Addition Of Si Research Articles

Structural heterogeneity, β relaxation and magnetic properties of Fe-Zr-B amorphous alloys with Si and Cu additions

Despite numerous studies on β relaxation of amorphous alloys, the relationship between structural heterogeneity and β relaxation and the effect on magnetic properties of Fe-based amorphous alloys is still unknow. In this work, the structure and magnetic properties of Fe84-xZr6B10Mx (M = Si, Cu; x = 0, 0.6, 1 at%) amorphous alloys were investigated. With the increase of Si content, the negative value of average chemical affinity (∆Hchem) of the alloys is increased, leading to the synergetic enhancement in structural heterogeneity and β relaxation. In addition to this, the increase of liquid-like regions (LLRs) reduces saturation magnetic flux density (Bs) and increases coercivity (Hc). However, the addition of Cu element decreases the negative value of ∆Hchem of the alloys, resulting in weakening of structural heterogeneity and β relaxation synergetically. The reduction of LLRs has resulted in increase in Bs. Moreover, Fe-Cu atom pairs with positive enthalpy of mixing increases the local density of quasi dislocation dipoles (QDDs), resulting in the increase of Hc.

Effects of Si on the stress rupture life and microstructure of a novel austenitic stainless steel

A novel Si-bearing austenitic stainless steel was designed as the cladding material for lead-cooled fast reactors with an operating temperature range of 450 °C–550 °C, and the effects of Si (1.0 wt% - 2.0 wt%) on the creep life and fractured microstructure of material were studied at 550 °C and 650 °C. Results show that the addition of Si increased the stress rupture life of steel at 550 °C whereas brought in an opposite effect at 650 °C, which was closely related to the precipitation behavior of σ phase at these two temperatures during creep. At 550 °C the precipitation of σ phase was quite slow, but it was substantially accelerated at 650 °C. Since the experimental materials used in this work were in a cold worked state, so deformation twins existed and they were discovered to be able to promote the nucleation and growth of σ phase on the grain boundaries. With the increase of Si content at 550 °C, σ phase became larger in quantity and denser in distribution on the grain boundary, which improved the anti-cracking ability of grain boundary and enhanced the creep life of material. However, at 650 °C, σ particles apparently coarsened and congregated on the grain boundary under thermal activation, leading to an evident drop in the stress rupture property of steel.

Surface morphology transformation and densification behaviour of conventionally sintered AlFeCoNiSi high entropy alloys

ABSTRACT A preliminary study has been performed to understand the effect of pressureless sintering on the surface morphology of the AlFeCoNi alloy by the addition of the Si element. This study aims to determine the possibility of achieving the densification of high entropy alloy using the conventional sintering technique. The results indicate that the HEAs have a single-phase BCC structure even with the addition of Si. The thermodynamic simulation (CALPHAD) was used to predict the phase formation. The variation of crystallite sizes and lattice strains caused by sintering temperatures was also discussed. In addition, densification mechanisms occurring with the different sintering temperatures have been discussed. The formation of porosity was observed, however, the density of HEAs improved with increasing sintering temperature. Ultimately, it was suggested that the present HEAs required higher sintering temperatures and a longer time to achieve high density.

Prior-β grain refinement of additive-manufactured Ti–6Al–4 V alloys via trace Si addition

Although Ti–6Al–4 V alloys have unique properties, their fabrication and treatment processes should be improved further to enhance their quality. Therefore, a novel method has been developed and applied to study the potential prior-β grain refinement of additive-manufactured Ti–6Al–4 V thin-wall components via trace Si addition. The microstructure and mechanical properties were analysed, and the results revealed that adding trace Si facilitates heterogeneous nucleation, contributing to the columnar-to-equiaxed transition of prior-β grains. The anisotropic percentages significantly decreased from 5.82% to 1.15% after trace Si addition. All the trace Si-added specimens exhibited dimples that are indicative of the ductile failure mode. Additionally, the possible phase-change process for the trace Si addition specimens was explored over several thermal cycles during the solidification process.

The Effect of Si Addition on the Microstructure, Mechanical Properties, and Wear Rate of the Co–Cr–Mo-Based Biomedical Alloys

The Effect of Si Addition on the Microstructure, Mechanical Properties, and Wear Rate of the Co–Cr–Mo-Based Biomedical Alloys

Effect of Si content on the oxidation behavior of CoCrWSi coatings at 900 °C

In this study, CoCrWSi coatings with different Si contents were prepared via high-velocity oxygen fuel spraying, and the effect of Si content on their high-temperature oxidation resistance in air was studied at 900 °C. The microstructures and chemical compositions of oxide scales were characterized through scanning electron microscopy, electron probe microanalysis, and X-ray diffraction. The results revealed that the oxidation resistance of the produced CoCrWSi coatings gradually increased with increasing Si content. Moreover, the beneficial effects of Si content on the oxidation resistance were discussed in terms of Si depletion. The addition of Si prevented the outward diffusion of Cr atoms, and inhibited internal oxidation, while the depth of internal oxidation varied significantly with the oxidation time.

Corrosion behavior of a Co−Cr−Mo−Si alloy in pure Al and Al−Si melt

Abstract Metallic phase change materials (MPCMs) are attracting considerable attention for their application in thermal energy storage. Al–Si alloys are considered potential MPCMs; however, to develop storage systems/modules, it is crucial to fabricate corrosion-resistant materials for MPCMs. In this study, the corrosion behavior of Co−28Cr−6Mo−1.5Si (wt%) alloy was examined via immersion tests in commercial Al−Si alloy (ADC12) melt at 700°C for 10 h. The results were compared to those obtained for pure Al. Substrate thickness loss measurements revealed that the liquid metal corrosion was more severe in the Al−Si melt than that in pure Al, suggesting an increased reactivity due to Si addition. Interfacial analysis elucidated a direct reaction between the alloy substrate and molten Al in both cases. Furthermore, the formation of oxides such as Al2O3 and SiO2 did not contribute to corrosion resistance.

On the mechanism of Si-promoted destabilization of TiCx particles in Al alloys

TiCx is an excellent composite strengthening particle and grain refiner for Al alloys. However, the stability of TiCx is poor when solute Si exists in Al alloy melts, which significantly depresses its strengthening and grain refining effects. In this work, the destabilization mechanisms of the TiCx particles in Al-Si alloy melt with a composition of Al-7Si-7.5TiC were explored via experiments, first-principles calculations and thermodynamic calculations. The experimental results show that Si atoms diffuse into TiCx and Ti atoms are released into the Al melt to form a Ti-rich transition zone during the insulation of TiCx in Al-Si melt, and the TiAlySiz and Al4C3 phases are solidified in the Ti-rich zone and at Ti-rich zone/TiCx interface, respectively. The first principles calculations show that the low formation energy of C vacancies facilitates the rapid diffusion of Si atoms in TiCx, while the doping of Si atoms reduces the energy barrier of diffusion of Ti atoms in TiCx and promotes the formation of Ti-rich zones. The thermodynamic calculations show that the wide crystallization temperature range of the destabilized product TiAlySiz phase is the key to continuous decomposition of TiCx particles. In addition, the driving force of the main destabilization reaction of TiCx in the Al-Si alloys is about 44 times higher than that in the Al alloys without Si addition. This indicates that the presence of solute Si remarkably promotes the subsequent decomposition process of TiCx in the Al-Si alloy melts.

Dry sliding wear and friction behavior of powder metallurgy FeCoNiAlSi0.2 high-entropy alloys

Wear behaviors of the FeCoNiAlSi0.2 high-entropy alloy (HEA) and Si-free FeCoNiAl during the reciprocating sliding wear process were evaluated for automotive piston rings. The HEAs were prepared using on advanced powder metallurgy route to solve the inherent shrinkage porosity and segregation defects during casting, poor densification, and wear resistance of HEAs while reciprocating motion. The morphology, hardness, wear, and friction properties were investigated. FeCoNiAl had biphasic face-centered cubic (FCC) and body-centered cubic (BCC) solid solution structures, whereas FeCoNiAlSi0.2 HEAs had a single BCC solid solution structure. Si addition significantly enhanced the hardness and wear resistance of the HEAs under reciprocating conditions because of the increased solid solution strengthening and lattice distortion effects. The maximum hardness of the FeCoNiAlSi0.2 HEA reached approximately 582 HV, which is higher than that of the equiatomic FeCoNiAl HEA (491 HV). The findings depict that FeCoNiAlSi0.2 maintains better tribological behavior than FeCoNiAl HEA against the tungsten carbide (WC) ball indenter. Oxidative delamination and cracking occurred in the FeCoNiAl/WC pair, whereas a mixed adhesive–abrasive wear mechanism was prominent in the FeCoNiAlSi0.2/WC pair because of the harder BCC solid solution phase. This study explores an effective strategy for enhancing the tribology behavior of automotive piston structures under varying loads and reciprocating dry sliding motions.

Microstructure and creep properties of cast hypereutectic Al-11Ce alloy with Si addition

This study systematically investigated the as-cast microstructure, heat and creep- resistance of Al-11Ce-xSi (x = 0, 0.9, 1.7 wt%) alloys. A tetragonal AlCeSi intermetallic compound was formed by adding Si. With the increase of Si content, the Chinese-script Al11Ce3/α-Al eutectics changed to rod-like structure and were gradually replaced by broad flaky AlCeSi/α-Al eutectics. The Al-11Ce alloy containing 0.9 wt% Si possessed the optimal tensile properties with an ultimate tensile strength of 129.2 MPa and an elongation of 4.3%, which were 8.4% and 65.4% higher than that of the based alloy, respectively. After aging at 400 ℃ for 1008 h, the microhardness of Si-containing alloys remained unchanged, indicating excellent coarsening resistance of the strengthening AlCeSi phases. Under tensile creep conditions at 300 ℃, the creep deformation of three alloys was the dislocation creep mechanism. Nevertheless, with the addition of 1.7 wt% Si, the steady-state strain rate of Al-11Ce alloy was decreased to 1/3 of its original value. The enhanced creep resistance was mainly attributed to the relatively high volume fraction of intermetallic phase, the reduced volume fraction of coarse primary phase, and the presence of AlCeSi precipitates that effectively hinder dislocations motion.

Crack reduction in Inconel 939 with Si addition processed by laser powder bed fusion additive manufacturing

Inconel 939 (IN939) suffers from cracking in the laser powder bed fusion based additive manufacturing (LPBF-AM) process. Previous studies attempted to minimize the crack density by reducing the Si content. In contrast, this paper demonstrates that Si addition can substantially lower the crack density in LPBF-AM IN939 alloy parts. Si addition into IN939 shows a moderate impact on the crystallographic orientation and grain size of LPBF-AM parts, and promotes the formation of precipitates. Mechanical tests show that Si addition increases tensile strength and indentation hardness but decreases ductility. This work suggests that the dominant cracking mode in LPBF-AM IN939 alloy parts is solidification cracking, the suppression of which by Si addition can be qualitatively rationalized using Clyne’s model and Kou’s model. The present findings provide a different perspective on designing defect-free LPBF-AM Ni-based superalloy parts with balanced mechanical properties, one which can be extended to other alloy systems.

Effect of Si and Nb additions on carbonitride coatings under proton irradiation: A comprehensive analysis of structural, mechanical, corrosion, and neutron activation properties

In the present study, understoichiometric TiZrCN, TiZrNbCN, and TiZrSiCN coatings were produced using the cathodic arc technique with a C/N ratio of approximately 0.5 to investigate their potential use in nuclear technology. The coatings were evaluated for their corrosion resistance in 3.5 % NaCl and neutron activation. The effect of adding Si and Nb to the quaternary TiZrCN system was also investigated. The results showed that the addition of Si (∼4.64 at.%) to the matrix of TiZrCN improved their electrochemical properties in NaCl solution, the protective efficiency was 92%, while the Nb addition (∼5.5 at%) lead to the decrease in corrosion resistance by 1.39 times comparing with TiZrCN. Furthermore, after fast neutron irradiation at a nominal power of 1450 kW, none of the coatings were activated, indicating good radiation resistance. It was determined from the structural analysis that the Ti6Al4V substrate before corrosion consists of hexagonal and cubic space groups with different lattice parameters. By adding Si and Nb, a small amount of ZrO2 and Si3N4 was detected along with the main phases in the TiZrCN structure.

Hot Corrosion Behavior of Co–Al–W Superalloys with Si Additions

The hot corrosion behavior of Co-9Al-9.5W-xSi (where x = 0%, 0.1%, 0.5%, at.%) alloys in a salt mixture at 900 °C was investigated. The effect of Si on hot corrosion resistance was examined using corrosion kinetics. The surface morphology of the corrosion products was explored via SEM with EDS and the phase constituents were examined using XRD. The results revealed that hot corrosion occurred as a combination of both sulfidation and oxidation behavior. With the increase in Si content, the hot corrosion resistance of the alloy was capable of remarkable advancement. Corrosion scales on the three Co-based alloys were mostly comprised of Co3O4, CoO, CoAl2O4, CoWO4, and Al2O3. The hot corrosion mechanism for the Co-based alloy in the presence of 75 wt.% Na2SO4 and 25 wt.% NaCl deposits were analyzed.

Improvement of Oxidation Resistance in Tungsten Heavy Alloys through Si, Y2O3, Ni, and Co Addition

Improvement of Oxidation Resistance in Tungsten Heavy Alloys through Si, Y2O3, Ni, and Co Addition

Microstructure and mechanical properties of Si micro-alloyed (Ti28Zr40Al20Nb12)100-xSix (x=0, 0.1, 0.2, 0.5) high entropy alloys

The effect of Si micro-alloying on the microstructure and mechanical properties of (Ti28Zr40Al20Nb12)100-xSix (x = 0, 0.1, 0.2, 0.5) high entropy alloys are studied. Coarse grains mainly composed of two ordered B2 phases are obtained during solidification, while the hcp-Zr5Al3 type precipitate formed after annealing at 1200 °C for 24h. As the Si content increases, the phase morphology at the grain boundaries gradually changes from a completely continuous structure to a partially continuous structure, and finally transfers to a necklace-type structure. The phase morphology inside the grains simultaneously transforms into merely randomly distributed particles. The changes in phase morphology are attributed to the stronger chemical affinity between Zr and Si. Owing to the severe inhomogeneity caused by the Si addition, a dramatic decrease of the fracture strain is observed in compression tests for as-cast samples. For Si micro-alloyed alloys, the annealed samples show an improvement of both compression strength and fracture strain. The obtained results suggest that Si micro-alloying has less impact on phase composition, but significantly affects elemental segregation, phase morphology and thus the mechanical properties.

On the Nb5Si3 Silicide in Metallic Ultra-High Temperature Materials

Refractory metal (RM) M5Si3 silicides are desirable intermetallics in metallic ultra-high temperature materials (UHTMs), owing to their creep properties and high Si content that benefits oxidation resistance. Of particular interest is the alloyed Nb5Si3 that forms in metallic UHTMs with Nb and Si addition. The choice of alloying elements and type of Nb5Si3 that is critical for achieving a balance of properties or meeting a property goal in a metallic UHTM is considered in this paper. Specifically, the different types of alloyed “normal” Nb5Si3 and Ti-rich Nb5Si3, namely “conventional”, “complex concentrated” (CC) or “high entropy” (HE) silicide, in metallic UHTMs with Nb and Si addition were studied. Advanced metallic UHTMs with additions of RMs, transition metals (TMs), Ge, Sn or Ge + Sn and with/without Al and with different Ti, Al, Cr, Si or Sn concentrations were investigated, considering that the motivation of this work was to support the design and development of metallic-UHTMs. The study of the alloyed silicides was based on the Nb/(Ti + Hf) ratio, which is key regarding creep, the parameters VEC and Δχ and relationships between them. The effect of alloying additions on the stability of “conventional”, CC or HE silicide was discussed. The creep and hardness of alloyed Nb5Si3 was considered. Relationships that link “conventional”, CC or HE bcc solid solution and Nb5Si3 in the alloy design methodology NICE (Niobium Intermetallic Composite Elaboration) were presented. For a given temperature and stress, the steady state creep rate of the alloyed silicide, in which TMs substituted Nb, and Al and B substituted Si, depended on its parameters VEC and Δχ and its Nb/(Ti + Hf) ratio, and increased with decreasing parameter and ratio value, compared with the unalloyed Nb5Si3. Types of alloyed Nb5Si3 with VEC and Δχ values closest to those of the unalloyed Nb5Si3 were identified in maps of alloyed Nb5Si3. Good agreement was shown between the calculated hardness and chemical composition of Nb5Si3 and experimental results.

Decomposition performance and kinetics analysis of magnesium hydroxide regulated with C/N/Ti/Si additives for thermochemical heat storage

MgO/Mg(OH)2 is a potential thermochemical material with high heat density and stable characteristics. Four additives including Carbons, Carbides, Nitrides and Titanates were selected for modification. The decomposition behavior and kinetics analysis were both investigated. It is found that Titanates has a catalytic effect, and the tested energy storage density can be increased to 971 J/g. The spectral absorption of candidate material with 6% graphite can reach to more than 50% and 6% CaTiO3 has an extremely high absorption in the short wave segment. In microscopic morphology, Mg(OH)2 particles sintered at high temperature will form adhesion agglomeration. Due to the special microstructures of candidate materials, agglomeration phenomenon is effectively alleviated and the candidate materials with 6% graphite or CaTiO3 are both ideal modified materials. The optimized mechanism of modified material with 6% CaTiO3 was illustrated by model. To investigate the kinetics mechanism, the model of non-isothermal decomposition was developed. The activation energies of pure material, sample with 6%graphite and sample with 6%CaTiO3 are 141 kJ/kg, 113 kJ/kg and 122 kJ/kg respectively. The shape factor polynomials and kinetic governing equations were obtained. According to the established kinetics equations, the heating processes at 8 K, 12 K and 25 K/min were predicted. The study can provide a necessary kinetics reference for the numerical calculation of thermochemical heat storage.

Tuning the microstructure and strengthening mechanism of the CoCrFeNi high-entropy alloy via doping metalloid and interstitial elements

The synergistic strengthening and toughening of metalloid and interstitial elements were demonstrated for the first time in the CoCrFeNi high-entropy alloy (HEA) by the sequential addition of Si, B, and N elements. The addition of Si simultaneously enhances the strength and ductility of the CoCrFeNi HEA, further, the doping of B promotes the formation of Cr2B precipitates and the grain refinement, and the addition of N further refines the grain size and increases the lattice distortion and the degree of chemical short-range order (SRO). The strengthening effect to yield strength of the addition of Si, B, and N elements is well described by the strengthening model. Doping Si, B, and N elements can regulate the stacking fault energy (SFE), lattice distortion, and short-range order of the alloy, and consequently induce the multiple twins and nanoscale transformation from the face-centered cubic (FCC) to the hexagonal close-packed (HCP) phase upon room-temperature tension, which enhances the work-hardening capability of the alloy. Finally, a dual heterostructure of grain size and precipitate phase distribution is formed in CCFN-Si-B-N-700 HEA by regulating heat treatment. The yield strength of the current alloy is further enhanced up to 1.2 GPa, and the ductility is maintained at 13 %. This study not only provides new insights into the strengthening and toughening mechanism of metalloid and interstitial elements but also a new paradigm for the development of HEAs with excellent strength-ductility synergy.

Role of Si segregation in the structural, mechanical, and compositional evolution of high-temperature oxidation resistant Cr-Si-B2±z thin films

This work investigates the influence of Si-alloying up to 17 at.% on the structural, mechanical, and oxidation properties of magnetron sputtered CrB2±z-based thin films. Density-functional theory calculations combined with atom probe tomography reveal the preferred Si occupation of Cr-lattice sites and an effective solubility limit between 3 to 4 at.% in AlB2-structured solid solutions. The addition of Si results in refinement of the columnar morphology, accompanied by enhanced segregation of excess Si along grain boundaries. The microstructural separation leads to a decrease in both film hardness and Young’s modulus from H ~ 24 to 17 GPa and E ~ 300 to 240 GPa, respectively, dominated by the inferior mechanical properties of the intergranular Si-rich regions. Dynamic thermogravimetry up to 1400 °C reveals a significant increase in oxidation onset temperature from 600 to 1100 °C above a Si content of 8 at.%. In-situ X-ray diffraction correlates the protective mechanism with thermally activated precipitation of Si from the Cr-Si-B2±z solid solution at 600 °C, enabling the formation of a stable, nanometer-sized SiO2-based scale. Moreover, high-resolution TEM analysis exposes the scale architecture after dynamic oxidation to 1200 °C (10 K/min heating rate) – consisting only of ~20 nm amorphous SiO2 beneath ~200 nm of nanocrystalline Cr2O3. In summary, the study provides detailed guidelines connecting the chemical composition with the respective thin film properties of high-temperature oxidation resistant Cr-Si-B2±z coatings.

Role of silicon on formation and growth of intermetallic phases during rapid Fe–Zn alloying reaction

The study of liquid metal embrittlement in Fe–Zn systems is challenging because of the high temperature and vapor pressure of Zn, which hinders in-situ investigations with sufficiently high spatial resolution. This is typically associated with subsecond processing steps and the coexistence of a solid substrate and a liquid Zn phase, which renders direct observations at the microstructural scale difficult. In this study, we comprehensively investigate the reactions occurring during the rapid heating and cooling stages of Fe–Zn systems using synchrotron X-ray diffraction. The phase transformation is analyzed for specimens with different interfacial structures, with focus on changes in the coating microstructure above and below the peritectic temperature (782 °C) of the Fe–Zn system. Advanced high-strength steel (AHSS) variants with 1.5 wt% Si content show a prominent destabilizing effect at the onset of Fe–Zn intermetallic compound formation and a simultaneous deceleration of the liquid Zn depletion rate compared with other AHSS alloys containing 0 wt% Si. Furthermore, the addition of Si suppresses the formation of the Γ phase, which is due to turbulence in the outburst Zn at temperatures above 773 K. Consequently, the fraction of Γ phase compounds with accumulated Zn deceases as the exposure time of the liquid Zn to the ferritic matrix increases owing to the solute redistribution of Si in the liquid Zn during rapid heating. Meanwhile, the absence of silicon in the substrate causes the formation of a ζ phase with a low melting point, which delays the formation of liquid Zn via an increase in the melting point of the Zn layer in the early stage of heating. Additionally, the amount of residual liquid Zn decreases due to the rapid depletion of Zn during the liquid initiation stage via an equilibrium Fe/Zn binary phase transformation. The results of this study provide a deeper understanding of the Fe–Zn intermetallic phase transformation and the sensitivity to liquid metal embrittlement of third-generation AHSSs based on silicon content.