The role of microstructural morphology on creep properties of a [0 0 1] oriented nickel‐base single crystal superalloy

The experimentally observed microstructure of nickel‐base single crystal alloy consists of a large volume of cuboidal γ′ precipitates coherently embedded in the γ matrix. In calculation, a representative volume element is usually used to represent the whole structures due to the regular γ/γ′ topological structures. Here, three experimentally found microstructures have been extracted to generate the representative volume elements. One is constituted by one cuboidal γ′ phase surrounded by γ phase. The other two consisted of two cuboidal γ′ phases and one rectangle γ′ phase with different arrangement of the two γ′ phases. The misfit stress is taken into consideration by different thermal expansion coefficients of the two phases. The influences of different microstructures on the macro‐creep strain evolution, rafting and stress distributions are discussed.     

Observation of the Dissolution of Topologically Close Packed Phases by an Additional Heat Treatment in Third Generation Nickel‐Based Single Crystal Superalloy Turbine Blades

Topologically close packed (TCP) phases degrade the superior creep and rupture properties of Ni‐based single crystal superalloys. Initially, small TCP phases are formed in the dendrite core during the early stages of the manufacturing process, such as solution heat treatment, and surrounded by a gamma prime (γ′) phase. Then, TCP phases continue to develop during full heat treatment. However, an additional heat treatment induces diffusion of refractory metals from the TCP phases into the γ′ phase and consequently, the TCP phases clearly disappear. After dissolution, the regions where the TCP phases existed are altered to a normal microstructure composed of γ channels and a normal cubic γ′ phase. Based on the observation result, the mechanism of the dissolution of TCP phases is discussed.

     

Ageing response of a Al–Cu–Li 2198 alloy

The effects of different ageing treatments on microstructure evolution, properties and fracture are investigated in the present study. 2198 alloy exhibits strong ageing response during ageing. It is found that tensile properties, hardness and conductivity of 2198 alloy are very sensitive to ageing temperatures, which corresponds to different microstructures. In the naturally-aged condition (T3), only δ′ (Al₃Li) was detected. After artificial ageing (T8), large amounts of precipitates emerged and major precipitates that were detected turned to be δ′, θ′ (Al₂Cu) and T₁ (Al₂CuLi) phase. Exposure to higher temperature caused greater amounts of the precipitation. The constitution and morphology of precipitates varies with different ageing temperature; the major precipitates are δ′, θ′ when ageing below 160 °C, while above 160 °C, T₁ phase comes out in large numbers, becoming dominate strengthening phases gradually. Fracture transforms from a typical dimple type to a dimple-intergranular mixed type with the rise of ageing temperature.     

Microstructural Characterization of Lamellar Features in TiAl by FIB Imaging

A novel experimental procedure is introduced to determine phase fractions and the distribution of individual phases of TiAl‐based two‐phase alloys using the focused ion beam (FIB) technique. Two γ‐titanium aluminide alloys with a fine‐grained duplex and a nearly lamellar microstructure are examined. The special FIB‐based preparation procedure results in high contrast ion beam‐induced images for all investigated alloys and allows to quantify the phase contents easily by automated microstructural analysis. Fine two‐phase structures, e.g. lamellar colonies in γ‐TiAl, can be imaged in high resolution with respect to different phases. To validate the FIB‐derived data, we compare them to results obtained with another method to determine phase fractions, electron back‐scatter diffraction (EBSD). This direct comparison shows that the FIB‐based technique generally provides slightly higher α₂‐fractions, and thus helps to overcome the limited lateral resolution near grain boundaries and interfaces associated with the conventional EBSD approach. Our study demonstrates that the FIB‐based technique is a simple, fast, and more exact way to determine high resolution microstructural characteristics with respect to different phase constitutions in two‐phase TiAl alloys and other such materials with fine, lamellar microstructures.     

Effect of different heat treatments on the microstructure and mechanical properties in selective laser melted INCONEL 718 alloy

The microstructure and tensile properties of selective laser melted (SLM) Inconel 718 alloy were studied in the as-printed and different heat treat conditions. The SLM as-print microstructures exhibited columnar grain structures with very fine dendritic structure with segregation of elements. Apart from the standard heat treatment, three other heat treat cycle variants were carried out in an attempt to remove the extensive segregation of elements and modify the textured grain structure of the SLM as-print microstructure. Increasing the homogenization temperature reduced the segregation and coarsened the grain structure. However, the grains still remained columnar, and the material became softer with reduction in strength. After the ageing treatment, the tensile strength improved significantly for all the heat treated samples, which is typical for precipitation hardening of IN718 alloy. The microstructures of the heat treated samples exhibited the needle shaped δ, carbides, and finely dispersed γ″, γ′ phases.     

Simulation of rafting type and stress distribution in nickel-based superalloys with different volume fractions of γ′ phase

A representative volume element is introduced to represent the microstructure of γ/γ′ morphology with periodic boundary conditions to formulate the full mode of the micromechanical analysis. [0 0 1]-oriented alloys with γ′ volume fractions from 60 to 70% are simulated under tensile loading. A raft criterion is implemented into the user subroutine to predict the rafting type. The misfit stress is considered by different thermal expansion coefficients of the two phases. It is very high in γ phase and slightly decreases with the increasing of γ′ volume fraction. The stress distributions in the two phases change during creep deformation. The creep crack initiation time slightly increases with the increase of γ′ volume fraction. The stress components decrease with the increasing of γ′ volume fraction at the beginning and change due to the stress redistribution during creep loading.     

Evolution of microstructure of nickel base superalloy at high temperatures

Abstract

The evolution of primary and secondary γ′ precipitates in the high γ′ volume fraction Rene 80 Ni based superalloy has been examined during aging at elevated temperatures for periods up to 1750 h. While the increase in average dimension of particles followed the cube rate Lifshitz, Slypzof and Wagner (LSW) law, r³_t – r³₀=Kt, there were significant discrepancies between the experimental and theoretical particle size distributions (PSDs) and inconsistency with the kinetic constants associated with the two populations of particles. These differences are attributed to the influence of elastic coherency strains which have not been considered in conventional capillarity driven coarsening models. During thermal exposure at 871°C, coalescence of primary cuboidal γ′ was predominant in early stages of aging, while the microstructure was relatively stable at longer aging times. The stability of the microstructure at longer aging times is attributed to the formation of the network of closely spaced dislocations at the γ/γ′ interface which would cause the loss of internal misfit stresses associated with the growth. Secondary spheroidal γ′ particles were initially coarsened and their volume fraction gradually decreased until they completely dissolved after 500 h at 871°C or 1 h at 982°C.     

Formation of microstructure resulting from quasi-peritectic reactions during freezing of Bi–Pb–Sn and Cu–Ni–Sn ternary alloys

Abstract

The quasi-peritectic reaction, frequently observed in ternary alloy systems, takes the form L + α?β + γ, where L indicates the liquid phase, and α, β and γ indicate solid phases. The formation of microstructure resulting from the kinetics of this reaction is considered from a theoretical point of view and compared with experimental observations of microstructures formed by quasi-peritectic reactions in two ternary alloy systems, Bi–Pb–Sn and Cu–Ni–Sn. Based on these considerations, an explanation is proposed for experimental observations previously reported in literature concerning phase transformations from the liquid in multicomponent ferrous alloys.     

Improved fatigue crack growth resistance by retrogression and re‐aging heat treatment in 7010 aluminum alloy

Aircraft grade 7010 aluminum alloy was heat treated to two different conditions: (1) standard peak aging (T6) and (2) retrogression and re‐aging (RRA). The microstructures of these alloys were characterized by using transmission electron microscope. Fatigue crack growth rate (FCGR) tests were conducted using standard compact tension specimens, following ASTM standards. Tests were conducted at various stress ratios, R ranging from 0.1 to 0.7. The RRA‐treated alloy was observed to contain coarsened η′ (MgZn₂) precipitates with higher inter‐particle spacing when compared with T6‐treated alloy. The grain boundary precipitates (GBPs) were also coarsened and discontinuous in RRA‐treated alloy as compared with continuous GBPs in T6 condition. The FCGR was lower and ΔK_th was higher in RRA‐treated alloy compared with T6‐treated alloy at all the stress ratios investigated. Improved fatigue crack growth resistance in RRA‐treated alloy was correlated to the modified microstructure and enhanced crack closure levels.     

Fabrication of Titanium Aluminides via Powder-Cored Wire Arc Additive Manufacturing: Microstructural and Mechanical Characterization

Herein, an innovative powder-cored wire arc additive manufacturing (PC-WAAM) process is proposed to fabricate γ-TiAl thin-walled intermetallic alloy. The metallography, phase composition, and mechanical properties at different thin-wall locations are characterized. The results show that the alternatively distributed layer-like microstructure composed of α₂ (Ti₃Al) and γ (TiAl) phases is obtained along the building direction. The content of α₂ phase exhibits the tendency of decreasing from the bottom to top region. This unique microstructure characteristic is closely related to the typical thermal cycling history during deposition. Moreover, the tensile strength and microhardness of the top region are lower than the middle and bottom region. In general, the current PC-WAAM technique shows promising capability of fabricating γ-TiAl intermetallic alloy with low cost. This work becomes a valuable reference for understanding the evolution mechanism of microstructure and paves the way for the flexible and customized additive manufacturing of γ-TiAl alloy.     

Directional coarsening of γ′ phase in single crystal nickel based superalloys during tensile creep

Abstract

The γ′ precipitate rafting kinetics and morphological evolution for two model single crystal superalloys have been studied. The microstructure of the alloys after different stages of tensile creep at 1040°C and under a range of stresses are examined using TEM and SEM. The chemical compositions of both γ and γ′ phases are analysed by energy dispersive spectrometry. Results show that a meshlike γ′ raft structure is formed along the direction normal to the stress axis during primary creep. The applied stress causes a decrease in the coherent strain energy at the γ′/γ interfaces in the (001) crystal plane. The released energy is the driving force for the diffusion of elements, leading to the formation of the γ′ rafts. A longer time is required for the formation of γ′rafts in alloy 2 owing to its higher content of the refractory element W which obstructs the migration of the other elements in the diffusion field of the γ′ rafts.     

Formation of rafting and cellular-like precipitates during creep

Abstract

Evolution of γ′ precipitates during creep has been investigated over a wide range of temperatures from 760 to 1100°C in a liquid metal cooling directionally solidified nickel-based superalloy. Cubic γ′ precipitates were found to remain in the specimens crept at low temperatures, while rafting and cellular-like precipitates remained at intermediate and high temperatures, respectively.     

Automated identification and characterisation of secondary and tertiary γ′ precipitates in nickel-based superalloys

Abstract

The use of different electron loss edges in energy-filtered transmission electron microscopy (EFTEM) has allowed researchers to capture images of the morphology and size of precipitates in nickel-based superalloys. In this work, the authors discuss a computational methodology for automated detection of secondary and tertiary γ′ precipitates in EFTEM images. The optimum parameters for the automated region growing technique were identified using a combination of visual inspection and intensity information from the EFTEM images. The microstructural statistics obtained from the segmented γ′ precipitates agreed with those of the manually segmented precipitates. Then, automated segmented precipitates are used to extract microstructural information about the distributions of equivalent diameters of 656 tertiary precipitates along with the distances to the nearest secondary precipitates. The significance of this technique is its ability to automate segmentation of precipitates in a reproducible manner for acquiring microstructural statistics that relate to both processing and properties.     

Recrystallisation textures in cold rolled low carbon steel containing ferritic and bainitic microstructures

Abstract

Low carbon steel strip was heat treated to generate four different starting microstructures (fine and coarse polygonal ferrite, acicular ferrite and bainite) for investigating their influence on texture development during cold rolling and annealing. The starting materials were cold rolled to 50–90% reduction and annealed for various times in the temperature range 853–953 K. The resultant microstructures and textures were examined mainly by electron backscatter diffraction and X-ray diffraction. The initial microstructure strongly influenced the crystallographic rotation paths during cold rolling, whereby high strain deformation generated strong {223}〈110〉 texture components in the polygonal ferritic microstructures, whereas a strong {001}〈110〉 texture was produced in the acicular/bainitic microstructures. Subsequent annealing generated, to varying degrees, the classic {111}〈uvw〉 (γ-fibre) recrystallisation texture in all materials. Unexpectedly, coarse polygonal ferrite produced the strongest γ-fibre recrystallisation texture after 70–90% cold rolling reduction. Based on arguments involving the effect of carbon in solution, initial grain size and deformation textures on recrystallisation texture development, it was shown that a strong γ-fibre texture can indeed be generated in coarse polygonal ferrite.     

Teilkohärente Ausscheidungen in einer Eisen‐Chrom‐Kohlenstoff‐Legierung

Semicoherent precipitates in a Fe‐Cr‐C alloy Precipitation processes in ferromagnetic materials can be recorded very well by measuring the sensitive coercive field strength. It should be tested, whether also semicoherent precipitates have a sufficient clear interaction with Bloch‐walls. For this purpose the mild‐magnetic alloy X1FeCr25 served. To carry out the evidence sensitively, a method based on differences between H_C^t (heat‐treated state at T = 600…︁700°C) – H_C⁰ (quenched state from high temperature) = Δ H_C was used. A quantitative record of the amount of precipitates (as particle size) is possible by a decomposition parameter Δ H_C/Δ t. Plate‐like β′‐precipitates with planes {100}∥{100} in the α‐Fe solid solution have been proved by transmission electron microscopic investigations; this is the preparation state for the transition into the stable fcc phase M₂₃C₆. As a result, the quantitative electron microscopic proof of the β′‐phase can be supported by magnetic measurements, qualitatively and quantitatively. The estimated values of the activation energy for the process in the 1st maximum of precipitation in X1FeCr25 are higher than for the stable phases as the orthorhombic M₃C or the cubic complex M₆C in other steels and give a hint to the difficult processes related to nucleation as to the transition into M₂₃C₆.     

Concurrent design for NiAl-based (β/γ′) two-phase alloys by controlling microstructure and texture

Control of the crystallography of Ni₃Al(γ′) precipitates along grain boundaries of NiAl(β) was systematically studied using β bicrystals with controlled orientations. γ′ phase preferentially precipitated along β grain boundaries showing a film-like shape. The variants of γ′ precipitates were uniquely selected, which satisfies the Kurdjumov–Sachs (K–S) relation with a neighboring β grain and deviates from the relation with another adjacent β grain. In the course of tensile deformation, fracture occurred preferentially at the (β/γ′-film) interface deviating from the K–S relation and the fracture stress decreased with increasing deviation angle from the K–S relation. For improvement of the coherency at the irrational (β/γ′-film) boundaries, the control of microstructure and crystal orientation distribution in (β/γ′) two-phase polycrystals was next attempted by thermomechanical processing. After hot-compression in β phase region and subsequently annealing in (β/γ′) two-phase region, γ′ phase transformed from β phase with 〈111〉_β fiber texture satisfying the K–S relation, resulting in the formation of 〈110〉_γ′ fiber texture. In particular, a large number of (β/γ′-film) boundaries became partially coherent. This thermomechanical processing was effective in controlling the crystallography of γ′-film along β grain boundaries and leads to the harmonic design of strength and ductility for (β/γ′) two-phase alloys.     

Microstructure-sensitive notch root analysis for dwell fatigue in Ni-base superalloys

Macroscopic viscoplastic constitutive models for γ–γ′ Ni-base superalloys typically do not contain an explicit dependence on the underlying microstructure. Microstructure-sensitive models are of interest in many applications since microstructure can vary in components, whether intentional or not. In such cases, the use of experiments from one microstructure condition to fit macroscopic models may be too limiting. The principal microstructure attributes that can significantly affect the cyclic stress–strain response of γ–γ′ Ni-base superalloys are the grain size and γ′ precipitate volume fraction and size distributions. An artificial neural network (ANN) is used to correlate the material parameters of a macroscale internal state variable cyclic viscoplasticity model with these microstructure attributes using a combination of limited experiments augmented by polycrystal plasticity calculations performed on other (virtual) microstructures within the range characterized experimentally. The trained model is applied to an example of a component fatigue notch root analysis with dwell periods at peak load to demonstrate the methodology and explore the potential impact of microstructure-sensitive constitutive models on life prediction for notched structures subjected to realistic load histories.     

Precipitation hardening in Inconel* 625

Abstract

The hardness of the nickel based superalloy Inconel 625, aged at 625, 700, and 760°C for different intervals of time ranging from 1 to 335 h, has been measured. Peak hardening is found to occur much earlier at 760°C than at 700°C. Also the peak hardness is higher at 700°C than at 760°C. The results have been discussed in terms of precipitation. Scanning electron microscopy revealed the presence of precipitates in specimens aged at 760°C for a longer time. Electron probe microanalysis results show these precipitates to be rich in Ni, Nb, and Mo indicating that these are γ″ precipitates of Ni₃ (Nb, Mo) type. Transmission electron microscopy confirmed that these are γ″ precipitates. It also suggests that nucleation takes place heterogeneously on dislocations and stacking faults. Longer aging causes somewhat uniform nucleation but still preferentially on the secondary defects. At 700°C γ′ precipitates have been observed in addition to γ″ precipitates. The orientation relationship between the precipitates and the matrix has also been determined.     

Micronotches for studying growth of small cracks

Well‐defined starter cracks at preselected sites in the microstructures are needed for systematic investigation of the characteristic features of microstructure controlled growth of small cracks. A kinked ellipsoidal micronotch with very high notch factor at the trailing kink is proposed, which triggers controlled crack initiation along the notch contour. These micronotches can be machined by femtosecond laser ablation with virtually no heat‐affected zone at the edges. Crack growth results obtained for an intermetallic γ‐TiAl alloy are presented as an illustrative example.     

Concurrent design for NiAl-based (β/γ′) two-phase alloys by controlling microstructure and texture

Control of the crystallography of Ni₃Al (γ′) precipitates along grain boundaries of NiAl(β) was systematically studied using β bicrystals with controlled orientations. γ′ phase preferentially precipitated along β grain boundaries showing a film-like shape. The variants of γ′ precipitates were uniquely selected, which satisfies the Kurdjumov–Sachs K–S) relation with a neighboring β grain and deviates from the relation with another adjacent β grain. In the course of tensile deformation, fracture occurred preferentially at the (β/γ′-film) interface deviating from the K–S relation and the fracture stress decreased with increasing deviation angle from the K–S relation. For improvement of the coherency at the irrational (β/γ′-film) boundaries, the control of microstructure and crystal orientation distribution in (β/γ′) two-phase polycrystals was next attempted by thermomechanical processing. After hot-compression in β phase region and subsequently annealing in (β/γ′) two-phase region, γ′ phase transformed from β phase with 〈111〉_β fiber texture satisfying the K–S relation,resulting in the formation of 〈110〉γ′ fiber texture. In particular, a large number of ′/γ′-film) boundaries became partially coherent. This thermomechanical processing was effective in controlling the crystallography of γ′-film along β grain boundaries and leads to the harmonic design of strength and ductility for (β/γ′) two-phase alloys.