Eutectic Layer Research Articles

Separation mechanism of a Si+TiSi2 eutectic alloy and metal impurity Fe segregation via electromagnetic directional solidification of Ti-80 wt%Si alloy melt

The separation mechanism of Si+TiSi2 eutectic alloy and Fe atom segregation were investigated during the electromagnetic directional solidification of Ti-80 wt%Si alloy melt at pull-down rates from 5 µm/s to 20 µm/s. The experimental results indicated that a fine lamellar Si+TiSi2 eutectic alloy was obtained in the middle of the ingot after electromagnetic directional solidification. Fe impurities were captured by primary SiC particles at the bottom of the ingot, which then segregated at the top of the ingot. When the pull-down rate was 5 µm/s, the solidification front was always close to the induction coil. Electromagnetic stirring produced forced convection that promoted Si+TiSi2 eutectic alloy precipitation by mixing the melt and offsetting the temperature gradient caused by the precipitation of the primary phase, thereby inhibiting segregation. Thus, the optimal pull-down rate for fabricating Si+TiSi2 eutectic alloy was 5 µm/s. At this pull-down rate, the maximum thickness of the Si+TiSi2 eutectic alloy layer was 27 mm, and the removal rate of impurity Fe in this layer reached 98.98%.

Formation mechanism and control of welding cracks in dissimilar materials of Ni50Ti50 SMA and Ti-6Al-4V

In this paper, the crack problem of welded joint between NiTi SMA and Ti-6Al-4V was studied. The crack morphology and propagation state were analyzed, which was found that the microstructure uniformity of the fusion zone was poor. And there was a eutectic reaction layer at the fusion line of Ti-6Al-4V. A large number of brittle compounds NiTi2 phase led to severe hardening of the welded joint. By means of thermodynamic calculation and finite element simulation, the precipitation of brittle phase and stress distribution were studied, and the formation mechanism of crack defects was clarified. According to the above research, the beam offset welding process was used to melt the NiTi SMA with low melting point, which by the fusion brazing method to achieve the reliable connection. Optimizing the microstructure of the fusion zone and eliminating crack defects, the most suitable electron beam offset amount was 0.4 mm.

Interfacial microstructure evolution of thick plate Al/Mg FSW: Effect of pin size

Thick plate Al/Mg alloy joints with an inclined butt configuration were fabricated by friction stir welding. The interfacial microstructure evolution was investigated using different pin size, namely A-Pin, B-Pin and C-Pin. The result shows that the joints without macro defects can be obtained by using B-Pin and C-Pin, but the A-Pin breaks during welding. The peak temperature of C-Pin reaches 449.8 °C, which is higher than that of B-Pin. For the B-Pin and C-Pin, there are Al3Mg2 layer and eutectic layer (Mg + Al12Mg17) along the upper of Mg side interface. Electron backscatter diffraction shows that the Al3Mg2 and Al12Mg17 layers are columnar structure, which indicates that the intermetallic compound layer is caused by atomic diffusion. The Al3Mg2 layer has higher kernel average misorientation, while the Al12Mg17 layer and eutectic layer have low kernel average misorientation. Cracks tend to occur near the interface between the Al3Mg2 layer and Al, which is due to the large stress and stress gradient between Al and Al3Mg2 layer. Nevertheless, the phase interface Al12Mg17/Mg presents a coherent relationship with small mismatch degree of 1.7%.

A Critical Conception of Hot-Tearing Susceptibility: How Controlled Diffusion Solidification Enables Shape-Casting with Wrought Aluminum Alloys

Instrumented Constrained Rod Casting (CRC) method and the Controlled Diffusion Solidification (CDS) process were used to study the high susceptibility of the aluminum alloy AA 7068 to hot-tearing. A precise analysis of the measured hot-tearing curves is provided considering the casting design and different feeding mechanisms. Scanning Electron Microscopy was used to study the hot-tear surfaces. It was found that the solidification of the eutectic liquid at the film stage and the phenomenon of solid-feeding strongly affects the hot-tear formation. The early formation of an absorbed eutectic layer on the primary phase due to low rate of back-diffusion is proposed to explain the loss of ductility at the film stage. The layer can serve as an efficient substrate for the eutectic nucleation and growth, hence its formation can advance the eutectic solidification. The isolation of liquid eutectic by its partial solidification results in development of solid-feeding stresses breaking the solid eutectic and provide the required tearing initiators. By increasing the back-diffusion rate, the CDS process avoids formation of the adsorbed solid layer which postpones eutectic solidification and thereby mitigating the hot-tearing susceptibility.

Interfacial Investigation of St13/Molten Zn–5% Al and Corrosion Behavior of Formed Layer via Hot-Dip Process

In the present study, the effect of temperature and duration of hot-dipping on structural properties, hardness, and corrosion resistance of Zn–5% Al coatings on ST13 low-carbon steel has been investigated. The samples were dipped in a molten metal bath at temperatures of 440, 480 and 520°C, during 20, 50 and 70 seconds. To investigate the microstructure and properties of samples, scanning electron microscopy, energy dispersive X-ray spectrometry, microhardness measurements, and potentiodynamic polarization corrosion tests were used. The results showed that the coating structure is an eutectic layer at 440°C, and at the interface of the substrate and the coating a thin inhibitory layer is formed that prevents the diffusion of the substrate atoms into the coating and from the molten bath into the substrate. With an increase of temperature to 480 and 520°C, with the diffusion of iron atoms of the substrate and of the molten bath, a triple metallic layer of Al–Fe–Zn forms, and the thickness of the coating increases. As the dipping time increases, the number of intermetallic particles in the coating also increases. At higher temperatures, the hardness went up from 86 to 121.73 Hv and the corrosion rate—from 7.381 to 37.53 Mpy. hot dip, Zn–5% Al coating, microstructure, intermetallic, corrosion

Microstructure formation and mechanical properties of isothermally-solidified titanium alloy joints brazed by a Ti – Zr – Cu – Ni – Be amorphous alloy foil

Two titanium alloys, OT4 and VT6-c, with a pseudo-α and α + β structure, respectively, were brazed using transient liquid phase (TLP) bonding. To obtain high strength joints an amorphous foil (Ti – 12Zr – 22Cu – 12Ni – 1.5 Be – 0.8V wt.%) was used. Based on microstructural studies and analysis of two- and three-component phase diagrams, the mechanism of the microstructural evolution of the brazed seams of titanium alloys OT4 and VT6-c is described. Brazing at 800 °C with exposure for 0.5 h leads to the formation of a heterogeneous structure consisting of Widmanstätten, eutectoid, and eutectic. Brazed OT4 and VT6-c joints with the presence of a eutectic layer in the centre show low mechanical properties; their ultimate strength lies in a range from 200 to 550 MPa. Increasing the brazing temperature to 840 °C and the exposure time to 2 h, leads to the disappearance of the brittle eutectic component from the seam. This structure typically consists of Widmanstätten with a small number of eutectoid fractions. Joints with the absence of a eutectic layer in the brazed seam demonstrate a strength equal to the base titanium alloys. In this case, failure occurs in the base metal. For brazed samples from the OT4 alloy, the tensile strength value is σb = 750 ± 3 MPa, and for samples from VT6-c, σb = 905 ± 3 MPa. This work was supported by Competitiveness Growth Programme of the Federal Autonomous Educational Institution of Higher Education National Research Nuclear University MEPhI (Moscow Engineering Physics Institute).

Сorrosion resistance of the welded junction of aluminum alloy of the Al-Mg-Si-Cu system

The results of complex investigations of corrosion resistance of aluminum alloy of the Al-Mg-Si-Cu doping system have been presented. By the method of metallography, it has been found that under the influence of the thermal cycle of welding, the secondary phases are separated and coagulation of insoluble phases occurs. The base alloying elements form metastable phases in the process of crystallization of seams, which are evenly distributed, but differ in size and shape. In the zone of thermal influence, the decomposition of the supersaturated solid solution and the dissolution of the strengthening phases, the characteristic feature of which is the instability of decomposition within the limits of one grain, the enrichment of the boundary areas with alloying elements, and the depletion of volume, are observed. This is accompanied by the formation of thin eutectic layers along the boundaries of crystallites. The results of electrochemical investigations have shown that the potential of the weld and the high effected zone (HAZ) is more noble compared to the base metal. Taking into account a larger area of the base metal in the construction compared to the weld (the weld area does not exceed 10% of the area of the base metal), a satisfactory resistance of the weld to the continuous corrosion can be expected in general. The depth of intergranular corrosion of the weld joint of the Al-Mg-Si-Cu system alloy is (0.245-0.350) mm, which is greater than for the base metal – from 0.082 to 0.086 mm. An increasing of the depth of destruction of the grain boundaries in the weld joint confirms the fact that the near-weld zone is weakening during welding. Welding does not worsen the resistance of welds to the exfoliation corrosion compared to the base metal, which is estimated by the ball 2-3 for the base metal and the ball 1 for the weld and HAZ. The stability of the weld under conditions of a simultaneous effect of constant loading and full immersion into the corrosive medium is reduced: the time until the destruction of the base metal is from 67 to 88 hours, of the weld – from 1 to 49 hours. Since the resistance to the exfoliation corrosion of the weld is satisfactory, the performance of the welded product as a whole, under conditions of a joint effect of corrosion-aggressive medium and mechanical stress, will be determined by the stability of HAZ against pitting and intergranular corrosion.

Ultrasound-assisted friction stir transient liquid phase spot welded dissimilar copper-aluminum joint

Ultrasound assistance was applied to friction stir transient liquid phase spot welding (FSTLPSW) to attain a high-quality dissimilar copper-aluminum joint in this study. Different ultrasound-assisted times were selected for comparing with the conventional condition without ultrasound. The interfacial microstructure evolution of conventional joint was analyzed combined with the measured temperature cycle. The ultrasound assistance improved the joint quality by regulating the interfacial microstructure. Cavitation erosion, broken IMC particles and thicker eutectic layer appeared in ultrasound-assisted joint. The highest tensile shear strength of 7.38 kN was obtained under the ultrasound-assisted time of 60 s which was 43.1 % higher than that of the conventional joint. The fracture location transferred from IMC layers to eutectic layer under the ultrasound assistance.

Microstructure and wear resistance of TiC reinforced AlCoCrFeNi2.1 eutectic high entropy alloy layer fabricated by micro-plasma cladding

In order to improve the wear resistance of AlCoCrFeNi2.1 high entropy alloy, AlCoCrFeNi2.1 layers with 0%, 5%, 10%, 15% weight addition of TiC were prepared by micro-plasma cladding technique. The results showed that micro-plasma cladding can effectively reduce decomposition of TiC particle and the lowest decomposition rate was 0.8%. Meanwhile, the hardness and wear resistance of the cladding layers were greatly enhanced with the addition of TiC. When added 15 wt% TiC, the hardness and wear resistance of the cladding layers were the most excellent. The main wear mechanisms of enhanced layers were adhesive wear, abrasive wear and oxidation wear.

Study on the interface of Al/Mg bimetal by static casting

In this study, fine aluminum cladding AZ91 magnesium alloy composite ingot was prepared under static casting. The microstructure and phase constituents of the interface were investigated by optical microscopy. The result shows that the interface from the Al matrix side can be divided into three parts. A diffusion layer made up by β-Al3Mg2, Mg-Al mtermetallic compound layer which is made up by γ-Mg17Al12 and a eutectic structure layer made up by α-Mg and γ-Mg17Al12. The element distribution is investigated by methods of electron probe microanalysis (EPMA). The component ratio of Al to Mg decreases in the whole interface. But the descent becomes very slow in the α-Mg and γ-Mg17Al12 eutectic structure layer. The Vickers hardness in the matrix and interface were also measured. The value of microhardness decrease with the phase changing in the whole interface from 120 HV to 275HV, but it’s still much higher than the Al matrix or AZ91 matrix as well.

Role of noble metal fission products on the microstrutural evolution and corrosion behaviour of ferritic steel-Zr based metal waste form alloys in the simulated repository environment

In the present work, the noble metal fission products (NMFPs) alloyed ferritic steel-Zr based metal waste form (MWF) alloys and its influence on microstructure and corrosion behavior in simulated ground water environment was investigated for pyrochemical nuclear spent fuel reprocessing applications. Ingots of 9Cr1Mo steel-12Zr-NMFPs MWF alloy with different NMFP (Ru, Rh and Pd) concentrations (1–4 wt%) were fabricated via vacuum arc melting. Microstructural examination revealed the eutectic layer microstructure consisting of α-Fe and Fe–Zr phases with the preferential distribution of NMFP along with Fe–Zr phases. XRD analysis confirmed that the noble metal does not show any discrete noble metal phases but as intermetallics phases. Electrochemical corrosion studies of the MWF alloys in the simulated Rajasthan Ground Water (RGW) and Kalpakkam Ground Water (KGW) media showed improved corrosion resistance with increase in noble metal content of the alloy due to cathodic modification. Higher noble metal-containing MWF alloys exhibited more protective passive film properties is attributed to the enhancement of Ru, Rh and Pd noble metals in the alloy which influences on the formation of more stable Fe3+ state than unstable Fe2+ state and vice versa with less noble metal-containing MWF alloy confirmed by XPS. Thus, the current investigation addressed the immobilization of noble metals fission product in the alloy microstructure and their beneficial impact on corrosion performance of ferritic steel-Zr based MWF alloy which are of primary concern in the high-level radioactive waste disposal in a geological repository.

Eutectic Formation, V/III Ratio and Controlled Polarity Inversion in Nitrides on Silicon[1

The crystallographic polarity of AlN grown on Si(111) by plasma assisted molecular beam epitaxy is intentionally inverted from N-polar to Al-polar at a planar boundary. The position of the inversion boundary is controlled by a two-step growth process that abruptly changes from Al-rich to N-rich growth conditions. The polarity inversion is induced by the presence of Si, which is incorporated from an Al-Si eutectic layer that forms during the initial stages of AlN growth and floats on the AlN surface under Al-rich growth conditions. When the growth conditions change to N-rich the Al and Si in the eutectic react with the additional N-flux and are incorporated into the solid AlN film. Relatively low levels of Al-Si eutectic formation combined with lateral variations in the Si incorporation lead to nonuniformity in the polarity inversion and formation of surprisingly narrow, vertical inversion domains. The results suggest that intentional incorporation of uniform layers of Si may provide a method for producing polarity engineered nitride structures.

Microstructural and tribological characterization of molybdenum–molybdenum carbide structures produced by spark plasma sintering

Abstract Pure molybdenum is one of the most important refractory metals owing to its high melting point, low thermal expansion coefficient and good thermal conductivity. Molybdenum carbide exhibits superior wear resistance to molybdenum owing to its low friction coefficient and high hardness; therefore, in this study, to endure abrasive conditions, the surface of molybdenum was modified as molybdenum carbide. In accordance with this purpose, pure molybdenum powder was sintered under constant pressure (40 MPa), various temperatures (1 650 °C, 1 700 °C and 1 725 °C) and holding periods (180 s, 360 s and 540 s) using the spark plasma sintering technique. The effects of sintering temperature and holding period on relative density, microhardness, microstructure and wear properties of the specimens were investigated. Moreover, carbide formation on the surface due to carbon diffusion was also investigated. Relative density values decreased with increasing sintering temperature and period, obtaining the highest value of 97.55 % at 1 650 °C and 180 s. A microhardness value of 15.20 GPa was reached on the surface while the maximum value measured at the cross-section was 2.17 GPa, showing the formation of molybdenum carbide structure on the surface of all specimens. Furthermore, microstructural analysis supported the microhardness results and showed a minimum 288-μm thick molybdenum carbide layer on the surface. In addition, a typical eutectic microstructure was observed between molybdenum and molybdenum carbide layers; however, the decrease in temperature decreased the thickness of the eutectic layer and it gradually disappeared from the centre to edge of disc-shaped specimens. Wear resistance of surfaces was improved by decreasing holding period where a non-porous carbide layer and highest microhardness values were achieved. The cross-sectional analysis corresponding to molybdenum base metal proved that the formation of molybdenum carbide layer because of carbon diffusion improved the wear properties of the sample with the low friction coefficient and low wear depth.

Effect of insert materials on microstructure and mechanical properties of Al/Mg bimetal produced by a novel solid-liquid compound process

A novel lost foam casting solid-liquid compound method was developed for the preparation of the A356 aluminum alloy/AZ91D magnesium alloy bimetal (Al/Mg bimetal), and the effect of insert materials on microstructure and mechanical properties of the Al/Mg bimetal was studied in this paper. Obtained results indicated that the insert materials played a great role in the bonding of the Al/Mg bimetal. The Al/Mg bimetal with a defect-free and uniform metallurgical interface was achieved by use of the A356 aluminum alloy solid insert. With an optimized parameter in the condition of the AZ91D magnesium alloy solid insert, the Al/Mg bimetal also obtained a metallurgical bonding. Nevertheless, the crack defects were always found in the interfaces of the bimetallic samples produced with the AZ91D solid insert. The formation mechanism of the crack were proposed by stress distribution, thermodynamics and intermetallic compounds (IMCs) analyses. The microstructures exhibited that the interfaces of the bimetallic samples prepared with different insert materials were constituted by Al3Mg2+Mg2Si layer, Al12Mg17+Mg2Si layer as well as Al12Mg17+δ-Mg eutectic layer. The Al/Mg bimetal produced with the A356 solid insert possessed a much higher bonding strength compared to the bimetal obtained with the AZ91D solid insert, whereas their hardnesses were basically same. The fractured samples produced with different insert materials showed brittle fracture morphologies, and the fractures primarily occurred between the Al3Mg2+Mg2Si layer and Al12Mg17+δ-Mg eutectic layer.

Interfacial bonding mechanism and pouring temperature effect on Al/Cu bimetal prepared by a novel compound casting process

In this work, the Al/Cu bimetal was prepared by a novel lost foam casting (LFC) compound process, and the effect of pouring temperature on microstructure and mechanical properties of the Al/Cu bimetal was studied. The formation mechanism of the Al/Cu interface was also discussed. Results show that the Al/Cu interface consisted of intermetallic compounds layers (IMCs layers) and eutectic layer. The IMCs layers were identified as Al2Cu, AlCu, and Al4Cu9 phase layers, and the eutectic layer was composed of Al2Cu, α(Al) and Si phases. The pouring temperature primarily influenced the morphology and size of the IMCs layers and eutectic layer as well as the pore defect in the interface. Based on the cooling curve and the morphology of the IMCs layers, a new formation mechanism of the Al/Cu interface was proposed. The Al/Cu bimetal obtained with a pouring temperature of 800 °C had the relatively highest bonding strength, up to 81 MPa, resulting from its improvement of the IMCs layers. Shear fracture exhibited a brittle fracture nature, mainly initiating with the IMCs layers, and the Si phase also played an important role in the fracture process.

Effect of Metallization Condition on Mechanical Strength of Multi Crystalline Silicon Solar Cell

During each processing step of multicrystalline (mc) silicon solar cells production, it requires a good mechanical stability to avoid cell breakage. Therefore, wafers and cells with a good mechanical stability are absolutely important. The cracking of silicon wafer solar cells is one of the major failures of solar module. Hence, it is important to evaluate the mechanical strength of solar cells and influenced factor. The present work focuses on determining the effect of metallization conditions on mechanical strength of multi crystalline silicon solar cells. A 4-point bending method was used to measure the fracture strength and data is statistically evaluated by a Weibull analysis for interpretation of the flaws within the specimen. It was observed that drying temperature at 250 °C provides better results than 350 °C. It was found that the contact formation between metal layer and silicon wafer (i.e. eutectic layer) has a significant effect on mechanical properties of mc-silicon solar cells. Moreover, the mesh sizes of screen printing produce different thickness of metal layer without altering the strength of silicon wafer. It was discovered that the metallization process was responsible to alter the mechanical strength of mc-silicon solar cell. It is also observed that the crack can initiate from the edge and surface defects and propagate along the preferred crystallographic plane. It was found that the edge defects look like more dominant as crack initiation.

The role of Si in GaN/AlN/Si(111) plasma assisted molecular beam epitaxy: polarity and inversion

The microstructure, polarity and Si distribution in AlN/GaN layers grown by plasma assisted molecular beam epitaxy on Si(111) was assessed by scanning transmission electron microscopy. Samples grown under both metal- and nitrogen-rich conditions contained defects at the AlN/Si interface which suggest formation of an Al-Si eutectic. Correlated with this, interfacial segregation of Si was found in the samples. It is proposed that Si is dissolved in a eutectic layer floating on the AlN surface under metal-rich conditions. This Si is then incorporated into the film if the growth becomes nitrogen-rich, either intentionally or due to plasma source transients. These Si-rich layers appear to induce inversion of the nitride from nitrogen- to metal-polarity, and uncontrolled variations in the Si concentration cause occasional nonuniformity in the resulting inversion.

Grain size dependence, mechanical properties and surface nanoeutectic modification of Al2O3-ZrO2 ceramic

Abstract The present work aims to provide fundamental insights into the grain size dependence and mechanical behavior of hot-pressed Al2O3-ZrO2 ceramic at its eutectic composition, and further to explore the hardening effect of laser-induced surface nanoeutectic layer. The underlying correlations between densification behavior, grain size distribution and mechanical properties were elucidated. Sintering at 1550 °C promotes the densification without extensive grain growth, and in this case the sample exhibits a critical density of 99.3 %. The average grain size is tailored into a range of 0.6–0.9 μm, and the measured flexural strength and toughness reach 1100 MPa and 11 MPa·m1/2, respectively. The metastable t-ZrO2 grains indeed play a pivotal role in energy dissipation at the crack tip through crack deflection and branching. In addition, the mechanical behavior is reasonably explained through constructing a multilevel toughening mechanism map associated with grain size distribution of ZrO2. Particularly, surface nanocrystallized Al2O3-ZrO2 eutectic layer with a thickness of 1000 μm free of pores and cracks is achieved by a rapid laser melting process. The outmost laser-modified nanoeutectic layer exhibits a fine cellular structure with an interphase spacing of only 105 nm and a hardness of as high as 26.1 GPa, which provides a promising potential in enhancing significantly the hardness and wear resistance for applications as sliding ceramic components.

Influence of Al-Si coating on resistance spot welding of Mg to 22MnB5 boron steel

Weld appearance, interfacial microstructure and mechanical properties with and without Al-Si coating were compared on resistance spot welding (RSW) of Mg to steel. No reaction layer was observed in Mg/bare steel joint. In Mg/coated steel joint, Al-Si eutectic layer improved the weld appearance and reduced welding defects, also was squeezed to border region and reacted with Mg as a metallurgical seal to ensure the clean contact between molten Mg and original intermetallic compounds (IMC) layer. Some of molten Al-Si eutectic diffused into Mg side facilitating a columnar-to-equiaxed dendrites evolution. The original IMC layer was influenced by both of growth mechanism and dissolution mechanism, which resulted in various interfacial reaction layers at different regions: new reaction layers at weld center and intermediate regions; original but grown IMC layer at weld border region. The metallurgical bonding between Mg and steel was enhanced by these new reaction layers because crack propagated inside of Mg alloy rather than the interface at weld center and intermediate regions. With Al-Si coating, the welding quality was improved 85% in peak load, a factor 6 increase in fracture elongation and a factor 9 increase in fracture energy absorption.

Interfacial bonding mechanism in Al/coated steel dissimilar refill friction stir spot welds

Defect-free dissimilar Al/zinc coated steel and Al/AlSi coated steel welds were successfully fabricated by refill friction stir spot welding. However, Al alloy and uncoated steel could not be welded under the same welding condition. Al-Zn eutectic layer formed at the Al/zinc coated steel interface showed non-uniformity in thickness and nanoscale intermetallic (IMC) produced was discontinuous. The bonding formation between the Al-Zn layer and the surrounding materials was attributed to a liquid/solid reaction mechanism. Bonding formation at Al alloy and AlSi coated steel interface was attributed to a solid/solid reaction mechanism, as the joining process did not involve with melting of base metals or AlSi coating materials. Kissing bond formed at the weld boundary acted as a crack initiation and propagation site, and the present study showed that weld strength of Al 5754/AlSi coated steel was greatly influenced by properties of original IMC layer.