Dislocation loops and phase decompositions of B2-FeAl(Co) compound in Fe–Al–Co ternary system

Phase decompositions in B2-FeAl(Co) compound were investigated by means of transmission electron microscopy. Precipitation behavior and hardening were investigated by measuring the hardness variation. The disordered α-Fe phase is present as a precipitate in B2-FeAl(Co) matrix and has a cubic-cubic orientation with the matrix. At the early aging periods prismatic dislocation loops formed in B2-FeAl(Co) matrix. The loops have a Burgers vector of a<100> and have a shape of square with their sides lying on the {100} plane of the B2-FeAl(Co) matrix. As the aging time is prolonged, the dislocation loops disappear and α-Fe precipitates form at the locations of dislocation loops. B2-FeAl(Co) matrix is hardened typically by the precipitation of α-Fe.     

Precipitation behavior of B2-ordered aluminide

Fine dispersion of disordered phases is obtained in Ni−Al−Cr and Fe−Al−Co temary systems. A transmission electron microscope investigation has been performed on the precipitation of α-Cr in B2-ordered β-NiAl with different stoichiometry and α-Fe in B2-FeAl(Co) compound. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of NiAl and FeAl increases appreciably with the fine precipitation of α-Cr and α-Fe, and over-age softening occurs after prolonged aging. In the case of B2-NiAl(Cr), perfect lattice coherency is maintained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface, followed by climbing around the particles. On the other hand, in the case of B2-FeAl(Co), the disordered α-Fe phase is present as a precipitate in the B2-FeAl(Co) matrix and has a cubic-cubic orientation with the matrix. At the early aging periods, prismatic dislocation loops formed in the B2-FeAl(Co) matrix. B2-FeAl(Co) matrix is typically hardened by the precipitation of α-Fe.     

RETRACTED ARTICLE: A study on the microstructure of precipitation strengthened B2-ordered NiAl

Microstructural control to produce a multiphase structure and there by improve the high temperature strength as well as low temperature ductility of intermetallics has received much attention. A transmission electron microscopy investigation has been performed in the present work on the precipitation of supersaturated B2-ordered (Ni,Co)Al and α-Cr in B2-ordered β-NiAl with different stoichiometry. Precipitation behavior and hardening were investigated by measuring the hardness variation. The hardness of (Ni,Co)Al and β-NiAl increases appreciably by the fine precipitation of (Ni,Co)₂Al and α-Cr, and overage softening occurs after prolonged aging. In the case of B2-ordered (Ni,Co)Al, the (Ni,Co)₂Al phase has a hexagonal structure and takes a rod-like shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)₂Al phase. The orientation relationship between the (Ni,Co)₂Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)_p//(111)B2 and [[`1]\bar 12[`1]\bar 10]_p//[[`1]\bar 110]_B2, where the suffixes p and B2 denote the (Ni,Co)₂Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by fine precipitation of the (Ni,Co)₂Al phase. On the other hand, in the case of B2-NiAl, perfect lattice coherency is retained at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, a loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles.     

Microstructure of Co precipitation strengthened B2-ordered NiAl

A transmission electron microscopy investigation on the phase decomposition of B2-ordered (Ni,Co)Al supersaturated with Ni and Co has revealed the precipitation of (Ni,Co)₂Al which has not been expected from the reported equilibrium phase diagram. The (Ni,Co)₂Al phase has a hexagonal structure and takes a rodlike shape with the long axis of the rod parallel to the 〈111〉 directions of the B2 matrix. By aging at temperatures below 873 K, a long period superlattice structure appears in the hexagonal (Ni,Co)₂Al phase. The orientation relationship between the (Ni,Co)₂Al precipitates and the B2-(Ni,Co)Al matrix is found to be (0001)_p//(111)_B2 and [ $ \bar 1 $ 2 $ \bar 1 $ 0]_p//[ $ \bar 1 $ 10]_B2, where the suffix p and B2 denote the (Ni,Co)₂Al precipitate and the B2-(Ni,Co)Al matrix, respectively. (Ni,Co)Al hardens appreciably by the fine precipitation of the (Ni,Co)₂Al phase.     

Microstructure and strength of B2-ordered (Ni,Co)Al

A transmission electron microscopy (TEM) investigation of the morphologies and crystal structures of precipitates in supersaturated B2-ordered (Ni,Co)Al has revealed that rod-like precipitates of the (Ni,Co)₂Al phase with a hexagonal structure form parallel to the <111> direction of the B2 matrix. By aging at temperatures below 973 K, a long period superlattice structure of hexagonal (Ni,Co)₂Al was formed. The (Ni,Co)Al hardens appreciably by the precipitation of these phases. An energy dispersive spectroscopy (EDS) was used to analyze the compositions of each phase formed in the B2-(Ni,Co)Al. The effects of the dispersion of the (Ni,Co)₂Al phase on the temperature dependence of the strength of the B2-(Ni,Co)Al have been investigated over a temperature range from 298 K to 1173 K.     

PRECIPITATION BEHAVIOR OF Co PHASES IN B2-ORDERED(Ni,Co)Al COMPOUND

且．工口止OOdllCt1OllThe BZ－ordered NIAI has received considerable attention because oflts pete尬lal forhightemperature applicatlonsll,2]．Its ad皿ntages are high melting temperature,rel幼Ivelylow density ofs．959/cm’and good皿idatlon resistance at high     

Microstructure of aluminized stainless steel 310 after annealing

The aluminized coating on type 310 stainless steel prepared by high-activity Al pack cementation method has been annealed at 900 °C for 12 h to transform the brittle δ-Fe₂Al₅ phase into the more ductile β-FeAl phase. The microstructure is studied in detail with transmission electron microscopy. The thick outer layer has β-(Fe, Ni)Al as matrix with cube-like Cr₂Al precipitates. The interfacial layer has a thin layer of metastable FCC phase (layer I) and then mixed β-(Fe, Ni)Al grains and α-(Fe, Cr) grains (layers II and III). The Cr₂Al precipitates are present in the β-(Fe, Ni)Al grains in layer II but not in those in layer III, while β-FeAl precipitates are present in the α-(Fe, Cr) grains in both layers. The orientation relationships between various phases, the formation of the layers, and the precipitation of Cr₂Al in β-(Fe, Ni)Al are discussed.     

Ni-Al-Fe系中NiAl(Fe)金属间化合物的析出强化

研究Ni-Al-Fe系中B2型金属间化合物NiAl(Fe)的硬度随时效时间的变化,同时测定时效处理后含析出相的NiAl(Fe)金属间化合物的屈服强度随温度变化。结果表明,在所有实验温度区域内,NiAl(Fe)化合物的屈服强度均远高于单相NiAl;析出相为体心立方结构的α-Fe相;在时效初期α-Fe相呈球状,过时效之后变成平行于有序基体{100}晶面的板条状。通过透射电镜观察还确定变形过程中位错滑移矢量为111。虽然α-Fe析出相的硬度低于NiAl(Fe)基体,但由于α-Fe析出相对运动位错有较强的钉扎作用而使基体得到强化。     

Phase equilibria in the T–Al–C (T: Co,Ni, Rh,Ir) and T–Al–B (T: Rh,Ir) systems for the design of E21–Co3AlC based heat resistant alloys

Phase equilibria in Co–Al–C ternary and Co–Ni–Al–C quaternary systems were investigated to establish the basis for designing new class of α(Co) based heat resistant alloys strengthened by the E2₁ type intermetallic compound Co₃AlC. Phase stability of E2₁ (Co, Ni)₃AlC was examined from the viewpoint of magnetic properties such as Curie temperature and saturation magnetization. The possibility of two-phase separation is indicated between E2₁(E2₁′) (Co,Ni)₃AlC and E2₁(L1₂) (Ni,Co)₃Al(C) in the Co–Ni–Al–C quaternary system, where we denote E2₁′ as standing for the ordered crystal structure of (Co,Ni)₃AlC_0.5 formed by the extra ordering of carbon atoms. Phase diagrams information was determined by means of electron probe microanalysis and microstructural observation for the T–Al–C (T: Co, Ni, Rh, Ir) and T–Al–B (T: Rh, Ir) systems to examine the phase equilibria in each alloy system focusing on the existence of E2₁ T₃AlC and T₃AlB. The existence of E2₁ Ir₃AlB (E2₁′ Ir₃AlB_0.5) phase has been revealed in the Ir–Al–B system by diffraction analysis of transmission electron microscopy and electron probe microanalysis.     

Precipitation of α-Cr in B2-ordered NiAl

A transmission electron microscope investigation has been performed on the precipitation of α-Cr in B2-ordered β-NiAl with different stoichiometry. By aging at temperatures around 1073 K after solution annealing at 1563 K, fine spherical particles of α-Cr appear in the NiAl matrix between the α-Cr which rapidly precipitated during quenching. The hardness of NiAl increases appreciably by the fine precipitation of α-Cr and overage softening occurs after prolonged aging. The perfect lattice coherency is kept at the interfaces between the α-Cr particles and the matrix during the initial stage of aging. After prolonged aging, the loss of coherency occurs by the attraction of matrix dislocations to the particle/matrix interface followed by climbing around the particles. The amount of age hardening of stoichiometric NiAl in which equal amounts of Ni and Al are replaced by Cr is smaller than that obtained in off-stoichiometric NiAl. Atom location by channeling enhanced microanalysis technique has been used to determine the site occupancy of Cr in NiAl.     

Precipitation of Cr-rich phases in a Ni–50Al–2Cr (at.%) alloy

An as-cast Al-rich B2-ordered Ni–50Al–2Cr alloy was heat-treated at 550 °C for 500 h and then analysed by using atom probe field-ion microscopy (APFIM), scanning electron microscopy (SEM), energy-dispersive X-ray analysis (EDX), electron backscatter diffraction (EBSD), and transmission electron microscopy (TEM). Chromium-rich precipitates with (unexpectedly) high Al contents were detected and identified as the so-called ‘X-phase’. The α-chromium phase was also present possessing a significantly lower Al content. Nanoscale B2-ordered NiAl precipitates present in the X-phase as well as nanoscale X-phase precipitates within B2-ordered NiAl were detected.     

Precipitation behavior of (Al,Ag)3Ti and Ti3AlC in L10-TiAl in Ti-Al-Ag system

A transmission electron microscopy (TEM) investigation has been performed on the morphologies of L1₂-(Al,Ag)₃Ti and Ti₃AlC precipitates in L1₀-ordered TiAl(Ag). During aging at temperatures around 1073 K after quenching from 1273 K, TiAl(Ag) hardens by the precipitation of (Al,Ag)₃Ti and Ti₃AlC. TEM observations revealed that plate-like (Al,Ag)₃Ti precipitates lie on {001} planes of TiAl(Ag) matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or high-temperature aging and finally to {112} of the matrix lattice. At the same time needle-like precipitates Ti₃AlC, which lie only in one direction parallel to the [001] direction of the TiAl(Ag) matrix, appear after long period aging or high-temperature aging. The anisotropic misfits between TiAl(Ag) matrix and L1₂-(Al,Ag)₃Ti and Ti₃AlC are considered to explain the morphologies of L1₂-(Al,Ag)₃Ti and Ti₃AlC precipitates.     

MICROSTRUCTURES AND MECHANICAL PROPERTIES OF β-NiA1 CONTAINING α-Fe PRECIPITATES

1. 'IntroductionThe BZ-structural intermetallic, NiAI, has received considerable attention because ofits potential for high temperature applicationsll)21. Its advalltages are the low density of5.95g/cm' (approximately two-thirds the density of nickel-base superalloys), excellent thermal conductivity (four to eight times that of nickeLbase superalloys) and good oxidationresistance at high temperature. However, before the compound can be exploited for thispurpose its low ductility at low tempe…     

Phase stability of the X2AlTi (X: Fe,Co, Ni and Cu) Heusler and B2-type intermetallic compounds

Ordering and phase separation between the B2 and L21 phases in the X2AlTi (X: Fe, Co, Ni, Cu) intermetallic compounds were investigated. The B2/L21 continuous ordering was determined using the diffusion couple technique in the temperature range of 1273–1573 K. It was found that the substitution of Co for Fe results in raising the B2/L21 ordering temperature and that of Cu for Ni brings about widening of the B2+L21 two-phase region on both the NiAl and NiTi sides. It is shown that the maximum temperatures of B2/L21 order–disorder transition (TcB2/L21max), the tricritical temperatures (TtB2/L21) and the phase boundaries of the miscibility gap between the B2 and L21 phases in the XAl–XTi pseudobinary systems can be well described by the 3d+4s electron concentration of the elements occupying the X site. On the basis of this finding, the phase stability and the interchange energies between Al and Ti in the next nearest neighbors of X–Al–Ti (X: Ti, V, Cr, Mn, Fe, Co, Ni and Cu) system are discussed.     

Morphology of L10-TiAl(Ag) precipitation in Ag-modified L12–Al3Ti

Transmission electron microscopy (TEM) examinations of Ag-modified Al₃Ti with an L1₂-ordered structure have revealed the precipitation of L1₀–TiAl upon aging after quenching from higher temperatures. TEM observations revealed that plate-like L1₀–TiAl precipitates lie on {001} planes of (Al,Ag)₃Ti matrix in the short aging period and the habit plane changed from {001} to {hhl} after a long period aging or higher temperature aging and finally to {225} of the matrix lattice. A quantitative X-ray microanalysis for determining the chemical compositions of precipitate and matrix has been done by using an analytical electron microscope. The L1₂ phase field in the Ti–Al–Ag system is severely skewed with respect to the temperature axis and is restricted into a much smaller field at lower temperatures. The coherency stresses across the precipitate/matrix interface is considered to be the main factor controlling the precipitate morphology.     

Combined ab initio and experimental study of A2 + L21 coherent equilibria in the Fe–Al–X (X = Ti,Nb, V) systems

A combined theoretical and experimental study of ordering and phase separation in α-Fe alloys in the Fe–Al–X (X = Ti, Nb, V) systems is presented. The theoretical part is dedicated to the assessment of the BCC-based phase equilibrium diagram in the iron-rich zone of the ternary systems via a truncated cluster expansion, through the combination of Full-Potential-Linear augmented Plane Wave (FP-LAPW) electronic structure calculations and of Cluster Variation Method (CVM) thermodynamic calculations in the irregular tetrahedron approximation. The stability and the solid solubility of Al in the Fe₂X Laves phases are also included in the discussion of the ternary BCC ground state diagrams. The approach was employed in order to explore particular vertical sections of the ternary systems where a coherent two-phase microstructure can be generated with an optimal combination of volume fraction of Fe₂AlX (L2₁) Heusler type precipitates and Al content in the α-Fe (A2) matrix. The results indicate that in the Fe–Al–Ti and Fe–Al–V systems there are two kinds of phase separations of the BCC phase, (A2+ L2₁) and (B2 (FeAl structure) + L2₁). A tie-line separates both two-phase fields that shrinks and moves toward the Fe-X binary system while its direction remains almost parallel as the temperature increases. Selected experiments were performed on three alloys of the Fe–Al–Ti system belonging to vertical sections that contain this tie-line. The microstructure and composition of the matrix and precipitate phases were characterized by transmission electron microscopy (TEM) and Energy Dispersive Spectroscopy (EDS), the theoretical predictions were borne out.     

The effect of the thermal decomposition reaction on the mechanical and magnetocaloric properties of La(Fe,Si,Co)13

We report on the influence of the Co content in the magnetocaloric system La(Fe,Si,Co)₁₃ on the thermal decomposition (TD) reaction, and subsequently on the magnetocaloric properties. In the course of the TD reaction, the magnetocaloric La(Fe,Si,Co)₁₃ phase reversibly decomposes into α-Fe(Co,Si) and the intermetallic LaFeSi phase, thus enhancing the mechanical properties and therefore the machinability of the compound. The addition of Co significantly speeds up the reaction kinetics. The optimum temperature range for the TD reaction was determined to be 973–1073 K, with the lower and upper limit at 873 K and 1173 K, respectively. With electron microscopy a lamellar microstructure has been found in the decomposed state, indicating a eutectoid-type phase reaction. The width of the lamellae is ～26 nm in LaFe₁₂Si and decreases with increasing Co content. Three-dimensional atom probe (3DAP) measurements show the enrichment of Co and Si in LaFeSi lamellae. We conclude that the addition of Co somehow decreases the lamellar spacing, which is the main reason for the enhanced TD kinetics. Finally, it is interesting to note that the highly ordered nano-scale mixture of strongly ferromagnetic α-Fe(Co) with the non-ferromagnetic phase induces a significant increase in coercivity, H_c. The shape anisotropy of the thin α-Fe(Co) lamellae yields a semi-hard permanent magnet with a coercivity of ～100 A cm^?1.     

Evolution of microstructure,hardness, and corrosion properties of high-entropy Al0.5CoCrFeNi alloy

In this study, we investigate the microstructure, hardness, and corrosion properties of as-cast Al_0.5CoCrFeNi alloy as well as Al_0.5CoCrFeNi alloys aged at temperatures of 350 °C, 500 °C, 650 °C, 800 °C, and 950 °C for 24 h. The microstructures of the various specimens are investigated using X-ray diffraction (XRD), scanning electron microscopy (SEM), and electron probe X-ray microanalysis (EPMA). The results show that the microstructure of as-cast Al_0.5CoCrFeNi comprises an FCC solid solution matrix and droplet-shaped phases (Al–Ni rich phases). At aging temperatures of between 350 and 950 °C, the alloy microstructure comprises an FCC + BCC solid solution with a matrix, droplet-shaped phases (Al–Ni rich phase), wall-shaped phases, and needle-shaped phases (Al–(Ni, Co, Cr, Fe) phase). The aging process induces a spinodal decomposition reaction which reduces the amount of the Al–Ni rich phase in the aged microstructure and increases the amount of the Al–(Ni, Co, Cr, Fe) phase. The hardness of the Al_0.5CoCrFeNi alloy increases after aging. The optimal hardness is obtained at aging temperatures in the range 350–800 °C, and the hardening effect decreases at higher temperatures. Both the as-cast and aged specimens are considerably corroded when immersed in a 3.5% NaCl solution because of the segregation of the Al–Ni rich phase precipitate formed in the FCC matrix. Cl^? ions preferentially attack the Al–Ni rich phase, which is a sensitive zone exhibiting an appreciable potential difference, with consequent galvanic action.     

Mechanical and Tribological Behavior of Ni(Al)-Reinforced Nanocomposite Plasma Spray Coatings

The mechanical and tribological behavior and microstructural evolutions of the Ni(Al)-reinforced nanocomposite plasma spray coatings were studied. At first, the feedstock Ni(Al)-15 wt.% (Al₂O₃-13% TiO₂) nanocomposite powders were prepared using low-energy mechanical milling of the pure Ni and Al powders as well as Al₂O₃-13% TiO₂ nanoparticle mixtures. The characteristics of the powder particles and the prepared coatings depending on their microstructures were examined in detail. The results showed that the feedstock powders after milling contained only α-Ni solid solution with no trace of the intermetallic phase. However, under the air plasma spraying conditions, the NiAl intermetallic phase in the α-Ni solid solution matrix appeared. The lack of nickel aluminide formation during low-energy ball milling is beneficial hence, the exothermic reaction can occur between Ni and Al during plasma spraying, improving the adhesive strength of the nanocomposite coatings. The results also indicated that the microhardness of the α-Ni phase was 3.91 ± 0.23 GPa and the NiAl intermetallic phase had a mean microhardness of 5.69 ± 0.12 GPa. The high microhardness of the nanocomposite coatings must be due to the presence of the reinforcing nanoparticles. Due to the improvement in mechanical properties, the Ni(Al) nanocomposite coatings showed significant modifications in wear resistance with low frictional coefficient.     

Origin of giant magnetoresistance in conventional AlNiCo5 magnets

Structure–property relationships in AlNiCo5 permanent magnets in the optimum magnetic state have been investigated so as to uncover the origin of giant magnetoresistance (GMR). According to analytical field ion and conventional transmission electron microscopy, AlNiCo5 consists of two coherent B2-ordered phases. One, α₁, corresponds to Fe₇Co₃ and the other, α₂, to Al_1.1Ni_0.9 with some substitution of Al, Ni, Cu and Fe, Co, Cu, respectively. Seventy percent of the GMR-effect can be attributed to superparamagnetic clusters, about 1 nm in size, positioned on the Al-rich α₂-sublattice. The remaining 30% result from 3–6 nm superparamagnetic α₁-clusters in the form of fine bridges interconnecting large α₁-particles normal to, or inclined to their direction of elongation.