Segregation Of Alloying Elements Research Articles

Effect of alloying element segregation on theoretical strength and structural low-energy transition of Ni grain boundaries: A first-principles computational tensile test study

Enhancing the stability and strength of nanocrystalline metals through the manipulation of grain boundary (GB) chemistry and structure is crucial for their engineering applications. In this study, first-principle computational tensile tests on Ni Σ5 [001] (210) GBs were conducted with considering Poisson's ratio and strengthening effect of alloying elements B, W, Zr, Ta, B-W and Zr-Ta segregation. The results indicate that the Zr-Ta co-segregated GB shows a remarkable increase in theoretical tensile strength by 195 % compared to the clean GB under small strain conditions. As tensile strain increases, both clean and segregated GBs undergo a low-energy structural transition from the Σ5 [001] (210) to the more stable Σ11 [110] (113). This work presents a promising approach for enhancing the stability of Ni nanograined materials through tensile deformation and GB segregation.

The influence of alloying elements segregation on heterogeneous nucleation of TiN on Al2O3 and its local corrosion resistance

A comprehensive study utilizing first-principles calculations has examined the heterogeneous nucleation of TiN on Al2O3 in steel and the impact of alloying elements on this process. Additionally, the study delved into how this nucleation affects local corrosion activity. The findings indicate that alloying elements such as Cr, Mn, Mo, and V tend to concentrate at the Al2O3 (0001)/TiN (001) interface, enhancing the interface's stability, thereby favoring the heterogeneous nucleation of TiN on Al2O3. Consequently, the nucleation leads to the formation of interfaces, namely Al2O3 (0001)/TiN (001) and TiN (100)/bcc-Fe (100), which are less prone to adsorbing chloride ions, replacing the Al2O3(0001)/bcc-Fe (110) interface that can stably adsorb chloride ions, consequently reducing the adsorption of chloride ions at grain boundaries. Moreover, it is difficult for Cr atoms to incorporate into the TiN structure, while Ti atoms readily replace Cr in CrN. TiN exhibits a lower formation energy compared to CrN, and the precipitation of TiN mitigates the formation of Cr-depleted zones, thereby inhibiting local corrosion.

First-principles modeling of corrosion current for passive magnesium alloys and its application in Mg-RE alloy

Theoretical calculation could simulate Mg alloy corrosion behavior from electrochemical thermodynamic factors. By considering the kinetic factor as the passivation oxide layer, a comprehensive corrosion current model has been set up in this work. The corrosion current modeling should belgJtot′=lgJa+∆lgJc+km×∑fm×∆Em+ki×∑(fi×∆Ei)−koxCPBR×Winterface−∆Gs−∆GF(ln10)RT)). The formula as koxCPBR×Winterface−∆Gs−∆GF(ln10)RT) is just for passivation factor. Alloying element segregation energy ∆Gs, oxide formation energy ∆GF, oxide PBR value CPBR, interface cleavage work Winterface have all been considered. By this more considerate modeling, the corrosion current lgJtot′ (related to polarization icorr) could be obtained for passivate Mg alloys such as Mg-RE series (RE =Sc, Y, La, Ce, Pr, Nd, Sm, Eu, Gd, Tb, Dy, Ho, Er, Tm, Yb, Lu). Moreover, the prediction is quite coherent to experimental electrochemical polarization result. The work provides a more precisely prediction for Mg alloy with passivation. It also proposes a passivation criterion for seeking corrosion-resistant Mg alloy in another way.

Effect of Secondary Phase on Passivation Layer of Super Duplex Stainless Steel UNS S 32750: Advanced Safety of Li-Ion Battery Case Materials.

Aluminum, traditionally the primary material for battery casings, is increasingly being replaced by UNS S 30400 for enhanced safety. UNS S 30400 offers superior strength and corrosion resistance compared to aluminum; however, it undergoes a phase transformation owing to stress during processing and a lower high-temperature strength. Duplex stainless steel UNS S 32750, consisting of both austenite and ferrite phases, exhibits excellent strength and corrosion resistance. However, it also precipitates secondary phases at high temperatures, which are known to form through the segregation of Cr and Mo. Various studies have investigated the corrosion resistance of UNS S 32750; however, discrepancies exist regarding the formation and thickness of the passivation layer. This study analyzed the oxygen layer on the surface of UNS S 32750 after secondary-phase precipitation. The microstructure, volume fraction, chemical composition, and depth of O after the precipitation of the secondary phases in UNS S 32750 was examined using FE-SEM, EDS, EPMA and XRD, and the surface chemical composition and passivation layer thickness were analyzed using electron probe microanalysis and glow-discharge spectroscopy. This study demonstrated the segregation of alloy elements and a reduction in the passivation-layer thickness after precipitation from 25 μm to 20 μm. The findings of the analysis aid in elucidating the impact of secondary-phase precipitation on the passivation layer.

Microstructure development of quenching and partitioning-processed martensitic stainless steels with different manganese content

This work presents an investigation of the microstructure development during the application of the quenching and partitioning (Q&P) process to two stainless steels with different Mn content. The results are compared with calculations based on the constrained carbon equilibrium theory, paying special attention to the presence of reactions competing for the carbon available for partitioning and to the effect of alloying element segregation. Results show that chromium carbides must be considered when accounting for the carbon available for austenite stabilisation. Moreover, manganese/chromium segregation bands play an important role in the microstructure development, particularly in martensite formation, with important consequences in the microstructure development during the following processing steps.

Segregation behavior of alloying elements at the fcc-Fe/TiC interface by first principles exploration

In this paper, the effects of rare earth elements on the bonding strength and stability of TiC/fcc-Fe interface are explored by using the first-principles method based on density functional theory. The results show that the Ti terminal is more stable than the C terminal in the process of forming the interface. The alloying elements tend to segregate at position 2 on the side of fcc-Fe. The segregation of Mo, Nb, Cr and Ce alloying elements increases the interatomic electron cloud enrichment and consumption between the interfaces and enhances the Fe–Ti interactions. The d orbitals of Mo, Nb, Cr and Ce and f orbitals of Ce have strong hybridization with Fe-d orbitals and Ti-d orbitals electrons near the Fermi energy level, indicating an increase in bonding strength and stability of the interfaces. When Fe atoms are replaced by W, Ni and Al atoms, the covalent bond strength between interfacial atoms is reduced, thus weakening the interfacial bonding strength. This provides solid theoretical foundation with regard to further application in austenitic heat-resistant steel fields.

Study on modification of primary carbides and improvement of properties in H13 steel by CeO2 nanoparticles addition

The large primary carbides in H13 steel significantly affect its service life. In this study, pre-dispersion technology is used to disperse CeO2 nanoparticles, which are then introduced into H13 steel. The effects of CeO2 nanoparticles on the inclusions, primary carbides, microstructures, and properties of H13 steel are investigated. Ce-containing inclusions with size of 0.5–2 μm are generated and distributed uniformly in H13 steel owing to the reduction of CeO2 by Al and C. Both the dendrite structure and primary carbides in H13 steel are refined by CeO2 nanoparticles. The secondary dendrite arm spacing decreases from 13.51 ± 2.34 to 6.90 ± 1.23 μm. The size and number of primary carbides decrease from 1 to 5 to 1–3 μm, and from 1125 to 725/mm2, respectively. This is attributable to the disregistry of Ce-containing inclusions and primary carbides/austenite. The segregation of alloy elements in H13 steel is calculated using the Clyne–Kurz model. The properties of H13 steel are improved via the addition of CeO2; in particular, its tensile strength, total elongation rate, and hardness are 726.38 MPa, 23.43%, and 243.49, respectively, which corresponds to increases by approximately 5.58%, 8.27%, and 10.11%, respectively. The results can facilitate improvements to the performance of H13 steel and the development of rare earth steel.

Evolution of microstructure, microhardness, crystallographic texture, and grain size variation in dissimilar joints used in concentrated solar power plants application

High carbon stainless steel 347H (SS347H) and Inconel 625 (IN625) are high-strength alloys, commonly used in high-temperature applications. These metals are commonly used in the fabrication of concentrated solar power energy storage tanks and pipelines. Construction of such tanks involves dissimilar welding as the primary manufacturing process. The critical factor to consider when joining two different materials is the development of microstructure, as it directly impacts the mechanical characteristics of the joints. According to the available literature, an increase in the heat input (HI) promotes alloying element segregation and susceptibility to weld cracking. Traditional arc welding processes, such as metal inert gas (MIG) welding, produce a higher HI. Therefore, there is a need to develop welding techniques that result in reduced HI. In the present study, a pulse-current MIG (PC-MIG) welding method was employed, utilizing current pulsing to minimize HI during the welding process. The welding was followed by metallographic and mechanical characterization. The microstructural examination found variations in microstructure at different regions. Electron backscatter diffraction analysis shows the crystallographic orientation at different regions. Analysis shows that both SS347H and IN625 have a face-centred cubic austenite structure. The inverse pole figure discloses strong texture formation. The average tensile strength of dissimilar weldments was found to be 479 MPa, while the average hardness value of the fusion zone was measured to be 235 HV. The research indicates that the PC-MIG technique is suitable for joining dissimilar materials with reduced HI, effectively eliminating the risk of cracking and enhancing the efficiency of the joint.

Estimation of the segregation in a high carbon cast steel by thermoelectric power means

This research study proposes the hot-cold tip thermoelectric power (TEP) method to estimate alloying elements segregation and the microstructure variation of high carbon steel cast in a mold. Optical Emission Spectrometry (OES) showed a higher concentration of carbon, nickel, chromium and aluminum at the ingot center. That elemental saturation produced an increase in perlite content as well as hardness. The non-destructive technique of thermoelectric power was applied varying the hot tip temperature (40 oC, 50 oC, 60 oC, 70 oC), where higher temperature values showed to be more sensitive to segregation and microstructural changes. The statistical analysis showed that the thermoelectric power technique is more sensitive to detect the nickel and chromium concentration changes.

Segregation of alloying elements at the TiC/V interface: A first-principles study

For precipitation/matrix coherent interface, the interfacial segregation behavior of solute atom plays a vital role in controlling the effectiveness of precipitation strengthening of the alloy. In this study, first-principles calculations based on density functional theory are used to figure out the interfacial structure of precipitates/vanadium and explain how solute atoms segregate at the interface and affect the interfacial strengthening. Firstly, according to the Baker-Nutting orientation relationship, the equilibrium interface structures are obtained. Meanwhiles, the segregation behavior of solute atoms at the different interfaces in vanadium alloys was studied. It is found that elements Sc, Ti, Y, Zr, Hf, and Ta, in the TiC/V(100) interface, tend to segregate at the interface, while other ones are not easy to segregate. The alloy elements show the same segregation trend as the TiC/V(100) interface, except for Nb, which also tends to segregate at the TiC/V(110) interface. According to the calculation, it is found that the alloying element with the larger atomic size and Voronoi volume is more likely to segregate into the interface, indicating that the atomic size effect dominates alloying element segregation at the two interfaces. The findings of this work can be used to develop theoretical approaches for future alloy designs by controlling alloying element doping.

The enhanced Zn and Ca co-segregation and mechanical properties of Mg–Zn–Ce alloy with micro Ca addition

The micro addition of Ca is demonstrated to noticeably increase the strength and ductility of Mg–Zn–Ce alloy, which is closely correlated with the enhanced alloying segregation at grain boundary. Addition of Ca is found to yield the enhanced Zn segregation and co-segregation of Zn, Ce, and Ca. The mechanism behind is that Ca decreases the segregation energy while promoting the diffusion process of Zn, ultimately accelerating the thermodynamic and kinetic process of segregation. The enhanced alloying segregation induced by Ca effectively stabilizes grain boundary, which contributes to the refined grain size, retardant mobility of dislocation and ultimately improved strength. Furthermore, the intergranular fracture energy can be noticeably increased by Ca owing to the promoted alloying element segregation at grain boundary, which contributes to the improved grain boundary cohesion and ductility. The controllable segregation at grain boundary via alloying is intended to contribute to the development of advanced magnesium alloys.

Microstructure and Mechanical Properties of Combined GTAW and SMAW Dissimilar Welded Joints between Inconel 718 and 304L Austenitic Stainless Steel

A dissimilar welded joint of Inconel 718 and 304L austenitic stainless steel was prepared using a combined procedure with the gas tungsten arc welding and shielded metal arc welding processes by employing the Ni-based fillers: ERNiCr-3 and ENiCrFe-3. The welded joints were investigated for metallographic testing and mechanical properties, and a relationship was established between the microstructure and the resultant mechanical properties. Microstructural observation revealed the formation of the unmixed zone on the 304L SS side. The weld metal showed that the fully austenitic microstructure consisted of the Nb- and Ti-rich carbide phases along the inter-dendritic spaces. The tensile test results at room temperature showed the failure from the weld metal which might be due to alloying element segregation along the inter-dendritic spaces. However, a tensile test at 600 °C showed the failure from the 304L SS base metal with a tensile strength and % elongation value of 377 MPa and 24%, respectively. The hardness plot showed the average hardness value of the weld metal of 236 ± 5 HV, which was higher than the 304L SS BM (204 ± 4 HV) but lower than the IN718 BM (243 ± 5 HV). The impact toughness of the weld metal was 109 J, which was significantly lower than the base metals. The poor impact strength of the weld metal might be due to the evolution of the NbC phase along inter-dendritic spaces.

Monitoring the effect of alloying elements segregation in Fe Mn Ni Al high Entropy alloy

Iron based high entropy alloy containing aluminum is facing a challenge in the liquid-solid transition regime due to its high affinity to segregation. In this research, the segregation of alloying elements in the designed high entropy alloy has been tracked after pouring in special investment mold that possesses different modulus with fluctuated solidification time. Different compositions of the targeted alloy were developed through melting of pure metals in induction furnace. X-ray Fluorescence (XRF), optical microscope, and SEM have been used for monitoring the chemical, and phase constituents among the different regions of solidification. In addition, the mechanical properties of the designed alloy have been determined using uni-axis tensile test, and impact toughness. It was well proved that the solidification time has a significant effect on promoting the characteristic properties of iron based high entropy alloy containing aluminum. Aluminum shows high percentage with increasing the solidification time. In addition, the mechanical observations proved that decreasing the solidification time enhance and improve the mechanical properties.

Influence of alloying element segregation at grain boundary on the microstructure and mechanical properties of Mg-Zn alloy

The role of alloying element segregation at grain boundary on the microstructure and mechanical properties is investigated in the case of Ca, Ce, and Nd addition in Mg-1Zn (wt.%) alloy. Segregation of alloying elements at grain boundary is demonstrated in all the designed ternary alloys, leading to a strong solute drag effect on the mobility of grain boundaries and ultimately the grain size. While the co-segregation of Zn and Ce is revealed in the Mg-Zn-Ce alloy, only the segregation of Zn is observed in the Mg-Zn-Ca alloy. The co-segregation of Zn and Nd in Mg-Zn-Nd alloy is demonstrated to lead to the relatively large solute drag pressure at grain boundary, which in combination with the Zener pressure emerging from nanoscale (Mg, Zn)3Nd precipitates significantly refines the grain size. The grain refinement in combination with the bimodal microstructure significantly contributes to the increased strength of the designed ternary alloys. Furthermore, the addition of Ca and Ce are found to simultaneously improve the strength and ductility, which is closely correlated with the segregation of alloying elements at the grain boundary. Among the considered alloying species, Ce is shown to efficiently improve the mechanical properties of Mg-Zn alloy.

Effect of segregation on the microstructure and properties of a quenching and partitioning steel

• Mn segregation decreases ferrite and increases secondary martensite fraction in the Q&P steel. • Mn segregation causes a lower fraction and stability of retained austenite in the Q&P steel. • Mn segregation degrades the strength and ductility of the Q&P steel due to insufficient TRIP effect. Effect of alloying element segregation on the microstructure and mechanical properties of an intercritically annealed quenching and partitioning steel is experimentally studied. Phase constitution of the segregation band and matrix was compared to elucidate the effect of Mn enrichment on the relevant phase transformation processes and the stability of retained austenite. Tensile specimens including and excluding the segregation band were carefully prepared to reveal the effect of segregation on the mechanical properties. The present study provides direct evidence of the influence of segregation on the microstructure and demonstrates the harmful effect of Mn segregation on both the strength and ductility in the studied quenching and partitioning steel.

Comparative study of microstructure and mechanical properties using a novel filler rod ER 4943 and autogenously butt welded joint during laser welding of AA 6061-T6 in 1G position

A 4 mm thick heat-treated aluminum alloy AA 6061-T6 has been butt welded in 1 G position using a 12-kW disk laser. A novel high magnesium content filler rod ER 4943 belonging to the 4xxx series of aluminum alloys has been used to investigate its effects on microstructure, mechanical properties and alloying elements segregation in the fusion zone. The results have also been compared with an autogenous laser butt welded joint case. A solidified microstructure has been analyzed by EBSD. It was found that additional solute content brought by filler rod into the molten pool caused a higher proportion of equiaxed grain zone after solidification due to an enhanced constitutional supercooling ahead of solid/liquid interface. For an autogenous butt welded joint, the columnar morphology sustained for a longer period and a narrower equiaxed grain zone were observed. Point analysis by an EDS revealed a higher retention of magnesium and silicon inside the solid solution with filler rod welding. In addition, the area map of magnesium also observed a denser distribution of magnesium inside the fusion zone. Both hardness and tensile strength of filler rod welded joint were higher than without filler rod welding. It is believed that a higher proportion of equiaxed grains and additional solute content within the solid solution are the primary causes of higher mechanical properties owing to hampered dislocation motion. The much desirable results obtained in terms of microstructure and mechanical properties could be of great significance to the welding industry.

Effect of alloying elements on the interface of fcc-Fe/Ni3Al by first principle calculations

The effect of alloying element segregation on the interface properties of the fcc-Fe/Ni3Al was investigated using the first-principles approach based on density functional theory, revealing the element segregation mechanisms from the perspectives of atoms and electrons. The results show that the alloying elements tend to replace position 1 on the interface of fcc-Fe phase side, and the order of stability after replacement is Ti, Nb, Mo, Cr and Mn. The state density diagrams show that the 4d orbital electrons of Mo and Nb contribute to increase the peak of Al atom 2p orbital at −5 eV, while the 3d orbital electrons of Cr and Ti tend to weaken the peak of Fe atom 3d orbital at 1 eV. The degrees of electron cloud enrichment and consumption increase between interface atoms of the difference charge diagrams indicate that the bonding strength increases and the interface stability is improved. The result of bond population is consistent with the differential charge density diagram. The study has a certain guiding significance for interfacial segregation behavior of alloying elements in heat-resistant steels.

Influence of Al/Ti Ratio and Ta Concentration on the As-Cast Microstructure, Phase Composition, and Phase Transformation Temperatures of Lost-Wax Ni-Based Superalloy Castings.

The as-cast microstructure, alloying element segregation, solidification behavior, and thermal stability of model superalloys based on Inconel 740 with various Al/Ti ratios (0.7, 1.5, 3.4) and Ta (2.0, 3.0, 4.0 wt%) concentrations were investigated via ThermoCalc simulations, scanning and transmission electron microscopy, energy-dispersive X-ray spectroscopy, dilatometry, and differential scanning calorimetry. The solidification of the superalloys began with the formation of primary γ dendrites, followed by MC carbides. The type of subsequently formed phases depended on the superalloys’ initial Al/Ti ratio and Ta concentration. The results obtained from solidification simulations were compared to the obtained microstructures. For all castings, the dendritic regions consisted of fine γ′ precipitates, with their size mainly depending on the initial Al/Ti ratio, whereas in the interdendritic spaces, (Nb, Ta, Ti)C carbides and Nb-rich Laves phase precipitates were present. In high Al/Ti ratio superalloys, β-NiAl precipitates, strengthened by η and α-Cr phases, were observed. Based on dilatometric results, the dissolution of γ′ precipitates was accompanied by a substantial increase in the coefficient of thermal expansion. The end of the dilatation effect took place around the γ′ solvus temperature, as determined via calorimetry. Moreover, the bulk solidus temperature was preceded by the dissolution of the Laves phase, which may be accompanied by local melting.

An investigation of precipitation strengthened Inconel 718 superalloy after triode plasma nitriding

In this study, we investigated the microstructural evolution, surface hardening and general corrosion properties of a precipitation-strengthened Inconel 718 Ni-superalloy after triode-plasma nitriding (TPN) at low treatment temperatures of 400–450 °C (i.e. thermodynamic paraequilibrium conditions) and a high treatment temperature of 700 °C. At low treatment temperatures, apart from the formation of nitrogen-expanded austenite (γN) from the high-Cr γ matrix, the pre-existing γ′ and γ″ intermetallic nano-precipitates appear to exhibit different nitriding responses. The spheroidal N-modified γ′ (or γ′N) precipitates were ‘slightly-expanded’, leading to slightly shifted XRD peaks, i.e. 2-theta angles of ~0.2° from γsubstrate(111) and ~0.5° from γsubstrate(200). In contrast, N-modified γ″ (or γ″N) could experience substantial lattice expansion close to that of the γN matrix. With increasing treatment temperature, nitride formation started as additional nano-sized precipitates (e.g. ~3–6 nm diameter as observed at 450 °C) and grew into laths (e.g. ~5–10 nm thick and ~15–30 nm wide as observed at 700 °C).Without changing core microstructure/properties, surface nitrogen modification and hardening were obtained on alloy 718 after TPN (e.g. from ~486 HV0.025 to ~1212 HV0.025 after TPN at 400 °C). No degradation of corrosion performance was observed for the nitrogen-supersaturated surface after TPN at 400 °C. However, the 450 °C TPN-treated surface showed a slightly increased current density in the anodic region, which can be associated with early-stage nitride formation. The significantly deteriorated corrosion performance after TPN treatment at 700 °C is due to pronounced nitride formation and segregation of substitutional alloying elements.

Study of defects in directed energy deposited Vanadis 4 Extra tool steel

A cold work tool steel with varied number of layers was deposited on a substrate of hot work tool steel for hard-facing by directed energy deposition technique. This study deals with the defects and microstructure in the as-cladded tool steels. Defects, including pores and cracks, were found in the deposited zone, the number of which increased with the building height or number of layers deposited. Large irregular pores were mainly located at the lower regions of the deposited layers. The formation of this type of pores was attributed to the segregation of alloy elements on the pore surface and insufficient heat input. Non-equilibrium eutectic microstructure was a characteristic feature in the regions adjacent to the pores. Cracking, on the other hand, tended to occur in the upper part of the deposited layers. Two important contributing factors were identified for cracking. The first one was the microstructural gradient, which was modified from fine cellular dendrite to columnar dendrite when moving from the bottom to the top deposited layer. Second, the deposited cold work tool steel exhibited a relatively large temperature range of solidification, as simulated by Thermocalc software, leading to high sensitivity to hot cracking.