The ordering reaction in the newly developed Ni-Cr-W-Ti alloy aged at 700 °C has been investigated using atom probe tomography and transmission electron microscopy. The results reveal that the existence of titanium in the superalloy induces and stabilizes the DO22 superlattices, which cannot precipitate in the ternary Ni-Cr-W superalloy. The stoichiometry of the DO22 phase has been determined to be Ni3(Cr0.2W0.4Ti0.4), and the interfaces between DO22 precipitates and matrix have been revealed to be fully coherent at the atomic scale.Download high-res image (549KB)Download full-size image

•Σ3n boundary distribution characteristics are changed during aging treatment at 600 °C.•The increase in fractions of Σ9 and Σ27 boundaries depends on interaction between Σ3n boundaries.•The consumption of Σ3c boundaries is modeled with migration of random HABs.•Generation mechanism of Σ3ic boundaries differs from traditional models.The evolution of Σ3n (n = 1,2,3) boundaries in a Ni-Cr-Mo alloy during an aging treatment at 600 °C was investigated based on electron backscatter diffraction (EBSD) measurements. Incoherent Σ3 (Σ3ic) and coherent Σ3 (Σ3c) boundaries were identified using the single-section trace analysis method. The results show that the Σ3n boundary distribution characteristics are changed and that a Pt2Mo-type Ni2(Cr,Mo) superlattice phase precipitates during the aging treatment. Through the analysis and comparison of the average grain diameters measured under the different conditions, the migration behaviors of both the Σ3ic boundaries and random high-angle boundaries (HABs) are found. The migration of the Σ3ic boundaries provides the probability of interaction between Σ3n boundaries. This interaction is responsible for some different types of triple junctions, the increase in Σ9 and Σ27 boundary fractions and interrupted connectivity of the network of random HABs. However, the Σ3c boundaries' consumption caused by the migration of random HABs and the difficulty of forming new Σ3c boundaries lead to the decreased fraction of Σ3c boundaries. The further discussion about the increase in the fraction of Σ3ic boundaries indicates that the generation mechanism of the Σ3ic boundaries during the aging treatment differs from the models applied to recrystallization and subsequent grain growth processes.

Ti2AlN particle reinforced Ti48Al2Cr2Nb composites were prepared by in-situ reactive arc-melting method. The tensile properties and fracture behavior of the composites at room temperature and 800 °C were investigated in comparison with those of the Ti48Al2Cr2Nb alloy. The results show that the microstructure of TiAl matrix is significantly refined due to the Ti2AlN particles which are distributed uniformly in the matrix. Compared with the unreinforced TiAl alloy, the tensile properties of the composites are enhanced both at room temperature and 800 °C. In particular, the optimum tensile properties are obtained in 4 vol% Ti2AlN/TiAl composite which shows the highest fracture strength of 670 MPa and fracture strain of 0.39% at room temperature and the highest yield strength of 552.4 MPa and ultimate tensile strength of 645.3 MPa at 800 °C. More specially, 3 vol% Ti2AlN/TiAl composite shows the highest elongation of 13.4% at 800 °C. The fracture surface analysis shows that the fracture mechanism of the composites is closely related to the deformation temperature and Ti2AlN content. The composites exhibit characteristics of cleavage fracture of the TiAl matrix and cracking of Ti2AlN particles at room temperature. While at 800 °C, the fracture type of the composites changes from a mixed mode of brittle cleavage and ductile fracture to brittle fracture as the Ti2AlN content increases from 3 vol% to 4 vol%.

•The effect of isothermal treatment on undercooling, liquid fraction and the grain growth was studied.•Microstructural refinement by isothermal treatment near the liquidus was explained.•The effect of different average grain sizes on mechanical properties was also analyzed.The solidification process of Ni-Cr-W superalloy was investigated by confocal scanning laser microscope (CSLM). The liquid fraction during solidification was obtained as a function of real time and temperature in reference with the in-situ observation. In this work, the effect of isothermal treatment near the liquidus on undercooling, grain initiation and the grain growth was studied, and an analytical model is developed to account for the effect of solute field on grain refinement. This study revealed that isothermal treatment near the liquidus restricted the growth of crystal and increased the number density of active particles for nucleation, giving rise to a finer grain size. The tensile results of Ni-Cr-W superalloy tested at different temperatures indicated fine grain microstructure can improve the mechanical properties at different temperatures.Cooling curves of Ni-Cr-W superalloy either isothermal treatment or not isothermal treatment near the liquidus temperature and solidification process in-situ observed by CSLM at different temperatures: (a) to (d) isothermal treatment near the liquidus temperature; (e) to (h) non-isothermal treatment near the liquidus temperature.

•An embedded serration appears in a Ni–Cr–Mo alloy at room temperature.•A new model is proposed to explain appearance of the embedded serration.•Both order–disorder transformation-induced twins and twinning of ordered phase itself induce the embedded serrations to occur.It is found that the Portevin–Le Chatelier (PLC) effect can occur in a Ni–Cr–Mo alloy containing ordered phase with Pt2Mo-type structure during uniaxial tensile tests at room temperature with strain rate of 10−3 s−1. The serrations exhibit an embedded characteristic, i.e. a small serration is embedded in two adjacent large serrations. Both formation of order–disorder transformation-induced twins and twinning of ordered phase itself are responsible for the occurrence of the serrations, which generally arise from dynamic strain aging (DSA) that is widely accepted as the underlying mechanism for the PLC effect.An embedded serration, i.e. a small serration is embedded in two adjacent large serrations is displayed in an ageing-treated Ni–Cr–Mo alloy containing ordered phase with Pt2Mo-type structure during uniaxial tensile tests at room temperature with strain rate of 10-3 s−1 as compared to the as-quenched alloy. Both formation of order–disorder transformation-induced twins and twinning of ordered phase itself are responsible for the occurrence of the embedded serrations.

•The DO22 phase precipitated in the Ni-Cr-W alloy has a high thermal stability.•The morphology of DO22 superlattice has been determined to be ellipsoid.•The interface between DO22 phase and matrix are fully coherent at the atomic scale.•Different variants of DO22 phase occur equiprobably.•The alloy strength can be improved dramatically by the nanoscale DO22 particles.The ordering transformation occurring in a model Ni-Cr-W superalloy during prolonged exposure to proper temperature has been investigated systematically. It is demonstrated that nanometer-sized precipitates with a DO22 structure can precipitate in the Ni-Cr-W alloy by means of simple aging treatment at 650–700 °C. The mechanism of transformation to DO22 superlattice has been determined to be continuous ordering based on the results of high resolution transmission electron microscopy investigation and variation trend in Vickers microhardness. Different variants of DO22 phase can coexist in the matrix with no signs of overaging as aging time increases, indicating it has a high thermal stability. The precipitates of DO22 superlattice has been found to be of ellipsoidal shape which results in the greatest reduction of strain energy. The interfaces between DO22 precipitates and matrix have been revealed to be coherent at the atomic scale, resulting in considerable coherency strain attributing to the lattice misfit between DO22 particle and matrix. Because of the high-density nanometer-sized DO22 phase, the microhardness of the alloy has been improved remarkably after aging treatment.Different variants of the DO22 superlattice can coexist in the matrix, and the interface between precipitate and the matrix remain coherence at the atomic scale. The three dimensional form of the DO22 precipitates constructed from three mutually perpendicular projections is an ellipsoidal stick, and the directions of elongations are along the longest axis of the unit cell for DO22 phase.

•The DF image of SRO cluster has been found for the first time experimentally.•The transformation of SRO to LRO gives rise to the Pt2Mo-type Ni2(Cr, W).•Variants of Ni2(Cr, W) occur equiprobably.•The interfaces between Ni2(Cr, W) and matrix are coherent at the atomic scale.It is demonstrated that a nanometer-sized Ni2(Cr, W) superlattice with a Pt2Mo-type structure can precipitate in an Ni–Cr–W alloy by means of a simple aging treatment at 550 °C. The dark-field image of short-range order domains has been found for the first time experimentally. The mechanism of short-range order to long-range order transformation has been revealed based on transmission electron microscopy result and static concentration waves theory and found to be continuous ordering. The randomness of the transformation of static concentration waves leads to equiprobable occurrence of the different variants. The transformation of short-range order to long-range order gives rise to the Pt2Mo-type Ni2(Cr, W) superlattice. The interfaces between Ni2(Cr, W) and Ni-based matrix and the different variants of Ni2(Cr, W) have been investigated by high resolution transmission electron microscopy. The results reveal that the interfaces between Ni2(Cr, W) and surrounding matrix are coherent at the atomic scale.

•Mechanism of ordering transition correlates with the microstructure of interface.•Continuous ordering mode leads to an obscure interface between particle and matrix.•The mechanism of nucleation and growth results in a sharp interface.•The ordering process can be known from the structure of interface.It is found that the microstructure of interfaces between superlattices and matrix correlates perfectly with the mechanism of ordering transformation. The mechanism of continuous ordering makes the ordering degree from the center of the superlattice to interface to surrounding disordered matrix become increasingly weaker, leading to an obscure interface which can be regarded as a transition region. In contrast, the mechanism of nucleation and growth makes the whole region of the superlattice completely ordered superstructure, resulting in a sharp interface between superlattice and matrix. The mechanism of ordering transformation can be identified by the microstructure of interface.Microstructure of interface between superlattice and matrix correlates with the mechanism of ordering transformation. The mechanism of ordering transformation leads to the obscure interface shown in (c) and (d); The mechanism of nucleation and growth results in the sharp interface shown in (b).

Various undercoolings 14-232 K of bulk K4169 superalloys were obtained by the method of molten glass fluxing combined with superheating cycling and the mechanical properties of undercooled K4169 with as-solidified state were tested. Microstructures and phases composition in undercooled bulk K4169 superalloy were identified by transmission electron microscope (TEM), scanning electron microscope (SEM) and optical microscopy (OM). The morphology of dendrites, grain size and intergranular phase all change with the increased undercooling. Meanwhile, the relationship between microstructure of undercooled K4169 superalloy and tensile properties was investigated. The experimental results show that the uniform distribution of Laves phase and the decrease of grain size and intergranular phase content are favorable for the improvement of mechanical properties. The maximum tensile strength and elongation obtained at undercooling of 232 K are 932.2 MPa and 6.5 %, respectively.

Microstructure stability of in situ synthesized Ti2AlN/Ti–48Al–2Cr–2Nb composite during aging at 900 °C was investigated by XRD, OM and TEM, and the unreinforced Ti–48Al–2Cr–2Nb alloy was also examined for comparison. The result showed that in the TiAl alloy, α2 lamellae thinned and were broken down, and became discontinuous with increasing aging time. The decomposition of α2 lamella to γ which was characterized by parallel decomposition and breakdown of α2 lamellae led to the degradation of the lamellar structure. While in the composite, lamellar structure remained relatively stable even after aging at 900 °C for 100 h. No breakdown of α2 lamellae except parallel decomposition and precipitation of fine nitride particles was observed. The better microstructural stability of the composite was mainly attributed to the precipitation of Ti2AlN particles at the α2/γ interface which played an important role in retarding the coarsening of lamellar microstructure in the matrix of composite.

The precipitation behavior of the second phase in Hastelloy C-2000 alloy matrix was studied after isothermal oxidation at 800 °C for 100 h in air. Mo-rich phase was precipitated in the alloy matrix after oxidation. According to the determination Mo-rich phase with diamond cubic structure is Mo3Ni3C type carbide. Morphology characteristics of Mo-rich phase with both no-continuous and continuous irregular strip are presented along grain boundaries and in matrix, respectively. However, some variation of morphologies will take place at both grains and grain boundaries after deep etching, i.e. a large number of white flocculent structure are distributed along grain boundaries, and the corrosion pits with a diamond structure appear in intragranular. Ni-rich and Cr-rich areas are more easily corroded during deep etching due to a more negative electrode potential in the areas.

By improving the method of B2O3 glass purification combined with cycle superheating based on traditional undercooling method, a stable undercooling above 200 K of 100 g bulk K4169 superalloy was obtained, and the largest undercooling went to 271 K. The effects of purification and superheating temperature on undercooling have been discussed. Moreover, the three-dimensional numerical analysis of temperature field inside the undercooled melt has been presented, and the maximum temperature deviation in different parts is only 14 K before nucleation. In contrast with the as-solidified microstructure, the grain size will increase with the increment of nucleating temperature. Generally, the difference of microstructure in 100 g undercooled K4169 superalloy is tiny and the average grain size of the superalloy with 271 K undercooling is 12±2 μm.

The influence of cooling rates on the dendritic growth and microstructure evolution of Ti48Al2Cr2Nb alloy is studied by electromagnetic levitation combined with copper mold casting. The different cooling rates of the conical as-cast sample with diameters from 4.7 to 0.8 mm were calculated by ANSYS software. The results show that primary dendrite arm spacing decreases with increase in cooling rate. Peritectic transformation (L + β → α) and the transformation of α → (α2 + γ) are restrained at cooling rate of 2.3 × 104 K s−1. With further increase in cooling rate (2.6 × 104 K s−1), a fine and homogeneous microstructure can be observed in the conical casting sample with the diameter of 0.8 mm. It consists of a large amount of massive γ phase, lath-like γ phase, and only few lamellar structures (α2 + γ). The formation of the microstructure in the alloy is attributed to the strong chilling, giving rise to the high undercooling and the high dislocation density during rapid solidification.

Microstructural evolution in nonequilibrium solidification of Ti–48Al–xNb alloys with Nb contents ranging from 2 to 8 at% has been studied by containerless electromagnetic levitation. Levitated drops of controlled undercooling were quenched onto chill copper substrates and subjected to phase and microstructure analysis. With increasing Nb content, the solidification path changes gradually from hyperperitectic solidification to hypoperitectic solidification and both solidification segregation (S-segregation) and β-solidification gradually increase. A transition from typical hypoperitectic solidification to a sole solidification of the β phase beyond a critical undercooling is revealed for the Ti–48Al–8Nb hypoperitectic alloy. For the Ti–48Al–2Nb alloy, the morphologies of the primary β dendrites are not observed. With increasing undercooling, the coarsening of the lamellar colonies occurs, which can be attributed to the transition of the primary β dendritic morphology. Furthermore, the solute concentration profiles for the final solidification microstructure are obtained to examine the segregation behaviors of alloying elements. With increasing Nb content, the undercooling eliminating S-segregation gradually increases.

The solidification behavior of Ti–48Al–8Nb alloy under nonequilibrium solidification conditions was studied by electromagnetic levitation technique. The solidification conditions are different undercooling (ΔT) under the same cooling condition and different cooling methods at the same undercooling condition, respectively. In different undercooling conditions, when the undercooling is above a critical value (ΔT* ≈ 211 K), a remarkable morphological transition from typical hypoperitectic solidification to a sole solidification of the β phase resulting in the suppression of the peritectic reaction occurs. For melts with different cooling conditions at the same undercooling (ΔT ≈ 85 K), the melt was rapidly cooled by quenching them in cooling media. With cooling rate increasing, a transition from β phase to peritectic α phase solidification mode is revealed for Ti–48Al–8Nb alloy.

The effect of Al content on the microstructure and solidification characteristics of Ti–Al–Nb–V–Cr alloys in as-cast and isothermally treated states was investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM) equipped with energy dispersive spectroscope (EDS), and transmission electron microscopy (TEM). The typical solidification characteristics are due to the joint influence of both the crystal temperature range and the solidification path. The wide crystallization temperature range contributes to obtaining coarse dendrites in the as-cast Ti47Al7Nb2.5V1.0Cr (at%) alloy solidifying through the peritectic reaction. The β-solidifying Ti46Al7Nb2.5V1.0Cr (at%) alloy with the narrow crystallization temperature range is attributed to the formation of a homogeneous fine-grained microstructure. However, the crystallization temperature range of Ti48Al7Nb2.5V1.0Cr (at%) alloy is equivalent to that of Ti46Al7Nb2.5V1.0Cr alloy, but it is solidified by peritectic reaction, leading to the formation of finer dendrites.

Solidification characteristics of Ti–46Al–7Nb melts were studied by the electromagnetic levitation technique. A maximum melt undercooling up to 240 K has been achieved. When the undercooling is lower than the critical value ΔT* = 205 K, the alloy possesses typical hypoperitectic solidification characteristic which can be evidenced by a peritectic layer observed in the as-solidified microstructure. However, the Widmanstätten structure can be observed at large undercooling regime of ΔT ≥ ΔT*, where peritectic reaction cannot proceed and γ lamellar precipitation within α plates is suppressed. Based on the BCT dendrite growth model, the dendrite growth velocities were calculated as a function of undercooling. Theoretical analysis indicates that the growth mechanism of the primary β phase transforms from solutal-diffusion-controlled to thermal-diffusion-controlled in the undercooling range of 188–205 K, which can be attributed to the onset of solute trapping at the critical undercooling. Meanwhile, with increasing undercooling, the solute trapping effect becomes more dominant as a consequence.

The microstructure, texture and microsegregation in conventional and fine grained ingots of Ni–Cr–W superalloy were experimentally charactered by metallography, field emission scanning electron microscope equipped with an Electron Backscatter Diffraction and electron probe microanalysis, respectively. Distribution of misorientation angles results indicates that the proportion of low-angle grain boundaries and high-angle grain boundaries is about 15% and 85%, respectively, demonstrating that the particles are genuinely separate grains with mostly high-angle boundaries. Further, inverse pole figure results supply evidence for elucidating the mechanism of grain refinement due to a high effective nucleation rate which is attributed to extremely uniform temperature and composition fields in the bulk liquid at early stages of solidification. Element segregation at the grain boundary of equiaxed fine grain microstructure is smaller than that of the conventional dendrite grain microstructure. The effect of different morphologies of microstructure on yield strength was investigated. The effect of different average grain sizes on micro-hardness and yield strength was also analyzed. Relationship between yield strength and the grain size of this alloy followed the Hall–Petch relationship: σs=700+422d−1/2.

It is demonstrated for the first time that a nanoscale superlattice with a DO22 structure can precipitate in an Ni–Cr–W alloy by means of a simple ageing treatment at 700 °C. The existence of tungsten in the superalloy stabilizes the DO22 superlattices, which are transient metastable phases in Ni–Cr–Mo alloy. Because of the fine, spherical and dispersed precipitates in the Ni–Cr–W matrix, the microhardness of the alloy is improved remarkably.

The grain refinement of Ni-Cr-W based superalloy was obtained by near liquidus casting. The melt was heated to the predetermined temperature near liquidus by induction heating and solidified directly in the crucible by furnace cooling. A fine-grained microstructure was obtained under a certain crucible nucleation undercooling and the related mechanisms are discussed. The solidification process of the undercooled melt is divided into two parts, a nonequilibrium process and a near-equilibrium process. The critical radius Rc for disturbance developing of spherical grains decreases greatly in the nonequilibrium process compared with the near-equilibrium process. Grains grow in dentritic manner in the nonequilibrium process and a crystal multiplication occurs in this process by remelting dentrite. Grain density is determined in the nonequilibrium process. In a near-equilibrium process, the disturbance of the spherical grains will develop with a larger Rc. In situation of high nuclei density, decided by the nonequilibrium process, the larger Rc and the solidification terminates before disturbance developing. Thus, granular grains are obtained.

Ni2Cr-type superlattices widely exist in Ni–Cr based and Ni–Mo based alloys. This work focus on the alloying behavior of M (M = Nb, Mo, Ta, and W) in Ni2Cr1−xMx superlattices and their electronic structures by first principles calculations based on the density functional theory. The energetic stability and elastic properties are studied by calculating the formation energy, cohesive energy, and independent elastic constants. The results reveal that all the studied alloying elements could not only improve the thermal stability of Ni2Cr superlattice but also modify its elastic anisotropy within the studied alloying range (that is, x ≤ 0.15).Highlights► Ab-initio calculations for Ni2Cr1−xMx (M = Nb, Mo, Ta, and W) superlattices have been performed. ► Geometry optimization, electronic, stability and elastic properties are investigated. ► The bonding characteristics of Ni2Cr1−xMx (M = Nb, Mo, Ta, and W) superlattices show strong covalent nature up to x = 0.25. ► The alloying elements M (M = Nb, Mo, Ta, and W) not only effectively improve the thermal stability of Ni2Cr superlattice but also modify its elastic anisotropy.

The effect of M23C6 carbides on the formation of grain boundary serrations (GBSs) has been systematically investigated in a solid solution strengthened Haynes 230 alloy. It is found that GBS occur in this alloy during the slow cooling process and are accompanied by the precipitation of intergranular planar M23C6 carbides. The amplitude and proportion of GBS increase with the rise of the solution temperature and time. If the specimens are cooled directly without any solution treatments, the grain boundaries remain planar and granular M23C6 carbides precipitate at them. The sequential evolutions of GBS and M23C6 carbides are investigated by scanning electron microscopy (SEM) examination. High-resolution transmission electron microscope (HRTEM) investigations reveal the coherent interfacial plane of M23C6 carbides formed at grain boundaries to be (1 1 1¯). These facts indicate that the nucleation and oriented growth of M23C6 carbides at grain boundaries play an important role in the formation of GBS. Based on the interfacial energy calculations and the tensions balance relation, a semi-quantitative model about the GBS formation is proposed.Highlights► The characteristics of GBS are dependent on the solution treatment conditions. ► The morphology of M23C6 carbides have a direct influence on the formation of GBS. ► The difference of diffusivity at interfaces cause M23C6 carbides and serrations.

Precipitation behavior of grain boundary (GB) M23C6 and its effect on tensile properties at elevated temperature were investigated systematically in a Ni–Cr–W based superalloy. The results show that the M23C6 precipitation behavior is influenced obviously by grain boundary character (GBC) and interfacial energy. The Σ≤9 GBs and low angle GBs have low interfacial energy, and no M23C6 carbide precipitates at these GBs. Plenty of M23C6 carbide particles precipitate at the large angle GBs with high interfacial energy. The coherent orientation relationship between M23C6 and the matrix plays an important role on the precipitation morphology of M23C6. M23C6 carbides with four typical morphologies distribute at the large angle GBs, including lamellar carbide which grows into the matrix near one side or both sides of the GBs, rod-like carbide and small lamellar carbide both of which grow along GBs. Moreover, the decrease of both tensile and yield strength of the aged alloy is mainly caused by the lamellar M23C6 carbide breaking. The tensile properties vary irregularly with increasing aging time.Graphical abstractHighlights► M23C6 precipitation can be inhibited by the Σ≤9 GBs and low angle GBs in Ni–Cr–W based superalloy. ► M23C6 carbides with four typical morphologies precipitate at the large angle GBs. ► The coherent orientation relationship between M23C6 and the matrix plays an important role on the precipitation morphology of M23C6. ► The decreasing strength of aged alloy can mainly be caused by lamellar M23C6 carbide breaking.

High undercooling has been achieved in Co80Pd20 melts by employing the method of molten glass denucleating combined with cyclic superheating, and the microstructure evolution with undercooling was systematically investigated. Within the achieved range of undercooling, 0–415 K, two kinds of grain refinements have been observed in the solidification microstructures. The three critical undercoolings are 72, 95, and 142 K, respectively. When undercooling is less than 72 K, the coarse dendritic morphology is formed, which is similar to the conventional as-cast microstructure. The first grain refinement occured in the range of undercooling, 72–95 K can be attributed to the breakup of dendrite-skeleton owing to remelting. When undercooling locates within 95–142 K, highly developed directional fine dendrite can be obtained because the severe solute trapping weakens the effect of solute diffusion during the dendrite growth. The second grain refinement occurred when undercooling exceeds the critical undercooling (∆T* = 142 K), the formation of fined equiaxed microstructure can be ascribed to the stress that originates from the extremely rapid solidification process, which resulted in the dendrite fragmentation finally.