Search

Computational superalloy development is a powerful alternative to the conventional experimental approach. Based on thermodynamic databases and the CALPHAD method, it is possible to estimate the properties of a large number of potential alloys and select the most promising ones. However, the accuracy of the databases and complementary property models can be unsatisfying. The accuracy of two mass density and γ/γ′$\gamma / \left(\gamma\right)^{&aposx;}$ lattice parameter models and the TTNI8 and TCNI10 databases is analyzed in detail on the experimental basis of computationally optimized single‐crystalline superalloys. Various properties are measured and compared to the results of the property models and databases. Neither of the databases is superior to the other and especially the γ′$\left(\gamma\right)^{&aposx;}$ solvus temperature is not accurately described in both. The new mass density model, a linear regression based on the molar mass, is more reliable for low‐density alloys. Both lattice parameter model versions slightly overestimate the room‐temperature γ$$ \gamma $$ lattice parameter. The γ′$\left(\gamma\right)^{&aposx;}$ lattice parameter, however, is more accurately calculated using the new model version. The results of this study can be readily used to improve a multicriteria alloy optimization tool for computational superalloy design. [ABSTRACT FROM AUTHOR]

To enhance the mechanical properties of NiAl–(Cr,Mo) in situ composites for high‐temperature structural use, achieving a regular eutectic microstructure is crucial. A novel alloy with a composition of Ni30.6Al36Cr31.4Mo2 is developed that exhibits a significantly reduced solidification interval compared to well‐studied alloys such as Ni33Al33Cr28Mo6. This advancement results from using the in‐house developed alloy design tool PyMultOpt and Calculation of Phase Diagrams analysis to minimize the solidification interval. The refined alloy demonstrates a theoretical solidification interval of 0 K. These results are verified with differential scanning calorimetry measurements at different heating rates for the new alloy and compared with the established alloy Ni33Al33Cr28Mo6. The small solidification interval leads to a uniform eutectic microstructure consisting of a (Cr,Mo) phase embedded in the NiAl matrix without primary NiAl dendrites present. This indicates that deviating from the stoichiometric NiAl composition results in a significantly lower solidification interval and thus in an actual eutectic composition of the NiAl–(Cr,Mo) system. [ABSTRACT FROM AUTHOR]

By increasing the density of interfaces in NiAl–CrMo in situ composites, the mechanical properties can be significantly improved compared to conventionally cast material. The refined microstructure is achieved by manufacturing through electron beam powder bed fusion (PBF‐EB). By varying the process parameters, an equiaxed or columnar cell morphology can be obtained, exhibiting a plate‐like or an interconnected network of the (Cr,Mo) reinforcement phase which is embedded in a NiAl matrix. The microstructure of the different cell morphologies is investigated in detail using scanning electron microscope, transmission electron microscopy, and atom probe tomography. For both morphologies, the mechanical properties at elevated temperatures are analyzed by compression and creep experiments parallel and perpendicular to the building direction. In comparison to cast NiAl and NiAl–(Cr, Mo), the yield strength of the PBF‐EB fabricated specimens is significantly improved at temperatures up to 1,027 °C. While the columnar morphology exhibits the best improved mechanical properties at high temperatures, the equiaxial morphology shows nearly ideal isotropic mechanical behavior, which is a substantial advantage over directionally solidified material. [ABSTRACT FROM AUTHOR]

Introducing Cu nanoparticles is an effective mechanism for strengthening and toughening Fe‐based materials such as ultra‐high‐strength steels. Herein, the effect of Cu on the mechanical properties of a novel Fe‐based α/α′/α″ superalloy is studied. Compared to a Cu‐free reference alloy, nanoindentation reveals an increase in hardness, which was associated with the formation of Cu nanoparticles. Both alloys show room temperature (RT) compressive plastic strain at maximum stress greater than 8%, irrespective of the heat‐treatment. At RT and at 750 °C, the Cu‐containing alloy exhibits a slightly higher strength, but the heat treatment has a more significant impact: A configuration of α‐matrix and intermetallic α′/α″‐phases forming an interpenetrating network is superior to a state with isolated precipitates. This difference vanishes in monotonic creep experiments, and under the same conditions, the Cu‐containing alloy exhibits a twice as high creep rate despite a slightly higher precipitate fraction. This is linked to a higher lattice misfit and faster‐coarsening kinetics. Post‐mortem transmission electron microscopy analysis of the creep‐deformed specimens identifies dislocation bypass as the dominant deformation mechanism. However, the presence of <010>{110} dislocations in the interfacial networks and evidence of dislocation activity within α′/α″ precipitates suggest the occurrence of shearing events. [ABSTRACT FROM AUTHOR]

Laser cladding of superalloys provides an attractive and cost‐efficient technique for near‐net shape manufacturing, remanufacturing, or individualized repair of components. While the weldability of γ' hardened Ni‐base superalloys is limited, Co‐base superalloys should have a lower susceptibility for cracking during welding due to a lower γ' solvus temperature and less segregation than Ni‐base superalloys. The weldability of the Co‐base superalloy CoWAlloy2Mod and the resulting mechanical properties at 950 °C are investigated. In the as‐welded condition of CoWAlloy2Mod, a fine dendritic microstructure without forming cracks and limited porosity is found. After a three‐step heat treatment, recrystallized, equiaxed grains and a high γ' volume fraction are obtained leading to a very good strength. CoWAlloy2Mod shows a significantly higher strength and creep strength than the conventional weld filler Co‐alloy PWA795. [ABSTRACT FROM AUTHOR]

MCrAlY bond coats are widely used as oxidation‐resistant coatings on superalloys. The major phase is the thermally stable intermetallic β‐NiAl phase. The addition of Cr increases the oxidation and corrosion resistance but leads to the formation of α‐Cr precipitates. Both phases are brittle and limit the fracture toughness of these coatings. To study the influence of thermal exposure and the resulting changing chemical composition on the fracture properties, the fracture toughness of both phases is measured by notched microcantilever bending, after processing and after oxidation by thermal cycling. The fracture toughness of β‐NiAl decreases during thermal cycling due to the depletion in Al, whereas the fracture toughness and the composition of α‐Cr stay constant. As the fracture toughness of α‐Cr is lower compared with β‐NiAl, the formation of α‐Cr precipitates is supposed to decrease the fracture toughness of the bond coat. [ABSTRACT FROM AUTHOR]

Choosing a properly optimized rotating scan strategy during the selective laser melting (SLM) process is essential to reduce residual stresses and thus to obtain homogeneous properties. Surprisingly, anisotropic material properties are found in several materials that are built with the often applied rotating stripes scan strategy of Electro‐Optical Systems (EOS) because the scan strategy avoids possible interactions of the laser beam with process by‐products and therefore excludes a range of scanning directions. Herein, the alloys Hastelloy X, Inconel 718, and stainless steel 316L are investigated. Vertically built specimens with a cylindrical gauge geometry show an oval deformation during tensile testing, indicating a mechanical anisotropy in the horizontal x‐ and y‐direction. Tensile tests along the x‐ and y‐direction reveal a deviation of the yield strength of 7% for Hastelloy X. Analyses of the microstructures show differences in the grain morphology, size, and texture in all three coordinate planes of the three materials. This anisotropic behavior can be explained by a detailed study of the texture and the calculated Schmid factors. Heat treatments can reduce the textural and mechanical anisotropy due to recrystallization of grains but requires annealing at sufficiently high temperatures and long times. [ABSTRACT FROM AUTHOR]

Allvac 718Plus is a derivative of alloy 718 for applications at higher temperatures in rotating and static components of gas turbines. According to literature, various phases as γ, γ′, η, δ, and γ″ can form with so far contradicting results. Herein, the precipitation behavior in different alloy conditions is investigated using transmission electron microscopy and atom probe tomography. The aim is to clarify how local variations in chemical composition alter the sequence of precipitation and type of precipitates that form. Material conditions with a minor volume fraction of η/δ phase and a high fraction of primary γ′ precipitates form γ″ phase due to excess of available Nb. Spherical γ′ precipitates with disc‐shaped γ″ precipitates at γ′ interfaces are observed. During discontinuous coarsening of η/δ plates mainly Nb is consumed which leads to an enrichment of γ′ forming elements and hence the heterogeneous nucleation of γ′ at the η/γ interface. The coexistence of thin η plates embedded in δ phase in grain boundary pinning precipitates could additionally be confirmed. These results show under which conditions certain primary phases form and how the resulting consumption and redistribution of phase promoting elements trigger the formation of other secondary phases. [ABSTRACT FROM AUTHOR]

Previous investigations revealed that Nb, Ta, and Zr improve the creep properties of fully lamellar titanium aluminides significantly. The enhanced creep properties can originate from different potential effects of the alloying elements. Herein, the elements' partitioning between α2‐Ti3Al and γ‐TiAl and their influence on the lattice parameters of ternary Ti–44Al–5X (X = Nb, Ta, Zr) alloys are investigated in comparison with a binary Ti−44Al alloy by atom probe tomography and high‐energy X‐ray diffraction. Ta partitions nearly equally between α2 and γ, Nb accumulates slightly in γ, and Zr enriches strongly in γ. As all alloying elements have a larger radius than Ti, a stronger partitioning to γ decreases the lattice misfit more. The synchrotron measurements show a clear influence of the alloying elements on the lattice parameters and resulting lattice misfits in γ‐⟨110] and γ‐⟨101] direction as well as the c/a ratio. In accordance with theoretical calculations based on the partitioning behavior, the lattice misfit decreases from Ti–44Al–5Ta over Ti–44Al–5Nb to Ti–44Al–5Zr. The c/a ratio decreases from Ti–44Al–5Nb over Ti–44Al–5Ta to Ti–44Al–5Zr. A correlation between the measured lattice misfits of the ternary Ti−44Al−5X with their primary creep strains is found, as the primary creep strain decreases with decreasing lattice misfit. [ABSTRACT FROM AUTHOR]

Additively manufactured austenitic stainless steel AISI 316L (EN 1.4404, X2CrNiMo17‐12‐2) is used at higher temperatures, e.g., in space applications. However, the high‐temperature properties of such materials have not been analyzed in detail yet. Thus, selective laser melted (SLM) 316L is tested in the solution‐annealed condition by compression and tensile tests at temperatures between 25 and 877 °C. The compressive strength of SLM 316L is higher in comparison with the conventionally produced reference material due to hardening by a high dislocation density and a fine substructure. However, tensile tests reveal a loss in ductility of the SLM material at temperatures between 300 and 627 °C, where the elongation to fracture is reduced from 65% to 39%. Alloying elements cause serrated yielding in the affected temperature range. Together with an increased normalized work‐hardening rate and a negative strain rate sensitivity, dynamic strain aging is found to cause the reduction of ductility. [ABSTRACT FROM AUTHOR]

Lamellar eutectic NiAl–Cr(Mo) alloys show an increased fracture toughness due to different toughening mechanisms. These mechanisms result from the fibrous or lamellar microstructure of the two constituting phases α‐Cr(Mo) and β‐NiAl. However, the fracture toughness of the individual phases and the evolution from early crack growth to the toughening mechanisms have not yet been systematically studied. Herein, bending tests on focused ion beam (FIB)‐notched microcantilever beams are used to characterize the small‐scale fracture properties. The micromechanical investigations reveal that the fracture toughness of the α‐Cr(Mo) phase (7.5−9.1 MPam) is much higher than the fracture toughness of β‐NiAl (2.2−2.9 MPam). Larger cantilevers in the crack arresting orientation show an enhanced fracture toughness with up to 14.4 MPam, which is still lower than the one of macroscopic experiments. This is attributed to the small interaction volume of the crack, which hinders the full exploitation of potential extrinsic toughening mechanisms. [ABSTRACT FROM AUTHOR]

The strength of superalloys is strongly influenced by γ′ precipitates, whose size and volume fraction which can be adjusted by heat treatments. According to classical precipitation strengthening models, an increasing precipitate diameter should lead to a transition from weak to strong coupling of the dislocation pairs that form superdislocations in the γ′ phase. We show that long‐term annealing of the Ni‐base superalloy A718Plus at 670 and 680 °C increases the alloy's strength without significantly changing the grain size and η fraction. To understand the effect of the slight increase in γ′ size, detailed atom probe tomography (APT) was performed. Here, different field evaporation rates of the phases strongly affect the determination of the volume fraction when using the usual isosurface construction. This can be mitigated by considering the number density of atoms inside and outside the γ′ precipitates. Using an approximation of the precipitate shapes and arrangements from the APT data in atomistic simulations revealed that precipitate shearing by both, weakly and strongly coupled dislocations can occur in the same sample due to the wide distribution of precipitate sizes. These results highlight the need for advanced strengthening models that take into account the γ′ size distribution. [ABSTRACT FROM AUTHOR]

The cover illustrates bcc Fe‐base superalloys strengthened by coherent B2/L21 precipitates. Consisting of two‐thirds Fe and Al, they represent a promising, sustainable and cost‐efficient alternative to Ni‐base superalloys at intermediate temperatures. In the corresponding article number 2201652, Luis Morales, Christopher Zenk and co‐workers investigate the effect of the microstructural configuration on the low‐ and high‐temperature mechanical properties of two such alloys. [ABSTRACT FROM AUTHOR]

The cover illustrates bcc Fe-base superalloys strengthened by coherent B2/L2 SB 1 sb precipitates. Consisting of two-thirds Fe and Al, they represent a promising, sustainable and cost-efficient alternative to Ni-base superalloys at intermediate temperatures. Influence of Cu Addition and Microstructural Configuration on the Creep Resistance and Mechanical Properties of an Fe-Based / '/ " Superalloy. [Extracted from the article]

The influence of Carbon addition on a ternary Co-Al-W alloy is investigated especially regarding phase stability and partitioning behavior of alloying elements. During casting and subsequent cooling, C-additon leads to the formation of an interdendritic carbide network and to the homogeneous precipitation of cuboidal E21 phase, Co3AlC, in the W-rich Co matrix. The solvus temperature of this phase is found to be about 1200 °C, which is significantly higher than the solvus temperature of the ternary L12 phase Co3(Al,W). In the course of solution heat treatment, the Co3AlC precipitates are dissolved and the carbides change in morphology and chemical composition. Further annealing leads to the precipitation of small cuboidal precipitates of L12 phase, Co3(Al,W), and to a discontinuous precipitation of further phases. [ABSTRACT FROM AUTHOR]

The nature and stability of intermetallic phases in a multi-component γ/γ′ Co-base superalloy is investigated. At least three kinds of unwanted intermetallic phases form due to the segregation of Al, Ta, Ti, Si, and Hf during casting in the interdendritic areas in addition to γ and γ′. One of the intermetallic phases that shows a blocky shape, contains high content of Ta, Hf, and Ti and has been identified as a topologically close-packed Laves phase. A B2 phase appears as relatively big pools and in some cases contains needle-shaped precipitates with high content of Al and Ti. The thermal stability of these intermetallic phases is studied in this work under defined heat treatments. The needle-shaped precipitates already dissolve at 1000 °C, whereas the B2 phase is dissolved only at higher temperatures of 1200 °C. Small amounts of Laves phases remained stable during aging at 1200 °C for 25 h. The prediction of phases as well as their stability is also checked by the CALPHAD method. The TCNi5 database, containing the description of Co3(Al,W), predicts the presence of the observed intermetallic phases; however, the predicted main transformation temperatures for these phases differ from the experimentally obtained values. [ABSTRACT FROM AUTHOR]

The influence of iridium on the phase stability, oxidation behavior, and high temperature creep strength of γ′-hardened Co-base superalloys is discussed. Ir increases the γ′-solvus temperature and γ′-volume fraction since it partitions slightly to the γ′-phase. However, Ir leads also to a discontinuous precipitation transformation of the metastable γ/γ′-microstructure into a lamellar microstructure of Co3W, CoAl, and a Co-rich fcc phase for the W-rich alloys. A reduction of W in presence of Ir leads to a higher phase stability. The constrained lattice misfit between the γ and γ′ phase in the Ir and high W containing alloy is +0.5%. The alloying element Ir strongly increases the creep strength at 850 °C as long as the γ/γ′-microstructure is not discontinuously transformed. Furthermore, the oxidation resistance is improved by the addition of iridium. [ABSTRACT FROM AUTHOR]