High corrosion resistance duplex fcc + hcp cobalt based entropic alloys: An experimental and theoretical investigation

A series of duplex fcc + hcp Co-based entropic alloys are being discovered as a new category of entropic alloys with outstanding mechanical properties, especially to overcome a typical mechanical trade-off between strength and ductility. In this work, CALPHAD-based (CALculation of PHAse Diagram) thermodynamic calculations were performed to facilitate alloy design and to understand corrosion behaviors. The kinetics of the electrochemical corrosion for designed alloys in typical aggressive anion Cl- was investigated by electrochemical tests, including open circuit potential (OCP), polarization and cyclic polarization curves, and electrochemical impedance spectroscopy (EIS). The valence state and the surface morphologies of the passive films were investigated by X-ray photoelectron spectroscopy (XPS) and atomic force microscope (AFM). High corrosion resistance materials with high strength and ductility performances were discovered in the present work. Except for Ni-oxides, various spinel compounds and many other oxides including Co2O3, Cr2O3, Fe2O3, MnO, MoO3, CoCr2O4, FeCr2O4, CoFe2O4, and CoMoO4 were observed in the passive films. The adsorbed and penetrated corrosive anion Cl- will be prone to breakdown the passive films with less Cr2O3, CoCr2O4 and MoO3 to form pitting corrosion (also include other localized corrosion, such as intergranular corrosion and crevice corrosion). The microstructure of the hcp martensite with the fcc matrix has played an important role in the propagation of the localized anodic dissolution in the form of cleavage and quasi-cleavage. The theoretical calculations are in good agreement with the experimental observations. This paper paves a way for the future development of high-performance Co-based entropic alloys served in some harsh environments

Microstructure and Inclusion Characteristics in Steels with Ti-oxide and TiN Additions

Non-metallic inclusions in steels are generally considered to be detrimental for mechanical properties. However, it has been recognized that certain inclusions, such as Ti-oxide and TiN, can serve as potent nucleation sites for the formation of intragranular ferrite (IGF) in low-alloy steels. The formation of IGF could improve the toughness of the coarse grained heat affected zone (CGHAZ) of weld metals. Thus, the present thesis mainly focuses on the effect of size of nucleation sites on the IGF formation. Quantitative studies on the composition, size distribution and nucleation probability for each size of the inclusions as well as the area fraction, starting temperature and morphology of an IGF have been carried out. In the present work, the Ti-oxide and TiN powders were mixed with metallic powders. The mixed powders were heated up to the liquid state and cooled with a slow cooling rate of 3.6 ºC/min. These as-cast steels with Ti-oxide and TiN additions were used to simulate the IGF formation in the CGHAZ of weld metals. Specifically, the inclusion and microstructure characteristics in as-cast steels have been investigated. The results show that the nucleant inclusion was identified as a TiOx+MnS phase in steels with Ti2O3 additions and as a TiN+Mn-Al-Si-Ti-O+MnS phase in steels with TiN additions. In addition, the TiOx and TiN phases are detected to be the effective nucleation sites for IGF formation. It is clearly shown that an increased inclusion size leads to an increased probability of IGF nucleation. This probability of IGF nucleation for each inclusion size of the TiOx+MnS inclusions is clearly higher than that of the complex TiN+Mn-Al-Si-Ti-O+MnS inclusions. In addition, the area fraction of IGF in the steels with Ti2O3 additions is larger than that of the steels with TiN additions. This result agrees with the predicted tendency of the probability of IGF nucleation for each inclusion size in the steels with Ti2O3 and TiN additions. In order to predict the effective inclusion size for IGF formation, the critical diameters of the TiO, TiN and VN inclusions, which acted as the nucleation sites of IGF formation, were also calculated based on the classical nucleation theory. The critical diameters of TiO, TiN and VN inclusions for IGF formation were found to be 0.192, 0.355 and 0.810 μm in the present steels. The calculation results were found to be in agreement with the experiment data of an effective inclusion size. Moreover, the effects of the S, Mn and C contents on the critical diameters of inclusions were also calculated. It was found that the critical diameter of the TiO, TiN and VN inclusions increases with an increased content of Mn or C. However, the S content doesn’t have a direct effect on the critical diameter of the inclusions for IGF formation. The probability of IGF nucleation for each inclusion size slightly decreases in the steel containing a higher S content. This fact is due to that an increased amount of MnS precipitation covers the nucleant inclusion surface. In the as-cast experiment, it was noted that an IGF can be formed in steels with Ti2O3 and TiN additions with a cooling rate of 3.6 ºC/min. In order to control the microstructure characteristics, such as the area fraction and the morphology of an IGF, and to investigate the starting temperature of IGF and grain boundary ferrite (GBF) formation, the dynamic transformation behavior of IGF and GBF was studied in-situ by a high temperature confocal laser scanning microscope (CLSM). Furthermore, the chemical compositions of the inclusions and the morphology of IGF after the in-situ observations were investigated by using scanning electron microscopy (SEM), electron backscatter diffraction (EBSD) and electron probe microanalysis (EPMA) which equipped wavelength dispersive spectrometer (WDS). The results show that the area fraction of IGF is larger in the steels with Ti2O3 additions compared to the steels with TiN additions, after the same thermal cycle has been imposed. This is due to that the TiOx phase provides more potent nucleation sites for IGF than the TiN phase does. Also, the area fraction of IGF in the steels is highest after at an intermediate cooling rate of 70 ºC/min, since the competing phase transformations are avoided. This fact has been detected by using a hybrid methodology in combination with CLSM and differential scanning calorimetry (DSC). In addition, it is noted that the morphology of an IGF is refined with an increased cooling rate.QC 20150325</p

Uncovering the generic and alloy-specific governing parameters of deformation-induced martensitic transformation in austenitic steel

In this work, a hybrid modeling approach, combining machine learning (ML)and computational thermodynamics, has been applied to predict deformationinduced martensitic transformation (DIMT) and explore the generic and alloyspecifc parameters governing DIMT in austenitic steels. The DIMT model wasestablished based on the ensemble ML algorithms and a comprehensive set ofphysical variables. The developed model is highly generalizable as validated onunseen alloys. The generic governing parameters of DIMT are in good agreementwith previous studies in the literature. However, the evaluated alloy-specifcgoverning parameters reveal large diferences between grades, e.g., 204 series ofaustenitic stainless steels has a quite balanced correlation between strain, stress,temperature, and DIMT, while the 301 series has much stronger correlationbetween stress and DIMT. The fndings in the current study emphasize theimportance that a general DIMT model for steels should include both stress andstrain, as well as other governing parameters, since DIMT can be both stressassisted and strain-induced transformation, and often the effect of appliedmechanical driving force and the formation of new nucleation sites interact.Validerad;2024;Nivå 2;2024-04-09 (hanlid);Funder: EIT Raw Material (18317); China Scholarship Council (CSC 202006080092);Full text license: CC BY 4.0ENDUREI

In-Situ High-Energy X-ray Diffraction Study of Austenite Decomposition During Rapid Cooling and Isothermal Holding in Two HSLA Steels

In-situ high-energy X-ray diffraction experiments with high temporal resolution during rapid cooling (280 °C s$^{−1}$) and isothermal heat treatments (at 450 °C, 500 °C, and 550 °C for 30 minutes) were performed to study austenite decomposition in two commercial high-strength low-alloy steels. The rapid phase transformations occurring in these types of steels are investigated for the first time in-situ, aiding a detailed analysis of the austenite decomposition kinetics. For the low hardenability steel with main composition Fe-0.08C-1.7Mn-0.403Si-0.303Cr in weight percent, austenite decomposition to polygonal ferrite and bainite occurs already during the initial cooling. However, for the high hardenability steel with main composition Fe-0.08C-1.79Mn-0.182Si-0.757Cr-0.094Mo in weight percent, the austenite decomposition kinetics is retarded, chiefly by the Mo addition, and therefore mainly bainitic transformation occurs during isothermal holding; the bainitic transformation rate at the isothermal holding is clearly enhanced by lowered temperature from 550 °C to 500 °C and 450 °C. During prolonged isothermal holding, carbide formation leads to decreased austenite carbon content and promotes continued bainitic ferrite formation. Moreover, at prolonged isothermal holding at higher temperatures some degenerate pearlite form

Solar selective reflector materials: Another option for enhancing the efficiency of the high-temperature solar receivers/reactors

The cavity wall is an important part of a cavity receiver in determining the receiver efficiency. Using solar selective reflector (SSR) materials with low solar absorptivity and high thermal emissivity for the cavity wall design is one efficient way to improve the receiver efficiency. In this work, we present a systematic study of the optical and high-temperature stability performances of six different SSR materials: one refractory ceramic fiber-based substrate material (Fiberfrax 140) and five metallic oxide coatings which are prepared by mixing metallic oxide powders of alumina, magnesium oxide and titanium dioxide with commercial inorganic adhesives. The thermal stability was studied by heating up and keeping the six candidate materials in atmospheric conditions at a temperature of 1250 ◦C for 200 h. The spectrum of hemispherical reflectance in the spectrum band 0.25–25 μmwas measured for analyzing the optical performance of the candidate materials. The obtained results show that all the six materials studied have good solar selective reflection characteristics, i.e, low solar absorptivity and relatively high thermal emissivity. Especially, the alumina-coated substrate material shows excellent performances both for thermal stability and solar selective reflection. The solar reflectivity can reach 94.6%.QC 20210623</p

Effect of Trace Magnesium Additions on the Dynamic Recrystallization in Cast Alloy 825 after One-Hit Hot-Deformation

Alloy 825 is widely used in several industries, but its useful service life is limited by both mechanical properties and corrosion resistance. The current work explores the effect of the addition of magnesium on the recrystallization and mechanical behavior of alloy 825 under hot compression. Compression tests were performed under conditions representative of typical forming processes: temperatures between 1100 and 1250 °C and at strain rates of 0.1–10 s−1 to a true strain of 0.7. Microstructural evolution was characterized by electron backscattered diffraction. Dynamic recrystallization was found to be more prevalent under all test conditions in samples containing magnesium, but not in all cases of conventional alloy 825. The texture direction ⟨101⟩ was the dominant orientation parallel to the longitudinal direction of casting (also the direction in which the samples were compressed) in samples that contained magnesium under all test conditions, but not in any sample that did not contain magnesium. For all deformation conditions, the peak stress was approximately 10% lower in material with the addition of magnesium. Furthermore, the differences in the peak strain between different temperatures are approximately 85% smaller if magnesium is present. The average activation energy for hot deformation was calculated to be 430 kJ mol−1 with the addition of magnesium and 450 kJ mol−1 without magnesium. The average size of dynamically recrystallized grains in both alloys showed a power law relation with the Zener–Hollomon parameter, DD~Z−n, and the exponent of value, n, is found to be 0.12. These results can be used to design optimized compositions and thermomechanical treatments of alloy 825 to maximize the useful service life under current service conditions. No experiments were conducted to investigate the effects of such changes on the service life and such experiments should now be performed.QC 20210517</p

Effect of ferrochromium (FeCr) and ferroniobium (FeNb) alloys on inclusion and precipitate characteristics in austenitic stainless steels

Lab-scale alloying experiments were carried out by first adding commercial low-carbon ferrochrome (LCFeCr) alloys and then adding ferroniobium (FeNb) alloys in 316-grade austenitic stainless steel in this study. The inclusion and precipitation characteristics in LCFeCr and FeNb were evaluated as well as in a 316 austenitic stainless steel after the alloy additions by using two- and three-dimensional characterization methods in combination with thermodynamic calculations. The results showed that MnCr2O4 spinels and pure Al2O3 were the main types of inclusions in LCFeCr alloys, while pure TiOx, Al2O3 inclusions and complex TiOx-Al2O3 aggregates were mainly found in FeNb alloys. After the addition of LCFeCr alloy to the steel, the SiO2 contents in liquid inclusions decreased to some extent, while more inclusions containing higher MnO contents were observed. Some MnCr2O4 spinel inclusions can be reduced by Si in steel and form liquid inclusions. Some MnCr2O4 spinel and Al2O3 inclusions from LCFeCr alloy can remain in the steel melt, which decreased the steel cleanliness. After the addition of FeNb alloy, pure TiOx inclusions present in this alloy can hardly be found in the steel melt. The inclusion types in steel were not changed so much but high Nb-containing phases were found around the inclusions and coarse Laves phases were formed in the matrix. Overall, this work aims to understand the impurity particle behavior during the alloying process when using ferroalloys to produce high-performance stainless steels

A machine learning model to predict the pyrolytic kinetics of different types of feedstocks

An in-depth knowledge of pyrolytic kinetics is vital for understanding the thermal decomposition process. Numerous experimental studies have investigated the kinetic performance of the pyrolysis of different raw materials. An accurate prediction of pyrolysis kinetics could substantially reduce the efforts of researchers and decrease the cost of experiments. In this work, a model to predict the mean values of model-free activation energies of pyrolysis for five types of feedstocks was successfully constructed using the random forest machine learning method. The coefficient of determination of the fitting result reached a value as high as 0.9964, which indicates significant potential for making a quick initial pyrolytic kinetic estimation using machine learning methods. Specifically, from the results of a partial dependence analysis of the lignocellulose-type feedstock, the atomic ratios of H/C and O/C were found to have negative correlations with the pyrolytic activation energies. However, the effect of the ash content on the activation energy strongly depended on the organic component species present in the lignocellulose feedstocks. This work confirms the possibility of predicting model-free pyrolytic activation energies by utilizing machine learning methods, which can improve the efficiency and understanding of the kinetic analysis of pyrolysis for biomass and fossil investigations

Hydrothermal carbonization coupled with pyrolysis: An innovative approach to digestate management

Anaerobic digestion presents a viable approach for managing biodegradable waste. However, the process generates a significant amount of digestate, which, if not appropriately managed, can contribute to eutrophication and salinization. This research explores the integration of hydrothermal carbonization (HTC) and pyrolysis as a potential solution for digestate management, emphasizing volume reduction and energy recovery. The study specifically focuses on the production of biochar from agricultural waste digestate (AWD) in an energy-efficient manner. Utilizing HTC as a pretreatment was found to enhance the heating value of both the AWD and the subsequent pyrolytic char, and it also improved the carbonization degree of the resulting char. Energy balance analysis revealed that the pyrolysis of AWD shifted from an endothermic to an exothermic reaction upon HTC pretreatment. This transition decreased the reaction's energy absorption from 41.1 to 1557.6 MJ to an energy release of 685.1 to 960.6 MJ per ton of digestate, thereby optimizing energy consumption in digestate management. Additionally, it was noted that using acetone as a solvent in gas chromatography for biooil samples with high ammonia content led to the formation of diacetonamine, an outcome that is deemed undesirable