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The effect of temperature over the range of ambient to 704 °C and strain rate from 10−4 to 10−2 s−1 on the tensile properties and fracture behavior of ATI 718Plus was investigated. The results showed that with increase in temperature at a strain rate 10−4 s−1, there is a small reduction in the yield strength, but a large drop in ductility at 704 °C. This reduction was accompanied by a change in fracture mode from ductile transgranular to brittle intergranular cracking. Detailed analysis of the microstructure and microchemistry of the areas around the crack using electron microscopy showed that the driving mechanism behind the failure at elevated temperatures and slow strain rates is oxygen-induced intergranular cracking, a dynamic embrittlement mechanism. In addition, the results suggest that the δ precipitates on the grain boundaries tend to oxidize and may facilitate the oxygen-induced intergranular cracking. Finally, an increase in strain rate at 704 °C caused a small increase in the yield strength and a huge increase in ductility. This increase in ductility was accompanied by a change in fracture mode from brittle-to-ductile failure. Possible mechanisms for the deformation, failure mechanisms, and strain rate dependence are discussed.

The development of high-temperature heavy-duty turbine disk materials is critical for improving the overall efficiency of combined cycle power plants. An alloy development strategy to this end involves superalloys strengthened by ‘compact’ γ′-γ″ coprecipitates. Compact morphology of coprecipitates consists of a cuboidal γ′ precipitate such that γ″ discs coat its six {001} faces. The present work is an attempt to investigate the microstructure and creep behavior of a fully aged alloy exhibiting compact coprecipitates. We conducted heat treatments, detailed microstructural characterization, and creep testing at 1200 °F (649 °C) on an IN718-variant alloy. Our results indicate that aged IN718-27 samples exhibit a relatively uniform distribution of compact coprecipitates, irrespective of the cooling rate. However, the alloy ruptured at low strains during creep tests at 1200 °F (649 °C). At 100 ksi (689 MPa) load, the alloy fails around 0.1% strain, and 75 ksi (517 MPa) loading causes rupture at 0.3% strain. We also report extensive intergranular failure in all the tested samples, which is attributed to cracking along grain boundary precipitates. The results suggest that while the compact coprecipitates are indeed thermally stable during thermomechanical processing, the microstructure of the alloy needs to be optimized for better creep strength and rupture life.

Using advanced aberration-corrected scanning transmission electron microscopy coupled with high-resolution energy-dispersive spectroscopy techniques, transitions to microstructure and composition occurring at a grain boundary (GB) crack-tip in Waspaloy after dwell-fatigue at 704 °C are characterized at the nano to atomic scale. The simple microstructure of this γ/γ′ superalloy, which enables elucidation of oxygen effect ahead of the oxide intrusion—dissolution of the precipitates fostering selective segregation of Al to the GB to form oxides—has been experimentally observed.

Recently, a class of refractory high-entropy alloys has been developed with a refined microstructure consisting of an ordered B2 matrix with cuboidal BCC precipitates resembling an “inverted superalloy-like” microstructure. In this paper, we have studied the evolution of the duplex microstructure of AlMo0.5NbTa0.5TiZr during aging at elevated temperatures, particularly with regard to the nature of the B2 and BCC phases. Samples were aged at 1000 °C for 6 h, followed by a water quench. It was found that while the relative volume fractions of the B2 and BCC phases remained nearly constant after ageing, compositional changes of both the B2 and BCC phases were determined. In the aged condition, there is evidence from high-resolution STEM HAADF imaging that suggests that the B2 phase in the aged condition may undergo a spinodal reaction, with the sub-lattice occupancies differing within the ordered precipitates. A change in size and shape of the BCC precipitates was also noted, and this was accompanied by a difference in the nature of the B2/BCC interfaces. Thus, a step-like B2/BCC interface is evidenced in the aged condition, in contrast with the planar {100} interfaces in the “superalloy-like” microstructure, likely adopted to accommodate a change in coherency resulting from an increase in misfit between the two phases from compositional changes occurring during the coarsening of the BCC precipitates.

Embossed parts, that contain a number of features, are desired for a range of components, such as, heat exchangers, bipolar plates, micro-reactors, and micro-fluidics. A comparison of low strain rate embossing and high strain rate embossing was investigated in this study. High strain rate deformation at the embossed surface induced quatitatively different properties than those on a quasi-statically formed part. A commercial hand file was chosen as a simple and available die surface due to its high hardness and relatively fine features. Commercially pure copper (110) was embossed using four methods: quasi-static (static, low strain rate), magnetic pulse (MP, high strain rate), direct vaporizing foil actuator (direct VFA, high strain rate), and urethane-assisted vaporizing foil actuator (urethane VFA, high strain rate). Embossed depth, mechanical properties, and microstructure evolution were studied for both low strain rate embossing and high strain rate embossing. The results showed that generally better conformity to the die features and higher surface hardness were achieved with high strain rate embossing, in part because higher pressures could be developed with these methods. The study of the microstructure revealed that besides the grain size and shape change, significant twinning appeared along the deformed surface in high strain rate embossed parts. Optical microscopy (OM), scanning electron microscopy (SEM), and transmission electron microscopy (TEM) were performed to study the resultant microstructure.

Electron microscopy, X-ray energy dispersive spectroscopy, and tomographic reconstructions were used to characterize a high entropy alloy predominantly composed of refractory elements. The intragranular microstructure was found to consist of a periodic two phase mixture, where a disordered bcc phase is aligned orthogonally in an ordered B2 phase. The phases were found to exhibit continuous lattice registry and an orientation relationship given by bcc// B2, {001}bcc//{001}B2. X-ray energy dispersive spectroscopy was used to determine the compositions of the phases, and spectral images were used in the tomographic reconstruction of the alloy to reveal the morphology and the elemental partitioning between phases.

A dissimilar weld between a low alloy steel (LAS) butter weld joined to a F65 steel pipe using a narrow groove hot wire gas tungsten arc welding (HW-GTAW) procedure with Alloy 625 filler metal was investigated. The weld interpass microstructure is comprised of large swirls formed by a macrosegregation mechanism involving partial, non-uniform mixing of liquid base metal with the lower melting temperature weld pool, followed by fast solidification. This mechanism produces steep gradients in composition and solidification behavior. The resulting swirls are composed of alternating iron-rich peninsulas and partially mixed zones (PMXZ) that are surrounded by planar and cellular zones exhibiting multiple solidification directions. Large austenitic grains, encompassing planar, cellular, and dendritic morphologies, nucleate off peninsulas in direct contact with the weld pool. The highest hardness was found in nickel and chromium rich PMXZs that exhibited a lath martensite microstructure. In the event of exposure to hydrogen containing environments, the PMXZs could serve as nucleation sites for hydrogen assisted cracking.

The microstructure of a high entropy alloy with composition of Mo0.5Al1Nb1Ta0.5Ti1Zr1 (the digits refer to molar volumes) has been characterised directly in three dimensions using TEM dark field (DF) imaging and by recording tilt pair micrographs using STEM high angle annular DF (HAADF) imaging. The microstructure contains disordered bcc precipitates that appeared as orthogonal stacks of plate-like features. A tapered needle sample was prepared in a focused ion beam/SEM and was used to acquire a 180° tomographic dataset of STEM/HAADF images in 2° increments. The tilt series images were registered, and the algebraic reconstruction technique was used to reconstruct the three-dimensional microstructure. The bcc precipitates were segmented using a combinative approach involving two threshold techniques. The precipitates were then visualised using commercial software, which revealed surprisingly the existence of both cuboidal and plate-like morphologies. Colouring each precipitate according to its morp...

Samples of Hot Isostatically Pressed (HIPped) powder of TIMETAL 6-4 (Ti-6Al-4V, compositions in wt pct unless indicated), which was HIPped at 1203 K (930 °C), and of forged bar stock, which was slowly cooled from above the beta transus, were both subsequently held at 773 K (500 °C) for times up to 5 weeks and analyzed using scanning and transmission electron microscopy and atom probe analysis. It has been shown that in the samples aged for 5 weeks at 773 K (500 °C), there is a high density of alpha2 (α2, an ordered phase based on the composition Ti3Al) precipitates, which are typically 5 nm in size, and a significantly smaller density was present in the slowly cooled samples. The fatigue and tensile properties of samples aged for 5 weeks at 773 K (500 °C) have been compared with those of the HIPped powder and of the forged samples which were slowly cooled from just above the transus, and although no significant difference was found between the fatigue properties, the tensile strength of the aged samples was 5 pct higher than that of the as-HIPped and slowly cooled forged samples. The ductility of the forged samples did not decrease after aging at 773 K (500 °C) despite the strength increase. Transmission electron microscopy has been used to assess the nature of dislocations generated during tensile and fatigue deformation and it has been found that not just is planar slip observed, but dislocation pairs are not uncommon in samples aged at 773 K (500 °C) and some are seen in slowly cooled Ti6Al4V.

Synchrotron X-ray diffraction and neutron diffraction techniques are employed to characterise the lattice parameters and volume fraction of γ and γ′ phases in a high strength nickel base superalloy. Samples of Rene88DT were solutionised at 1150°C and cooled under three different rates to provide fine and coarse γ′ size distributions. Samples were aged at 760°C to precipitate tertiary γ′ and coarsen secondary precipitates. Lattice parameter misfit and coefficient of thermal expansion parameters were also determined. Results indicate significant microstructure changes within the samples during the initial 25 h of aging.

The compositional and microstructural evolution of different generations of precipitates of the ordered γ′ phase during the continuous cooling, followed by isothermal aging, of a commercial nickel-base superalloy, Rene 88DT, has been characterized by three-dimensional atom probe (3DAP) tomography coupled with energy-filtered transmission electron microscopy (EFTEM) studies. After solutionizing in the single γ-phase field, during continuous cooling at a relatively slow rate (~24 °C/min), the first-generation primary γ′ precipitates, forming at relatively higher temperatures, exhibit near-equilibrium compositions, while the smaller-scale secondary γ′ precipitates, forming at lower temperatures, exhibit nonequilibrium compositions often containing an excess of Co and Cr while being depleted in Al and Ti content. The compositions of the γ matrix near these precipitates also exhibit similar trends, with the composition being closer to equilibrium near the primary precipitates as compared to the secondary precipitates. Subsequent isothermal aging at 760 °C leads to coarsening of the primary γ′ precipitates without affecting their composition significantly. In contrast, the composition of the secondary γ′ precipitates is driven toward equilibrium during the isothermal aging process.

Constant-load creep experiments at a temperature of 750 °C and a nominal stress of 300 MPa were conducted on a fine-grained Ti–45Al–5Nb–0.2B–0.2C (in at.%) alloy with a duplex microstructure. Microstructures before and after creep (accumulated strain: 9.6%) were analyzed using scanning and transmission electron microscopy (SEM and TEM). TEM analysis after creep indicates that the individual microstructural constituents of the fine duplex microstructure, namely, the equiaxed γ and the lamellar α2/γ colonies, undergo varying degrees of deformation and develop various substructures. Lamellar grains deform by dislocation creep. They show clear evidence for dislocation and twin activity. In contrast, only few dislocations are found in the equiaxed grains. We show that the regions with small equiaxed γ grains, representing 65–75 vol.% of the microstructure, deform by grain-boundary sliding.

In the present study, the relationship between the crystallographic orientations and growth directions of grain boundary-allotriomorphic-α (GB α) and secondary Widmanstatten α laths growing from the GB α at grain boundaries separating β grains with specific misorientations has been examined. These relationships have been determined using a variety of characterization techniques, including scanning electron microscopy, orientation imaging microscopy, transmission electron microscopy (TEM) and a dual-beam focused ion beam instrument to provide site-selected TEM foils. Two very interesting cases, one in which the two adjacent β grains are rotated mutually by approximately 10.5° about a common 〈1 1 0〉 direction and the other in which the two β grains are in a twin relationship, i.e. a 60° rotation about a common 〈1 1 1〉 direction, have been studied. It was discovered that the α laths growing into two adjacent β grains from the common grain boundary may have the same orientation in both grains, while they may have either large (∼88.8°) or small (28.8°) angular differences in growth directions in the two adjacent β grains, depending on the relative misorientation of the β grains. The growth directions of the α laths growing from such boundaries are explained on the basis of the Burgers orientation relationship between the Widmanstatten α and the β phases and the interfacial structure proposed previously by various workers.

The development of a set of computational tools that permit microstructurally based predictions for the tensile properties of commercially important titanium alloys, such as Ti-6Al-4V, is a valuable step toward the accelerated maturation of materials. This paper will discuss the development of neural network models based on a Bayesian framework to predict the yield and ultimate tensile strengths of Ti-6Al-4V at room temperature. The development of such rules-based model requires the population of extensive databases, which in the present case are microstructurally based. The steps involved in database development include producing controlled variations of the microstructure using novel approaches to heat treatments, the use of standardized stereology protocols to characterize and quantify microstructural features rapidly, and mechanical testing of the heat-treated specimens. These databases have been used to train and test neural network models for prediction of tensile properties. In addition, these models have been used to identify the influence of individual microstructural features on the tensile properties, consequently guiding the efforts toward development of more robust mechanistically based models. Based on the neural network model, it is possible to investigate the influence of individual microstructural features on the tensile properties, and in certain cases these dependencies can point toward unrecognized phenomena. For example, the apparently unexpected trend of increase in tensile strength with increasing prior β-grain size has led to the determination of the pronounced role of the basketweave microstructure in strengthening these alloys, especially in case of larger prior β grains.

The hardness of α-titanium grains as a function of both indentation depth and orientation has been assessed using nanoindentation. Direct observations and analyses of the dislocation substructures have been achieved by cutting thin-foil membranes exactly through given indents with a dual-beam focused-ion-beam instrument and from diffraction-contrast experiments in a transmission electron microscope. It was found, as expected, that the hardness varied with the depth of indentation. Regarding the orientation dependence of hardness, the nature of the statistically stored dislocations as well as that of the geometrically necessary dislocations has been identified. Thus, the occurrence of the majority of the former dislocations can be predicted on the basis of Schmid’s law, while noting the presence of minor densities of other dislocations required presumably because of the arbitrary shape change imposed by the nanoindenter. The geometrically necessary dislocations have been identified as the appropriate combinations of slip dislocations such that an overall displacement parallel to the direction of the indentation results.

Creep deformation substructures in the superalloy Rene 88 DT have been investigated after small-strain (0.2–0.5%) creep at 650 °C using conventional and high resolution transmission electron microscopy. Clear differences in creep strength and deformation mechanisms have been observed as a function of applied stress and precipitate microstructure. Both coarse and fine bimodal precipitate microstructures have been tested, produced by relatively slow and fast cooling from the supersolvus solutionizing temperature. The finer γ′ microstructure exhibited significantly lower creep rates. It has been established that microtwinning caused by the passage of Shockley partial dislocations on successive {1 1 1} planes is the dominant deformation process at low applied stress, and changes to shearing by 1/2[1 1 0] dislocations and Orowan looping around the larger secondary precipitates at higher applied stress. In the coarser microstructure, the dominant deformation mode is isolated faulting where 1/2[1 1 0] dislocations shear the matrix while superlattice extrinsic stacking faults are created in the secondary γ′ particles. The detailed mechanisms by which these deformation modes proceed are discussed, leading to the proposition that the thermally activated process for both microtwinning and isolated faulting is similar, involving diffusion-mediated re-ordering within the γ′ particles in the wake of shearing 1/6〈1 1 2〉 Shockley partials. Based on the present evidence, it is proposed that the tertiary γ′ volume fraction is crucial in dictating the transition in mechanism and the creep strength of these alloys.

In this paper, we describe an on-going effort to develop a thin film combinatorial method for identifying alloy compositions of potential interest as structural materials. To investigate this idea, compositionally graded Ti–Al thin films have been sputter deposited onto Si substrates, and the mechanical properties of the “library” of compositions are then probed using nanoindentation. This combinatorial method offers the possibility of rapidly varying the compositions and microstructures of alloys and quickly determining the compositions and microstructures of greatest interest for further development as bulk structural alloys. Nanoindentation experiments are used to extract the properties of the film so that the bulk properties of the material may be estimated. A new method for extracting the hardness of films on substrates from nanoindentation experiments has been developed and is applied to the compositionally graded Ti–Al thin films. Together with the nanoindentation data and a compositional analysis, we are able to establish a relationship between the hardness of the film and the composition. We discuss the results of these combinatorial experiments in connection with estimating the properties of bulk materials.

Crept microstructures in g-TiAl based alloys reveal a preponderance of 1/2[110]-type jogged-screw dislocations, suggesting that the rate of creep deformation is controlled by the glide of such dislocations. A creep model based on these microstructural observations has been recently developed. This leads to an excellent prediction of creep rates and stress exponents. In this paper, the framework of this model including the verification and validation of the functional dependencies of various microstructural model parameters is reviewed. It has also been observed that creep phenomenology is extremely sensitive to microstructure – fully lamellar g-based alloys exhibit lower creep rates and higher stress exponents even though the deformation microstructure is similar to that in equiaxed alloys. The modifications made to the model that account for the constrained nature of deformation in lamellar alloys are discussed. The applicability of the model is explored in materials systems, including a-Ti and a+b Ti alloys where similar creep exponents and deformation structures have been observed. Finally, the relevance, applicability and shortcomings of the model are critically analyzed.

The present study has examined for α/β-Ti alloys the relationship between the morphology and crystallography of Widmanstatten plates of α-Ti in colonies within a prior grain of β-Ti. Thus, optical metallography, scanning electron microscopy and transmission electron microscopy have been used to characterize the morphological features of the microstructure, whereas orientation-imaging microscopy (OM) and transmission electron microscopy (TEM) have been employed to reveal crystallographic information. It has been discovered that within a prior β-Ti grain, although the growth direction of the Widmanstatten plates in given colonies may differ by large angles from α-plates in other colonies, they may exhibit very close crystallographic relationships. For example, inclined α-plates may share common basal planes and be related by a rotation of ~10.5° about the c-axis of the crystals. This phenomenon has been interpreted on the basis of variant selection of the Burgers orientation relationship commonly adopted between the α and β phases in these alloys. Similar relationships have been observed in α-colonies growing either side of a given prior β grain boundary. These latter observations have been used to draw conclusions concerning the precipitation of α on prior β grain boundaries.

The effect of trace levels of Ni on the intermediate temperature creep behavior of the alloy Ti–6Al–2Sn–4Zr–2Mo (wt%) has been investigated. Creep experiments were performed in tension over the temperature range 510–565 °C at stress range 138–413 MPa. Two heats of commercial grade Ti–6Al–2Sn–4Zr–2Mo with Ni levels of 0.006 and 0.035 wt% were studied. The high Ni material uniformly exhibited higher primary creep strains and minimum strain rates than the lower Ni material. Stress exponents in the range 5–7 and 4–6 were obtained for the high Ni and low Ni material respectively. At 565 °C a transition to a low stress region with a stress exponent equal ∼1 is found for both materials. At all stress levels, the apparent activation energy was lower for the high Ni material. The apparent activation energy is in excellent agreement with those reported for lattice self-diffusion in α-titanium in the presence of fast diffusing impurities. The results also suggest that creep in the higher stress regime is controlled by dislocation motion within the α-phase. We suggest that trace levels of Ni in the α-phase accelerate self-diffusion therefore increasing the rate of dislocation climb leading to the higher creep rates observed in the high Ni material. In Part II, direct evidence in support of dislocation-based creep being important in both low and high stress regimes is presented.

Transmission Electron microscopy studies on crept samples of an equiaxed Ti–48Al alloy deformed to strains near the minimum strain rate show a microstructure dominated by unit 1/2[110] type dislocations. These dislocations are pinned by jogs of varying heights. The jogged-screw model is adopted, where the rate controlling step is assumed to be the non-conservative dragging of the jogs along the length of the screw dislocations. The presence of tall jogs and the existence of a stress-dependent upper bound to the height of tall jogs which can be dragged have been incorporated into the model. These modifications lead to excellent agreement with experimental data. The evolution of the creep curve is also qualitatively predicted. In contrast, lamellar structures show a highly inhomogeneous deformation behavior with dislocation activity in lamellae above a critical thickness and negligible activity below that limit. This limit is related to the minimum stress required to cause channeling of dislocations. The observation of jogged segments in the thicker lamellae suggests that a modification of the jogged-screw velocity law could be used by incorporating an effective stress approach. Qualitative analysis of a probable method of evaluating the creep rate is discussed. Finally, microstructural changes during the aging process in K5 and K5SC alloys have also been studied. Aging causes the dissolution of metastable α2. The effect of Si and C on the aging behavior and precipitation of the silicide and carbide particles during aging and following creep is discussed. These results suggest that microstructural stability is critically important in order to achieve the highest possible creep strengths.

Stress exponents for creep, in the range of 5, are typically associated with dislocation creep processes, normally associated with a strong tendency for subgrain formation. In this article, we will demonstrate that there are several important alloy systems that have similar stress dependence and, yet, lack this tendency for subgrain formation. Specifically, dislocations in the intermetallic compound γ-TiAl and the hexagonal close-packed (hcp) α phase of the commercial Ti alloy, Ti-6242, tend to be homogeneously distributed with a tendency for alignment along screw orientation. In both alloy systems, the screw dislocations exhibit a large density of pinning points, which detailed transmission electron microscopy (TEM) investigation indicate are locations of tall jogs. These observations suggest that the jogged-screw model for creep should be appropriate after suitable modification for the presence of these tall jogs. This modified jogged-screw (MJS) model is presented, together with a discussion of the assumptions made, and the results of this model are shown to compare favorably with experiment for both alloy systems. The possible criteria for the formation of tall jogs are also described, and the potential application of this modified model to other alloy systems is discussed.

The objective of this paper is to revisit the present status of the high temperature creep of alloys with particular reference to models based on the motion of jogged-screw dislocations. Models based on this concept have been previously attempted to describe the creep in pure metals and alloys (Barrett and Nix, Acta Metall., 13 (1965) 1247). Now, in light of emerging new experimental evidence, a modified version of this model appear to be appropriate for the case of creep in both disordered α-Ti alloys and ordered Ti–Al based intermetallic alloys based alloy. TEM investigations have shown that in both cases dislocations of screw character dominate the deformation microstructures in these alloys and that jogs much taller than atomic dimensions form along the dislocations. The predictions of the modified jogged-screw model are compared with creep data for Ti-6242 and γ-TiAl alloys.

Minimum creep rates as a function of stress (100–300 MPa) and T=815 °C have been obtained for a Ti–48Al–2Cr–2Nb alloy with a fully lamellar microstructure. Transmission electron microscopy (TEM) investigation reveals that the deformation structures are dominated by jogged screw 1/2[110] dislocations in γ-laths. It is suggested that the creep in this alloy is controlled by the non-conservative motion of 1/2[110] unit dislocations, which are normally seen in varying proportions depending on the width of the γ-laths. The creep results are then compared with a binary Ti–48Al alloy. The difference in creep rates and stress exponents between these alloys (although differing in composition) have been explained in terms of the active volume fraction of γ-laths that are participating in deformation for a given applied stress. In this context, the effect of lamellar spacing on the creep rates are also been discussed.

During the high-temperature creep of the γ-phase (L10 structure) of a “near-gamma” Ti–48Al microstructure, observations using transmission electron microscopy indicate that a/2〈110] or “unit” dislocation activity is a dominant deformation mode. These unit dislocations tend to be elongated along the screw orientation, and exhibit a large number of localized pinning points. Tilting experiments demonstrate that these pinning points are associated with jogs on the screw dislocations, suggesting that the jogged-screw model for creep should be appropriate in this case. However, it is shown that in its conventional formulation, the jogged-screw model is not capable of reproducing the measured creep response (i.e. stress exponents or absolute creep rates). Microscopic observations also demonstrate that a spectrum of jog heights are present, with some as large as 40 nm, based on present observations. A modification of the jogged-screw model is proposed in which the average jog height is assumed to depend on stress. This modified model results in good agreement between predicted and measured creep rates while using reasonable model parameters. Additional implications of the model and required experiments to further validate the model are also discussed.

The interfaces between the phase separated regions in the dendritic grains of laser-deposited samples of the high entropy alloy CoCrCuFeNiAl have been studied using aberration-corrected analytical (scanning) transmission electron microscopy ((S)TEM). The compositional variations have been determined using energy dispersive x-ray spectroscopy (EDS) in (S)TEM. It was found that between B2, consisting mainly of Al, Ni, Co, and Fe, and disordered bcc phase, consisting mainly of Cr and Fe, there is a transition region, approximately 1.5 nm in width, over which the chemical composition changes from the B2 to that of the bcc phase. The crystal structure of this interfacial region is also B2, but with very different sublattice occupancy than that of the adjacent B2 compound. The structural aspects of the interface between the ordered B2 phase and the disordered bcc phase have been characterized using high angle annular dark-field (HAADF) imaging in STEM. It has been determined that the interfaces are essentially coherent, with the lattice parameters of the two B2 regions and the disordered bcc phase being more or less the same, the uncertainty arising from possible relaxations from the proximity of the surfaces of the thin foils used in imaging of the microstructures. Direct observations show that there is a planar continuity between all three constituent phases.

Deformation mechanisms, operative during intermediate temperature creep of Ni-based polycrystalline superalloys, are poorly understood. The creep deformation substructure has been characterized in Rene 88DT following rapid cooling from the super-solvus temperature, yielding a fine $\gamma^{\prime}$-precipitate microstructure. After creep to modest strain levels (up to 0.5% strain) at 650°C and an applied tensile stress of 838 MPa, microtwinning is found to be the predominant deformation mode. This surprising result has been confirmed using diffraction contrast and high-resolution transmission electron microscopy. Microtwinning occurs via the sequential movement of identical 1/6[11-2] Shockley partials on successive (111) planes. This mechanism necessitates reordering within the $\gamma^{\prime}$ precipitates in the wake of the twinning partials, so that the $L1_2$ structure can be restored. A quantitative model for creep rate has been derived on the basis that the reordering process is rate-limiting. The model is in reasonable agreement with experimental data. The results are also discussed in relation to previous studies under similar deformation conditions.

Empirical datasets of volume fractions and size distributions of small gamma' precipitates from "real" multi-component engineering nickel-based superalloys are vital to calibrate and validate the computer models which predict high sensitivities of mechanical properties to size and volume fraction of these fine gamma precipitates, and in order to accelerate microstructure and alloy development. Consequently, we investigated a number of imaging techniques available in a Tecnai F-20 FEG/TEM and selected the technique which best enabled rapid and extensive acquisition of these datasets using the engineering alloy, René 88'DT. The EFTEM technique was found to be the most appropriate method for imaging fine gamma' precipitates while further investigation showed that the Cr-M-edge, in comparison with other ionization-edges provided the best images based largely on contrast-to-noise ratio. Imaging of the Cr-M-edge elemental maps were further improved by investigating the effects of microscope parameters, imaging filter parameters and analysis of the experimental electron energy loss spectra obtained from this alloy. In addition, a novel technique to determine the volume fraction of the fine gamma' precipitates without the need to determine the absolute thickness of the TEM foil is proposed.

The dislocation substructures developed after small-strain (0.5%) creep at 650°C and 121.6 ksi in Rene 88 DT have been analyzed using conventional and high resolution transmission electron microscopy. Clear differences in creep strength and substructures have been observed for samples heat treated with two different cooling rates from the supersolvus heat treatment temperature. For the more rapidly cooled (400°F/min) microstructure exhibits very low initial creep rates, and deformation microtwinning is the dominant deformation process. For the more slowly cooled (75 °F/min) microstructure, the creep rate is much faster, and isolated faulting of individual secondary � ’ precipitates is observed. The detailed mechanisms by which these two mechanisms proceed is discussed, and a possible explanation for the transition in creep mechanism based on the effect of the tertiary � ’ volume fraction is also offered.

Minimum creep rates as a function of stress have been obtained for Ti-48Al binary alloy with a near gamma and a fully lamellar microstructure. TEM investigation reveals that deformation structures in both microstructures are dominated by jogged screw 1/2[110] dislocations in γ phase. A modif ied jogged screw model is adopted to predict minimum creep rates where the rate controlling step is assumed to be the non-conservative motion of 1/2[110] unit dislocations. In the case of equiaxed microstructure where the deformation was mostly uniform, the creep rates predicted by this model were in agreement with experimental values. Conversely, deformation in lamellar microstructures were highly inhomogeneous where the density of jogged 1/2[110] unit dislocations were seen in varying proportions depending on the width of the γ laths. The creep rates and stress exponents in these microstructures is explained in terms of active volume fractions of γ laths participating in deformation for a given applied stress.

Extended abstract of a paper presented at Microscopy and Microanalysis 2004 in Savannah, Georgia, USA, August 1–5, 2004.