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A deformation mechanism, rarely reported in Ti alloys, has been observed involving α/β interfacial and colony boundary sliding. This paper reports the initial characterization of this mechanism, involving a comparative study of the deformation behaviors of the new alloy (Ti–0.85Al–4.0V–0.25Fe–0.25Si–0.15O, in wt%) and an established α/β alloy. Both alloys exhibited slip steps on their surfaces. However, the new alloy may also deform by a mechanism which manifests itself as sliding at α/β interfaces and colony boundaries. This mechanism is consistent with the provision of significant ductility during very high strain rate testing, a property exhibited by the new alloy.

The US plan for high-level nuclear waste includes the immobilization of long-lived radionuclides in glass or ceramic waste forms in stainless-steel canisters for disposal in deep geological repositories. Here we report that, under simulated repository conditions, corrosion could be significantly accelerated at the interfaces of different barrier materials, which has not been considered in the current safety and performance assessment models. Severe localized corrosion was found at the interfaces between stainless steel and a model nuclear waste glass and between stainless steel and a ceramic waste form. The accelerated corrosion can be attributed to changes of solution chemistry and local acidity/alkalinity within a confined space, which significantly alter the corrosion of both the waste-form materials and the metallic canisters. The corrosion that is accelerated by the interface interaction between dissimilar materials could profoundly impact the service life of the nuclear waste packages, which, therefore, should be carefully considered when evaluating the performance of waste forms and their packages. Moreover, compatible barriers should be selected to further optimize the performance of the geological repository system. Immobilization of radionuclides in glass or ceramic forms in stainless steel in deep geological repositories is planned in the US for disposal of nuclear waste. Under simulated repository conditions, corrosion could be significantly accelerated at the interfaces of different barrier materials.

The strength of a commercial β‑titanium alloy was tuned by systematically controlling the size-scale of α precipitation via two-step heat treatments. While the first step annealing at 350 °C forms the precursor metastable ω phase, the subsequent annealing at 600 °C/1H resulted in fine-scale α precipitation. We find that annealing duration at 350 °C greatly impacts the α size-scale and, this consequently can be used to tune the ultimate tensile strength (UTS) from ~1 to 2 GPa. A combination of high volume-fraction of non-shearable super-refined α precipitates, coupled with their pyramidal arrangement, and a constrained β matrix leads to very high strength.

Due to the competing influences of configurational entropy and enthalpy of mixing, in recent years, secondary (including intermetallic) phases have been reported in many high entropy alloy (HEA) systems. These secondary phases offer great potential in terms of strengthening the HEA beyond the solid solution strengthening effects, and as such are of great interest in regards to alloy design for engineering applications. The present research investigates novel nano-scale core–shell precipitates forming within the disordered bcc matrix phase of an Al2CrCuFeNi2 HEA, utilizing complementary high-resolution microscopy techniques of atom probe tomography (APT) and transmission electron microscopy (TEM). The size, morphology, and local chemistry of these core–shell precipitates was measured by APT, and the composition was further corroborated by high-resolution scanning transmission electron microscopy–energy dispersive spectroscopy in an aberration-corrected TEM. Furthermore, high-resolution TEM imaging of the core–shell structure indicates that the Cu-rich core exhibits a bcc crystal structure.

Hydrogen embrittlement behavior of four heats of alloy 725 (H1, H2, H3, and H4) were characterized by measuring the crack growth rate response over a range of cathodic potentials. Significant grain boundary precipitation in two heats (H1 and H3) may have contributed to higher susceptibility, as evidenced by higher values of crack growth rate. The crack growth rate increased with increasing twin fraction. The crack morphology exhibited evidence of intergranular as well as slip band cracking. The crack growth rate response of the two heats that were less susceptible (H2 and H4) were evaluated in more detail over a range of applied potentials and stress intensity factors. Evidence of nano void formation at the intersection of dislocation slip bands, and grain boundaries suggests that hydrogen stabilized vacancies may play a role in the crack growth mechanism. Crack growth rate exhibited a strong response with applied potential varying by as much as 100 times in the potential range of −1050mV SCE to −850mV SCE. Over the range of K values evaluated, the crack growth rate increased by a factor of 3to 30 with applied K depending on the applied potential. The crack growth rate was related to the water adsorption on fresh metal surfaces of alloy 725 in the anticipated crack tip conditions. A crack tip strain rate based approach was applied to model the measured crack growth response.

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.

The compositional and order-disorder transition widths of γ′/γ interfaces in a Co–Al–W alloy have been established for the first time by coupling of aberration corrected high resolution scanning transmission electron microscopy and orientation-specific atom probe tomography. While the compositional width is approximately 2.5 nm, the order-disorder transition width is very sharp measuring 0.5 nm. A comparative study revealed that the γ′/γ interfaces, in a model Ni–Al–Cr alloy, are both compositionally and structurally diffuse with nearly the same width (~ 2.5–3 nm). These differences between the γ′/γ interfaces in Co and Ni-base alloys can impact the coarsening behavior of the ordered precipitates.

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.

To reduce self-corrosion of Al-Zn-In pigments in an Al-rich primer, trivalent chromium process (TCP) was applied to the pigments before mixing in the epoxy binder. The formation of TCP and its effects on both the Al-Zn-In bulk alloy and pigment particles were investigated in this study. The TCP layer on the Al-Zn-In alloy has a bilayer structure similar to TCP layers formed on other alloys. The corrosion rate of bulk Al-Zn-In alloy is reduced by the TCP treatment. The TCP layer likely blocks the access of Cl- to the Al-Zn-In alloy, which reduces corrosion attack initiation.

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...

Microcompression testing was conducted on the cast single crystal nickel-base superalloy Rene N5. Microcrystals were selectively fabricated from either dendrite core or interdendritic regions. The compression axis was oriented for single-slip deformation and microcrystal diameters ranged from 2.5 to 80 μm. All microcrystals displayed several hallmarks of size-affected plastic flow, including a size-affected and stochastic flow-stress and initial strain hardening rate, as well as an intermittent flow response. The magnitude of size-affected flow-stress scaling behavior was dependent upon the plastic strain level of the flow-stress measurement, with increasing size-dependence for increasing strain levels. TEM analysis demonstrated the activation of multiple slip-systems, despite the microcrystals being oriented for single-slip deformation. Zig-zag slip was also observed in microcrystals that achieved flow stresses of ∼1300 MPa or higher. For microcrystals fabricated within interdendritic regions the flow-stress values are, on average, lower compared to dendrite core microcrystals. This difference in flow-stress is especially pronounced for microcrystals which are 5 μm in diameter. The microcrystal diameter for which bulk-like properties are estimated to be observed is approximately 350 μm, which is approaching the measured primary dendrite arm spacing for this crystal (430 μm).

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.

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.

Single crystal sheets of niobium with initial orientations of (0 0 1) [ 1 1 ¯ 0 ] , (1 1 0) [ 1 1 ¯ 0 ] and (1 1 1) [ 1 1 ¯ 0 ] were rolled at room temperature in the strain range of 25–50%. The deformed specimens were vacuum annealed at temperatures of 800 °C, 1000 °C, and 1200 °C for 3 h. TEM, SEM-OIM and optical microscopy revealed orientation stability in (0 0 1) and (1 1 0) rolled samples with no recrystallization observed after annealing. Samples rolled along (1 1 1) partially recrystallized after annealing at 1000 °C and 1200 °C. A relatively small increase was observed in hardness of (0 0 1) rolled crystals between 25% and 50% strain, implying low work hardening rates. (1 1 1) rolled samples showed higher hardening rates, and enhanced recovery in hardness values after annealing, due to partial recrystallization. Conditions have been identified for the deformation and annealing of niobium single crystals, enabling the preservation of single crystal structure and near-complete recovery of mechanical properties. A simple crystallographic model is proposed, giving an explanation for the observed orientation stability in (0 0 1) and (1 1 0) rolled samples, and the tendency towards instability and recrystallization in (1 1 1) rolled samples.

A creep deformation mechanism in a polycrystalline nickel-base superalloy (Rene 88 DT) has been examined using crystallographic analysis and comparison between simulated and experimental high-resolution transmission electron microscopy (HRTEM) images. Confirmation of the magnitude of the Burgers vector of the partial dislocations involved in the creep deformation was critical in conclusively determining its nature. A new approach to determining a complex stacking fault was developed using superlattice image contrast from an experimental HRTEM image.

It has been recently shown that microtwinning is the principal deformation mechanism under certain creep conditions in several polycrystalline nickel-base superalloys. A creep model based on this mechanism is developed in this paper. Model parameters, supplied by microscopy and by atomistic calculations, are used to compare the model against experiment.

This paper reports a pioneering investigation of the orientation change of the α phase after recrystallization and the mode of transformation from the α to the β phase on heating above the β transus in the α/β alloy Ti–6Al–4V by in situ texture measurements done at room temperature, 800 °C, 1020 °C and again at room temperature using the HIPPO neutron time-of-flight diffractometer at the Los Alamos Neutron Science Center.

Titanium alloys are widely used in various industrial, domestic, and medical applications such as turbine blades, bicycle frames, knee implants, etc. The two-phase titanium alloy Ti-6Al-4V (wt. percent) is considered to be a workhorse alloy for many applications in these diverse fields. Despite the large body of work on this alloy, the question of the transformation mechanism from the hcp a to the bcc b phase, occurring on heating to temperatures above the a/b transus at ~980°C, is still unresolved. Due to experimental difficulties, it has not yet been clearly determined whether the increase in b volume fraction occurs by fresh nucleation of b crystals within a phase grains or the growth of preexisting b grains. Since the Burgers orientation relationship holds only if the b grains are nucleated within the a grains, the outcome of this question greatly affects texture-modeling efforts for this system. The Burgers orientation relationship predicts that the {0001} crystal direction in a grain of the a phase becomes a {110} crystal direction in a grain of the b phase after the transformation. In this work we present experimental results from in-situ texture measurements performed on the HIPPO neutron diffractometer at LANSCE. Using the combination of time-offlight neutrons and full-pattern Rietveld analysis allowed us to determine the orientation distribution functions of both phases at room temperature, 800°C, 1020°C and again at room temperature. We found strong indications that the b phase indeed grows from grains preexisting at room temperature. Upon re-transformation from b to a we found that the Burgers relationship is followed.

Creep deformation substructures in superalloy Rene 88 DT have been investigated at two applied stress levels after small-strain (0.5%) creep at 650 °C using conventional and high resolution transmission electron microscopy. Clear differences in creep strength and substructures have been observed as a function of applied stress. It has been established that at intermediate temperatures microtwinning caused by the passage of Shockley partial dislocations on successive {1 1 1} planes is the dominant deformation process at low applied stress. At higher applied stress the mechanism changes to planar shearing of the matrix by 1/2〈1 1 0〉 unit dislocations and Orowan looping of the precipitates. Detailed experimental evidences for these operating processes are shown and possible explanation is provided.

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.

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.

Using state-of-the-art energy dispersive spectroscopy, it has been established for the first time that there exists significant compositional variation (enrichment of Co and Cr and deficiency of Ni and Al) associated with superlattice intrinsic stacking faults created in the ordered γ′ precipitates following intermediate temperature deformation of two commercial superalloys. The results indicate that long range diffusion of these elements is intimately involved in the precipitate shearing process and is therefore closely linked to the time-dependent deformation of the alloys.

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.

This paper reviews the presently proposed mechanisms for creep of {\gamma^{\prime}} strengthened Ni-base superalloys that are typically used for disk applications. Distinct creep strength controlling modes, such as dislocation-coupled antiphase-boundary shearing, shearing configurations involving superlattice stacking faults, Orowan looping, climb by-pass, and microtwinning have been observed. These are strongly influenced by the scale of the {\gamma^{\prime}} precipitating phase and are operative within specific ranges of temperature and stress. Insight from more recent experimental findings concerning microtwinning and extending stacking fault mechanisms suggest important similarities between these deformation modes. It is suggested that local atomic reordering in the wake of Shockley partials is responsible for the temperature dependence exhibited in this regime.