Eutectic Microstructure Research Articles

A novel strategy for promoting corrosion and wear resistance of Mg-Li alloys: Gradient eutectic high-entropy alloy coating induced by in-situ bidirectional diffusion

In this work, a novel surface strengthening strategy for Mg-Li alloys was proposed, called cold spraying assisted high-speed laser cladding. CuAl9 aluminum bronze coating was firstly deposited on Mg-Li alloy by cold spraying, and then CoCrFe0.5Ni1.5Mo0.1Nb0.68 eutectic high-entropy alloy (EHEA) coating was prepared on the CuAl9 coating utilizing high-speed laser cladding. A gradient coating consisted of in-situ transition region and EHEA layer formed by bidirectional diffusion. The in-situ transition region was composed of β-Li, Cu2Mg and Cu3Al2 phases. TEM analysis indicated that Cu2Mg and Cu3Al2 phases were well matched with the β-Li matrix phase. The EHEA coating had a nano-lamellar eutectic microstructure with relatively small lamellar-spacing (< 100 nm). Metallurgical bonding interfaces formed between the EHEA coating, transition region and Mg-Li substrate. The evolution mechanism of the coating was revealed from the perspectives of mixing enthalpy, atomic radius difference and laser energy distribution. In 3.5 wt.% NaCl solution, the corrosion potential of the EHEA coating (-24 mVSHE) was 1345 mVSHE higher than that of Mg-Li alloy (-1369 mVSHE), while the corrosion current density of the EHEA coating (3.13 × 10−7 A·cm−2) was almost three orders of magnitude lower than that of Mg-Li alloy (1.25 × 10−4 A·cm−2). The wear rate of Mg-Li alloy (1.11 × 10−3 mm3/N·m) was about 36 times higher than that of the EHEA coating (3.05 × 10−5 mm3/N·m).

High-strength AlCoCrFeNi2.1 eutectic high entropy alloy skeleton with hollow brick wall structures by selective laser melting

In this work, a hollow brick wall structure was introduced into the preparation of AlCoCrFeNi2.1 EHEA to realize a skeleton structure by taking advantage of the good castability of eutectic high entropy alloys (EHEAs) and high cooling rate of selective laser melting (SLM). The effects of processing parameters on eutectic microstructure and mechanical properties of SLM-ed AlCoCrFeNi2.1 EHEA skeletons were investigated. With increasing volume energy density (VED), the microstructure along the building direction evolved from cellular structure to the coexistence of cellular and lamellar structures. The phases in the SLM-ed AlCoCrFeNi2.1 EHEA skeletons contained BCC/B2 and FCC phases. Under the VED of 137.25 J/mm3, SLM-ed skeletons with hollow brick wall structures had a compressive strength of 241.2 ± 4.2 MPa. The mechanical properties from tensile tests exhibited a yield strength of 1349.4 ± 12.4 MPa and an ultimate tensile strength of 1529.5 ± 12.8 MPa. The superior performance of the selective laser melting method was attributed to fine grain strengthening, dislocations strengthening, and heterogeneous structure of FCC and BCC/B2. This work provides possibilities for microstructure design and performance improvement of EHEAs and expands the avenue for developing advanced high-strength lightweight structural and energy-absorbing alloys.

Mechanisms underlying the influence of Co and Ti on the microstructure, mechanical and wear properties of A2/B2 typed AlCrFeNi alloy

AlCrFeCoNi is one of widely studied high-entropy alloys, in which Co is considered to be beneficial. Here we demonstrate in detail that Co does not play a positive role. Instead, Co-free AlCrFeNi possesses higher ductility than AlCrFeCoNi while maintaining a similar mechanical strength. It is of interest to observe that removing Co results in a two-fold increase in the wear resistance. The improvements are related to the formation of a soft-hard alternating lamellar structure which is absent in the Co-containing alloy. We further tailor the alloy by replacing Co with Ti, leading to further enhanced wear resistance. AlCrFeCoxNi (x = 0.5 and 1) and Co-free AlCrFeNiTix (x = 0, 0.2, 0.5, 0.7 and 1) all contain a Fe-Cr-rich disordered BCC phase (A2) and a NiAl (Ti)-rich ordered BCC phase (B2). For x = 0.5, a small amount of L21 phase with embedded nanoscale A2 phase is observed. For x ≥ 0.7, an additional CrFeTi Laves phase forms mainly at grain boundaries. The increase in Ti content alters the lamellar eutectic microstructure of AlCrFeNi, forming a cellular eutectic microstructure. Ti considerably increases hardness and compressive strength at the expense of ductility. AlCrFeNiTi0.7 appears to have a proper balance between hardness and ductility with a superior wear resistance comparable to that of a well-known industrial wear-resistant high-Cr cast iron (Cr30C2.85). This HEA has demonstrated itself a promising candidate for the mechanical and anti-wear applications.

Microstructure evolution at the interface between Cu and eutectic Sn–Bi alloy with the addition of Ag or Ni

The study investigated the growth kinetics and rate-controlling processes of intermetallic layers formed at the interface between Cu and eutectic Sn–Bi alloys with Ag or Ni addition at solid-state temperatures. Isothermal sections of the equilibrium phase diagram were calculated to understand phase stability at the interface. The thicknesses of the intermetallic layers were plotted against annealing time, and a power function equation with an exponent slightly smaller than 0.25 described their relationship, indicating boundary diffusion with grain growth as the predominant control mechanism. The relationship between the exponent and annealing temperature suggested that the grain growth in the intermetallic layer followed an approximate parabolic law. The eutectic microstructure in Sn–57Bi–1Ag and Sn–57Bi–1Ni alloys changed with annealing, with differences in the Bi region width. However, the contact surface area between the Bi region and Cu6Sn5 compound was similar in both alloys. Activation enthalpies for the rate-controlling process of intermetallic layer growth were evaluated, providing insights into the energy barrier for atomic diffusion or transport across the interface.

The microstructure and mechanical properties of novel Fe-rich Fe–Cr–Ni–Ta eutectic multi-principal element alloys

Co–Cr–Fe–Ni–Ta eutectic multi-principal element alloys (MPEAs) demonstrate great potential to replace structural alloys in engineering applications. However, the cost of these alloys is high due to the usage of a number of precious metals. To decrease the cost, the present work developed a set of novel Fe55Cr15Ni30−xTax (x = 0, 5, 8, 10 and 15 at.%) eutectic MPEAs and the effect of Ta on microstructure and compressive mechanical properties were investigated for the first time. As the Ta content increases, the microstructure changes from a single-phase FCC solid solution (x = 0) to hypoeutectic microstructure (x = 6), then to eutectic microstructure (x = 8), and eventually to hypereutectic microstructure (x = 10 and 15). The yield strength and the hardness increase at the expense of ductility reduction with increasing Ta content. The strengthening mechanism of this alloy system is mainly second-phase strengthening caused by Laves phase and grain boundary strengthening produced by eutectic interface, supplemented by solid solution strengthening. The Fe55Cr15Ni22Ta8 and Fe55Cr15Ni20Ta10 alloys present the excellent comprehensive mechanical properties. The yield strength, fracture strength and fracture strain of the former are 838 MPa, 1994 MPa and 45.1%, respectively. The yield strength, fracture strength and fracture strain of the latter are 1029 MPa, 1956 MPa and 43.0%, respectively. The phase formation of Fe55Cr15Ni30−xTax alloys can be predicted by Md criterion. When Md ≥ 0.89, the alloys consist of FCC phase and Laves phase. Otherwise, the alloys consist of FCC single phase.

Liquid Metal Embrittlement Susceptibility of Hot Formed Zn-Al-Mg Coated Steel with Eutectic Coating Microstructure

Liquid metal embrittlement (LME) in Zn-based coating plates during hot stamping is an abnormal phenomenon where intimate contact between liquid Zn and a steel matrix results in the penetration of liquid Zn into the matrix, causing ruptures. In order to alleviate LME phenomenon, this paper designed a series of eutectic Zn-Al-Mg coating alloys to improve the uniformity of the Zn element distribution in the coating during heat treatment and inhibit the reaction between Fe and Zn. The high temperature mechanical properties of the coated steels are determined using thermal simulation to calculate the relative reduction in fracture energy, which is used to evaluate the LME level of the different composition coatings. It is suggested that the Zn-4.5Al-3.0Mg coating shows the highest LME resistance at 920 °C. The microstructure of these Zn-Al-Mg coated steels is observed after austenitization at 850 °C~920 °C for 3 min, which shows that the uniformity of the microstructure after austenitizing is affected by the composition of the coating and the austenitizing temperature. The higher temperature benefits the homogenization of the coating and steel and inhibits LME. The findings of this study provide valuable insights for the development of ternary Zn based LME resistant coatings.

Effect of Undercooling on the Microstructure and Mechanical Properties of Hyper-eutectic Ni–Sn Alloy

In this study, container-less solidification of hyper-eutectic Ni–Sn alloy has been performed by using the electromagnetic levitation technique. The effect of undercooling on the formed microstructure and on the mechanical properties have been investigated. Growth velocities were determined by high-speed video-imaging of the solidification process. A step change in the growth velocities that are increasing with increasing undercooling is observed. This aligns with an observed first change in the microstructure between low and intermediate undercoolings. At the lower undercoolings, a pro-eutectic Ni3Sn phase along with lamellar eutectic structure in the inter-dendritic region is found. At intermediate undercooling of 100–150 K, a divorced eutectic microstructure is observed whereas at undercoolings above 165 K α-Ni precipitates are observed within β-Ni3Sn dendrites. Microhardness testing revealed higher strength for the lamellar phase as compared to the non-lamellar phase. Nano-indentation has been performed to determine the hardness and strength of individual phases in the microstructure.

Effect of friction stir welding parameters on microstructure and mechanical properties of the dissimilar alloys of AZ91D and AA7075

Friction stir welding is an efficient joining process for light metal alloys, offering benefits such as high joint efficiency and reduced grain sizes. This work is to investigate the process parameters that can improve the welding quality and reduce the brittle intermetallic compounds, including Al3Mg2 (β) and Al12Mg17 (γ) in the welded region. It was perceived that the process parameters influenced the morphological characteristics and input heat, thereby affecting the materialization of intermetallic compounds. The primary diffraction peaks of both Al and Mg were present in all the joints. Two specific intermetallic phases, Al3Mg2 and Al12Mg17, were formed as a result of the diffusion of Al and Mg atoms during welding. These intermetallic compounds were found in all the joints. The Al12Mg17 phase, when mixed with the Mg phase, tended to migrate towards the weld surface where the AZ91D plate was positioned. This migration might lead to constitutional liquation, which refers to the melting of specific phases during solidification, resulting in the materialization of eutectic microstructures. The highest ultimate tensile strength (UTS) value of 116.64 MPa was achieved with a rotational tool speed of 800 r/min and a traverse speed of 20 mm/min. On the other hand, the lowest UTS value of 68.32 MPa was observed with a rotational tool speed of 400 r/min and a traverse speed (TS) of 20 mm/min. It was noted that lower UTS values were associated with the materialization of dense layers of intermetallic compounds (Al12Mg17 and Mg + Al12Mg17) in the stir zone when using lower rotational tool speed and TS. It suggests that improper processing parameters can lead to the materialization of intermetallic compound in the stir zone, which subsequently decreases the mechanical properties of the weldments.

Experimental cooling rates during high-power laser powder bed fusion at varying processing conditions

High-power laser power bed fusion (HP-LPBF) with a large flat-top laser beam allows additive manufacturing of components at much higher build-up rates than conventional LPBF, since thicker powder layers can be processed. This makes this technology attractive for industry due to the augmented productivity. Here, we have utilized HP-LPBF to fabricate Al-33Cu (wt%) specimens at differing layer thickness and laser power. Based on the average spacing of the lamellae of the eutectic microstructure, the cooling rate inherent to HP-LPBF was experimentally determined as a function of the processing conditions. At the lowest layer thickness (50 µm) and laser power (500 W), the cooling rate amounts to about 90000 K/s, whereas it drops to about 15000 K/s for HP-LPBF with the largest layer thickness (200 µm) and highest laser power (1000 W). When the base plate is additionally pre-heated, the cooling rate prevailing during solidification decreased to about 5000 K/s. Our findings demonstrate that relatively low cooling rates are effective during HP-LPBF at high build-up rates being tantamount to high productivity. It should be considered that such drastic changes in cooling rate may strongly affect the microstructure formation, the properties and hence performance of the corresponding additively manufactured component.

Influence of Sr and Mn Elements on the Microstructural Characteristics, Mechanical, and Anticorrosion Properties of Zn‐Based Biodegradable Materials

This study specifically focuses on investigating the effects of Mn and Sr additions on the microstructure, mechanical properties, and anticorrosion behavior of as‐extruded Zn–Mn–Sr alloys, with the ultimate goal of developing biodegradable implants. The microstructural observations reveal that the incorporation of Mn and Sr elements plays a crucial role in refining the hcp (Zn) grains within the alloys. Moreover, the intermetallic phase SrZn13 is observed to precipitate along with the eutectic microstructure. Mechanical testing demonstrates that the addition of Sr shows the potential to enhance the mechanical properties of Zn–Mn alloys, and also results in a trade‐off with ductility. Notable alloys, BZA920, exhibit the highest values of yield strength at 219.8 ± 5.2 MPa and ultimate tensile strength at 240.6 ± 4.3 MPa; alloys of BZA315, BZA615, and BZA915 exhibit excellent ductility of over 40%. The electrochemical and immersion tests conducted in simulated body fluid (SBF) solution demonstrate the alloys’ favorable corrosion resistance. The corrosion rates range from 0.05 to 0.15 mm year−1, indicating their potential suitability as implant materials. Overall, this study highlights the exceptional mechanical properties, favorable corrosion resistance, and excellent biocompatibility of the investigated Zn–Mn–Sr alloys.

Making as-cast (FeCoNi)NbB alloys excellent mechanical and magnetic properties via refined eutectic microstructures

Making as-cast (FeCoNi)NbB alloys excellent mechanical and magnetic properties via refined eutectic microstructures

Microstructure and mechanical properties of ZrC modified Ni60 hard-facing alloy fabricated by laser metal deposition

Crack-free specimens were prepared from ZrC-modified Ni60 alloy by laser metal deposition. The effects of ZrC powder adulteration on the mechanism of microstructure refinement were investigated. The results demonstrated that the adulteration of ZrC powder did not provide nucleation sites for the hardening phase. The ZrC powder was added to Ni60 alloy powder to reduce the content of columnar dendrites and the eutectics of laser metal deposition specimens. The transformation of the dendritic hardening phase into a massive hardening phase was induced. The fine Ni–B–Si eutectic microstructure changed to a massive NiZr eutectic and peritectic microstructure with increasing ZrC powder mass fraction. The microstructure transformation mechanism constrained the initiation and propagation of cracks in the deposited specimens. The present research provides a method for improving the cracking defect of laser additive manufacturing Ni–Cr–B–Si system alloys and a theoretical basis for promoting the application of the alloy in additive manufacturing.

Microstructural Evolutions and Strengthening Mechanism according to the Aging Temperatures of a High Si Cast Aluminum Alloy

A356 cast aluminum alloy contains 7 at.% Si and 0.3 at.% Mg, producing an approximately 50% eutectic microstructure. This high Si content and various casting conditions play a significant role in strengthening A356 alloy, by controlling the eutectic morphology and precipitates of other intermetallic compounds. Understanding how Si-related precipitates and clusters are soluble in the α-matrix is necessary to provide high strength and fatigue resistance to A356 alloys. The aging heat-treatment temperature in the A356 alloy most likely promotes the formation of these precipitates and clusters. The A356 samples were differently aged at temperatures of 110 oC and 130 oC for 2 h, and were labeled 110A, and 130A, respectively. 110A was found to have improved mechanical properties, such as high strength and elongation, compared to 130A, which may be attributed to the formation of secondary phases in the α-phase matrix. Scanning and transmission electron microscopy and atom probe tomography analyses demonstrated Ti2Si precipitation and various-sized cluster formations in 110A. In contrast, 130A had fewer clusters than 110A. Therefore, different aging heat-treatment temperatures relate to a change in the behavior of atoms, affecting the mechanical properties.

Growth behavior of intermetallic layers at the interface between Cu and eutectic Sn–Bi by grain boundary diffusion with the grain growth at solid-state temperatures

The growth kinetics and rate-controlling processes of intermetallic layers formed in Cu/(Sn-58 wt% Bi) diffusion couples were investigated. Isothermal annealing of diffusion couples was conducted in the temperature range of 363–393 K for various times up to 384 h. In the diffusion couple, an intermetallic layer composed of Cu6Sn5 with irregular scallop shapes and Cu3Sn with relatively uniform thickness forms at the Cu/(Sn–Bi) interface. Isothermal sections of the equilibrium phase diagram in the ternary Bi–Cu–Sn system were calculated at the isothermal annealing temperatures and the peak temperature of the reflow process by a CALPHAD (calculation of phase diagrams) method. The diffusion path passes through the three-phase tie-triangle, including Cu6Sn5 and Cu3Sn. The obtained thicknesses were plotted against the annealing time, and a power function equation was found to describe their relationship. The values of the exponent in the power function were close to 0.25, indicating that the layer growth was predominantly controlled by boundary diffusion with the grain growth. Furthermore, the exponent values were relatively insensitive to the annealing temperature. The relationship between the exponent and the annealing temperature was analyzed, and it was concluded that the grain growth in the intermetallic layer followed a parabolic law. The growth and morphology evolution of the intermetallic compound and Sn–Bi eutectic microstructure at the interface were depicted, shedding light on the widening of the eutectic microstructure. Additionally, the activation enthalpy for the rate-controlling process of intermetallic layer growth at solid-state temperatures was evaluated using a least-squares method, providing insights into the energy barrier for the diffusion or transport of atoms across the interface. This study contributes to a better understanding of intermetallic layer formation and growth in Cu/(eutectic Sn–Bi) diffusion couples, offering valuable information for developing reliable solder joints and high-temperature solders.

Microstructural formation and evolution of near-eutectic Al2O3–ZrO2 powders prepared by a rapid cooling technique: An exploration on coupled eutectic growth range

Regulation of eutectic compositions is a difficult challenge due to the limited formation range of this microstructure. Al2O3–ZrO2 powders were successfully prepared through a modified high-temperature melt-assisted air atomization technique. Micron-sized raw powders of α-Al2O3 and m-ZrO2 were used to regulate the temperature of system and the composition of final products. The high-temperature melt of 3500 K (3227 °C) was obtained based on the exothermic reaction between Al and O2, which was subsequently atomized and quenched to form composite powders. The phase composition and microstructure of the as-prepared powders were characterized by X-Ray diffraction and scanning electron microscopy. Eutectic colonies with nanolamellae (62–79 nm) were observed in the as-prepared Al2O3-(28–58 mol%) ZrO2 powders, with growth rates ranging from 1.9 to 2.8 mm s−1. Importantly, the powders with ZrO2 contents of 38 mol% and 42 mol% showed complete eutectic microstructures without primary dendrites, which indicated an extended coupled growth range deviating by ∼4.9 mol% from the eutectic composition (CE). The microstructural formation mechanism was discussed based on solidification behavior analysis. Furthermore, the evolution of the microstructure and phase in Al2O3-42 mol% ZrO2 powders was investigated. This work aims to develop a novel strategy for preparing oxide powders with eutectic structures and provide a reference for the composition regulation of eutectics.

Development of eutectic high entropy alloy by addition of W to CoCrFeNi HEA

High entropy alloys have shown a remarkable combination of physical and mechanical properties. The introduction of eutectic microstructure, consisting of a tough fcc phase, and a hard-intermetallic phase, can help in obtaining even better synergy of strength and ductility. The presence of multiple principal alloying elements in HEAs and absence of corresponding multicomponent phase diagrams makes designing of eutectic high entropy alloys a tedious task. In the present study, systematic investigation of CoCrFeNi-Wx system has been carried out for the development of eutectic microstructure. Experimental results validated the presence of eutectic reaction in the calculated phase diagram. CoCrFeNi-Wx HEAs remained single fcc phase alloys at smaller amount of W (x = 0.1) but changed to hypoeutectic (x = 0.25, 0.5, 0.75) and hypereutectic alloys (x = 1.0) with increase in the amount of tungsten. It has been shown that calculated pseudo binary phase diagrams can provide a very good starting point for the development of eutectic HEAs. Mechanical characterization of the developed HEAs revealed that development of eutectic mixture of a soft (fcc) and a hard phase (intermetallic/bcc) can help in obtaining outstanding combination of mechanical properties.

The Effect of Tool Rotation Speed on the Formation of Eutectic Structure during Friction Stir Welding of Aluminum to Magnesium

Friction stir welding (FSW) is a solid-state welding process capable of joining a wide range of light metals. However, liquation and solidification may occur during joining of dissimilar metals which leads to eutectic formation. This article aims to discover the influence of tool rotation speed on the formation of eutectic structure during friction stir welding of aluminum to magnesium. To do so, friction stir welding was performed at 600 and 950 rpm to join pure aluminum and ECO-AZ91 magnesium alloy in a lap configuration. In order to investigate the influence of the welding speed, the welding speeds of 23.5 and 37.5 mm/min were also chosen. Scanning electron microscopy (SEM) was used to study the microstructure of the joints. A shear-tensile test was used to evaluate the joints’ strengths. The fracture surfaces were also studied by SEM. The results revealed that changing the rotation speed directly affects the eutectic formation, whereas the welding speed had no influence. A lower rotation speed resulted in a thin, continuous intermetallic layer, whereas a higher speed led to the formation of a massive Mg-Al12Mg17 eutectic microstructure. The formation of eutectic, as an indicative of liquation, may affect the material flow during the process due to decreasing the friction coefficient between the tool and material. The macrostructure analyses showed that the phase evolution as well as the mechanism of material flow are highly affected by liquation.

Atomistic simulations of nano-fiber-confined metal plasticity

Atomistic simulations are used to explore dislocation mechanisms in nano-confined aluminium (Al) containing silicon (Si) nanofibers. Our simulations revealed that dislocations on four {111} slip planes nucleate at Al-Si interfaces, propagate in Al nano-channels, loop and bow around Si nanofibers. Due to multiple active slip systems and dislocation cross-slips in the confined volume, a high density of dislocations intersects to form dislocation junctions and promote the formation of dislocation forests, resulting in high flow strength and strain hardening. The plasticity of Si nanofibers is accommodated by local shears and fracture triggered by the accumulated dislocation loops. Simulations show that the cracking of Si nanofibers can be efficiently buffered by dislocation mobility in the surrounding Al. Our results are consistent with experiments on nano-fibrous Al-Si eutectic microstructure.

A genome dependence of metastable phase selection on atomic structure for undercooled liquid Nb90Si10 hypoeutectic alloy

The phase selection mechanism within undercooled liquid Nb90Si10 hypoeutectic alloy was investigated by electrostatic levitation technique combined with deep neural network molecular dynamics. A stepwise-solidification procedure was conducted, where the primary phase and eutectic microstructure successively solidified from undercooled liquid alloy and undercooled residual liquid, respectively. The intermetallic phase of the eutectic structure transfers from Nb3Si to βNb5Si3 and finally into αNb5Si3 compound with the increase in liquid undercooling. The deep neural network molecular dynamic simulations have shown that the phase selection between Nb3Si and Nb5Si3 is mainly controlled by the short-range order of residual liquid, considering that the predominant short-range configuration transforms from Nb3Si-like to Nb5Si3-like structures. The αNb5Si3-like medium-range order, which is characterized by vertex-connected ⟨0,2,8,4⟩ clusters, is shown to significantly influence the competitive nucleation of the αNb5Si3 and βNb5Si3 phases. The residual liquid favors the αNb5Si3-like medium-range order rather than βNb5Si3 at large undercoolings, which explains the transformation from βNb5Si3 to αNb5Si3.

A simplistic accelerated design methodology for eutectic multi-principal element alloys

In this work, the limitations of the current design strategies of eutectic multi-principal element alloys (EMPEAs) have been investigated and addressed using a novel design approach that utilizes Scheil solidification simulations. The proposed design approach was experimentally verified by demonstrating a eutectic in the Al-Fe-Ti-V-Zr alloy system, in which Fe was identified as the eutectic forming element (EFE). Vacuum-arc-melting was utilized to fabricate the precisely designed Fex(AlTiVZr)100−x (x = 70, 73, and 75) alloys. The alloy, with x = 73, was found to possess a fully eutectic microstructure corroborating the proposed design approach.