Solid Solution Matrix Research Articles

Investigation on the Corrosion Resistance of Laser Cladding Fe-Based Alloy Coating Against Molten Zinc

In this paper, laser cladding technology was used to prepare an Fe-based coating on H13 steel substrate and its corrosion behavior in molten zinc was studied. The results show that a laser-cladding Fe-based coating can effectively protect the substrate from the corrosion of molten zinc, which is mainly related to its microstructure. The typical microstructure of the coating is composed of α-(Fe, Cr) solid solution matrix and CrFeB eutectic phases continuously distribute around the matrix. When molten zinc is in contact with the surface of the coating, it corrodes the α phase matrix preferentially and CrFeB eutectic phases with better corrosion resistance interweave with each other to form a three-dimensional skeletal structure that can play the role of diffusion barrier and slow down the diffusion rate of liquid zinc. The corrosion by molten zinc leads to the formation of a transition layer and an outer corrosion layer above the coatings. With the prolongation of the corrosion time, a large number of microcracks are generated inside the transition layer and fracture gradually occurs under the action of thermal stress. The partial spalling of the transition layer and the corrosion of α phase matrix occur at the same time, making the corrosion depth of the coating increase continuously. However, the dense corrosion layer above the coating and the dispersed boride fragments can still function as a barrier to the inward diffusion of molten zinc.

The microstructure, mechanical-thermal properties and softening resistance of Y4Al2O9 dispersion-strengthened Cu alloy

Oxide dispersion-strengthened copper (ODS-Cu) alloys with yttrium oxide nanoparticles are promising heat sink materials in fusion reactor due to their high strength, thermal conductivity and thermodynamic stability. The particle size and number density of oxide nanoparticles can be improved via addition of microalloying element. In this study, the atomized Cu-Y alloy, Al and Cu2O were used as the Y source, the microalloying element and the oxidant material, respectively, to fabricate ODS-Cu alloy by mechanical alloying method. After spark plasma sintering and annealing, a ODS-Cu alloy with a microstructure containing Y4Al2O9 nanoparticles homogeneously distributed in Cu(Al) solid solution matrix was successfully fabricated. It is found that a crystallographic orientation relationship of (31¯2)Y4Al2O9∥(11¯1)Cu(Al) was formed between Y4Al2O9 nanoparticles and Cu(Al) matrix and the lattice mismatch was 0.337%. The small lattice mismatch reduced the nucleation barrier for Y4Al2O9 nanoparticles so a mean particle size of 4.8 nm and a number density of 2.16×1024 m−3 were obtained in the alloy. Consequently, the microhardness was enhanced via Hall-Petch strengthening, dislocation strengthening and dispersion strengthening. The softening temperature of ODS-Cu alloy was improved to higher than 1000 °C due to the thermodynamic stability and the pinning effect of Y4Al2O9 nanoparticles. In addition, the thermal conductivity of Cu-1Y-9Al-0.3Cu2O alloy is close to other copper alloys as the volume fraction of Y4Al2O9 nanoparticles is limited. Cu-1Y-9Al-0.3Cu2O alloy provides a good overall performance, which may be used as heat sink materials under severe conditions.

Interpretation of Specific Strength-Over-Resistivity Ratio in Cu Alloys.

Reaching simultaneously high mechanical strength and low electrical resistivity is difficult as both properties are based on similar microstructural mechanisms. In our previous work, a new parameter, the tensile strength-over-electrical resistivity ratio, is proposed to evaluate the matching of the two properties in Cu alloys. A specific ratio of 310 × 108 MPa·Ω−1·m−1, independent of the alloy system and thermal history, is obtained from Cu-Ni-Mo alloys, which actually points to the lower limit of prevailing Cu alloys possessing high strength and low resistivity. The present paper explores the origin of this specific ratio by introducing the dual-phase mechanical model of composite materials, assuming that the precipitate particles are mechanically mixed in the Cu solid solution matrix. The strength and resistivity of an alloy are respectively in series and parallel connections to those of the matrix and the precipitate. After ideally matching the contributions from the matrix and the precipitate, the alloy should at least reach half of the resistivity of pure Cu, i.e., 50%IACS, which is the lower limit for industrially accepted highly conductive Cu alloys. Under this condition, the specific 310 ratio is related to the precipitate-over-matrix ratios for strength and resistivity, which are both two times those of pure Cu.

Effects of yttrium addition on microstructures and mechanical properties of CoCrNi medium entropy alloy

Element addition is an effective way to improve the properties of alloys. In this work, (CoCrNi)100-xYx (x = 0, 0.1, 0.3, 0.7 and 1.0, named Y-0, Y-0.1, Y-0.3, Y-0.7 and Y-1.0 accordingly) medium entropy alloys (MEAs) were fabricated by arc melting, and the effects of yttrium addition on the microstructures and mechanical properties of CoCrNi MEAs were studied. For Y-0 and Y-0.1, the alloys remained an FCC solid solution. Due to the most negative mixing enthalpy of Y–Ni element pair in the alloy, the HCP YNi5 precipitates formed in Y-0.3, and the volume fraction of precipitates increased with the increasing Y content. Both Y in the solid solution matrix and YNi5 precipitates resulted in grain refinement, and the grain size decreased with the increasing Y content. Annealing twins were observed; however, the twin boundary fraction showed only slight increase with the increasing Y content (from 69.2% for Y-0 to 73.1% for Y-1.0). The nanoindentation tests showed that the hardness of the HCP precipitates was about two times of the FCC matrix. Compared to Y-0, Y-0.1 showed only slight improvements in microhardness and tensile strength. However, in the presence of precipitates, the microhardness, yield strength and tensile strength showed significant enhancements with the increasing Y content in the alloy. Both the microhardness and strength scaled with the volume fraction of precipitates in the alloy. While precipitation strengthening had the major contribution, both the solid solution strengthening and grain boundary strengthening had only the minor contribution to our system.

Ultra-small-angle neutron scattering study on temperature-dependent precipitate evolution in CoCrFeNiMo0.3 high entropy alloy

Phase evolution of CoCrFeNiMo0.3 high entropy alloy (HEA) under different annealing temperatures was investigated by means of ultra-small-angle neutron scattering (USANS), X-ray diffraction (XRD), and scanning electron microscopy (SEM). For the as-cast CoCrFeNiMo0.3 alloy, a dendritic structure consisting of mainly face-centered cubic (FCC) solid solution matrix and small amount of micro-sized σ precipitates was observed. Upon isothermal annealing, secondary phase precipitation occurred and precipitates with two size ranges (i.e., microscale and nanoscale) generated in the alloy. These precipitates were identified as newly formed μ and σ phases. Specifically, the size distribution of the nanoscale μ precipitates was determined by USANS measurements. It was found that the size of the nanoscale precipitates increased with the annealing temperature, jumping from 72 nm at 750 °C to 258 nm at 800 °C, which contributed to the sharp drop in hardness of the alloy. The present alloy maintained high hardness within a wide temperature range of 550–750 °C, which makes it a promising material for structural applications at intermediate temperatures.

Proton irradiation induced chemical ordering in an Al0.3CoFeNi high entropy alloy

The radiation resistance of single-phase high entropy alloys has been reported to be superior to conventional alloys, due to their high lattice distortion and sluggish diffusion kinetics. The current study combines the beneficial effects of a concentrated multi-component solid solution and chemical ordering on the parent lattice of a candidate alloy, Al0.3CoFeNi, to enhance proton radiation resistance. The strong ordering tendency in this alloy results in the formation of Ni-Al-rich short-range ordered (SRO) domains when it is annealed in a single FCC phase field and water quenched. The irradiation of these microstructures with high-fluence MeV-energetic protons aids the transformation of the prior metastable single FCC solid-solution with SRO domains toward a more stable condition with L12 long-range ordered (LRO) domains embedded within the FCC solid solution matrix. Potentially, the creation of radiation-induced vacancy cascades within the FCC solid-solution enhances local diffusivity aiding the transition from SRO domains to LRO L12 domains. Therefore, this can be considered as a recovery mechanism, since the radiation-induced damage is not allowed to accumulate and is minimized via nanometer-scale precipitation of the ordered intermetallic phase. Additionally, preferential Co segregation to defect clusters or dislocation loops was also observed. In comparison, purely thermal activation via annealing at 500 °C for 30 min induces a similar transformation from SRO to LRO in this alloy, driving the system closer to equilibrium.

Additive manufacturing of in-situ strengthened dual-phase AlCoCuFeNi high-entropy alloy by selective electron beam melting

The application scope and market demand for additive-manufactured high-entropy alloys (AM HEAs) have broadened of late. However, a long-standing problem associated with AM HEAs is their limited ductility. In this study, a dual-phase AlCoCuFeNi HEA, consisting of body-centered cubic (BCC) solid solution matrix with uniformly dispersed face-centered cubic (FCC) structured precipitates, was fabricated by selective electron beam melting (SEBM). SEBM involved a preheating process can enable the formation of Cu-rich FCC phases with needle-like and spherical morphologies, as well as nanotwins that in-situ precipitated from the metastable BCC(B2) matrix. The compressive strength and ductility of the SEBM HEA were superior to those of the HEAs processed by selective laser melting (SLM) AM technique. Furthermore, using selective electron beam remelting (SEB-RM) during SEBM could result in a higher relative density, finer microstructure, and enhanced compressive properties. Particularly, the SEB-RM sample exhibited a better compressive strength of 2572 MPa, a yield strength of 870 MPa, and a strain of 18.3%. The improved mechanical properties of SEB-RM samples could be ascribed to the refined grains and the formation of FCC precipitates, mostly along the grain boundaries. This provides new insights into the dual-phase HEAs—fabricated via a combination of the SEBM additive manufacturing process and selective electron beam remelting—that exhibit in-situ strengthening.

State and effect of oxygen on high entropy alloys prepared by powder metallurgy

To reveal the state and effect of oxygen on high entropy alloys (HEAs) prepared by powder metallurgy, a VNbMoTaW refractory HEA was prepared by mechanical alloying and vacuum hot-pressing sintering. During mechanical alloying, with increasing ball milling time, the elemental distribution of the VNbMoTaW powders gradually became homogeneous, and the particle size decreased, while ~0.2 atom% O was introduced. After sintering at 1700 °C and 40 MPa for 2 h, a Ta-rich oxide with a grain size of approximately 15 nm was formed, leading to brittle fracture of the sintered VNbMoTaW HEA. When the sintering temperature was increased to 1900 °C, the grain size of the oxides decreased to ~8 nm and was dispersed on the BCC solid solution matrix. The yield strength of the VNbMoTaW HEA at room temperature and 1600 °C reached 2800 MPa and 350 MPa, respectively, due to Orowan strengthening caused by dispersed nano-sized oxides; additionally, the plasticity deformations at room temperature were ~2%. Although oxygen combined with the constituent metals to form oxides, dispersed nanosized oxides could improve the mechanical properties of HEAs prepared by powder metallurgy. The feasibility of powder metallurgy for HEAs was proven by this work.

Laser gas alloying of Ti-6Al-4V in air for tribological applications

Hard and wear-resistant TiOxNy/α-Ti composite coatings were fabricated on Ti-6Al-4V alloy via laser gas alloying in air for tribological applications. The effects of laser power, scan speed, and hatch spacing on the surface morphology, geometry and microstructure of the composite coatings were investigated systematically. The formation of laser-sustained plasma above the melt surface could lead to the dissociation of N2 and O2 molecules into N and O atoms. The atoms diffused and mixed into the melt, and subsequently reacted with molten Ti to form TiOxNy dendritic reinforcements, while the remaining melt solidified to form the α-Ti solid solution matrix. A compositional gradient was observed in the transitional zone between the TiOxNy dendrite and α-Ti matrix. The transitional zone was also found to have a crystallographic relationship with the TiOxNy dendrite. The microhardness and wear resistance of the composite coating were 2.8 and 10.8 times those of Ti-6Al-4V base metal, respectively. The excellent wear resistance of the composite coatings was attributed to the formation of hard TiOxNy reinforcements and α-Ti solid solution, as well as the strong TiOxNy/α-Ti interface.

Dual-wavelength responsive color-tunable upconversion luminescence in Na(Y, Gd)F4: Yb3+, Er3+ phosphors

A series of Na(Y, Gd)F4:Yb3+,Er3+ phosphor samples co-doped with different concentrations of Gd3+, Yb3+ and Er3+ ions were synthesized at 550 °C for 2 h by a solid-state reaction method. The crystal structure and upconversion (UC) luminescence properties of the samples upon 980/1550 nm wavelength excitation were characterized by XRD and emission spectral analysis methods. The XRD analysis showed that the synthesized samples were all pure hexagonal β-NaYF4 phase. The doping of Gd3+ ions into β-NaYF4 solid solution matrix can obviously enhance the UC luminescence intensity of the NaYF4: Yb3+, Er3+ samples both upon 980 and 1550 nm laser diode excitation. Moreover, the UC emission of the 1550 nm excited Na(Y, Gd)F4:Yb3+,Er3+ samples had a better color tunability than that of the UC emission of the 980 nm excited samples. Meanwhile, based on the experimental results, the UC luminescence mechanisms of the sample upon 980 and 1550 nm excitation were also discussed.

Tribocorrosion characteristics of CrMoSiCN/Ag coatings on Ti6Al4V alloys in seawater

CrMoSiCN/Ag coatings were deposited on Ti6Al4V alloys at the trimethylsilane flow of 15 sccm using closed-field unbalanced magnetron sputtering, and their microstructures were observed and analyzed using SEM, XRD, XPS and TEM, respectively. The coatings’ mechanical properties were measured using nano-indenter. The tribocorrosion characteristics of Ti6Al4V and CrMoSiCN/Ag coatings were investigated in seawater using tribocorrosion tester. The results revealed that the nanocomposite coatings consisted of (Cr, Mo)N solid solution, Ag nanocrystal and amorphous SiCNx matrix. As the Ag target current increased to 1.0 A, a large amount of Ag nanoparticles were observed on the coating surface. The coating hardness initially increased to 21.0 ± 0.7 GPa at the Ag target current of 0.4 A and then declined. After the Ag element was added into coatings, their tribocorrosion characteristics were improved. The tribocorrosion characteristics of coatings were much better than those of Ti6Al4V. The tribocorrosion characteristics of CrMoSiCN/Ag coating at the Ag target current of 1.0 A were the best in seawater.

Effects of aging treatment on the evolution of precipitated phase in CoCrFeNiTi0.6 high entropy alloys

In this study, the effect of aging time on the microstructure of precipitates in a face-centered cubic (FCC) CoCrFeNiTi0.6 high-entropy alloy (HEA) was investigated. The microstructure of precipitates was analyzed by SEM and TEM. The results showed that the CoCrFeNiTi0.6 alloy exhibited a typical dendritic structure, whose phase consisted of an ordered FCC solid-solution matrix (nanoscale L12-(Ni, Co)3Ti type particles) and intermetallic phases at 900 °C for different aging times. Meanwhile, the intermetallic compounds were composed of (Cr, Fe)-rich σ phases with a tetragonal structure and (Ni, Ti)-rich η phase with a long period stacking ordered (LPSO) structure. Additionally, the influence of aging time on the shape and structure of γ' precipitates was revealed. It was noticeable that the morphological transformation of nanoscale γ' precipitates evolved from an initially spheroidal to spheroidal-like shapes, following cuboidal-like shapes with flat edges, and finally, γ' precipitates with cuboidal shape were formed simultaneously with the appearance of the split with increasing aging time. Furthermore, the morphological evolution of γ' precipitates was quantitatively analyzed using a characteristic parameter's value of precipitates based on lattice misfit between the matrix and precipitates. This research would provide guidance for designing the precipitate structure in the L12-strengthened HEA and facilitate to establish the relationships between the microstructure of the precipitates and properties.

In-situ Ti(C, N) reinforced AlCoCrFeNiSi-based high entropy alloy coating with functional gradient double-layer structure fabricated by laser cladding

In-situ Ti(C, N) reinforced AlCoCrFeNiSi-based high entropy alloy coating with functional gradient double-layer structure is successfully fabricated on the H13 substrate using laser cladding. The microstructures and wear resistance of the gradient coating are characterized using SEM, EBSD, TEM, EDS and wear test, respectively. The results show that FCC solid-solution matrix is formed in first layer, while a mixture of disordered BCC phase (Fe-Cr) and ordered B2 phase (Al-Ni-Ti) with coherent interface is found as the matrix in second layer. Ti(C, N) ceramic particles are in-situ synthesized in the coating due to the large negative mixing enthalpy of Ti-C and Ti-N atom pairs. The solid-solution strengthening and dislocation strengthening caused by the dissolution of Ti into B2 phase, accompanying with the fine grain strengthening caused by the microstructure refinement and dispersion strengthening of Ti(C, N) particles, contribute to the high surface hardness and wear resistance. The dominant wear mechanism of this coating is oxidation wear with a small amount of abrasive wear. It can be concluded that AlCoCrFeNiSi-based high entropy alloy coating with in-situ Ti(C, N) reinforcement dramatically improves the wear resistance of the H13 substrate.

A comparative study on microstructures and tensile behaviours of a novel polycrystalline and single-crystal Co–9Al–9W–2Ta-0.02B-0.02Ce alloy at room and high temperatures

A comparative investigation in microstructure and mechanical properties of a novel as-cast polycrystalline and single-crystal (SC) Co–9Al–9W–2Ta-0.02B-0.02Ce (at.%) alloy with a growth direction of [0 0 1] has been performed. Aged at 900 °C for 100 h, the polycrystalline and SC alloy consisted of a Co solid solution (γ-CoSS) matrix and nano-scale cuboidal γ′-Co3(Al, W) intermetallic precipitates, while the two phases cohered with each other. For the polycrystalline alloy, Ce was observed to segregate primarily at the grain boundaries and the γ′-Co3(Al, W) phase, and it could exclude impurity O from the grain boundaries. From room temperature (RT) to 900 °C, the SC sample always showed higher yield strength and elongation under tension than the polycrystalline sample except strength at 700 °C, while the flow stress anomalies took place at 700 °C for the polycrystalline alloy, and 800 °C for the SC alloy. Without the grain boundary, the SC alloy had excellent elongation, larger than 15.6% from RT to 900 °C. However, the polycrystalline alloy lost the ductility at temperatures above 600 °C, which was caused by transformation of the failure from a mixed transgranular dimple plus intergranular brittle mode to fully intergranular brittle mode.

Microstructure evolution and mechanical properties of in-situ multi-component carbides reinforced FeCoNi alloy

In-situ carbides dispersion is an effective reinforcement strategy for alloys. However, introducing traditional single-principal element carbides often lead to a severe loss of strength and ductility. Based on the design principle of high entropy alloy, (Fe40Co50Ni10)-(VNbTaZrCx) alloys strengthened by in-situ high entropy multi-component carbides (MCCs) were fabricated by introducing refractory metal elements (V, Nb, Ta, Zr) and their carbides into FeCoNi alloy via vacuum arc melting. The dispersed MCCs with rock-salt structure was formed in the FeCoNi matrix with BCC structure. At grain boundaries, an irregular lamellar/granular eutectic structure formed composed of BCC-type secondary solid solution and FeCoNi matrix. With the increase of C content, the microstructure of the alloys was refined significantly, and the volume fraction of in-situ MCCs increased while the opposite trend happened in the eutectic structure. Due to the excellent properties of high entropy MCCs combining with the dispersion strengthening and grain refinement strengthening, remarkable strengthening and toughening effects were achieved for the (Fe40Co50Ni10)-(VNbTaZrCx) alloys. The yield strength, ultimate strength, and elongation of C2 were 787 MPa, 1176.5 MPa, and 6.54%, respectively. Compared with the Fe40Co50Ni10 alloy, the ultimate strength was increased by 90.1% and the elongation was increased by 5 times. The good combination of the strength and ductility was attributed to the dislocation pile-ups and the deformation twins appeared in the MCCs during deformation.

Laser powder bed fusion of high-entropy alloy particle-reinforced stainless steel with enhanced strength, ductility, and corrosion resistance

In additive manufacturing of metal matrix composites, it is difficult to ensure high strength as well as high ductility simultaneously. In this study, a FeCoNiCr high-entropy alloy was used as the strengthening phase to improve the performance of austenitic stainless steel processed via laser powder bed fusion. The microstructure of the as-printed parts was composed of high-entropy alloy nanoparticles, a solid solution matrix, and high-density dislocations. The strengthening effect caused by the microstructures resulted in a tensile strength of 627 MPa with a maximum elongation of 50%. The semi-obstructive effect of the interface on the dislocations improved the ductility, as proved through molecular dynamics analysis. The Cr, Ni, and Co elements in the high-entropy alloy melted into the stainless-steel matrix, facilitating the formation of a passive film; this improved the corrosion resistance of the composite. Thus, simultaneous improvements in strength, ductility, and corrosion resistance of the stainless-steel composites were realized. The composites inherited the performance characteristics of high-entropy alloys. This research also provides a new strategy to select the strengthening phase for composite materials. High-performance high-entropy alloy particles with lattice structures and element types similar to the stainless steel matrix have substantial potential to be used as strengthening phases.

Transformation of structure during friction stir welding

In comparison with low carbon steels, there is increased interest in the use of aluminium-based alloys as materials for the manufacture of welded structures rolling stock of railway transport. During friction stir welding aluminium-based alloy, against the background of the analysis structural transformations, issues of development hardening processes are considered. Under conditions of existence, a temperature gradient at zone of weld formation, shown degree approximation alloy to the conditions of superplastic flow and influence from presence particles of the second phase on grain size of matrix is estimated. Evaluation of the separate influence grain size of matrix and state of solid solution at total hardness of the weld showed dependence of their contributions on temperature of hot plastic deformation. As the temperature of plastic deformation of alloy at area of the weld increases, contribution to the total hardness from grain size increase and on state of the solid solution decreases.

Thermal activation process of plastic deformation in Fe–18Cr single-crystal micropillars with high-density dislocations

Micropillar compression testing was performed on bcc solid-solution single-crystal micropillars with two diameter (d) ranges (2–3 and 5–6 μm) prepared on the surfaces of fully annealed and subsequently 10% cold-rolled 18Cr ferritic steel specimens. This was done to investigate the effect of initial dislocations on the plastic deformation of micron-sized crystals. The 10% cold rolling process significantly increased the stress level (provided by an activated single slip system) required to initiate slip in the annealed micropillars with different diameter ranges. In the case of the annealed micropillars, the strain-rate sensitivity (m) of the yield stress becomes lower for larger specimens (the m value decreased from 0.12 to 0.04). However, the 10% cold-rolled micropillars exhibited a much lower strain-rate sensitivity regardless of the specimen size. The activation volume (v*) of the annealed micropillars showed a significant specimen size dependency (v*∝d1.47), consistent with published data on the micropillar compression experiments of single-crystals of the bcc metals and alloys. For the 10% cold-rolled micropillars, the high v* values (88b3 and 112b3) showed a slight dependency on the specimen size (v*∝d0.33). The v* values appeared somewhat higher but were equivalent to those of bulk-size specimens, suggesting that the initial high-density dislocations would provide sufficient sources for dislocation multiplication in the micron-sized specimens under compression. The observed high v* values might be associated with the strong dislocation interactions in the α-(Fe, Cr) solid-solution matrix.

Neutron Diffraction Study of Residual Stresses in a W–Ni–Co Heavy Alloy Processed by Rotary Swaging at Room and High Temperatures

Residual stresses were studied in tungsten heavy alloy bars produced by powder metallurgy and deformed by rotary swaging at room temperature (RT) and at 900 °C. Neutron diffraction technique was used to scan the residual stresses across the bars. Both tungsten particles and NiCo2W solid solution matrix were analysed. Maximum axial stresses of ~ 300 MPa and ~ 200 MPa were observed for the tungsten phase at the centre in the RT and in the high-temperature deformed samples, respectively. Compressive residual axial stresses were found close to the sample surface, showing that rotary swaging is a suitable deformation method for tungsten heavy alloys to provide an appropriate surface modification for its use in metallic parts undergoing, e.g., fatigue. Residual stresses developed in the NiCo2W-phase are larger than those found in the tungsten particles although with a secondary role in the overall equilibrium conditions due to its lower strength and smaller volume fraction. Total stresses for each phase were separated into macro- and microstresses. Macrostresses can be mainly influenced by the incompatibility of the elliptical cross-section of the sintered sample with the head of the rotary machine while microstresses are mainly developed by the elastic mismatch between the constituent phases.

Influence of in-flight particle characteristics and substrate temperature on the formation mechanisms of hypereutectic Al-Si-Cu coatings prepared by supersonic atmospheric plasma spraying

Hypereutectic Al-Si-Cu coatings were prepared by supersonic atmospheric plasma spraying to enhance the surface performance of lightweight alloys. To find out optimum process conditions and achieve desirable coatings, this work focuses on the influence of three important parameters (in-flight particle temperature, impact velocity, and substrate temperature) on the collected splats morphology, coatings microstructure and microhardness. Results show that appropriate combinations of temperature and velocity of in-flight particles cannot only completely melt hypereutectic Al-Si-Cu particles, especially the primary Si phase, but also provide the particles with sufficient kinetic energy. Thus, the optimized coating consists of 98.6 % of fully-melted region with nanosized coupled eutectic and 0.9 % of porosity. Increasing the substrate deposition temperature promotes the transition from inhomogeneous banded microstructure to homogeneous equiaxed microstructure with a lower porosity level. The observations are further interpreted by a newly developed phase-change heat transfer model on quantitatively revealing the solidification and remelting behaviors of several splats deposited on substrate. Besides, phase evolutions including the formation of supersaturated α-Al matrix solid solution, growth of Si and Al2Cu phases at different process conditions are elaborated. An ideal microstructure (low fractions of unmelted/partially-melted regions and defects) together with solid solution, grain refinement, and second phase strengthening effects contributes to the enhanced microhardness of coating. This integrated study not only provides a framework for optimizing Al-Si based coatings via thermal spraying but also gives valuable insights into the formation mechanisms of this class of coating materials.