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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