Roger C. Reed

The precipitation of topologically close-packed phases in rhenium-containing superalloys

Abstract Most modern single crystal alloys are thermodynamically unstable with respect to the formation of topologically close-packed (TCP) phases due to the addition of solid solution strengtheners, such as rhenium, molybdenum and tungsten. The occurrence, nucleation and growth of TCP phases in a number of rhenium-containing second generation alloys have been studied. It has been found that all the alloys form the σ phase; in some cases as a meta-stable intermediate. Most go on to form either the μ of P phase, depending on the alloy composition, the second phase nucleating from the σ phase. The structure of the σ phase is shown to depend on the fit between the γ matrix and the σ phase and it is suggested that alloys which show good lattice fit suffer accelerated TCP precipitation.

Heat treatment of UDIMET 720Li: the effect of microstructure on properties

Abstract The evolution during heat treatment of the as-forged microstructure of the high strength superalloy UDIMET 720Li has been studied. Particular emphasis has been placed on the characterisation of γ ′ precipitation kinetics using optical and transmission electron microscopies (TEM) and subsequent image analysis. The observations are interpreted using thermodynamic, phase transformation and precipitate hardening theories. The results have implications for the gas turbine manufacturers. Through a better understanding of the evolution of the microstructure during ageing, a heat treatment of 24 h at 700°C is proposed, which is believed to be optimal. This allows full advantage to be taken of the properties of the alloy, whilst reducing the costs and time associated with the heat treatment schedules. Moreover, the data presented allows the variation in properties across a U720Li forging to be estimated.

Creep of CMSX-4 superalloy single crystals: effects of rafting at high temperature

The creep performance of (001)-orientated CMSX-4 superalloy single crystals at temperatures beyond 1000 C is analyzed. Rafting of the {gamma}{prime} structure occurs rapidly, e.g., for the 1150 C/100 MPa tests rafting is completed within the first 10 h. At this stage and for a considerable time thereafter the creep strain rate decreases with increasing strain, implying a creep hardening effect which is absent at lower temperatures when the kinetics of rafting is less rapid. Once a critical strain {epsilon}* of (0.7 {+-} 0.3)% is reached, the creep strain increases dramatically and failure occurs within a few tens of hours. It is demonstrated that methods of interpretation which, assume a proportionality between the creep strain rate and creep strain, are unable to account for creep hardening which occurs as a consequence of rafting. A modification is proposed which accounts for the blocking of the glide/climb of {l{underscore}brace}111{r{underscore}brace}{l{underscore}angle}1{bar 1}0{r{underscore}angle} creep dislocations which occurs as the number of vertical {gamma} channels is reduced and cellular dislocation networks become stabilized. Consequently, failure must be taken to be associated with creep cavitation, which occurs predominantly around casting porosity. It is emphasized that more work is required to quantify the interaction between the various creep damagemorexa0» mechanisms.«xa0less

Interdiffusion in the face-centred cubic phase of the Ni-Re, Ni-Ta and Ni-W systems between 900 and 1300°C

Abstract In order to deduce the temperature and composition dependence of the interdiffusion coefficient in the nickel-rich end of the Ni–Re, Ni–Ta and Ni–W systems, a number of diffusion couples have been fabricated and heat treated at temperatures between 900°C and 1300°C. The concentration profiles so-obtained have been determined using an electron microprobe. Subsequently, values of the interdiffusion coefficients were determined using a modified form of the Boltzmann–Matano analysis, and these are presented. Only a very weak concentration dependence of the interdiffusion coefficients was found. The results are compared with the very limited data which are available in the literature.

Interdiffusion of the platinum-group metals in nickel at elevated temperatures

Rates of interdiffusion of the platinum group metals Ir, Pd, Pt, Rh and Ru with nominally pure nickel have been characterised. Interdiffusion coefficients are estimated by analysing the measured diffusion profiles. In the composition range studied (no greater than 10 wt.% of the platinum group metal), the interdiffusion coefficients display little concentration dependence, but the values can differ by an order of magnitude. In order to rationalise the dependence of the interdiffusion coefficient on atomic number, the data are compared with values for other elements which are already in the literature. A systematic trend is observed: elements furthest from the centre of the period display the largest values and elements at the centre the smallest. It appears therefore that the magnitude of the interdiffusion coefficient correlates strongly with the atomic radius of the interdiffusing transition metal species. Our results suggest the existence of a binding energy between the vacancies and the impurity species. We believe that the interdiffusion data are of considerable use to those designing new and novel grades of superalloy for high temperature applications.

On the kinetics of rafting in CMSX-4 superalloy single crystals

Abstract The kinetics of γ′ rafting in CMSX-4 single crystals at 950°C has been studied using scanning and transmission electron microscopies. The crept material was subjected to further heat treatment in the absence of applied stress, in order to deduce the threshold strain ϵth for rafting to continue; this is shown to be 0.10±0.03%. For strains smaller than ϵth, rafting occurs exceedingly slowly; once ϵth is exceeded the kinetics are largely unaffected by the absence of applied stress. The magnitude of ϵth confers a reduction in γ/γ′ interfacial coherency and a relaxation of interfacial misfit stresses. It is concluded that the threshold strain is a quantity suitable for distinguishing between rafting occurring in the “elastic” and “plastic” regimes. The results confirm conclusively that plasticization of the horizontal γ-channels by (a/2)〈1 1 0〉{111} creep dislocations and their subsequent adsorption at the γ/γ′ interfaces, with a concomitant loss of perfect coherency and reduction in elastic misfit strains, is responsible for providing the kinetic path which enables rafting to occur at a reasonable speed. An argument is put forward to explain the magnitude of ϵth.

Creep of CMSX-4 superalloy single crystals: Effects of misorientation and temperature

Abstract In order to better understand the creep performance of CMSX-4 superalloy single crystals, creep strain testing has been carried out on various specimens systematically misaligned by up to 20° from 〈001〉. At 750°C, significant primary creep is observed, the extent of which depends strongly upon small misorientations away from the 〈001〉/〈011〉 symmetry boundary. In this regime there is evidence of secondary creep which is associated with primary creep strain sufficient for two or more further {111}〈11 2 〉 systems to become active. At 950°C, tertiary creep is prevalent, there being little primary creep exhibited under the stress levels tested. Measurements of the shape change and the lattice rotation, as deduced from analysis of electron backscatter patterns in the scanning electron microscope (SEM), confirms that the macroscopic shape deformation is due to {111}〈11 2 〉 at 750°C but that this changes to {111}〈1 1 0〉 at 950°C. This conclusion is supported further by observations made using transmission electron microscopy (TEM). The extent of creep as a function of misorientation away from 〈001〉 has been rationalised with a numerical model which takes account of the underlying deformation mechanisms. In the case of primary creep, the model accounts for the initial orientation of the tensile axis, the lattice rotation, the hardening on the primary slip system, the extent of primary creep, the onset of secondary creep and the evolution of the macroscopic creep strain. There appears to be evidence to suggest that the maximum creep rate in the primary regime can be correlated to the secondary rate, which is invariant with increasing creep strain, i.e. a steady state is reached. At higher temperatures where tertiary creep is prevalent, the misorientation dependence of creep deformation is less strong; its extent is rationalised on the basis of Schmids law. For the purposes of engineering design, it is suggested that the occurrence of {111}〈11 2 〉 and {111}〈1 1 0〉 deformation be associated with primary and tertiary creep; these should be modelled as two separate curves which are associated with hardening and softening, respectively.

Process modelling of grain selection during the solidification of single crystal superalloy castings

Abstract A process model is described for the grain selection occurring during the solidification of single crystal investment castings, which are now used widely for a number of critical applications in gas turbine engines. The basis of the model is a thermal analysis of the heat transfer in the vicinity of the chill region onto which the molten metal is poured. Subsequently the competitive growth of grains during directional solidification is simulated via a cellular-automaton technique. For the purpose of model validation, processing trials have been carried out on a commercial single crystal casting furnace. The thermal cycles set up in and around the vicinity of the grain selector have been measured, and these are used to choose a number of critical parameters in the thermal model. The evolution of grain structure during competitive growth has been characterised using a number of analytical techniques, including orientation imaging microscopy. The results are compared critically with the predictions from the model. It is shown that the model is able to reproduce the statistical distribution describing the final casting orientation, measured with respect to the 〈001〉 crystallographic pole. The model is used to study the geometrical factors influencing competitive growth and the efficacy of two designs of grain selector, and in particular the conferral of any control of the secondary 〈001〉 orientation.

The role of stacking fault shear in the primary creep of [001]-oriented single crystal superalloys at 750°C and 750 MPa

The structure of the a〈112〉 dislocation ribbons in CMSX-4 single crystal superalloy crept at 750°C and 750 MPa has been characterised using transmission electron microscopy. Calculations are performed which confirm that the macroscopic creep deformation occurring under these conditions, i.e. the accumulation of primary creep, is principally a result of the movement of the ribbons through the γ/γ′ structure. Emphasis is placed on clarifying the factors which control the movement of the fault: it appears that the rate controlling step is the entry of the final a/3〈112〉 dislocation into the γ′ phase, which has the effect of removing the superlattice extrinsic fault. Further observations suggest that the a/2〈110〉 dislocations play a vital role not only in nucleating the ribbons, but also in terminating their long-range movement through the microstructure. It is proposed that this latter effect results in the breakdown of primary creep and thus the transition to secondary ‘steady-state’ creep.

Intergranular and interphase microstresses

Recent advances in the characterisation and modelling of microstresses developed between grains and phases in a polycrystal are reviewed. The evolution of microstresses in materials needs to be understood and acknowledged when interpreting diffraction-based measurements of residual stress fields. Measurements of the associated microstrains can be used to test self-consistent models for polycrystalline deformation and texture evolution. Microstress development in face-centred cubic metals is now well understood, and progress is being made for other crystal systems and for two phase materials.