Eutectic Reaction Research Articles

High temperature brazing of C/C using Hastelloy C-276 as filler and joint formation mechanism

To increase servicing temperature of brazed C/C joint, a commercial Hastelloy C-276 (Ni-15.5Cr-16Mo-5Fe-4W, wt. %) with high melting point was used as filler in high-temperature brazing (HTB). The fast HTB process with a soaking time of 2 min was performed at four levels 1250 °C, 1300 °C (bellow solidus), 1350 °C and 1400 °C (over liquidus). Joint shear strength of 23.5 MPa (under 1400 °C) and 20 MPa (under 1250 °C) were obtained due to significant dissolution of C/C, dense interface, infiltration, and MC in-situ reinforced C-rich Ni-based bond seam. Joint microstructure evolution examination indicated that carbon could rapidly diffuse into C-276, inducing segregation of the active elements Cr and Mo at C-276 grain boundaries, triggering C-(Ni, Fe)-(Cr, Mo) eutectic reaction in the grain. All joints fractured mainly within infiltrated zone. The study demonstrated that using Ni-based alloy containing Cr and Mo as filler was feasible.

Microstructural evolution during sintering of Fe-Cr-C steels prepared from admixed elemental powders

ABSTRACT In the upcoming years, a reduction in the use of critical elements, such as Ni and Cu, with unstable prices and high demand from the electromobility sector will become increasingly important for the PM-industry. Cr-alloyed sintered steels offer attractive properties at a moderate cost, but so far mostly Cr-prealloyed grades have been used. This work analyses the microstructural homogenisation process when Cr is introduced as admixed elemental powder. It is shown how – due to its high carbon affinity – Cr particles act as ‘internal carbon-getters’. There is an intermediate ‘heterogenization’ of the microstructure, i.e. the iron matrix is decarburised due to the formation of (Cr, Fe)-carbides. Final homogenisation depends on the formation of a transient liquid phase through the eutectic reaction between carbides and the iron matrix. Thus, the microstructure is not only sensitive to aspects such as sintering temperature or Cr-particle size but also to the heating rate and small variations in nominal carbon.

Phase and properties prediction of Al–Ag binary system using thermo-calc

The temperature phase diagram stability and physical properties of a binary Silver–Aluminium (Al–Ag) alloy system have been calculated using Thermo-Calc 2021b software at a temperature range of 250–1300 K. Six single-solid phases other than the liquid phase, elemental Ag (fcc_Ag), elemental Al (fcc_Al), hcp_A3, cub_A13, and bcc_Ag were obtained from the equilibrium calculation. Ten regions of the double phase were obtained and evaluated. Three intermediate phases were also obtained from the phase diagram with five invariant reactions, namely one eutectic reaction (at 840.00 K), two peritectic reactions (at 1051.22 K and 1003.18 K), and two peritectoid reactions (at 730.58 K and 881.40 K) were equally found. Sixteen regions other than elemental Ag and Al at 0, and 100, respectively, were obtained. Maximum solid solubility obtained was 5.60 Al mass percent at 730. 58 K for Al in Ag and 58.17 Ag mass percent at 840.00 K for Ag in Al. From the temperature–composition phase diagram, the melting point for pure silver and aluminium were equally inferred as 1234.96 K and 933.50 K, respectively, which agrees with experimental standards. These results play a significant role in the processing of Al–Ag alloy for different industrial-related applications.Graphical abstract

Heterogeneous nucleation behavior of solidified α-Al phase on agglomerated TiB2 particles in aluminum-copper alloy: Study by synchrotron radiation multiscale tomography

The size of the TiB2 particles and the orientation relationship (OR) between the Al-TiB2 particles in Al solidified grains in Al-Cu hypo-eutectic alloy are characterized by a combined method of synchrotron multiscale-CT and SEM/EBSD to identify the true nucleation particle. Not only single TiB2 particles but also TiB2 agglomerates were observed in each of the four analyzed grains (A-D). The OR of {0001}TiB2∥{111}Al was confirmed for the largest TiB2 particle in each grain of A to C, corresponding to the free growth model. Such OR was absent in Grain D and EDS analysis revealed Cu segregation at the basal plane of its largest TiB2 particle. It is possible that the eutectic reaction of Al and Cu affects the nucleation and growth of Al, resulting in the change of OR. This paper evidently demonstrates that the eutectic reaction occurs on the nucleant TiB2 particles at the initial stage of nucleation.

Oxidation behavior of CrAl-Mo coated Zircaloy-4 in DB and BDB scenarios

Cr coatings have been widely applied on Zircaloy for accident tolerant fuel materials. However, it has long been a great challenge in the nuclear community for Cr coatings surviving in the beyond design basis conditions (BDB) because of the unstable chromia layer and eutectic reactions between Cr and Zr. In this study, bi-layered CrAl-Mo coatings have been designed to improve the performance of Cr coatings in the BDB conditions. Results showed that the CrAl-Mo coated samples can tolerant in steam at temperatures as high as1400 °C because of the formation of a more stable alumina layer on the surface and the inhibition of eutectic reactions near the coating/substrate interface. Moreover, diffusion of Mo facilitated the formation of a CrAlMo layer, which stabilized the outer alumina layer and reserved the Al inventory within the coating.

Influence of gadolinium on solidification behavior of Ni–Mo-Cr-Gd alloy

The influence of Gd content on the solidification behaviors and as-cast microstructures of Ni–Mo-Cr-Gd alloy have been investigated systematically. It was found that the γ/Ni5Gd eutectic phases were formed in the interdendritic region, which exhibits the divorced character. With the Gd adding, the γ/M6C eutectic phases were replaced by the γ/M2C eutectic ones. Most M2C carbides locate at the interfaces of γ/Ni5Gd eutectic phases or are enclosed by Ni5Gd phases. The segregation of alloying elements was mitigated by Gd adding. Based on the thermal analysis, composition analysis and thermodynamic calculation, the solidification behaviors, and the affecting mechanism of Gd adding were briefly discussed. During the solidification process, the γ/Ni5Gd eutectic reaction occurs first, which interrupts the solute redistribution process and thus mitigates the segregation of some alloying elements. The Ni5Gd phases accommodate a mass of Si and keep the Si content in the residual liquid phase low, which decreases the nucleation rate of M6C carbides. Then, the γ/M2C eutectic phases become the dominant ones at the final stage of the solidification process. Our results are useful in future optimization and processing research of Ni-Mo-Cr-Gd alloy for applications in molten salt reactors.

Physico-chemical study of nitrazepam and citric acid eutectic mixture

The thermal and structural properties of nitrazepam and citric acid have been studied alone and in the corresponding binary mixture, using differential scanning calorimetry (DSC), Fourier Transform Infrared spectroscopy (FTIR), and X-ray diffraction (XRD). The thermodynamic study of the binary mixture nitrazepam-citric acid was made by DSC analysis. The mixtures were obtained by mechanosynthesis. For the binary mixtures, an eutectic reaction was observed in all DSC curves at 396 K. The phase diagram and Tamman's plot suggest an eutectic composition at the ≈0.4 molar fraction of nitrazepam. XRD and FTIR have been used as complementary methods to demonstrate that the obtained compound is eutectic. The solubility tests have been performed on the eutectic composition and nitrazepam. The results showed that the parameter influencing the solubility of nitrazepam is the acidic character of the sample.

Cracking suppression in selective electron beam melted WMoTaNbC refractory high-entropy alloy

WMoTaNb refractory high-entropy alloy (RHEA) with its brilliant thermal resistance at 1600 ℃ becomes increasingly attractive to researchers. To accelerate the processing stage of WMoTaNb RHEA before fulfilling diverse demands, additive manufacturing was employed for fabricating WMoTaNb RHEA. However, recent study of the additively manufactured WMoTaNb RHEA frequently encountered serious cracking behavior, which hinders the potential development of this alloy. In an attempt to mitigate such cracking behavior, graphite with a concentration of 0.5 wt% (5.43 at%) was introduced to modify the microstructure in the as-deposited WMoTaNbC RHEA. The experimental results revealed that C is responsible for prolonged solidification range and small quantity of eutectic reaction into WMoTaNb RHEA, which caused formation of dense dendrites and posed significant effect on lowering the cracking susceptibility. Such an effect of cracking suppression positively provides a new window for fabrication of refractory alloys in high quality with additive manufacturing.

Influences of Different Isothermal Temperatures on Microstructural Evolution of Mg2Si/AZ91D Composite Fabricated by Semi-solid Treatment Process

Due to the eutectic reaction, the Mg2Si phase is inclined to creating an unfavorable fishbone shape at low solidification rates. This can significantly degrade the mechanical characteristics and become the main barrier to the practical application of Mg2Si/AZ91D composites. It is regarded that semi-solid treatment process is an efficient technique to modulate the fishbone-shaped Mg2Si phases in Mg2Si/AZ91D composite. In this work, a new type of Mg2Si/AZ91D composite is successfully fabricated by semi-solid treatment process, and the influences of different isothermal temperatures on microstructural evolution of Mg2Si/AZ91D composite are studied. The findings illustrate that the mean size of the α-Mg grains increases and their trendency of spherification becomes more evidently after held for 30 min at different isothermal temperatures from 550° C to 580°C. Additionally, through semi-solid treatment process, the Mg2Si phases in Mg2Si/AZ91D composite are totally changed from their original fishbone shape into polygon or granule shapes. With increasing of the isothermal temperatures, the mean size and morphologies of Mg2Si phases have no significantly changes. The morphological change theory of Mg2Si phases in Mg2Si/AZ91D composite by semi-solid treatment process is also studied.

Modification of Iron-Rich Phase in Al-7Si-3Fe Alloy by Mechanical Vibration during Solidification

The plate-like iron-rich intermetallic phases in recycled aluminum alloys significantly deteriorate the mechanical properties. In this paper, the effects of mechanical vibration on the microstructure and properties of the Al-7Si-3Fe alloy were systematically investigated. Simultaneously, the modification mechanism of the iron-rich phase was also discussed. The results indicated that the mechanical vibration was effective in refining the α-Al phase and modifying the iron-rich phase during solidification. The forcing convection and a high heat transfer inside the melt to the mold interface caused by mechanical vibration inhibited the quasi-peritectic reaction: L + α-Al8Fe2Si → (Al) + β-Al5FeSi and eutectic reaction: L → (Al) + β-Al5FeSi + Si. Thus, the plate-like β-Al5FeSi phases in traditional gravity-casting were replaced by the polygonal bulk-like α-Al8Fe2Si. As a result, the ultimate tensile strength and elongation were increased to 220 MPa and 2.6%, respectively.

Improved flame retardancy and ceramifiable properties of PE composites by the combination of aluminum hypophosphite and zinc borate

AbstractCeramifiable polyolefin materials protect circuits by forming compact ceramic layers under fire conditions, and can have an excellent application prospect in the refractory cable field. In this paper, Aluminum hypophosphite (AHP) and zinc borate (ZB) were added to further improve flame retardancy and ceramifiable properties of polyethylene (PE)/silicon powder (SP)/wollastonite (WS)/glass frits (GF) composites. The LOI values of composites with AHP/ZB can reach 23.5%, significantly higher than that of PE/SP/WS/GF composites (19.6%). The thermal stability behavior and char yield behaviors of composites could also be characterized by the TG test. The incorporation of AHP/ZB enhances the flexural strength of residue formed at 1000°C from 0.1 to 20.5 MPa. In addition, a new crystal is formed at high temperatures and is identified as the calcium aluminum phosphate phase [CAP, Ca9Al (PO4)7] by XRD analysis. The flow of molten ZB accelerates the reaction of AHP and WS, and this eutectic reaction promotes the formation and stability of ceramics. Furthermore, the SEM analysis reveals the fluxing effect of ZB at low temperatures and AHP at high temperatures. The incorporation of AHP/ZB with a ratio of 1:1 could effectively avoid the vitrification of ceramics and improve their dimensional stability.

Microstructural evaluation in dissimilar liquid phase bonding of cast Al–Si alloy and stainless steel using pure zinc filler metal

The metallurgical joining between cast A359 aluminium alloy and AISI321 stainless steel using pure Zn as the interlayer was performed slightly above Al–Zn eutectic temperature. The liquid phase was formed by a eutectic reaction between Al and Zn and subsequently isothermally solidified by diffusion of Zn into the A359 base metal (i.e. reactive transient liquid phase bonding). The formation of a Zn-rich diffusion affected zone in A359, incorporation of Si particles into the bond zone, formation of a thin Al–Fe–Si intermetallic at the steel interface, and isothermal solidification of the liquid phase and subsequent Al–Zn eutectoid transformation govern the microstructure of the joint. The resultant bond microstructure does not deleteriously affect the heat sink function of Al base metal.

Towards a dependable TEM characterization of hot-dip galvanized steels with low and high Si content

Designing Advanced High-Strength Steels assumes fine-tuning of alloying elements to enhance performance properties. This affects the kinetics of Fe-Zn interaction during further galvannealing. A key role here belongs to the inhibition layer at the steel/coating interface. Applying mutually-reinforcing TEM techniques, we traced the evolution of Zn-Fe phases in low- and high-Si steels during galvannealing. It has been shown that conventional room temperature Ga FIB for TEM sample preparation is applicable only to annealed low-Si steel, a coating of which comprises equilibrium Zn-Fe compounds. The phases were validated via electron microscopy and diffraction methods. In contrast, even relatively long annealing of the high-Si steel was not sufficient to form stable Zn-Fe phases pointing to a hampering of the interphase reactions. As-galvanized and annealed high-Si specimens contain almost pure Zn and are affected by Ga via a eutectic reaction during FIB preparation. Thus, low-temperature FIB has been applied for reliable Zn-coated steel preparation. We refined with HRTEM the inhibition layer structures specific for low- and high-Si steel grades. While the typical Fe2Al5-xZnx interface layer has been found in the low-Si samples, for high-Si ones the formation of a quite stable Si-Mn mixed oxide membrane, which drastically hinders Fe-Al reaction, has been disclosed.

Interface regulation strategy of Al-CNTs composite induced by Al-Si eutectic reaction and its strengthening mechanism

Carbon nanotubes (CNTs) reinforced aluminum matrix composites (AMCs) show broad application prospects in the fields of aerospace and transportation because of their lightweight, high specific strength and specific modulus. A new alloying strategy in this study is proposed to regulate its weak intrinsic interface and uncontrollable interfacial reaction. Our results show that Si element could hinder the dissolution and diffusion of carbon atoms in Al matrix, inhibiting Al-CNTs interface reaction. Grain refinement and dislocation increment induced by the reinforcements promote the contributions to the strength of AMCs. The wettability and interface bonding of Al-CNTs could be effectively improved by Al/Si/CNTs composite interface, the formed discontinuous strong interface is advantageous to coordinate the plastic deformation of AMCs. Si particles can also firmly fix CNTs through pinning effect during deformation, which leads to a better load transfer. Therefore, the tensile strength of Al-5Si-0.5CNTs composite can be enhanced up to 391 MPa while maintaining an acceptable plasticity of 7.5%. It brings the strengthening effect of CNTs into full play in AMCs and achieves the strengthening effect of 1+1>2 under the comprehensive effect of grain refinement strengthening, dislocation strengthening, dispersion strengthening of Si particles and load transfer to CNTs.

Achieving grain refinement of α-Al and Si modification simultaneously by La‒B‒Sr addition in Al‒10Si alloys

As-cast Al‒Si alloys always face low ductility because of coarse α-Al grains and large lamellar eutectic Si. Although B-containing refiners and Sr modifiers could refine the α-Al grains and eutectic Si respectively, simultaneous addition leads to an invalid effect of Sr because B-containing refiner has a poisoning effect (PE) on Sr modifier in the Al‒10Si‒B‒Sr alloys. The present work achieves both refinement of α-Al grains and modification of Si merely by the addition of 500 ppm La in Al‒10Si‒0.02B‒0.015Sr alloy, which greatly enhances the ductility and tensile strength. Sr‒B interaction in the Al‒10Si‒0.02B‒0.015Sr melt results in a consumption of Sr by forming Sr‒B intermetallic compound. CALPHAD-type calculations reveal that Sr concentration in liquid before eutectic reaction becomes a key parameter to estimate the alloying modification effect. The addition of La forms LaB6 and thereby releases Sr to protect the Sr modification effect. First-principles calculations illustrate that both Sr and La are beneficial to the formation of twin boundaries in Si particles due to their negative formation energy of twin boundaries. The formed LaB6 has a semicoherent interface with α-Al with the orientation relationship of [11¯0]LaB6//[11¯0]Al and (11¯1)LaB6//(11¯1)Al, demonstrating that LaB6 is an efficient nucleation site for α-Al, which contributes to the refinement of α-Al in the Al‒10Si‒0.05La‒0.02B‒0.015Sr alloy. Our findings reveal the micro-mechanism of refinement during the solidification process and develop an effective and simple way to obtain high-performance Al alloys.

Interfacial intermetallic compound layer in friction stir welded Mg/Al joints: Relationship between thickness and the welding temperature history

Thickness of the intermetallic compounds (IMC) layer at the interface has a significant effect on the mechanical properties of Mg/Al dissimilar joints. However, the thickness of IMC layer can be only obtained by metallurgical microscopy, which is destructive and has to break down the weld. Therefore, it is crucial to find a reliable approach that can non-destructively predict the thickness of IMC layer in practical application. In the current study, Mg alloy and Al alloy were friction stir butt welded (FSW) under different tool rotation speeds (TRS) to obtain different thicknesses of IMC layers. As the TRS increased from 400 rpm to 1000 rpm, thickness of the IMC layer increased from 0.4 μm to 1.3 μm, the peak welding temperatures increased from 259 °C to 402 °C, and the Z-axis downforces decreased from 10.5 kN to 3.2 kN during welding process. Higher TRS would generally induce higher welding heat input, which promotes the growth of the IMC layer and the softening of base materials. The IMC layer formed through solid-state diffusion and transformation instead of eutectic reaction according to the welding temperature history and interfacial microstructure, and its evolution process was clearly observed by plan view. In order to incorporate the effect of dramatic change of welding temperature which is the characteristic feature of FSW, PsdVoigt function was used to fit the welding temperature histories. A new prediction formula was then established to predict thicknesses of IMC layers with considering sharp welding temperature change. Predicted thicknesses gave good agreement with measured thicknesses obtained experimentally under different welding parameters, which confirmed the accuracy and reliability of the new prediction formula. Based on this prediction formula, the time period of temperature higher than 200 °C during welding was found critical for the thickening of interfacial IMC layers.

Thermal behaviour and microstructure evolution of new ternary eutectic alloy in Al-Cu-Si-Ni system

Eutectic alloys were fabricated from the quaternary Al-Cu-Si-Ni system via arc melting and suction casting. An invariant ternary eutectic reaction (α-Al+Si+θ-Al2(CuNi)) was found in the quaternary alloy system with a composition of Al67.2Cu24Si8Ni0.8 (wt%). The dissolution of Ni (∼1.7 at%) into tetragonal θ-Al2Cu takes place during this ternary eutectic reaction. Density functional theory (DFT) calculations show that the configurational entropy stabilises this level of randomly substituted Ni with Cu sites in the θ-Al2Cu lattice at high temperatures. The as-solidified eutectic microstructure exhibits a lamellar θ-Al2(CuNi) phase showing fragmented lamellar morphology with a lamellar thickness of 130 ± 30 nm and Si exhibits fibrous morphology with a fibre diameter below 100 nm. The thermal stability of the Al-Cu-Si-Ni eutectic alloy after post-solidification annealing was investigated, and the thermal stability of the ternary eutectic microstructure is better than the corresponding Al33Cu (wt%) binary eutectic microstructure. It was found that Ni solution in θ-Al2(CuNi) phase contributes to the thermal stability of this ternary eutectic microstructure and β2-Al3(CuNi)2 (β1-(Cu2.9Ni0.1)Al type, Fm-3 m) phase can transform from θ-Al2(CuNi) phase after annealing at different temperatures. The Al-Cu-Si-Ni eutectic alloy has excellent as-cast hardness together with thermal stability. It is potentially valuable for the design of new aluminium alloys for serving at elevated temperatures.

Changes in the physical and mechanical properties of aluminum depending on the composition of the alloying elements

Subject of research: the influence of the concentration and composition of alloying elements Mn, Ti, Cr, Y, Mo, W, V on the change in the aluminum recrystallization temperature.
 Purpose of research: to find the optimal concentration of alloying additives, at which the recrystallization temperature reaches its maximum value.
 Methods and objects of research: using the methods of thermodynamic analysis of binary state diagrams of aluminum with alloying additives, it has been established that the nature of the change in the recrystallization temperature depends on what type of eutectic or peritectic reaction the formation of intermetallic compounds belongs to. It is shown that aluminides act as alloy modifiers and impart high mechanical properties to products.
 Main results of research: it was found that for the AlMn system, the optimal content of Mn should be considered 0.2%, for the AlCr system, the maximum increase in the Al recrystallization temperature is observed at 0.6% Cr, for the AlY system, the mechanical properties of Al at elevated temperatures maximum at 1% Y.

Study on Slagging Characteristics of Co-Combustion of Meager Coal and Spent Cathode Carbon Block

The harmless disposal of spent cathode carbon blocks (SCCBs) has become an urgent issue in the primary aluminum industry, and the disposal of SCCBs by co-combustion in pulverized coal boilers is expected to be the most effective treatment method. A muffle furnace at 815 °C was used in this study to perform a co-combustion experiment of meager coal and SCCBs. The ash fusion characteristics (AFTs), microscopic morphology, and minerals composition of co-combustion ash were characterized. The interaction mechanism of different mineral components and the change in AFTs and viscosity-temperature characteristics were investigated using FactSage software. Results show that the change in the ash deformation temperature (DT) is correlated linearly with the SCCB addition ratio, whereas other characteristic temperatures exhibit a nonlinear relationship. The contents of SiO2, Al2O3, and Na2O collectively determine the DT in the ash, and the influence degree from high to low is in the order of SiO2, Na2O, and Al2O3. The phase diagram of Na2O–Al2O3–SiO2 is used to accurately predict the changing trend of the melting point of co-combustion ash. The ratio changes between refractory and fusible minerals in the ash, as well as the degree of low-temperature eutectic reaction between sodium- and calcium-containing minerals, are the main factors affecting the melting point of ash. When the blending amount of SCCBs is 5%, mostly complete combustion is achieved, and slagging does not occur easily. The optimal blending ratio of SCCBs is obtained using the co-combustion method from the aspect of AFTs and viscosity-temperature characteristics. This work lays a theoretical foundation for industrial application.

A new method for fabricating Mg/Al alloy composites by pulse current-assisted rolled welding

• The Mg/Al composite plates are produced by a pulse current-assisted rolled welding. • The metal droplets are squeezed in the interfacial cross section. • The eutectic reaction mechanism is found at the interface. • A “serrated” structure interlayer is formed at the interface during the process. • The press-shear strength of the Mg/Al composite plate reaches 70.37 MPa. Pulse current-assisted rolled welding (PCARW) technology was applied for the fabrication of an Mg/Al alloy composite plate. Compared with traditional hot rolling, this process is characterized by rolling in an atmospheric environment instead of vacuum heat rolling or pack-rolling. The results showed that molten metal droplets were squeezed in the interfacial cross section, and the interfacial transition layer was formed with a serrated morphology. The formation of this special structure was due to element diffusion and eutectic reactions at the interface. A recrystallized fine-grained microstructure and shear bands (SBs) were found at the Mg alloy side near the interface. To summarize, it can be proven that the instantaneous transition liquid phase was formed at the bonding interface during rolling, and then the welded interface was obtained. Furthermore, the press-shear strength of the Mg/Al composite plate reached 70.37 MPa. Combined with the fracture morphologies, the fracture failure path was speculated to be along the interface between the transition layer and Mg alloy matrix and then pass through the transition layer along the cracks to the Al alloy matrix side.