Modification of NiAl–Cr(Mo)–0.15Hf alloy by Sc addition

The effect of Sc addition on the microstructure and room temperature compressive properties of NiAl–28Cr–5.85Mo–0.15 (at pct) Hf?x (wt pct) Sc (x = 0,0.05, 0.1, 0.2, 0.3) alloys was investigated. The results show that appropriate Sc addition (no more than 0.10 wt pct) leads to the refinement of interlamellar and intercellular spacings of the eutectic NiAl/Cr(Mo) cell, and the improvement of the compressive ductility and ultimate compressive strength at room temperature. When the addition of Sc is more than 0.10 wt pct, the typical NiAl/Cr(Mo) cell structure becomes broken. With the fragment of Cr(Mo) rods embedded in NiAl matrix instead of the alternating NiAl and Cr(Mo) plates, which damages the compressive properties. In addition, when the Sc addition content increases to 0.20 wt pct, Sc-containing phase is found and tentatively identified as ScO.     

Microstructure and high-temperature mechanical behavior of the NiAl–27 at.% Cr intermetallic composite

A NiAl–27 at.% Cr composite material was prepared by a powder metallurgical route, involving argon atomization and consolidation by hot isostatic pressing at 1350°C for 4 h at 400 MPa. The consolidated material exhibited a fine-grained microstructure consisting of a fine dispersion of Cr particles of about 1.7 μm in a NiAl matrix. The mechanical behavior at temperatures ranging from 650 to 1100°C was investigated by tensile-strain-rate-change tests. Analysis of the strain–stress data with both power law creep and Garofalo’s hyperbolic sine relation shows the transition to a low stress exponent creep regime with decreasing stress and/or increasing testing temperature. The measured activation energy for deformation of 300 kJ/mol is consistent with the activation energy for Ni self-diffusion in Ni–50Al. Experiments with coarse grain sizes established that the creep rate is independent of grain size which suggests that the deformation mechanisms must be associated with the motion of lattice dislocations.     

The effect of Ti-addition on plastic deformation and fracture behavior of directionally solidified NiAl/Cr(Mo) eutectic alloys

To improve the high-temperature strength of NiAl/Cr(Mo) eutectic alloys, the effect of Ti-addition on microstructure and mechanical properties was examined. Three directionally solidified (DS) alloys with the composition of Ni–(33 − x)Al–31Cr–3Mo–xTi (x = 0, 3 and 5 at.%, respectively), denoted 0Ti-, 3Ti- and 5Ti-alloys hereafter, were prepared. Temperature dependence of the yield stress and the room temperature fracture toughness of these DS alloys was examined. The aligned lamellae with B2-NiAl and A2-Cr(Mo) were formed in 0Ti-alloy, but the formation of lamellar structure was hindered by the Ti-addition. Cellular microstructures containing short plate shapes of Cr(Mo) phases were obtained in 3Ti- and 5Ti-alloys. In 5Ti-alloy, the precipitation of the L2₁-Ni₂AlTi was confirmed in NiAl matrix phase after the DS treatment. The Ti-addition induced a significant increase in high-temperature strength accompanied by a large deterioration of room temperature fracture toughness. The fracture toughness of 5Ti-alloy showed the low value of about 4 MPa m^1/2 because of the disturbance of microstructure.     

Microstructure and mechanical properties of hot pressed Mo–Cr–Si–Ti in-situ composite,and oxidation behavior with silicide coatings

The present study deals with the synthesis of Mo–16Cr–4Si–0.5Ti (wt.%) alloy by means of the reactive hot pressing method. The microstructure of the synthesized alloy consisted of (Mo, Cr, Ti)₃Si, and the discontinuous α-(Mo, Cr, Ti)_SS phases. The isothermal oxidation behavior of the alloy was investigated in air at 1273 K for 50 h. The alloy exhibited superior oxidation behavior in comparison with single phase molybdenum alloys, because of the formation of SiO₂ and Cr₂O₃ over the alloy surface. The flexural strength determined from three-point bend testing of single edge notch bend specimens was 615 ± 15 MPa. The dominant mechanism of fracture was identified as transgranular mode of crack propagation. To extend the life of the alloy under oxidizing atmosphere, silicide based oxidation resistant coatings were developed, using halide activated pack cementation process. The kinetic behavior of growth of the coating was established and the activation energy of the coating process was determined to be 52.5 kJ/mol. Isothermal oxidation tests of the coated alloy at 1273 K for 50 h, revealed a small weight gain at the initial stages of oxidation followed by no change of weight, indicating the protective nature of the coating.     

Fabrication and characterization of bulk glassy Co40Fe22Ta8B30 alloy with high thermal stability and excellent soft magnetic properties

In this work, Co₄₀Fe₂₂Ta₈B₃₀ alloy as a new bulk metallic glass with a wide supercooled liquid region of 74 K and excellent soft magnetic properties was prepared by the powder metallurgy method. Glassy Co₄₀Fe₂₂Ta₈B₃₀ powders were obtained by ball milling of melt-spun glassy ribbons at a cryogenic temperature and subsequently consolidated by hot pressing into disk-shaped specimens 10 mm in diameter and 2 mm thick. It was found that the new glassy alloy exhibited the largest diameter compared with the other Co-based bulk metallic glasses produced in the well-known Co–Fe–Ta–B alloying system up to now. The influence of the consolidation time on the microstructure and magnetic properties of the bulk samples was investigated by X-ray diffraction, differential scanning calorimetry, vibrating sample magnetometry and Faraday magnetometry. The results indicate that the new alloy exhibits a long incubation time before crystallization upon annealing above the glass transition temperature: noticeably longer than for other known (Co,Fe)-based amorphous alloys. The glassy bulk sample consolidated for 600 s at 923 K had a relative density of 99.2%, a saturation magnetization of 46.6 A m² kg^?1, a Curie temperature of 425 K and a low coercivity of 6 A m^?1. In addition, the new bulk glassy alloy exhibited ultra-high hardness of 13.48 GPa. The mechanisms by which the thermal stability and incubation time prior to crystallization increase are explained in accordance with pair correlation function analysis.     

Tensile creep of directionally solidified NiAl–9Mo in situ composites

Well-aligned Mo fiber-reinforced NiAl in situ composites were produced by specially controlled directional solidification. The creep behavior parallel to the growth direction was studied in static tensile tests at temperatures between 900 °C and 1200 °C. A steady-state creep rate of 10^?6 s^?1 was measured at 1100 °C under an initial applied tensile stress of 150 MPa. Compared to binary NiAl and previously investigated NiAl–Mo eutectics with irregularly oriented Mo fibers, this value demonstrates a remarkably improved creep resistance in NiAl–Mo with well-aligned unidirectional Mo fibers. A high-resolution transmission electron microscope investigation of the NiAl/Mo interface revealed a clean semi-coherent boundary between NiAl and Mo, which enabled an effective load transfer from the NiAl matrix to the Mo fibers, and thus leads to the remarkably increased creep strength. The stress exponent, n, was found to be between 3.5 and 5, dependent on temperature. The activation energy for creep, Q_c, was measured to be 291 ± 19 kJ mol^–1, which is close to the value for self-diffusion in binary NiAl. Transmission electron microscopy observations substantiated that creep occurred by dislocation climb in the NiAl matrix. The Mo fiber was found to behave in a quasi-rigid manner during creep. A creep model for fiber-reinforced metal matrix composites was applied for an in-depth understanding of the mechanical behavior of the individual components and their contribution to the creep strength of the composite.     

Study on the solid solubility extension of Mo in Cu by mechanical alloying Cu with amorphous Cr(Mo)

This paper presents the extension of the solid solubility of Mo in Cu by a mechanical alloying technique. Two binary systems, Cu–10 wt.% Mo and Cr–50 wt.% Mo, and one ternary system, Cu–20 wt.% Cr(Mo), are investigated. The solid solubility of Mo in Cu has been shown to be less than 4.3 at.% when the Cu–Mo system is mechanically alloyed, whereas when the Cr–Mo system is mechanically alloyed all of Mo dissolves into Cr, forming an amorphous Cr(Mo). Similarly, all of 10 wt.% Mo dissolve into Cu when Cu–20 wt.% amorphous Cr(Mo) is mechanically alloyed. Based on Miedema’s model, the Gibbs free-energy changes in these three alloy systems during the formation of solid solutions are calculated to be positive, which means that thermodynamic barriers exist for the formation of these three alloy systems in solid solution states. The mechanism of solid solubility extension in these mechanical alloyed systems is discussed. The conclusion is that the extension of solid solubility is favoured by adding a third element, such as Cr, to the Cu–Mo system.     

Atomic-scale compositional characterization of a nanocrystalline AlCrCuFeNiZn high-entropy alloy using atom probe tomography

We have studied a nanocrystalline AlCrCuFeNiZn high-entropy alloy synthesized by ball milling followed by hot compaction at 600 °C for 15 min at 650 MPa. X-ray diffraction reveals that the mechanically alloyed powder consists of a solid-solution body-centered cubic (bcc) matrix containing 12 vol.% face-centered cubic (fcc) phase. After hot compaction, it consists of 60 vol.% bcc and 40 vol.% fcc. Composition analysis by atom probe tomography shows that the material is not a homogeneous fcc–bcc solid solution but instead a composite of bcc structured Ni–Al-, Cr–Fe- and Fe–Cr-based regions and of fcc Cu–Zn-based regions. The Cu–Zn-rich phase has 30 at.% Zn α-brass composition. It segregates predominantly along grain boundaries thereby stabilizing the nanocrystalline microstructure and preventing grain growth. The Cr- and Fe-rich bcc regions were presumably formed by spinodal decomposition of a Cr–Fe phase that was inherited from the hot compacted state. The Ni–Al phase remains stable even after hot compaction and forms the dominant bcc matrix phase. The crystallite sizes are in the range of 20–30 nm as determined by transmission electron microscopy. The hot compacted alloy exhibited very high hardness of 870 ± 10 HV. The results reveal that phase decomposition rather than homogeneous mixing is prevalent in this alloy. Hence, our current observations fail to justify the present high-entropy alloy design concept. Therefore, a strategy guided more by structure and thermodynamics for designing high-entropy alloys is encouraged as a pathway towards exploiting the solid-solution and stability idea inherent in this concept.     

Improvement in oxidation resistance of a Ni3Al-based superalloy IC6 by rhenium-based diffusion barrier coatings

The oxidation behavior of a Ni₃Al-based superalloy IC6 coated with a duplex Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer was investigated in air at 1423 K for up to 1080 ks. The diffusion barrier layer was formed by electroplating Re(Ni) and Ni films on the alloy, followed by Cr pack cementation at 1573 K, and as a result, forms a continuous inner Re–Cr–Ni–Mo diffusion barrier layer and an outer Ni(Cr,Mo,Al) layer. Then a Ni film was electroplated on the Ni(Cr,Mo,Al) layer, followed by Al-pack cementation at 1273 K for 18 ks, to form an Al reservoir layer with a duplex Ni₂Al₃ and γ-Ni(Cr,Mo,Al) layers. After oxidation at 1423 K in air for 1080 ks, the Al reservoir layer changed to a γ-Ni–4Cr–5Mo–12Al (all in at%) layer, on which a protective α-Al₂O₃ scale formed. The Re–Cr(Mo)–Ni layer was stable and effectively retarded the interdiffusion between the Al reservoir layer and the alloy, as a result, the depth of the microstructural change zone of the alloy was less than 15 μm. In contrast, the bare and the coated IC6 superalloy only with an Al reservoir layer were significantly oxidized, accompanied by serious spallation of oxide scales. After oxidation at 1423 K for 1080 ks, the depth of the microstructural change zone of the alloy was about 200 μm for the bare and coated alloy only with an Al reservoir layer. These results indicate that the oxidation resistance of IC6 superalloy can be effectively improved by coating with a Re–Cr–Ni–Mo diffusion barrier layer and an Al reservoir layer.     

Deformation behavior of a two-phase Mo–Si–B alloy

Mo–Si–B alloys are being considered as possible candidates for high-temperature applications beyond the capabilities of Ni-based superalloys. In this paper, the high-temperature (1000–1400 °C) compression response over a range of quasi-static strain rates, as well as the monotonic and cyclic crack growth behaviors (as a function of temperature from 20 °C to 1400 °C) of a two-phase Mo–Si–B alloy containing a Mo solid solution matrix (Mo(Si,B)) with ∼38 vol% of the T2 phase (Mo₅SiB₂) is discussed. Analysis of the compression results confirmed that deformation in the temperature–strain-rate space evaluated is matrix-dominated, yielding an activation energy of ∼415–445 kJ/mol. Fracture toughness of the Mo–Si–B alloy varies from ∼8 MPa√m at room temperature to ∼25 MPa√m at 1400 °C, the increase in toughness with temperature being steepest between 1200 °C and 1400 °C. S–N response at room temperature is shallow whereas at 1200 °C, a definitive fatigue response is observed. Fatigue crack growth studies using R = 0.1 confirm the Paris slope for the two alloys to be high at room temperature (∼20–30) but decreases with increasing temperature to ∼3 at 1400 °C. The crack growth rate (da/dN) for a fixed value of ΔK in the Paris regime in the 900–1400 °C range, increases with increasing temperature.     

Co–Cr–Mo-based composite reinforced with bioactive glass

The powder metallurgy (PM) technology was used to produce a porous Co–Cr–Mo-based composite material with the bioactive glass (S2) addition of 5 wt.%, 10 wt.% and 15 wt.%. The results show that the addition of bioglass to the matrix of Co–Cr–Mo alloy, as well as rotary cold repressing and heat treatment of sintered specimens can cause significant changes in the microstructure, mechanical and corrosion properties of composite materials in comparison with the pure porous Co–Cr–Mo alloy. A significant increase in the hardness, yield strength and corrosion resistance of the composites was observed with increasing the bioglass volume fraction. Although all PM samples are in a passive state, the higher corrosion resistances were obtained in the case of the composites with bioglass additions. Superior mechanical properties were achieved in the case of composite with 10 wt.% of bioglass.     

Effects of boron on the microstructure and thermal stability of directionally solidified NiAl–Mo eutectic

Microalloying with 0.01 at.% B decreases the range of growth speeds over which a well-aligned fibrous eutectic microstructure can be obtained in directionally solidified NiAl–Mo. Compared to the undoped alloy, the size/spacing of the Mo fibers is larger, and the fiber density smaller, in the B-doped alloy. Annealing at 1400 °C coarsens the fibers by a mechanism involving fault migration and annihilation driven by diffusion along the fiber–matrix interface. The coarsening kinetics, given by the decrease in Mo fiber density with time, is exponential, and microalloying with B decreases the coarsening rate.     

Structure and yield strength of directionally solidified Ni3Al intermetallic premelted with MoSi2 phase

Ni₃Al and MoSi₂ intermetallic phases were arc melted in Ni₃Al/MoSi₂ molar proportion of ten. Pure binary Ni₃Al and the quaternary alloy were subjected to directional solidification using the floating zone method at growth rates of 10–50 mm h⁻¹. The phase composition and structure of the crystals were analyzed using optical microscopy, scanning electron microscopy, energy dispersive spectroscopy and X-ray diffractometry. The yield strength was studied in compression at 293–1293 K and in tension at room temperature. Compared to the binary Ni₃Al crystals, the quaternary Ni–Al–Mo–Si crystals revealed up to 5 times higher compressive yield strength at 400–800 K. Ni–Mo–Si C14 Laves phase precipitation in the L1₂ Ni₃Al+L1₀ NiAl matrix duplex phase was found in quaternary crystals. This precipitation is assumed to cause the observed mechanical behaviour.     

Microstructure and mechanical properties of Ni3Al base alloy reinforced with Cr particles produced by powder metallurgy

The microstructural evolution of a powder metallurgy (PM) Ni₃Al–8Cr (at.%) alloy reinforced with Cr particles has been correlated with its mechanical properties. The material was synthesised using rapidly solidified Ni₃Al–8Cr powders which were mixed with a Cr volume fraction of 10% and milled for 20 h. Consolidation by HIP was carried out at 150 MPa for 2 h at 1250 °C. For comparative purposes the unreinforced Ni₃Al–8Cr alloy was processed following the same route. After consolidation by HIP both materials show a bimodal microstructure consisting of coarse and fine grain regions in which fine particles are heterogeneously distributed. Besides Cr reinforcement, the difference between the two materials is the presence of β phase and higher volume fractions of γ+γ′ regions and α-phase precipitates in the reinforced material. The reinforced material presents the highest hardness, yield stress and the ultimate tensile strength values. The yield stress and ultimate tensile strength of the reinforced material at room temperature is 1286 and 1335 MPa, respectively. The strength of the composite is determined by the strength of the Cr particles and the good bonding between the matrix and Cr reinforcement. Although the ductility loss as the temperature increases is not suppressed, an improvement in ductility is obtained at temperatures above 500 °C compared with the unreinforced material.     

Manufacturing of Al–Mg–Si alloy foam using calcium carbonate as foaming agent

Aluminium foams can be manufactured by two main methods: casting and powder metallurgy. When the latter route is used, a foaming agent (usually TiH₂) is mixed with the aluminium or aluminium alloy powders, followed by powder mixture consolidation (usually hot extrusion) into a precursor and finally its foaming treatment. In this research, two calcium carbonate powders were used as foaming agents on an Al–Mg–Si (AA6061) alloy. Their different characteristics (particle size and chemical composition) modified the manufacturing process to achieve the final foam. AA6061 powders were then mixed with 10% calcium carbonate and, after cold isostatic pressing into green cylinders, hot extruded at different temperatures (475–545 °C). The foaming treatment was carried out in a furnace preheated to 750 °C using several heating times. The density changed from 2.03 to 2.10 g/cm³ after cold isostatic pressing to 2.64–2.69 g/cm³ in precursor materials obtained by hot extrusion. Foaming behaviour depends on the carbonate powder as well as the extrusion temperature. Thus, natural carbonate powder (white marble) produces a foam density close to 0.65 g/cm³ after a shorter time than when chemical carbonate is used. The foam structure showed a low degree of aluminium draining, no wall cell cracks and a good fine cell size distribution. Compressive strength of 6.11 MPa and 1.8 kJ/m³ of energy absorption were obtained on AA6061 foams with a density between 0.53 and 0.56 g/cm³.     

Mechanical properties and hardening mechanism of Fe–Al–Ni single crystals containing NiAl precipitates

The microstructures and mechanical properties of Fe–23.0 Al–6.0 Ni (at.%) single crystals containing NiAl precipitates were investigated and the hardening mechanism due to the precipitates was discussed, focusing on the activated slip systems. When these alloys were slowly cooled to room temperature after homogenization at 1373 K, the NiAl phase with the B2 structure precipitated in the body-centered cubic (bcc) Fe–Al matrix, satisfying the cube-on-cube relationship with a small misfit strain. The single crystals containing the NiAl precipitates exhibited a high yield stress above 1 GPa at room temperature. In addition, the activated slip system and deformation behavior depended strongly on the loading axis. For instance, 〈1 1 1〉 slip, which is the primary slip for the bcc matrix, occurred at 〈1 4 9〉 and 〈0 0 1〉 orientations and the NiAl precipitates were sheared by the slip. A critical resolved shear stress of 〈1 1 1〉 slip in the NiAl phase was known to be extremely high, which led to strong precipitation hardening. On the other hand, at 〈5 5 7〉 and 〈0 1 1〉 orientations, 〈0 0 1〉 slip, which is the primary slip system for the NiAl precipitates, forcibly sheared the bcc Fe–Al matrix, also leading to strong hardening. Thus, in the Fe–Al–Ni alloys, the difference in the primary slip system between the bcc Fe–Al matrix and the NiAl precipitates resulted in extreme hardening. This hardening mechanism caused by the NiAl precipitates effectively increased the yield stress even at high temperatures. In fact, the crystals exhibited a high yield stress at ～1 GPa up to 823 K.     

Site preference of transition-metal elements in B2 NiAl: A comprehensive study

First-principles supercell calculations based on density functional theory were performed to study the T = 0 K site preference of 3d (Ti–Cu), 4d (Zr–Ag) and 5d (Hf–Au) transition-metal elements in B2 NiAl. By adopting a statistical-mechanical Wagner–Schottky model within the canonical ensemble, the effects of finite temperature on site preference were further considered. The calculations showed that, at all alloy compositions and temperatures, Co, Tc, Ru, Rh, Re, Os, Ir and Pt have a consistent preference for the Ni sublattice, while Ti, Zr, Nb, Hf and Ta have a consistent preference for the Al sublattice. In contrast, the site preference of V, Cr, Mn, Fe, Cu, Mo, Pd, Ag, W and Au was found to depend on both composition and temperature. The present calculated results compare favorably with existing theoretical and experimental studies in the literature.     

Deformation twins and related softening behavior in nanocrystalline Cu–30% Zn alloy

Nanocrystalline Cu–30% Zn samples were produced by high energy ball milling at 77 K and room temperature. Cryomilled flakes were further processed by ultrahigh strain high pressure torsion (HPT) or room temperature milling to produce bulk artifact-free samples. Deformation-induced grain growth and a reduction in twin probability were observed in HPT consolidated samples. Investigations of the mechanical properties by hardness measurements and tensile tests revealed that at small grain sizes of less than ～35 nm Cu–30% Zn deviates from the classical Hall–Petch relation and the strength of nanocrsytalline Cu–30% Zn is comparable with that of nanocrystalline pure copper. High resolution transmission electron microscopy studies show a high density of finely spaced deformation nanotwins, formed due to the low stacking fault energy of 14 mJ m^–2 and low temperature severe plastic deformation. Possible softening mechanisms proposed in the literature for nanotwin copper are addressed and the twin-related softening behavior in nanotwinned Cu is extended to the Cu–30% Zn alloy based on detwinning mechanisms.     

Mössbauer effect measurement evidence for magnetic transition in ordered Fe-doped NiAl

Mössbauer effect measurements at room and cryogenic temperatures on powdered Fe-doped NiAl materials (designated as Ni–40Al–9Fe and Ni–50Al–9Fe) show that both were paramagnetic down to17 K, displaying one Fe site in the host. At 4.2 K, Ni–40Al–9Fe remained paramagnetic, while Ni–50Al–9Fe showed a magnetic transition, with a clearly resolved Fe environment. The determined internal magnetic field was 185 ± 8 kOe in comparison to 330 kOe for α-Fe (bcc) reference at room temperature. This shows that development and evolution of electronic and magnetic interactions associated with the dopant Fe in the ordered NiAl depend on its site preference tendencies. The temperature dependence of magnetic state of the dopant Fe, in conjunction with its site preference tendencies from Mössbauer effect measurements, confirms the effectiveness of this technique in the study of the hardening/softening behaviors of Fe-doped NiAl.     

Deformation behavior of isothermally forged Ti–5Al–2Sn–2Zr–4Mo–4Cr powder compact

The isothermal deformation behavior of hot isostatic pressed (HIPed) Ti–5Al–2Sn–2Zr–4Mo–4Cr(Ti-17) powder compact was investigated by compression testing in the temperature range of 810–920 °C and constant strain rate range of 0.001–1 s^?1. The true stress–true strain curves of the powder compact exhibit flow oscillation and flow softening phenomenon in both beta field and beta + alpha field. The flow softening behavior is related to the globularization of the primary acicular microstructure and deformation heating. The apparent activation energy for deformation in beta field is estimated to be 149 kJ mol^?1, indicating that the deformation is controlled by diffusion. The high apparent activation energy of 537 kJ mol^?1 for deformation in beta + alpha field may be related to the dynamic recrystallization of the primary acicular microstructure. Constitutive equations with the form of Arrhenius-type hyperbolic-sine relationship are proposed to delineate the peak flow stress as a function of the strain rate and the temperature for isothermal forging HIPed Ti-17 powder compact.