Si-free Alloy Research Articles

Atomic mechanism of enhanced thermal stability in Al-Cu-Mg-Si alloys with a low Cu/Mg ratio

The Si-modified Guinier-Preston-Bagaryatsky (GPB) zones are claimed to be responsible for the high-thermal stability of Al-Cu-Mg-Si alloys with a low Cu/Mg ratio. However, the structure of this phase and its formation mechanism remain unclear. In the present study, the precipitation behaviors of two Al-Cu-Mg alloys with or without Si addition upon thermal aging at 200 °C are characterized using atomic-resolution imaging techniques and the ab initio calculation. It is found that the Si-modified GPB zone is needle-shaped β'/GPB composite with core/shell structure. The core part is β' phase (Mg9Si5), while the shell is composed of the conventional GPB zones. In the early stage of aging, β' phase precipitates rapidly and acts as the heterogeneous nucleus of GPB zones. With the aging time extending to 60 h, the diameter of the β'/GPB composites remains almost constant, since the GPB zones effectively hinder the coarsening of the internal β' phase, thus promoting the thermal stability of the alloy as compared with the Si-free alloy. Our results clarify the mechanism of Si, at atomic scale, to improve the thermal stability of Al-Cu-Mg alloys with a low Cu/Mg ratio.

Microstructural and mechanical properties of precipitation-strengthened Al-Mg-Zr-Sc-Er-Y-Si alloys

We study the precipitation behavior of two L12-strengthened alloys with Mg and Y additions: ultralow-Sc, Si-free, Al-1Mg-0.09Zr-0.007Sc-0.006Er-0.02Y-0.01Si, and low-Sc, Si-added, Al-1Mg-0.09Zr-0.013Sc-0.006Er-0.02Y-0.08Si (at.%), and their ambient temperature strength and high-temperature creep resistance. Scanning transmission electron microscopy analyses reveal that β-Mg2Si precipitates or their precursors (β’, β’’) form in the Si-added alloy at ∼ 200°C, which act as preferential nucleation sites for the L12-nanoprecipitates and cause partial depletion of Si solute atoms, which decelerates L12 precipitation-kinetics, resulting in a microhardness peak at the same isochronal temperature, 475°C, in both alloys. Atom-probe tomography analyses reveal that the L12-nanoprecipitates in both alloys exhibit Sc/Er/Y-rich cores and Zr-rich shells, as well as Mg segregation at the interfacial regions. The L12-nanoprecipitates in the Si-added alloy has a smaller Sc/Er/Y and higher Zr and Si concentrations. The Si-free alloy exhibits superior creep properties at 300°C, due to a larger lattice parameter mismatch of the L12-nanoprecipitates with the Al(f.c.c.) matrix provided by higher Sc/Er/Y concentrations. Both alloys exhibit slower L12 nanoprecipitation-kinetics, extremely high coarsening resistance, and similar high-temperature creep resistance compared to a recently developed Mg/Y-free low-Sc, Al-0.08Zr-0.014Sc-0.008Er-0.09Si (at.%) alloy. The extremely high coarsening resistance of the Si-added alloy is attributed to the smaller Si concentration in the Al(f.c.c.) matrix. Silicon is scavenged by the β-Mg2Si precipitates, thermodynamically stable at < 475°C, which is then not available in the matrix to accelerate the coarsening of the L12-nanoprecipitates.

Influence of Si on the microstructure and C redistribution in martensitic steels

In martensitic steels the distribution of C is fundamental for the technological material properties. However, the effect of Si on C segregation is not known in detail. In this study, the impact of Si on the microstructure of martensitic steels is investigated using scanning electron microscopy and the martensite/austenite orientation relationships are determined using electron backscatter diffraction and transmission Kikuchi diffraction. Transmission electron microscopy and selected area diffraction pattern analysis were used to analyse the intragranular substructure of the martensitic phase. Further, the effect of Si on the redistribution of C is investigated by atom probe tomography in combination with transmission Kikuchi diffraction measurements in order to obtain the local C distribution and correlate it with the crystallography. It was found that the Si addition forms a sigmoidal distribution at the phase boundary and functions as a barrier for C segregation, hence reduces C partitioning into the austenite, while a Si free alloy shows clear C partitioning. Further it was observed that stable C cluster formation occurs along fine twin boundaries in the martensite, inhibiting carbide formation and partitioning of the C into the austenite.

Revealing trace Si effect on precipitation behavior and properties of Cu-Cr-Zr alloy: Experiments and first-principles calculations

High-performance Cu-Cr-Zr-X alloys are regarded as novel structural materials for nuclear fusion reactors. Through combining the experiments and density functional theory (DFT), the effect of Si addition on precipitation behavior and related properties of Cu-Cr-Zr-(Si) alloy was investigated for higher performance. Compared with the Si-free alloy, precipitation of Cr-rich phase is accelerated in the Cu-Cr-Zr-Si alloy (aging time<30 min at 450 °C). The partial radial distribution functions of the Cu-rich matrix reveal that the formation of Si-Cr clusters is resulted from the attractive Si-Cr binding energies at the nearest-neighbor distance. Moreover, the addition of Si can reduce the Cr migration energy in Cu. At the initial stage of aging at 550 °C, the precipitation kinetics is dominated by temperature. By TEM analysis, it is indicated that the growth of Cr-rich phase in Cu-Cr-Zr-Si alloy is delayed, which may be attributed to the decrease of Cu/Cr interface energy due to the addition of Si, as confirmed by first-principles calculations. The optimized performance of aged Cu-Cr-Zr-Si alloy can be achieved at 450 °C/4 h, and Orowan strengthening is dominant. This work can provide a guideline for design of novel high-performance Cu-Cr-Zr-X alloys.

Silicon Contamination During Alloy Oxidation in Water Vapour at 650 °C

A model Si-free alloy, Fe-30Cr (wt.%), was exposed to Ar-10H2O mixture (in vol.%) at 650 °C, forming a protective Cr2O3 scale, which contained silica. Liquid water used to form water vapour in a bubbler was either deionised or distilled. In both cases, low-level water impurities of Si, F, Cl, S, and Ca matched those detected in the Cr2O3 scales by TEM/EDX and ToF–SIMS. Consideration of water droplet and residual solute particle sizes, together with Stokes’ Law shows that entrainment of Si in the gas stream is likely. Calculation of mass transfer rates from the gas to the growing chromia scale show that the amount of silica in the scale is accounted for. Many laboratory experiments could be affected in this way.

The effect of pre-deformation on the precipitation behavior of AlCuMg(Si) alloys with low Cu/Mg ratios

The addition of a trace amount of Si significantly affects the precipitation sequence of the Al–Cu–Mg alloy with a low Cu/Mg ratio, such as forming Si-modified Guinier-Preston-Bagaryatsky (GPB) zones to suppress the precipitation of the S phase. The introduction of pre-deformation also affects the precipitation behavior during the subsequent aging process. In this paper, the atomic-resolution high-angle-annular-dark-field (HAADF) imaging technique and first-principles calculation are used to study the effect of pre-deformation on the precipitation behavior of an Al-3.0Cu-1.8Mg-0.5Si alloy during aging at 180 °C. In order to learn the effect of minor Si-addition, a Si-free alloy with similar chemical composition is also studied. It is found that for Si-free alloy, the 6% pre-deformation promotes the precipitation of the S phase dispersedly during the subsequent aging, leading to an increase of the peak hardness. For Si-containing alloy, the 6% pre-deformation prior to aging suppresses the precipitation of Si-modified GPB zone acting as the original main strengthening precipitates and promotes the formation of successive composite precipitates including the S phase, various GPB zones, C phase and sub-units of C/Qʹ phase. The successive composite precipitates are easy to grow up and coarsen, which weakens the contribution of precipitation strengthening compared to the small Si-modified GPB zone. As such, the 6% pre-deformation prior to aging at 180 °C provides a positive strengthening effect for Si-free alloy, but a negative strengthening effect for the Si-containing alloy.

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.

Simultaneously improving elastic modulus and damping capacity of extruded Mg-Gd-Y-Zn-Mn alloy via alloying with Si

Developing magnesium (Mg) alloys with both high elastic modulus and damping capacity is a long-term challenge in the field of lightweight metals. Herein, it is shown that alloying with Si in Mg-Gd-Y-Zn-Mn simultaneously increased the elastic modulus and damping capacity. After Si alloying, the LPSO phase was greatly reduced and a large number of (RE + Si)-rich particles were formed. Due to the contribution of the high modulus second phases, the elastic modulus of the extruded Mg-Gd-Y-Zn-Mn alloy with a trace amount of Si was 49.3 GPa, 8 GPa greater than that of the extruded pure Mg. In addition, the extruded Mg-Gd-Y-Zn-Mn-Si alloy possessed good damping capacity at both room temperature (RT) and high temperature. At high strain amplitude, the extruded Mg-Gd-Y-Zn-Mn-Si alloy achieved a RT damping value of Q−1 > 0.01, significantly higher than the extruded pure Mg, which was related to the high density of thermal mismatch dislocations in the vicinity of the interfaces between the reinforcing phase and the α-Mg matrix. With the increasing temperature, the extruded Mg-Gd-Y-Zn-Mn-Si alloy showed obviously higher damping capacity than the extruded Si-free alloy, which was attributed to the incoherent phase interfaces, weakened texture and thermal mismatch dislocations.

Aging and recrystallization behavior of quaternary Al-0.25Zr-0.03Y-0.10Si alloy

Aging behavior of Al-0.25Zr-0.03Y-0.10Si were investigated to evaluate the role of 0.10%Si usually contained in commercially pure Al. Compared with the Si-free ternary alloy (Al-Zr-Y), accelerated aging kinetics, enhanced hardening effect and higher conductivity were obtained. Aging precipitates with tripled number density were observed in alloy Al-0.25Zr-0.03Y-0.10Si, whereas size and coarsening resistance were not affected by Si content. Contribution of Si provides a large number of nuclei, rather than accelerating the diffusion of Zr. Formation of primary Al3Y during solidification resulted in a 50 °C drop in recrystallization temperature.

Formation of nano-sized M2C carbides in Si-free GH3535 alloy

GH3535 alloy is one of the most promising structural materials for molten salt reactors (MSRs). Its microstructure is characterized by equiaxed grains and coarser primary M6C carbide strings. In this study, stable nano-sized M2C carbides were obtained in GH3535 alloy by the removal of Si and thermal exposure at 650 °C. Nano-sized M2C carbide particles precipitate preferentially at grain boundaries during the initial stage of thermal exposure and then spread all over the grain interior in two forms, namely, arrays along the {1 1 1} planes and randomly distributed particles. The precipitate-free zones (PFZs) and the precipitate-enriched zones (PEZs) of the M2C carbides were found to coexist in the vicinity of the grain boundaries. All M2C carbides possess one certain orientation relationship (OR) with the matrix. Based on microstructural characterizations, the formation process of M2C carbides with different morphologies was discussed. The results suggested that the more-stable morphology and OR of M2C carbides in the Si-free alloy provide higher hardness and better post-irradiation properties, as reported previously. Our results indicate the preferential application of Si-free GH3535 alloy for the low-temperature components in MSRs.

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.

Enhanced mechanical properties and structure stability induced by Si in Ti–8.5Al–1.5Mo alloys

The crystalline microstructures, mechanical properties and structural stability of Ti-8.5Al-1.5Mo-xSi (wt%, x=0%, 0.2%, 0.4%, and 0.6%) alloys were comprehensively investigated. It is shown that with the addition of certain amount of Si, Widmanstatten microstructure was refined and high density of dislocation lines was accumulated in β film and at the grain boundaries between α and β phase. The addition of 0.2% and 0.4% Si induced suppression of the transformations of α→α2, α2→α and α→β in heating process and slow-down of the coarsening tendency of Widmanstatten microstructure during the annealing process. It has been found that the addition of Si results in enhanced tensile mechanical properties and overcomes the strength-ductility trade-off, e. g. the fracture strength and elongation of the alloy with 0.2% Si were increased by 10.5% and 68.5%, respectively, as compared with those of Si free alloy. The strengthening and toughening mechanism are attributed to the solid solution strengthening of Si in α and β phase, the grain boundary and Cottrel atmosphere strengthening, and remaining β phase induced toughening.

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.

Effect of silicon on microstructure and stress rupture properties of a corrosion resistant Ni-based superalloy during long term thermal exposure

The influence of silicon on microstructure and stress rupture properties of GH3535 nickel-based superalloy during long term thermal exposure at 700°C was investigated. After long term thermal exposure, the secondary carbides precipitated along grain boundaries. Si addition induced the formation of M12C carbides, while only M6C carbides were observed in Si free alloy. The secondary M12C carbides exhibited particular orientation relationship with matrix for the alloy with Si, but the secondary M6C carbides are incoherent with matrix for the Si free alloy. It is believed that the difference in the types of the secondary carbides was originated from Si. The stress rupture life decreased after 1000h thermal exposure for the Si free alloy, but no noticeable changes could be detected for the alloy with Si. The interface between the secondary M6C carbides and matrix became the main site for crack initiation after 1000h thermal exposure for the Si free alloy. But no cracks was detected at the interface between the secondary M12C carbides and matrix even after 10,000h thermal exposure, which may be related to the strong interfacial cohesive force between secondary carbides and matrix.

Improving wear performance of dual-scale Al–Sn alloys by adding nano-Si@Sn: Effects of Sn nanophase lubrication and nano-Si polishing

In this work, an attempt was made to further promote the wear and friction behavior of dual-scale structured Al–12wt% Sn bearing alloys by coupling with a nano-Si@Sn composite. A so-called tri-modal Al–Sn–Si (denoted as CG-30 Si-containing alloy), which exhibited increased bulk hardness and improved distribution and refinement of the Sn phase, was produced by combining mechanical alloying and conventional powder sintering. The wear performance of the Si-containing and Si-free alloys was differentiated by performing both block-on-ring and ball-on-disk testing. Moreover, the addition of nano-Si@Sn composites to the dual-scale Al–Sn matrix resulted in significant reductions of the friction coefficient and wear volume. These improvements in wear behavior stemmed from the polishing effect of fine Si particles on the counterface, and Sn-rich clusters that acted as lubricants on the worn surface; together, the polishing effect and lubrication led to the formation of a smooth tribolayer, and hence, a decrease in the tendency for abrasive wear. Abrasive wear was also reduced by an increase in the alloy hardness with the addition of Si. As such, Sn-phase modification via Si addition constitutes a promising method for preparing self-lubricating Al-based alloys.

A comparative study of powder metallurgical (PM) and wrought Fe–Mn–Si alloys

In this study Fe–Mn–Si alloys with Si content up to 4wt% were developed as a degradable biometal. Fe–Mn–Si alloys were fabricated by both powder metallurgy (PM) and wrought processes. In the PM route, mixtures of elemental Fe, Mn and Si powders were ball-milled, pressed under 400MPa and subsequently sintered in a high vacuum furnace at 1200°C for 3h. In the wrought route, Fe–Mn–Si alloys were arc melted in high vacuum and subsequently hot forged. The PM alloys attain a relative density of 80.2–82.5% and the powder densification increases from 36% for the Si-free alloy to 53.1% for the Fe–28Mn–4Si alloy. The phase constituents depend on the Si content and forging process. Duplex ε-martensite and γ-austenite phases exist in the sintered Fe–28Mn–xSi (where x>3wt%) and forged ternary alloys. The tensile strength of both sintered and forged alloys increases with increased Si content. The fracture strain for the powder sintered alloys is generally <4.2%, while the forged alloys demonstrate significantly high ductility, in the range of 13.6–40.4% depending on the Si content. The electrochemical measurements reveal that the corrosion rate of the powder sintered alloys decreases, whereas that of the forged alloys increases with Si. The discrepancy of corrosion behaviour in the powder sintered and the forged samples is attributed to the presence of duplex phases and also to the porosity.

Effects of Annealing Temperature on the Electrochemical Hydrogen Storage Behaviors of La-Mg-Ni-Based A2B7-Type Electrode Alloys

In an attempt to improve the cyclic stability of La-Mg-Ni-based A2B7-type electrode alloys, La0.8Mg0.2Ni3.3Co0.2Six (x = 0−0.2) electrode alloys were fabricated by casting and annealing, and the effects of annealing temperature on the structures and electrochemical hydrogen storage performances of the alloys were systematically investigated. The results indicate that the as-cast and annealed alloys exhibit multiple structures that contain two major phases, (La,Mg)2Ni7 with a Ce2Ni7-type hexagonal structure and LaNi5 with a CaCu5-type hexagonal structure; and one residual phase, LaNi3. Both the lattice constants and cell volumes of the two major phases increase with the increasing annealing temperature. Instead of altering the phase composition, the annealing treatment causes the abundances of these two major phases to vary. Based on electrochemical measurements, the cycle stabilities of the alloys are found to be considerably improved by annealing, and the alloy’s discharge capacity yields a maximum value with the increasing annealing temperature due to the variation in phase abundance and the homogenization of the composition, respectively. The influence of the annealing treatment on the electrochemical kinetics of the alloys is associated with the alloy’s composition; the electrochemical kinetics of the Si-free alloy become slower with the increasing annealing temperature, whereas those of the Si-added alloys first mount up and then go down under the same conditions.

Short Term Oxidation Behavior of Si-Free Low-Cr Alloy Sputtered Coating

A sputtered coating of a low-Cr alloy without Si was deposited on the cast alloy with the same composition. The short term (100 h) oxidation behavior of the sputtered coating and the cast alloy was evaluated in air at 800 °C. The results indicated that the sputtered coating exhibited a higher oxidation resistance than the cast alloy. It was found that the mass gain of the cast alloy increased continuously with oxidation time and was higher than that of the sputtered coating, which demonstrated only a slight increase in mass gain with oxidation time after 5 h thermal exposure. During the initial thermal exposure of 0.5 h, the oxide scale formed on the cast alloy consisted of Fe2O3 and (Fe,Co,Cr)3O4 spinel with a small amount of Cr. However, (Fe,Co,Cr)3O4 spinel and Fe2O3 were thermally grown on the sputtered coating. After oxidation for 100 h, the oxide scale formed on the cast alloy consisted of Co3O4 and (Fe,Co)3O4 with internal oxide of Cr, while a double-layer oxide consisting of an outer (Fe,Co,Cr)3O4 spinel layer and an inner Cr2O3 layer was developed on the sputtered coating.

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).

Microstructure and magnetic properties of (Fe100−xCox)84.5Nb5B8.5P2 alloys

Partial substitution of Fe with Co in gradually devitrified Si-free Nanoperm-type alloys improves their soft magnetic properties, increases the Curie temperature of the amorphous matrix, and allows casting in air. In the present work we report the temperature dependence of structural and magnetic properties of (Fe100−xCox)84.5Nb5B8.5P2 (x=20, 40, 60) alloys using differential scanning calorimetry, magnetic, thermo-magnetic and X-ray diffraction measurements, to get complementary information on the microstructure, composition of the precipitated crystalline phase and their correlation with magnetic properties. Higher Co concentrations improve the magnetic softness of the alloy, whose properties can be further optimized by suitable thermal treatments. Magnetic data is discussed in terms of the precipitation of a Fe–Co crystalline phase consisting of different Co content.