The effect of off-stoichiometry on the microstructures and tensile properties of Ni3Al–Ni3V pseudo-binary alloys was investigated by a scanning electron microscope, a transmission electron microscope, Vickers hardness test, and high-temperature tensile test. As the alloy deviates from a just-stoichiometric composition toward Ni-rich one, the microstructures constituted by two ordered phases, Ni3Al and Ni3V changed to those constituted by two ordered phases, Ni3Al and Ni3V, and one disordered phase, Ni solid solution. Also, the deviation from the stoichiometric composition resulted in a decrease in flow strength as well as Vickers hardness and conversely increase in tensile elongation. Higher tensile elongation in the off-stoichiometric alloys was induced by the transition from intergranular fracturing to transgranular fracturing. The trade-off relation in the yield strength (or hardness) versus tensile elongation curve, which was drawn plotting the data obtained from the alloys with different off-stoichiometric compositions, was most excellent at 600 °C but rapidly became worse at high temperatures beyond 600 °C. It was demonstrated that the deviation to the off-stoichiometric composition in the two-phase Ni3Al–Ni3V pseudo-binary alloy system was a useful alloying parameter to improve the balance of the flow strength and tensile ductility.

Mechanism for the hardening of two-phase Ni3Al-Ni3V intermetallic alloy to which 2 at.% Ta was added in different substitution manners for Ni, Al and V was presented, based on the microstructural observation, alloying behavior and lattice properties of the additive in the constituent phases. The hardening behavior was explained in terms of solid solution hardening in which the mixture rule in the volume fraction of the two constituent phases and the atomic size misfit evaluated from the changes of the lattice parameters were incorporated. Consequently, the hardening for the alloys in which the additives were substituted for Ni and V was attributed to solid solution hardening. On the other hand, the hardening for the alloy in which the additive was substituted for Al was attributed to the hardening due to microstructural refining in addition to the solid solution hardening.

The two-phase Ni3Al and Ni3V intermetallic alloy laser clad on substrate of SUS304 was evaluated by hardness measurement, scanning electron microscopy, electron probe microanalyzer and transmission electron microscopy observations, focusing on the effect of post annealing after laser irradiation. The laser clad coating layer was diluted with approximately 5.4 at.% Fe and 1.6 at.% Cr stemming from the substrate. In the as-clad coating layer, the inhomogeneous eutectoid microstructure due to incomplete phase separation into two intermetallic phases Ni3Al and Ni3V took place. The hardness in the as-clad coating layer was lower than that in post annealed coating layers. In the coating layer annealed at 1553 K for 5 h, a dual two-phase microstructure composed of the primary cuboidal Ni3Al surrounded by the channel regions in which the major constituent is the Ni3V phase was observed, indicating that the complete eutectoid microstructure was developed. In the coating layer annealed at 1248 K for 24 h, the developed microstructure was lamellar-like one composed of the Ni3Al and Ni3V phases. The hardness in the coating layer annealed at 1248 K was the highest in the coatings observed in this study.

Ni base intermetallic alloy coating was fabricated by laser cladding, controlling the laser power and powder feed rate. Atomized powder of the Ni base intermetallic alloy was laser-cladded on the substrate of stainless steel 304. The hardness and microstructure of the clad layers were investigated by Vickers hardness test, SEM, XRD and TEM observations. The hardness of the cladding layer was affected by the dilution with the substrate; it increased with decreasing laser power and increasing powder feed rate. By optimizing the dilution with the substrate, the cladding layer with an almost identical hardness level to that of the Ni base intermetallic alloy fabricated by ingot metallurgy was obtained. The TEM observations revealed that a very fine-sized microstructure composed of Ni3Al and Ni3V was partially formed even in the as-cladded state. After annealing, the two-phase microstructure composed of Ni3Al and Ni3V was developed in the cladding layer, resulting in enhanced hardness in the cladding layers fabricated in the majority of cladding conditions.

The microstructures and mechanical properties of ternary (Ni75Al9V16) and multicomponent (Ni69Al9V9.5Nb3Ti1.5Co4Cr4) dual two-phase intermetallic alloys charged with C were investigated by scanning electron microscope, electron probe microscopic analyzer, x-ray diffraction, Vickers hardness, and tensile tests. Solid solubility limits of C in the dual two-phase microstructures were small, mostly less than 0.1 at.%. When C was charged exceeding the solid solubility limit, large-sized carbides were solidified with no structural coherency with the dual two-phase microstructure. Major transition metals constituting the carbides changed from Nb and Ti to V with increasing charged C content. This transition was correlated with the capability of the carbide formation. C dissolving in the dual two-phase microstructure enhanced hardness and flow strength through solid solution hardening without sacrificing tensile ductility. The formed carbides little contributed to strengthening, rather, contributed to softening through depleting constituent element V as well as alloying metals Nb, Ti, and Cr from the dual two-phase microstructures.

The effect of Si addition on microstructure and mechanical properties of dual two-phase intermetallic alloys was investigated. Si was added to the base alloy composition Ni75Al9V13Nb3 + 50 wt. ppm B by three substitution ways in which Si was substituted either for Ni, for Al and for V, respectively. The alloys added with 1 at.% Si showed a dual two-phase microstructure composed of Ni3Al (L12) and Ni3V (D022) phases, while the alloys added with over 2 at.% Si exhibited the same dual two-phase microstructure but contained third phases. The third phases were G phase (Ni16Si7Nb6) and A2 phase (the bcc solid solution consisting of Nb and V). Yield and tensile strength of the 1 at.% Si-added alloys were high in the alloy in which Si was substituted for Al but low in the alloys in which Si was substituted for Ni or for V, in comparison with those of the base alloy. Tensile elongation was lower than that of the base alloy irrespective of substitution ways. The density of the Si added alloys was close to or slightly lower than that of the base alloy. Oxidation resistance of the Si added alloy was increased. Si addition to the dual two-phase intermetallic alloys is beneficial for reducing the density and enhancing the oxidation resistance without a harmful reduction of strength properties.

The effect of W addition on microstructure and mechanical properties of Ni3Al (L12) and Ni3V (D022) two-phase intermetallic alloys has been investigated. W was added to the base alloy composition, Ni75Al10V12Nb3 (at. %) in place of either Ni, Al or V. The W-added alloy ingots were heat-treated in vacuum at 1575 K for 5 h. The majority of W-added alloys showed a dual two-phase microstructures while the alloy in which 3 at. % W substituted for Ni exhibited the dual two-phase microstructure containing W solid solution dispersions. Vickers hardness was significantly enhanced by W addition, which is primarily due to solid-solution strengthening.

A Ni3(Si,Ti) intermetalic alloy was synthesized by the powder metallurgy method using elemental powders. The raw powder mixtures with various compositions were sintered by a spark plasma sintering apparatus and then homogenized at high temperatures. Microstructure, hardness, tensile properties and density of the sintered alloys were investigated as functions of the chemical composition and sintering temperature. It was found that a highly-densified Ni3(Si,Ti) sintered alloy was obtained by choosing proper chemical composition and sintering temperature. Also, the Ni3(Si,Ti) sintered alloy with an L12 single-phase microstructure exhibited high hardness and tensile strength.

The objective of this study is to establish alloy designing which can reduce the amount of V for a Ni-base dual two-phase intermetallic alloy, without degenerating the dual two-phase microstructure. It was demonstrated that the favorable dual two-phase microstructure will be maintained as far as the valence electron concentration (e/a) of the alloys added by Cr is not so much different from that of the base alloy (i.e. the alloy without additive elements). Consequently, it was found that the dual two-phase microstructure was maintained even though the amounts of V were reduced by 7 at.%, 7 at.%, and 10at.% by substituting of Cr for V, Cr for both of Ni and V, and Cr for Ni, respectively. The hardness of the alloys with reduced V content was higher than that of the base alloy.

The effect of fine M2C particles on the recrystallization temperature and high temperature strength of warm rolled Fe3Al base alloys was investigated. Fe-27Al-1.2C-2Cr-xMo (x: 0.3, 0.9) alloys (in at.%) were arc melted, warm rolled and annealed. TEM observations have revealed that fine M2C particles were present in the alloy containing 0.9% Mo but not in the alloy with 0.3% Mo after warm rolling. The recrystallization temperature increased from 740 °C to 810 °C when the Mo content is increased from 0.3 to 0.9 due to the presence of fine M2C particles. Tensile tests conducted on annealed samples with fine sub-grained matrix have shown that the introduction of fine M2C particles is effective to enhance the proof stress at 600 °C.

The aim of this study is two-fold: first we reexamine the thermodynamic stability of γ’-Co3(Al,W) phase in the Co-Al-W ternary system. Secondly, we investigate the effect of a fourth alloying element (Ti or Ta) on the thermodynamic stability of the γ’ phase through microstructure observation, DSC measurement and EPMA analysis. Coarsened areas with γ/CoAl/Co3W phases are formed after annealing at 900 ºC for 2000 h in Co-Al-W ternary alloys with different Al/W ratios, which confirms that the three phases are in equilibrium with each other and that the γ’ phase is metastable at this temperature. The addition of a fourth alloying elements does not drastically change the microstructure formed after long term annealing at 900 ºC, indicating that the alloying elements do not improve the stability of the γ’ phase.

The composition areas of the A2/L21 two-phase field in the Fe-Al-Ti-Cr quaternary system at 800 ºC and 700 ºC were investigated using a diffusion-multiple (DM) technique for which Fe-30Al-7Ti, Fe-30Cr-7Ti and Fe-7Ti alloys were diffusion-coupled and heat-treated. The A2/L21 two-phase field was found to expand slightly toward lower Al content when the Cr content is increased. The field is slightly shifted toward lower Al content when the temperature is decreased.

The effect of microstructure on cold-rolling workability and tensile properties of Ni3Si (L12)−Ni3Ti (D024)−Ni3Nb (D0a) multi-phase intermetallic alloys was investigated. The cast alloys with different microstructures containing the D024 phase and/or the D0a phase particles in the L12 matrix were homogenized and then cold rolled. For the alloys with the microstructure consisting of coarse plate-like D024 particles in the L12 matrix, serious cracks initiated at the coarse D024 particles in the early stage of the cold rolling process and then propagated, resulting in failure of the rolled plate. On the contrary, for the alloys with the microstructure consisting of fine needle-like D024 precipitates and/or granular-shaped D0a particles, these second phase particles did not spoil the cold workability, leading to successful cold rolling to 90 % reduction. After 90 % cold rolling, the rolled sheets were fully recrystallized at 1173 K for 1 h, resulting in the formation of a fine-grained microstructure. The room-temperature tensile strength and the yield stress of the recrystallized sheet were remarkably enhanced compared with that of the unrolled alloys, possibly due to the fine-grained microstructure as well as the particle hardening. Also, the high-temperature tensile strength and the elongation were improved in the recrystallized sheets compared to an L12 single-phase Ni3(Si,Ti) alloy sheet. Consequently, it was found that the cold rolling and annealing process was beneficial to improve the tensile properties for the present multi-phase intermetallic alloys.

The effect of Nb addition on phase equilibria among Ni (A1), Ni3Al (L12) and Ni3V (D022) phases was investigated in the Ni-rich Ni-Al-V ternary system. The addition of Nb to the Ni-Al-V ternary system shifts the three-phase coexisting region of A1 + L12 + D022 towards the Ni-rich side at 950 ºC. Nb is partitioned into the D022 phase more strongly than the A1 and L12 phases. These results suggest that the addition of Nb stabilizes the D022 phase against the A1 and L12 phases in the systems. The alloying element raises the temperature of a eutectoid-like reaction (A1→L12+D022) by ~30 ºC in the vertical section of Ni-Al-V ternary system at 75 at. % Ni.

The effect of grain boundary (GB) and matrix precipitates on high temperature strength was investigated in Fe3Al base alloys containing Cr, Mo and C. Tensile tests were conducted at 600°C for three types of microstructures consisting of: (I) film-like κ phase precipitates covering GBs and fine M2C particles in the matrix, (II) only fine M2C particles in the matrix and (III) no second-phase particles in the matrix. It was found that κ films on GBs are more than twice as effective as finely dispersed M2C particles for improving the proof stress.

The refractory element Ta was added to L12-type Ni3(Si,Ti) intermetallic alloys in order to substitute for Ti. The microstructure and the mechanical properties of the alloys were investigated as a function of the Ta content. All the alloys were doped with 50 wt.ppm boron to suppress intergranular fracture. The alloys containing up to 5 at.% Ta showed an L12 single-phase microstructure, while the alloys containing more than 5 at.% Ta exhibited a two-phase microstructure consisting of Ni3Ta particles in the L12 matrix. At room-temperature (RT), the hardness of the alloys with the L12 single-phase microstructure increased with increasing Ta content due to solid solution hardening of Ta. RT yield stress (0.2% yield strength) and tensile ultimate strength of the alloys with the L12 single-phase microstructure increased with increasing Ta content keeping a high level of tensile elongation. At high-temperature (HT), the positive temperature dependence of hardness was observed in all the alloys, irrespective of Ta addition. HT hardness was also enhanced by the addition of Ta. It was consequently demonstrated that Ta is a remarkable solid solution hardening element in the Ni3(Si,Ti) alloys.

A two-phase intermetallic alloy composed of Ni3Al (L12) and Ni3V (D022) was plasma-nitrided or -carburized in dependence of temperature and time. It was found that the hardness of the surface layer of the present intermetallic alloy was enhanced by both plasma-nitriding (PN) and -carburizing (PC), and primarily depended on treating temperature; the maximum surface hardness of the alloy was shown by nitriding at around 850 K and by carburizing at 1025 K. In addition, the hardened layers due to PN and PC effectively kept their hardness up to a high temperature. The XRD analysis revealed that vanadium nitride (VN) and vanadium carbide (VC) were formed in the surface of the nitrided and carburized alloy, respectively, suggesting that the enhanced surface hardness was attributed to the dispersion hardening due to the nitrides and carbides.

Four kinds of L12-type Ni3(Si,Ti) intermetallic alloys with a quaternary element X (X: Al, Cr, Co and Mo) were warm rolled accompanied by intermediate annealing and then cold rolled to thin foils. The effects of alloying element on microstructure, tensile properties and oxidation resistance of the cold-rolled Ni3(Si,Ti) foils were investigated. The Al-added Ni3(Si,Ti) alloy showed an L12 single-phase microstructure, while the Cr-, Co- and Mo-added Ni3(Si,Ti) alloys exhibited a two-phase microstructure consisting of L12 and fcc Ni solid solution phases. Room-temperature strength of the Ni3(Si,Ti) foils was slightly enhanced by the addition of quaternary element, whereas high-temperature strength was significantly enhanced especially by the addition of Mo and Co. High-temperature tensile elongation was remarkably improved by the addition of all the elements investigated. On the other hand, oxidation resistance was improved by the addition of Al and Cr.

The effects of annealing temperature on the room temperature tensile properties were investigated in a fine-grained Fe3Al based-alloy, and the correlation between microstructure, texture and tensile properties was discussed. Tensile elongation showed a peak as a function of annealing temperature. The highest elongation was obtained in the recrystallized samples annealed at 700°C. The decrease in the elongation with increasing annealing temperature above 700°C is attributed to the increase in the fraction of <100> oriented grains with respect to the tensile direction. A high sensitivity to environmental embrittlement was observed in the partially recrystallized samples annealed at 650°C.