Spherical Precipitates Research Articles

A High-Resolution Population Dynamic Model for Predicting Homogeneous Precipitation Kinetics of Spherical Precipitates During Thermal Aging

A High-Resolution Population Dynamic Model for Predicting Homogeneous Precipitation Kinetics of Spherical Precipitates During Thermal Aging

Flow behavior and dynamic recrystallization of a power metallurgy nickel-based superalloy during hot compression in (γ + γ′)-phase region

Flow behavior and dynamic recrystallization of a powder metallurgy EP962NP nickel-based superalloy in the (γ + γ′)-phase region have been investigated under the isothermal compression at 1075–1150 °C with strain rate of 0.0005–0.1 s−1. The results show that predicated solvus temperature of γ’ precipitates is as high as ~1196 °C. Flow stress behavior of experimental alloy exhibits typical characteristics of dynamic recrystallization (DRX) and can be well described by hyperbolic-sine Arrhenius-type model with a higher thermal deformation activation energy of 1181.7 kJ/mol. Discontinuous dynamic recrystallization can be identified as a dominant nucleation mechanism for DRX of the alloy, as evidenced by the prevalent observation of necklace structures. In addition, the refined recrystallized γ grains can be obtained with a higher strain rate due to the increasing deformation stored energy. During hot compression, the morphology of primary γ′ transforms from micron-scale irregular blocks to nano-scale spherical precipitates via the dissolution and re-precipitation process.

Microstructure and mechanical properties of difficult to weld Rene 77 superalloy produced by laser powder bed fusion

Fabrication of γ′ precipitation strengthened nickel-based superalloys via laser powder bed fusion still remains a challenge. In this study, Rene 77, a high γ′ containing superalloy that is considered as difficult to weld, was processed by laser powder bed fusion. Crack-free parts with high density were fabricated without any compositional modifications or preheating of the built plate. This defect-free structure was maintained upon solutionizing and aging heat treatment. The microstructure of the samples has been characterized in detail following the fabrication and after the heat treatment. Scanning electron microscopy analysis revealed that the as-built microstructure consists of columnar grains mainly aligned in the <100> direction along with extremely fine γ′ precipitates and spherical cell boundary carbides. The grain structure and texture were unaffected by the applied heat treatment due to the pinning effect exerted by the carbide particles. Development of a bimodal γ′ distribution including cuboidal primary and spherical secondary precipitates was observed in the heat-treated sample. Additional carbide formation as a discontinuous grain boundary film was also seen. Tensile deformation behavior for both conditions was also tested at room temperature and 810 °C. Measured strength values for all test conditions were higher compared to a wrought and heat-treated alloy tested at room temperature. The as-built sample showed hardening and loss of ductility during elevated temperature testing due to γ’ precipitation at the test temperature. The microstructure of the heat-treated sample was not altered during testing at 810 °C. However, improved elongation behavior and transition in fracture mode from cleavage to ductile fracture were observed due to microtwin formation at elevated temperatures.

Preparation of Cu-Cr-Zr Alloy by Laser Powder Bed Fusion: Parameter Optimization, Microstructure, Mechanical and Thermal Properties for Microelectronic Applications

Laser powder bed fusion (LPBF) technology is beneficial for the fabrication of thermal conductive materials, integrating with the predesigned structure, which shows a great potential for high heat dissipation applications. Here, a Cu–Cr–Zr alloy with relative density of 98.53% is successfully prepared by LPBF after process optimization. On this basis, microstructure, phase identification, precipitates, mechanical and thermal properties are investigated. The results demonstrate that the surface morphology of microstructure is affected by laser energy density, the α-Cu is the main phase of the LPBF sample and the virgin powder, the size of Cr spherical precipitates in some areas is about 1 μm, and the tensile fracture mode is a mixed ductile–brittle mode. Furthermore, the Vickers hardness of the LPBF Cu–Cr–Zr sample is 70.7 HV to 106.1 HV, which is higher than that of LPBF Cu and a wrought C11000 Cu, and the difference in Vickers hardness of different planes reflects the anisotropy. Ultimately, the two types of Cu–Cr–Zr alloy heat sinks are successfully fabricated, and their heat transfer coefficients are positively correlated with the volume flow. The heat dissipation performance of the cylindrical micro-needle heat sink is better, and its maximum heat transfer coefficient is 3887 W/(m2·K).

Carbon transformations in rapidly solidified nickel–carbon ribbon

We report the structural transformations of carbon via the melt spinning and subsequent annealing of nickel-carbon alloys. We attained metastable solid solubility of carbon in nickel ribbon by achieving a rapid solidification rate of up to 1.6 × 106 K/s. Excess carbon atoms were found to be dissolved in the nickel lattice causing up to 1.4% strain for an alloy spun at 70 m/s tangential wheel speeds. High temperature heat treatments led to precipitation of carbon from the nickel lattice on the ribbon free surfaces but also led to growth of spherical precipitates within the nickel matrix, an effect consistent with bulk diffusion-driven Ostwald ripening. Carbon was excavated from the ribbons via chemical dissolution of the metal and characterized by electron microscopy and Raman spectroscopy. We found that the microstructure of carbon precipitated from the rapidly quenched ribbon could be tuned by varying the carbon content from 4 to 12 at. % in the precursor and annealing the ribbon at temperatures that ranged from 400 to 1200 °C. Via the step-wise variation of these two parameters, we sequentially transformed amorphous carbon nanospheres with a high BET surface area of 203 m2/g into thick, highly crystalline flakes of graphite that conformed to the shape of the as-spun ribbon.

Red mud regulates arsenic fate at acidic pH via regulating arsenopyrite bio-oxidation and S, Fe, Al, Si speciation transformation

Red mud (RM) as waste of industrial aluminum production is piling up in huge ponds. RM could be a cost-effective adsorbent for heavy metals, but adsorption is vulnerable to pH changes, metal ions speciation and the occurrence of iron bearing minerals. In this study, the precipitation and elemental speciation transformation relevant to arsenic fate in responding to the addition of RM during arsenopyrite bio-oxidation by Sulfobacillus thermosulfidooxidans was investigated. The results show that the addition of RM significantly changed the arsenic precipitation and the solution chemistry and thus affected the arsenopyrite bio-oxidation and arsenic fate. An addition of a small amount (≤ 4 g/L) of RM substantially promoted arsenopyrite bio-oxidation with formation of SiO2 @ (As, Fe, Al, Si) spherical nanoparticles that can enhance the stability of the immobilized arsenic. The SiO2-based spherical nanoparticles precipitate was mainly composed of jarosites, amorphous ferric arsenate and crystalline scorodite, and its formation were controlled by Fe3+ concentration and solution pH. An addition of increased amount of RM (≥ 6 g/L) resulted in a significant increase of the solution pH and a decrease in the Fe2+ bio-oxidation activity, and spherical nanoparticles were not formed. Consequently, the dissolution of arsenopyrite was inhibited and the release of arsenic was blocked. This study suggests the applicability of RM in mitigation of arsenic pollution from bio-oxidation of As-bearing sulfide minerals.

Simultaneously achieving excellent mechanical properties and high thermal conductivity in a high Mn-containing Mg-Zn-Ca-Al-Mn sheet alloy

Tensile properties, formability and thermal conductivity of Mg-3Zn-0.5Ca-0.5Al-xMn (x: 0.1–1.5 in wt%) sheet alloys with different Mn contents were investigated. Benefitting from dense Al-Mn nanoscale spherical precipitates, yield strength was significantly enhanced by 39 MPa with increasing the Mn content from 0.1 wt% to 1 wt%. In the meanwhile, excellent formability was maintained with Erichsen values larger than 8 mm. Further increasing the Mn content to 1.5 wt% did not contribute to strengthening due to the formation of coarse β-Mn particles. In spite of larger additive amount, thermal conductivity improved for the 1 wt% Mn added alloy compared to the 0.1 wt% Mn added alloy due to the increased amount of Al-Mn nanoscale spherical precipitates, which reduced a content of Al solute atoms. High strength (average yield strength: 180 MPa), high formability (Erichsen value: 8.5 mm) and high thermal conductivity (133 W/(m K)) were simultaneously achieved in the 1 wt% Mn added alloy.

Multicomponent Ni-rich high-entropy alloy toughened with irregular-shaped precipitates and serrated grain boundaries

A novel precipitation-strengthened multicomponent Ni-rich Ni46.23Co23Cr10Fe5Al8.5Ti4W2Mo1C0.15B0.1Zr0.02 (at.%) high-entropy alloy (HEA) was designed with strain-hardening after yielding rather than strain-softening even at the temperature of 1000°C. Due to the rapid onset of intergranular cracking, the HEA with spherical precipitates and straight grain boundaries was brittle, exhibiting an inferior tensile strength of ~220 MPa, and an insufficient uniform elongation of only ~1.9%. The serrated-grain-boundary architecture effectively overcomes this intergranular cracking issue. The resultant HEA with irregular-shaped precipitates and serrated grain boundaries showed a brittle-to-ductile transition, giving in a superior strength up to as high as ~260 MPa while maintaining a uniform elongation of ~6.5%. Such an improvement of the strength and ductility has been attributed to the enhanced resistance of the serrated grain boundaries to the intergranular crack nucleation and propagation. Our current results provide a new method for the innovative design of high-temperature structural materials with superior mechanical performances.

The effect of heat treatments on mechanical properties of M789 steel fabricated by laser powder bed fusion

The microstructural evolution during the aging heat treatment of a newly developed steel (designed to be employed in the laser powder bed fusion), known as M789, was evaluated. The electron-backscatter diffraction (EBSD) analysis suggests that the as-printed sample contains a fully martensitic phase with random crystallographic texture. The samples were then solutionized at 1000 °C for 1 h, followed by aging at a temperature range of 400–600 °C and holding times from 40 to 120 min. The maximum hardness and tensile strength were achieved at aging temperatures of 450 °C and 500 °C for 2 h, with the latter temperature generated the highest amount of Ni3Ti precipitates (η-phase) with no traces of reverted austenite. Then, the heat-treated sample underwent transmission electron microscopy (TEM) and atom probe tomography (APT) analyses, which revealed the nano-scale, plate-like and spherical precipitates with number densities of 1.11 × 1024 m−3 and 0.93 × 1024 m−3, respectively. These precipitates contributed to the increase in tensile strength, from 1019 MPa (as-printed) to 1798 MPa (heat-treated), a net increase of 779 MPa. As a consequence of enhanced strength, the elongation decreased from 16% to 9%. Generally, the results of all experiments are consistent with simulations; the errors were identified to be less than 5%.

Numerical modeling of dissolution process of spherical precipitates in alloys by differential quadrature method (DQM)

پیش بینی سینتیک انحلال رسوب در فرایند های متالورژیکی مختلف نظیر جوشکاری، عملیات همگن سازی و پیشگرم کردن آلیاژهای رسوب سخت شونده حائز اهمیت است. مسئله انحلال رسوب کروی در زمره مسائل با مرزهای متحرک است و تابحال حل تحلیلی دقیق برای آن ارایه نشده است. در تحقیق پیش رو یک مدل عددی بر مبنای روش دیفرانسیل کوادریچر برای حل مسئله انحلال رسوب با هندسه کروی در زمینه ای با ابعاد متناهی ارایه شده است. در مدل پیشنهاد شده سینتیک انحلال بصورت تابعی از کسر حجمی رسوب، غلظت عنصر آلیاژی در زمینه و رسوب، غلظت تعادلی در فصل مشترک رسوب/زمینه و نیز دمای عملیات آنیل بیان شده است. همگرایی مدل عددی ارایه شده در حل مسئله انحلال از طریق بررسی اثر اندازه گام زمانی و تعداد نقاط شبکه بر نتایج حل عددی، مورد ارزیابی قرار گرفته است. همچنین دقت مدل پیشنهادی از طریق مقایسه نتایج مدل با نتایج یک مدل تحلیلی تقریبی و نیز داده های آزمایشگاهی مورد بررسی قرار گرفته است. نتایج بدست آمده نشان می دهد که مدل عددی ارایه شده حتی با تعداد نقاط شبکه کم، نیز همگرا بوده و انطباق خوبی با داده های آزمایشگاهی مربوط به انحلال رسوب کروی حین عملیات آنیل هم دما دارد.

Role of elastic strain energy in spheroidal precipitates revisited

Improving the treatment of thermodynamic modelling of precipitation kinetics makes it necessary to account for the stored elastic and interface energies of non-spherical strengthening particles. The existing kinetic concepts available for spherical precipitates can be directly adapted to spheroidal precipitates by introducing shape factors, thus approximating shapes differing from spheres. This paper provides shape factors for the stored elastic and interface energies over a wide range of aspect ratios as well as of elastic material properties.

A New Approximate Solution for Diffusional Growth and Dissolution of Spherical Precipitates

A New Approximate Solution for Diffusional Growth and Dissolution of Spherical Precipitates

Investigation on Mechanical Properties of GH4720Li at High Strain Rates at Wider Temperature Range

In this paper, the dynamic mechanical properties of GH4720Li nickel-base alloy under a large temperature range and high and low strain rates were studied by the hot compression test. The difference of mechanical properties of GH4720Li alloy under high and low strain rates was analyzed from the perspective of microstructure. The hot compression test experimental results showed that the true stress of GH4720Li alloy decreased at a low strain rate as the trial temperature elevated. Nevertheless, it was abnormal that the true stress increased at high strain rate condition as temperature elevated. By comparing the microstructure under high and low strain rates, it was found that the precipitates under low strain conditions contained a large amount of Cr (Mo). However, the content of Cr (Mo) in the precipitates at a high strain rate decreased, while the content of Fe increased. It would be concluded that Cr (Mo) would reduce the compressive strength and plasticity of GH4720Li alloy, while Fe would increase the compressive strength and plasticity of GH4720Li alloy. In addition, under the condition of a low strain rate, the shape of Cr (Mo) precipitates obtained at 20°C was lamellar, but it was spherical at 800°C. The compressive strength of GH4720Li composites with lamellar precipitates was higher than that of spherical precipitates.

Utilisation of Titanium and Titanium Dioxide as Scaffolds for Proliferating Coral Reef

Estimated 30 percent or more of coral reefs are now in danger of extinction by coastal construction increases and global temperatures rise. Several restoration techniques such as fragmentation, forming, Biorock have been developed in the past few years. In vertebrates such as mammals, osteoblast is known to form the bones composed of hydroxyapatite. Therefore, bone substitutional devices are generally surface modified to improve the adhesion of osteoblasts on the surfaces. Titanium dioxide film is often employed as the surface material for hard tissue substitutes made of titanium and its alloys. In hard corals, on the other hand, the soft tissue covered on the skeletons made of calcium carbonate has osteoblasts as well. The purpose of this work was to investigate the potential of titanium (Ti) and titanium dioxide (TiO2) as scaffolds for proliferating coral reefs by analysing the several interfacial reactions. The rods of pure Ti were anodised in aqueous phosphoric acid at a constant voltage of 80 V. The surfaces were confirmed to be anatase type TiO2. The coral fragments were kept in contact with the rods in a lab-scale aquarium with artificial seawater for several days. The colony of polyps vigorously expanded on the surfaces. Fragments of coral were placed on pure Ti, TiO2 coated pure Ti in Petri dishes and were reared in artificial seawater. Fine spherical precipitates of calcium carbonate with aragonite structure, which is the same inorganic substance as corals, were observed radially and regularly on the surfaces of TiO2. In addition, the adherence of planula larva to the sputtered TiO2 film was observed by using a QCM (Quartz Crystal Microbalance) method. The approach and adhesion of planula larva to the surface could be detected by monitoring the resonance frequency and resistance. The surfaces might have a great potential in coral reef regenerations.

Heterogeneous precipitates facilitate excellent mechanical properties in non-equiatomic medium-entropy alloy

In this work, a novel non-equiamotic Ni47.5Cr23.75Co23.75Ti5 medium-entropy alloy strengthened by heterogeneous precipitates was successfully fabricated. Our results demonstrate heterogeneous precipitation behaviors, i.e. nanosized spherical particles with L12 superlattice in inner grains plus nano-lamellar precipitates with long-period stacking ordered structure nearby grain boundaries, could bring out outstanding strength-ductility synergy properties. This heterogeneous precipitation-strengthened medium-entropy alloy exhibits high yield strength and ultimate tensile strength of ~840 MPa and ~1220 MPa, respectively, and remains an excellent ductility of ~34.8%. Further analyses indicate this heterogeneous precipitation result in effective precipitation-strengthening enhancement (~603 MPa), stacking-faults activated in spherical precipitates region and deformation nanotwins induced in lamellar precipitates region synergy contribute to high work-hardening capacity and ductility. Our work provides not only a useful insight of precipitation behaviors caused by Ti element addition in non-equiatomic high/medium-entropy alloys, but also a further understanding of precipitation-strengthening in high/medium-entropy alloys.

Effects of aluminum sulfate soaking pretreatment on dimensional stability and thermostability of heat-treated wood

Acidic aluminum sulfate hydrolysis solutions can be used to catalyze the thermal degradation of wood in a mild temperature environment, and thus reduce the temperature required for heat treatment process. To improve the dimensional and thermal stability of Chinese fir during heat treatment at 120 °C, 140 °C and 160 °C, this study investigated the effects of soaking pretreatment with 5%, 10% and 15% aluminum sulfate on the chemical and structural changes of the heat-treated Chinese fir. The results indicated that the samples treated at 15% aluminum sulfate concentration and 160 °C heat treatment achieved the best dimensional and thermal stability. Chemical analyses by Fourier transform infrared spectroscopy (FTIR) and X-ray diffraction (XRD) indicated that the catalysis of aluminum sulfate resulted in degradation of hemicelluloses during the heat treatment, and an increase in the soaking concentration and heat treatment temperature also affected the thermal degradation of celluloses. The scanning electron microscope (SEM) and mass changes test results proved that the hydrolyzed aluminum flocs mainly adhered to the inner wall of the wood tracheid as spherical precipitates, and when the soaking concentration reached 10% and 15%, a uniform soaking effect could be achieved. The thermogravimetric (TG) analysis revealed the soaking pretreatment effectively improved the thermal stability of the heat-treated wood by physically wrapping and promoting the formation of a carbon layer on the wood surface during heat treatment. Thus, aluminum sulfate soaking pretreatment exerted a great effect on the dimensional and thermal stability of wood, allowing heat treatment to be performed at a lower temperature.

Additive manufacturing of the ferritic stainless steel SS441

Abstract In this study, the ferritic stainless steel SS441 was produced with excellent mechanical properties using laser powder bed fusion (L-PBF) compared to samples produced by conventional casting and hot-rolling. In addition, thermodynamic calculations were utilized to study the phase stability at elevated temperatures and to understand the solidification behavior. The hot-rolled sample showed a grain size up to several hundred μm with additional precipitates of TiN and Nb(C,N). In contrast, the as-built L-PBF samples displayed a grain size in the μm range. Spherical precipitates with a size of around 50 nm could be observed and were attributed to a corundum phase from the thermodynamic calculations. The printed material shows superior mechanical properties, with more than 30 times higher impact energy compared to the hot-rolled alloy (217 ± 5 J vs. 7 ± 0.5 J). Furthermore, the properties are anisotropic for the L-PBF produced alloy, with the highest tensile strength vertical to the build direction. The superior mechanical properties of the L-PBF produced sample can be attributed to a smaller grain size, giving a higher strength according to the Hall-Petch relationship. The anisotropy of the material can be eliminated by heat treatments at 900 °C followed by water quenching, but the absolute strength decreases slightly due to formation of intermetallic phases such as Nb(C,N) and the Fe2Nb Laves phase. The results clearly illustrates that L-PBF provides a promising manufacturing route for enhanced strength of ferritic stainless steels.

Discrete stochastic model of point defect-dislocation interaction for simulating dislocation climb

Dislocation climb is an important high temperature process in metals plasticity, responsible for the phenomena such as creep, swelling, or hardening. Climb is defined by the ability of dislocations to leave their original glide plane by interacting with point defects. As such, dislocation climb is controlled by point defect diffusion/absorption/emission, all of which involve thermal activation. The existing thermodynamically consistent models for climb are generally formulated in a continuum framework, through the definition of effective defect fluxes and climb propensities in response to thermodynamic driving forces. However, the point-wise discrete nature of vacancies (and/or self-interstitials) confers a highly discrete nature to the climb dynamics, which is also strongly affected by elastic forces. The combination of discreteness, thermal activation, and elasticity makes this process too challenging for direct atomistic methods such as molecular dynamics. Here we develop a kinetic Monte Carlo model that captures vacancy generation and transport kinetics acting in conjuction with the evolving elastic fields provided by discrete dislocation dynamics simulations. The two models are coupled via the applied stresses and stress gradients generated by dislocation structures at vacancy locations. Our simulations reveal two surprising results. First that climb is dominated by vacancy emission even when the background vacancy concentration is much higher than the equilibrium one. And, second, that climb velocities might be much faster than otherwise believed when one uses the classical theories of climb. These effects are due to the locality of vacancy-dislocation processes, which are not captured in classical treatments that assume smooth vacancy fluxes and homogeneous concentrations. We apply the method to study elementary climb processes in body-centered cubic iron and furnish climb mobility functions to be used in parametric dislocation dynamics and/or crystal plasticity simulations. We apply the technique to study non-conservative plastic bypass of spherical precipitates by edge dislocations and point out the differences between our discrete approach and existing continuum formulations.

Stress orientation dependence for the propagation of stacking faults and superlattice stacking faults in nickel-based superalloys

Superlattice stacking fault propagation dominates the creep deformation behaviour of nickel-based superalloys at intermediate temperatures. These planar defects may appear under many different configurations depending on the dislocation arrangements and their interactions with the precipitates. Whilst these have been spotted and described before, no systematic way to explain their configurations has been provided. The current study quantifies the types of faults in multiple grains within a tensile crept polycrystalline alloy via a combination of scanning transmission electron microscopy and electron backscatter diffraction. A new defect consisting of a superlattice intrinsic stacking fault in the precipitates and an extrinsic stacking fault in the matrix is observed and a mechanism for its formation is proposed. In combination with data from the literature on single crystals, the results are incorporated into a robust framework to discern the orientation dependencies of these faults. A comprehensive analytical model based on a series of one-dimensional force balances on different dislocation configurations is developed first for the case of athermal stacking fault propagation for the cases of cuboidal and spherical precipitates. The model is then extended to include six configurations of superlattice faults and microtwinning. This results in novel mechanistic maps that account for stress, orientation and microstructure, with excellent qualitative agreement with experiments.

Plastic yielding and deformation bursts in the presence of disorder from coherent precipitates

Alloying metals with other elements is often done to improve the material strength or hardness. A key microscopic mechanism is precipitation hardening, where precipitates impede dislocation motion, but the role of such obstacles in determining the nature of collective dislocation dynamics remains to be understood. Here, three-dimensional discrete dislocation dynamics simulations of FCC single crystals are performed with fully coherent spherical precipitates from zero precipitate density upto $\rho_p = 10^{21}\,\text{m}^{-3}$ and at various dislocation-precipitate interaction strengths. When the dislocation-precipitate interactions do not play a major role the yielding is qualitatively as for pure crystals, i.e., dominated by "dislocation jamming", resulting in glassy dislocation dynamics exhibiting critical features at any stress value. We demonstrate that increasing the precipitate density and/or the dislocation-precipitate interaction strength creates a true yield or dislocation assembly depinning transition, with a critical yield stress. This is clearly visible in the statistics of dislocation avalanches observed when quasistatically ramping up the external stress, and is also manifested in the response of the system to constant applied stresses. The scaling of the yielding with precipitates is discussed in terms of the Bacon-Kocks-Scattergood relation.