Si Precipitation Research Articles

The effect of bonding time on the microstructure and mechanical properties of dissimilar transient liquid phase bonding between UNS N08825 alloy and UNS S32750 super duplex stainless steel using the BNi-2 interlayer

• The dissimilar transient liquid phase (TLP) bonding of the UNS N08825 alloy to UNS S32750super duplex stainless steel was successfully accomplished using the BNi-2 interlayer. • The complete isothermal solidification occurred at 1050 °C and the bonding time of 300 s. • Shear strength of the TLP joints improved by increasing the holding time. In this study, the microstructure and mechanical properties of transient liquid phase, dissimilar joints between UNS N08825 alloy and UNS S32750super duplex stainless steel was investigated. The joining process was carried out at 1050 °C for 0, 60, 120, and 300 s using an BNi-2 interlayer. The transient liquid phase bonding area was divided into three zones: (i) the isothermal solidification zone which contained γ-Ni solid solution, (ii) the a-thermal solidification zone which consisted of Ni- and Cr-rich borides and Ni 3 Si precipitates, (iii) and the diffusion affected zone in which Cr-Ni-Fe-Mo-Cu-rich borides constituents existed. The hard and brittle eutectic compounds in the bonding zone posed deleterious effects on the mechanical properties of the joints. By increasing the holding time, the amount of these detrimental phases in the bonding zone decreased before finally a eutectic-free joint was achieved at the bonding time of 300 s, which was notably low as compared to the previous studies. The joint held for 0 s had the widest a-thermal solidification zone and the highest hardness was measured at the center of it (546 Hv). The average hardness of the diffusion affected zone on the UNS S3750 super duplex stainless steel side was approximately 450 Hv, which was markedly higher than that of the diffusion affected zone on the UNS N08825 side (302 Hv). The joint held for 0 s showed the lowest shear strength (239 MPa), while the maximum shear strength was obtained for the joint with a holding time of 300 s, approximately 487 MPa. Moreover, it was demonstrated that eutectic compounds were the principal cause of the brittle fracture in the joints containing the a-thermal solidification zone, whereas the joint with a complete isothermal solidification experienced a brittle fracture that occurred at the diffusion-affected zone of UNS N08825 alloy.

Effects of processing parameters and heat treatment on thermal conductivity of additively manufactured AlSi10Mg by selective laser melting

This work investigates the thermal conductivity of parts which have been additively manufactured using the aluminium alloy AlSi10Mg by selective laser melting, a laser-based powder bed fusion technique. Thermal conductivity characterisation is of particular importance to thermal engineers wishing to make use of additive manufacturing in next generation thermal management solutions.A number of processing parameters and scanning strategies were employed to fabricate samples for experimental characterisation. While the porosity of produced parts had a significant impact on thermal conductivity, after an anneal heat treatment post-processing step, thermal conductivity increased by 18–41% without any measurable change in porosity. Even though the parts produced with the “points” strategy have higher levels of porosity compared to the “contour-hatch” strategy, it has been found that after the heat treatment step, its thermal conductivity can be increased up to the “contour-hatch” strategy. Analysis of the resulting microstructures using scanning electron microscope and energy-dispersive X-ray showed precipitation and coalescence of Si with increasing heat treatment temperature, with dwell time having a lower impact.While there is a desire for additively manufactured parts with little to no porosity, it has been shown in this study that it is possible to reduce laser energy density requirements by approximately one order of magnitude and still produce parts with acceptable levels of thermal conductivity which could be used for components that are not subjected to strenuous loading conditions, such as heat sinks.

Effects of Silicon and Heat-Treatment on Microstructure and Mechanical Properties of Biomedical Ti-39Nb-6Zr Alloy

The cytotoxic tissue reactions of alloying elements (Al, V) of Ti-6Al-4V have been reported, whereas the Ti-39Nb-6Zr (TNZ40) alloy developed by adding β-phase stabilizing elements is known to have no cytotoxicity and exhibits excellent biocompatibility. In addition, there is a slight modulus difference between the TNZ40 alloy and human bones as the elastic modulus of the TNZ40 alloy is very low. This can inhibit detrimental effects such as osteoblast loss due to a stress-shielding effect. In this study, various Si contents were added and heat treatment under various conditions was performed to control the microstructure and mechanical properties of the TNZ40 alloy. In the β-type titanium alloy, the ω phase is commonly observed by quenching from the solution-treatment or aging-treatment temperature. These ω precipitates can typically increase the elastic modulus, hardness, and embrittlement of the β-type titanium alloy, which are important to control this phase. The correlation between Si content and precipitation and the effects of solution treatment and aging condition on the mechanical properties such as tensile strength, and hardness, were analyzed.

Rapid homogeneous precipitation of nano-sized Si in Al–1%Si alloy by electric pulses

Rapid and homogeneous precipitation of nano-sized Si particles with high density was achieved in Al–1%Si alloy by electric pulses treatment (EPT) in a liquid nitrogen bath (∼77 K). In contrast, when processed by EPT in air or by high-temperature (527–720 K) salt bath treatment, the precipitate density decreases dramatically. The homogeneous precipitation of Si could be facilitated by both the low-temperature environment and the non-thermal effects of EPT. The low-temperature environment provides a high driving force for nucleation, and retains the quenched-in vacancies serving as potential nucleation sites. The non-thermal effects of EPT can significantly promote local migration of solutes and vacancies at such low temperatures.

Influence of Residual Stress Around Constituent Particles on Recrystallization and Grain Growth in Al-Mn Based Alloy During Annealing

The recrystallization behavior of the homogenized AA3003 aluminum alloy with reduction rate of 20%, 50% and 90% was investigated. The change in the microstructure and dislocation density during annealing was characterized by TEM image analysis and Williamson-Hall plot. It was found that as the reduction rate increased, the microstructure evolves into a lamellar shape parallel to the rolling direction with the increased dislocation density. By applying subsequent annealing process, the dislocation density under the recovery stage was decreased by about 10%, which suggests the dislocation rearrangement. In the meanwhile, the dislocation density under the recrystallization stage was rapidly decreased due to the evolution of recrystallized grains. The effect of particle stimulated nucleation on the recrystallization was discussed in terms of residual stress around the α-Al(Mn, Fe)Si precipitate by solving inhomogeneous inclusion problem under cold rolling.

Effects of W micro-additions on precipitation kinetics and mechanical properties of an Al–Mn–Mo–Si–Zr–Sc–Er alloy

A micro-addition (0.025 at.%) of slow-diffusing W to an Al-0.26Mn-0.11Mo-0.11Si-0.08Zr-0.02Sc-0.01Er (at.%) alloy accelerates L12-Al3(Zr,Sc) precipitation but retards α-Al(Mn,Mo)Si precipitation. The W micro-addition increases the peak-microhardness of the alloy during isothermal aging but does not improve the coarsening resistance and creep resistance.

Microstructural Characteristics of Unidirectionally Solidified Cast Al–Si–Cu Alloy

The properties of a conventional Al–Si–Cu alloy (ADC12) material, fabricated using a unidirectional casting process and slowly solidified in the range 0.02–0.14 °C s−1, were determined. The microstructural characteristics of the cast sample were dependent on sample area and resulted from changes in the amount of Si present during solidification. In the lower and middle regions, where the α-Al phase formed relatively organized crystal structures of different patterns, e.g., 〈101〉, columnar grain growth of α-Al dendrites with a low eutectic Si content was observed. Although columnar grain growth was also found in the upper region, it was randomly formed and the area was narrower. Random crystal orientation (i.e., weak control of unidirectional solidification) was created by interrupting columnar α-Al dendrite growth, which resulted from changes in the dynamics of the alloyed Si atoms. Eutectic Si is considered the only Si precipitate in ADC12; however, primary Si was also formed in the middle and upper regions, which was attributed to high Si concentrations resulting from Si migration to the upper region. Fine and coarse microstructures were observed in the lower and upper regions, respectively, with the middle region acting as a transition zone in which the amount of Si rapidly increased following transport between the lower and upper regions. A high amount of hard Si precipitate in the upper region of the sample resulted in high hardness values. In contrast, due to its fine microstructure with unidirectional crystal formation, the lower region exhibited high tensile strength and high ductility.

Effects of CeO2 on the Si Precipitation Mechanism of SiCp/Al-Si Composite Prepared by Powder Metallurgy.

SiCp/Al-Si composites with different CeO2 contents were prepared by a powder metallurgy method. The effect of CeO2 content on the microstructure of the composites was studied. The mechanism of CeO2 on the precipitation of Si during sintering was analyzed by theoretical calculations. The results show that the appropriate amount of CeO2 can significantly refine the size of precipitated Si particles in the composite and increase the number of Si particles. With the increase of CeO2 content from 0 to 0.6 wt%, the number of Si particles precipitated in the composites increases gradually, and the average particle size of Si particles decreases gradually. When the CeO2 content is 0.6 wt%, the number of Si particles precipitated in the composites reaches the maximum, and the average particle size reaches the minimum. However, with the increase of CeO2 content from 0.6 wt% to 1.8 wt%, the number of Si particles precipitated in the composites began to decrease, and the average size of Si particles gradually increased. CeO2 can be used as heterogeneous nucleation substrate of precipitated Si, and the nucleation rate of precipitated Si on a CeO2 substrate is higher than that on an aluminum substrate. The proper addition of CeO2 can improve the nucleation efficiency of precipitated Si, thus increasing the amount and refining the size of precipitated Si.

Microstructural evolution and mechanical performance of in-situ TiB2/AlSi10Mg composite manufactured by selective laser melting

In-situ TiB2/AlSi10Mg composites were prepared by casting, gas atomization, and selective laser melting (SLM), respectively. Microstructural evolution of the composites in as-cast, atomized, and SLMed were investigated. Compared with the as-cast composite, the SLMed sample has no texture and is composed of extremely fine grains and crystal cells. The in-situ TiB2 particles uniformly distribute over the grain boundaries, the acicular Si phase is replaced by a network of fine Si particles and there are nano-sized Si precipitations in the crystal cell. The formation of this microstructure is due to the continuous solidification of non-equilibrium eutectic Al–Si during the rapid cooling process of SLM. The SLMed TiB2/AlSi10Mg composite obtains excellent strength (tensile strength ∼ 451 MPa, bending strength ∼ 674 MPa), hardness (∼151 HV0.1) and good friction coefficient (∼0.4), which are higher than that of the as-cast TiB2/AlSi10Mg composite and the SLMed AlSi10Mg alloy. The formation mechanism of this particular structure and its strengthening effect were discussed.

Bulk Si production from Si–Fe melts under temperature gradients, part I: Growth and characterization

Silicon growth from Si–Fe melts was investigated in view of producing high-quality Si using induced temperature gradients. Scanning electron microscopy coupled with energy-dispersive spectroscopy and micro-computed tomography analyses were used to characterize the quality of bulk Si, in particular its microstructure and purity. The migration of Si and Fe atoms along a temperature gradient in the mushy zone of the Si–Fe alloy was evidenced, which became the source of Si precipitation on the C substrate, and ultimately forming bulk Si. Furthermore, the content of residual Fe–Si can be decreased in the bulk Si by controlling the temperature gradient, the holding time, and the alloy composition.

Effect of Precipitation and Dissolution of Si on the Thermal Diffusivity in the Al-Si Alloy System

The effect of precipitation and dissolution of Si on the thermal diffusivity in the Al-Si alloy system is reported in this study and solution heat treatment followed by aging treatment is carried out to determine the effects of heat treatment on the thermal characteristics. The solution treatment is performed at 535 ºC for 4 and 10 h and then the specimens are cooled by rapid quenching. The samples are aged at 300 ºC for 4 h to precipitate Si solute. The addition of 9 wt% silicon contents makes the thermal diffusivity decrease from 78 to 74 mm/s2 in the cases of solid solution treated and quenched samples. After quenching and aging, the Si solute precipitates on the Al matrix and increases the thermal diffusivity compared with that after the quenched state. In particular, the increase of the thermal diffusivity is equal to 10 mm/s2 without relation to the Si contents in the Al-Si alloy, which seems to corresponded to solute amount of Si 1 wt% in the Al matrix.

Understanding the formation mechanism of supersonic atmospheric plasma sprayed in-situ hypereutectic Al-25 wt%Si coating with nanostructured coupled eutectic: From powder, in-flight droplet, splat to coating

A comprehensive study on the formation mechanism of supersonic atmospheric plasma sprayed hypereutectic Al-25 wt%Si coating was performed by investigating microstructural evolution from powder, in-flight droplet, splat, to coating. Results show that feedstock Al-20 wt%Si powder exhibits sub-micron eutectic and micro-sized primary Si. Elements loss, surface oxidation, breakup and chemical diffusion occur when in-flight droplets are exposed to plasma jet. The splat exhibits an ideal structure of nano-sized coupled eutectic. The coating consists of featureless (~84.7 vol%) and ultrafine (~14.2 vol%) zones. Four Si morphologies coexist in the coating from scale of angstrom to sub-micron: Si atoms/clusters in supersaturated solid solution; round/rod-shape Si precipitations (~6.23 nm); nano-Si particulates (~34.37 nm) in featureless zone; and sub-micron Si particulates (~107.32 nm) in ultrafine zone. Further, physical/chemical changes mechanisms of in-flight droplets, rapid solidification mechanisms of splat and coating, formation mechanisms of featureless and ultrafine zones, precipitation and growth mechanisms of Si phase are extensively elucidated. Accordingly, the coating demonstrates ultra-high hardness (~259.8 HV0.1, better than previously reported values of hypereutectic Al-Si alloys with similar silicon content), and superior wear resistance. The unique microstructure with solid solution and grain refinement strengthening mechanisms, and in-situ fabricating higher Si-content Al-Si alloys contribute to the exceptional mechanical properties.

In-situ TEM study of the crystallization sequence in a gold-based metallic glass

The composition Au49Ag5.5Pd2.3Cu26.9Si16.3 (at.%) is of interest as the basis for the development of gold-based bulk metallic glasses for application in jewellery. In-situ heating in transmission electron microscopy (TEM) and differential scanning calorimetry (DSC, both conventional and fast) are used to obtain a comprehensive characterization of the decomposition on heating a melt-spun glass of this composition. Linking TEM with DSC over a range of heating rates 0.083‒2000 K s‒1, allows the sample temperature in the TEM heating stage to be calibrated. On heating up to melting, the glass decomposes in up to four stages: (1) complete transformation to single-phase nanocrystalline (Au,Cu)7Si; (2) grain growth of this phase; (3) precipitation of (Pd,Ag)Si, reducing the supersaturation of silicon in the (Au,Cu)7Si matrix; (4) with the precipitate phase remaining stable, decomposition of the matrix to a mixture of (Au,Ag)8Cu2, AuCu and Cu3Au phases. At all stages, grain diameters remain sub-micrometre; some of the stable nanocrystalline microstructures may themselves be of interest for applications. The characterization of the decomposition can assist in the optimization of the glass composition to improve tarnish-resistance, while retaining adequate glass-forming ability, formability in thermoplastic processing, and resistance to crystallization. For materials in general, the close correlation of in-situ TEM and DSC results should find wide use in characterizing complex transformation sequences.

In-Situ TEM Study of the Crystallization Sequence in a Gold-Based Metallic Glass

The composition Au49Ag5.5Pd2.3Cu26.9Si16.3 (at.%) is of interest as the basis for the development of gold-based bulk metallic glasses for application in jewellery. In-situ heating in transmission electron microscopy (TEM) and differential scanning calorimetry (DSC, both conventional and fast) are used to obtain a comprehensive characterization of the decomposition on heating a melt-spun glass of this composition. Linking TEM with DSC over a range of heating rates 0.083‒2000 K s‒1, allows the sample temperature in the TEM heating stage to be calibrated. On heating up to melting, the glass decomposes in up to four stages: (1) complete transformation to single-phase nanocrystalline (Au,Cu)7Si; (2) grain growth of this phase; (3) precipitation of (Pd,Ag)Si, reducing the supersaturation of silicon in the (Au,Cu)7Si matrix; (4) with the precipitate phase remaining stable, decomposition of the matrix to a mixture of (Au,Ag)8Cu2, AuCu and Cu3Au phases. At all stages, grain diameters remain sub-micrometre; some of the stable nanocrystalline microstructures may themselves be of interest for applications. The characterization of the decomposition can assist in the optimization of the glass composition to improve tarnish-resistance, while retaining adequate glass-forming ability, formability in thermoplastic processing, and resistance to crystallization.

Impact of ambient conditions on the Si isotope fractionation in marine pore fluids during early diagenesis

Abstract. Benthic fluxes of dissolved silicon (Si) from sediments into the water column are driven by the dissolution of biogenic silica (bSiO2) and terrigenous Si minerals and modulated by the precipitation of authigenic Si phases. Each of these processes has a specific effect on the isotopic composition of silicon dissolved in sediment pore fluids, such that the determination of pore fluid δ30Si values can help to decipher the complex Si cycle in surface sediments. In this study, the δ30Si signatures of pore fluids and bSiO2 in the Guaymas Basin (Gulf of California) were analyzed, which is characterized by high bSiO2 accumulation and hydrothermal activity. The δ30Si signatures were investigated in the deep basin, in the vicinity of a hydrothermal vent field, and at an anoxic site located within the pronounced oxygen minimum zone (OMZ). The pore fluid δ30Sipf signatures differ significantly depending on the ambient conditions. Within the basin, δ30Sipf is essentially uniform, averaging +1.2±0.1 ‰ (1 SD). Pore fluid δ30Sipf values from within the OMZ are significantly lower (0.0±0.5 ‰, 1 SD), while pore fluids close to the hydrothermal vent field are higher (+2.0±0.2 ‰, 1SD). Reactive transport modeling results show that the δ30Sipf is mainly controlled by silica dissolution (bSiO2 and terrigenous phases) and Si precipitation (authigenic aluminosilicates). Precipitation processes cause a shift to high pore fluid δ30Sipf signatures, most pronounced at the hydrothermal site. Within the OMZ, however, additional dissolution of isotopically depleted Si minerals (e.g., clays) facilitated by high mass accumulation rates of terrigenous material (MARterr) is required to promote the low δ30Sipf signatures, while precipitation of authigenic aluminosilicates seems to be hampered by high water ∕ rock ratios. Guaymas OMZ δ30Sipf values are markedly different from those of the Peruvian OMZ, the only other marine OMZ setting where Si isotopes have been investigated to constrain early diagenetic processes. These differences highlight the fact that δ30Sipf signals in OMZs worldwide are not alike and each setting can result in a range of δ30Sipf values as a function of the environmental conditions. We conclude that the benthic silicon cycle is more complex than previously thought and that additional Si isotope studies are needed to decipher the controls on Si turnover in marine sediment and the role of sediments in the marine silicon cycle.

Influence of Si content on thermal expansion characteristic of Al–Si alloys

ABSTRACTThe objective of this study was to find out how Si precipitation affects the linear thermal expansion behaviors of Al–Si alloys with various Si content. These Al–Si alloys were manufactured by gravity casting using 99.98 wt-% pure Al and 98.5 wt-% Si pellets. Solution treatment was carried out at 530°C for 10 h for each specimen. As-quenched specimens were subjected to microstructure observation and linear thermal expansion analysis. Si precipitation and additional linear thermal expansion occurred at lower temperature when Si content was increased to 9.5 wt-%. The activation energy of Si precipitation was also lower when Si content was higher. Eutectic Si reduced diffusion distance and acted as a nucleation site of dissolved Si atoms in the Al matrix during aging. These Si phases decreased Si precipitation temperature and activation energy of Si precipitation.

Enhancing the Mechanical Properties of Si Particle Reinforced ZA22 Composite by Ti–B Modification

In situ precipitation of primary Si (SiP) particles within the microstructure of Si-containing Zn–22Al alloys, also known as ZA22/SiP composites, can improve their hardness and mechanical strength, provided that the size, morphology, and distribution of SiP particles are optimized. In this study, Ti–B treatment was employed to modify the microstructure and improve the mechanical properties of ZA22–xSi (x = 4 and 8 wt%) composite. According to the results, Ti–B treatment eliminated the primary α-Al dendrites, reduced the volume fraction of remaining α-Al phase within the microstructure, reduced the size of SiP particles, and improved their distribution within the matrix. Moreover, the size of SiP particles is reduced from 25.2 ± 9.5 to 30.1 ± 4.4 μm in non-modified samples containing 4 and 8 wt% Si to 14.7 ± 6.3 and 19.6 ± 5.3 μm in the samples modified by Ti–B, respectively. Ti–B addition also substantially refined and homogenized the distribution of the other second phases and porosities. The mechanical properties analyses revealed that, at a given Si concentration, the hardness of the alloys modified by Ti–B was relatively less than that of the non-modified ones. Moreover, the tensile strength and fracture strain of modified ZA22–4Si and ZA22–8Si composites are increased by about 15 and 59, and 60 and 82%, respectively.

The Cu photoluminescence defect and the early stages of Cu precipitation in Si

This theoretical–experimental study focuses on the formation of the substitutional-tri-interstitial cluster Cus1Cui3, which has been proposed as the photoluminescence defect CuPL. The configurations and electronic properties of the intermediate defects Cus1Cui1 and Cus1Cui2 are calculated, and their electrically active levels are obtained from conventional and Laplace deep-level transient spectroscopy. The vacancy formation energy near copper-related defects is calculated and found to be much smaller than in the perfect crystal. Then, we show how Cus1Cui3 could become the seed of agglomerates of Cus1Cui3 “units.” The discussion focuses mostly on unanswered questions about the discrepancies between the calculated and measured properties of CuPL and Cus1Cui3.

Fabrication of Surface Layer Enriched with Zinc on AlSi17 Aluminium Cast Alloy by Hot-Dip Galvanizing

The paper deals with the fabrication of the surface layer enriched with Zn on AlSi17 aluminium alloy to modify the microstructure and surface properties of the alloy. The continuous surface layer was fabricated on the AlSi17 substrate by the hot-dip galvanizing of AlSi17 for 15min in a Zn bath heated to 450°C. The thickness of the layer was about 100 μm. The layer was characterised by a multi-component microstructure containing the regions of a solid solution of Al in Zn and dendrites of a eutectoid composed of a solid solution of Al in Zn and a solid solution of Zn in Al. In the layer, fine particles of Si with a regular shape were distributed. The results indicated that these Si particles formed by the action of Zn on the eutectic Si precipitations in the AlSi17 substrate. In the microstructure, large primary Si crystals and multi-phase precipitations, originating from the substrate, were also observed. The surface layer had much higher microhardness than the AlSi17 substrate. The results showed that hot-dip galvanizing can be used to modify the microstructure and properties of the surface layer of AlSi17. The study indicates the possibility of conducting further research on the fabrication of joints between AlSi17 and other metallic materials using a Zn interlayer fabricated by hot-dip galvanizing.

Microstructural evolution and EBSD analysis of AlSi10Mg alloy fabricated by selective laser remelting

This study characterizes microstructural evolution and texture orientation of AlSi10Mg components through selective laser melting (SLM) by adopting different strategies during remelting. SLM selectively melts the powders and solidified layer, generating a moving melt pool and resulting in unique microstructure. The microstructural evolution, grain sizes and the texture orientation of the fabricated samples were investigated using different scanning strategies by optical microscopy, scanning electron microscopy, and electron back-scattered diffraction. Experimental results show that the inter-connected net shape of the AlSi eutectic structure tends to be widen as a result of the increase of the Si precipitation phase. The grain sizes become smaller and produce more subgrains effected by high thermal conductivity of the solidified layer. The average hardness and Young's modulus increased to 2.32 GPa and 93.26 GPa, respectively. When the remelting scanning direction is rotated to 90°, the cellular microstructure comes into domination. Due to the remelting processing, the preferential grain growth orientation is 〈100〉 both along the scanning and building direction, and the crystallographic texture is enhanced. Results show distinct epitaxial growth along the heat flow with <100> orientation can be obtained and hence isotropy is strengthened.