Search

A risk based inspection and asset management process has been implemented on a number of HRSGs for a client in Australasia. This process has been undertaken in a phased approach consisting of: data gathering on site, detailed risk assessment of all critical items of static plant, risk calculation and implementation of a risk based inspection plan based upon the results of the prior aspects of the assessment. The project has been streamlined through the use of dedicated risk based asset management software, RMS, developed specifically for the tasks described.Following the programme of work major inspections have been undertaken, the results of which have been uploaded to the program to allow a subsequent detailed review of the new plant profile and hence risk reduction. This not only allows a clear report on progress to senior management, but ensures that the process is continually maintained and future maintenance is most efficiently deployed to ensure optimum plant efficiency. Furthermore the rig...

Maintenance and/or replacement of hot gas path (HGP) components in gas turbines represent a major fraction of the cost of installation and operation. Condition assessment of HGP components can increase reliability and minimise costs, based on sound metallurgical and mechanical analysis. In this paper relevant improvements to condition assessment procedures and analyses are described, including γ’ precipitate sizing and lifing techniques used on precipitation-hardened Ni-based superalloys. Non-destructive testing, optical metallography and miniature creep testing are also covered. Understanding the operation and possible failure mechanisms of the component in service is the key to selecting an appropriate assessment procedure to facilitate optimisation of component lives and repair cycles

Nanowires can be fabricated from bismuth nanoclusters. The structure of bismuth nanoclusters of 40–60 nm diameter has been observed by high resolution transmission electron microscopy (HRTEM) and matched to multislice image simulations for B = [−2, 2, 0, −1] and B = [1, 0, −1, 0]. The hexagonal structure matches that of bulk bismuth embedded in a 5 nm thick shell of β-Bi2O3.

Since it was first proposed in the early eighties, the local approach to cleavage fracture, or Beremin model, has been applied to a wide range of engineering problems. However, several results have cast doubts on both the predictive capabilities and fundamental assumptions of the Beremin model. This three-part paper shows that most problems encountered with the original Beremin model result from an unnecessarily oversimplified description of the local cleavage event. A new statistical local criterion for cleavage is proposed, which expresses the necessity of maintaining a critical dynamic connectivity between microcrack nucleation and unstable propagation in order for cleavage fracture to occur.

In the first part of this paper, a new model for cleavage fracture in steel was presented, based on a new statistical local criterion, which expresses the necessity of simultaneously fulfilling the conditions for both cleavage microcrack nucleation and propagation. In this second part, the assumptions and predictive capabilities of the new model are assessed using a modern offshore structural steel plate (Grade 450EMZ). It is shown that the model assumptions are consistent with the cleavage fracture behaviour of the steel and that the new model has the potential of correctly quantifying the effects of size, constraint, temperature and strain rate on cleavage fracture risk.

Physically-based constitutive equations for uniaxial creep deformation in nickel alloy C263 [Acta Mater. 50 (2002) 2917] have been generalised for multiaxial stress states using conventional von Mises type assumptions. A range of biaxial creep tests have been carried out on nickel alloy C263 in order to investigate the stress state sensitivity of creep damage evolution. The sensitivity has been quantified in C263 and embodied within the creep constitutive equations for this material. The equations have been implemented into finite element code. The resulting computed creep behaviour for a range of stress state compares well with experimental results. Creep tests have been carried out on double notched bar specimens over a range of nominal stress. The effect of the notches is to introduce multiaxial stress states local to the notches which influences creep damage evolution. Finite element models of the double notch bar specimens have been developed and used to test the ability of the model to predict correctly, or otherwise, the creep rupture lifetimes of components in which multiaxial stress states exist. Reasonable comparisons with experimental results are achieved. The γ′ solvus temperature of C263 is about 925 °C, so that thermo-mechanical fatigue (TMF) loading in which the temperature exceeds the solvus leads to the dissolution of the γ′ precipitate, and a resulting solution treated material. The cyclic plasticity and creep behaviour of the solution treated material is quite different to that of the material with standard heat treatment. A time-independent cyclic plasticity model with kinematic and isotropic hardening has been developed for solution treated and standard heat treated nickel-base superalloy C263. It has been combined with the physically-based creep model to provide constitutive equations for TMF in C263 over the temperature range 20–950 °C, capable of predicting deformation and life in creep cavitation-dominated TMF failure.

The effect of minor additions of carbon and hafnium on the microporosity of single crystal superalloy RR2086 has been investigated. The results provided additional evidence that carbon has a positive effect on the alleviation of microshrinkage in this alloy system.

The microstructure of a 20Cr32Ni1Nb alloy in the as-cast and Ex-Service conditions has been fully characterised using scanning electron microscopy (SEM)/transmission electron microscope (TEM) and EDX. Nb-rich MC and Cr-rich M23C6 carbides were identified in both the as-cast and Ex-Service samples. Minor addition element Mn is present more in M23C6 carbides than in other phases. In addition, a Si-rich intermetallic compound formed on the MC carbides along the grain boundaries in the cast alloy which had been in service around 760 °C for about 4 years. The Si-rich phase was identified as G-phase by the experimental and simulated TEM diffraction patterns. EDX of TEM revealed that its molecular formula is Nb6 (Ni,Fe)16 (Si,Cr)7.

Severe embrittlement has been experienced in a number of cast manifold components. This has manifested itself as cracking at tee to manifold connections. Attempts to weld repair proved futile leading to concern about the integrity of the entire system. This experience contrasts with similar components that have successfully remained in service for many years. The paper describes the investigations into these failures and laboratory investigations into the properties of cast 20Cr32Ni1Nb alloys. Results indicate that variations in alloy chemistry within the stated allowable range are sufficient to cause embrittlement.

A crystal slip system model has been developed for the analysis of gas turbine single crystal Ni-base superalloys and has been implemented in finite element analysis software using the established theory of crystal plasticity. The model incorporates many of the dominant deformation mechanisms seen in these high temperature materials, which have been determined from mechanical testing and microscopic analysis. In particular slip is considered on the {1 1 1}〈0 1 1〉 and {1 1 1}〈1 1 2〉 slip systems, the former are denoted type I systems and the latter type II. Type I systems are dominant at high temperature in orientations close to [0 0 1], in other orientations or at lower temperatures (700–800 °C) type II systems become active. When the type II systems are activated they give rise to significant transient creep strains at low temperature, which can be more than 5–6% in magnitude.

Creep deformation defects from tensile creep tests carried out at 750 °C under 750 MPa in the approximate orientations [ 1 3 6], [0 0 1], [0 1 1] and [1 1 1] for CMSX-4 single crystals have been identified carefully, using transmission electron microscope combined with computer simulated microimages. A series of results have been obtained. (i) It was confirmed that 〈1 1 2〉/3 slip in γ′ phase mainly involved the reaction and dissociation of two 〈1 1 0〉/2 dislocations which had different Burgers vectors and met at the γ/γ′ interfaces where the product 〈1 1 2〉/6 partials were trapped. (ii) At 750 °C and 750 MPa, the selectivity among {1 1 1}〈1 1 2〉 slip systems was sensitive to Schmid factors. The reason for the anisotropy was that the resolved shear stress of highly stressed {1 1 1}〈1 1 2〉 slip systems in the [ 1 3 6], [0 0 1] and [0 1 1] orientations were just around the critical resolved shear stress of 〈1 1 2〉/3 faulting in γ′ phase which was estimated to be ∼300 MPa. Hence, a slight decrease of a RSS could switch off the operation of a {1 1 1}〈1 1 2〉 slip system, or vice versa. (iii) The activation of {1 1 1}〈1 1 0〉 slip systems was a necessary but not sufficient factor of initiating 〈1 1 2〉/3 faulting in the γ′ particles. The operation of 〈1 1 2〉/3 slip and density of superlattice stacking faults were controlled by the RSSs acting on those {1 1 1}〈1 1 2〉 slip systems.

This paper describes further insight which has been gained into the formation of deformation twins during high temperature creep in a high volume fraction single crystal superalloy (CMSX-4) and correlation with the nature of superlattice stacking faults also observed. In general it is found that the formation of high temperature twinning can always be associated with the loading orientation in which SESF formation is expected from a determination of the sign of the shear stress. One can rationalise the reason for this twinning being associated with extrinsic stacking faults, by consideration of the critical radius of stacking fault loop nucleation. In particular calculations at 1223 K demonstrate a minimum in radius being associated with four overlapping faulted planes for the extrinsic case. Although the process by which twin formation occurs has not been observed, it is shown that twin formation can take place by the passage of extrinsic stacking faults within the precipitate which operate on every alternate plane of the structure. If one considers formation to occur via a pole mechanism similar to that in a face centred cubic structure a mechanism for this occurrence is postulated due to the fact that climb of the helix structure as it rotates around a pair of suitable matrix dislocations, will amount to double that which occurs in the single pole case.

A study of crack growth in vacuum and air at 725 °C (T/T m=0.6) highlights the relative importance of creep and environmental crack-tip damage mechanisms in Powder Metallurgy (P/M) disc alloy RR1000. Both of these mechanisms are associated with a transition to intergranular fracture during fatigue crack growth at 0.25 Hz. Crack growth under sustained loads reveals the precise nature of these mechanisms in RR1000. The severity of creep and environmental mechanisms is controlled by the grain-boundary microstructure and the crack-tip stress. Near-tip cavitation leads to fracture in vacuum. Sigma-phase precipitation causes an increase in crack growth rate through increased crack-tip cavity nucleation. Rapid near-tip stress relaxation induced by γ′ coarsening has a beneficial effect on the severity of this type of damage. In air, increases in crack growth rates are associated with near-tip intergranular oxidation. It is proposed that the extent of this damage and subsequent growth rates are increased by sigma-phase precipitation through enhanced oxidation due to chromium depletion and subsequent decreased passivation. Again, a beneficial effect of rapid near-tip stress relaxation due to selective γ′ coarsening is apparent and environmental damage is reduced under these conditions.

The significant creep anisotropy associated with crystallographic orientation in single crystal Ni-base superalloys in the temperature range 1023–1123 K has been well documented over the past 20 years. The mechanisms primarily responsible for this anisotropy are established and are related to proximity to symmetry boundaries and the rotation that occurs during creep in single slip orientations. However, quantification and modelling of this behaviour has not been sufficiently addressed. It is not yet possible to determine the effect of lower temperature anisotropy and transient behaviour in macroscopic models and component analysis. In this paper this situation is addressed through the development of a mechanism based slip system model capable of accounting for the major effects associated with lower temperature anisotropy, including transient behaviour and rupture life. The model has been fitted to the first and second-generation single crystal superalloys RR2000 and CMSX-4. The results of creep and rupture tests on these alloys in many crystallographic orientations at 1023 K are presented.

The polycrystalline nickel-base superalloy (C263) is used for stationary components in aero-engines such as combustion chambers, casings, liners, exhaust ducting and bearing housings. It is a fine-precipitate strengthened alloy at 800°C, with a precipitate solvus temperature of 925°C. Below the solvus, the precipitate coarsens at elevated temperature. A critical precipitate size exists below which particle cutting is the rate-controlling creep mechanism. Above the critical size, it is dislocation pinning and climb. Once the solvus temperature has been exceeded, however, the rate-controlling mechanism becomes pinning and climb within a dislocation network. This paper presents physically-based constitutive equations for creep deformation in C263. In the presence of the γ′ precipitate, populations of climbing and gliding dislocations are assumed to exist in a similar way to that proposed by Dyson and Osgerby [3] . Climbing dislocations are assumed to be pinned at precipitates. The dependence of steady state glide dislocation flux can then be obtained as a function of precipitate volume fraction. Above the γ′ solvus, the pinning process is different and results from the establishment of a dislocation network, the size of which determines the pinning distance. The physical constants arising in the equations have been determined by the conventional minimisation of errors between experimental and calculated creep curves. They have also been determined quite independently using fundamental data and experiments. Remarkable agreement between the two sets of physical constants is achieved. The constitutive equations have been shown to capture the material’s creep deformation characteristics over a broad range of temperature, both below and above the γ′ solvus. In addition, the effect of precipitate coarsening on creep rate is correctly captured, together with the effect of prior ageing on subsequent creep rate. The damage processes of cavitation and multiplication of mobile dislocation density have been coupled with the constitutive equations and used to predict failure in constant and variable stress creep.

A number of slip systems are known to operate in commercial Ni-base single crystal superalloys, one of these being the {111}〈112〉 system which is particularly influential in the primary creep regime. The asymmetric nature of this dislocation activity with regard to the direction of slip under creep conditions is highlighted in this paper. Testing has been undertaken on CMSX-4 at temperatures of 1223 and 1023 K. The [001] and [256] orientations have been investigated under tensile and compressive loading conditions. It has been demonstrated that in addition to stacking fault generation, twinning of the complete two-phase structure is a major source of creep strain accumulation under certain conditions. These are dependent upon activation of suitable a/3〈112〉{111} partials which are a function of the sign of the resolved shear stresses. TEM analysis and EBSD measurements have shown that twin nucleation is dependent upon the activation of suitable 〈110〉{111} type slip within the matrix channels, but once it has taken place deformation can proceed rapidly, only being hindered if a suitable network of dislocations within the matrix channels can be created to halt the progress of the twinning partials.

This paper describes the analysis of a turbine blade component using a slip system model developed for modern single-crystal superalloys. Structural elasto-viscoplastic calculations are carried out for the component. The emphasis throughout is on the effect of micromechanisms of deformation, accounted for in the material model, on the predicted overall behaviour of the component. With the recent proliferation in detailed material models that are available, it is prudent to take a step back and investigate the implications of such models for component analysis and design. This effect is manifested through the determination of a stabilised and redistributed stress state throughout the component. While some components are creep-limited in design, many are fatigue-limited and it is stabilised stresses which control the cyclic life of these components. The accuracy of the material model, incorporating various micromechanisms as a function of stress and temperature, can significantly effect these stabilised stresses. The effect of the crystallographic orientation on blade behaviour is illustrated and the implications of shakedown simulations for fatigue lifing of turbine blades are discussed.

This paper describes a slip system model developed for the analysis of modern single crystal superalloys - specifically, the first generation alloys RR2000 and SRR99, and the second generation alloy CMSX-4. The single crystal model is implemented as an ABAQUS User MATerial (UMAT) subroutine, the framework is based on the classical theory of single crystal plasticity. The constitutive equations used have different formulations based on the micromechanisms of deformation and experimental measurements. The emphasis is on the effect of incorporating the micromechanisms of material behaviour on predicted macroscopic results. Several important phenomena and mechanisms, which are required for explaining creep properties as a function of stress, temperature and orientation are identified and included in the model. These include: activation of {111} and {111} slip systems, rigid body rotation, continuum damage, slip system softening, dislocation interaction, threshold behaviour and rafting. Model simulations are compared with experimental data in various deformation regimes. In a later part of the paper, the model is used to analyse the performance of a single crystal turbine blade. This enables the effect of the micromechanisms of deformation on overall component behaviour to be quantified.

Investigation into the addition of minor grain boundary strengthening elements (C, Hf and B) on the properties of an experimented single crystal nickel superalloy has been undertaken. The alloys were subjected to a variety of conditions including thermal exposure at 950°C and creep over 850–1050°C. Detailed microstructural analysis has revealed the presence of script and blocky MC carbides in the eutectic regions. On prolonged exposure σ precipitation occurred close to the MC carbides in the eutectic areas. A significant observation with regard to creep is that the principal crack initiation sites, namely casting pores, are greatly reduced owing to the precipitation of the MC carbides during solidification. This improves the intermediate temperature rupture life, although the presence of retained eutectic regions due to incomplete solidification degrades the high temperature performance of the modified alloys. The morphology of the carbides is a strong function of composition, with the presence of significant levels of Hf leading to the more blocky morphology, which is less prone to fracture during creep.

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.

A high temperature fatigue crack growth study on an experimental disc alloy of approximate composition Ni-14.75Cr-14-19Co-4.75Mo-3Al-3.75Ti-1.75Ta-0.7Hf-0.06Zr-0.02C-0.0175B wt- has been undertaken. Comparison of constant load fatigue crack growth tests conducted in air and a vacuum at 725C and in air at room temperature indicate that an oxidising environment has a major influence on crack growth rates over a wide range of applied stress intensity range. In particular it contributes to enhanced embrittlement of grain boundary regions, promoting an early transition to intergranular failure along with a concomitant increase in growth rate. Constant stress intensity factor range tests at high R ratio showed that a decrease in frequency at 725C in both air and a vacuum caused an increase in the crack growth rate per cycle due to time dependent crack growth. This was most significant in air at 725C rather than in vacuo, although in both instances low frequency tests were accompanied by wholly intergran...

A model has been developed to predict the high temperature cyclic life of single crystal superalloys RR2000 and CMSX-4 under conditions of creep and fatigue. A combined creep-fatigue model is used, although it is found that failure always occurs by creep or fatigue separately, and that creep-fatigue interaction has a minor influence. Microstructural investigation of a series of interrupted high- and low-frequency tests are presented, these are combined with the results of a series of interrupted creep tests to identify the separate and interactive mechanisms of creep and fatigue. When creep damage is present the material behaves homogeneously. Under these conditions crack growth is initiation controlled, the mechanism of failure is surface or casting pore-initiated planar crack growth followed by shear on crystallographic planes. As the temperature is lowered or the cyclic frequency increased, the material behaves less homogeneously and shear bands are formed during cycling. Crack growth under these conditions is again initiation controlled and failure is by rapid crystallographic crack growth along shear bands. Such a failure is a distinct fatigue failure and occurs when little creep damage is present. Under certain cyclic conditions, mainly those where the crystallographic failure mechanism is dominant, the material shows an anomalous increase in fatigue resistance with temperature up to approximately 950 °C, This behaviour has been quantified by relating it to the effect of strain rate and temperature on the yield strength of the material.

This paper is concerned with modelling and prediction of the stress rupture properties of several single crystal superalloys. The stress rupture theory is based on a modified form of the damage mechanics equations of Kachanov and Rabotnov. The ultimate tensile strength (UTS) is included and acts as an upper bound in these equations, which corrects for the fact that failure should occur when the effective stress reaches the UTS, rather than infinity, and allows the curvature in stress rupture data to be modelled effectively. The interpretation of damage on a microscopic scale is discussed. The time–temperature behaviour of these alloys is shown to obey a stress modified Sherby–Dorn parameter. A consideration is made for the changing strength of the material with temperature by comparing rupture data at constant fractions of the UTS rather than at constant stress. The activation energy used in the Sherby–Dorn parameter is similar to that for self-diffusion of nickel. By basing the rupture model on physical properties (namely the UTS and activation energy for diffusion), predictions of rupture behaviour of new alloys can be made when these corresponding physical properties are determined. However, such predictions of behaviour can only be made accurately between alloys of relatively similar composition. A major distinction is shown to exist between non-rhenium containing first generation alloys, and alloys containing rhenium, due to the increased rupture strength and different rafting kinetics of the latter.

A damage mechanics based model has been developed to model stress rupture and creep behaviour of the first and second generation single crystal superalloys SRR99 and CMSX-4. In this article the creep behaviour of CMSX-4 in several different orientations at 950°C is simulated using finite elements, these simulations are compared with the results of creep tests. In order that the effects of rotation and specimen bending can be accounted for in the analysis the entire creep specimen is modelled. The FE program ABAQUS has been used and the slip system model is written using a User MATerial subroutine (UMAT). EBSD (electron back scattered diffraction) measurements of the lattice rotations occurring during creep indicate that the active slip systems at 950°C are the {111} and {111} systems, our simulations show that the creep results can be explained by activating these two families of slip system. There is strong microstructural evidence that the significant components of the hardening matrix should be those causing self and latent hardening of the {111} systems and latent hardening by the {111} systems on the {111} systems.

Constant load creep tests have been performed on the single crystal nickel base superalloy CMSX-4 in crystallographically unstable orientations distant from the 〈001〉 pole of the standard stereographic triangle at 1023 K (750°C) and varying stress levels. Orientations distant from the 〈001〉–〈111〉 boundary had high creep rates and relatively low rupture lives, while the orientations closer to it had relatively low initial creep rates and longer rupture lives. Electron back scatter diffraction patterns, cross section shape change measurements on creep tested specimens, and optical microscopy indicate that the operating slip system is of the type {111}〈112〉 in orientations far from the 〈001〉–〈111〉 boundary. The results are explained based on the resolved shear stress acting on the operating slip system taking into consideration the rotations and changes in cross section. A theory based on the ease of generation of superlattice intrinsic stacking faults is proposed to account for the observed creep rates in various orientations. A finite element model based on the slip system approach is used to demonstrate the validity of the experimental results.

A new cladding material based on the Fe–Cr–Mo–V–C alloy system, suitable for submerged arc welding, has been designed for the refurbishment of forged and cast backup rolls used in the finishing stands of hot strip rolling mills. The work undertaken includes mechanical analysis, mechanical testing, and microstructural characterisation. The mechanical analysis indicated the nature and level of stresses operating near the surface of rolls; mechanical testing allowed material performance to be anticipated. An optimal post-weld heat treatment procedure, which maximises strength while minimising material strain hardening, was subsequently chosen. The microstructure of the candidate cladding material is a mixture of lower bainite and martensite, containing a very fine distribution of molybdenum carbides. In situresults have shown that welded rolls outperform traditional rolls, as the amount of steel rolled per millimetre of cladding material is 40% higher than with forged rolls and double that obtained w...

An investigation has been undertaken into the creep behavior of the single-crystal superalloy CMSX-4. Creep deformation in the alloy occurs largely through dislocation activity in the γ channels. Shearing of the γ′ dislocations is observed, but, at higher temperatures, this does not occur until late in life via the passage of superpartial dislocation pairs. At lower temperatures (1023 K) and high stress levels, shearing of the γ′ precipitates is observed relatively early in the creep curve through the passage of {111}〈112〉 dislocations, which leave superlattice stacking faults (SSFs) in the precipitates. The stress-rupture behavior of CMSX-4 has been modeled using a damage-mechanics technique, where the level of damage required to cause failure is defined by the effective stress reaching the material’s ultimate tensile strength (UTS). This technique ensures that short-term rupture data extrapolate back to the UTS. High-temperature steady-state and tertiary creep are modeled using modified damage-mechanics equations, where the strain and damage rates are similar functions of stress. At intermediate operating temperatures of 1023 to 1123 K, the material exhibits pronounced sigmoidal primary creep of up to 4 pct strain, which cannot be modeled using a conventional approach. This transient behavior has been explained by the effect of internal stresses acting on dislocations in the gamma matrix; such an internal stress has been included in the creep law and evolves as a function of the damage-state variable.

The damage mechanisms of several kinds of coatings on a single crystal nickel base superalloy under thermomechanical fatigue (TMF) are described. The systems investigated were diffusion platinum aluminide coatings, Co–Ni–Cr–Al–Y overlay coatings, and thermal barrier coatings (TBCs). The TMF experiments were carried out on hollow specimens over a temperature range from 300 to 1050°C, at strain ranges Δɛ = 0·5 and 0·7%, and at a strain ratio R = -∞. No coating cracking was found for the platinum aluminide coating. Instead, specimens failed owing to oxidation induced crack initiation from the uncoated inner surface of the hollow testpieces, although coating surface roughening caused by non-homogeneous oxidation was observed. For the overlay coating, roughening in terms of coating rumpling and coating cracking occurred, resulting in reduced TMF life. For TBC specimens with a thin ceramic coating processed by electron beam–physical vapour deposition (EB–PVD), TMF life was comparable with that of speci...

The microstructure and the microstructural development of a Pt-aluminide coating on CMSX-4 superalloy during thermal mechanical fatigue (TMF) over a temperature range from 300 to 1050 °C have been characterised. Two kinds of precipitates have been observed in the as-coated condition; submicron-sized tungsten structured particles in the outer layer of the coating and micron-sized μ phase in the inner layer of the coating. During TMF testing, the coating extended into the original substrate with the initial γ / γ ′ substrate, transforming to a mixture of γ ′, β -NiAl and a certain amount of μ-phase precipitates. β -NiAl grains mostly exhibit a lath shape in this region. Another important microstructural change is the precipitation of many σ-phase needles in the substrate adjacent to the interface with the coating. The microstructural changes became more evident as the number of TMF cycles increased. No detrimental effect on TMF lives in terms of crack initiation and growth from these TCP phases was found. The distribution of alloying elements in precipitates, β -NiAl phase and substrate has been analysed, and their influence on the precipitation of some phases is discussed.

In order to clarify the fatigue behaviour of polycrystalline Ni-base superalloy Udimet 720 Li and its statistical characteristics, four point bending fatigue test of plain specimens were carried out at room temperature. Initiation and propagation behaviour of all the cracks in each specimen was observed using the plastic replication technique. At extremely early stages of cycling, cracking of some TiN particles occurred. Fatigue cracks were initiated from these cracked particles. The growth rate of a small crack can be determined by a term σ n a l , not by Δ K , where n is the material constant and is 3.6 for this alloy. Moreover, the relationship between the small crack growth law and fluctuation of growth rate caused by the inhomogeneity of the microstructure was studied. With regard to statistical characteristics of fatigue behaviour, the investigation of initiation lifetime of cracks and crack density was performed. The distribution characteristics of crack length, microcrack propagation life and growth rate of microcracks was examined by assuming a Weibull distribution and log-normal distribution.

In order to clarify the fatigue behavior of Ni-base superalloy Udimet 720Li, four point bending fatigue tests of plain specimens were carried out at room temperature. Initiation and propagation behavior of major cracks was observed by plastic replica technique. At extremely early stages of cycling, cracking of TiN particles was generated. Fatigue cracks were always initiated from cracked TiN particles. The growth rate of a small crack can be determined by a term σnal, not by ΔK. The n is a material constant and was 3.6. On the other hand, the statistical characteristics of fatigue behavior were investigated to quantify the scatter in fatigue damage. Namely, the distribution for crack length, microcrack growth life and crack growth rate were analyzed by assuming a Weibull distribution. Moreover, the cracking time and cracking orientation of all the TiN particles were measured. The measurements suggested that the TiN particle were fractured by the principal tensile stress which is related to the variation in local strain around a particle.

Ni-base single crystal superalloy turbine components show better creep resistance than their polycrystal counterparts and are used as blades and vanes in aircraft and gas turbine engines. In order to achieve higher operating temperatures combined with good environmental performance, a range of coatings and intermediate coatings are being examined. In this respect it has been realized that the thermomechanical fatigue (TMF) response is a major life limiting factor. The aim of this paper is to determine the influence of an aluminide coating on the TMF lives of a CMSX4 single crystal superalloy and to identify the dominant failure mechanism operating under out-of-phase TMF testing conditions. 300--1,050 C, 135{degree} out-of-phase mechanical loading cycles have been chosen to represent the harshest conditions faced at a critical location across on a blade in service. This information is of great significance to aero-space industries and provides a sound basis for predicting the TMF lives of superalloy components.

In order to clarify the fatigue behavior of Ni-base superalloy Udimet 720 Li, four point bending fatigue tests of plain specimens were carried out at room temperature. Initiation and propagation behavior of major cracks was observed by plastic replica technique. At the extremely early stage of cycling, cracking of TiN particles was generated. The cracked TiN particle always became the starting site of a micro crack. Though some particles embedded in the surface were cracked due to the stress cycling, the generation of cracking TiN particles was stopped before 30% of fatigue life. On the other hand, the growth rate of a small crack can be determined by a term σanl, not by ΔK. The n is a material constant and was 3.6. The relation between the fluctuation of dl/dN due to the microstructure and the small crack growth law was investigated. It was concluded that the dl/dN of microcracks can be evaluated based on the small crack growth law and dl/dN calculated from a smoothed crack growth curve.

The effects of a thermal residual stress field on fatigue crack growth in a silicon carbide particle-reinforced aluminum alloy have been measured. Stress fields were introduced into plates of material by means of a quench from a solution heat-treatment temperature. Measurements using neutron diffraction have shown that this introduces an approximately parabolic stress field into the plates, varying from compressive at the surfaces to tensile in the center. Long fatigue cracks were grown in specimens cut from as-quenched plates and in specimens which were given a stress-relieving overaging heat treatment prior to testing. Crack closure levels for these cracks were determined as a function of the position of the crack tip in the residual stress field, and these are shown to differ between as-quenched and stress-relieved samples. By monitoring the compliance of the specimens during fatigue cycling, the degree to which the residual stresses close the crack has been evaluated.

A study of the influence of macroscopic quenching stresses on long fatigue crack growth in an aluminium alloy-SiC composite has been made. Direct comparison between quenched plate, where high residual stresses are present, and quenched and stretched plate, where they have been eliminated, has highlighted their role in crack closure. Despite similar strength levels and identical crack growth mechanisms, the stretched composite displays faster crack growth rates over the complete range of ΔK, measured at R = 0.1, with threshold being displaced to a lower nominal ΔK value. Closure levels are dependent upon crack length, but are greater in the unstretched composite, due to the effect of surface compressive stresses acting to close the crack tip. These result in lower values of ΔKeff in the unstretched material, explaining the slower crack growth rates. Effective ΔKth values are measured at 1.7 MPa√m, confirmed by constant Kmax testing. In the absence of residual stress, closure levels of approximately 2.5 MPa√m are measured and this is attributed to a roughness mechanism.

A study has been made of the influence of the reinforcement/matrix interfacial strength on fatigue crack propagation in a powder metallurgy aluminum alloy 8090-SiC particulate composite. The interfacial region has been altered by two separate routes, the first involving aging of the 8090 matrix, with the subsequent formation of precipitate free zones at the boundaries, and the second consisting of oxidizing the surface of the SiC particles before their incorporation into the composite. In the naturally aged condition, oxidation of the SiC leads to a reduction in fatigue crack growth resistance at higher values of stress intensity range ΔK. This is due to a proportion of the crack growth occurring through voids formed in association with many of the weak SiC interfaces which have retained a layer of thick surface oxide after processing. On overaging no difference in crack growth rate is discernible between the oxidized and unoxidized SiC composites. It is proposed that this is due to similar levels of interfacial weakening having occurred in both composites, indicating that this is an important factor in the reduction of the high ΔK crack growth resistance of the unoxidized SiC composite on aging.

The effect of residual stresses, induced by cold water quenching, on the morphology of fatigue crack fronts has been investigated in a powder metallurgy 8090 aluminium alloy, with and without reinforcement in the form of 20 wt-%SiC particles. Residual stress measurements reveal that the surface compressive stresses developed in these materials are significantly greater than in conventional metallurgy ingot 8090, because surface yielding occurs on quenching. The yield stresses of the powder route materials are greater than those of ingot produced 8090 and hence greater surface stresses can be maintained. In fatigue, severe crack front bowing is observed in the powder formed materials as a result of the reduction of the R ratio (minimum load/maximum load) by the compressive residual stresses at the sides of the specimen, causing premature crack closure and hence reducing the local driving force for fatigue crack growth ΔKeff. This distortion of the crack fronts introduces large errors into measurements of crack growth rate and threshold values of ΔK.

A study of the influence of SiC-particulate reinforcement on ageing and subsequent fatigue crack growth resistance in a powder metallurgy 8090 aluminium alloy-SiC composite has been made. Macroscopic hardness measurements revealed that ageing at 170°C in the composite is accelerated with respect to the unreinforced alloy, though TEM studies indicate that this is not due to the enhanced precipitation of S′. Fatigue crack growth rates in the naturally aged condition of the composite and unreinforced matrix are similar at low to medium values of ΔK, but diverge above ≈ 8 MPa√m owing to the lower fracture toughness of the composite. As a result of the presence of the reinforcement, planar slip in the composite is suppressed and facetted crack growth is not observed. Ageing at or above 170°C has a deleterious effect on fatigue crack growth. Increased ageing time decreases the roughness of the fracture path at higher growth rates. These effect are though to be due to microstructural changes occurring at or near to the SiC/matrix interfaces, providing sites for static mode failure mechanisms to operate. This suggestion is supported by the observation that as ΔK increases, crack growth rates become Kmax dependent, implying the crack growth rate is strongly influenced by static modes.

The use of single crystal turbine blade components has made many traditional models for creep unsuitable for application in certain loading situations. Such models generally assume deformation is isotropic. Since the introduction of single crystal components a generation of crystal plasticity or slip system models has emerged. Amongst the challenges of such models is to explain the orientation dependence of uniaxial creep behaviour and to attempt to make correlation between uniaxial and multiaxial stress states so that confidence can be’gained as to the performance of a model when it is applied to complex loading conditions. In this paper an attempt is made to address these issues. A slip system based finite element creep model has been fitted to uniaxial data at 1223 K in a range of crystallographic orientations. Based on experimental evidence the operative slip systems incorporated are the {111} and {111}412> families of slip system. Analysis of the data and a consideration of the dislocation mechanisms likely to occur suggest that the significant component of the hardening matrix is latent hardening by the {111} systems on the { 11 1} systems, although hardening between the { 11 l} systems may also occur. The model can describe creep deformation as a function of orientation to a reasonable degree of accuracy. It is also shown to have reasonable predictive capability when used to analyse the results of thin cylinder biaxial creep tests on CMSX-4 and SRR99. The reason for this success is explained in terms of activated slip systems and the magnitude of the cumulative shear strain rate on different types of slip system as a function of orientation and stress state. Superanoys zoo0 @ited by T.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. M&an, S. Olson, and J.J. Schirra TMS (The h4inemls. Metals & Materials Society), 2ooO

A study has been conducted to examine the behaviour of isothermal low cycle fatigue (LCF) and thermomechanical fatigue (TMF) of Ni-base superalloy C263 over a temperature regime from 25 to 950°C for several strain ranges. The macroscopic performance was correlated by detailed microstructural analysis using TEM and SEM. Initial cyclic hardening in isothermal LCF occurred at all temperatures. At room temperature, slight cyclic softening occurred following the hardening stage. This effect is more evident with increasing strain amplitude. TEM analysis revealed that this was related to dislocation shearing of y’ precipitates. Continuous cyclic hardening and considerable softening were observed respectively at 600 and 800°C. As at room temperature, planar slip was pronounced, but more slip systems operated resulting in a higher dislocation density as temperature increased. A gradual change of fracture mode fkom planar transgranular at room temperature to more wavy tansgranular failure at 800°C was observed. At 950°C, the load carrying capacity of C263 alloy was significantly reduced and the material was essentially cyclically stable. During LCF at this temperature, carbides precipitate in the matrix, promoted by plastic deformation, while the y’ phase was dissolved. IP and OP TMF tests suggested that considerable high temperature creep deformation occurred followed by heavy plastic deformation at low temperatures. Plastic recovery and recrystallization produced a microstructure with elongated grains and subgrains which is different from that in isothermal fatigue. In all TMF tests and isothermal LCF at 950°C, fracture was predominantly via an intergranular mechanism. Edited by T.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S. Olson, aad J.J. Schim TMS (The Minerals, Metals & Materials Society), Zoo0

In this paper the microstructural instability of a recently developed powder-processed alloy, RRlOOO, is investigated and the corresponding influence on the creep-fatigue crack growth behaviour scrutinised. Equilibrium phase calculations of the alloy composition predict the presence of the TCP (Topologically Close Packed) phase sigma at the intended operating temperature of the alloy. Subsequent exposures at elevated temperatures reveal intergranular sigma phase precipitation. The kinetics of sigma phase formation has been determined experimentally by quantitative x-ray diffraction of exposed specimens. The effect of this grain boundary modification during prolonged exposure on the mechanical properties of the alloy was then investigated by testing specimens with two thermal histories: as-heat-treated (with a standard solution and ageing heat treatment developed for RRlooO) and exposed to contain 1 wt% sigma phase on the grain boundaries. To enable the discrimination of creep and environmental damage mechanisms during crack growth, fatigue and sustained load crack growth tests were performed in both vacuum and air. The influence of exposure on tensile aud creep rupture behaviour was also investigated. In order to provide a suitable reference, a single fatigue crack propagation test was performed on the as-heat-treated material at room temperature. All other tests were performed at 725 “C. From the crack growth rates recorded and fractographlc studies it is concluded creep aud environmental mechanisms are responsible for increases in crack growth rates observed when the temperature is increased from 20 “C to 725 “C. Sigma phase precipitation during alloy exposure increases the severity of creep and environmental processes leading to increases in crack growth rates. Superalloys 2ooo Edited byT.M. Pollock, R.D. Kissinger, R.R. Bowman, K.A. Green, M. McLean, S. Olson, and J.J. Schirm TMS (The Minerals, Metals &Materials Society), 2OC0