We present the first report of a tungsten-free cobalt-based superalloy having a composition Co–10Al–5Mo–2Nb. The alloy is strengthened by cuboidal precipitates of metastable Co3(Al,Mo,Nb) distributed ...throughout the microstructure. The precipitates are coherent with the face-centred cubic γ-Co matrix and possess ordered L12 structure. The microstructure is identical to the popular γ–γ’ type nickel-based superalloys and that of recently reported Co–Al–W-based alloys. Being tungsten free, the reported alloy has higher specific proof stress compared to existing cobalt-based superalloys.
(left) Plot showing effect of Ta occupancy on formation energy for Co0.75Al0.125Mo0.125−xTax intermetallic phase in competing DO19 and L12 ordered structures. (middle) Dark field image taken along ...001 zone axis using 100 superlattice L12 ordered spot and (right) APT reconstruction delineated using 10% Al isosurface showing γ′ precipitates (red color) in Co–10Al–5Mo–2Ta heat treated alloy. Display omitted
The present paper reports a new class of Co based superalloys that has γ–γ′ microstructure and exhibits much lower density compared to other commercially available Co superalloys including Co–Al–W based alloys. The basic composition is Co–10Al–5Mo (at%) with addition of 2at% Ta for stabilization of γ′ phase. The γ–γ′ microstructure evolves through solutionising and aging treatment. Using first principles calculations, we observe that Ta plays a crucial role in stabilizing γ′ phase. By addition of Ta in the basic stoichiometric composition Co3(Al, Mo), the enthalpy of formation (ΔHf) of L12 structure (γ′ phase) becomes more negative in comparison to DO19 structure. The ΔHf of the L12 structure becomes further more negative by the occupancy of Ni and Ti atoms in the lattice suggesting an increase in the stability of the γ′ precipitates. Among large number of alloys studied experimentally, the paper presents results of detailed investigations on Co–10Al–5Mo–2Ta, Co–30Ni–10Al–5Mo–2Ta and Co–30Ni–10Al–5Mo–2Ta–2Ti. To evaluate the role alloying elements, atom probe tomography investigations were carried out to obtain partition coefficients for the constituent elements. The results show strong partitioning of Ni, Al, Ta and Ti in ordered γ′ precipitates.
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.
A novel high-strength steel design is proposed, with a fine bainitic microstructure free from inter-lath carbides, for railway crossings applications. The design is based on the phase transformation ...theory and avoids microstructural constituents like martensite, cementite and large blocky retained austenite islands in the microstructure which are considered to be responsible for strain partitioning and damage initiation. The designed steel consists of fine bainitic ferrite, thin film austenite and a minor fraction of blocky austenite which contribute to its high strength, appreciable toughness and damage resistance. Atom probe tomography and dilatometry results are used to study the deviation of carbon partitioning in retained austenite and bainitic ferrite fractions from the T0/T0ʹ predictions. A high carbon concentration of 7.9 at.% (1.8 wt%) was measured in thin film austenite, which governs its mechanical stability. Various strengthening mechanisms such as effect of grain size, nano-sized cementite precipitation and Cottrell atmosphere at dislocations within bainitic ferrite are discussed. Mechanical properties of the designed steel are found to be superior to those of conventional steels used in railway crossings. The designed steel also offers controlled crack growth under the impact fatigue, which is the main cause of failure in crossings. In-situ testing using micro digital image correlation is carried out to study the micromechanical response of the designed microstructure. The results show uniform strain distribution with low standard deviation of 1.5% from the mean local strain value of 7.7% at 8% global strain.
This article reports the microstructural stability and consequent phase decomposition including the appearance of topologically close-packed (TCP) phases at high temperature of recently discovered ...tungsten-free γ–γ′ alloys of base composition Co–10Al–5Mo–2Nb with or without the addition of Ni and Ti. On prolonged aging at 800 °C of the Co–10Al–5Mo–2Nb alloy, needle-shaped DO
19
-ordered precipitates with stoichiometry of Co
3
(Mo, Nb) start appearing in the microstructure. In addition, growth of cellular domains from the grain boundaries featuring a three-phase composite lamellar structure could be observed. These phases are fcc γ-Co with composition different from the original matrix, CoAl with B2 ordering and Co
3
(Mo, Nb) with DO
19
ordering. All the phases exhibit well-defined crystallographic orientation relationships. The decomposition of the alloys depends on the solvus temperature of the γ′ phase. The Ni-containing alloy exhibits no phase decomposition until 100 h of aging at 800 °C without any significant effect on γ′ volume fraction (76 %). However, at 950 °C, the alloy decomposes leading to the appearance of four different phases including TCP phases: a Cr
3
Si-type cubic phase, a hexagonal Laves phase, rhombohedral μ phase, and solid solution of Co phase. The γ–γ′ microstructure in the Co–10Al–5Mo–2Nb and Co–30Ni–10Al–5Mo–2Ta alloys is not stable at 800 and 950 °C, respectively, on long-term aging. This shows that the measured solvus temperatures (i.e., 866 and 990 °C) are metastable solvus temperatures. We also report that the Ti-containing alloy exhibits superior stability with no evidence of either TCP phase formation or any other decomposition of γ′ precipitates, even after aging at 950 °C for 100 h.
Pt-modified β-NiAl bond coats are applied over the superalloys for oxidation protection in jet engine applications. However, as shown in this study, it also enhances the growth of the interdiffusion ...zone developed between the bond coat and the superalloy along with brittle precipitates. Location of the Kirkendall plane indicates that a precipitate free sublayer grows from the bond coat, whereas another sublayer grows from the superalloy containing very high volume fraction of precipitates. With increasing Pt content, thickness of both the sublayers increases because of an increase in diffusion rates of the components. Quantitative electron probe microanalysis indicates high concentration of refractory components in the precipitates. Transmission electron microscopy shows that René N5 superalloy produces TCP phases μ and P, whereas CMSX-4 superalloy produces μ and σ in the interdiffusion zone. With increasing Pt content in the bond coat, the average size of the precipitates decreases when coupled with René N5. Precipitates become much finer when the same bond coats are coupled with CMSX-4.
Quantitative EPMA analysis of the interdiffusion zone between René N5 superalloy and Ni(Pt)Al superalloy. Display omitted
The paper reports the design and development of wrought Al-Cu alloys that retain high strength until 250 °C. It is achieved by minor alloying of Zr (<0.15 at%) and Nb (<0.1 at%) and inducing ...precipitation of stable nanometric dispersion of L12 ordered precipitates during controlled thermomechanical treatment (rolling) of cast alloys before conventional heat treatment (solutionizing and ageing). The rolling temperature was optimized to 450 °C by quantitatively evaluating the size distribution of the dispersions at different rolling temperatures. The dispersions influence the microstructure and the nature of the precipitation of the strengthening θ′ plates during subsequent conventional heat treatment. The microstructure of the processed alloy show stability at high temperatures. The atom probe tomography reveals Zr enrichment at the broad faces of θ′ plates. The presence of Zr plays a critical role in the high-temperature strength by promoting the stability of θ′ precipitates and resisting its coarsening. The composite microstructure of Al-Cu-Nb-Zr alloy displayed yield strength (YS) of 415 MPa and 220 MPa at room temperature and at 250 °C, which are higher by 26 % and 45 % respectively as compared to a commercial Al-Cu based alloy (2219-T851). The present results are promising for developing wrought Al alloys for high-temperature applications.
•Effect of Zr and Nb addition in Al-Cu alloy processing through thermo-mechanical route.•Dynamic precipitation of nanometric L12 ordered phase in Al matrix during wrought processing•L12 ordered precipitate promotes the nucleation of θ′ and retard the growth and coarsening.•APT study reveals solute (Zr/Nb) segregation at the Al/θ′ interface.
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 γ–γ′ cobalt-based superalloys with ...base alloy compositions of Co–30Ni–10Al–5Mo–2Ta (2Ta) and Co–30Ni–10Al–5Mo–2Ta–2Ti (2Ta2Ti). Cr addition leads to a change in the morphology of the strengthening cuboidal-shaped γ′ 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 γ matrix relative to γ′. However, Cr also has a strong effect on the Ta and Mo partitioning coefficient across γ/γ′ interfaces. The value of partition coefficient for Mo (
K
Mo
) becomes <1 with Cr addition to the alloys. Results from ab initio calculations show that the Cr atoms prefer to replace Mo atoms in the sublattice sites of the L1
2
unit cell. The solvus temperature of about 1038 and 1078 °C was measured for 10Cr2Ta and 10Cr2Ta2Ti alloy, respectively, and these Cr-containing alloys have very low densities in the range of ~8.4–8.5 gm/cm
−3
. The 0.2% compressive proof strength of 10Cr2Ta2Ti alloy yields a value of 720 MPa at 870 °C, substantially better than most Co–Al–W-based alloys and many of the nickel-based superalloys (e.g., MAR-M-247).
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.
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