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We report on comparative atom probe tomography investigations of I³/I³â�²-forming Co-12Ti-4Mo-Cr alloys. Moderate additions of Cr (2 and 4 at) reduced the I³/I³â�² lattice misfit and increased the I³â�² volume fraction of a Co-12Ti-4Mo alloy significantly. These microstructural changes were accompanied by changes in the elemental partitioning between I³ and I³â�² and site-occupancy in I³â�². Spatial distribution maps revealed that Mo occupied both Co and Ti sub-lattice sites in I³â�². In agreement with the experimental data, thermodynamic calculations predicted a stronger tendency for Mo to occupy the Co-sites than for Cr and an increase in Cr fraction on the Ti-sites with increasing Cr content. Copyright © The Author(s), 2021. Published by Cambridge University Press on behalf of the Microscopy Society of America.

In this work, a simple rectangular milling technique was demonstrated to prepare needle shape atom probe tomography (APT) specimens from Al alloys by focused-ion-beam (FIB) milling using Ga

Titanium is an important alloying addition to γ/γ′ cobalt-based superalloys that enhances the high temperature microstructural stability and make the alloys lighter. In this work, we probe the role of Ti composition on the phase stability and oxidation behavior of Co–30Ni–10Al–8Cr–5Mo–2Nb superalloys. With Ti addition, the γ′-solvus temperature is enhanced and the γ′-precipitate shape changes from spherical to rounded cuboids. Addition of 4 at. pct Ti to the alloy promotes topologically-close-packed (TCP) phase formation that are rich in Co, Cr, and Mo. During oxidation at 900 °C, Ti was found to facilitate the early formation of passivating oxide layers (spinel CoCr2O4/CoAl2O4) on the exposed surfaces, however, it was not effective in reducing the oxidation-induced mass gain. Microstructural analysis reveals that Ti delays the Al2O3 layer formation eventually leading to faster oxidation kinetics. Additionally, we also found formation of (Ti,Nb)N in the γ′ denuded zones near the alloy-oxide interface.

Co base superalloys strengthened by coherent L12 ordered γ′ precipitate in a disordered face-centered cubic γ matrix represent a new opportunity for high-temperature alloy development. The emergence of alloys with low density and high specific yield strength at elevated temperatures has further energized the research and development efforts in the last 5 years. Initially stabilized by the addition of small amounts of Nb and Ta, these new generations of alloys with multiple alloying additions to form basic quaternary and ternary alloys have steadily expanded the property envelopes to raise hope for a modern class of superalloys with higher-temperature capabilities. This article reviews the work of a vibrant set of researchers across the globe whose findings are constantly unlocking the potential of these alloys. These developments have achieved high-temperature strength (at 870°C) >0.6 GPa, γ′ solvus temperature exceeding 1,100°C, and densities between 7.8 and 8.6 g/cm3.

In this work, the nanometer-scale mass flow coupled to dislocation processes near the γ/γ′-interface during high temperature and low stress creep of a model Ni-base single crystal superalloy is investigated. In the early creep stages, the dislocation networks in the γ-phase at γ/γ′-interfaces attract γ-stabilizing elements like Cr, Co and in particular Re, resulting in compositional gradients close to the interface. At larger strains, where dislocations frequently cut into the γ′-phase, this local interfacial enrichment in these elements is no longer observed. The cutting dislocations take part of the segregated atoms away, whilst the remaining atoms are released and diffuse back into the γ-channels.

We investigated the flow behavior of γ/γʹ-strengthened Co-12Ti and Co-12Ti-4Mo (at.%) alloys at room and elevated temperatures (up to 900°C) by electron microscopy and density functional theory. The Mo-added alloy exhibited an enhanced compressive yield strength and strain hardening behavior as compared to the reference binary alloy. This behavior could be attributed to a ~25% larger γʹ volume fraction and ~7% higher planar fault energies in Co-12Ti-4Mo. Using electron channeling contrast imaging, we observed interrupted slip bands in the Co-12Ti-4Mo alloy deformed to a strain of 6%, which led to enhanced strain hardening, in contrast to extended slip bands along {111} planes in the Co-12Ti alloy. Interrupted slip band formation in Co-12Ti-4Mo could be explained by rapid exhaustion of dislocation sources and a higher energy barrier required to cut the γʹ precipitates. These effects are due to a reduced γ channel width and substantial hardening effect of γʹ-Co3(Ti,Mo) in the ternary alloy as well as due to the large shear modulus difference between γʹ and γ.

We report new compositions of CoNi-based superalloys that have low mass densities (7.84--8.20 g/cc), negative $\ensuremath{\gamma}/{\ensuremath{\gamma}}^{\ensuremath{'}}$ lattice misfit, and high ${\ensuremath{\gamma}}^{\ensuremath{'}}$ solvus temperature up to 1094 \ifmmode^\circ\else\textdegree\fi{}C with promising high-temperature mechanical properties. The $\ensuremath{\gamma}/{\ensuremath{\gamma}}^{\ensuremath{'}}$ in the base Co-30Ni-10Al-5Cr alloy was stabilized with Nb and further developed by varying Ti and Cr. The \ensuremath{\gamma}\ensuremath{'} precipitates in these alloys are richer in Ni than the \ensuremath{\gamma} matrix phase, unlike for ${\ensuremath{\gamma}}^{\ensuremath{'}}$ in other CoNi-based superalloys. The atomic-scale compositional analysis indicates that the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ has a stoichiometry of ${(\mathrm{Ni},\mathrm{Co})}_{3}(\mathrm{Al},\mathrm{Nb},\mathrm{Ti},\mathrm{Cr})$. In these alloys, the ${\ensuremath{\gamma}}^{\ensuremath{'}}$ microstructure is stable at 900 \ifmmode^\circ\else\textdegree\fi{}C up to 1000 h. In an alloy with a ${\ensuremath{\gamma}}^{\ensuremath{'}}$ solvus of 1089 \ifmmode^\circ\else\textdegree\fi{}C, the coarsening kinetics of \ensuremath{\gamma}\ensuremath{'} at 900 \ifmmode^\circ\else\textdegree\fi{}C follows the Lifshitz-Slyozov-Wagner model with a rate constant of $6.6\ifmmode\pm\else\textpm\fi{}0.7\phantom{\rule{0.16em}{0ex}}\mathrm{n}{\mathrm{m}}^{3}/\mathrm{s}$, comparable with high-density Co-Al-W-based superalloys. The alloys also show an anomalous increase in strength, with temperature having a peak of 0.2% proof stress (PS) and specific PS (SPS) of \ensuremath{\sim}710 MPa and $\ensuremath{\sim}91\phantom{\rule{0.16em}{0ex}}\mathrm{MPa}\phantom{\rule{0.16em}{0ex}}{\mathrm{g}}^{--1}\mathrm{cc}$, respectively, at 770 \ifmmode^\circ\else\textdegree\fi{}C. The low density and high strength make this class of alloys a potential and cost-effective alternative to the other Co-based superalloys and comparable with Ni-based superalloys.

We investigated the solute diffusional behavior active during compressive creep deformation at 150 MPa / 975 °C of a Co-Al-W-Ta single crystal superalloy in the [001] orientation. We report the formation of shear-bands that involves re-orientation of γ/γʹ rafts to {111} from {001} planes, referring to as γ/γ′ raft-rotation. In the shear-band regions, we observed abundant micro-twins, stacking faults (SFs), disordered zones within the γʹ termed as ‘γ pockets’ and also few geometrically-close-packed (GCP) phases. We used a correlative approach blending electron microscopy and atom probe tomography to characterize the structure and composition of these features. The SFs were identified as intrinsic and exhibit a W enrichment up to 14.5 at.% and an Al deficiency down to 5.1 at.%, with respect to the surrounding γʹ phase. The micro-twin boundaries show a solute enrichment similar to the SFs with a distinct W compositional profile gradients perpendicular from the boundaries into the twin interior, indicating solute diffusion within the micro-twins. The γ-pockets have a composition close to that of γ but richer in W/Ta. Based on these observations, we propose (i) a solute diffusion mechanism taking place during micro-twinning, (ii) a mechanism for the γ/γʹ raft-rotation process and evaluate their influence on the overall creep deformation of the present Co-based superalloy.

We demonstrate the formation of a hierarchical microstructure by optimized combination of Mo-doping and thermo-mechanical processing in a NiCoCr medium-entropy-alloy (MEA) that results in a significant improvement in yield-strength (~775 MPa) and ultimate-tensile-strength (~1200 MPa) with an elongation to fracture up to 35%. Addition of Mo slows down the recrystallization kinetics of the cold-rolled alloy at 700 °C. After 4 h annealing, the microstructure comprises soft recrystallized and hard non-recrystallized zones, dispersed σ precipitates, annealing twins and retained stacking faults/nanotwins that are shown to contribute to the overall mechanical behavior.

In this article, we probe the strain partitioning between the microstructural features present in a continuously cooled carbide-free bainitic steel together with damage nucleation and propagation. These features mainly comprise of phases (bainitic ferrite, martensite, and blocky/thin film austenite), interfaces between them, grain size and grain morphology. A micro Digital Image Correlation (μ-DIC) technique in scanning electron microscope is used to quantify the strain distribution between these microstructural features. The results show a strong strain partitioning between martensite, bainitic ferrite and retained austenite that provides weak links in the microstructure and creates conditions for the crack initiation and propagation during deformation. Blocky austenite islands accommodate maximum local strains in the global strain range of 0–2.3% and undergo strain-induced austenite to martensite transformation governing the local strain evolution in the microstructure. However, the local strains are minimum in martensite regions during entire in-situ deformation stage. Narrow bainitic ferrite channels in between martensitic islands and martensite-bainitic ferrite interfaces are recognised as primary damage sites with high strain accumulation of 30 ± 2% and 20 ± 3% respectively, at a global strain of 9%. The inclination of these interfaces with the tensile direction also affects the strain accumulation and damage.

Atom probe tomography (APT) provides three-dimensional compositional mapping with sub-nanometre resolution. The sensitivity of APT is in the range of parts per million for all elements, including light elements such as hydrogen, carbon or lithium, enabling unique insights into the composition of performance-enhancing or lifetime-limiting microstructural features and making APT ideally suited to complement electron-based or X-ray-based microscopies and spectroscopies. Here, we provide an introductory overview of APT ranging from its inception as an evolution of field ion microscopy to the most recent developments in specimen preparation, including for nanomaterials. We touch on data reconstruction, analysis and various applications, including in the geosciences and the burgeoning biological sciences. We review the underpinnings of APT performance and discuss both strengths and limitations of APT, including how the community can improve on current shortcomings. Finally, we look forwards to true atomic-scale tomography with the ability to measure the isotopic identity and spatial coordinates of every atom in an ever wider range of materials through new specimen preparation routes, novel laser pulsing and detector technologies, and full interoperability with complementary microscopy techniques. This Primer on atom probe tomography introduces the fundamentals of the technique and its experimental set-up, describes recent developments in specimen preparation, highlights aspects of data reconstruction and analysis, and showcases various applications of atom probe tomography in the materials sciences, geosciences and biological sciences.

A systematic study of the compression creep properties of a single-crystalline Co-base superalloy (Co-9Al-7.5W-2Ta) was conducted at 950 {\deg}C, 975 {\deg}C and 1000 {\deg}C to reveal the influence of temperature and the resulting diffusion velocity of solutes like Al, W and Ta on the deformation mechanisms. Two creep rate minima are observed at all temperatures indicating that the deformation mechanisms causing these minima are quite similar. Atom-probe tomography analysis reveals elemental segregation to stacking faults, which had formed in the $\gamma\prime$ phase during creep. Density-functional-theory calculations indicate segregation of W and Ta to the stacking fault and an associated considerable reduction of the stacking fault energy. Since solutes diffuse faster at a higher temperature, segregation can take place more quickly. This results in a significantly faster softening of the alloy, since cutting of the $\gamma\prime$ precipitate phase by partial dislocations is facilitated through segregation already during the early stages of creep. This is confirmed by transmission electron microscopy analysis. Therefore, not only the smaller precipitate fraction at higher temperatures is responsible for the worse creep properties, but also faster diffusion-assisted shearing of the $\gamma\prime$ phase by partial dislocations. The understanding of these mechanisms will help in future alloy development by offering new design criteria.

The antagonism between strength and resistance to hydrogen embrittlement in metallic materials is an intrinsic obstacle to the design of lightweight yet reliable structural components operated in hydrogen-containing environments. Economical and scalable microstructural solutions to this challenge must be found. Here, we introduce a counterintuitive strategy to exploit the typically undesired chemical heterogeneity within the material’s microstructure that enables local enhancement of crack resistance and local hydrogen trapping. We use this approach in a manganese-containing high-strength steel and produce a high dispersion of manganese-rich zones within the microstructure. These solute-rich buffer regions allow for local micro-tuning of the phase stability, arresting hydrogen-induced microcracks and thus interrupting the percolation of hydrogen-assisted damage. This results in a superior hydrogen embrittlement resistance (better by a factor of two) without sacrificing the material’s strength and ductility. The strategy of exploiting chemical heterogeneities, rather than avoiding them, broadens the horizon for microstructure engineering via advanced thermomechanical processing., Typically undesired chemically heterogeneous microstructures are shown to enhance the resistance of high-strength steel against hydrogen embrittlement, with no loss in strength or ductility.

A dual-anneal diffusion multiple was utilized to generate a composition gradient via a first anneal at a high temperature followed by a second anneal at a lower temperature to induce phase precipitation as a function of composition/supersaturation. By adjusting the interfacial energy value in simulations using the classical nucleation and growth theories as implemented in the Kampmann-Wagner numerical (KWN) model and matching the simulated average precipitate sizes at different compositions with the experimental measurements along the composition gradient, the Ni3Al/fcc interfacial energy in the Ni-Al system at 700 °C was effectively determined to be ~12 mJ/m2.

We explore the effects of the elemental site occupancy in γ′-A3B (L12) intermetallic phases and their partitioning across the γ/γ′ interface in a class of multicomponent W-free Co-based superalloys. Atom probe tomography and first principles density functional theory calculations (DFT) were used to evaluate the Cr site occupancy behavior in the γ′ phase and its effect on the γ/γ′ partitioning behavior of other solutes in a series of Co-30Ni-10Al-5Mo-2Ta-2Ti-XCr alloys, where x is 0, 2, 5, and 8 at.% Cr, respectively. The increase in Cr content from 0 to 2 to 5 at.% leads to an inversion of the partitioning behavior of the solute Mo from the γ′ phase ( K M o > 1 ) into the γ matrix ( K M o 1 ) . At 5 at.% Cr, the Cr also has a preference to replace the excess anti-site Co atoms from the B-sites. At 8 at.% Cr, the Cr develops an additional preference to replace Co atoms from the A-sites. These compositional changes in the phases and the site partitioning behavior in the γ′ phase are accompanied by an overall decrease in the lattice misfit (δ) across the γ/γ′ interfaces as measured by high-resolution X-ray diffraction at room temperature. The reduction in misfit triggers a change in morphology of the γ′ phase from cuboidal (δ ∼ +0.48% at 0 at.% Cr) to round-cornered (δ ∼ +0.34% at 5 at.% Cr) to spheroidal shaped (δ ∼ +0.19% at 8 at.% Cr) precipitates. We also observed an increase in the solvus temperature from 1066 °C to 1105 °C when adding 5 at.% Cr to the alloy. These results on the effects of Cr in Co-base superalloys enable tuning the microstructure of these alloys and widening the alloy spectrum for designing improved high temperature alloys.

We report on the full three-dimensional compositional partitioning among the features of a planar imperfection comprising a superlattice intrinsic stacking fault (SISF) with its leading and trailing partials, as well as the antiphase boundary (APB) in the wake of the trailing partial formed in the L12-ordered γ′ phase of a CoNi-based single crystal superalloy. The partial dislocations and the APB are found to be Cr/Co rich relative to the surrounding γ′ and richer in W/Ta/Ti compared to the γ matrix phase. Solute diffusion mechanisms are derived from the compositional gradients in the vicinity of the imperfection.

We report evidence of Re and Mo segregation (up to 2.6 at.% and 1 at.%) along with Cr and Co to the dislocations inside of {\gamma}' precipitates in a second generation Ni-based single crystal superalloy, after creep deformation at 750{\deg}C under an applied stress of 800 MPa. The observed segregation effects can be rationalized through bridging the solute partitioning behavior across the {\gamma}/{\gamma}' interface and the pipe diffusion mechanism along the core of the dislocation line. This understanding can provide new insights enabling improved alloy design.

Atom probe tomography (APT) is rising in influence across many parts of materials science and engineering thanks to its unique combination of highly sensitive composition measurement and three-dimensional microstructural characterization. In this invited article, we have selected a few recent applications that showcase the unique capacity of APT to measure the local composition at structural defects. Whether we consider dislocations, stacking faults, or grain boundary, the detailed compositional measurements tend to indicate specific partitioning behaviors for the different solutes in both complex engineering and model alloys we investigated.

We report on the sub-nanometer scale characterization of Co-12Ti-4Mo and Co-12Ti-4Cr (at.%) model alloys. Atom probe tomography reveals that Co and Cr partition to γ, whereas Ti and Mo to γ′. Additions of Mo and Cr to the reference Co-12Ti system lead to strong increases in γ′ volume fraction by about 25% and 12%, respectively. Element-specific spatial distribution maps along the [001] direction of the L12-ordered γ′ phase reveal that both Mo and Cr preferentially replace Ti on its sublattice. The remaining excess Ti is available for formation of additional γ′, resulting in enhanced γ′ volume fractions.

We propose here a deformation-induced diffusive phase transformation mechanism occurring during shearing of γ′ ordered phase in a γ/γ′ single crystalline CoNi-based superalloy. Shearing involved the creation and motion of a high density of planar imperfections. Through correlative electron microscopy and atom probe tomography, we captured a superlattice intrinsic stacking fault (SISF) and its associated moving leading partial dislocation (LPD). The structure and composition of these imperfections reveal characteristic chemical – structural contrast. The SISF locally exhibits a D019 ordered structure coherently embedded in the L12 γ′ and enriched in W and Co. Interestingly, the LPD is enriched with Cr and Co, while the adjoining planes ahead of the LPD are enriched with Al. Quantitative analysis of the three-dimensional compositional field in the vicinity of imperfections sheds light onto a new in-plane diffusion mechanism as the LPD moves on specific {111} planes upon application of stress at high temperature.

Mo-Si-B alloys are potential creep resistant materials for accessing harsh loading scenarios beyond Ni-based superalloys due to their excellent mechanical performance at ultra-high temperatures (> 1200 °C). Here, we report on the fabrication through laser additive manufacturing of a Mo rich Mo-Si-B alloy with and without dispersion of oxide (La2O3) particles. The major phase in the solidified material is dendritic α-Mo. The inter-dendritic regions contain a mixture of the Mo5Si3 (T1) + Mo5SiB2 (T2) phases, and not the expected equilibrium Mo3Si + Mo5SiB2 (T2) phases. This combination of phases is shown to yield improved high temperature creep resistance but was only accessible through by addition of Nb, W or Ti that substitute Mo in the intermetallic phases. Whereas here it is attributed to the large undercooling in the small melt pool produced during laser processing. We show that this phase mixture, upon annealing, is stable at 1200 °C for 200 h. We also demonstrate successful dispersion of oxide particles mainly in the inter-dendritic regions leading to a high indentation fracture toughness of ∼18 MPa√m at room temperature. Toughening originates from crack trapping in the ductile α-Mo and the formation of micro-cracks and crack deflection in the vicinity of oxide particles.

A set of advanced single crystalline γ′ strengthened Co-base superalloys with at least nine alloying elements (Co, Ni, Al, W, Ti, Ta, Cr, Si, Hf, Re) has been developed and investigated. The objective was to generate multinary Co-base superalloys with significantly improved properties compared to the original Co-Al-W-based alloys. All alloys show the typical γ/γ′ two-phase microstructure. A γ′ solvus temperature up to 1174 °C and γ′ volume fractions between 40 and 60 pct at 1050 °C could be achieved, which is significantly higher compared to most other Co-Al-W-based superalloys. However, higher contents of Ti, Ta, and the addition of Re decrease the long-term stability. Atom probe tomography revealed that Re does not partition to the γ phase as strongly as in Ni-base superalloys. Compression creep properties were investigated at 1050 °C and 125 MPa in 〈001〉 direction. The creep resistance is close to that of first generation Ni-base superalloys. The creep mechanisms of the Re-containing alloy was further investigated and it was found that the deformation is located preferentially in the γ channels although some precipitates are sheared during early stages of creep. The addition of Re did not improve the mechanical properties and is therefore not considered as a crucial element in the design of future Co-base superalloys for high temperature applications. Thermodynamic calculations describe well how the alloying elements influence the transformation temperatures although there is still an offset in the actual values. Furthermore, a full set of elastic constants of one of the multinary alloys is presented, showing increased elastic stiffness leading to a higher Young’s modulus for the investigated alloy, compared to conventional Ni-base superalloys. The oxidation resistance is significantly improved compared to the ternary Co-Al-W compound. A complete thermal barrier coating system was applied successfully.

Nanoscale solute segregation to or near lattice defects is a coupled diffusion and trapping phenomenon that occurs in superalloys at high temperatures during service. Understanding the mechanisms underpinning this crucial process will open pathways to tuning the alloy composition for improving the high-temperature performance and lifetime. Here, we introduce an approach combining atom probe tomography with high-end scanning electron microscopy techniques, in transmission and backscattering modes, to enable direct investigation of solute segregation to defects generated during high-temperature deformation such as dislocations in a heat-treated Ni-based superalloy and planar faults in a CoNi-based superalloy. Three protocols were elaborated to capture the complete structural and compositional nature of the targeted defect in the alloy.

The present work explores the effect of ruthenium (Ru) addition to the γ/γ′ strengthened Co-30Ni-10Al-5Mo-2Nb superalloy. A 2 at.% Ru increases the γ/γ′ microstructural stability at 900°C. XRD and APT revealed a reduction of γ/γ′ lattice misfit by ∼ 26% and segregation of Ru at γ/γ′ interface, respectively that contribute to the decrease of γ/γ′ interfacial energy. The temporal evolution of γ′ precipitates at 900°C shows the classical matrix diffusion controlled LSW mechanism with a coarsening rate constant (Kr) ∼ 6.03 nm3/s similar to the values for Co-Al-W based superalloys. Furthermore, Ru addition increases the degree of anomalous strengthening with a peak 0.2% proof stress (PS) value of ∼ 630 MPa at 770°C. The results show Ru as a promising alloying addition to increase the γ/γ′ microstructural stability at high temperatures for the low-density class of CoNi-based superalloys.

A dynamic strain aging (DSA) regime is established for a near-α titanium alloy with a microstructure consisting of 80% equiaxed α, 15% lamellar α and 5% β with silicon in solid solution through a series of constant strain rate tests carried out in tension from 5 × 10−2 to 5 × 10−6 s−1 in the temperature range of 623–823 K. Transmission electron microscopy shows jogged screw dislocations within slip bands dominating the dislocation structure in this domain. Strain accumulation occurs by conservative jog glide along the length of screw dislocations due to line tension forces. We estimate the solute concentration accumulating at these edge jogs on arrest during their thermally activated glide over static solute obstacles. The domain of DSA is then predicted using Friedel's model to obtain the stress required to break the jogs free from the solute atmosphere. There is a good agreement between model and experimental data showing a DSA peak in the temperature range of 673–723 K. The solute species responsible for DSA from the model are C and Si, but dominated by C in this temperature and strain rate regime. Atom probe tomography demonstrates C and Si segregation at the dislocations, strongly supporting the model.

Boron is an essential solute element for improving the grain boundary strength in several high temperature metallic alloys especially in Ni- and Co-base superalloys although the detailed strengthening mechanisms are still not well understood. In superalloys, boron leads to the formation of borides and precipitate depleted zones around the grain boundaries and alters the bond strength among the grains directly. In this paper, we explore in detail the role of the boron content in ternary γ/γ′ Co-9Al-9W alloys. Local as well as bulk mechanical properties were evaluated using nanoindentation and compression testing and correlated to near-atomic scale microstructure and compositions obtained from electron microscopy and atom probe tomography. The alloy variant with low B content (0.005 at.% B) reveals an increase in yield strength at room temperature and 600 °C and atom probe tomography investigations show that solute B segregates to the grain boundaries. However, in the bulk B exclusively partitions to the γ′ phase. Additionally, the γ′/γ′ grain boundaries are depleted in W and Al with the concentration locally shifted towards the γ composition forming a very thin γ layer at the γ′/γ′ grain boundaries, which supports dislocation mobility in the γ′/γ′ grain boundary region during deformation. Higher content of B (0.04 at.% B) promotes formation of W-rich borides at the grain boundaries that leads to undesirable precipitate depleted zones adjacent to these borides that decrease the strength of the alloy drastically. However, it was also found that a subsequent annealing heat treatment eliminates these detrimental zones by re-precipitating γ′ and thus elevating the strength of the alloy. This result shows that, if a precipitate depleted zone can be avoided, B significantly improves the mechanical properties of polycrystalline Co-base superalloys by strengthening the γ′ phase and by improving grain boundary cohesion.

Strengthening by precipitation of second phase is the guiding principle for the development of a host of high strength structural alloys, in particular, aluminium alloys for transportation sector. Higher efficiency and lower emission demands use of alloys at higher operating temperatures (200 °C–250 °C) and stresses, especially in applications for engine parts. Unfortunately, most of the precipitation hardened aluminium alloys that are currently available can withstand maximum temperatures ranging from 150–200 °C. This limit is set by the onset of the rapid coarsening of the precipitates and consequent loss of mechanical properties. In this communication, we present a new approach in designing an Al-based alloy through solid state precipitation route that provides a synergistic coupling of two different types of precipitates that has enabled us to develop coarsening resistant high-temperature alloys that are stable in the temperature range of 250–300 °C with strength in excess of 260 MPa at 250 °C.

The present article deals with effect of Cr addition (10 at.%) on the partitioning behavior and the consequent effect on mechanical properties for tungsten-free gamma-gamma' cobalt-based superalloys with base alloy compositions of Co-30Ni-10Al5Mo- 2Ta (2Ta) and Co-30Ni-10Al-5Mo-2Ta-2Ti (2Ta2Ti). Cr addition leads to a change in the morphology of the strengthening cuboidal-shaped gamma' precipitates to a spherical shape. The site preference of Cr atoms in two alloy systems (with and without Ti) has been experimentally investigated using atom probe tomography with the supportive prediction from first principles DFT-based computations. Cr partitions more to the c matrix relative to gamma'. However, Cr also has a strong effect on the Ta and Mo partitioning coefficient across gamma/gamma' interfaces. The value of partition coefficient for Mo (K-Mo) becomes

An amendment to this paper has been published and can be accessed via a link at the top of the paper.

In the present work, we show the feasibility of microstructural control by additions of Ti and Cr to a γ/γ′ Co-30Ni–10Al–5Mo–2Nb superalloy. Solutioning at 1300 °C followed by aging at 900 °C leads to homogenous distribution of L12 ordered cuboidal γ′ precipitates in face-centered-cubic (fcc) γ matrix. Compositional measurements show Al, Mo, and Nb partition to γ′ that indicates the γ′ can be described as (Co, Ni)3(Al, Mo, Nb). An addition of 2 at.% Ti leads to increases in the γ′ volume fraction from 56 to 70% and solvus temperature from 990 to 1030 °C. Ti strongly partitions to γ′ with respect to γ matrix. Similarly, an addition of 10 at.% Cr to the base alloy leads to a morphological transition of γ′ precipitates from cuboidal to near spherical shape, indicating a direct influence on the γ/γ′ lattice misfit. Unlike Ti, Cr partitions to γ matrix, and additionally, Cr influences Mo to partition into γ matrix. A combined addition of Ti and Cr leads to high γ′ volume fraction ~76% and an increase in γ′ solvus temperature to 1045 °C, while maintaining the spherical γ′ morphology. These superalloys show 0.2% proof strength comparable to those of Co–Al–W-based superalloys. At 870 °C, 10 at.% Cr and 2 at.% Ti added alloys show higher specific 0.2% proof stress than Co–Al–W-based superalloys. The obtained results show the microstructural sensitivity of these Co-based superalloys toward their designing for better performance.

We report atomic structures and chemical compositions of defects associated to planar faults in a creep deformed Co-base superalloy and discuss their formation and contribution to plastic deformation. The multinary single crystalline Co-base superalloy was creep deformed under tension along \( \left[ {001} \right] \)-direction at 850 °C and 400 MPa. The creep microstructure comprises a high density of planar defects. Solute segregation to superlattice intrinsic stacking faults (SISF) is characterized via EDXS analysis of a statistically relevant number of faults and compared at different creep stages. The amount of solute segregation shows negligible difference at different creep stages indicating that segregation directly occurs during planar fault formation and does not significantly evolve afterward. Based on the observation and analysis of Frank partial dislocations with \( {\raise0.7ex\hbox{$a$} \!\mathord{\left/ {\vphantom {a 3}}\right.\kern-0pt} \!\lower0.7ex\hbox{$3$}}{\langle111\rangle} \) Burgers vectors terminating SISF, we discuss a new route to SISF formation via dislocation climb. Additionally, two more complex fault structures are analyzed, and potential formation mechanisms are discussed. The first of these structures is a terminating end of an SISF where an \( {\raise0.7ex\hbox{$a$} \!\mathord{\left/ {\vphantom {a 3}}\right.\kern-0pt} \!\lower0.7ex\hbox{$3$}}{\langle112\rangle} \) partial dislocation splits up into two closely spaced \( {\raise0.7ex\hbox{$a$} \!\mathord{\left/ {\vphantom {a 6}}\right.\kern-0pt} \!\lower0.7ex\hbox{$6$}}{\langle112\rangle} \) partials separated by an SESF. The second structure consists of two parallel SISFs connected by an anti-phase boundary (APB). All deformation mechanisms described in this study show an involvement of solute segregation directly affecting formation and propagation of creep defects by changing planar fault energies and chemical environments of dislocations. Solute segregation is therefore expected to be a key to future alloy design by enabling control of creep deformation mechanisms in specific temperature and stress regimes.

The effect of Zr addition to Al rich binary α-Al - Al 3 Ni eutectic cast alloy (Al-3.1 at. Ni) in enhancing the microstructural stability and strength at high temperature is demonstrated. On subsequent heat treatment after casting, nanometric coherent L1 2 ordered Al 3 Zr precipitates form inside the α-Al that strengthen the alloy. Additionally, remarkable stability of eutectic microstructure was observed even after 100 h of annealing at 400 °C. The synergetic effect of the strengthening of the α-Al matrix by coherent Al 3 Zr precipitates and the low coarsening rate of the Al 3 Ni rods results in a significant increase in high temperature hardness and yield strength of the alloy. The tensile yield strength of the annealed Al-3.1Ni-0.15Zr alloy (400 °C, 10 h) tested at 250 °C is found to be 185 ± 10 MPa, which is 1.5 times higher than the corresponding binary Al-3.1Ni alloy. The experimentally determined average rod size (radius) during annealing at 400 °C follows the classical matrix diffusion controlled LSW-based coarsening model for both binary Al-3.1Ni and ternary Al-3.1Ni-0.15Zr alloys. The calculated coarsening rate constant values based on modified LSW coarsening model are 10.3 and 4.1 nm 3 /s for Al-3.1Ni and Al-3.1Ni-0.15Zr alloys, respectively. Atom probe tomographic (APT) investigations of the heat-treated ternary alloy unambiguously reveal segregation of Zr solute at the αAl/Al 3 Ni interface in addition to the presence of the strengthening Al 3 Zr ordered precipitates in the α-Al matrix. The segregation hinders the interdiffusion of Al and Ni in the eutectic and, thereby, increasing the stability of the eutectic phase at high temperature.

In sensitized (700°C for 10 h) AISI 316L stainless steel samples attacked by an HF solution, coincidence site lattice (CSL) ∑7 grain boundaries (GBs) always show corrosion while ∑5 GBs never do. This distinct corrosion behaviour can be related to the distinct precipitation behaviour of the different GBs, which itself depends on the GB characters. It is found that ∑5 GBs, although being curved both macro- and microscopically, neither show facets nor precipitates. The amount of segregation is enhanced in sensitized ∑5 GBs compared to solution-annealed ones. In contrast, ∑7 GBs show clear faceting in both the solution-annealed and sensitized state, with one facet orientation showing strong segregation while the other not. GB energy anisotropy is applied to explain the faceting phenomenon. The formation of two kinds of precipitates, i.e., C14 Laves phase and M23C6 carbide, holding responsible for two distinct morphologies of corrosion, are believed to be both linked to intensive faceting and the anisotropic segregation of ∑7 GBs. For the carbide, a preferred {111} orientation out of all ∑7 facets measured by trace analyses is presumed to facilitate its nucleation.

The paper explores the influence of Ti addition to the Ta stabilized γ/γ′ two-phase Co-based superalloys having the compositions of Co-30Ni-10Al-5Mo-2Ta-xTi (x = 0, 1, 2 and 4 at.%). The microstructure of the alloys after solutionizing (1300 °C for 25 h) and quenching show fine distribution of γ′ precipitates with spherical shape within the γ matrix. Aging of these alloys at 900 °C results in a morphological transition from spheroidal to cuboidal with a concomitant increase in γ/γ′ lattice misfit as compared to the solutionized alloys. However, the γ/γ′ lattice misfit of cuboidal particles decreases with an increase in Ti in the alloys. Atomic-scale compositional analysis by atom probe tomography across γ/γ′ interface for the aged alloys indicates a preferential partitioning of Ti to the γ′. This can be correlated with the observed increase in the γ′ solvus temperature up to 1166 °C for alloys with 4 at.% Ti addition. The increase in Ti content also decreases the partitioning of Mo to the γ′. Despite the rise in solvus temperature, the coarsening study reveals an escalation in the γ′ coarsening rate at 900 and 950 °C with a maximum for the alloy containing the highest Ti content. The initial stages of γ′ coarsening follow diffusion-controlled LSW kinetics. However, during later stages, a propensity for particle coalescence could be observed. An addition of 3 and 4 at.% Ti is shown to improve the high-temperature strength with 0.2% proof stress at 1000 °C of 360 and 400 MPa, respectively.

In this work, we discussed the atomic-scale mass transport occurring in a Mo-doped NiCoCr medium-entropy alloy to form σ-precipitates during partial recrystallization at 700°C. We reveal the segregation of Cr/Mo to the fault planes of nano-twins/stacking faults in non-recrystallized zones. Upon annealing, the migration of recrystallized grain boundaries (GBs) redistributes the structure and composition, leading to the annihilation of faults and leaving behind the Mo-rich σ-precursors for precipitation. The segregated Cr/Mo solutes can also diffuse to the moving GBs and create additional σ-nucleation sites. The present results highlight the role of Mo in overcoming strength-ductility trade-off of the alloy.

We report a rare experimental evidence of an exceptional increase in grain-boundary diffusion-controlled growth kinetics of product phase by grain-refinement due to minor alloying. Ti-addition during growth of V3Ga superconductor by coupling V and Cu(Ga)-solid solution by bronze process increases the critical current density (Jc) significantly. In this manuscript, the exact role of Ti-addition is studied based on systematic and quantitative diffusion-couple experiments that mimic the bronze-method. Metallurgical aspects like microstructural features and growth mechanism are assessed critically. Results are compared when Ti is added either to V or Cu(Ga) and to both for a fixed Ga content. Ti-addition to Cu(Ga) compared to V increases growth kinetics of V3Ga more significantly. Quantitative EPMA-analysis indicates that a mixture of Ti-free and Ti-containing V3Ga phase grows when Ti is added to V. Small precipitates grow along with Ti-containing V3Ga when Ti is added only to Cu(Ga). TEM-study indicates that these precipitates are rich in V and Ti. Much finer grains found in the precipitate-containing region lead to higher growth kinetics due to grain-boundary diffusion. EBSD-analysis shows equiaxed grains and relatively random in orientation because of alloying. This study indicates that Ti-addition to Cu(Ga) is most effective considering metallurgical aspects that influence Jc. Keywords: Superconductor material, Microstructure, Grain size distribution, Grain boundary diffusion, Microtexture