Interdendritic Phase Research Articles

Evaluation of the corrosion and oxidation resistance of NiCoCrAlY cladding on CMSX-4 by laser cladding

This study uses laser cladding to investigate the hot corrosion and oxidation behavior of CMSX-4 claddes with NiCoCrAlY powder. High-temperature oxidation tests were conducted at 1050°c for 50, 100, 150, and 200 h on 4 samples, while 6 samples underwent hot corrosion testing at 850°c for 6 h. The innovative application of laser cladding enables the creation of a γ′ and β interdendritic phase microstructure. High-temperature oxidation tests reveal a growth rate of 0.05 μm/h in oxide layer thickness between 50 and 200 h, accompanied by a surface weight increase rate of approximately 0.8 mg/cm2.h. The oxide layer's regeneration, continuity, and density are observed at 200 h, highlighting the innovative potential of this technique. Fracture oxidation is observed at 150 h in some TGO layer regions, but at 200 h of oxidation, the TGO layer's regeneration, continuity, and dense TGO was observed at the cladding interface. The TGO forms at 1050°c consist of Al2O3, Cr2O3, and CoO oxides, with Al2O3 as the main oxide phase. The study's findings demonstrate the ability of laser cladding to enhance the thermal protection of CMSX-4, showcasing a promising innovation in the field.

Cooling Rate and Compositional Effects on Microstructural Evolution and Mechanical Properties of (CoCrCuTi)100-xFex High-Entropy Alloys.

This study investigates the impact of cooling rate and alloy composition on phase formations and properties of (CoCrCuTi)100-xFex (x = 0, 5, 10, 12.5, 15) high-entropy alloys (HEAs). Samples were synthesized using arc-melting and electromagnetic levitation, followed by quenching through the use of a Cu chill or V-shaped Cu mold. Cooling rates were evaluated by measuring dendrite arm spacings (DASs), employing the relation DAS = k ɛ-n, where constants k = 16 and n = ½. Without Fe addition, a microstructure consisting of BCC1 + BCC2 phases formed, along with an interdendritic (ID) FCC Cu-rich phase. However, with the addition of 5-10% Fe, a Cu-lean C14 Laves phase emerged, accompanied by a Cu-rich ID FCC phase. For cooling rates below 75 K/s, alloys containing 10% Fe exhibited liquid phase separation (LPS), characterized by globular Cu-rich structures within the Cu-lean liquid. In contrast, for the same composition, higher cooling rates of 400-700 K/s promoted a dendritic/interdendritic microstructure. Alloys with 12.5-15 at. % Fe displayed LPS irrespective of the cooling rate, although an increase in uniformity was noted at rates exceeding 700 K/s. Vickers hardness and fracture toughness generally increased with Fe content, with hardness ranging from 444 to 891 HV. The highest fracture toughness (5.5 ± 0.4 KIC) and hardness (891 ± 66 HV) were achieved in samples containing 15 at. % Fe, cooled at rates of 25-75 K/s.

Tailoring the Crystalline structures and mechanical properties of IN718 alloy repaired by wire-fed electron beam directed energy deposition through a custom-designed heat treatment process

Wire-fed electron beam directed energy deposition (EB-DED) is a cost-effective technique for repairing damaged metal components. However, a large number of Laves phases precipitated in the deposited zone impair the mechanical properties of the repaired IN718 alloy parts. This study investigated the microstructural evolution and mechanical properties of IN718 alloy repaired by EB-DED, emphasizing a custom-designed heat treatment combining homogenization and double aging to enhance deformation coordination. Two homogenization durations, 15 min (HA15) and 60 min (HA60), were tested. The research utilized advanced characterization techniques, including scanning electron microscopy, electron backscatter diffraction, transmission electron microscopy, and digital image correlation, to analyze microstructural evolution and deformation behavior. These findings revealed that, in the as-repaired condition, the deposited zone exhibited columnar grains with inter-dendritic Laves phase, while the substrate zone contained equiaxed grains and δ phase precipitation at the grain boundaries. Post-HA treatment led to the dissolution of Laves and δ phases and the precipitation of γ'/γ'' strengthening phases. Furthermore, the HA60-repaired sample showed recrystallization in the deposited zone, markedly improving plastic deformation uniformity. Tensile tests exhibited that the HA samples achieved yield strength and elongation matching wrought standards, with ultimate tensile strength reaching 95 % of wrought values. This work presents a novel method for achieving high-quality repair in IN718 alloy, advancing the potential for broader industrial applications.

Effect of Alloying on Microstructure and Mechanical Properties of AlCoCrFeNi2.1 Eutectic High-Entropy Alloy.

In order to explore the effect of alloying on the microstructures and mechanical properties of AlCoCrFeNi2.1 eutectic high-entropy alloys (EHEAs), 0.1, 0.2, and 0.3 at.% V, Mo, and B were added to the AlCoCrFeNi2.1 alloy in this work. The effects of the elements and contents on the phase composition, microstructures, mechanical properties, and fracture mechanism were investigated. The results showed that the crystal structures of the AlCoCrFeNi2.1 EHEAs remained unchanged, and the alloys were still composed of FCC and BCC structures, whose content varied with the addition of alloying elements. After alloying, the aggregation of Co, Cr, Al, and Ni elements remained unchanged, and the V and Mo were distributed in both dendritic and interdendritic phases. The tensile strengths of the alloys all exceeded 1000 MPa when the V and Mo elements were added, and the Mo0.2 alloy had the highest tensile strength, of 1346.3 MPa, and fracture elongation, of 24.6%. The alloys with the addition of V and Mo elements showed a mixed ductile and brittle fracture, while the B-containing alloy presented a cleavage fracture. The fracture mechanism of Mo0.2 alloy is mainly crack propagation in the BCC lamellae, and the FCC dendritic lamellae exhibit the characteristics of plastic deformation.

Effect of multi-element synergistic addition on the microstructure evolution and performance enhancement of laser hot-wire cladded Fe-based alloy

To satisfy the requirements of hardness and corrosion resistance for laser additive manufacturing of hydraulic supports, this study applied the synergistic addition of C, B, Cr, Ni, Nb and Mo elements in Fe-based alloys. The multi-phases, martensite + austenite + ferrite were designed. The microstructure, hardness and corrosion resistance of the coatings were analyzed. Increasing the C and B contents could significantly increase the hardness of the coatings, while the corrosion resistance was decreased. The corrosion resistance of the coatings was determined by the Cr contents. However, increasing the Cr contents to 20.0 wt % resulted in the ferrite structure with lower hardness (21 HRC). The coatings with 0.25 wt % C, 1.2 wt % B and 19.0 wt % Cr showed the optimal matching of hardness (56 HRC) and corrosion resistance (survived in neutral salt spray test≥300h). The resulted coatings were mainly consisted of dendritic structure. Fine lath martensite phase was dominant in the dendrite regions. The interdendritic regions were consisted of nano-sized intermetallics with a mixture of σ+Nb + FeNb + Cr2Nb+(Fe,Cr)2B + NbC compounds. These interdendritic regions (14.2 GPa) showed higher hardness than that of the dendritic regions (7.1 GPa). The high Cr contents with finer dendritic structures were the major mechanisms for the excellent combination of hardness and corrosion resistance. The precipitation and growth mechanisms of the interdendritic phases were elaborated. This work provides a valuable reference for laser hot-wire cladding to prepare Fe-based alloys with high hardness and excellent corrosion resistance.

Microstructural Evolution of Quaternary AlCoCrNi High-Entropy Alloys during Heat Treatment.

This study examines the microstructural evolution and mechanical properties of quaternary AlCoCrNi high-entropy alloys after heat treatment at 873 K for 72 and 192 h. The changes in nanostructure and phase transformation based on the heat treatment duration were as follows: B2 dendrite + BCC interdendrite and sigma phases after 72 h; B2 dendrite and interdendritic sigma phases + BCC after 192 h. After annealing, the morphology of the dendritic region shifted from spherical to needle-like, and the interdendritic region transformed from a spinodal-like to a plate-like morphology. Additionally, a phase transformation was observed in the dendritic regions of the annealed alloys at the nano-scale. The presence of the sigma phase in AlCoCrNi high-entropy alloys significantly improved the yield strength to around 1172 MPa; nevertheless, it decreased the compressive strain rapidly to 0.62%.

The Impact of Multiple Thermal Cycles Using CMT® on Microstructure Evolution in WAAM of Thin Walls Made of AlMg5

Wire Arc Additive Manufacturing (WAAM) of thin walls is an adequate technology for producing functional components made with aluminium alloys. The AlMg5 family is one of the most applicable alloys for WAAM. However, WAAM differs from traditional fabrication routes by imposing multiple thermal cycles on the material, leading the alloy to undergo cyclic thermal treatments. Depending on the heat source used, thermal fluctuation can also impact the microstructure of the builds and, consequently, the mechanical properties. No known publications discuss the effects of these two WAAM characteristics on the built microstructure. To study the influence of multiple thermal cycles and heat source-related thermal fluctuations, a thin wall was built using CMT-WAAM on a laboratory scale. Cross-sections of the wall were metallographically analysed, at the centre of a layer that was re-treated, and a region at the transition between two layers. The focus was the solidification modes and solubilisation and precipitations of secondary phases. Samples from the wall were post-heat treated in-furnace with different soaking temperatures and cooling, to support the results. Using numerical simulations, the progressive thermal cycles acting on the HAZ of one layer were simplified by a temperature sequence with a range of peak temperatures. The results showed that different zones are formed along the layers, either as a result of the imposed thermal cycling or the solidification mode resulting from CMT-WAAM deposition. In the zones, a band composed of coarse dendrites and an interdendritic phase and another band formed by alternating sizes of cells coexisted with the fusion and heat-affected zones. The numerical simulation revealed that the thermal cycling did not significantly promote the precipitation of second-phase particles.

Microstructural Analysis of K-TIG-Welded New Ni-Based Superalloy VDM Alloy 780

The fusion zone microstructures in K-TIG-welded and post-weld solution heat-treated new superalloy VDM Alloy 780 were examined. In addition, the kinetics of the base metal grain growth during solution heat treatments were analyzed. (S)TEM analyses show that major interdendritic microconstituents formed in the fusion zone due to elemental microsegregation are MC carbides and coarse irregularly shaped Laves phase. Additionally, minor secondary interdendritic phases are found to include γ′, γ″, and tiny plate-like Laves particles. To prevent any potential deterioration of mechanical properties caused by the irregular Laves phase, post-weld solution heat treatments (PWSHTs) at 954 °C to 1060 °C/1 hours were performed to remove the Laves phase. PWSHT at 954 °C only partially eliminates the Laves particles while forming an abundance of interdendritic δ/η phase. Laves phase is dissolved entirely without forming δ/η platelets after PWSHT at 1060 °C. It is proven that Laves eutectics in VDM Alloy 780’s fusion zone can be eliminated through PWSHT without significantly coarsening the base metal’s grain size in comparison to Alloy 718 as a result of substantial grain growth inhibition likely caused by solute segregation at grain boundaries.

Experimental characterization and 3D sharp-interface model simulation of grain refinement during solidification of nano-TiCN/Al–7Si composites

Significant grain refinement was achieved in nano-TiCN/Al-7 wt.% Si composites through growth control induced by nano-TiCN particles (NPs). The microstructures displayed two types of NP distributions: within interdendritic eutectic Si phase and along grain boundaries. This refined microstructure contributed to enhancing mechanical properties. To elucidate the grain refinement mechanism in nano-TiCN/Al–7Si composites, a three-dimensional (3D) sharp interface model (SIM) was developed, integrating the cellular automaton (CA) technique with a transformation matrix comprising three Euler angles. This 3D SIM effectively replicated the dendritic morphology of α-Al. The exact impact of latent heat on grain refinement was clarified. The observed grain refinement induced by NPs could not be numerically reproduced without considering latent heat. The grain refinement in TiCN/Al–7Si composites is governed by multiple factors. As NPs layers inhibits solute diffusion along the solid-liquid interface, the dendrite growth rate reduces, leading to the reduced rate of latent-heat release. Consequently, constitutional undercooling in the surrounding liquid increases, activating more heterogeneous nuclei. The developed SIM model offers deeper insights into the grain refinement mechanism induced by TiCN NPs in Al–7Si alloy.

Effect of Ce and solidification cooling rate on the microstructure and mechanical properties of AA2017 aluminum alloy

Controlling dendritic growth and grain size in new Ce-containing Al-based alloys becomes crucial due to new envisioned applications and Ce rising demand in casting processes. In the present work Ce effect on the 2xxx Al series microstructures at various industrial-scale solidification cooling rates was investigated. The directional solidification technique is crucial in this endeavor since can generate several solidified samples related to several cooling rates. The AA2017 alloy and a modified version containing 3 wt % of Ce were both produced under directional solidification and various microstructure aspects were characterized. The mechanical behavior was analyzed by microhardness and compression tests. The Ce addition reduced primary and secondary dendritic spacing without impacting grain size. Cells were only observed to form on the Ce-containing alloy at cooling rates of approximately 19 °C/s; while dendritic configurations dominated all other conditions. While round pockets composed of Al+Al2Cu+Mg2Si ternary eutectic formed the interdendritic zones in the AA2017 alloy, the AA2017-Ce alloy microstructure was mainly constituted by elongated AlCeSi + Al8CeCu4 interdendritic phases. Finally, the addition of Ce favored an increase in the microhardness and compressive strength values of the AA2017 alloy, which is attributed to the refinement of the solidification structure, as well as the formation of a greater fraction of secondary reinforcement phases.

Simultaneously improving thermal conductivity, mechanical properties and metal fluidity through Cu alloying in Mg-Zn-based alloys

Mg-Zn-based alloys have been widely used in computer, communication, and consumer (3C) products due to excellent thermal conductivity. However, it is still a challenge to balance their mechanical performance and thermal conductivity. Here, we investigate microstructure, mechanical performance, thermal conductivity and metal fluidity of Mg-5Zn (wt.%) alloy after Cu alloying by experimental and simulation methods. First, Mg-5Zn alloy consist of α-Mg matrix and interdendritic MgZn phases. As the Cu content increases, however, MgZn phases disappear but intragranular Mg2Cu and interdendritic MgZnCu phases appear in Mg-5Zn-Cu alloys. Besides, the grain size of α-Mg phase is refined and the volume fraction of MgZnCu phase increases as the Cu content increases. Second, Cu addition is found to improve thermal conductivity of Mg-5Zn alloy remarkably. Especially, Mg-5Zn-4Cu alloy exhibits the best thermal conductivity of 124 W/(m·K), which is mainly due to the significant reduction in both solid solubility of Zn in the α-Mg matrix and lattice distortion of α-Mg matrix. Moreover, a stable crystal structure of MgZnCu phase also contributes to an increased thermal conductivity based on first principles and molecular dynamics simulations. Third, Cu addition simultaneously enhances strength and ductility of Mg-5Zn alloy. Tensile yield strength and elongation of Mg-5Zn-6Cu alloy reach 117 MPa and 18.0 %, respectively, which is a combined result of refinement, solution, second phase, and dislocation strengthening. Finally, combined with a phase field simulation, we found that Cu addition enhances metal fluidity of Mg-5Zn alloy. On the one hand, Cu alloying not only delays dendrite growth but also prolongs solidification time. On the other hand, MgZnCu phase stabilizes the dendrite growth of the α-Mg phases by reducing energy consumption during solidification of liquid metal. This work demonstrates that Cu alloying is an ideal strategy for synergistically improving the thermal conductivity, mechanical performance and metal fluidity of Mg-based alloys.

Effect of Ti additions on the mechanical properties and thermal expansion properties of Ni9Cr9Co9Fe9Tix (x=1, 2, 3) high entropy alloy

The mechanical properties and thermal expansion properties of high entropy alloys (HEAs) are crucial for practical applications. However, there is a scarcity of research on how to optimize the relationship between the mechanical and thermal expansion properties, which currently restricts the application of HEAs as precision structural materials. To address this critical issue, the present study investigates the mechanical properties and thermal expansion properties of Ni9Cr9Co9Fe9Tix (x=1,2,3) HEAs fabricated via repeated arc melting and casting in a vacuum environment. The results show that Ni9Cr9Co9Fe9Tix HEAs exhibit a typical microstructure characteristic of dendritic and inter-dendritic phase, and a small amount of Ni3Ti phase is formed in the Ni9Cr9Co9Fe9Tix HEAs. The addition of Ti notably increases the tensile strength of Ni9Cr9Co9Fe9Tix HEAs, while the presence of the Ni3Ti phase contributes to a reduction in its thermal expansion coefficient. Notably, HEAs with higher Ti content demonstrate increased yield strength but diminished ductility. The primary mechanisms that strengthen the HEA are solid-solution strengthening, secondary phase hardening and dislocations strengthening of the FCC matrix. This study provides superior mechanical properties for HEAs structural components and reduces the thermal expansion coefficient of the matrix alloy, which also indicated the potential of this material for the design and development of HEAs for precision structural components.

Laser direct forming submicron Cu-rich particle structural TiZrNbCux medium-entropy alloy coatings to achieve desirable anti-bacterial property

To develop high-performance medical alloy coatings that can reduce the risk of postoperative infection, TiZrNbCux refractory medium-entropy alloy (RMEA) coatings are designed and prepared on the Ti6Al4V alloy. The effects of varying Cu additions on the microstructure, micro/nano-hardness, elastic modulus, wear resistance, corrosion resistance, and antibacterial properties are investigated. The microstructure reveals that the RMEA coatings comprise body-centered-cubic dendrites and an Mo2Si-type interdendritic (Ti, Zr)2Cu phase. Also, the contents and dimensions of (Ti, Zr)2Cu gradually increase with the increasing Cu contents. The increased amounts of Cu atoms are beneficial for the improvement of hardness and elastic modulus, but contribute little to wear resistance. Meanwhile, the electrochemical polarization curve reflects that Cu-containing RMEA coatings show superior corrosion resistance. The antibacterial test on the Cu0.1 RMEA coating demonstrates a 99.95% antibacterial rate against S. aureus after coculturing for 18 h, indicating its novel antibacterial property. Thus, TiZrNbCux RMEA coatings present huge potential in medical applications for implants.

A novel prediction model of the freckle defects for single-crystal superalloy blades

The freckle is a typical surface defect formed during the directional solidification of SC (single crystal) components of Ni-based superalloys. It generally appears as a long and narrow trail of equiaxed grains aligned roughly parallel to the direction of gravity, which breaks the integrity. Once appears, the freckle can never be avoided by further treatment. With the turbine blade requirements increasing, the state-of-the-art methods include adding more refractory elements into Ni-based SC superalloys, and the design of blades with a more complex geometric shape make the freckle defects grow easier at the special surface zones. This leads to a significant challenge in controlling grain defect formation in SC blades and vanes for freckles. The current consensus to freckle formation is described as the Rayleigh-Taylor instability (RTI) flow that occurred in the interdendritic zone. During solidification, the compositional segregation (CS) occurred as soon as the solid interface forwarding led to the density changes between the interdendritic melting phases and the residual liquids. For nickel-based superalloys, the enrichment of low-density solutes like Al, and Ti at the interdendritic zone will make the liquid lighter. However, the widely used Ra model can’t correctly match the freckle tendency of the components with complex shapes (called geometrical effects of freckles), especially for blades. Recent reports indicate that the freckles occur at the preferred positions in the casting. In this work, a novel model is designed to quantitatively discuss the geometrical effects on freckle formation and to combine existing Ra number models with solidification models to enable freckle predictions at a smaller scale. The proceeding of this work can make the design of complex blades easier.

Long-period stacking ordered phase kinking, twinning behavior and dynamic recrystallization of Mg-Gd-Y-Zn-Zr alloy under severe shear deformation

A novel severe shear specimen with a maximum shear strain of 3 was designed to further clarify kinking and fragment behavior of long period stacking ordered (LPSO) phases, as well as their interaction with twinning behavior and dynamic recrystallization (DRX) behavior under severe shear deformation (SSD). The results indicate that promoted kinking and fragment behavior for LPSO phases is obseved for hihger activation of basal <a> dislocations induced by shear deformation. Besides the known crack causd by LPSO kinking, a new fragment process without kink bands formation is found for block-shaped LPSO phases under SSD, which follows three stages of micro-crack at the interface of LPSO/α-Mg matrix, crack and crush. Moreover, the actvition of tension twinning detected at kinked boundaries is reveled to be controlled by the combined effect of LPSO kinking and shear stress state. Specifically, crystal rotation related to LPSO kinking reorient grains to be possbile for twinning, and then high effective Schmid factor (ESF) under shear stress furter prioritise the twining acivaiton. Additionally, orientation dependence of DRX behaviors was weakened due to the particle-stimulated nucleation (PSN) mechanism caused by interdendritic block-shaped LPSO phases, which can accelerate the formation of low angle grain boundaries and DRX grains by activated pyramidal <c+a> slip systems even at basal grains.

A D019 precipitate strengthened laser additively manufactured V and Nb bearing CoCrFeNi based high entropy alloys

Two novel HEA compositions of [Nb¯-(FeCoNi)12]Cr3 (Nb¯ = Nb, V) were designed using a high-entropy alloying strategy by analyzing traditional IN718 superalloy based on a cluster-plus-glue-atom model. And their thin-wall-shaped bulks were prepared by laser additive manufacturing, with an emphasis on the effects of using V atoms to substitute for Nb atoms on their microstructures and mechanical properties. Also, the strengthening mechanism induced by D019 precipitates of as-aged V2Nb4 HEA was discussed. After using V to substitute for Nb, the contents of inter-dendritic C14-Laves phases are effectively inhibited. The ductility is significantly improved with a small sacrifice of strength (i.e., σb ∼ 851 MPa and δ ∼ 16.8 % for V2Nb4 HEA). After aging treatment, the main microstructure transforms from fine dendrites to recrystallized equiaxed grains. The intercrossed needle-shaped D019 precipitates with a volume fraction of 7.2 % are uniformly distributed in the FCC matrix, which hinders the dislocation slip. Thereto, the as-96-hr-aged V2Nb4 HEA exhibits the most excellent strength-ductility trade-off (e.g., σs ∼ 721.5 MPa, σb ∼ 979.5 MPa with a sufficient δ of 7.5 %). The above findings provide references for the development of LAM high-performance HEAs originating from traditional alloys.

Microstructural evolution and improved corrosion resistance of NiCrSiFeB coatings prepared by laser cladding

Abstract Inconel 625 (IN 625) is widespread in the manufacturing of critical components such as nuclear reactors, control rods, steam turbines, supercritical boilers, rotary shafts, aerospace engines, etc., that operate in severe harsh environments. However, if the service environments consist of sulphur (fuel tanks), chlorine (supercritical boilers and heavy water plants), H2S, HCl, etc., this alloy will suffer from localized corrosion attacks that minimize its resistance towards corrosion, followed by sudden failure. This study is aimed to facilitate the anti-corrosion characteristics of IN 625 by cladding it with Colmonoy 5 (NiCrSiFeB) alloy particles. The clad microstructure was revealed by micrographs captured by means of optical and field emission scanning electron microscopy followed by the nanoindentation study to analyze the hardness offered. Corrosion testing was carried out on both IN 625 and Colmonoy 5 clad samples at various intervals (0, 13, 27 and 56 h) for interrogating the corrosion behavior in terms of Tafel and impedance plots along with the surface roughness examination using scanning probe microscopy. The results showed that the clad region consists of dendritic microstructure along with the segregation of interdendritic Cr-rich precipitates after solidification. These interdendritic precipitates aid in improving the hardness at the clad region. Moreover, the clad samples have better anti-corrosion characteristics because of the existence of dendritic and interdendritic phases compared to the IN 625 samples in terms of current density, polarization resistance and average surface roughness values.

Microstructure evolution of Ni-Cr-Mo superalloy fabricated by microwave-assisted combustion synthesis

In this study, the microwave-assisted self-propagation high-temperature synthesis (SHS) technique was utilized to successfully synthesize the Nistelle super C commercial alloy with the chemical composition of Ni59Cr23Mo18. The precursors used for this purpose were NiO, Cr2O3, and MoO3 oxide compounds. The thermodynamicall calculations were carried out by the Factsage software. The synthesized alloys' chemical composition, phase, and microstructure were evaluated by induction-coupled plasma, X-ray diffraction, optical microscopy, and scanning electron microscopy. The findings revealed considerable residual silicon in the final product due to using Si as the reducing agent. The presence of residual Si in the chemical composition led to forming a eutectic Niγ-FCC+ Mo2Ni3Si microstructure in the alloys produced by the alumino/silicothermic method. The high silicon content in the excess Al-containing sample led to forming of proeutectic Mo2Ni3Si phases with a petaloid morphology. Besides, this alloy displayed small islands of the interdendritic nickel silicide phase. Conversely, the aluminothermic sample showed a nearly single-phase structure with low amounts of the precipitates of chromium and molybdenum-rich phases. The phase diagram plotted for this sample indicated that the precipitates were probably TCP phases like σ and P. In all the samples, the segregation of Mo occurred significantly, while Cr was distributed almost uniformly in all regions. The formation of the Mo2Ni3Si intermetallic phase resulted in the significant segregation of Mo and depletion of the Niγ-FCC matrix from this element. However, the segregation of Mo was also observed near the grain boundaries in the single-phase sample. The results showed that the dissolution of the alloying elements, especially molybdenum, caused severe offset in some peaks of the Niγ-FCC XRD pattern.

Microstructure and mechanical properties of the Mg98.5Zn0.5RE1 (RE=Gd and/or Y) alloys processed by equal channel angular pressing

In this work, Mg98.5Zn0.5RE1 (RE = Gd and/or Y) and Mg97Zn1Y2 (at%) alloys were subjected to homogenization and equal channel angular pressing (ECAP) to systematically compare the microstructure evolution and mechanical properties. The results indicate that, instead of the lamellar LPSO phases in the as-cast Mg-Zn-Y series alloys, the β-Mg5(Gd, Zn) phases appear in the as-cast Mg98.5Zn0.5Gd1 alloy which tend to be dissolved during homogenization. The furnace cooling after homogenization promotes the precipitation of numerous lamellar LPSO phases in all the studied alloys which inhibit the DRX behavior but facilitate the operation of kinking deformation during ECAP for the Mg98.5Zn0.5RE1 alloys, especially the Mg98.5Zn0.5Gd1 one. After the 5p-ECAP process at 350 °C, the larger density of kink bands, more LAGBs (low-angle grain boundaries) and lower SFB (Schmid factor for basal slip) combine to endow the Mg98.5Zn0.5Gd1 alloy with the higher strength-ductility synergy, exhibiting the tensile yield strength (TYS) of 214 MPa and elongation of 9.6%. The Mg97Zn1Y2 alloy with the larger volume fraction of interdendritic LPSO phases and dynamically recrystallized (DRXed) regions exhibits the superior ultimate tensile strength (UTS) of 277 MPa and moderate elongation of 7.2%.

Hot isostatic pressing and heat treatments of LPBFed CoCuFeMnNiTi0.13 high-entropy alloy: microstructure and mechanical properties

The present work explores the possibility of processing a CoCuFeMnNiTi0.13 high-entropy alloy by laser powder bed fusion (LPBF). The alloy, produced under optimised processing conditions, presents good densification but also hot cracks, caused by the liquation of an inter-dendritic Cu-rich phase. Microstructure of the as-built alloy is characterised by face centred cubic (FCC) columnar grains, containing Cu-poor dendrites and Cu-rich inter-dendritic areas. The alloy, which was designed to be strengthened by spinodal decomposition and precipitation, was subjected to different thermo-mechanical treatments to try and improve its properties. Direct ageing and solution treatment and ageing produced a strong but brittle material (tensile strength of 683 MPa and elongation to failure of 1.3%), whereas hot isostatic pressing followed by controlled cooling was able to heal pores and cracks while triggering the desired microstructural transformations (spinodal decomposition and precipitation). This resulted into a balanced set of mechanical properties (tensile strength of 473 MPa and elongation to failure of 7.6%). This work shows that proper post-processing can mitigate the issues typically affecting LPBF fabricated HEAs, producing tailored microstructures with satisfactory mechanical performances.