Search

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.

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 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.

This work investigates two nominally similar polycrystalline alloys, with a subtle difference in Nb content, intended to elucidate its effect on local phase transformation strengthening during high temperature creep. Tests were conducted at 750 °C and 600 MPa to target the creep regime dominated by superlattice intrinsic and extrinsic stacking faults, as well as microtwinning. Alloy A, with higher Nb and lower Al, was found to be superior in creep strength to Alloy B, with lower Nb and higher Al, as well as previously investigated ME3 and LSHR. Atomic resolution scanning transmission electron microscopy and energy-dispersive spectroscopy found that this increased creep strength was due to a novel local phase transformation occurring along microtwin boundary interfaces as a result of the Nb increase. Complementary density functional theory calculations helped to confirm that this was χ phase formation. It is hypothesized that this transformation was the cause of the increased creep strength exhibited by Alloy A.

Titanium and titanium alloys are excellent candidates for aerospace applications owing to their high strength to weight ratio. Alpha/beta titanium alloys are used in nearly all sections of the aircraft, including the fuselage, landing gear, and wing. Ti-6Al-4V is the workhorse alloy of the titanium industry, comprising of nearly 60% of total titanium production. TIMETAL® 407, Ti-0.85Al-3.9V-0.25Si-0.25Fe (Ti-407) is an excellent candidate for alloy applications requiring excellent machinability and increased energy absorption. These properties are a result of the alloy’s increased ductility while maintaining moderate levels of strength. In this study, the deformation mechanisms of Ti-407 have been studied at high strain rates using split-Hopkinson bar testing. Utilizing post-mortem characterization, Ti-407 has been shown to deform significantly by ⟨c+a⟩ slip and deformation twinning. The observation of ⟨c+a⟩ slip is in contrast with other studies and will be discussed further.

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.

The crystallography and structure of the interfaces formed between refined hexagonal close packed (hcp) α precipitates and the body centered cubic (bcc) β matrix in a solution-treated and quenched metastable β titanium alloy, Ti-5Al-5Mo-5V-3Cr (wt%, Ti-5553), were investigated after heating to 600 °C and subsequently aging for 0 and 30 min, respectively. The two phases adopted the classical Burgers orientation, and the observed interfacial structures were analyzed using a topological model, i.e. they contained disconnections, as well as β crystal lattice dislocations acting as a lattice invariant deformation (LID). The structure of the α/β interface was observed along two mutually perpendicular directions, in high angle annular dark-field scanning transmission electron microscopy (HAADF-STEM). Viewed along the [ 0001 ] α : [ 110 ] β directions, an array of disconnections was observed to accommodate the misfit between the α and β crystals. While the spacing of these disconnections varied, the step height associated with them was always twice the { 112 } β interplanar spacing, and their Burgers vector b was [ − 0.019 0.019 0.034 ] β . When the interface was viewed along the [ 1 ¯ 1 ¯ 20 ] α : [ 1 ¯ 11 ] β direction, in the sample aged for 0 min at 600 °C the misfit between the two lattices along [ 0001 ] α : [ 110 ] β was so small that no misfit or lattice invariant dislocations were observed within the field of view, in contrast with the interface structure reported in other more conventional titanium alloys. After aging the sample for 30 min, however, the interfacial structure equilibrated, and crystal lattice dislocations were observed on the terrace planes, accommodating the LID, with b = 1 2 [ 1 ¯ 1 ¯ 1 ¯ ] β . Growth of the α precipitates is shown to occur principally by the emission of disconnections from the tip of the existing α precipitates into the β matrix, and its resulting conversion to α. These results, combined with analysis using the topological model, show that the nucleation and growth of the hcp α phase from the bcc β matrix in this alloy can be described in terms of the motion of disconnections, accompanied by the displacive diffusion necessary to accommodate the differences in composition between the α and β phases.

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.

Novel imaging strategies in the scanning electron microscope aimed at significantly improved image contrast of second phases in metal alloys are described. These include the use of low accelerating voltages, small working distances, and a novel detection system. Contrast is assessed as a function of voltage and optimized imaging conditions which result in much improved image quality are presented. These strategies are applied to two precipitation hardened Ni-base alloys, a cast single crystal and a hot isostatically pressed sample.

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.

In titanium alloys, variant selection (VS) of grain boundary α (GBα) at prior β grain boundaries (GBs) during α precipitation has a significant influence on the subsequent transformation pathway and transformation texture development of the α phase, and thus on the final mechanical properties. In this paper, we assess systematically the applicability of all current empirical rules for VS of GBα at prior β GBs using experimental characterization of GB misorientation, boundary plane inclination, and orientation relationships between the GBα and adjacent β grains in Ti-5553. We find that all these VS rules may be violated for a given β grain boundary. Based on the experimental observations, the frequencies of each of these empirical rules being either violated or followed have been assessed, and from this assessment, we analyze theoretically whether the arguments underlying each of these rules are physically sound and why rules are violated, and then discuss when a sound prediction may be made using these empirical VS rules.

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.

In this study, we applied a series of surface analysis techniques to explore the structure and chemistry of the corrosion products built up at the interface of different materials, which is relevant to the permanent disposal of world-wide high-level nuclear waste. Metallic alloys, amorphous glass, and crystalline ceramics are typical candidate nuclear waste forms planned for immobilizing and isolating the dangerous radionuclides enriched in the waste. The near-field corrosion interactions between these dissimilar materials are the focus of this study. The experiments were performed by corroding metallic alloys in intimate proximity to glass or ceramics in Cl--containing solutions at 90 oC. The exposed materials were subsequently subjected to comprehensive surface characterization using X-ray photoelectron spectroscopy, scanning transmission electron microscopy, energy dispersive spectroscopy, time-of-flight secondary ion mass spectrometry, and Raman/infrared spectroscopy analysis. The systematic study revealed localized damage on the alloy surface due to crevice corrosion. Interestingly, localized corrosion was also identified on the surface of some glasses and ceramics that were present nearby, while an inhibition effect was observed in other systems. These interactions were found to strongly depend on the type of materials and environment, which led to different corrosion phenomena.

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.

In this paper, the local compositional and structural changes occurring in association with stacking faults in a Ni-base superalloy are characterized and related to the possible rate-controlling processes during creep deformation at intermediate temperatures. These rate-controlling processes are not presently understood. In order to promote stacking fault shearing, compression creep tests on specially prepared single crystals of an exploratory Ni-base superalloy were conducted at 760 °C in the [0 0 1] orientation. Scanning transmission electron microscopy (STEM) imaging was coupled with state-of-the-art energy dispersive X-ray (EDX) spectroscopy to reveal for the first time an ordered compositional variation along the extrinsic faults inside the γ′ precipitates, and a distinct solute atmosphere surrounding the leading partial dislocations. The local structure and chemistry at the extrinsic fault is consistent with the η phase, a D024 hexagonal structure. Density Functional Theory (DFT) and high angle annular dark field (HAADF)-STEM image simulations are consistent with local η phase formation and indicate that a displacive–diffusive transformation occurs dynamically during deformation.

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...

New technologies and applications for Mg require further improvements of its corrosion resistance. Fundamental comprehension of the Mg corrosion mechanisms is necessary to address the issue, as the Mg corrosion behavior is unexpected from the theory of electrochemical kinetics. The anomalous hydrogen evolution phenomenon (anomalous HE) exhibit by Mg, also known as the negative difference effect (NDE), is still an unresolved issue. Recent research suggests that the anomalous HE is a consequence of a surface change during dissolution. The proposed mechanisms debate the location of the phenomenon on the surface. The phenomenon is proposed to happen at the corroded areas behind the corrosion front, in the dark corrosion products left behind the corrosion front and/or at the active dissolving sites. New techniques and experimental data are needed to further address the discussion. In this work, the 2D pit growth method is used to study the anomalous HE phenomenon on Mg with the aim to provide further experimental evidence towards an explanation. The method is called 2D pitting because the sample is a thin metallic film deposited on an inert substrate, so the pit depth is constant and limited to the film thickness. By analyzing 2D pit growth, it is possible to accurately track the active dissolving surface and to measure the anodic, net and cathodic current densities. The hydrogen evolution current density was shown to increase with increasing applied potentials in the ohmic/activation control region, which is direct evidence for the anomalous HE behavior. In addition, the results clearly demonstrated that the anomalous HE phenomenon happens at the active dissolving site, which in this case is the 2D pit wall. Anomalous HE did not occur at the corroded areas behind the corrosion front, which in the case of a 2D pit is the inert substrate. It also did not occur in the corrosion product, which in this case was broken and lifted out by the hydrogen bubbles being electrically disconnected from the Mg matrix. The 2D pits were also evaluated at high applied potentials where a salt film precipitated on the active dissolving surface. The results showed that the anomalous HE rate decreased in the transport control region approaching zero at the limiting current density. In this case, the rate of hydrogen evolution was limited by water diffusion through the salt film to reach the active surface. It was also shown by TEM/EDX analysis that impurities do not accumulate at the pit wall and cannot account for anomalous HE. The results support the idea that the actively dissolving Mg surface is the site of anomalous HE, and that, in the absence of a salt film, the dissolving surface is extremely catalytic to the HE process.

Ni3Al alloys exhibit remarkable deformation characteristics at small scales that are tied to their unique dislocation core structures. The present work examines microcrystal strengthening and flow for a binary Ni3Al alloy and two alloys containing −0.25% Hf and −1.0% Ta, and evaluates their response relative to known dislocation mechanisms for this material and sample size effects in other materials. The work includes analysis of the flow-stress anomaly mechanisms, dislocation velocity, the single-arm source model, exhaustion strengthening and dislocation multiplication for describing the response of Ni3Al alloy microcrystals. Primary remaining needs are to understand the nature of double cross slip and dislocation sources that trigger the onset of flow and limit size-dependent strengthening.

Atomistic processes governing the high-temperature strength of structural materials were accurately characterized and modeled., It has long been known that solute segregation at crystalline defects can have profound effects on material properties. Nevertheless, quantifying the extent of solute segregation at nanoscale defects has proven challenging due to experimental limitations. A combined experimental and first-principles approach has been used to study solute segregation at extended intermetallic phases ranging from 4 to 35 atomic layers in thickness. Chemical mapping by both atom probe tomography and high-resolution scanning transmission electron microscopy demonstrates a markedly different composition for the 4–atomic-layer–thick phase, where segregation has occurred, compared to the approximately 35–atomic-layer–thick bulk phase of the same crystal structure. First-principles predictions of bulk free energies in conjunction with direct atomistic simulations of the intermetallic structure and chemistry demonstrate the breakdown of bulk thermodynamics at nanometer dimensions and highlight the importance of symmetry breaking due to the proximity of interfaces in determining equilibrium properties.

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.

This paper presents experimental evidence of homogeneous precipitation of the α-phase within the β matrix of a titanium alloy, and then accounts for this phase transformation by a new, non-classical mechanism involving compositional fluctuations, based on the pseudo-spinodal concept [1] . This mechanism involves local compositional fluctuations of small amplitude which, when of a certain magnitude, can favor thermodynamically certain regions of the β matrix to transform congruently to the α-phase but with compositions far from equilibrium. Subsequently, as measured experimentally using the tomographical atom probe, continuous diffusional partitioning between the parent β- and product α-phases during isothermal annealing drives their compositions towards equilibrium. For a given alloy composition, the decomposition mechanism is strongly temperature dependent, which would be expected for homogeneous precipitation via the compositional fluctuation-mediated mechanism but not necessarily for one based on classical nucleation theory. The applicability of this mechanism to phase transformations in general is noted.

The present work investigates the effect of grain boundary chemistry and crystallography on creep and on creep damage accumulation in Cu–0.008 wt.% Bi and Cu–0.92 wt.% Sb at stresses ranging from 10 to 20 MPa and temperatures between 773 and 873 K. Small additions of Bi and Sb significantly reduce the rupture strain and rupture time during creep of Cu. High stress exponents (Cu–Bi) and high apparent activation energies for creep (Cu–Bi and Cu–Sb) are obtained. Sb promotes creep cavitation on random high-angle grain boundaries. Bi, on the other hand, causes brittle failure when small crack-like cavities cause decohesion. Both elements suppress dynamic recrystallization, which occurs during creep of Cu at high stresses and temperatures.

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.

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.

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.

Rene 88 DT samples were subjected to different cooling rates after a supersolvus treatment, and aged for varying periods of time from 25 to 200 h at 760 °C. Primary and secondary γ′ precipitate sizes were measured after each heat treatment. Coarsening rate constants were calculated and reported based on the measured precipitate sizes. When describing the change in radius (r) as a function of time (t), fits between the experimental results and analysis were investigated for two types of functional relationships, r3 vs. t and r2 vs. t. The experimental rate constants derived from this analysis were compared with analytical values deduced from two different models: volume diffusion and bulk diffusion through the interface. The applicability of the two mechanisms for γ′ coarsening is discussed based upon the comparison between the analytically derived and experimentally observed values of these rate constants.

The chemical partitioning of alloying elements between the disordered γ and ordered γ′ phases in the nickel-base superalloy Rene 88 DT, has been characterized in detail using three-dimensional atom probe tomography (3DAP) coupled with energy-filtered transmission electron microscopy (EFTEM) studies. After a homogenization treatment followed by a water quench, it is observed that the morphology of the γ′ precipitates remains near spherical even after long aging times. Compositional variation, observed between small (

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.

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.

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.

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.

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.

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.