Microstructure characterization of Al<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>CoCrCuFeNi high-entropy alloy system with multiprincipal elements

A new approach for the design of alloy systems with multiprincipal elements is presented in this research. The Al _x CoCrCuFeNi alloys with different aluminum contents (i.e., x values in molar ratio, x=0 to 3.0) were synthesized using a well-developed arc-melting and casting method. These alloys possessed simple fcc/bcc structures, and their phase diagram was predicted by microstructure characterization and differential thermal analyses. With little aluminum addition, the alloys were composed of a simple fcc solid-solution structure. As the aluminum content reached x=0.8, a bcc structure appeared and constructed with mixed fcc and bcc eutectic phases. Spinodal decomposition occurred further on when the aluminum contents were higher than x=1.0, leading to the formation of modulated plate structures. A single ordered bcc structure was obtained for aluminum contents larger than x=2.8. The effects of high mixing entropy and sluggish cooperative diffusion enhance the formation of simple solid-solution phases and submicronic structures with nanoprecipitates in the alloys with multiprincipal elements rather than intermetallic compounds.     

Effect of vanadium addition on the microstructure, hardness, and wear resistance of Al0.5CoCrCuFeNi high-entropy alloy

The authors studied the effect of vanadium addition on the microstructure and properties of Al_0.5CoCrCuFeNi high-entropy alloy. The microstructure of Al_0.5CoCrCuFeNiV _x (x=0 to 2.0 in molar ratio) alloys was investigated by scanning electron microscopy, energy dispersive spectrometry, and X-ray diffraction. With little vanadium addition, the alloys are composed of a simple fcc solid-solution structure. As the vanadium content reaches 0.4, a BCC structure appears with spinodal decomposition and envelops the FCC dendrites. From x=0.4 to 1.0, the volume fraction of bcc structure phase increases with the vanadium content increase. When x=1.0, fcc dendrites become completely replaced by bcc dendrites. Needle-like σ-phase forms in bcc spinodal structure and increases from x=0.6 to 1.0 but disappears from x=1.2 to 2.0. The hardness and wear resistance of the alloys were measured and explained with the evolution of the microstructure. The hardness values of the alloys increase when the vanadium content increases from 0.4 to 1.0 and peak (640 HV) at a vanadium content of 1.0. The wear resistance increases by around 20 pct as the content of vanadium increases from x=0.6 to 1.2 and levels off beyond x=1.2. The optimal vanadium addition is between x=1.0 and 1.2. Compared with the previous investigation of Al_0.5CoCrCuFeNi alloy, the vanadium addition to the alloy promotes the alloy properties.     

Wear resistance and high-temperature compression strength of Fcc CuCoNiCrAl<Subscript>0.5</Subscript>Fe alloy with boron addition

This study discusses the wear resistance and high-temperature compression strength of CuCoNiCrAl_0.5Fe alloy with various amounts of boron addition. Experiments show that within the atomic ratio of boron addition from x=0 to x=1.0 in CuCoNiCrAl_0.5FeB _x (referred to as B-0 to B-1.0 alloys), the alloys are of fcc structure with boride precipitation. The volume fraction of borides increases with increasing boron addition. The corresponding hardness increases from HV 232 to HV 736. Wear resistance and high-temperature compression strength are significantly enhanced by the formation of boride. The alloys with boride are less tough. The superior wear resistance of B-1.0 alloy, which is even better than SUJ2 wear-resistant steel, indicates that the CuCoNiCrAl_0.5FeB _x alloys have potential applications as ambient- and high-temperature mold, tool, and structural materials.     

[(Fe<Subscript>0.5</Subscript>Co<Subscript>0.5</Subscript>)<Subscript>0.75</Subscript>B<Subscript>0.20</Subscript>Si<Subscript>0.05</Subscript>]<Subscript>96</Subscript>Nb<Subscript>4</Subscript> Metallic Glasses with Small Cu Additions

Recently, (Fe-Co)-B-Si-Nb bulk metallic glasses (BMGs) were produced. Such BMGs exhibit high glass-forming ability (GFA) as well as good mechanical and magnetic properties. These alloys combine the advantages of functional and structural materials. The soft magnetic properties can be enhanced by nanocrystallization. To force the nanocrystallization, small content of Cu was added to the starting composition. In this article, {[(Fe_0.5Co_0.5)_0.75Si_0.05B_0.20]_0.96Nb_0.04}_100–x Cu _x glassy alloys (x = 1, 2, and 3) were chosen for investigation. The GFA and the thermal stability of these alloys were evaluated. The effects of crystallization during heat-treatment processes on the phase evolution and the magnetic properties, including M _s , H _c , and T _c , in these alloys were investigated. The phase analyses were done with the help of the X-ray diffraction patterns recorded in situ by using the synchrotron radiation in transmission configuration.     

Influence of Ni Content on the Microstructure and Dry Sliding Wear Properties of Cr<Subscript>13</Subscript>Ni<Subscript>5</Subscript>Si<Subscript>2</Subscript>/<Emphasis Type="Italic">γ</Emphasis> Intermetallic Alloys

The effect of Ni content on microstructure, hardness, and wear resistance was studied for the Cr₁₃Ni₅Si₂-base intermetallic alloys toughened by Ni-base solid solution (γ). Volume fraction and microhardness of the Cr₁₃Ni₅Si₂ primary dendrite as well as the average hardness of the Cr₁₃Ni₅Si₂/γ alloy decrease with the increasing Ni content. The Cr₁₃Ni₅Si₂/γ alloys have excellent wear resistance under dry sliding wear test conditions, which increases under high contact load wear conditions and decreases under low contact load wear test conditions with the increasing Ni content. The high wear resistance is due to the combination of high toughness of γ and high hardness of Cr₁₃Ni₅Si₂ and formation of a transferred cover layer on the worn surface during wear process. The wear rate of the Cr₁₃Ni₅Si₂/γ alloy is governed by the slow process of microspalling or pullout of the cracked Cr₁₃Ni₅Si₂ primary dendrites. The Cr₁₃Ni₅Si₂/γ alloys have extremely low load sensitivity of wear and the load-sensitivity coefficient of wear decreases drastically as the Ni content increases.     

Mechanochemical preparation of Ni<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>Al<Subscript><Emphasis Type="Italic">y</Emphasis></Subscript>/Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanocomposites

The mechanochemical preparation of nickel aluminide/corundum (Ni _x Al _y /Al₂O₃) powder nanocomposites is shown to be possible during the mechanochemical aluminum reduction of nickel oxide at various weight proportions of the components.     

Stabilizing the L1<Subscript>2</Subscript> structure of Pt<Subscript>3</Subscript>Al(r) in the Pt-Al-Sc system

The Pt-Al system has high potential to act as alloy base for so-called refractory superalloys. Although the envisaged strengthening phase Pt₃Al(r) has favorable L1₂ crystal structure only at high temperatures, even small amounts of Sc stabilized L1₂ crystal structure at low temperature. Pt-Al-Sc alloys were arc melted, heat treated, and examined by means of scanning electron microscopy and X-ray diffraction (XRD). Pt₃Al_1−x Sc _x (r) forms a continuous phase field from the Al-rich side to the Sc-rich side of the Pt-Al-Sc ternary system. The absolute value of the lattice misfit between cubic Pt₃Al_1−x Sc _x (r) and the matrix decreases with increasing Sc content.     

Phase composition and some properties of titanium carbonitride-titanium nickelide alloys with Al<Subscript>2</Subscript>O<Subscript>3</Subscript> nanoparticles

The phase formation in and the microstructure of titanium carbonitride-titanium nickelide alloys with aluminum oxide Al₂O₃ nanopowder additions are studied by X-ray diffraction, electron-microscopic, and electron-probe microanalyses. The phase interaction is characterized by the redistribution of nonmetallic elements and aluminum between refractory and binding phases with the formation of a nonstoichiometric titanium-aluminum (Ti,Al)(C,N) carbonitride and a titanium-aluminum nickelide. The number of forming phases and their compositions are controlled by the kinetic parameters of the process.     

Specific features of formation of nanostructured electrospark protective coatings on the OT4-1 titanium alloy with the use of electrode materials of the TiC-Ti<Subscript>3</Subscript>AlC<Subscript>2</Subscript> system disperse-strengthened by nanoparticles

The effect of nanodispersed additives of ZrO₂, Al₂O₃, NbC, W, WC, and WC-Co into the composition of the TiC-Ti₃AlC₂ electrode on the kinetics of formation of coatings by electrospark doping (ESD) on the titanium alloy of the OT4-1 grade on varying the duration of pulsed discharges in a wide range is investigated. The structure, phase composition, properties (continuity, thickness, microhardness, friction coefficient, wear resistance, and heat resistance) are investigated. The optimum energy mode of the ESD treatment of the OT4-1 titanium alloys by new electrode materials is determined. It is found that the introduction of a nanodispersed component is favorable to an increase in continuity and the microhardness of electrospark coatings and a substantial increase in wear resistance and heat resistance of titanium alloys.     

Thermodynamics of MnO-SiO<Subscript>2</Subscript>-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MnS Liquid Oxysulfide: Experimental and Thermodynamic Modeling

The activities of MnO and MnS in a MnO-SiO₂-Al₂O₃(or AlO_1.5)-MnS liquid oxysulfide solution were investigated by employing the gas/liquid/Pt-Mn alloy chemical equilibration technique under a controlled atmosphere at 1773 K (1500 °C). Also, the sulfide capacity, defined as C _S = (wt pct S)(pO₂/pS₂)^1/2, in MnO-SiO₂-Al₂O₃ slag with a dilute MnS concentration was obtained from the measured experimental data. As X _SiO2/(X _MnO + X _SiO2) in liquid oxysulfide increases, the activity coefficient of MnO decreases, while that of MnS first increases and then decreases. As X(AlO_1.5) in liquid oxysulfide increases, the activity coefficient of MnS increases, while no remarkable change is observed for the activity coefficient of MnO. The behavior of the activity coefficient of MnS was qualitatively analyzed by considering MnO + A _x S _y (SiS₂ or Al₂S₃) = MnS + A _x O _y (SiO₂ or Al₂O₃) reciprocal exchange reactions in the oxysulfide solution. The behavior was shown to be consistent with phase diagram data, namely, the MnS saturation boundary. Quantitative analysis of the activity coefficient of the oxysulfide solution was also carried out by employing the modified quasichemical model in the quadruplet approximation.     

Wear of Plasma Sprayed Conventional and Nanostructured Al<Subscript>2</Subscript>O<Subscript>3</Subscript> and Cr<Subscript>2</Subscript>O<Subscript>3</Subscript>, Based Coatings

An ever increasing demand for high-performance ceramic coatings has made it inevitable for developing techniques with precise control over the process parameters to enable the fabrication of coatings with the desired microstructure and improved structural properties. The literature on plasma sprayed nanostructured ceramic coatings such as of Al₂O₃, Cr₂O₃, and their composites obtained using reconstituted nano sized ceramic powders has been reviewed in this study. Ceramic coatings due to their enhanced properties are on the verge of replacing conventional ceramic coatings used for various applications like automotive systems, boiler components, power generation equipment, chemical process equipment, aircraft engines, pulp and paper processing equipment, land-based and marine engine components, turbine blades etc. In such cases, the advantage is greater longevity and reliability for realizing the improved performance of ceramic coatings. It has been observed that the plasma sprayed nanostructured ceramic coatings show improvement in resistance to wear, erosion, corrosion, and mechanical properties as compared to their conventional counterparts. This article reviews various aspects concerning the plasma sprayed ceramic coatings such as (i) the present understanding of formation of plasma-spray coatings and factors affecting them, (ii) wear performance of nanostructured Al₂O₃, Cr₂O₃ and their composite ceramic coatings in comparison to their conventional counterparts, and (iii) mechanisms of wear observed for these coatings under various conditions of testing.     

Microstructural Evolution and Mechanical Properties of Nanointermetallic Phase Dispersed Al<Subscript>65</Subscript>Cu<Subscript>20</Subscript>Ti<Subscript>15</Subscript> Amorphous Matrix Composite Synthesized by Mechanical Alloying and Hot Isostatic Pressing

The structure and mechanical properties of nanocrystalline intermetallic phase dispersed amorphous matrix composite prepared by hot isostatic pressing (HIP) of mechanically alloyed Al₆₅Cu₂₀Ti₁₅ amorphous powder in the temperature range 573 K to 873 K (300 °C to 600 °C) with 1.2 GPa pressure were studied. Phase identification by X-ray diffraction (XRD) and microstructural investigation by transmission electron microscopy confirmed that sintering in this temperature range led to partial crystallization of the amorphous powder. The microstructures of the consolidated composites were found to have nanocrystalline intermetallic precipitates of Al₅CuTi₂, Al₃Ti, AlCu, Al₂Cu, and Al₄Cu₉ dispersed in amorphous matrix. An optimum combination of density (3.73 Mg/m³), hardness (8.96 GPa), compressive strength (1650 MPa), shear strength (850 MPa), and Young’s modulus (182 GPa) were obtained in the composite hot isostatically pressed (“hipped”) at 773 K (500 °C). Furthermore, these results were compared with those from earlier studies based on conventional sintering (CCS), high pressure sintering (HPS), and pulse plasma sintering (PPS). HIP appears to be the most preferred process for achieving an optimum combination of density and mechanical properties in amorphous-nanocrystalline intermetallic composites at temperatures ≤773 K (500 °C), while HPS is most suited for bulk amorphous alloys. Both density and volume fraction of intermetallic dispersoids were found to influence the mechanical properties of the composites.     

Surface Treatment of S355JR Carbon Steel Surfaces with ZrB<Subscript>2</Subscript> Nanocrystals by CO<Subscript>2</Subscript> Laser

To improve mechanical properties of S2355JR carbon steel, pre-synthesized ZrB₂ nanocrystals were used to coat the metal surface by laser cladding using 2000 W CO₂ laser. ZrB₂ nanocrystals were synthesized by mechanochemical process. The effect of laser power on the coating layers was examined for optimizing the most effective coating conditions. Microstructural studies were carried out using optical microscope, scanning electron microscope and X-ray diffraction to analyze phase structures of the coated layers. Mechanical characteristics of the laser coated layers were evaluated by studying microhardness, wear and scratch resistance properties. Maximum hardness of the coated layers was observed while cladding with 75 and 125 W laser powers, when other processing parameters and conditions were kept at optimum levels. EDS analysis of these laser cladded layers indicated the formation of complex boro-nitrides, nitrides and carbides of Zr and Fe that contributed to vast increase in hardness of the laser-clad coating on S2355JR steel. Depending upon the laser powers used, the thickness of the coated layers was found to be in the range of 15–37 µm. The wear and micro-scratch tests results revealed significant improvement in wear properties.     

Effect of CaO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript>-MgO slags on the formation of MgO-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> inclusions in ferritic stainless steel

A thermodynamic equilibrium between the Fe-16Cr melts and the CaO-Al₂O₃-MgO slags at 1823 K as well as the morphology of inclusions was investigated to understand the formation behavior of the MgO-Al₂O₃ spinel-type inclusions in ferritic stainless steel. The calculated and observed activities of magnesium in Fe-16Cr melts are qualitatively in good agreement with each other, while those of aluminum in steel melts exhibit some discrepancies with scatters. In the composition of molten steel investigated in this study, the log (X _MgO/X _{Al 2}O₃) of the inclusions linearly increases by increasing the log [a _Mg/a _Al ² ·a _O ² ] with the slope close to unity. In addition, the relationship between the log (X _MgO/X _{Al 2}O₃) of the inclusions and the log (a _MgO/a _{Al 2}O₃) of the slags exhibits the linear correlation with the slope close to unity. The compositions of the inclusions are relatively close to those of the slags, viz. the MgO-rich magnesia-spinel solid solutions were formed in the steel melts equilibrated with the highly basic slags saturated by CaO or MgO. The spinel inclusions nearly saturated by MgO were observed in the steel melts equilibrated with the slags doubly saturated by MgO and MgAl₂O₄. The spinel and the Al₂O₃-rich alumina-spinel solid solutions were formed in the steel melts equilibrated with the slags saturated by MgAl₂O₄ and MgAl₂O₄-CaAl₂O₄ phases, respectively. The apparent modification reaction of MgO to the magnesium aluminate inclusions in steel melts equilibrated with the highly basic slags would be constituted by the following reaction steps: (1) diffusion of aluminum from bulk to the metal/MgO interface, (2) oxidation of the aluminum to the Al³⁺ ions at the metal/intermediate layer interface, (3) diffusion of Al³⁺ ions and electrons through the intermediate layer, and (4) magnesium aluminate (MgAl₂O₄ spinel, for example) formation by the ionic reaction.     

Microstructures and properties of CoCrCuFeNiMox high-entropy alloys fabricated by mechanical alloying and spark plasma sintering

CoCrCuFeNiMox (x values in molar ratio, x?=?0, 0.2, 0.4 and 0.8) high-entropy alloys were prepared by mechanical alloying and spark plasma sintering method. The effects of Mo addition on microstructure and mechanical properties were investigated. The X-ray diffraction (XRD) result showed that the addition of Mo into CoCrCuFeNi high-entropy alloy (HEA) changed the original phase constitution from FCC to FCC?+?σ?+?μ and the peak intensity of (1 1 1) shifted to the left and decreased steadily. The field emission scanning electron microscope confirmed that the Cu-rich second FCC phase disappeared and the σ phase with a tetragonal structure expanded as the Mo content was increased. Additionally, a high density of dimple-like features were seen in CoCrCuFeNi HEA while typical quasi-cleavage facets could be observed from the fracture surfaces of the HEAs with the addition of Mo. The Mo0.8 alloy showed a good wear resistant and appropriate strength with fracture strain 22.70%, fraction coefficient 0.65, hardness 530?HV and compressive strength 1448?MPa.

Special theme block on high entropy alloys, guest edited by Paula Alvaredo Olmos, Universidad Carlos III de Madrid, Spain, and Sheng Guo, Chalmers University, Gothenburg, Sweden.     

Structure and stability of a new ternary hexagonal phase in Al<Subscript>3</Subscript> Ti-based Al-Ti-V alloys

A novel hexagonal phase (designated H₍₂₎) has been detected as a major constituent of both Al₆₂Ti₁₀V₂₈ and Al₅₅Ti₁₀V₃₅ alloys, following chill casting and after homogenization at 1523 K. The phase H₂ has an ordered hexagonal crystal structure (space group P6₃/mmc, α=0.558±0.001 and c=0.450±0.001 nm), similar to that of α ₂-Ti₃Al, and an atomic composition of 54±1 Al−11±1 Ti−35±1 V in the chill-cast Al₅₅Ti₁₀V₃₅ alloy. A fine-scale, duplex lamellar structure, developed within the ordered H₂ phase in the solid state, was composed of parallel-sided multivariants of ξ-Ti₅Al₁₁ phase, formed parallel to (0001)_{H 2}. The orientation relationship between constituent phases was of the form

Effect of Internal Interfaces on Hardness and Thermal Stability of Nanocrystalline Ti<Subscript>0.5</Subscript>Al<Subscript>0.5</Subscript>N Coatings

The effect of microstructure on the thermal stability and hardness of the cathodic arc evaporated Ti_0.5Al_0.5N coatings was investigated with the aid of the in-situ high-temperature X-ray diffraction experiments, which were accompanied by high-resolution transmission electron microscopy (HRTEM) and nanoindentation measurements. The microstructure of the coatings was modified through the choice of the bias voltage in the deposition process. It was found that the bias voltage affects strongly the uniformity of the local distribution of titanium and aluminum in the coatings. The nonuniform distribution of the elements contributes to the formation of lattice strains at the crystallite and phase boundaries. The lattice strains at the crystallite boundaries increase the hardness of the coatings; the lattice strains at the phase boundaries improve their thermal stability. A certain nonuniformity of the distribution of the metallic species in the coatings is regarded as advantageous. However, a great nonuniformity in the distribution of the metallic species accelerates the degradation of the coatings at high temperatures. As a measure for the nonuniformity of the distribution of the atomic species in the as-deposited (Ti, Al) N samples, the stress-free lattice parameter of fcc-(Ti, Al) N is suggested.     

Development of Wear-Resistant Cu-10Cr-3Ag Electrical Contacts with Nano-Al<Subscript>2</Subscript>O<Subscript>3</Subscript> Dispersion by Mechanical Alloying and High Pressure Sintering

Cu-10Cr-3Ag (wt pct) alloy with nanocrystalline Al₂O₃ dispersion was prepared by mechanical alloying and consolidated by high pressure sintering at different temperatures. Characterization by X-ray diffraction and scanning electron microscopy or transmission electron microscopy shows the formation of nanocrystalline matrix grains of about 40 nm after 25 hours of milling with nanometric (<20 nm) Al₂O₃ particles dispersed in it. After consolidation by high pressure sintering (8 GPa at 400 °C to 800 °C), the dispersoids retain their ultrafine size and uniform distribution, while the alloyed matrix undergoes significant grain growth. The hardness and wear resistance of the pellets increase significantly with the addition of nano-Al₂O₃ particles. The electrical conductivity of the pellets without and with nano-Al₂O₃ dispersion is about 30 pct IACS (international annealing copper standard) and 25 pct IACS, respectively. Thus, mechanical alloying followed by high pressure sintering seems a potential route for developing nano-Al₂O₃ dispersed Cu-Cr-Ag alloy for heavy duty electrical contact.     

Crystallization of Co<Subscript>100-<Emphasis Type="Italic">x</Emphasis></Subscript>Pt<Subscript><Emphasis Type="Italic">x</Emphasis></Subscript>B<Subscript>10</Subscript>Si<Subscript>12</Subscript> amorphous metallic alloys

Alloys of Co_78-x Pt _x B₁₀Si₁₂ were produced using the melt-spin process in order to study the crystallization behavior and ensuing magnetic properties of the Co-Pt amorphous alloys as a function of the Pt content. We showed that when x>15, well below its stoichiometric composition, CoPt intermetallic compound crystallized in the amorphous alloy. Below this composition, the main crystallization product was Co with Pt dissolved in its lattice. The nucleation of CoPt greatly altered the crystallized microstructures and magnetic properties of the Co-Pt amorphous alloys during annealing. In spite of the nucleation of CoPt with its high magnetic anisotropy, the highest coercivity was obtained when x was 15, free of the CoPt grains. It was also concluded that the Pt addition, in general, triggered crystallization to occur at a progressively lower temperature.