld metal, removed from service after 72337 h, have been examined using optical and scanning electron microscopy. In addition, microhardness measurements have been made and local chemical compositions have been analysed using the energy dispersive X-ray attachment on a scanning electron microscope. Temperature accelerated creep rupture tests have been carried out between 590 and 625°C at stresses of 31–62 MN m−2 on cross–weld tensile specimens machined longitudinally from the walls of the joints. Detailed metallographic examinations showed the same failure mode as that found in long–term service failures. Therefore, the use of post-exposure temperature accelerated testing of uniaxial cross-weld specimens appears to be a viable method of assessing the remanent life of nickel-base transition joints operating at elevated temperatures. The applicability of various multiaxial stress rupture equati...