The Fe–Ni–Al phase diagram in the Al-rich (>50 at.% Al) corner

The ternary Fe–Ni–Al phase diagram between 50 and 100 at.% Al was investigated by a combination of powder X-ray diffraction (XRD), differential thermal analysis (DTA) and electron probe microanalysis (EPMA). Ternary phase equilibria and accurate phase compositions of the respective equilibrium phases were determined within the isothermal section at 850 °C. The crystal structure of τ1 (Fe_4−xNi_xAl₁₀) and τ2 (Fe_2−xNi_xAl₉) was determined by means of single crystal X-ray diffraction. The decagonal quasi-crystalline phase q (Fe_4.9Ni_23.4Al_71.7) was found to be stable between 850 °C and 930 °C. All experimental data were combined to yield a ternary reaction scheme (Scheil diagram) involving 10 ternary invariant reactions in the investigated composition range, and a liquidus surface projection was constructed based on DTA results.     

Grain-boundary diffusion of nickel in Cr-, Fe-, and Zr-modified Ni3Al intermetallic

Tracer grain-boundary diffusion of ⁶³Ni in slightly hypostoichiometric (x_Al < 25 at.%) Ni₃Al alloys containing several percents of Cr, Fe and Zr was studied using both serial sectioning and residual activity methods. Measurements of grain-boundary diffusivity P were carried out in the temperature interval 873–1273 K. It was found that the additions of Cr, Fe and Zr decrease the P and increase the activation enthalpy of ⁶³Ni grain-boundary diffusion.     

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.     

The effects of iron on the creep properties of NiAl

A comparative study has been made of the creep behaviour of stoichiometric Ni₅₀Al₅₀ and (Ni₄₀Fe₁₀)Al₅₀ over the temperature range 600–750 °C. It is confirmed that the addition of Fe improves the creep resistance of NiAl, decreasing appreciably its steady state creep rate. The rate controlling process is dislocation climb in Ni₅₀Al₅₀, which gives rise to a well developed subgrain substructure while dislocation glide appears to control the creep in (Ni₄₀Fe₁₀)Al₅₀. Almost, all the dislocations have Burgers vectors with b=〈100〉, except for a few short segments with b=〈110〉 found in the dislocation networks. In addition, numerous prismatic dislocation loops are observed in crept NiAl; they may be associated with a supersaturation of vacancies and the precipitation of impurity particles at intermediate temperatures. Lastly, it is found that the strengthening mechanisms are solid solution hardening as proved by transient compressive test results and the decrease of the diffusion coefficient by Fe additions according to the evidence provided in the literature.     

Long term oxidation resistance and thermal stability of Ni-aluminide/Fe-aluminide duplex diffusion coatings formed on ferritic steels at low temperatures

A coating with a duplex structure consisting of an outer Ni₂Al₃ layer and an inner Fe₂Al₅ layer was formed on a commercial type of ferritic steel P92 using a two step process of electroless Ni/B plating followed by pack aluminising at 650 °C. Nearly 11,000 h oxidation test in air and more than 13,000 h isothermal annealing test in argon atmosphere were carried out to assess the long term oxidation resistance and thermal stability at 650 °C. The amount of oxidation was only about 0.66 mg/cm² for the coating as compared to 10.3 mg/cm² for the uncoated steel after nearly 11,000 h oxidation test. Inward Al diffusion took place during oxidation test, which led to the transformation of the outer Ni₂Al₃ layer to NiAl and increase in the Al diffusion depth. However, once the outer Ni₂Al₃ layer was completely transformed to NiAl, it stayed stable during the remaining period of oxidation test, providing long term oxidation resistance. Kirkendall voids formed and grew and then finally disappeared in the coating layers due to interdiffusion processes taking place at the oxidizing temperature at the interfaces between different layers of the duplex coating. No spallation was observed in the coating during the entire period of the oxidation or isothermal annealing tests.     

Martensitic transformation and magnetic properties in Ni–Fe–Ga–Co magnetic shape memory alloys

Ni–Fe–Ga–Co is a promising system for magnetic shape memory alloy applications, due to its good ductility, mobile twin boundaries and high transformation temperatures. Unlike previous studies which focused on compositions with a Ga content of 27 at.%, here the martensitic transformation and magnetic properties over a large composition range of Ni_54−xFe₂₀Ga₂₆Co_x, Ni_54−xFe₁₉Ga₂₇Co_x, Ni_56−xFe₁₇Ga₂₇Co_x and Ni_54−xFe₁₈Ga₂₈Co_x (x = 0, 2, 4) are investigated. The martensitic transformation temperature T_m and the Curie temperature T_c can be tailored in a wide range by changing composition and heat treatment. A coupling of martensitic and magnetic transformations at ∼90 °C is found for Ni₅₂Fe₁₇Ga₂₇Co₄. Additionally, the effect of thermal cycling on the martensitic transformation of single- and two-phase Ni–Fe–Ga–Co alloys is discussed. Furthermore, an intermediate face-centered cubic phase induced by powderization and transformed into a body-centered cubic phase by aging is reported. The saturation magnetization is significantly decreased by powderization, while recovered by the subsequent aging.     

Glass formation in La-based La–Al–Ni–Cu–(Co) alloys by Bridgman solidification and their glass forming ability

Six La-based La–Al–Ni–Cu–(Co) alloys were subjected to a systematical study of glass formation by Bridgman unidirectional solidification at growth velocities between 0.1 and 4.82 mm/s at a temperature gradient of 15 K/mm. With increased Cu content the critical growth velocity for glass formation in La₅₅Al₂₅Cu_20−xNi_x (x=0–20) alloys shows a steep minimum at 10 at.% Cu, indicating the largest glass forming ability for the La₅₅Al₂₅Ni₁₀Cu₁₀ alloy. However, replacement of Ni by Co leads to a further decrease in the critical cooling rate for the La₅₅Al₂₅Ni₅Cu₁₀Co₅ alloy. Critical cooling rates for glass formation in the present alloys were also obtained through a study of their melting and solidification behaviours by thermal analytical measurement. The effect of alloying addition and the significance of reduced glass transition temperature for the glass forming ability of these alloys is discussed.     

Development and characterization of cube-textured Ni–Cu–Co substrates for YBCO-coated conductors

Ni–Cu–Co alloys were studied for the development of textured substrates for YBCO-coated conductor application. Three compositions were obtained by adding a fixed amount of 3 at.% Co to the binary Ni_xCu_100?x, where x = 40, 50 and 60. Cube texture was induced by conventional cold rolling followed by high-temperature annealing. The structural, microstructural, morphological, electrical, magnetic, mechanical and oxidation properties were evaluated and compared with those exhibited by the binary Ni–Cu alloy, as well as by Ni–W and pure Ni. A low Ni content is detrimental for the development of the cube texture with respect to higher concentrations. Nevertheless, the use of high annealing temperatures enabled an area fraction of cube orientation as high as 95% to be obtained for x = 40, and >97.5% in the case of Ni-richer alloys. Compared with Ni and Ni–W, Ni–Cu–Co alloys oxidize more easily and exhibit higher electrical resistance. In addition, the presence of copper enables the Curie temperature to be reduced to 60 K for x = 40 and to 155 K for x = 50. Furthermore, the introduction of cobalt reduces the oxidation rate at temperatures normally used for the deposition of ceramic buffer layers, thus allowing the successful development of a CeO₂/YSZ/CeO₂ architecture on ternary Ni–Cu–Co alloy. YBCO/buffer multilayer architecture deposited by pulsed laser deposition on a selected alloy tape exhibits a critical current density exceeding 1 MA cm^?2 at 77 K in self-field, indicating that this alloy substrate is suitable for YBCO-coated conductor application.     

Correlation between composition and phase transformation temperatures in Ni–Mn–Ga–Co ferromagnetic shape memory alloys

The effect of Co addition on the phase transformation temperatures (martensitic and Curie point) and crystal structure of Ni–Mn–Ga–Co shape memory alloys has been investigated on (Ni_50.26Mn_27.30Ga_22.44)_100−xCo_x (x = 0, 2, 4, 6) alloys as well as on alloys having different Ni/Mn/Ga ratios and a fixed amount of Co. Alloying by Co affects the martensitic transformation temperature and the transformation enthalpy change mainly through the change on the valence electron concentration (e/a), but the transformation entropy is almost unaffected. On the other hand, the composition (analyzed through the e/a ratio) shows a different influence on the Curie temperature depending on the crystallographic phase (austenite or martensite) in which the magnetic ordering takes place. It is also reported that in Ni–Mn–Ga–Co alloys the Curie temperature of the martensitic phase is lower than that of the austenitic phase, opposite to what occurs in ternary Ni–Mn–Ga alloys.     

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.     

Modeling of phase stability of the fcc phases in the Ni–Ir–Al system using the cluster/site approximation method coupling with first-principles calculations

A combined CSA (cluster/site approximation)/FP (first-principles) calculation approach was employed to investigate the phase stability and thermodynamic properties of the face-centered cubic (fcc) phases of the Ni–Ir–Al system. For the constituent binaries of the Ni–Ir–Al system, enthalpies of formation of the Ni_xIr_1−x, Ni_xAl_1−x and Ir_xAl_1−x fcc compounds were calculated by first-principles approach at x = 0.75, 0.5 and 0.25 at 0 K, respectively. The pair exchange energies of the Ni–Ir and Al–Ir systems in the CSA model were obtained from FP calculated enthalpies of formation, while those for the Ni–Al binary were adopted from previous work. Thermodynamic model parameters of the fcc phases for the Ni–Ir–Al ternary system were then obtained from the constituent binaries via extrapolation. The calculated isothermal section at 1573 K is in good agreement with the experimental data within the uncertainties of the calculations and experiments.     

Hydrogen permeation and structural features of melt-spun Ni–Nb–Zr amorphous alloys

Amorphous (Ni_0.6Nb_0.4)_100−xZr_x (x = 0, 20, 30, 40 and 50 at.%) alloys were prepared by the melt-spinning technique, and the hydrogen permeation through those alloy membranes was examined. The local atomic structure in these alloys was also investigated by radial distribution function (RDF) analysis. Moreover, hydrogen solubility and diffusivity were also measured in order to discuss the mechanism for hydrogen permeation. The permeability of the Ni–Nb–Zr amorphous alloys increases with Zr content and temperature. The maximum hydrogen permeability is 1.59 × 10⁻⁸ mol m⁻¹ s⁻¹ Pa^−1/2 at 673 K for the (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy. The (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy showed larger hydrogen solubility and diffusivity than the (Ni_0.6Nb_0.4)₇₀Zr₃₀ amorphous alloy. As the result, the (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloy showed higher hydrogen permeability than the (Ni_0.6Nb_0.4)₇₀Zr₃₀ amorphous alloy at 673 K. The RDF analysis shows that the atomic distance between the Zr atoms increases by hydrogenation. The chemical ordering such that the number of Zr coordinates is much higher than that of Ni and Nb coordinates was found in the (Ni_0.6Nb_0.4)₇₀Zr₃₀ and (Ni_0.6Nb_0.4)₅₀Zr₅₀ amorphous alloys. The relation between the amorphous local structure and the permeation was discussed in detail.     

The mechanical properties of intermetallic Ni50−xTi50Alx alloys (x = 6,7,8,9)

High-strength, heat- and oxidation-resistant low density Ti–Ni–Al intermetallic alloys have recently attracted attention competing with some conventional high temperature structural superalloy such as Ni-based superalloy. In the present study, the mechanical properties of Ti-rich Ni_50−xTi₅₀Al_x (x = 6,7,8,9) alloys were examined by compression tests at room temperature and at high temperature from 400 °C to 800 °C. X-ray diffraction, scanning electron microscopy as well as microhardness tester were utilized to characterize the microstructure as well as the structural evolution with the increasing Al additions. The systematic analyses of the mechanical behavior were made according to compression test at different temperatures. A yield stress of 1800 MPa and more than 10% of compression strain were achieved at room temperature; and a yield stress of 400 MPa at 800 °C. It is suggested that controlling the shape, the volume percent and the distribution of second phases in the matrix is most important to obtain good mechanical properties in these alloys. The strengthening mechanism of aluminum addition on the mechanical properties was discussed systemically according to the microstructure evolution and solution hardening and precipitation hardening upon Al addition.     

Strain glass in Fe-doped Ti–Ni

We report the existence of strain glass in Ti₅₀Ni_50?xFe_x, beyond a critical Fe doping level x > x_c(5 < x_c < 6). The strain glass state is confirmed by the appearance of a frequency-dependent anomaly in the AC mechanical modulus/loss at a freezing temperature T₀ and by the breaking of ergodicity shown in a zero-field-cooling/field-cooling experiment. Based on the experimental results, a phase diagram is established in which both the normal martensitic transformations (for x < x_c) and the strain glass transition (for x > x_c) are indicated. The new phase diagram allows for explanations of two long-standing puzzles in Ti₅₀Ni_50?xFe_x and Ti–Ni alloys: (i) the origin of nano-domains present prior to the martensitic transformation (for x < x_c) and (ii) the negative temperature dependence of electrical resistivity in abnormal non-transforming compositions (for x > x_c).     

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.     

On the constitution of the ternary system Al–Ni–Ti

The constitution of the ternary system Al–Ni–Ti is investigated. Phase equilibria at 1000 °C and 900 °C are clarified confirming the stability of the four ternary phases τ₁ to τ₄ already described in the literature. An additional ternary phase τ₅ occurs as an equilibrium phase in the Al-rich corner. The composition of this phase is Al₆₅Ni₂₀Ti₁₅. The ternary phases τ₃ and τ₄ melt congruently at 1289 °C and ∼1500 °C, respectively. The ternary phases τ₁, τ₂ and τ₅ are found to melt incongruently at 1347 °C, 1225 °C, and 1107 °C, respectively. Two ternary eutectics occur: E₁ (τ₄ + Ni₃Al + TiNi₃) at 1269 °C and E₂ (τ₃ + τ₄ + NiAl) at 1221 °C. The composition reported for E₁ in the literature is corroborated. The composition for the newly discovered E₂ is determined to be 43 at%Al, 31 at% Ni, and 26 at% Ti. The section NiAl–NiTi is pseudobinary, but NiTi melts incongruently into L + τ₄ rather than congruently as binary NiTi. The section Ni₃Al–Ni₃Ti is eutectic, but not pseudobinary. By adding aluminium apparently all invariant equilibria of the binary system Ni–Ti are shifted to higher temperatures.     

Topological instability and glass forming ability of Al–Ni–Sm alloys

The thermal crystallization of Al-based metallic glasses can be described in association with the topological instability λ criterion. In the present work, we report on the crystallization behavior and glass forming ability of Al-rich, Al–Ni–Sm alloys, designed with compositions corresponding to the same topological instability condition of λ ≈ 0.1. Amorphous melt-spun alloys were prepared with the following compositions, varying the ratio of Ni and Sm elements: Al_87.5Ni₄Sm_8.5, Al_83.5Ni₁₀Sm_6.5, Al_80.5Ni_14.5Sm₅ and Al_76.5Ni_20.5Sm₃. The glass forming ability of each alloy composition was evaluated based on the thermal parameters obtained from DSC runs and on X-ray diffraction patterns. Better glass forming ability was observed in compositions whose Sm content was increased and Ni content reduced. Thermal crystallization of the alloys with low Sm content showed only one crystallization peak and no glass transition event. In alloys with higher rare-earth content, a glass transition event was clearly detected before the crystallization event. The results are interpreted considering the different types and proportions of Sm–Al and Ni–Al clusters that can be formed in the alloys along the λ ≈ 0.1 line. They also emphasize the relevance of these different types of clusters in the amorphous phase in defining the stability of the glass and the types of thermal crystallization.     

Effect of solute segregation on the strength of nanocrystalline alloys: Inverse Hall–Petch relation

We have used a high-energy ball mill to prepare single-phased nanocrystalline Fe, Fe₉₀Ni₁₀, Fe₈₅Al₄Si₁₁, Ni₉₉Fe₁ and Ni₉₀Fe₁₀ powders. We then increased their grain sizes by annealing. We found that a low-temperature anneal (T < 0.4 T_m) softens the elemental nanocrystalline Fe but hardens both the body-centered cubic iron- and face-centered cubic nickel-based solid solutions, leading in these alloys to an inverse Hall–Petch relationship. We explain this abnormal Hall–Petch effect in terms of solute segregation to the grain boundaries of the nanocrystalline alloys. Our analysis can also explain the inverse Hall–Petch relationship found in previous studies during the thermal anneal of ball-milled nanocrystalline Fe (containing ∼1.5 at.% impurities) and electrodeposited nanocrystalline Ni (containing ∼1.0 at.% impurities).     

Ternary Sm–Al–Ni bulk metallic glasses

The present paper is concerned with the formation of the ternary Sm-based Sm–Al–Ni bulk metallic glasses. Composition design is carried out using our e/a- and cluster-related criteria. Three bulk metallic glasses, Sm₅₄Al₂₃Ni₂₃, Sm₅₆Al₂₂Ni₂₂ and Sm₅₈Al₂₁Ni₂₁, are obtained by suction casting into rods with diameter of 3 mm. All of them share a constant e/a = 1.5 and fall along the e/a-constant composition line in the ternary composition diagram. The Sm₅₄Al₂₃Ni₂₃ BMG exhibits the best thermal stability and glass-forming ability, which is located at the intersecting point of the e/a-constant line and the Sm₇Ni₃–Al cluster line.     

An investigation of the Al–Ni–Rh phase diagram between 50 and 100 at% Al

The Al-rich part of Al–Ni–Rh was studied between 800 and 1080 °C. The Al₉Rh₂ phase was found to contain up to 8 at% Ni. The orthorhombic Al–Rh ɛ₆-phase extends up to 17.5 at% Ni, high-temperature cubic C-Al₅Rh₂ up to about 10 at% Ni while low-temperature hexagonal H-Al₅Rh₂ extends up to 4 at% Ni. The Al₇Rh₃ phases contained up to 3 at% Ni. The solubility of Rh in Al₃Ni is up to 3 at% and in Al₃Ni₂ up to 5 at%. The isostructural binary AlNi and AlRh phases probably form a continuous β-range of the CsCl-type solid solutions. A ternary hexagonal phase similar to Al₂₈Ir₉ (a = 1.213 and c = 2.626 nm) was found to be formed between Al₇₆Ni₄Rh₂₀ and Al₇₆Ni₁₃Rh₁₁. The formation of the high-temperature stable decagonal phase was confirmed. Another ternary phase, whose structure is not yet clarified, was revealed around Al₇₀Ni₁₁Rh₁₉. Partial 1080, 1000, 900 and 800 °C isothermal sections of the Al–Ni–Rh phase diagram are presented.