Pct Si Alloy Research Articles

The Effect of Hidden Damage on Local Process Variability in Al-10 Pct Si Alloy High-Pressure Die Castings

The Effect of Hidden Damage on Local Process Variability in Al-10 Pct Si Alloy High-Pressure Die Castings

Preparation of C54-TiSi2 by the Electromagnetic Directional Solidification of Hypoeutectic Ti-53.98 Wt Pct Si Alloy Melt

Preparation of C54-TiSi2 by the Electromagnetic Directional Solidification of Hypoeutectic Ti-53.98 Wt Pct Si Alloy Melt

Fe Nitride Formation in Fe\u2013Si Alloys: Crystallographic and Thermodynamic Aspects

A Fe–3wt pctSi alloy was gas nitrided to study the effect of Si on the Fe nitride formation. Both ε-Fe3N1+x and γ′-Fe4N were observed at nitriding conditions only allowing to form single-phase γ′ layers in pure α-Fe. During short nitriding times, ε and γ′ simultaneously grow in contact with Si-supersaturated α-Fe(Si). Both nitrides almost invariably exhibit crystallographic orientation relationships with α-Fe, which are indicative of a partially displacive transformation of α-Fe being involved in the initial formation of ε and γ′. Due to Si constraining the Fe nitride growth, such transformation mechanism becomes highly important to the nitride layer formation, causing α-Fe-grain-dependent variations in the nitride layer morphology and thickness, as well as microstructure refinement within the nitride layer. After prolonged nitriding, α-Fe is depleted in Si due the pronounced precipitation of Si-rich nitride in α-Fe. The growth mode of the compound layer changes, now advancing by conventional planar-type growth. During nitriding times of 1 to 48 hours, ε exists in contact with the NH3/H2-containing nitriding atmosphere at a nitriding potential of 1 atm−1/2 and 540 °C, only allowing for the formation of γ′ in pure Fe, indicating that Si affects the thermodynamic stability ranges of ε and γ′.

Hot Tearing Behavior in Double Ternary Eutectic Alloy System: Mg-Ce-Al Alloys

The hot tearing behavior of the double ternary eutectic alloy system, Al-6Mg-xSi (0 to 6.0 wt pct Si) alloys, was evaluated by a constrained rod casting mold equipped with a thermocouple and a load cell connected to a data acquisition system. Experimental results were compared with the results calculated by Scheil’s model using Pandat software to predict the occurrence of hot tearing in Al-6Mg-xSi alloys. The curve of hot tearing susceptibility contained two peaks, which were mainly affected by the double ternary eutectic reactions and the solidification phases evolution. And it agreed with the Kou’s hot tearing index precisely. The types and fractions of solidification phases (Al3Mg2, Mg2Si, and eutectic Si) determined by the addition of Si content changed the freezing range and eutectic liquid fraction greatly. In addition, the types, fractions, sizes, and distributions of solidification phases affected the nucleation, propagation, and healing of hot tear cracks. The maximum contraction force value decreased as the reduction of volume shrinkage coefficient due to the addition of Si content. The drop-in force value on the contraction force curve could indicate the occurrence of hot tearing, and predict the hot tearing susceptibility when considering the shrinkage coefficient.

Unusual Iron Nitride Formation Upon Nitriding Fe-Si Alloy

The influence of Si, substitutionally dissolved in ferritic Fe-2 at. pct Si and Fe-4.5 at. pct Si alloys, on the nucleation and growth of γ′ iron nitride upon controlled gaseous nitriding was investigated. The nitrided specimens were characterized by XRD, light microscopy, TEM, EELS, EPMA and EBSD. The combination of difficult and thus delayed precipitation of (1) silicon nitride, because of a large misfit with the ferrite matrix, and (2) γ′ iron nitride, because of the negligible solubility of Si, was shown to lead to a series of unusual, nonequilibrium phenomena: high nitrogen supersaturation, development of Si-containing metastable γ′ phase of peculiar morphology, precipitation of amorphous silicon nitride in γ′ and development of ε nitride at thermodynamic conditions unlikely to allow its formation.

Grain Refinement Mechanism of the δ-Ferrite in Steels Through Cerium Addition

To investigate and clarify the roles and mechanism of Ce addition in grain refinement of primary δ-ferrite during solidification of steels, a Fe-4wt pct Si alloy, which can retain the primary δ-ferrite to room temperature after solidification, was used to evaluate the roles of Ce in refining the δ-ferrite. Experimental results showed that addition of small amount of Ce led to significant reduction in average grain size of the as-cast Fe-4wt pct Si alloy from 1906 ± 160 µm without Ce addition down to 998 ± 70 µm at 0.0260 wt pct Ce addition. For the purpose of investigating the grain refinement mechanism of Ce, the Equilib and Phase Diagram modules in the FactSage Software System were used to simulate the formation sequence and calculate fractions of the Ce-containing inclusions above the liquidus in various Fe-4wt pct Si alloys with different Ce, S and O contents. The predicted formation sequence was Ce2O3 > Ce2O2S > Ce2S3/Ce3S4. However, due to the low fraction of the oxide and sulfides formed, only the oxysulfide, Ce2O2S, was observed and verified in the experimental alloys at room temperature in a transmission electron microscopy and a scanning electron microscope that was equipped with the electron backscatter diffraction and energy-dispersive X-ray spectroscopy. Based on the crystallographic calculations using the edge-to-edge matching model together with previously determined orientation relationships between the δ-ferrite and Ce2O2S, it is considered that the heterogeneous nucleation of δ-ferrite on the in situ-formed Ce2O2S particles in the Fe-4wt pct Si alloy melt played a key role in grain refinement of the δ-ferrite. An inductively coupled plasma mass spectrometry was used to measure the amount of Ce solute in the ferrite after conducting low-temperature electrolysis to separate and filter out the Ce compounds. The calculation of the growth restriction parameter (β) using the Phase Diagram module in FactSage Software System indicated an almost negligible role of the Ce solute in the grain refinement of δ-ferrite. The present work clarifies previous uncleanness in understanding the grain refinement mechanism of δ-ferrite through Ce addition.

Fabrication of the τ5c Intermetallic Compound Monoliths by a Novel Powder Metallurgy and Hot-Dipping Approach

The precipitation of discrete iron-based intermetallic compounds (IMC) is an inevitable phenomenon in a steel strip alloy coating pot due to the continuous supply of iron from the steel strip and temperature perturbations in the pot. These IMC [τ5c: Al20Fe5Si2(Zn) or Al8Fe2Si(Zn)] particles grow by coarsening and agglomeration and are the primary source of coating inclusions for coated steel products. They are further responsible for excessive bottom dross build-up in the alloy coating pot. It is hypothesized that they will re-dissolve into the pot during the heating cycle of the coating bath, but their dissolution kinetics remains unclear. The underlying hold-up is the lack of large IMC samples for quantitative studies of their dissolution kinetics in the molten coating alloy. To quantify and understand the dissolution of the τ5c IMC phase, this study reports the fabrication of monolith samples of the τ5c phase by a novel powder metallurgy and hot-dipping approach. Firstly, τ5c IMC particles were extracted by acid dissolution of the bottom dross collected from a commercial alloy coating pot. A powder metallurgy route was then used to fabricate an intermediate form of the monolithic τ5c IMC disks of 14.80 mm in diameter and 17 mm in height. X-ray diffraction (XRD) and electron transmission microscopy (TEM) analyses revealed that the initial τ5c IMC phase transformed into α-Al8Fe2Si (τ5H) and Al3Fe phases during isothermal sintering at 900 °C due to devolatilization of Zn from the τ5c phase. However, when these intermediate IMC monoliths were immersed in the Zn-concentrated and iron-saturated 55 pct Al-43.4 pct Zn-1.6 pct Si (AZ) alloy coating bath at 595 °C, both the α-Al8Fe2Si (τ5H) and the Al3Fe phases transformed back into the τ5c IMC phase through diffusion of Zn into the as-sintered monoliths. The fabrication of the τ5c monolith samples, which can be shaped in any geometry, provides an essential basis for quantifying and understanding the dissolution of the τ5c intermetallic phase in a commercial steel strip coating pot. Dissolution analysis of the IMC monoliths revealed a mass transfer coefficient (k) of 5.3 × 10−8 m/s in an unsaturated AZ alloy coating bath at 595 ± 2 °C. It is suggested that dissolution kinetic of IMC monoliths is sluggish; therefore, IMC(τ5c) particles precipitated in the AZ alloy bath will unlikely dissolve back at industrial steel strip coating processing conditions.

Inhomogeneous Warm Deformation Behavior and Its Effect on the Deformability of an Fe-6.5PctSi Alloy with Ce Doping

The inhomogeneous deformation behavior generated by deformability differences between soft and hard phases is a critical factor that influences the deformation and processability of materials. For Fe-6.5pctSi alloys with coexisting disordered and ordered phases, this work studied warm (300 °C to 500 °C) tensile deformation property differences and mechanisms for non-Ce-doped and 0.02 wt pct Ce-doped alloys. The warm deformation behavior was analyzed according to the stress and strain partitioning of the two-phase alloy by establishing a two-phase (A2 disordered phase + B2/D03 ordered phase) system model based on the structural characteristics of the alloy. The results show that the plastic deformation is mainly attributed to the A2 disordered phase, and the existence of a hard and ordered phase leads to local and inhomogeneous deformation behavior, which decreases the uniform plastic elongation and deformability of the alloy. Ce doping improves the warm deformation homogeneity and deformability by reducing the size and amount of the B2 ordered phase, which improves the warm rolling deformation and increases subsequent cold rolling deformability. Cold rolling with a total thickness reduction greater than 63 pct was achieved after the Ce-doped hot-rolled specimen with a 2 mm thickness was warm rolled to an 82.5 pct reduction, and the warm-rolled and cold-rolled sheets had good surface integrity and no obvious cracking.

Microstructure Solidification Maps for Al-10 Wt Pct Si Alloys

Hypo-eutectic Al-Si alloys are widely used in both the automotive and aerospace industries; however, they still have limited usage as structural materials, due to the inherent morphology of the Si phase that forms within the eutectic structure. This non-ideal Si morphology can be modified, via alloy additions and/or rapid solidification (RS), but the underlying mechanism(s) behind this is poorly understood. This work focused on understanding the influence of RS on the eutectic structure, for hypo-eutectic Al-10 wt pct Si alloys produced by Impulse Atomization and Differential Scanning Calorimetry. This study found that the eutectic Si forms into four distinct morphologies: (1) flaky, (2) fibrous, (3) globular + fibrous and (4) globular, depending on the solidification conditions. As a result, two solidification maps of the Si morphology are proposed, one based on local eutectic solidification conditions and another based on a solidification continuous cooling diagram (SCCT). Both maps help identify the required conditions for certain Si morphologies to form. Hardness measurements were also carried out and it was found that the Si morphology would influence the alloy hardness, with the highest value being achieved when the eutectic Si was globular. This result indicates that the Si morphology is an important factor that can alter the mechanical properties of hypo-eutectic Al-Si alloys.

Effect of Solutes on Grain Refinement of As-Cast Fe-4Si Alloy

Grain size is one of the key microstructural factors that control the mechanical properties of steels. The present work aims to extend the theories of grain refinement which were established for cast light alloys to steel systems. Using a designed Fe-4 wt pct Si alloy (all-ferrite structure during whole solidification process), the solute effect on grain refinement/grain coarsening in ferritic systems was comprehensively investigated. Experimental results showed that boron (B), which is associated with the highest Q value (growth restriction factor) in ferrite, significantly refined the as-cast structure of the Fe-4 wt pct Si alloy. Cu and Mo with low Q values had no effect on grain refinement. However, although Y and Zr have relatively high Q values, addition of these two solutes led to grain coarsening in the Fe-4Si alloy. Understanding the results in regards to the growth restriction factor and the driving force for the solidification led to the conclusion that in addition to the grain growth restriction effect, the changes of thermodynamic driving force for solidification due to the solute addition also played a key role in grain refinement in ferritic alloys.

Two-Zone Microstructures in Al-18Si Alloy Powders

Hypereutectic Al-18 wt pct Si alloy is widely used in automotive industry as a wear-resistant alloy for engine components. However, in the last few years, this traditional composition is being considered for processing by different rapid solidification methods. Positive points include its low thermal expansion and uniform distribution of surface oxides. Nevertheless, the microstructural aspects of Al-Si powders of 18 wt pct Si are still need to be addressed, such as, the eutectic Si morphology, size, and distribution generated by different process conditions during rapid solidification. Based on a detailed quantitative analysis of the microstructures of rapid solidified Al-18 wt pct Si in this work, solidification conditions that yield specific Si morphologies, Si spacing, and thermal cooling conditions are outlined. The focus is determining the solidification conditions that will yield a specified shape of eutectic Si. It is shown that Si morphology is dependent on a combination of growth velocity (based on modified JH model) and temperature gradient. Furthermore, the highest hardness is achieved with globular morphologies of Si. The processing conditions required to achieve these properties are outlined.

Removal of Phosphorus in Silicon by the Formation of CaAl2Si2 Phase at the Solidification Interface

To fully understand the role of CaAl2Si2 phase in concentrating the non-metallic impurity phosphorus, an experiment of directional solidification of Al-70 at. pct Si alloy with extreme small lowering rate 0.05 mm min−1 was carried out. With good dynamic condition for the diffusion of impurity (Ca, Al, P) from silicon to the S/L interface, the CaAl2Si2 phase with 0.6-0.7 at. pct P was successfully observed by Electron Probe Micro Analyzer (EPMA), and its distribution character was originally presented. This impurity phase was widely detected in the refined sample but only at the interface of silicon crystal and Al-Si alloy which contributed to the deep removal of impurity P. The formation mechanism of CaAl2Si2-P phase was thus explored, in which the microsegregation and concentration of element P, Ca, Al in front of S/L interface were crucial. After acid leaching, the P content decreased from the original 23 ppm to below 5 ppm. Compared with normal solidification, a 16 pct higher removal efficiency of P was obtained in this study.

Wettability of Molten Aluminum-Silicon Alloys on Graphite and Surface Tension of Those Alloys at 1273 K (1000 °C)

The wettability of molten aluminum-silicon alloys with silicon contents of 0, 6, 10, and 20 mass pct on graphite substrates by changing the placing sequence of aluminum and silicon and the surface tension of those alloys were investigated at 1273 K (1000 °C) using the sessile drop method under vacuum. The results showed that the wetting was not affected by changing the placing sequence of the Al-Si alloys on the graphite substrates. The wettability was not improved significantly upon increasing the Si content from 0 to 10 mass pct, whereas a notable decrease of 22 deg in the contact angle was observed when increasing the Si content from 10 to 20 mass pct. This was attributed to the transformation of the interfacial reaction product from Al4C3 into SiC, provided the addition of Si to Al was sufficient. It was verified that the liquid Al can wet the SiC substrate very well in nature, which might explain why the occurrence of SiC would improve the wettability of the Al-20 mass pct Si alloy on the graphite substrate. The results also showed that the surface tension values of the molten Al-Si alloys decreased monotonously with an increase in Si content, being 875, 801, 770, and 744 mN/m for molten Al, Al-6 mass pct Si, Al-10 mass pct Si, and Al-20 mass pct Si alloys, respectively.

The Strength–Grain Size Relationship in Ultrafine-Grained Metals

Metals processed by severe plastic deformation (SPD) techniques, such as equal-channel angular pressing (ECAP) and high-pressure torsion (HPT), generally have submicrometer grain sizes. Consequently, they exhibit high strength as expected on the basis of the Hall–Petch (H–P) relationship. Examples of this behavior are discussed using experimental data for Ti, Al, and Ni. These materials typically have grain sizes greater than ~50 nm where softening is not expected. An increase in strength is usually accompanied by a decrease in ductility. However, both high strength and high ductility may be achieved simultaneously by imposing high strain to obtain ultrafine-grain sizes and high fractions of high-angle grain boundaries. This facilitates grain boundary sliding, and an example is presented for a cast Al-7 pct Si alloy processed by HPT. In some materials, SPD may result in a weakening even with a very fine grain size, and this is due to microstructural changes during processing. Examples are presented for an Al-7034 alloy processed by ECAP and a Zn-22 pct Al alloy processed by HPT. In some SPD-processed materials, it is possible that grain boundary segregation and other features are present leading to higher strengths than predicted by the H–P relationship.

Direct Resistance Joule Heating of Al-10 pct Si-Coated Press Hardening Steel

Various rapid heating methods have been developed to increase the productivity of press hardening steel. One of these methods is direct resistance Joule heating. This heating method results in the melting of the surface coating and the formation of a persistent liquid trail as a result of the high thermal conductivity and low melting temperature of the Al-10 pct Si alloy coating. This can be addressed by an alloying preheating treatment prior to the press hardening process.

Application of the Eyring Equation in the Evaluation of Semi-Solid Forming-Induced Si Particle Refinement in the Hypereutectic Al-Si Alloys

On the basis of Eyring’s theory of absolute reaction rate, an approach to modeling Si particle refinement acceleration in the semi-solid forming of a hypereutectic Al-Si alloy has been developed. The acceleration variable data used in the present analysis were obtained from a semi-solid compression test using Al-25 mass pct Si alloy cylindrical specimens with a diameter of 15 mm and a height of 15 mm; the test conditions comprised a combination of compression displacements ∆h = 5, 10, and 12 mm; compression rates v = 5, 25, and 125 mm/min; and test temperatures T = 853 K and 863 K (580 °C and 590 °C). The coarse primary Si particle refinement depends on a complex interaction among variables, such as compression displacement, compression rate, and test temperature. The performance of Si particle refinement degraded under higher temperature, slower strain rates, and slower shear rates. The results of the Si particle size are suitably summarized by the Eyring equation as a function of the temperature and the shear rate. The baseline Si particle size and the baseline temperature of Si particle refinement, i.e., the reference temperature, were GN = 0.27 mm and TN = 866.4 K (593.4 °C), respectively. The calculated results using this equation correlated well with the observed results. An acceleration factor of Si particle refinement was successfully derived on the basis of this equation and indicated that operating at a higher shear rate and a temperature just above the melting point of eutectic Al-Si alloy are the optimum conditions for refining Si particles.

Microstructural Evolution and Phase Formation in Rapidly Solidified Ni-25.3 At. Pct Si Alloy

The drop-tube technique was used to solidify droplets of the Ni-25.3 at. pct Si alloy at high cooling rates. XRD, SEM, and TEM analysis revealed that the metastable phase, Ni25Si9, formed as the dominant phase in all ranges of the droplets, with γ-Ni31Si12 and β 1-Ni3Si also being present. Three different microstructures were observed: the regular and anomalous eutectic structures and near single-phase structure containing small inclusions of a second phase, termed here as heteroclite structure. Both eutectic structures comprise alternating lamellae of Ni25Si9 and β 1-Ni3Si, which, we conjecture, is a consequence of an unobserved eutectic reaction between the Ni25Si9 and β 1-Ni3Si phases. The matrix of the heteroclite structure is also identified as the metastable phase Ni25Si9, in which twined growth is observed in the TEM. As the cooling rate is increased (particle size decreased), the proportion of droplets displaying the entire heteroclite structure tends to increase, with its fraction increasing from 13.91 pct (300 to 500 µm) to 40.10 pct (75 to 106 µm). The thermodynamic properties of the Ni25Si9 phase were also studied by in-situ heating during XRD analysis and by DTA. This showed the decomposition of Ni25Si9 to β 1 and γ-Ni31Si12 for temperatures in excess of 790 K (517 °C).

In Situ Study of Microstructure Evolution in Solidification of Hypereutectic Al-Si Alloys with Application of Thermal Analysis and Neutron Diffraction

Understanding of the kinetics of solid-phase evolution in solidification of hypereutectic aluminum alloys is a key to control their as-cast microstructure and resultant mechanical properties, and in turn, to enhance the service characteristics of actual components. This study was performed to evaluate the solidification kinetics for three P-modified hypereutectic Al-19 pct Si alloys: namely, Al-Si binary alloy and with the subsequent addition of 2.8 pct Cu and 2.8 pct Cu + 0.7 pct Mg. Metallurgical evaluation included thermodynamic calculations of the solidification process using the FactSage™ 6.2 software package, as well as experimental thermal analysis, and in situ neutron diffraction. The study revealed kinetics of solid α-Al, solid Si, Al2Cu, and Mg2Si evolution, as well as the individual effects of Cu and Mg alloying additions on the solidification path of the Al-Si system. Various techniques applied in this study resulted in some discrepancies in the results. For example, the FactSage computations, in general, resulted in 281 K to 286 K (8 °C to 13 °C) higher Al-Si eutectic temperatures than the ones recorded in the thermal analysis, which are also ~278 K (~5 °C) higher than those observed in the in situ neutron diffraction. None of the techniques can provide a definite value for the solidus temperature, as this is affected by the chosen calculation path [283 K to 303 K (10 °C to 30 °C) higher for equilibrium solidification vs non-equilibrium] for the FactSage analysis; and further complicated by evolution of secondary Al-Cu and Mg-Si phases that commenced at the end of solidification. An explanation of the discrepancies observed and complications associated with every technique applied is offered in the paper.

Effects of Small Addition of Ti on Strength and Microstructure of a Cu-Ni-Si Alloy

The effect of addition of 0.04 or 0.2 mass pct Ti on the mechanical properties of a Cu-2.0 mass pct Ni-0.5 mass pct Si alloy has been investigated. The addition of 0.04 mass pct Ti enhances the strength of the Cu-Ni-Si alloy without reducing its electrical conductivity. This increase in strength is caused by the decrease in inter-precipitate spacing of δ-Ni2Si precipitates. The addition of trace Ti reduces the equilibrium concentration of Ni and Si atoms in the alloy bearing the δ precipitates, resulting in an increase in the volume fraction of δ precipitates and decrease in the inter-precipitate spacing. However, the addition of 0.2 mass pct Ti to the Cu-Ni-Si alloy decreases the strength of the alloy. The reduction in strength is attributed to the decrease in the volume fraction of δ precipitates caused by the reduction in Ni and Si atoms in the Cu matrix resulting from the formation of Ni16Si7Ti6 particles.

Evaluation of the Deformation Behavior of a Semi-solid Hypereutectic Al-Si Alloy Compressed in a Drop-Forge Viscometer

The rheological behavior of an Al-25 mass pct Si alloy, i.e., a hypereutectic Al-Si alloy, was investigated to determine its processability under semi-solid forming conditions. To measure the viscosity of the semi-solid alloy, a parallel-plate drop-forge viscometer similar to that devised by Yurko and Flemings was developed. Drop-forge experiments revealed that the viscosity initially decreased as the shear rate increased and subsequently increased as the shear rate decreased. Thus, the viscosity reached a minimum at approximately the maximum shear rate. The summarized relationship between the viscosity, μ [Pa s], and the shear rate, \( \dot{\gamma } \) [s−1], can be described by the power-law model μ = 1.78 × 107 \( \dot{\gamma } \) −1.5. The decrease in viscosity as a function of the shear rate derived from this equation depends on both the temperature and the applied force but not the duration of deformation. A convex curve was obtained when the effective duration of deformation, i.e., the actual compression time, was plotted as a function of the viscosity and the effective duration of deformation reached a maximum at approximately μ = 30 kPa s (\( \dot{\gamma } \) = 70 s−1). The origin of this profile can be attributed to a combination of both a moderate working time and an adequate deformation, which resulted from a decrease in the deformation resistance accompanied by a lowering of the viscosity. The viscosity at the maximum effective duration of deformation thus corresponds to the transition point for the change in the flow process dominant factor from plastic forming (forging) to casting. Therefore, the viscosity μ = 30 kPa s is believed to be the optimum viscosity for the semi-solid forming of the Al-25 mass pct Si alloy. The approximate temperature condition can be ranged from 855 K to 859 K (582 °C to 586 °C).