Effect of carbon on formation of mixed solid solutions during mechanochemical synthesis of Ni-Al-Mo-C mixtures and ordering of solutions during heating

Solid solutions Ni(Al, Mo, C) are formed via milling the Ni_2.8Al₁Mo_0.2 and Ni₃Al_0.8Mo_0.2 and graphite-containing Ni_2.8Al₁Mo_0.2C_{(0.25, 0.5)} and Ni₃Al_0.8Mo_0.2C_{(0.25, 0.5)} mixtures. In this case, some amount of Mo remains beyond the solid solution. Graphite added to a starting mixture decreases the Mo solubility and favors the amorphization of solid solutions. The complete amorphization was found for the mixture with the 5 at % C and 5 at % Mo, which was added instead of Ni. The heating of mechanically synthesized (MS) powder alloys leads to the ordering of carbon-free and carbon-containing solid solutions with the formation of the L1₂ and E2₁ structure, respectively. In the course of the ordering of the Ni(Al, Mo, C) solid solutions, Mo and carbon precipitate in the form of the molybdenum carbide (Mo₂C) second phase. The hardness of the MS three-phase Ni-Al-Mo-C solid solutions subjected to hot isostatic pressing is determined by the mass fraction of the formed Mo₂C carbide. It is shown that the carbon content in the multicomponent antiperovskite can be estimated by analyzing the ratio of integral intensities of superlattice reflections I ₍₁₀₀₎/I ₍₁₁₀₎.     

The synthesis of nanocrystalline Ni75Nb12B13 alloys by high energy ball milling of elemental components

Ni₇₅Nb₁₂B₁₃ alloys were synthesized by mechanical alloying (MA) of individual Ni, Nb and B components. X-ray investigation showed the formation of Ni (Nb, B) solid solution and amorphous phase at the intermediate stage of milling. Metastable phases formed by MA turned into Ni (Nb), Ni₂₁Nb₂B₆ and Ni₃Nb stable phases during heating up to 720 °C. The exothermal effects on DSC curves were caused with these processes. The disintegration of Ni (Nb, B) solid solution and crystallization of an amorphous phase resulted in the stable phases formation during the milling prolongation as well as after thermal treatment.     

Effects of annealing at 800 and 1000 °C on phase precipitates and hardness of Al7Cr20FexNi73−x alloys

The effects of Fe content on the microstructure, phase constituents and microhardness of the as-cast, 800 °C- or 1000 °C-annealed Al₇Cr₂₀Fe_xNi_73?x (x=13?66) alloys were investigated. Not all these alloys are composed of the single FCC phase. The BCC and B2 phases are found. It is confirmed that the BCC phase in the Al₇Cr₂₀Fe₆₆Ni₇ alloy is transformed from the FCC phase at about 900 °C during cooling. While in the 800 °C-annealed Al₇Cr₂₀Fe₆₀Ni₁₃ alloy, the FCC phase is stable and the hardness decreases. After annealing at 1000 °C, for the precipitation of the B2 particles, the Al content in the FCC phase decreases, which results in decreasing of the alloy hardness. Moreover, after annealing at 800 °C, a small amount of Al-rich B2 particles precipitate at the phase boundary and some nanocrystal BCC phase precipitates in the FCC matrix, which increases the hardness of the Al₇Cr₂₀Fe_xNi_73?x (x=41?49) alloys. These results will help to the composition design and processing design of the Al?Cr?Fe?Ni based high-entropy alloys.     

The correlation of microstructural development and thermal stability of mechanically alloyed multicomponent Fe-Co-Ni-Zr-B alloys

The microstructural evolution of multicomponent Fe_70-x-yCo_xNi_yZr₁₀B₂₀ (x = 0, 7, 21; y = 7, 14, 21, 28) alloys during mechanical alloying (MA) has been studied using XRD, SEM and TEM. Mixtures of elemental and pre-alloyed powders have been transformed initially into the single supersaturated bcc α-Fe solid solution phase for the alloys investigated. Subsequently, an amorphous phase has been obtained in Co-free alloys and Co-containing alloys with high Ni/Co ratios of 1 and 3. However, no amorphous phase was detected in another Co-containing alloy with a lower Ni/Co ratio (e.g. 0.33). The thermal stability of the as-milled powders has been investigated by a combination of DSC and the Pendulum magnetometer experiments. The DSC studies provide information on the thermodynamics and kinetics of crystallization of amorphous structure as a function of alloying contents. The Pendulum magnetometer studies reveal the phase transformation from nanocrystalline bcc α-Fe solid solution to amorphous structure during MA and the thermomagnetization behavior of the as-milled powder.     

Mechanical alloying as method for introducing carbon in Ni3Al intermetallide

The method for the mechanical alloying of Ni-Al-C and Ni₃Al-C mixtures was used to obtain nonequilibrium solid Ni(Al,C) solutions in which the carbon content varies from 2.9 to 8.5 at %. The relationship between carbon dissolution and the probability of appearance of deformation-induced stacking faults (SFs) in the formation of mixed (substitutional and interstitial) solid Ni(Al,C) solutions has been found based on an analysis of the diffraction spectra. SFs are assumed to serve as pathways of carbon penetration in nickel-based solid solutions. The effective carbon radius was found to be about 0.0616 nm in the formation of an antiperovskite phase Ni₃AlC _x . The method of calculating the amount of interstitial carbon was proposed based on the experimental lattice parameters of fcc solid Ni(Al,C) solutions and ordered phases L1₂ Ni₃Al and E2₁ (Ni₃AlC _x ). The temperature stability of the nonequilibrium solid Ni(Al,C) solutions was established. It was shown that the decomposition of the solid solutions proceeded according to a spinodal mechanism at a temperature of 400°C with separation into two phases, i.e., an antiperovskite carbide (Ni₃AlC _x ) and Ni(Al,C). At higher temperatures (600?C800°C), carbon precipitates from these phases with the formation of an antiperovskite Ni₃AlC_0.16, solid Ni(Al) solution, and nanocrystalline graphite.     

Mechanical alloying of Ni3Al with molybdenum and arrangement of Mo atoms in sublattices of the intermetallic

Mechanochemical synthesis (MS) of Ni₇₀Al₂₅Mo₅ (composition 1) and Ni₇₅Al₂₀Mo₅ (composition 2) mixtures, in which 5 at % Mo substitutes for the equal amount of Ni or Al, leads to the formation of Ni-based nanocrystalline (coherent domains are ～7–12 nm in size) solid solutions; in this case, some amount of molybdenum remains free. A comparison of the lattice parameters of solid solutions, which were determined experimentally, with the magnitudes determined theoretically using Vegard law and Bozollo-Ferrante simulation, which takes into account volume modules of elasticity of elements, showed an increase in interactions between atoms composed the solid solution and the formation of regions characterized by short-range order. The heating of mechanically synthesized three-component Ni(Al, Mo) solid solutions to 720°C in a calorimeter chamber forms the ordered γ′ phase (L1₂) at T ～ 450°C. An analysis of the ratio of relative intensities of superlattice and fundamental reflections showed that, whatever the composition of initial mixture, Mo atoms always occupy positions in the Al sublattice. This arrangement of Mo atoms was confirmed by calculations of coefficients of concentrational variations of the lattice parameters. When molybdenum is added to Ni₃ Al, Mo atoms, rather than Ni atoms, complete the Al sublattice. In this case, vacancies compensate for the lack of atoms in the Ni sublattice.     

Stability,phase transformation and deformation behavior of Ti-base metallic glass and composites

Melt-spun ribbons and copper-mold cast cylinders of (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)_100−xMo_x bulk glass-forming alloys are prepared. Both Ti₅₀Cu₂₃Ni₂₀Sn₇ and (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)₉₅Mo₅ melt-spun glassy ribbons exhibit large supercooled liquid regions, high reduced glass transition temperatures, and good thermal stabilities. During continuous heating of the melt-spun ribbons, both alloys present a two-stage crystallization behavior. Mo slightly lowers the glass-forming ability but significantly decreases the temperature of the second stage crystallization. For both alloys, the stable phases after heating are Ti₂Ni, TiCu, Ti₃Sn and β-(Cu,Sn). As-cast Ti₅₀Cu₂₃Ni₂₀Sn₇ cylinders contain dendritic hcp-Ti solid solution precipitates, as well as interdendritic glassy and Sn-rich crystalline phases. The ultimate compression stress reaches 2114 MPa with 5.5% plastic strain for 2-mm diameter cylinders. Yielding occurs at 1300 MPa, and Young’s modulus is 85.3 GPa. Mo improves and stabilizes the precipitation of a β-Ti solid solution but prevents glass formation in as-cast (Ti_0.5Cu_0.23Ni_0.2Sn_0.07)₉₅Mo₅ bulk alloys. The bulk samples contain dendritic β-Ti solid solution precipitates, Ti₂Ni particles and Sn-rich phases. The ultimate compression stress is 2246 MPa with about 1% plastic strain for a 3-mm diameter cylinder. σ_0.2 is about 1920 MPa and Young’s modulus is 104 GPa. The high strength is attributed to both Mo solution strengthening and Ti₂Ni particle strengthening. The limited ductility is induced by the precipitation of brittle Ti₂Ni particles.     

Structure and phase transformations in Fe–Ni–Mn alloys nanostructured by mechanical alloying

Ternary Fe₈₆Ni_xMn_14−x alloys, where x = 0, 2, 4, 6, 8, 10, 12, 14, 16 at.%, were prepared by the mechanical alloying (MA) of elemental powders in a high-energy planetary ball mill. X-ray diffraction analysis and Mössbauer spectroscopy were used to investigate the structure and phase composition of samples. Thermo-magnetic measurements were used to study the phase transformation temperatures. The MA results in the formation of bcc α-Fe and fcc γ-Fe based solid solutions, the hcp phase was not observed after MA. As-milled alloys were annealed with further cooling to ambient or liquid nitrogen temperatures. A significant decrease in martensitic points for the MA alloys was observed that was attributed to the nanocrystalline structure formation.     

Microstructures,Compressive Properties,and Microhardness of NiAl-Cr(Mo) Eutectic Alloys With Various Ni Contents

The microstructures and mechanical properties of 66(Ni_xAl)-28Cr-6Mo (x?=?1.0, 1.5, 2.0, 2.5, 3.0, and 3.5) alloys were investigated using scanning electron microscopy, energy dispersive spectroscopy, transmission electron microscope, microhardness, and compression tests. The microstructure of NiAl-28Cr-6Mo (Ni_1.0) eutectic alloy consists of NiAl and Cr(Mo) phases. With increasing the Ni content to 2.0, the microstructure changes from eutectic (Ni_1.0) to eutectic?+?primary NiAl dendrite (Ni_1.5 and Ni_2.0), and the morphologies of part of precipitates in primary NiAl dendrite evolve from granular to needle-like. When the Ni content increases further, besides eutectic and primary NiAl dendrite, the gray phase forms and is identified as an ordered FCC (L1₂) (Ni,Cr)₃(Al,Mo) phase. Moreover, the more needle-like precipitates emerge in the primary NiAl dendrite of Ni_2.5, Ni_3.0, and Ni_3.5 alloys, and the precipitate is identified as a bcc Cr(Mo) phase. The deep etching reveals that the three-dimensional morphology of Cr(Mo) precipitate is not needle-like but lath-like. Among the investigated alloys, both Ni_2.0 and Ni_2.5 alloys possess the higher fracture strength and microhardness. The relevant strengthening mechanisms are discussed.     

Microstructure and oxidation behavior of Nb–Mo–Si–B alloys

The microstructure and oxidation behavior of sintered Nb–Mo–Si–B alloys were investigated for the phase assemblies T₁ (Mo₅Si₃B_x)–MoSi₂–MoB, T₁–T₂ (Mo₅SiB₂)–Mo₃Si, and Mo–T₂–Mo₃Si in the Mo–Si–B system. In the Nb–Si–B system, T₂ (Nb₅(Si,B)₃) and D8₈ (Nb₅Si₃B_x) were investigated, while in the quaternary Nb–Mo–Si–B system, T₁–T₂–D8₈ was investigated. Alloys were oxidized at 1000 °C in flowing air. For Mo–Si–B compositions, the alloys showed excellent oxidation stability and initial mass loss of the alloy varied according to its Mo content. Minor quantities of MoO₂ were observed in the Mo–Si–B scales. The oxidation rates of Nb–Si–B and Nb–Mo–Si–B alloys were much larger than that of Mo–Si–B alloys. Their overall mass gains were significantly dependent on the initial heating atmosphere. In the Nb–Si–B system, T₂ and D8₈ alloys were more resistant to oxidation when heated to the test temperature in high purity argon. The quaternary Nb–Mo–Si–B alloy containing less D8₈ phase showed lower mass gains than that containing more D8₈ phase. Scales of the order of 10–50 μm thick were observed on Mo–Si–B alloys while much thicker scales, of the order of 200–600 μm, were observed on Nb–Si–B and Nb–Mo–Si–B alloys. Initial heating in argon resulted in denser scales and reduced the oxidation rate of Nb–Mo–Si–B alloys.     

Structure analysis of the constitutional phases in liquid phase sintered W–Mo–Ni–Fe heavy alloys

The crystal structures of constitutional phases in two different tungsten heavy alloys processed via different conditions were examined. The compositions of these two alloys were W–11.9Mo–17.0Ni–7.7Fe and W–29.6Mo–17.0Ni–7.7Fe (at.%), which were liquid phase sintered at 1500 °C for 5 or 240 min, and followed by either furnace-cooling or water-quenching. Increase in isothermal hold caused increased concentration of Mo but decreased concentration of W in the matrix phase, which did not affect the lattice parameter of the matrix phase to a significant extent. Quenching the specimen in water caused increase in the concentrations of both W and Mo in the matrix phase, and, consequently, increases in the lattice parameter of the matrix phase. A tungsten heavy alloy with a high alloying concentration of Mo was prone to induce the precipitation of an intermetallic phase during cooling, which was enhanced by increasing the isothermal hold at the liquid phase sintering temperature and decreasing the cooling rate. The structure of this intermetallic phase is analogous to that of MoNi, and can be designated as (W_xMo_1−x)(Fe_yNi_1−y). The composition of this intermetallic phase varied with the composition of the alloy and its cooling rate subsequent to sintering. For a furnace-cooling condition, the atomic ratio of W to Mo (x/1−x) in this intermetallic phase was about 0.47 times the atomic ratio of W to Mo of the original alloy composition. Such a proportional constant decreased to about 0.30 when the specimen was water-quenched.     

Measurement of 900 °C Isothermal Section in the Mo-Ni-Zr System

The isothermal section of the Mo-Ni-Zr system at 900 °C was investigated by characterization of eighteen equilibrium alloys. Electron probe microanalysis (EPMA) and x-ray diffraction (XRD) were used to identify the phases and obtain their compositions. The existence of two ternary compounds, Zr₆₅Mo_18?x Ni_16.5+x (τ₁, cF96-Ti₂Ni) and Zr₆₅Mo_27.3Ni_7.7 (τ₂, hP28-Hf₉Mo₄B), was confirmed in the Zr-rich corner, and the compositions of the two phases were determined. The isothermal section of the Mo-Ni-Zr system at 900 °C consists of 15 three-phase regions and 29 two-phase regions. The following three-phase equilibria were well established: (1) (Ni) + Ni₇Zr₂ + Ni₅Zr, (2) MoNi + MoNi₃ + Ni₇Zr₂, (3) Ni₇Zr₂ + MoNi + (Mo), (4) (Mo) + Ni₇Zr₂ + Ni₃Zr, (5) (Mo) + Ni₃Zr + Ni₂₁Zr₈, (6) (Mo) + Ni₂₁Zr₈ + Ni₁₀Zr₇, (7) (Mo) + Ni₁₀Zr₇ + NiZr, (8) (Mo) + Mo₂Zr + NiZr, (9) NiZr₂ + Mo₂Zr + τ₁, (10) τ₁ + Mo₂Zr + τ₂, (11) τ₂ + Mo₂Zr + (Zr)ht, (12) NiZr₂ + τ₁ + (Zr)ht and (13) τ₁ + τ₂ + (Zr)ht. Several binary phases, such as MoNi₃, Ni₇Zr₂ and Mo₂Zr, dissolve appreciable amount of the third component.     

Influence of Gd and B on solidification behaviour and weldability of Ni–Cr–Mo alloy

Abstract

The influence of Gd and B on the solidification behaviour and weldability of Ni–Cr–Mo alloy UNS N06455 has been investigated by Varestraint testing, differential thermal analysis and microstructural characterisation. These alloys are currently being developed as structural materials for nuclear criticality control in applications requiring transportation and disposition of spent nuclear fuel owned by the US Department of Energy. The Gd containing alloys were observed to solidify in a manner similar to a binary eutectic system. Solidification initiated with a primary L→y reaction and terminated at ~1258°C with a eutectic type L→y+Ni₅Gd reaction. The solidification cracking susceptibility of the Gd containing alloys reached a maximum at ~1 wt-%Gd and decreased with both higher and lower Gd additions. Low cracking susceptibility at Gd concentrations below ~1 wt-% was attributed to a relatively small amount of terminal liquid that existed over much of the crack susceptible solid+liquid zone. Low cracking susceptibility at Gd concentrations above ~1 wt-% was attributed to a reduced solidification temperature range and backfilling of solidification cracks. The addition of B above the 230 ppm level leads to the formation of an additional eutectic type reaction at ~1200°C and the secondary phase within the eutectic type constituent was tentatively identified as Mo₃B₂. The B containing alloys exhibited a three step solidification reaction sequence consisting of primary L→y solidification, followed by the eutectic type L→y+Ni₅Gd reaction, followed by the terminal eutectic type L→y+Mo₃B₂ reaction. Boron additions had a strong, deleterious influence on solidification cracking susceptibility. The high cracking susceptibility was attributed to extension of the crack susceptible solid+liquid zone induced by the additional eutectic type L→y+Mo₃B₂ reaction and extensive wetting of the grain boundaries by the solute rich liquid. Simple heat flow equations were combined with solidification theory to develop a relation between the fraction liquid f _L and distance x within the solid+liquid zone. Information on the phenomenology of crack formation in the Varestraint test were coupled with the calculated f _L–x curves and were shown to provide useful insight into composition–solidification–weldability relations.     

Reassessment of Ni-Al and Ni-Fe-Al solidus temperatures

Solidus temperatures of the B2 NiAl phase have been determined by high-temperature differential thermal analysis for binary melt compositions Ni_xAl_100?x (45<x<57) and for ternary alloys Fe_yNi_50?yAl₅₀ (0≤y≤50). It was shown that the melting temperature of the stoichiometric Ni₅₀Al₅₀ phase is 1681 °C, which is 43 K higher than some literature data. The solidus line at the Ni-rich side of the Ni-Al phase diagram exhibits a steeper slope than that reported previously. Substituting Fe for Ni, the decrease of solidus temperature along the isoplethal section with 50 at.% Al of the ternary Ni-Fe-Al phase diagram exhibits a steep initial slope of ?13 K/at.% Fe for small Fe-fractions, which changes into a nearly linear decrease with an average slope of ?8.5 K/at.% Fe.     

Phase continuity in high temperature Mo–Si–B alloys: A FIB-Tomography Study

The microstructure of mechanically alloyed Mo–Si–B materials of different compositions has been studied by X-ray diffraction and FIB-tomography. Three different phases were found in all alloys: α-Mo solid solution, T2-phase (Mo₅SiB₂-type) and A15-phase (Mo₃Si). The Mo–6Si–5B and Mo–9Si–8B (at%) alloys reveal a continuous morphology of the α-Mo phase. In Mo–9Si–8B the α-Mo phase is distributed more homogeneously compared to the Mo–6Si–5B alloy. In contrast, Mo–13Si–12B does not possess a continuous α-Mo phase. The consequences for the mechanical properties and oxidation resistance are discussed.     

Corrosion behavior of nanocrystalline (Ni70Mo30)90B10 alloys in 0.8 M KOH solution

Nanocrystalline materials are claimed to exhibit improved properties as a result of their novel microstructure. The corrosion behavior of nanocrystalline (Ni₇₀Mo₃₀)₉₀B₁₀ alloys prepared by crystallization from the amorphous state was studied and compared with that of their amorphous and coarse-grained counterparts. Special emphasis was given to the relationship between microstructure and corrosion resistance. It was concluded that nanocrystalline (Ni₇₀Mo₃₀)₉₀B₁₀ alloys are less sensitive to corrosion in alkaline solutions than the coarse-grained material. This behavior was related to their small grain size and homogeneous single phase microstructure. They provide a uniform substrate where it is easy to form a passive film, consisting essentially of Ni(OH)₂ for the alloy studied.     

Role of Mo in the local configuration and structure stabilization of amorphous steels,a Synchrotron X-ray diffraction and Mössbauer study

Amorphous steels are promising materials with potential structural applications. The glass-forming ability (GFA) and mechanical properties of metallic glasses are intimately related to the local structure. In Fe-based alloys, Cr and Mo content seem to play a key role in stabilizing the amorphous atomic-level structure. Here we present a study on the effects of changing Mo content in Fe_72?xC₇Si_3.3B_5.5P_8.7Cr_2.3Al₂Mo_x amorphous steels. We study the local structure of these alloys by Synchrotron X-ray diffraction and Mössbauer spectroscopy. The results show how the amorphous phase evolves from a ferromagnetic Fe-rich structure to a structure with predominance of paramagnetic environments with the increase of Mo content. The changes in the distribution of magnetic environments cannot be attributed only to the Fe–Mo substitution but to a change of local configuration in the amorphous phase.     

不同铸造工艺条件下B含量对Fe0.25Co0.25Ni0.25Cr0.125Mn0.125高熵合金组织及性能的影响

以(Fe_0.25Co_0.25Ni_0.25Cr_0.125Mn_0.125)_100-xB_x (x=8-16, at.%)合金为研究对象,采用真空电弧熔炼、真空水冷铜模吸铸和真空熔融甩带的方法制备出该系列合金的铸锭、细棒和带材。研究不同B含量对Fe_0.25Co_0.25Ni_0.25Cr_0.125Mn_0.125高熵合金在不同铸造工艺条件下的相组成、显微组织演变和力学性能变化。结果表明通过控制B含量和冷却速率,能够实现该体系高熵合金从fcc结构向非晶结构转变,制备出伪高熵合金。并且在相同的铸造工艺下,B含量的增加能细化晶粒,从而使得合金的强度和硬度随B含量增加而增大。     

Structure and properties of titanium-free maraging alloys

The changes in the structure, phase composition, and physicomechanical properties of titanium-free maraging alloys based on the Fe-15–23% Ni-(Co, Mo, V) system after heating to the single-phase α field and two-phase α + γ field have been studied. It has been established that the strengthening of N15K10M5F5-type maraging alloys is caused by the precipitation of fine particles (20–50 nm) of intermetallic phases such as the fcc Ni₃(Mo, V) phase and the Fe₂(Mo, V) Laves phase (in the N23K9M6 alloys, with the formation of the Ni₃Mo and Fe₂Mo phases). It has been shown that the two-step aging of the N15K10M5F5 alloy leads to an additional strengthening by 200–250 MPa and provides the achievement of the ultimate tensile strength σ_u=2400?2500 MPa. The high-strength N15K10M5F5 maraging alloys are obtained with two levels of the coercive force H _c: (a) semihard maraging alloys with H _c=20?50 Oe and σ_u=2100?2400 MPa; and (b) hard magnetic maraging alloys with H _c=180?230 Oe and σ_u=1500?1800 MPa. The high-strength titanium-free N15K10M5F5 and N23K9M6 maraging alloys possess many properties characteristic of structural, elastic, and magnetic alloys and are thus multifunctional materials. These alloys can be used for advanced high-tech articles and as high-strength magnetic materials.     

Structure and thermal behavior of multicomponent Fe68−xNixZr15Nb5B12 (x = 5, 10, 15, 20) alloys

Multicomponent Fe_68−xNi_xZr₁₅Nb₅B₁₂ (x = 5, 10, 15, 20) alloy powders milled for 60 h were prepared by mechanical alloying (MA). The structure and crystallization behavior were investigated by X-ray diffraction (XRD), scanning electron microscopy (SEM) and differential thermal analysis (DTA). Ni enhances the amorphisation of alloy powders. Particle size increases with increasing Ni content. Both onset crystallization temperature T_x and the first crystallization peak temperature T_p of the four alloys shift to a higher temperature with increasing heating rate while melting temperature (T_m) is just the opposite. Fe_68−xNi_xZr₁₅Nb₅B₁₂ (x = 5, 10, 15, 20) alloys all have a large supercooled liquid region ΔT_x. The supercooled liquid region ΔT_x increases and the crystallization activation energy E decreases with increasing Ni content.