Si Segregation Research Articles

Effect of strain rate, soaking time and alloying elements on hot ductility and hot shortness of low alloy steels

Susceptibility to hot shortness and hot cracking of Cu bearing steels is generally minimized with the addition of Ni. However, in this study, attempts have been made to reduce the hot working problems in such steels by the addition of Si, P and either very low content of Ni or no Ni, which will be understood after evaluating their hot ductility and hot shortness properties. The main techniques employed for this study are hot tensile test, analysis of fractography and characterization of scale/steel interface after long exposure at high temperature. The steels having 0.80–0.86% Si exhibited adequate hot ductility at all deformation temperatures ranging from 850 °C to 1200 °C during slow strain rate (0.005 s−1) testing conditions and the fracture morphology exhibited both ductile conical dimples and transgranular fracture. On the other hand, ~0.40% Si containing steels exhibited significantly reduced hot ductility at 850 °C and failed in intergranular brittle fracture mode. The improved hot ductility of high Si containing steels is explained using Fe-P phase diagram along with the proposition of inverse segregation of Si and repulsive interaction between P and Si. On increasing strain rate to 6.5 s−1, ductile failure was observed in all steels. The long duration of soaking at 1200 °C lead to intergranular decohesion and hot tearing during tensile testing in ~0.12% and ~0.19% P containing steels, respectively. A separate study of oxidation in air at 1200 °C indicated that presence of ~ 0.12% P in steel is beneficial in reducing hot shortness by keeping Cu enriched phase away from the scale/steel interface. In contrary to this, oxidation of ~0.19%P containing steel is detrimental to hot working as the grain boundaries of this steel were severely oxidized to the depth of few grains. Other alloying elements such as Ni and Cr in experimental steels had no effect on hot ductility and hot shortness properties.

基于激光诱导击穿光谱技术的生铁中硅锰钛偏析的同步分析

基于激光诱导击穿光谱技术的生铁中硅锰钛偏析的同步分析

First principles study of oxidation of Si-segregated α-Ti(0001) surfaces

The oxidation of the α-Ti(0001) surface was studied using the density functional theory. To enhance the oxidation resistance, we substituted Ti atoms with Si atoms on the Ti(0001) surface. We observed that Si prefers to segregate at the surface layer of Ti(0001) compared with other subsurface layers. The Si solubility on the Ti(0001) surface, based on the segregation energy, was found to be much larger than the corresponding bulk solubility. Si segregation was found to reduce the binding between the oxygen atom and the Ti(0001) surface and hinder the diffusion of oxygen atoms into the slab. The dissociation of the oxygen molecule on the clean and Si-segregated surfaces of Ti was found to be barrierless. Overall, the Si segregation on the Ti(0001) surface was found to hinder the dissolution of oxygen in Ti.

Influence of heat input in electron beam process on microstructure and properties of duplex stainless steel welded interface

The influence of heat input in electron beam (EB) process on microstructure, mechanical properties, and pitting corrosion resistance of duplex stainless steel (DSS) welded interface was investigated. The rapid cooling in EB welding resulted in insufficient austenite formation. The austenite mainly consisted of grain boundary austenite and intragranular austenite, and there was abundant Cr2N precipitation in the ferrite. The Ni, Mo, and Si segregation indicated that the dendritic solidification was primarily ferrite in the weld. The weld exhibited higher hardness, lower toughness, and poorer pitting corrosion resistance than the base metal. The impact fractures of the welds were dominated by the transgranular cleavage failure of the ferrite. The ferrite was selectively attacked because of its lower pitting resistance equivalent number than that of austenite. The Cr2N precipitation accelerated the pitting corrosion. In summary, the optimised heat input slightly increased the austenite content, reduced the segregation degree and ferrite texture intensity, decreased the hardness, and improved the toughness and pitting corrosion resistance. However, the effects were limited. Furthermore, optimising the heat input could not suppress the Cr2N precipitation. Taking into full consideration the microstructure and properties, a heat input of 0.46 kJ/mm is recommended for the EB welding of DSS.

Segregation of Mn, Si, Al, and Oxygen During the Friction Stir Welding of DH36 Steel

This work investigates the role of welding speed in elemental segregation of Mn, Si, Al, and oxygen during friction stir welding (FSW) in DH36 steel. The experimental work undertaken showed that when the speed of the FSW process exceeds 500 RPM with a traverse speed of 400 mm/min, then elemental segregation of Mn, Si, Al, and O occurred. The mechanism of this segregation is not fully understood; additionally, the presence of oxygen within these segregated elements needs investigation. This work examines the elemental segregation within DH36 steel by conducting heat treatment experiments on unwelded samples incrementally in the range of 1200–1500 °C and at cooling rates similar to that in FSW process. The results of heat treatments were compared with samples welded under two extremes of weld tool speeds, namely W1 low tool speeds (200 RPM with traverse speed of 100 mm/min) and W2 high tool speeds (550 RPM with traverse speed of 400 mm/min). The results from the heat treatment trials showed that segregation commences when the temperature exceeds 1400 °C and Mn, Si, Al, and oxygen segregation progress occurs at 1450 °C and at a cooling rate associated with acicular ferrite formation. It was also found that high rotational speeds exceeding 500 RPM caused localized melting at the advancing-trailing side of the friction stir-welded samples. The study aims to estimate peak temperature limits at which elemental segregation does not occur and hence prevent their occurrence in practice by applying the findings to the tool’s rotational and traverse speed that correspond to the defined temperature.

Effects of Silicon Content on the Properties of Gas-Atomized Fe–Si–Cr Powders

Fe-7%, 8%, 9%, and 11%Si–Cr powders were gas atomized and then annealed at 25 °C–750 °C to investigate the effects of Si content, DO3 phase, and B2 phase on the high-frequency permeability of the powders. The formation of the B2 phase could not be suppressed during the atomization process of all the powders. The evolution temperatures of the DO3 phase in the Fe-7%, 8%, and 9%Si–Cr powders as measured by X-ray diffractometer were 550 °C, 550 °C, and 450 °C, respectively. As the annealing temperature was increased, the DO3 phase grew rapidly, and the lattice parameters of the powders were decreased except for those of the Fe-11%Si–Cr powder. The coercivity of all powders decreased with increasing temperature; however, the coercivity of the Fe-8%, 9%Si–Cr powders increased sharply after annealing at 450 °C. Permeability decreased rapidly with increasing temperature except for the Fe-11%Si–Cr powder; thus, the highest real permeability was obtained for the as-atomized Fe-8%Si–Cr powder. The changes in the coercivity and permeability of the powders with annealing temperature could be attributed to the evolution of the DO3 phase, Si segregation, and changes in electrical resistivity.

Effect of Trace Levels of Si on Microstructure and Grain Boundary Segregation in DOP-26 Iridium Alloy

The thermodynamics and kinetics of Silicon (Si) segregation to grain boundaries in Iridium alloy DOP-26 with added trace levels of Si of 6, 11, 29, and 36 wppm was studied by Auger Electron Spectroscopy. The four alloys were annealed at 1500 or 1535 °C for 19 or 76 hours followed by cooling at three different rates. Si enrichment at the grain boundaries (GB) increased with increasing bulk Si content, with the grain boundary Si enrichment factors ranging from 62 to 344, depending on the bulk Si content and the cooling rate. Grain boundary Si contents increased with decreasing cooling rate in all alloys, indicating that Si GB segregation is influenced by both thermodynamic and kinetic factors in the alloys and temperature ranges of the study. A Langmuir-McLean isotherm-based model was successfully used to predict the temperature dependence of GB Si segregation in DOP-26 alloys with Si additions and estimate the temperature independent free energy of Si segregation to grain boundaries in DOP-26.

Segregation and mechanical properties of Si, Fe and Ti on the Al/Al2.5X0.5Zr (X = Cu, Zn, Ag) coherent interfaces: First-principles calculations

Based on the HRTEM observations about the Al/Al2.5Cu0.5Zr coherent interface, the effects of Si, Fe and Ti on the adhesion and ductility of the Al2.5X0.5Zr are investigated using first-principles methods. Six possible interfacial structural models are provided. The interfacial separation obtained by Model V is well in agreement with experimental results. This means Model V is the optimal structure for calculations. The work of adhesion reveals the strongest adhesion of Al/Al2.5Cu0.5Zr. The unstable stacking fault energies show that the preferred slip direction is 〈110〉. By analyzing the Rice criterion, the Al/Al2.5Zn0.5Zr interface has the best ductility. For Si segregation, Al−11 site is favorable for Al/Al2.5X0.5Zr (X=Cu, Zn) interfaces. The adhesion of these interfaces are reduced due to the segregation of Si. The ductility is improved along 〈110〉 for Al/Al2.5X0.5Zr (X=Cu, Zn) interfaces. Fe prefers to occupy Zr site in Al/Al2.5X0.5Zr interfaces. The results demonstrate that the adhesion are improved for all interfaces. The ductility is decreased for Al/Al2.5X0.5Zr (X=Cu, Zn) interfaces. For Ti segregation, Ti prefer to substitute Zr, comparing with Al and Cu sites, in Al/Al2.5X0.5Zr interfaces. Ti substitutions will enhance the adhesion with the reduction of ductility. The charge density difference are also used to understand the adhesion and the results are in accordance with the predicted results.

Atomic-scale analysis of early-stage precipitation in Ti(Al,Si)3 alloy

The early-stage precipitation behavior of Si in Ti(Al,Si)3 alloy heated to 800°C and then down to room temperature (RT) is characterized at the atomic scale by in-situ transmission electron microscope (TEM). Three types of nanoparticles were observed in Ti(Al,Si)3 alloy during the early-stage precipitation, which contain Si-rich nanoclusters, Ti7Al5Si12 precipitates as well as Ti5Si3 precipitates. The precipitation sequence and orientation relationship (OR) for the early-stage precipitation in Ti(Al,Si)3 alloy are as follows: (i) small Si-rich nanoclusters firstly form in TiAl3 matrix due to the segregation of Si, coexisting with larger Si-rich nanoclusters; (ii) Si-rich nanoclusters that reach the critical size (20–30nm) serve as nucleation sites for metastable Ti7Al5Si12 precipitates with an OR: 002TiAl3//11¯6Ti7Al5Si12, 1¯10TiAl3//1151¯Ti7Al5Si12; and then (iii) the formation of Ti5Si3 precipitates is by transformation of metastable Ti7Al5Si12 with an OR: 101¯0Ti5Si3//001Ti7Al5Si12, 12¯13Ti5Si3//010Ti7Al5Si12.

Resolving mass spectral overlaps in atom probe tomography by isotopic substitutions – case of TiSi15N

Mass spectral overlaps in atom probe tomography (APT) analyses of complex compounds typically limit the identification of elements and microstructural analysis of a material. This study concerns the TiSiN system, chosen because of severe mass-to-charge-state ratio overlaps of the 14N+ and 28Si2+ peaks as well as the 14N2+ and 28Si+ peaks. By substituting 14N with 15N, mass spectrum peaks generated by ions composed of one or more N atoms will be shifted toward higher mass-to-charge-state ratios, thereby enabling the separation of N from the predominant Si isotope. We thus resolve thermodynamically driven Si segregation on the nanometer scale in cubic phase Ti1-xSix15N thin films for Si contents 0.08 ≤ x ≤ 0.19 by APT, as corroborated by transmission electron microscopy. The APT analysis yields a composition determination that is in good agreement with energy dispersive X-ray spectroscopy and elastic recoil detection analyses. Additionally, a method for determining good voxel sizes for visualizing small-scale fluctuations is presented and demonstrated for the TiSiN system.

Precipitation kinetics in a 10.5%Cr heat resistant steel: Experimental results and simulation by TC-PRISMA/DICTRA

The precipitation kinetics of secondary phases in a 10.5%Cr heat resistant steel, designed by the authors, was studied experimentally and theoretically. Experimental data of nucleation, growth and coarsening stages for M23C6 carbides, V-MX, Nb-MX and Laves phase were obtained by HRTEM-characterization on samples after tempering (780°C/2 h) and isothermally aging for 1440 h and 8760 h at 650 °C. Theoretical studies of precipitation behavior of M23C6 and Laves phase were carried out by TC-PRISMA and complemented with DICTRA. A good fit between TC-PRISMA simulation and experimental results was obtained for M23C6 carbides considering a heterogeneous nucleation in grain boundaries, a semi-coherent interfacial energy of 0.3 J/m2, and decreasing the atomic mobility along grain boundary in order to include the effect of B. Experiments and simulation indicate a low coarsening rate for M23C6 carbides. Furthermore, precipitation of Laves phase at 650 °C was simulated by TC-PRISMA considering the effect of W and Si segregation at grain boundary, prior to the beginning of the nucleation and growth processes. Therefore, thermodynamic and kinetic boundary conditions were changed purposely in TC-PRISMA. Best agreement with the experimental results was obtained for an interfacial energy of 0.6 J/m2 and heterogeneous nucleation in grain boundary.

Effects of bubbles on high-temperature corrosion of helium ion-irradiated Ni-based alloy in fluoride molten salt

Samples of a Ni-Mo-Cr-Fe-Si alloy subject to helium ion irradiation damage were corroded in a eutectic mixture of FLiNaK at 750°C. It was found that the helium bubbles that formed increased the number of surface defects so greatly increasing the physical contact area between the alloy and the molten salt, resulting in acceleration of corrosion damage. Segregation and depletion of chemical elements at bubble surfaces were also identified, and became more severe with increasing bubble size. Significant segregation of Si resulted in the formation of Ni-Si precipitates at large bubbles. This segregation enhanced the chemical corrosion damage to the alloy.

Peritectic Solidification Path of the La(Fe,Si)13 Phase in Dual-Phase Directionally Solidified La-Fe-Si Magnetocaloric Alloys

NaZn13-type La(Fe,Si)13 alloy is a promising magnetocaloric material for solid refrigeration. Currently, several days of high-temperature annealing are typically required to transform both the α-Fe phase and LaFeSi phase to a La(Fe,Si)13 phase by diffusion. However, no bulk casting has yet been reported. In this study, directional solidification is used to prepare LaFe11.6Si1.4 alloys with a dual-phase microstructure consisting of α-Fe and LaFeSi phases. It was found that the volume fraction of La(Fe,Si)13 phase in directionally solidified LaFe11.6Si1.4 alloys can be increased to ~62 pct. It was also found that the volume fraction of the La(Fe,Si)13 phase is dependent primarily on the temperature gradient at different growth rates, which can be the result of their wide temperature range between solidus and liquidus. A significant Si segregation was observed in directionally solidified LaFe11.6Si1.4 alloys. Furthermore, the solidification path was discussed, focusing on the volume fraction increase of the La(Fe,Si)13 phase by directional solidification.

Master curve generation and modeling of ac conductivity for Mn0.7+xZn0.3SixFe2–2xO4 spinel ferrite system

The compositional dependence of ac conductivity ([Formula: see text]), real ([Formula: see text]′) and imaginary ([Formula: see text]′′) parts of complex electric conductivity ([Formula: see text]*) was investigated as a function of temperature ([Formula: see text]) and frequency ([Formula: see text] for Mn[Formula: see text]Zn[Formula: see text]SixFe[Formula: see text]O4, [Formula: see text], 0.1, 0.2 and 0.3 spinel ferrite system. The compositional dependence of lattice constant values suggested that the most of the substituted Si[Formula: see text]-ions reside at grain boundaries and only a few Si-ions are inside grains. The variation of [Formula: see text], [Formula: see text], [Formula: see text] is explained on the basis of segregation and diffusion of Si[Formula: see text] ions at grain boundaries and grains, respectively, and the electrode effect. Thermal variation of ac conductivity at fixed frequency suggested two different mechanisms which could be responsible for conduction in the system. It is found that [Formula: see text]* is not the preferred presentation for dielectric data and the scaling process of real part of conductivity by normalized frequency and the scaled frequency were found unsuccessful. The fitting results of ac conductivity data with path percolation approximation were found suitable in low-frequency regime while in high-frequency regime, effective medium approximation (EMA) was found successful.

Reactive wetting of Ti-6Al-4V alloy by molten Al 4043 and 6061 alloys at 600–700°C

Abstract Wetting of Ti-6Al-4V alloy by two industrial grade Al alloys (i. e., Al 6061 and 4043 alloys) was studied using the sessile drop method at 600–700 °C under high vacuum. Al/Ti-6Al-4V is a typical reactive wetting system with good final wettability accompanied by the formation of precursor film which is actually an extended reaction layer. The formation mechanism for the precursor film is “subcutaneous infiltration”. The small amount of alloying element Si in the alloys can cause significant segregation at the liquid/solid interface which satisfies the thermodynamic condition. The wetting behavior can be described by the classic reaction product control models, and Ti7Al5Si12 decomposition and Al3Ti formation correspond to the two spreading stages. The small difference in alloying elements in Al 6061 and 4043 resulted in distinctly different interface structures, formation of precursor film and spreading dynamics, especially for the Si segregation at the interface.

Radiation induced segregation and precipitation behavior in self-ion irradiated Ferritic/Martensitic HT9 steel

In this study, Ferritic/Martensitic (F/M) HT9 steel was irradiated to 20 displacements per atom (dpa) at 600 nm depth at 420 and 440 °C, and to 1, 10 and 20 dpa at 600 nm depth at 470 °C using 5 MeV Fe++ ions. The characterization was conducted using ChemiSTEM and Atom Probe Tomography (APT), with a focus on radiation induced segregation and precipitation. Ni and/or Si segregation at defect sinks (grain boundaries, dislocation lines, carbide/matrix interfaces) together with Ni, Si, Mn rich G-phase precipitation were observed in self-ion irradiated HT9 except in very low dose case (1 dpa at 470 °C). Some G-phase precipitates were found to nucleate heterogeneously at defect sinks where Ni and/or Si segregated. In contrast to what was previously reported in the literature for neutron irradiated HT9, no Cr-rich α′ phase, χ-phases, η phase and voids were found in self-ion irradiated HT9. The difference of observed microstructures is probably due to the difference of irradiation dose rate between ion irradiation and neutron irradiation. In addition, the average size and number density of G-phase precipitates were found to be sensitive to both irradiation temperature and dose. With the same irradiation dose, the average size of G-phase increased whereas the number density decreased with increasing irradiation temperature. Within the same irradiation temperature, the average size increased with increasing irradiation dose.

Mesoscale modeling of vacancy-mediated Si segregation near an edge dislocation in Ni under irradiation

We use a continuum method informed by transport coefficients computed using self-consistent mean field theory to model vacancy-mediated diffusion of substitutional Si solutes in FCC Ni near an $\frac{a}{2}[1\overline{1}0](111)$ edge dislocation. We perform two sequential simulations: first under equilibrium boundary conditions and then under irradiation. The strain field around the dislocation induces heterogeneity and anisotropy in the defect transport properties and determines the steady-state vacancy and Si distributions. At equilibrium both vacancies and Si solutes diffuse to form Cottrell atmospheres with vacancies accumulating in the compressive region above the dislocation core while Si segregates to the tensile region below the core. Irradiation raises the bulk vacancy concentration, driving vacancies to flow into the dislocation core. The out-of-equilibrium vacancy fluxes drag Si atoms towards the core, causing segregation to the compressive region, despite Si being an oversized solute in Ni.

The effect of alloying elements on the ductility of Al-Mg-Si alloys

Experiments were conducted to document the combined effect of different alloying elements on the microstructure and associated stress-strain behavior of extruded Al-Mg-Si alloys, with a special emphasis on the ductility to fracture characteristics. Four alloys with different concentrations and relative amounts of Mg, Si, Fe, Cu, and Mn were selected for examination. In order to obtain an accurate description of the strength-ductility relationship, tensile testing was performed with a laser extensometer, and scanning (SEM) and transmission microscope (TEM) studies were carried out to correlate the microstructure and fracture characteristics with the stress-strain behavior. Excess-Si (Mg/Si<1.73) was found to have a detrimental effect on the ductility of Al-Mg-Si alloys without the presence of additional alloying elements, leading to fracture occurring partly intergranularly. This was linked to the segregation of free Si at grain boundaries, forming coarse Si-particles, and the formation of Mg2Si particles during solution heat treatment. Adding Fe and Cu improved the ductility and strength, which could be attributed to a lower solvus temperature and increased formation of primary particles, resulting in less free Si for embrittlement of grain boundaries. The highest ductility was found by introducing Mn, which in addition to tie up free Si to second-phase particles, suppressed recrystallization by forming dispersoids. The most desirable combination of tensile strength and ductility was found in an excess-Mg alloy (Mg/Si>1.73) with additions of both Mn and Cu.

Microstructural Characterization of Melt Extracted High-Nb-Containing TiAl-Based Fiber.

The microstructure of melt extracted Ti-44Al-8Nb-0.2W-0.2B-1.5Si fiber were investigated. When the rotation speed increased from 2000 to 2600 r/min, the appearance of the wire was uniform with no Rayleigh-wave default. The structure was mainly composed of fine α2 (α) phase dendritic crystal and a second phase between dendrite arms and grain boundaries. The precipitated second phases were confirmed to be Ti5Si3 from the eutectic reaction L→Ti5Si3 + α and TiB. As the lower content of Si and higher cooling rate, a divorced eutectic microstructure was obtained. Segregation of Ti, Nb, B, Si, and Al occurred during rapid solidification.

Si segregation and its role in reaching transparent YAG

AbstractIt is well‐known that doping YAG with Si dramatically affects densification and grain growth, and as a result Si is commonly added to YAG as a sintering additive to achieve full density and transparency. In recent studies, the influence of Si was explored, but segregation of Si to grain boundaries in YAG was not detected. The present article contradicts previous findings by revealing an excess of Si at grain boundaries in YAG. The findings were corroborated using atom probe tomography and energy‐dispersive spectroscopy in a scanning transmission electron microscope. To the best of the author's knowledge, this is the first time Si segregation was discovered in SiO2‐doped YAG, and the first time a dopant concentration profile at grain boundaries in a ceramic material was characterized by atom probe tomography. Finally, a change is proposed for the method to calculate grain‐boundary mobility in the presence of a solute (Lucke and Detert), showing a good correspondence to the experimental results. These results can change the view on solute distribution in ceramic grain boundaries from theoretical and characterization aspects.