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.     

Partial phase relationships in Ir-Nb-Ni-Al and Ir-Nb-Pt-Al quaternary systems and mechanical properties of their alloys

Two quanternary systems, Ir-Nb-Ni-Al and Ir-Nb-Pt-Al, were successively investigated to assess their possible use in ultra-high-temperature applications. The phase relationships concentrated on the fcc/L1₂ two-phase region were primarily established, and the mechanical properties were studied. Ir-Nb-Ni-Al quaternary alloys around the Ir-rich or Ni-rich corners of the Ir-Nb-Ni-Al tetrahedron showed a coherent fcc/L1₂ two-phase structure, analogous to that of Ni-base superalloys; however, most of the alloys presented three or four phases with two types of L1₂ phases. Although these alloys showed a high compressive strength at high temperature, they exhibited a higher creep rate than Ir-base binary and ternary alloys. Another quanternary system, Ir-Nb-Pt-Al, showed promising results. Only an fcc/L1₂ two-phase structure was found in all the alloys investigated with compositions ranging from the Ir-rich side to the Pt-rich side, and the lattice misfit between the fcc and L1₂ phases was small. The high-temperature strength at 1200 °C of Ir-Nb-Pt-Al alloys was higher than that of Ir-Nb-Ni-Al alloys with the same Ir content (at. pct). Moreover, Ir-Nb-Pt-Al alloys exhibited excellent creep resistance at 1400 °C and 100 MPa. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Influence of Pr-Al-Co alloy diffusion source on magnetic properties and microstructure of sintered Nd-Fe-B magnets processed by grain boundary diffusion

In this study, the influence of the content of Al and Co in the diffusion source on the magnetic performance and microstructure of the diffused magnet was studied by grain boundary diffusion treatment with Pr₇₀Al_30–xCo_x (x = 0 at%, 10 at%, 15 at%, 20 at%, 30 at%) alloys. When the Co content in the diffusion source increases from 0 at% to 10 at%, the coercivity enhancement in the Pr₇₀Al₂₀Co₁₀ diffused magnet is the highest, increased from 1.62 to 2.24 T, higher than 2.01 T of the Pr₇₀Al₃₀ diffused magnet. With further increase of Co content in the diffused source, the coercivity of the diffused magnet decreases gradually, the coercivity of Pr₇₀Al₁₅Co₁₅, Pr₇₀Al₂₀Co₁₀ and Pr₇₀Co₃₀ diffused magnet is 2.15, 1.99 and 1.81 T, respectively. Microstructural analysis shows that plenty of continuous grain boundary phases (CGBPs) can be formed in the Pr₇₀Al₂₀Co₁₀ diffused magnet under the synergistic effect of Al and Co, which leads to the enhancement of magnetic isolation between more adjacent grains. However, the amount of CGBP in the diffused magnets gradually decreases with the further increase of Co content in the diffusion source.     

Microstructure and phase identification in type 304 stainless steel-zirconium alloys

Stainless steel-zirconium alloys have been developed at Argonne National Laboratory to contain radioactive metal isotopes isolated from spent nuclear fuel. This article discusses the various phases that are formed in as-cast alloys of type 304 stainless steel and zirconium that contain up to 92 wt pct Zr. Microstructural characterization was performed by scanning electron microscopy (SEM) and energy dispersive X-ray spectroscopy (EDS), and crystal structure information was obtained by X-ray diffraction. Type 304SS-Zr alloys with 5 and 10 wt pct Zr have a three-phase microstructure—austenite, ferrite, and the Laves intermetallic, Zr(Fe,Cr,Ni)_2+x. whereas alloys with 15, 20, and 30 wt pct Zr contain only two phases—ferrite and Zr(Fe,Cr,Ni)_2+x. Alloys with 45 to 67 wt pct Zr contain a mixture of Zr(Fe,Cr,Ni)_2+x and Zr₂(Ni,Fe), whereas alloys with 83 and 92 wt pct Zr contain three phases—α-Zr, Zr₂(Ni,Fe), and Zr(Fe,Cr,Ni)_2+x. Fe₃Zr-type and Zr₃Fe-type phases were not observed in the type 304SS-Zr alloys. The changes in alloy microstructure with zirconium content have been correlated to the Fe-Zr binary phase diagram.     

Role of Si in Improving the Shape Recovery of FeMnSiCrNi Shape Memory Alloys

The effect of Si addition on the microstructure and shape recovery of FeMnSiCrNi shape memory alloys has been studied. The microstructural observations revealed that in these alloys the microstructure remains single-phase austenite (γ) up to 6 pct Si and, beyond that, becomes two-phase γ + δ ferrite. The Fe₅Ni₃Si₂ type intermetallic phase starts appearing in the microstructure after 7 pct Si and makes these alloys brittle. Silicon addition does not affect the transformation temperature and mechanical properties of the γ phase until 6 pct, though the amount of shape recovery is observed to increase monotonically. Alloys having more than 6 pct Si show poor recovery due to the formation of δ-ferrite. The shape memory effect (SME) in these alloys is essentially due to the γ to stress-induced ε martensite transformation, and the extent of recovery is proportional to the amount of stress-induced ε martensite. Alloys containing less than 4 pct and more than 6 pct Si exhibit poor recovery due to the formation of stress-induced α′ martensite through γ-ε-α′ transformation and the large volume fraction of δ-ferrite, respectively. Silicon addition decreases the stacking fault energy (SFE) and the shear modulus of these alloys and results in easy nucleation of stress-induced ε martensite; consequently, the amount of shape recovery is enhanced. The amount of athermal ε martensite formed during cooling is also observed to decrease with the increase in Si.     

Microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys

In this paper, we systematically investigated the microstructure evolution and coercivity mechanism of hydrogenation-disproportionation-desorption-recombination (HDDR) treated Nd-Fe-B strip cast alloys by transmission electron microscopy (TEM) and three-dimensional atom probe (3DAP) analyses. The rod-like NdH_2+x phases with diameters of 10–20 nm are embedded into α-Fe matrix, which hereditarily leads to textured grains in HDDR alloy. The migration of NdH_2+x from Nd-rich region to α-Fe matrix during hydrogen absorption process contributes to the uniform redistribution of Nd-rich phases after HDDR treatment. The HDDR alloy with single domain grain sizes of 200–300 nm exhibits relatively low coercivity of 1.01 T that arises from pinning magnetic domain motion. The weak c-axis orientation of HDDR alloy results in a lower reverse magnetic field (coercivity) to reduce remanence to 0. Moreover, the direct contact of Nd₂Fe₁₄B grains and the high concentration of ferromagnetic elements (Fe content ≈ 66.06 at%, Co content ≈ 0.91 at%) in Nd-rich grain boundary layer lead to strong magnetostatic coupling effect among Nd₂Fe₁₄B grains. The nano-sized α-Fe inside Nd₂Fe₁₄B matrix makes the magnetization reversal easily and decreases the coercivity of HDDR alloy.     

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.     

Formation and Stability of Equiatomic and Nonequiatomic Nanocrystalline CuNiCoZnAlTi High-Entropy Alloys by Mechanical Alloying

Nanocrystalline equiatomic high-entropy alloys (HEAs) have been synthesized by mechanical alloying in the Cu-Ni-Co-Zn-Al-Ti system from the binary CuNi alloy to the hexanary CuNiCoZnAlTi alloy. An attempt also has been made to find the influence of nonequiatomic compositions on the HEA formation by varying the Cu content up to 50 at. pct (Cu _x NiCoZnAlTi; x = 0, 8.33, 33.33, 49.98 at. pct). The phase formation and stability of mechanically alloyed powder at an elevated temperature (1073 K [800 °C] for 1 hour) were studied. The nanocrystalline equiatomic Cu-Ni-Co-Zn-Al-Ti alloys have a face-centered cubic (fcc) structure up to quinary compositions and have a body-centered cubic (bcc) structure in a hexanary alloy. In nonequiatomic alloys, bcc is the dominating phase in the alloys containing 0 and 8.33 at. pct of Cu, and the fcc phase was observed in alloys with 33.33 and 49.98 at. pct of Cu. The Vicker’s bulk hardness and compressive strength of the equiatomic nanocrystalline hexanary CuNiCoZnAlTi HEA after hot isostatic pressing is 8.79 GPa, and the compressive strength is 2.76 GPa. The hardness of these HEAs is higher than most commercial hard facing alloys (e.g., Stellite, which is 4.94 GPa).     

Microstructure Related Hydrogen Storage Capacity of Cast Mischmetal-Nickel Alloys Substituted with Aluminium,Manganese and Tin

Abstract

The morphology and compositions of the phases present in the microstructure of cast hydrogen storage MmNi_5-xAl_x (x= 0.4 and 0.8) MmNi_4.6 Sno₄ Mn, alloys (Mm = Indian Mischmetal) have been studied. Al, Mn and Sn substitutions for Ni in MmNis produce two-phase microstructures. The hydrogen storage capacity of these alloys is related with the volume fraction of the matrix phase. Higher the matrix volume fraction, higher the hydrogen storage capacity. The hydride forming elements that constitute Mm are present in greater proportion in the major constituent matrix phase of the two-phase structure. Only trace amounts of these elements are found in the precipitate phase in the Al- and Mn substituted alloys. However, for the Sn-substituted composition, the proportion of these elements is nearly same in matrix and precipitate phase.     

Evolution of Constitution,Structure, and Morphology in FeCo-Based Multicomponent Alloys

Constituent phases, melting behaviors, and microstructure of multicomponent (Fe_0.5Co_0.5) _x (Mo_0.1C_0.2B_0.5Si_0.2)_100–x alloys (x = 95, 90, 85, 80, and 70) produced by copper mold casting were evaluated by various analysis techniques, i.e., X-ray diffractometry, scanning electronic microscopy with energy dispersive X-ray spectrometry, and differential scanning calorimetry. Metastable Fe₃C- and Cr₂₃C₆-type phases were identified in the chill-cast alloys. A schematic illustration was proposed to explain the evolution of constituent phases and microstructure for the alloys with x = 95, 90, and 85 during the solidification process, which could be applicable to controlling microstructural formation of other multicomponent alloys with similar microstructures by artificially adjusting the composition.     

On the development and investigation of quaternary Pt-based superalloys with Ni additions

The objective of this work is to mimic the microstructure and strengthening mechanisms of Ni-based superalloys in a new group of high-temperature alloys based on the system Pt-Al. The elements Cr and Ni were chosen as further alloying components. Having a face-centered cubic (fcc) crystal structure with an Ll₂-ordered and coherently embedded phase, these new alloys should increase creep and corrosion resistance beyond Ni-based superalloys. After arc melting and heat treatment, the alloys were investigated by means of scanning electron microscopy (SEM) and X-ray diffraction (XRD). In the aged condition, the alloy composition 13 at. pct Al, 3 at. pct Cr, 7 at. pct Ni, and balance Pt showed the most promising microstructure with cubical precipitates, 30 pct precipitate volume fraction, and a lattice misfit of about −0.1 pct at room temperature. This article is based on a presentation made in the symposium entitled “Beyond Nickel-Base Superalloys,” which took place March 14–18, 2004, at the TMS Spring meeting in Charlotte, NC, under the auspices of the SMD-Corrosion and Environmental Effects Committee, the SMD-High Temperature Alloys Committee, the SMD-Mechanical Behavior of Materials Committee, and the SMD-Refractory Metals Committee.     

Microstructure and mechanical properties of sputter-deposited Cu1−x Ta x alloys

The microstructures and mechanical properties of a family of sputter-deposited Cu_1−x Ta _x (0<x<0.18) alloys have been investigated. The as-deposited microstructures for all film compositions consisted of a polycrystalline, face-centered-cubic (fcc) Cu matrix, with varying levels of Ta in solid solution, plus a very high density of discrete, 1 to 3 nm, fcc Ta particles. Decreased deposition temperature (−120 °C vs 100 °C) increased the level of Ta in solid solution. After annealing (900 °C for 1 hour) the as-deposited 6 at. pct Ta films, the Cu matrix grains remained submicron and the Ta particles remained fcc with no apparent particle coarsening. Additionally, the fcc Ta particles were found before and after annealing to be oriented identically with the Cu matrix and aligned on {111} and {100} habit planes. Annealing 17 at. pct Ta films at 900 °C for 1 hour resulted in the formation of body-centered-cubic (bcc) Ta particles (>50-nm diameter) in addition to the much smaller fcc Ta particles. Annealing the low and high Ta composition films at 900 °C for as long as 100 hours produced no observed change in either the Cu matrix grain size or the size and distribution of the fcc and bcc Ta particles. Microhardness and nanoindentation mechanical property evaluations of bulk hot-pressed materials indicated that the high strengths of the composites were unchanged, even after annealing for 100 hours at 900 °C.     

New Economical 19Cr Duplex Stainless Steels

New economical duplex stainless steels (DSSs) containing 19Cr-6Mn-xNi-1.0Mo-0.5W-0.5Cu-0.2N (x = 0.5 to 2.0) were developed, and the microstructure, impact property, and corrosion resistance of the alloys were studied. The ferrite content increases with the solution treatment temperature, but decreases with an increase in nickel. The sigma phase is not found precipitating in the alloys treated with solution from 1023 K to 1523 K (750 °C to 1250 °C). The low-temperature impact energy of the experimental alloys increases first and then decreases rapidly with an increase in nickel, which is mainly due to the martensite transformation with an increase in austenite. The alloys have a better mechanical property and pitting corrosion resistance than AISI 304. Among the designed DSS alloys, 19Cr-6Mn-1.3Ni-1.0Mo-0.5W-0.5Cu-0.2N is found to be an optimum alloy with proper phase proportion, a better combination of mechanical strength and elongation, and higher pitting corrosion resistance compared with those of the other alloys.     

Deformation Behavior of Duplex Zircaloy-4-Oxygen Alloys

The mechanical properties of two-phase Zircaloy-4-oxygen alloys at thermal equilibrium have been determined. The strength of these alloys depends to a large extent on their microstructure. The strengthening behavior for alloys having isolateda grains in the softer β matrix is similar to the dispersed particle strengthening. The yield strength of these alloys is found to obey the Petch relationship, (MPa), whereλ _β is the mean free path of β phase inμm. As the volume fraction ofa phase increases, its aspect ratio also increases. This allows more effective load transfer from the matrix to the hardera phase. It has been shown that the strength of these alloys obeys a modified rule of mixtures. The alloys having equiaxeda grains in the β matrix show large strain rate sensitivities at low strain rate. The deformation behavior is interpreted in terms of dislocation slip in the β matrix and diffusion assisted climb near the interphase boundary.     

Influence of gadolinium and dysprosium substitution on magnetic properties and magnetocaloric effect of Fe78−xRExSi4Nb5B12Cu1 amorphous alloys

Amorphous Fe_78−xRE_xSi₄Nb₅B₁₂Cu₁ (RE = Gd, Dy) ribbons with different RE contents were prepared by melt spinning to investigate the effect of heavy rare earth (Gd, Dy) substitution on the hyperfine structure, magnetic properties and magnetocaloric effect. The Curie temperature of RE substituted alloys, hyperfine field and magnetic moments of Fe atoms initially increase up to 1 at% RE content and then decrease monotonously for increasing RE content up to 10 at%. The dependence of magnetic entropy change (–ΔS_M) and refrigeration capacity (RC) of the alloys on RE contents displays the same tendency. The RC_AREA values of the alloys substituted with 1 at% Gd and Dy are similar to those of recently reported Fe-based metallic glasses with enhanced RC values compared with those of Gd₅Ge_1.9Si₂Fe_0.1. Enhanced –ΔS_M and RC values, negligible coercive force and hysteresis commonly make these Fe_78−xRE_xSi₄Nb₅B₁₂Cu₁ amorphous alloys as low-cost candidates for high-temperature magnetic refrigeration.     

Characterization of microstructure in laser-surface-alloyed layers of aluminum on nickel

In order to obtain basic understanding of microstructure evolution in laser-surface-alloyed layers, aluminum was surface alloyed on a pure nickel substrate using a CO₂ laser. By varying the laser scanning speed, the composition of the surface layers can be systematically varied. The Ni content in the layer increases with increase in scanning speed. Detailed cross-sectional transmission electron microscopic study reveals complexities in solidification behavior with increased nickel content. It is shown that ordered B2 phase forms over a wide range of composition with subsequent precipitation of Ni₂Al, an ordered ω phase in the B2 matrix, during solid-state cooling. For nickel-rich alloys associated with higher laser scan speed, the fcc γ phase is invariably the first phase to grow from the liquid with solute trapping. The phase reorders in the solid state to yield γ′ Ni₃Al. The phase competes with β AlNi, which forms massively from the liquid. The β AlNi transforms martensitically to a 3R structure during cooling in solid state. The results can be rationalized in terms of a metastable phase diagram proposed earlier. However, the results are at variance with earlier studies of laser processing of nickel-rich alloys.     

Titanium-Based Metallic Glass Composites with Good Plasticity

Three types of Ti-based alloys (an amorphous material, an amorphous composite with intermetallic crystals, and an intermetallic compound) of the compositions Ti_41.5Zr_2.5Hf₅Cu_42.5−x Ni_7.5+x Si₁ (x = 0, 5, and 15) were fabricated to study the effect of composition on glass formability and microstructure, and the dependence of mechanical properties on microstructure were investigated at room temperature. The results show that the amorphous composite has an excellent combination of both ultrahigh strength (2245 MPa) and large plastic strain (9 pct), which is a significant improvement compared to both the fully amorphous and intermetallic structures. In addition, it is also found that the crystal phases in the amorphous matrix can obstruct the shearing-off of the shear bands by inducing them to interact, deflect, and branch, resulting good plasticity in the amorphous composite. This article is based on a presentation given in the symposium entitled “Bulk Metallic Glasses IV,” which occurred February 25–March 1, 2007 during the TMS Annual Meeting in Orlando, Florida under the auspices of the TMS/ASM Mechanical Behavior of Materials Committee.     

Enhancement of superelasticity in Cu-Al-Mn-Ni shape-memory alloys by texture control

A significant improvement in the degree of superelasticity in Cu-Al-Mn ductile polycrystalline alloys has been achieved through the addition of Ni and control of the recrystallization texture by thermomechanical processing, which contain the annealing in the fcc (α) + bcc (β) two-phase region, followed by heavy cold reductions of over 60 pct. The addition of Ni to the Cu-Al-Mn alloys shows a drastic effect on the formation of the strong {112} 〈110〉 recrystallization texture. Superelastic strains on the order of 7 pct, 3 times larger than those in other Cu-based shape-memory alloys (SMAs), have been realized in the textured Cu-Al-Mn-Ni alloys. The superelastic strains obtainable in the textured Cu-based SMAs are on a par with those attainable in Ni-Ti-based alloys.