Microstructure and mechanical properties of a directionally solidified and aged intermetallic Ni–Al–Cr–Ti alloy with β-γ′-γ-α structure

Microstructure and mechanical properties were investigated in a directionally solidified (DS) Ni–21.7Al–7.5Cr–6.5Ti (at.%) alloy. The dendrites of the as-grown alloy were composed of β(B2)-matrix (NiAl), coarse γ′(L1₂)-particles (Ni₃Al), fine γ′-needles and spherical α(A2)-precipitates (Cr-based solid solution). The majority of fine γ′-precipitates was found to be twinned. The interdendritic region contained γ(A1)-matrix (Ni-based solid solution) separating ordered domains of γ′-phase and fine lath-shaped α-precipitates. Ageing in the temperature range 973–1373 K decreased the volume fraction of dendrites from about 50 vol.% measured in the as-grown material to about 38 vol.% in the material aged at 1373 K for 300 h. During ageing in the temperature range 973–1273 K the γ-phase transformed to the γ′-phase in the interdendritic region. This transformation was connected with precipitation of lath-shaped α-precipitates. Ageing at higher temperatures of 1373 and 1473 K resulted in stabilisation of the γ-phase and precipitation of spherical γ′-particles in the interdendritic region. Ageing at 973 K significantly increased the microhardness, hardness and decreased room-temperature tensile ductility. Neither ageing nor finer dendritic microstructure were found to be effective in increasing the ductility of the alloy. The measured tensile ductility up to 1.1% can be attributed to the effect of extrinsic toughening mechanisms operating in the β-phase such as blunting and bridging of cracks by the α- and γ′-precipitates.     

Effect of ageing on the microstructure and mechanical behaviour of a directionally solidified Ni3Al-based alloy

The effect of ageing in the temperature range from 1023 to 1373 K on the micro-structure and mechanical behaviour of a directionally solidified (DS) Ni₃Al-based alloy modified with additions of chromium and iron was investigated. The microstructure of the as-grown alloy consisted of well-aligned and equally spaced lamellas composed of β(B2) intermetallic compound NiAl (Cr, Fe), some β′(L1₀) martensite and spherical -Cr precipitates. The matrix consisted of γ′(L1₂) intermetallic compound Ni₃Al (Cr, Fe), γ-phase (Ni-based solid solution) and lath-shaped -Cr precipitates. Ageing at 1123 and at 1173 K was found to be the most effective in transforming the unstable lamellae to γ′-phase and -Cr precipitates. The change of microstructural characteristics such as volume fraction of lamellae, size, morphology and distribution of γ′-phase, γ-phase and -Cr precipitates significantly influenced the room-temperature yield strength and elongation of DS alloy after ageing. The strain-hardening exponent varied with the ageing temperature between 0.30 and 0.46 and the quasi-steady work-hardening rate between 2710 and 5340 MPa. In the specimens with the lowest amount of disordered regions, the strain-hardening exponent was found to be 0.46 and the quasisteady work hardening rate was determined to be 3340 MPa.     

The oxidation of nanocrystalline Ni3Al fabricated by mechanical alloying and spark plasma sintering

Nanocrystalline Ni₃Al was fabricated through mechanical alloying of elemental powders and spark plasma sintering. The nanocrystalline Ni₃Al has a nearly full density after being sintered at 1223 K for 10 min under a pressure of 65 MPa. Isothermal and cyclic oxidations of nanocrystalline Ni₃Al were tested at 1173–1373 K with intervals of 100 K. The results indicate that nanocrystalline Ni₃Al exhibits excellent isothermal and cyclical oxidation resistance. The oxide scales consist primarily of dense and continuous -Al₂O₃. The grain refinement is beneficial for improving the oxidation resistance of Ni₃Al by providing more nucleation centers for the Al₂O₃ formation, promoting the selective formation of Al₂O₃ and improving the adhesion of oxide scales to the matrix.     

Structure and anelasticity of Fe3Ge alloy

The structure and anelastic properties of Fe-27 at.%Ge alloy are studied. Long-term annealing of the as-cast alloy at 1273 K leads to homogenising and several transformations take place below 873 K. These low temperature transitions are studied by several methods: X-ray diffraction, calorimetry, vibrating-sample magnetometry and internal friction, and are related to magnetic transitions in the different phases. A high stability of the hexagonal (D0₁₉) phase at room temperature is recorded. The hexagonal β (B8₁) phase is also detected in the alloy at room temperature, while the presence of the ′ and phases is doubtful. A broad internal friction relaxation peak with the relaxation strength of Δ = 0.0036, the activation energy of about 1.78 eV and the preexponential relaxation time of τ₀ = 2 × 10⁻¹⁷ s was discovered and classified as the Zener peak in both the and β phases.     

Influence of deformation on precipitation process in Mg–15 wt.%Gd alloy

Mg–Gd is a promising light hardenable alloy with a high creep resistance at elevated temperatures. The supersaturated solid solution of Gd in Mg decomposes in a sequence of the following phases: β″ (D0₁₉) → β′ (c-base centered orthorhombic-c-bco) → β (fcc) stable. Formation of the metastable β′ phase causes a strong hardening. Dislocations facilitate nucleation of precipitates. Dislocation density is, therefore, an important parameter which influences the precipitation process. This effect was examined in the present work by comparison the decomposition sequences in Mg–15 wt.%Gd alloy cold rolled to various thickness reductions. It was found that precipitation of the β′ phase starts at lower temperatures in the cold rolled specimens.     

Formation of interfacial voids in cast and micro-grained γ′-Ni3Al during high temperature oxidation

Specimens of cast and micro-grained γ′-Ni₃Al, which were produced with vacuum casting and unbalanced magnetron sputter deposition, respectively, were isothermally oxidised in air at 1473 K for different periods of time. The formation of interfacial voids at the alloy/oxide interface was observed with SEM, which indicated that there were more interfacial voids formed in the cast Ni₃Al than in the micro-grained alloy under the same oxidation conditions. A phenomenological equation describing the fraction of the void projected areas was established, in which the impingement and coalescence between voids during their growth was taken into consideration. It was elucidated that low vacancy density in the micro-grained Ni₃Al due to the high creep, re-crystallisation and the enhanced Al diffusion reduced the void percentage. Also, it was confirmed that aluminium evaporation, perhaps supplemented by surface diffusion, supplied most Al to the oxide scales formed above the interfacial voids.     

Laser beam weld-metal microstructure in a yttrium modified directionally solidified Ni3Al-base alloy

The microstructure of laser beam weld-metal of an yttrium doped directionally solidified alloy IC 6A, with chemical composition Ni–16Al–8.5Mo–0.12B–0.05C–0.03Y (at.%) was studied. The dendritic microsegregation observed within the fusion zone indicated that dendrite cores were slightly depleted in molybdenum and aluminum and the interdendritic regions were also considerably enriched in yttrium. Severe cracking in the weld-metal was observed and was found to be closely associated with interdendritic eutectic-type microconstituents identified as consisting of γ, γ′ and Ni–Mo phases. An yttrium-rich phase (Ni₃Y) was observed in some interdendritic regions containing the eutectic γ, γ′ and Ni–Mo products. Their formation was discussed in relation to plausible microsegregation induced alteration of primary solidification path during cooling from welding temperatures.     

The strengthening effect of (Ni,Fe)Al precipitates on the mechanical properties at high temperatures of ferritic Fe–Al–Ni–Cr alloys

Strengthening through a homogeneous distribution of a second phase is a concept that is widely employed in high-temperature materials. The most prominent among this group are nickel-based superalloys which owe their high-temperature strength to finely dispersed Ni₃Al particles. Similar microstructures can be obtained in the Fe–Al–Ni–Cr system with B2-ordered (Ni,Fe)Al precipitates in a ferritic matrix. These precipitates lead to an increase of high-temperature strength compared to conventional iron-base high-temperature alloys. However, secondary precipitates form during air cooling from high temperatures and affect the ductility. The results show that the ductility can be improved by a two-step aging treatment. Within the stress and temperature range investigated, the dependence of the secondary creep rate on the applied stress of aged alloys can be described by a power law if a threshold stress is introduced.     

Formation of 14H long period stacking ordered structure and profuse stacking faults in Mg–Zn–Gd alloys during isothermal aging at high temperature

This paper proposes a time–temperature-transformation diagram of an Mg–Zn–Gd alloy. An Mg₉₇Zn₁Gd₂ (at.%) alloy shows different precipitation sequences at low, medium and high temperatures. Low-temperature aging at <523 K brings about coherent β′-phase precipitation to a -Mg solid solution, resulting in increase in strength of the alloys. However, aging at medium and high temperatures >623 K led to strengthening of the Mg–Zn–Gd alloy, owing to the formation of profuse stacking faults and 14H long period stacking ordered structure from the supersaturated -Mg matrix, respectively.     

Evaluation of Ni–Ni3Al(5 wt.%)–Al(3 wt.%) as an anode electrode for molten carbonate fuel cell: Part I: Creep and sintering resistance

To develop an anode electrode of molten carbonate fuel cell (MCFC) that has higher creep and sintering resistance and more stable reactivity than the traditional anode electrode, the five kinds of nickel anode electrodes such as Ni–Al(5), Ni–Cr(10), Ni–Ni₃Al(7), Ni–Ni₃Al(5)–Cr(5) and Ni–Ni₃Al(5)–Al(3) (number in parenthesis means the weight percent of its component) were prepared and their performance were investigated under the same conditions of MCFC operation. In part I of our study, their relative creep and sintering resistance were compared.

The Ni–Ni₃Al(5)–Al(3) anode electrode was the most resistant against the sintering and its porosity was kept over 60% even at 1000 °C. And its porosity decrease and thickness shrinkage by creep were the least among the five kinds of anode electrodes. Thus, the effects of the aluminum and nickel–aluminum intermetallic compound (Ni₃Al) addition to nickel as the dispersant strengthening and the solid solution strengthening agents were confirmed by comparative tests for sintering and creep resistance of five kinds of anode electrodes.

Besides these results, we could also know that the creep of anode electrode for MCFC mostly proceeded within 60 h from the start of operation irrespective of the kinds of anode electrodes. And in part II of paper, for more information about the wetting ability and cell performances of these five kinds of anode electrodes, the measurement of wetting ability and unit cell operations were carried out.     

Reaction synthesis of Ni/Ni3Al multilayer composites using Ni and Al foils: High-temperature tensile properties and deformation behaviour

The effects of temperature on the tensile properties and deformation behaviour of Ni/Ni₃Al multilayer composites have been systematically investigated. With increasing tensile test temperature from room temperature to 600 °C, the ultimate tensile strength decreased. The ‘abnormal’ strengthening of Ni₃Al gave rise to a reduction in the capability for cooperative deformation between the Ni and the Ni₃Al layers at elevated temperatures. As a result, the ultimate tensile strength of the multilayer composites decreased at elevated temperatures. A mixture of transgranular cleavage and intergranular failure at relatively low temperatures, and an intergranular failure at elevated temperatures were observed in the fracture surfaces of the Ni₃Al layers in the multilayer composites. The splitting of the coarse precipitates along the {0 0 1} planes at 800 °C resulted from the differences in solubility of Al in Ni between room temperature and 800 °C.     

Microstructures and mechanical properties of Fe3Al-based Fe–Al–C alloys

In this paper results on the microstructures and mechanical properties of Fe₃Al-based Fe–Al–C alloys with strengthening precipitates of the perovskite-type κ-phase Fe₃AlC_x are presented. The alloys are prepared by vacuum induction melting and cast into Cu-moulds. The composition of the Fe₃Al matrix of the investigated Fe–Al–C alloys varies between 23 and 29 at.% Al. The ternary C-additions range from 1 to 3 at.%. The microstructures of the alloys are characterised by means of light optical microscopy (LOM). Phase identification is performed by means of X-ray diffraction (XRD). The strength of the alloys as a function of temperature is determined through compression tests. The room-temperature ductility is evaluated by tensile tests. The fracture surfaces of the tensile specimens are analysed using scanning electron microscopy (SEM).     

Effect of lamellar plates on creep resistance in near gamma TiAl alloys

Effect of interlamellar spacings on creep rate in Ti-48 at% Al alloy with fully transformed lamellar structure (FL) has been investigated at 1123 K/98 MPa. In addition, the correlation between creep rate and lamellar orientation to stress axis was elucidated by conducting creep tests at 1148 K/68.6 MPa for three single crystal (SC) specimens of Ti-48 at% Al with different lamellar orientations to stress axis. The difference in creep rate among FL specimens having different interlamellar spacings could not be defined in the early stage of transient creep. The onset of accelerating creep was slightly retarded by decreasing the inter-lamellar spacing. While the creep rate of the SC specimen whose lamellar orientation to stress axis 30–63 ° gradually decreases with increasing strain, and is larger than that of the FL specimen, the creep rate of the SC specimen with lamellar orientation nearly parallel to stress axis drastically decreases within small strain, and is 1/50 smaller than that of the FL specimen. This strong lamellar orientation dependence of creep rate is interpreted by the correlation between dislocation slip system and the lamellar plate.     

Control of microstructure and orientation distribution in Ni–Al-based (β/γ′) two phase alloys by thermomechanical processing

Microstructure and orientation distribution of two phase Ni–Al(β)/Ni₃Al(γ′) alloys obtained by thermomechanical processing were examined using electron back-scatter diffraction pattern technique. Cylindrical specimens were hot-compressed in the β phase region and subsequently annealed in the (β/γ′) two phase region. After the hot deformation, equiaxed β grains surrounded by high angle boundaries were homogeneously formed due to dynamic recrystallisation under adequate condition. Moreover, strong 1 1 1 fibre texture parallel to the compressive axis developed in the β phase because of the lattice rotation during hot deformation. After annealing in the two phase region, γ′ phase transformed from β phase with 1 1 1_β fibre texture satisfying the Kurdjumov–Sachs relationship and resulting in the formation of 1 1 0_γ′ fibre texture. Film-shaped γ′ phase preferentially often precipitated along the β grain boundaries and a large number of (β/γ′) boundaries were partially coherent. This thermomechanical processing was effective in controlling the crystallography of γ′ along the β grain boundaries.     

Microstructure control and high temperature properties of TiAl base alloys

An equiaxed fine grain structure, a γ grain structure with the precipitated ₂ laths, and a fully lamellar structure were obtained by the microstructure control using thermomechanical processing and heat treatment. The key to obtaining the equiaxed fine grain structure using isothermal forging is to decompose the lamellar structure and then produce the fine grain microstructure through dynamic recrystallization. TiAl base alloys consisting of fine equiaxed grains, in particular, Ti-39Al-9V consisting of the γ and B2 phases exhibited superplastic elongation of more than 600% at 1423 K. Creep rupture properties of TiAl binary alloys with various microstructures were studied in purified He in the temperature range from 1073 to 1373 K. Above 1173 K the precipitated ₂ phase improved the steady state creep rate and creep life. At 1023 K, the ₂ phase did not improve the creep rate, although the steady state creep rate decreased and the creep life increased as the γ grain size increased.     

Phase transformation and morphology of the intermetallic compounds formed at the Sn–9Zn–3.5Ag/Cu interface in aging

The morphology and phase transformation of the intermetallic compounds (IMCs) formed at the Sn–9Zn–3.5Ag/Cu interface in a solid-state reaction have been investigated by X-ray diffraction (XRD), transmission electron microscopy (TEM), electron diffraction (ED), scanning electron microscopy (SEM) and energy dispersive spectrometry (EDS). The monoclinic η′-Cu₆Sn₅ transforms to the hexagonal η-Cu₆Sn₅ and the orthorhombic Cu₅Zn₈ transforms to the body-centered cubic (bcc) γ-Cu₅Zn₈ as aged at 180 °C. The scallop-shaped Cu₆Sn₅ layer is retained after aging at 180 °C for 1000 h. In the solid-state reaction, Ag is repelled from η′-Cu₆Sn₅ and reacts with Sn to form Ag₃Sn, and the Cu₅Zn₈ layer decomposes. Kirkendall voids are not observed at the Sn–9Zn–3.5Ag/Cu interface even after aging at 180 °C for 1000 h.     

Coarsening of γ (Ni–Al solid solution) precipitates in a γ′ (Ni3Al) matrix; a striking contrast in behavior from normal γ/γ′ alloys

Coarsening of γ precipitates in a γ′ (Ni₃Al) matrix depends strongly on volume fraction, in dramatic contrast with coarsening of γ′ precipitates. Coalescence of γ precipitates is easy, unlike the resistance to coalescence of γ′ precipitates. We regard this as indirect proof that anti-phase relationships among γ′ precipitates inhibit coalescence.     

Deformation behaviour and oxidation resistance of single-phase and two-phase L21-ordered Fe–Al–Ti alloys

Single-phase Fe–Al–Ti alloys with the Heusler-type L2₁ structure and two-phase L2₁ Fe–Al–Ti alloys with MgZn₂-type Laves phase or Mn₂₃Th₆-type τ₂ phase precipitates were studied with respect to hardness at room temperature, compressive 0.2% yield stress at 20–1100 °C, brittle-to-ductile transition temperature (BDTT), creep resistance at 800 and 1000 °C and oxidation resistance at 20–1000 °C. At high temperatures the L2₁ Fe–Al–Ti alloys show considerable strength and creep resistance which are superior to other iron aluminide alloys. Alloys with not too high Ti and Al contents exhibit a yield stress anomaly with a maximum at temperatures as high as 750 °C. BDTT ranges between 675 and 900 °C. Oxidation at 900 °C is controlled by parabolic scale growth.     

Evaluation of Ni–Ni3Al(5 wt.%)–Al(3 wt.%) as an anode electrode for molten carbonate fuel cell. Part II: wetting ability and performance in unit cell operation

In part I of this paper, the sintering and creep resistances of the five kinds of anode electrodes were compared and those of Ni–Ni₃Al(5)–Al(3) were even better than any other electrodes. In part II of this paper, the wetting abilities of the same five kinds of anode electrodes to the electrolyte and their performance in unit cell operation were investigated. Their contact angles, which indicate the wetting ability, were within the range between 77 and 84°. The contact angles of Al- and/or Ni₃Al-added electrodes such as Ni–Al(5), Ni–Ni₃Al(7) and Ni–Ni₃Al(5)–Al(3) were relatively lower than those of Cr-added electrodes. Although there was no evidence that the effect of Al and/or Ni₃Al addition to pure Ni could enhance the number of pores or improve their structure for more wetting ability, it could be clearly known that the component of Al and/or Ni₃Al in anode electrode could make the electrode be wetted by the electrolyte very well.

In unit cell operation, the electric resistance of Ni–Al(5) and Ni–Ni₃Al(5)–Al(3) were relatively lower than those of any other electrodes. After 120 h operation of the unit cell, the cell performance and the endurance of Ni–Ni₃Al(5)–Al(3) were even better than those of any other electrodes. And also its thickness shrinkage and porosity changes after unit cell operation were the least in five kinds of electrodes. The performance of Ni–Ni₃Al(5)–Al(3) as anode seems to be caused by the synergy effect between the strengthening characteristics of Ni₃Al and electric conductivity of Al.     

The 773 K isothermal section of the Nd–Ni–V ternary system

The 773 K isothermal section of the phase diagram of Nd–Ni–V ternary system was investigated by X-ray diffraction (XRD), optical microscopy, scanning electron microscopy (SEM) and energy dispersion spectroscopy (EDS) techniques. There are in total 14 single-phase regions, 25 two-phase regions and 12 three-phase regions in the 773 K isothermal section. The maximum solid solubilities of V in Ni, NdNi₅, Nd₂Ni₇ and NdNi₂, is about 16.5 at.% V, 2.0 at.% V, 1.0 at.% V and 3.0 at.% V at 773 K, respectively, and that of Nd in Ni, Ni₃V, Ni₂V, Ni₂V₃, NiV₃ and V does not exceed 1 at.% Nd. No ternary compounds have been observed in this work.