Si-containing Alloys Research Articles

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.

The effect of Si addition on the structure and mechanical properties of equiatomic CoCrFeMnNi high entropy alloy by experiment and simulation

The effect of Si addition on the mechanical properties and deformation behaviour of the Cantor alloy in varying compositions from 1 to 5 atomic % has been investigated by experiments and first principle calculations. All alloy compositions exhibit single-phase FCC crystal structures with average grain size ranges from 125–140 μm. Cantor alloy with 1 and 2 atomic % of Si shows an excellent combination of strength and ductility while 5 atomic % Si exhibits the lowest strength and highest ductility (⁓100 %). Detailed TEM analysis of deformed samples revealed the formation of high densities of closely spaced deformation twins in 1 and 2 atomic % Si at room temperature which was observed in Cantor alloy at cryogenic temperatures, while 5 atomic % Si shows the slip-mediated deformation with no observations of the twins. The dependency of the generalized stacking fault energy as a function of Si concentration was investigated by the first principle calculation for Si-containing alloys. The change in deformation mode from twinning for low Si content ( < 4 atomic %) to the dislocation-mediated slip for high Si content ( ≥ 4 atomic %) can be explained based on effective energy barrier, average interlayer distance, and interlayer distance distortion, which is in good agreement with experiments. The study showcases the unique contribution of Si in triggering multiple strengthening and strain hardening mechanisms by stacking fault mediated plasticity in Cantor alloy, thus highlighting the potency of minor alloying additions to single phase HEAs to achieve a large spectrum of strength-ductility paradigm.

Synergy effect of Si addition and pre-straining on microstructure and properties of Al–Cu–Mg alloys with a medium Cu/Mg ratio

The synergy effect of Si addition and pre-straining prior to artificial aging heat treatment on microstructure and mechanical properties of Al–Cu–Mg alloys with a medium Cu/Mg ratio have been investigated by Vickers hardness, room temperature tensile test combined with X-ray diffraction and transmission electron microscopy characterization. It is indicated that the strength was greatly enhanced via adding Si and introducing pre-straining due to the formation finer θ′ precipitates. The pre-straining was more effective for Si-free alloys on improving strength compared with Si-containing alloys. The combined mechanism of Si addition and pre-straining has been discussed in detail.

Influence of Al and Si additions on the microstructure and mechanical properties of Mg-4Li alloys

The microstructure and mechanical properties of four hot-rolled alloys, Mg-4Li, Mg-4Li-3Al, Mg-4Li-3Al-0.38Si and Mg-4Li-6Al-0.76Si, (weight percent) were investigated. The results showed that the addition of Al and Al-Si eutectic results in the precipitation of Al3Li(Mg) within the hot-rolled alloys. The Al3Li(Mg) is present as a solid solution of Mg in Al3Li with an L12 superlattice structure, which precipitates as small spherical particles. Another minor constituent present as a precipitate in the Si-containing alloys, Mg-4Li-3(Al-Si) and Mg-4Li-6(Al-Si), is a Mg2Si phase which is broken into polygonal particles in the hot-rolled alloys. Ultimate tensile strength (UTS) up to 295MPa was measured for the Mg-4Li-6(Al-Si) alloy, compared with 143MPa for the Mg-4Li alloy. This improvement is attributed to precipitation strengthening imparted by the Al3Li(Mg) and Mg2Si phases, as well as solid-solution strengthening due to Al atoms dissolved in the α-Mg matrix of the alloys.

Air-oxidation of FeCoNiCr-based quinary high-entropy alloys at 700–900 °C

The oxidation of three quinary equimolar high-entropy alloys, containing FeCoNiCrAl, FeCoNiCrMn, and FeCoNiCrSi was studied in dry air at 700–900°C. The Mn-containing alloy was single-phase, while both Al- and Si-containing alloys were dual-phase. The oxidation kinetics of all the alloys followed the parabolic rate law, regardless of temperature and alloy composition. The oxidation rates of both Al- and Si-containing alloys were lower than those of the quaternary FeCoNiCr alloy, while those of the Mn-containing alloy went to an opposite direction. Multiple scales formed on the alloys were strongly composition-dependent.

Microstructure and mechanical properties of refractory HfMo0.5NbTiV0.5Six high-entropy composites

HfMo0.5NbTiV0.5Six (x = 0, 0.3, 0.5, 0.7) high-entropy alloys are synthesized by induction levitation melting with the aim of achieving a balanced combination of excellent strength at elevated temperature and reasonable ductility at room temperature (RT). The microstructure, phase evolution and compression mechanical properties of the alloys from 20 °C to 1200 °C are reported in this paper. It is found that the HfMo0.5NbTiV0.5 matrix forms a simple disordered body-centered cubic (BCC) phase. After adding the Si element, multi-component silicide (Hf, Nb, Ti)5Si3 is generated inside the alloys and exhibits a transition from hypoeutectic structure to eutectic structure and then to hypereutectic structure as the Si content increases. The addition of Si significantly improves the hardness and strength but reduces the ductility. At room temperature, The HfMo0.5NbTiV0.5 and HfMo0.5NbTiV0·5Si0.7 alloys show yield strengths of 1260 MPa and 2134 MPa, respectively, and the compressive mechanism transitions from ductile deformation to brittle fracture from x = 0 to x = 0.7. Strain softening and silicide segmentation are found to be typical during compression deformation of these alloys at elevated temperatures. In these conditions, the alloys survive at least 35% of engineering compression strain without fracture. During deformation at 1200 °C, the yield strengths of HfMo0.5NbTiV0.5 and HfMo0.5NbTiV0·5Si0.7 alloys are 60 MPa and 235 MPa, respectively. The attractive strength of the Si-containing alloys at elevated temperatures is strongly dependent on the strengthening effect caused by the silicides.

Role of silicon in the precipitation kinetics of dilute Al-Sc-Er-Zr alloys

The precipitate nanostructure and the strength of an Al-0.055Sc-0.005Er-0.02Zr at% alloy with Si additions, in the range 0–0.18at%, were investigated utilizing micro-hardness, electrical conductivity, scanning electron microscopy and atom-probe tomography techniques. Si-containing alloys are cost-effective due to the existence of Si in commercial purity Al. In all studied alloys, homogenization for at least 0.5h at 640°C is needed to eliminate Al3Er primary precipitates. Alloys containing the higher Si concentrations achieve higher microhardness by increasing the heterogeneous nucleation current of (Al, Si)3 (Sc, Zr, Er) precipitates. The alloy containing 0.18at% Si achieves an 60% improvement in peak-microhardness compared to the Si-free alloy, during isothermal aging at 400°C. Silicon additions reduce the peak-aging time in the temperature range 300–400°C, indicating that the Er and Sc diffusion kinetics are accelerated. Silicon also enhance the Zr diffusion kinetics, accelerating precipitate growth during aging at 300°C and precipitate coarsening at 400°C. Addition of Si modifies the concentration profiles within the nanoprecipitates, enhancing the chemical homogeneity of Sc and Er in their cores, rather than forming Er-enriched-cores/Sc-enriched-shells that we have observed in prior research. Finally, the microhardness of the alloys, containing 0.12 and 0.18at% Si, only diminishes slightly from the peak values after isothermal aging at 375°C for about 2000h, suggesting that the studied alloys can be practically utilized at this operating temperature.

The high-temperature performance of low-cost La[sbnd]Ni[sbnd]Fe based hydrogen storage alloys with Si substituting

Various properties of La0.78Ce0.22Ni3.73Co0.30Mn0.30Al0.17Fe0.5−xSix (x = 0, 0.05, 0.075 and 0.1) hydrogen storage alloys at 20–80 °C have been investigated systematically. With the increase of Si content, the hydrogen storage capacity increases and the equilibrium pressure decreases, thus the improved thermodynamic stability of the metal hydrides makes the Si-containing alloys more suitable for working at higher temperatures. Appropriate addition of Si (x = 0–0.075) improves the anti-corrosion capability of the alloy, as a result, the temperature insensitivity, high-temperature dischargeability and high-temperature recoverability increase accordingly, particularly the cycling stability (S150) increases from 43.7% to 71.8%. In addition, the charge-transfer process is suppressed gradually by Si substituting (x = 0–0.075) at 20 °C, but gets promoted due to the improved surface catalytic ability when that at 60 °C, consequently the high-rate dischargeability and high-power delivery deteriorate at room temperature, but increase gradually at higher temperature. Therefore, an optimum alloy electrode is got when x = 0.075, the deterioration of x = 0.1 alloy on high-temperature performance is attributed to the appearance of the second A2B7 phase.

Microstructures of the Activated Si-Containing AB2 Metal Hydride Alloy Surface by Transmission Electron Microscope

The surface microstructure of an activated Si-containing AB2 metal hydride (MH) alloy was investigated by transmission electron microscopy (TEM) and X-ray energy dispersive spectroscopy (EDS). Regions of the main AB2 and the secondary TiNi (B2 structure) phases directly underneath the surface Zr oxide/hydroxide layers are considered electrochemically inactive. The surface of AB2 is covered, on the atomic scale, by sheets of Ni2O3 with direct access to electrolyte and voids, without the buffer oxide commonly seen in Si-free AB2 alloys. This clean oxide/bulk metal alloy interface is believed to be the main source of the improvements in the low-temperature performance of Si-containing AB2 alloys. Sporadic metallic-Ni clusters can be found in the surface Ni2O3 region. However, the density of these clusters is much lower than the Ni-inclusions found in most typical metal hydride surface oxides. A high density of nano-sized metallic Ni-inclusions (1–3 nm) is found in regions associated with the TiNi secondary phase, i.e., in the surface oxide layer and in the grain boundary, which can also contribute to enhancement of the electrochemical performance.

Effects of Si Addition and Long-Term Thermal Exposure on the Tensile Properties of a Ni–Mo–Cr Superalloy

The effect of long-term thermal exposure on the grain boundary carbides and the tensile behavior of two kinds of Ni-Mo-Cr superalloys with different silicon contents (0 and 0.46 wt%) was investigated. Experimental results showed granular M2C carbides formed at the grain boundaries after exposure for 100 h for the non-silicon alloy. Furthermore, these fine granular M2C carbides will transform into plate-like M6C carbides as exposure time increases. For the Si-containing alloys, only the granular M6C carbides formed at the grain boundaries during the whole exposure time. The coarsening of the grain boundary carbides occurred in both alloys with increasing exposure time. In addition, the coarsening kinetics of the grain boundary carbides for the non-silicon alloy is faster than that of the standard alloy. The tensile properties of both alloys are improved after exposure for 100 h due to the formation of nano-sized grain boundary carbides. The grain boundary carbides are coarsened more seriously for non-silicon alloys than that of Si-containing alloys, resulting in a more significant decrease in the tensile strength and elongation for the former case. Silicon additions can effectively inhibit the severe coarsening of the grain boundary carbides and thus avoid the obvious deterioration of the tensile properties after a long-term thermal exposure.

Improving aging and creep resistance in a dilute Al–Sc alloy by microalloying with Si, Zr and Er

Nanosized precipitates in an Al–0.06Scat.% alloy containing various microalloying additions were studied, with the goal of developing cost-effective aluminum alloys for high-temperature applications, using micro-hardness, electrical conductivity and atom-probe tomography measurements. Substituting 0.005at.% Er for the more expensive Sc maintains high ambient-temperature strength, and dramatically improves the high-temperature creep resistance, as anticipated from the increase in lattice parameter mismatch between the α-Al(fcc) matrix and the coherent L12-ordered Al3(Sc,Zr,Er) precipitates. A concentration of the slow-diffuser Zr as low as 0.02at.% is sufficient to provide coarsening resistance at 400°C (an homologous temperature of 0.72) for up to 66days by forming a Zr-enriched outer shell encapsulating the precipitates. Finally, adding 0.05at.% Si enhances ambient-temperature strength by increasing the number density of precipitates, while decreasing the homogenization and peak-aging heat-treatment times, which is caused by the Si atoms accelerating the Er and Sc diffusion kinetics. Si-containing alloys are also cost effective, owing to the existence of Si in commercial purity Al. But addition of Si reduces the precipitate coarsening resistance: the magnitude of this effect is, however, determined by the Si concentration.

Effects of Si addition on the microstructure evolution of Al–Cu–Mg alloys in the α + S + T phase field

The precipitation behaviour and age-hardening response of Al–1.5Cu–4.0Mg (wt.%) alloys microalloyed with Si have been investigated by means of hardness measurement, TEM and HRTEM. Compared to the ternary alloy, the quaternary alloys exhibit a higher hardness. It is found that the underaged microstructure in the Al–1.5Cu–4.0 Mg alloy contains some fine precipitates, which are identified as the T phase by FFT spectra. The peak-aged microstructures of the ternary alloy is dominated by the T phase, while the peak-aged microstructures of the Si-containing alloys are dominated by the S phase. The volume fraction of the S phase is found to increase as more Si is added.

Electrochemical performance of AB2 metal hydride alloys measured at −40°C

The low-temperature (−40 °C) electrochemical performance of Y-, Si-, Fe-, Cu-, and Mo-partially substituted AB2 metal hydride alloys was studied. The surface area and catalytic ability of these alloys were obtained from the double-layer capacitance and the product of the double-layer capacitance and charge-transfer resistance from AC impedance measurement, respectively. With the increase in amount of substitution, the surface areas in these substituted alloys showed different degrees of increase: Y-addition exhibited the largest increase, followed by Si-addition, and then Cu-, Mo-, and Fe-additions. The surface catalytic abilities of Si-containing alloys were the highest and followed by those of Mo-containing alloys. Cu-, Y-, and Fe-additions reduced the surface catalytic ability. The resistance at −40 °C benefited from the incorporations of Si, Mo, and Y, and deteriorated with the incorporations of Cu and Fe. In the best scenario of a single element-substituted AB2 alloy, the resistance at −40 °C is still higher than that from a commercially available AB5 alloy. Since the contributions to the low temperature performance of AB2 alloys from various modifiers are not the same, the combination of several modifiers is possible for further optimization.

Role of Si in high Bs and low core-loss Fe85.2B10−XP4Cu0.8SiX nano-crystalline alloys

The effects of Si element on structural and magnetic properties of Fe-rich Fe85.2B10−XP4Cu0.8SiX (x = 0–2.5 at. %) alloys were investigated. Our results show that addition of Si significantly reduces the activation energy for nucleation of α-Fe and increases the activation energy for grain growth. As a result, it is much easier to obtain a finer and uniform nanogranular structure (grain size ∼18 nm) made from densely packed α-Fe grains after annealing in the case of Si-containing alloys (Fe-B-P-Si-Cu) in comparison to Si-free alloys (Fe-B-P-Cu). However, addition of Si on the expense of B reduces the amorphous forming ability of the alloy, which results in lower reproducibility. The reproducibility improves significantly in Si-free alloy, but the structure of the alloy is relatively unstable on annealing, which means more strict annealing treatment is required. After optimum annealing treatment, Si-free alloys (Hc ∼ 6 A/m, Js ∼ 1.83 T) show superior soft magnetic properties than the Si-containing alloys (Hc ∼ 10 A/m, Js ∼ 1.78 T). Results show that the excellent soft magnetic cores can be obtained only if the extra heat generated on nano-crystallization of as-quenched amorphous phase can be released efficiently. The toroidal core of Si-free alloy (core-loss, W ∼ 0.58 W/kg at ∼1.7 T, 50 Hz) exhibits lower magnetic core loss than the Si containing alloys (core-loss, W ∼ 1.51 W/kg at ∼1.7 T, 50 Hz).

High Temperature Corrosion Behavior of Si-Containing Alloys in the Liquid Phase of Na&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt; 25.7 Mass% NaCl

The high temperature corrosion behavior of Si-containing alloys consisting of Cr-Si-Ni and CoNiCrAlY-Si alloys fabricated by spark plasma sintering technique was investigated in the liquid phase of Na2SO4 + 25.7 mass% NaCl at temperatures ranging from 923-1273 K. The purpose of this study is to develop excellent corrosion resistant alloys for coating applications. Our experimental results show the CrSi2 alloy with 10 mass% Ni content and the CoNiCrAlY alloy with 30 mass% Si content are the most promising materials for applications in this atmosphere. This is due to the formation of a protective SiO2 and Al2O3/SiO2 scale, respectively. The formation of a dense and continuous oxide layer composed and/or consisted of SiO2 plays a significant role in hindering the inward diffusion of chlorine and sulfur to the alloys substrate. Particularly, the corrosion mechanism of Cr-Si-Ni alloys and the influence of Ni addition on the corrosion resistance of CrSi2 alloy are discussed in the present paper.

High Temperature Corrosion Behavior of Si-Containing Alloys in the Gas Phase of Air-(Na&lt;sub&gt;2&lt;/sub&gt;SO&lt;sub&gt;4&lt;/sub&gt;+25.7mass%NaCl)

The objective of this study is to develop an excellent corrosion resistant alloy for high temperature coating applications. The Si-containing alloys consisting of CoNiCrAlY and CrSi2 alloys with varying Si and Ni content respectively were prepared by spark plasma sintering (SPS) technique. The corrosion behavior of these alloys was investigated in the gas phase of air-(Na2SO4+25.7mass%NaCl) at elevated temperatures of 923, 1073 and 1273K. The results showed that CoNiCrAlY alloy with 30mass% Si content and CrSi2 alloy with 10mass% Ni content were the most effective materials for application in the gas phase of air-(Na2SO4+25.7mass%NaCl) due to the formation of protective Al2O3/SiO2 and SiO2 scale, respectively. Therefore, it is realized that CoNiCrAlY-30mass% Si and CrSi2-10mass% Si coating are very effective for improving of high temperature corrosion resistance of STBA21 steel.

Half-Metallicity and High-T c Ferromagnetism in Si-containing Half-Heusler Alloys

The structural and magnetic properties of XYSi (X = Ni, Pd, Pt; Y = Mn and Cr) are investigated through the ab initio calculation of the electronic structure, equilibrium lattice constant, magnetic exchange interaction and Curie temperature. The role of X and Y elements in the opening of the minority spin gap is discussed. The Fermi level lies closer to the conduction band for slighter X or Y. XYSi is half-metallic ferromagnets at equilibrium lattice constant, except PdMnSi. The Curie temperature of 1076, 785 and 634 K has been predicted by Monte Carlo simulations for NiMnSi, PtMnSi and NiCrSi, respectively.

Three-dimensional atom probe study of Fe–B-based nanocrystalline soft magnetic materials

Solute clustering and partitioning in new Fe–B-based soft magnetic materials with high saturation magnetic flux density ( B s), (Fe 0.85B 0.15) 100− x Cu x ( x = 0.0, 1.0, and 1.5) and Fe 82.65Cu 1.35Si y B 16− y ( y = 0.0, 2.0, and 5.0) melt-spun alloys, were investigated by three-dimensional atom probe and transmission electron microscopy. Although Cu clusters form after annealing in all the samples, it was found that only the clusters of 4–6 nm can serve as heterogeneous nucleation sites for α-Fe. While annealing the Si-free alloys at 410 °C led to the precipitation of Fe 3B, only α-Fe nanocrystals were observed in the Si-containing alloys. Lorenz TEM observation indicated the Fe 3B particles pin magnetic domain walls. The Fe 82.65Cu 1.35Si y B 16− y alloy with y = 2.0 crystallized by annealing at 400 °C exhibited optimal nanocrsytal/amorphous microstructure without the precipitation of Fe 3B, which led to the lowest coercivity while keeping a high B s ∼1.85 T.

The Effect of Si Content on the Mechanical Properties of Rheocast Al Components Using the RHEOMETAL&lt;sup&gt;TM&lt;/sup&gt; Process

The RHEOMETALTM process is a commercially used semi-solid process for production of high integrity cast components. The process differs from most other semi-solid casting processes in that temperature control is not necessary during processing and large amount of slurry with required solid fraction can be quickly produced. The simplicity of this process has led to a large commercial interest during the last year. This work is based on an investigation regarding the variation of as-cast mechanical properties for secondary Al-Si based alloys (~2.5 % Cu) with a Si content varying from 4.55 to 8.90 % using the RHEOMETALTM process. The purpose was to find the most suitable Al-Si alloy for rheocasting, in comparison with the common HPDC-alloy A380 (EN-AC46000). It was found that lower Si containing alloys exhibited better elongation but slightly lower yield strength. The alloy containing 5.39 wt% Si showed the highest ultimate tensile strength in this investigation. The lower Si containing alloys also demonstrated better feedability in the rheocasting process, which is contrary to what normally is found for normal liquid casting processes. Based on the results in this investigation it is recommended to use an alloy containing about 5-7 wt% Si for rheocasting purposes.

Kinetic and Thermodynamic Aspects of High-Temperature Oxidation of Selected Ti-Based Alloys

In the presented work, high-temperature cyclic oxidation of pure Ti, Ti-7.8wt.%Si, Ti- 45.6wt.%Al and Ti-11.7wt.%Al-7.2wt.%Si alloys at 750 – 1050 °C in air were studied. It was found that both Si and Al strongly reduce the oxidation rate of titanium, mainly up to 850 °C. They incorporate into the oxide scales and reduce penetration rate of oxygen towards the metallic substrate. Mechanism of the barrier effects of the elements is discussed with respect to their distribution in alloys. It was observed by comparison of Al-containing and Si-containing alloys, that the influence of Si on the oxidation kinetics is significantly stronger. It can be supposed that a synergic effect of silicon and nitrogen is a reason for this behavior.