Needle-like Precipitates Research Articles

Simulation-guided design of novel precipitation-strengthened eutectic high entropy alloy

Eutectic high entropy alloys (EHEAs) with hard and ductile phases are promising for high-temperature applications. The mechanical properties of EHEAs can be improved by incorporating precipitates in the ductile phase. The study focuses on the CALPHAD-guided alloy design approach for developing EHEA with cuboidal L12 precipitates in the ductile phase. The newly designed alloy (Al0.17CoCrFeNiTa0.22) is subjected to a simulation-guided heat treatment cycle. The Al0.17CoCrFeNiTa0.22 alloy shows FCC phase with a needle-like Ta-rich precipitate at 12 h and Ni-Al-rich cuboidal precipitate at 24 h of heat treatment. The calculated entropy of mixing of cuboidal precipitate is higher than that of needle-like precipitate. The detailed TEM characterisation confirms the characteristics of precipitates and microstructural changes correlated with microhardness measurements. The study proposes a novel EHEA system with a nano-scale precipitation-strengthened FCC phase and eutectic colony.

Enhanced mechanical properties and anti-wear performance of Cu45Mn8Ni40Sn7 medium entropy bronze alloy by multi-stage heat treatment

High/ medium entropy brass and bronze alloys exhibit considerable potential for application as structural materials. This study focuses on a Cu45Mn8Ni40Sn7 medium entropy bronze alloy that demonstrates excellent hardness, compressive strength, ductility, and wear resistance through a multi-stage heat treatment technique. The effects of heat treatment on the phase composition, microstructural evolution, mechanical properties, and tribological performance of the alloy were systematically investigated. The alloy, treated by homogenization (820 ℃/4 h)-solution (900 ℃/1 h)-aging (450 ℃/5 h), exhibits a hardness of 362 HV, yield strength of 626 MPa, ultimate strength of 1229 MPa, compressive strain of 47.2 %, fracture toughness of 20.7 MPa·m1/2, and maintains a low wear rate of 1.72×10−5 mm3·N−1·m−1. The processes of homogenization and solution treatment reduce dendrite segregation of elements and eliminate the detrimental lamellar structure at the grain boundaries. During the aging treatment of the alloy, sub-micron scale needle-like precipitates are formed, which play a significant role in enhancing its mechanical properties.

Unexpected effects on creep resistance of an extruded Mg-Bi alloy by Zn and Ca co-addition: Experimental studies and first-principles calculations

In the present work, a new Mg-Bi based alloy is developed by the addition of Zn and Ca in equivalent atom fraction with Bi. Mg-Bi and Mg-Bi-Zn-Ca alloys were prepared by extrusion at a ram speed of 20 mm/s. Room temperature mechanical properties and creep behaviors at 423 K were investigated. The results show that Zn and Ca co-addition shows little influence on average grain size and texture intensity but changes the dispersive Mg3Bi2 into Mg2Bi2Ca particles in different sizes and a lower density. Twinning is largely activated during room-temperature deformation. Consequently, a slightly decreased proof strength but tripled elongation is shown at room temperature. Unexpectedly, large enhancement in creep resistance is detected after the co-alloying of Zn and Ca and the minimum creep rate is reduced by 10 to 20 times in the BZX621 alloy. Stress exponent n = 4–5 indicates that the creep is a dislocation-climb controlled type. Post-mortem characterization on microstructure shows slip of dislocation 〈c + a〉 are also largely found in B6 as well as BZX621 alloy and cross-slip is detected more severe in B6 alloy. Dynamic segregation and precipitation are also seen in both alloys. Bi-clusters are seen dispersive across the grains in B6 and so did the PFZs that could undermine creep resistance at the grain boundaries. By contrast, Zn-rich needle-like precipitates are developed at most “ends” of 〈c + a〉 dislocations, which would hinder the further dislocation motions and thus improve the creep resistance. First-principles calculations were adopted and the results show that the thermal stability and thermomechanical properties of Mg2Bi2Ca are much better than that of Mg3Bi2. Stacking faults energy is lowered down with the co-addition of Ca and Zn, which could inhibit the rate of dislocation climb and cross-slip. As a result, the improved creep resistance is obtained in the Mg-Bi-Zn-Ca alloys. Microstructural and controlling mechanism changes by thermal activation result in the unexpected enhancement in creep resistance with decreased room-temperature proof strength after co-addition. These findings could contribute to the development and optimization of creep-resistant Mg alloys in the future.

Temperature-sensitively dissolving of GGBS in neutral and alkali media

The sensitivity of temperature on the decomposition/polymerisation of ground granulated blast-furnace slag (GGBS) was investigated through the dissolution experiments at high water to solid ratio under controlled temperatures (5, 20, and 40 °C). The aqueous compositions during the dissolution in neutral and alkali media were determined by ICP-OES (Inductively coupled plasma-optical emission spectrometry) and the saturation conditions with respect to the possible precipitations were estimated by thermodynamic modelling. The depolymerisation of silicate groups and the chemically combined water in the residual GGBS was examined by FTIR/SEM (Fourier Transform Infrared spectroscopy/Scanning Electron Microscopy) and TG (Thermogravimetric) analysis respectively. The results demonstrated that the effect of temperature on the dissolution of GGBS is more pronounced from 5 °C to 20 °C than that from 20 °C to 40 °C. The extremely low temperature (∼ 5 °C) restricts the hydration of GGBS even in a high pH environment. The temperature does not affect the depolymerising sequences of silicate groups in GGBS in a neutral environment but changes the overall dissolution rate. The major silicate spices (Q0 to Q3) in GGBS tend to dissolve evenly in DI water, whereas the Q2 tends to depolymerise faster than others in the NaOH solution. The decreasing Ca concentration in the NaOH solution corresponds to the formation of ∼ 50 nm needle-like precipitates regardless of temperature. Based on the results collected, the dissolving potential and the temperature sensitivity of the major components in GGBS can be divided into three batches: the easiest/most sensitive group: Na-Ox; intermediate group: K-Ox, Si-Ox, Ca-Ox, Al-Ox; the hardest/least sensitive group: Mg-Ox. The intermediate batch required an initial activation energy of ∼ 15 kJ/mol in DI water. The relevant data could be considered as experimental evidence in the thermodynamic modelling of the hydration of GGBSs.

A strategy of fabricating efficient Mg2Ni nano-catalysts in Mg-Ni-Ag fibers and their excellent hydrogen storage properties at near-room temperatures

In this study, a new strategy was utilized to fabricate Mg-based hydrogen storage fibers with uniform elemental distribution and diameters of 30–50 μm. The in-situ formation of nano-precipitates in Mg85Ni14.8Ag0.2 metallic glass fibers was visually observed at varied annealing temperatures. It indicates the metastable Mg6Ni phase is formed from an amorphous microstructure initially. Then, the needle-like Mg2Ni nanoparticles precipitate at 140–160 °C and subsequently grow into spherical shapes. This results in the formation of the fiber structure with well-distributed nano-Mg2Ni particles. The alloy fibers can absorb H2 at only 40 °C, and the activation energy for hydrogenation is lowed to be 46.9 kJ/mol. The onset desorption temperature of hydride is reduced to 220 °C due to the improved the catalytic efficiency by uniformly distributed Mg2Ni nanoparticles. The abundant interfaces, as well as the shortened distance by micropores, serve as paths for the fast diffusion of H atoms.

Microstructure and strengthening of Al-6Ce-3Ni-0.7Fe (wt%) alloy manufactured by laser powder-bed fusion

An Al-6Ce-3Ni-0.7Fe (wt%) alloy was fabricated via laser powder-bed fusion and its microstructure, thermal stability, tensile properties, and creep properties are investigated for two rapid-solidification rates with different eutectic spacings. For both faster- and slower-cooled states, the as-fabricated alloy mostly shows elongated grains with very fine eutectic networks (∼40 nm lamellar width) comprising a high-volume fraction (∼18 %) of intermetallic phases. Faster-cooled samples, however, display a less continuous eutectic network with finer spacing (∼100 nm), leading to higher Orowan strengthening and therefore superior microhardness and yield stress up to ∼350 °C. Upon aging (300 – 450 °C), micron-size Al9(Ni,Fe)2 needle-like precipitates form within grains, and the Al11Ce3 eutectic network spheroidizes, while retaining a submicron width and spacing, resulting in 33–37 % drop in microhardness after 144 h aging at 400 °C. Limited tensile ductility (∼6 %) and creep ductility (∼1–2 %) are measured for both faster- and slower-cooled alloys due to an inhomogeneous microstructure, where cavitation preferentially initiates at melt-pool boundaries, precipitate-free zones, and/or denuded zones, eventually leading to local fracture. Denuded zones, induced by stress, form due to diffusional flow with stress-dependent orientations, and are identified as microstructurally weak regions leading to strain localization. The present alloy does not show a significant difference in 300 °C creep resistance between: (i) tensile and compressive loading, (ii) faster- and slower-cooled samples, and (iii) continuous and spheroidized eutectics. Creep resistance at 300 °C is comparable to that of a Ce-richer Al-10.5Ce-3.1Ni-1.2Mn (wt%) alloy, despite the formation of denuded zones and needle-like Al9(Ni,Fe)2 precipitates.

Evolution in microstructure, properties of the laser cladding CoCrFeMoNi coatings at different annealing temperatures

Laser-cladding CoCrFeMoNi high-entropy alloy coating is composed of face-centred cubic, σ and µ phases. After annealing at 700 °C, many needle-like precipitates of the σ phase are found in the FCC solid solution, and the granular µ phase precipitated at the edge of the primary net-structural σ phase. After annealing at 800 °C, granular µ phases are found inside the primary σ phase. The friction coefficients of the coatings (as-cladded, as-annealed at 700 °C and 800 °C) decrease dramatically compared with the substrate of H13 steel. Obvious spheroidising of the σ phase at 900 °C leads to a decrease of hardness and an increase of friction coefficient. Furthermore, the CoCrFeMoNi high-entropy coatings show better corrosion resistance than H13 steel in 3.5 wt-% NaCl solution.

Laves phase and equiaxed grains formation in directed energy deposited AlCuFeNiTi high entropy alloy

In this study, an equiatomic AlCuFeNiTi high entropy alloy (HEA) was successfully fabricated for the first time using pre-alloyed powder with the directed energy deposition method on a Ti-6Al-4V substrate. The alloy was characterized using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), transmission electron microscopy (TEM), and nanoindentation to provide insight into the relationship between its microstructural and nanoscale mechanical properties. The microstructure has three distinct regions, resulting from variation in the alloy's local elemental and crystallographic composition that influences its mechanical properties. The alloy was found to be multiphase with a mostly ordered Heusler structure (L21) and a small amount of FCC structure with traces of C14 Laves in them. Needle-like and spherical-shaped Cu-rich precipitates were also observed in the dendrites. During the solidification process, evidence of dendritic fragmentation was observed, contributing to the alloy having mostly equiaxed grains rather than the commonly observed columnar grains in previously reported additively manufactured (HEAs). Nanoindentation results showed that the dendrites constitute a hard zone, whereas the inter-dendritic regions typify a continuous soft zone, with the dispersed Laves phase having the highest hardness. The presence of Laves phase inside the grain boundaries and its distribution as small-size grains strengthens the grain boundaries. Results highlight the synthesis of an equiaxed HEA with multiple distinct regions that influence its overall mechanical properties.

Effect of heat treatment regime on microstructure and phase evolution of AlMo0.5NbTa0.5TiZr refractory high entropy alloy

This paper reports the effect of heat treatment process on the ensuing microstructure evolution, and hardness variation of a cast and homogenized AlMo0.5NbTa0.5TiZr refractory high-entropy alloy (RHEA). Heat treatments were carried out on the homogenized samples at 1000, 1100, and 1200 °C for 10, 24, and 48 h. It was revealed that the phase decomposition in the alloy took place in a much shorter time than that reported in the literature. The significant tendency of Zr and Ti in respect of separation from the solid solution was found as the main reason for the formation of multi-phase structure and causing thermal instability. The phase decomposition occurred though the formation of Zr-rich needle-like phase constituents and Ti-rich globular ones. The semi-stable Zr-rich phase constituents are believed to form on B2 lamellas, and during their growth, the crystal structure gradually transforms from body-centered cubic (BCC) to a hexagonal close-packed (HCP) configuration. The electron back-scattered diffraction (EBSD) analyses confirmed that the HCP phase fraction increased significantly with increase in annealing temperature and/or time. A remarkable increase of 78% in Vickers hardness was measured for the sample heat treated at 1100 °C for 24 h compared to that of the homogenized state. It turned out that the morphology, size, interface coherency, and volume fraction of the needle-like precipitates played the leading role in influencing the alloy’s hardness. A comprehensive mechanism has been proposed for the phase decomposition of the alloy, based on the microstructure and chemical composition variations.

Microstructure and Mechanical Properties of Selective Laser Melted Reduced Activation Ferritic/Martensitic Steel

Cube and tensile samples of reduced activation ferritic/martensitic steel were formed at different laser powers and scanning velocities using a selective laser melting process; the microstructural characteristics and tensile properties of the cube and tensile samples were investigated in this study. The experimental results showed that the SLMed CLF-1 samples that formed with selected laser melting were near-fully dense, and the relative density of the SLMed CLF-1 samples exceeded 99%. Meanwhile, there were numerous nano-sized spherical and needle-like precipitate dispersions distributed in the grains and boundary of the grains, and the precipitates were mainly composed of M23C6 carbide and MX carbide. The microstructure was composed of columnar grains and equiaxed grains arranged in a sequence, and the smallest average size of the grains was 15 ± 2.1 µm when measured at 320 W of power and 800 mm/s scanning velocity. In addition, the sample at 320 W of power and 800 mm/s scanning velocity exhibited higher yield strength (875 ± 6.0 MPa) and higher elongation (25.6 ± 0.8%) than that of the sample at 200 W of power, 800 mm/s scanning velocity, yield strength of 715 ± 1.5 MPa, and elongation of 22.6 ± 1.2%.

Cellular transport of uranium and its cytotoxicity effects on CHO-k1 cells

Uranium is a radioactive heavy metal and a significant public health concern; however, its associated underlying toxicological mechanisms remain largely unknown. In this work, the uptake and efflux processes of uranium in CHO-k1 cells were studied and the cytotoxicity effects were explored. It was found that both the uptake and efflux processes took place rapidly and half of the internalized uranium was expelled within 8 h. The uranium exposure caused a decrease of cell viability and adhesion ability in a dose-dependent manner and blocked the cell cycle at the G1 stage. In addition, gene expression analysis revealed relative changes in the transcription of metabolism related genes. Further studies revealed that the cytotoxicity of uranium could be alleviated by exposing cells to a lower temperature or by the addition of amantadine-HCl, an endocytosis inhibitor. Interestingly, after uranium exposure, needle-like precipitates were observed in both intracellular and extracellular regions. These findings collectively suggest that the cellular transport of uranium is a rapid process that disturbs cell metabolism and induces cytotoxicity, and this impact could be reduced by slowing down endocytic processes.

Effect of Re and C on mechanical properties of NbTaW0.4 refractory medium-entropy alloy at elevated temperature

The NbTaW0.4, Re0.1NbTaW0.4, NbTaW0.4C0.25 and Re0.1NbTaW0.4C0.25 refractory medium entropy alloys (RMEAs) were fabricated by arc melting. The effect of Re, C andRe + C on the microstructure and mechanical properties of NbTaW0.4 alloy was investigated. The NbTaW0.4 alloy consisted of disordered body-centered cubic (BCC) solid solution phase. The yield strength at room temperature and 1450 ℃ of NbTaW0.4 alloy were 912 MPa and 198 MPa, respectively. A small addition of Re (4 at%) has no significant effect on the microstructure and room temperature mechanical properties of the alloy, but can increase the yield strength by about 69% at 1450 ℃; when C (9.4 at%) or both Re and C (Re is 3.6 at%; C is 9.1 at%) were added to the alloy, the microstructure of the alloy was fine and uniform hypo-eutectic structure, and needle-like precipitates were dispersedly distributed within the grains in the matrix. The mechanical properties of the alloy were significantly improved when C or both Re and C were added. Compared with NbTaW0.4 alloy, the yield strength of NbTaW0.4C0.25 alloy and Re0.1NbTaW0.4C0.25 alloy increased by 316.6% and 233.3% at 1450 ℃, respectively. Re element can affect the morphology, quantity and distribution of Ta2C phase. Consequently, element Re shows an antagonistic effect on the enhancement of NbTaW0.4 alloy by C element. The NbTaW0.4C0.25 alloy exhibits excellent yield strength of 825 MPa at 1450 ℃ with decent ductility of 12.4% at room temperature.

Microstructure evolution, phase decomposition and transition of Al0.8Co0.5Cr1.5CuFeNi HEA during high temperature sintering and mechanical properties corresponding to different microstructures

Microstructure evolution, phase determination, phase decomposition and transition of mechanically alloyed Al0.8Co0.5Cr1.5CuFeNi during 950–1150 ℃ high temperature sintering, and the effect of sintered microstructures on mechanical properties were investigated. The microstructure was characterized by X-ray diffraction (XRD), scanning electron microscope (SEM) and transmission electron microscope (TEM). Cu separation occurred first, needle-like Cu rich precipitation in the BCC matrix formed the nucleation sites of Al-Ni rich decomposition, while fine lath separation and Cu rich decomposition region surrounding the FCC matrix were formed in the FCC matrix, and the remaining FCC matrix formed L12 order. Ordered Al-Ni rich decomposition in BCC matrix appeared after 1000 ℃ sintering. As sintering temperature increased, Al-Ni rich decomposition increased and coarsened and length–width ratio increased. σ phase precipitated after 950 ℃, 1000 ℃ and 1050 ℃ sintering, and disappeared as sintering temperature was higher than 1100 ℃. Yield strength of sintered alloy decreased from 1445.9 MPa to 1151.5 MPa, and hardness decreased from 714.2 HV to 536.3 HV as sintering temperature increased. The significant increase in plasticity after 1100 ℃ and 1150 ℃ was due to dissolution of σ phase. Dislocation accumulation around σ phase caused development of micro cracks and reduced the plasticity.

Texture features and strengthening mechanisms in welding nugget zone of SSFSWed thick-plate Al–Li alloy joint

In order to investigate the microstructure evolution, texture features, and strengthening mechanisms at different positions in the welding nugget zone (WNZ) of thick-plate joints in depth, stationary shoulder friction stir welding (SSFSW) tests were conducted on a 12-mm thick 2195-T8 Al–Li alloy with rotating speeds of 300–500 rpm. At 300 rpm, the results demonstrate that WNZ is mostly made up of equiaxed grains orientated in <111> and <101>//TD, with a homogeneous grain size distribution along the plate thickness. In contrast, at 500 rpm, the crystal orientations convert to <001> and <101>//TD, and the grain size at the bottom of WNZ reaches only 39.3% of that at the top. At 300 rpm, WNZ is predominantly composed of shear textures and remains a large number of needle-like precipitates T1 and θ′, both of which contribute to the improvement in mechanical properties. The weld zone exhibits higher microhardness and tensile strength along the plate thickness at a lower rotating speed, retaining a high level within 8 mm from the weld surface but gradually decreasing from 8 to 12 mm as the rotating speed increases. In addition, the strengthening mechanism of the whole WNZ is dominated by precipitation strengthening at 300 rpm, which is replaced by dislocation strengthening at 400 rpm. As the rotating speed increases to 500 rpm, grain refinement strengthening takes over as the dominant strengthening mechanism in the WNZ.

Phase-field modeling of the clustering of transmutation element rhenium in irradiated tungsten

Incident neutrons in irradiated W can cause the formation of solid transmutation elements, of which Rhenium (Re) is the most abundant. We apply the phase-field method to investigate the clustering and growth of the Re-rich precipitate in irradiated W based on the spinodal decomposition mechanism, and their effects on the mechanical and thermal properties of W. Needle-like precipitates are reproduced and comparable to experimental observations. We then vary the irradiation dose and temperature to study their influences on the microstructure evolution of the Re-rich precipitate. Simulation results show that the average diameter, the number density, and the coverage rate of the precipitates significantly increase with the increase of the irradiation doses but slightly increase with the increase of the temperature. The effects of the Re-rich precipitate on the mechanical and thermal properties of W are also investigated. Results show that the Vickers hardness increase and the thermal conductivity degrade due to the formation of the Re-rich precipitate, especially at high irradiation doses. Conventional simulation methods can hardly handle the effect of the needle-like precipitates on the mechanical and thermal properties. In this work, we compare these two properties by using needle-like precipitates and circular precipitates. Our results clearly show that the needle-like precipitates give a better consistency with experimental results of the Vickers hardness increase, and reveal the anisotropic ability of the heat transfer in neutron-irradiated W. The current results can provide a systematic understanding of the Re clustering behavior from the microstructure evolution to its influences on the mechanical and thermal properties of W materials.

A precipitation-hardened AlSi10Mg alloy fabricated using selective laser melting

The microstructure and mechanical properties of AlSi10Mg alloy fabricated by selective laser melting were systematically investigated in this study. Substantial hardening of this alloy was achieved by aging heat treatment. The alloy exhibits an excellent combination of strength and ductility, attributed to its fine cellular structure with nanoscale needle-like precipitates.

Partitionless solidification and anomalous triradiate crystal formation in drop-tube processed Al-3.9 wt%Fe alloys

Drop tube atomized Al-3.9 wt% Fe alloy has been investigated. Spherical samples were collected and sieved into 9 different size fractions ranging between 850 + µm to 38 µm, with corresponding estimated cooling rates ranging between 100 and 20,000 K s−1. X-ray diffraction analysis showed the presence of Al, Al13Fe4 and Al6Fe in all size fractions. Scanning electron microscopy and optical microscopy have been employed for microstructural evolution. Large proeutectic crystals of Al13Fe4 surrounded by α-Al, dendritic α-Al with interdendritic lamellar eutectic, lamellar eutectic and rod-like eutectic was observed in samples with d > 212 µm. In the smaller samples (d < 212 µm) primary Al13Fe4 disappeared and featureless Y-shaped structures with thin, triradiating arms start to emerge. These Y-shaped structures can be heavily fragmented in nature and appear to be the first phase to nucleate in the droplets, followed by divorced eutectic, microcellular α-Al, dendritic α-Al with lamellar interdendritic eutectic and lamellar and rod-like eutectics. Serial sectioning with a cumulative depth of 20.2 µm has revealed that the Y-shaped features have an internally connected sheet-like morphology. Transmission electron microscopy reveals that these Y-shaped features are composed of nano-sized needle-like and spherical precipitates. TEM diffraction from a Y-shaped region has revealed the presence of AlmFe in this region. EDX analysis shows that these Y-shaped features have the same bulk composition as the liquid from which they grew, suggesting they form via partitionless solidification of highly supersaturated α-Al, which subsequently undergoes solid-state decomposition. The formation of these feature also slightly increases the eutectic spacing from 0.35 µm in the 300–212 µm size fraction to 0.45 µm in the 212–150 µm size fraction. In order to understand the effect of nonequilibrium the solidification on the mechanical properties microhardness of the droplets was measured. The microhardness has risen from 50 HV0.01 to 83 HV0.01 for 850+µm and 53≤d≤38µm droplets respectively.

Effect of yttrium content on microstructure and irradiation behavior of V-4Cr-4Ti-xY alloys

V-4Cr-4Ti-xY alloys with different yttrium (Y) contents (x = 0, 0.1, 1 wt. %) were prepared and irradiated with Fe2+ at 550 ℃ to the peak damage dose of 35 dpa (displacement per atom). The microstructure and hardness of the alloys before and after irradiation were obtained with transmission electron microscopy (TEM), scanning electron microscopy (SEM) and nano-indentation test. With the increase of Y contents, the grain size decreased monotonously, and the formation of needle-like Ti-rich precipitates was suppressed. After irradiation, voids were not observed in Y added alloys but in the V-4Cr-4Ti alloy, showing that the addition of Y could inhibit the formation of voids and irradiation-induced swelling. Dislocation loops and obvious irradiation hardening were observed in all V-4Cr-4Ti-xY alloys, and the size of the dislocation loops and the irradiation hardness increment decreased with the increase of the Y content. The irradiation-induced growth of Ti-rich precipitates was observed, which may be an important factor contributing to the irradiation hardening besides the dislocation loops and voids. The possible mechanism of the effect of the Y element on the irradiation behavior was discussed. Among the three Y contents, V-4Cr-4Ti-1Y alloy exhibits the best performance in the suppression of the formation of Ti-rich precipitates and irradiation hardening.

Splitting of needle-like precipitates in grain-oriented silicon steel manufactured by the acquired inhibitor method

Needle-like precipitates are formed in high-permeability grain-oriented (HGO) silicon steel manufactured by the acquired inhibitor method after nitriding. A sawtooth splitting of the needle-like precipitate was observed in high-temperature annealing. To understand this, the needle-like precipitates at different temperatures have been analyzed using scanning electron microscope (SEM) and transmission electron microscope (TEM). The needle-like precipitate is defined as (Al,Si,Mn)N having a hexagonal crystal structure similar to AlN, and its longer direction correspond to the [0001] (c-axis) direction. Its needle shape is closely related to the growth conditions including high supersaturation of nitrogen and low nitriding temperature. With the increase of temperature, the needle-like (Al,Si,Mn)N gradually transforms to AlN, and it splits along the gaps at c-axis edges into 6–8 hexagonal regular particles finally. In this process, the crystal structure and orientation stay the same. The evolution of morphology shows the sawtooth unique characteristic because the dissolution of unstable (Al,Si,Mn)N precipitate occurs simultaneously with the formation of regular AlN precipitates under the near-equilibrium condition. The splitting results in the dispersing and increasing of precipitates in steel, which is significant for studying secondary recrystallization

Phase Formation during Heating of Amorphous Nickel-Based BNi-3 for Joining of Dissimilar Cobalt-Based Superalloys.

Phase transformations and the melting range of the interlayer BNi-3 were investigated by differential scanning calorimetry, which showed three stages of crystallization during heating. There were three exothermic peaks that indicated crystallization in the solid state. The cobalt-based X-45 and FSX-414 superalloys were bonded with interlayer BNi-3 at a constant holding time of 10 min with bonding temperatures of 1010, 1050, 1100, and 1150 °C using a vacuum diffusion brazing process. Examination of microstructural changes in the base metals with light microscopy and scanning electron microscopy coupled with X-ray spectroscopy based on the energy distribution showed that increasing temperature caused a solidification mode, such that the bonding centerline at 1010 °C/10 min included a γ-solid solution, Ni3B, Ni6Si2B, and Ni3Si. The athermally solidified zone of the transient liquid phase (TLP)-bonded sample at 1050 °C/10 min involved a γ-solid solution, Ni3B, CrB, Ni6Si2B, and Ni3Si. Finally, isothermal solidification was completed within 10 min at 1150 °C. The diffusion-affected zones on both sides had three distinct zones: a coarse block precipitation zone, a fine and needle-like mixed-precipitation zone, and a needle-like precipitation zone. By increasing the bonding temperature, the diffusion-affected zone became wider and led to dissolution.