Yasuyuki Kaneno

Phase relation and microstructure in Ni3Al–Ni3Ti–Ni3Nb pseudo-ternary alloy system

Abstract The phase relation and microstructure of alloys based on the Ni 3 Al–Ni 3 Ti–Ni 3 Nb pseudo-ternary alloy system at 1273 K were investigated by an optical microscope, an X-ray diffraction and a scanning electron microscope (attached with a wavelength dispersive spectroscope). As the constituent intermetallic phases, L1 2 (Ni 3 Al), D0 24 (Ni 3 Ti), D0 a (Ni 3 Nb) and D0 19 (Ni 3 Ti 0.7 Nb 0.3 ) were identified and then their phase fields were shown to depend upon the electron concentration ( e/a ) and the atomic size factor ( R x / R Ni ) of constituent atoms. Among four intermetallic phases identified, five kinds of two-phase relations (i.e. L1 2 –D0 24 , D0 24 –D0 19 , D0 19 –D0 a , D0 a –L1 2 and D0 24 –D0 a ) and two kinds of three-phase relations (i.e. L1 2 –D0 24 –D0 a and D0 24 –D0 19 –D0 a ) were found to exist. Also, D0 24 (Ni 3 Ti) phase extended up to concentration field in which a majority of constituent Ti elements were replaced by Al and Nb elements, keeping their concentration ratios identical. The prepared alloys exhibited widely different microstructures, depending on whether their alloy compositions exist in a single phase, a two-phase or a three-phase region, and also on what kind of intermetallic phases their alloy compositions are composed of.

Microstructures and mechanical properties of NbCr2 and ZrCr2 Laves phase alloys prepared by powder metallurgy

Microstructures, mechanical properties and oxidation behavior were investigated on NbCr2 and ZrCr2 Laves phase alloys prepared by powder metallurgy (P/M), and also by arc-melting, i.e. ingot metallurgy (I/M). These properties were also evaluated, in terms of alloying, heat treatment and alloy stoichiometry. High-temperature yield strength and brittle ductile transition temperature (BDTT) were generally lower in alloys prepared by P/M process than in those prepared by I/M process while micro hardness and fracture toughness were higher in alloys prepared by P/M process than in those prepared by I/M process, irrespective of NbCr2 or ZrCr2 alloys. Also, high-temperature strength and micro hardness were higher in NbCr2 alloys than in ZrCr2 alloys while fracture toughness was lower in NbCr2 alloys than in ZrCr2 alloys, irrespective of P/M or I/M process. For oxidation behavior at 1223 K, NbCr2 alloys showed linear increase with increasing time accompanied with irregular fluctuation, while ZrCr2 alloys showed parabolic increase with increasing time. It was also found that alloy stoichiometry greatly affected micro hardness, fracture toughness and oxidation behavior in ZrCr2 alloys.

Microstructures and defect structures in ZrCr2 Laves phase based intermetallic compounds

Microstructures of Zr–Cr binary alloys based on the ZrCr2 Laves phase are characterized by optical and scanning electron microscopy, electron probe analysis, X-ray diffraction, transmission electron microscopy, and density measurement. Eutectic compositions corresponding to the reactions of liquid→Cr solid solution+ZrCr2 and liquid→Zr solid solution+ZrCr2 exist at approximately 20.5 at.%Zr and 72 at.%Zr, respectively. The ZrCr2 Laves phase has the solid solution range between 31 at.%Zr and 34 at.%Zr, and more extends in Cr-rich side of stoichiometry. Alloy stoichiometry and heat treatment affect the phase stability and microstructure of the ZrCr2 Laves phase. Atomistic defect structures introduced in Cr-rich and Zr-rich sides of stoichiometry are of the anti-site type and a mixture of the anti-site type with the vacancy-type, respectively. The determined defect structures are discussed, on the basis of the geometrical model, i.e. the atomic size factor.

Tensile properties of L12 intermetallic foils fabricated by cold rolling

Abstract Polycrystalline L12-type Ni3(Si,Ti), Ni3Al and Co3Ti alloys prepared through thermomechanical processing from arc-melted ingots were successfully cold-rolled to thin foils with a thickness of less than 200 m. The cold-rolling with over 90 % reduction in thickness was possible without providing intermediate annealing. The cold-rolled foils showed high tensile strength (∼2 GPa) at room temperature although no plastic elongation was observed. The tensile strength of the annealed foils generally decreased acquiring a certain level of fracture strain. Room temperature fracture strain increased with increasing annealing temperature, and reached to 30 – 40 % by a high temperature annealing at 1173 K. Among three kinds of intermetallic alloys, the Ni3(Si, Ti) foil annealed around at 900 K exhibited an extremely high tensile strength and yield strength (over 2 GPa) with a reasonable fracture strain. Also, it was found that the fully-recrystallized Ni3(Si, Ti) and Co3Ti foils showed a strength anomaly at intermediate testing temperature. The observed tensile properties, especially tensile strength at low temperature as well as at high temperature for the present L12 intermetallic foils, were found to be superior to those for the conventional alloys such as nickel based alloys and stainless steels.

The effect of Cr addition on mechanical and chemical properties of Ni3Si alloys

Abstract The alloying behavior and microstructure of the Ni–Si–Cr ternary alloys were firstly characterized by optical microscopy, X-ray diffraction (XRD) and scanning electron microscopy (SEM) with electron probe analysis. The microstructures of the Ni–Si–Cr ternary alloys consisted of largely dispersed Ni 5 Si 2 phase and finely precipitated Ni 3 Si phase in Ni solid solution. Then, the high-temperature mechanical properties, three-point bend test, oxidation and corrosion properties were investigated. The Ni–Si–Cr ternary alloys showed significant strengthening over a wide range of temperatures, and also large compressive plastic deformation at high temperatures. The strength and fracture toughness at ambient temperatures were correlated with the volume fraction of the Ni 5 Si 2 phase. The Ni–Si–Cr ternary alloys showed substantially improved oxidation resistance in air at 1173 K, comparing with the Ni 3 Si and Ni 3 (Si,Ti) alloys. Also, the Ni–Si–Cr ternary alloys showed corrosion resistance comparable to the Ni 3 Si and Ni 3 (Si,Ti) alloys.

Texture evolution during cold rolling and recrystallization of L12-type ordered Ni3(Si,Ti) alloy

Abstract Texture evolution during cold rolling and recrystallization of L12-type ordered Ni3(Si,Ti) (Ni79Si11Ti10, denoted by at.%) alloy was investigated. The starting material with no preferential orientation was cold rolled up to 80% reduction, and then fully recrystallized by annealing at 1173 K for 1 h. After cold rolling, a copper type texture composed of the α- and β-fiber textures with the strongest component of {011}〈211〉 orientation was developed. The recrystallization textures depended on rolling reduction. For the 30% reduction, no textural change was observed, while above 50% reduction the recrystallization textures were quite weak, but {011}〈100〉 and {001}〈110〉 orientations apparently existed. Based on these results, formation mechanism for cold rolling and recrystallization textures of the Ni3(Si,Ti) alloy was discussed and compared with those of other L12-type ordered alloys.

Tensile property and fracture behavior of hot-rolled CoTi intermetallic compound

Abstract B2-type CoTi intermetallic compound was arc-melted, hot-rolled at 1273 K and then recrystallized at 1323 K. Tensile tests were carried out at temperatures from room temperature to 1223 K. Tensile behavior and fracture mode of CoTi were divided into three regions depending on temperature. From room temperature to ∼773 K, almost no plastic deformation was observed. In this region, fracture surfaces showed a mixed mode of quasi-cleavage and intergranular fracture. Above 773 K, a considerable necking began to occur, and tensile elongation increased with increasing temperature, accompanied by a brittle quasi-cleavage fracture. From 1073 to 1223 K, ∼50% tensile elongation took place, accompanied by almost 100% reduction in area. At this temperature region, a chisel edge type fracture took place. The brittle–ductile transition temperature (BDTT) of CoTi was defined as ∼800 K under the condition used in the present study. Also, the influence of thermal vacancies on mechanical properties of CoTi was briefly discussed.

Plastic flow instabilities of L12 Co3Ti alloys at intermediate temperatures

The serrated plastic flow of L12 Ni3Al alloys at intermediate temperatures was investigated using tensile tests. The effects of temperature, strain rate and composition were examined. The serrated plastic flow accompanied by the lowest (negative) strain-rate sensitivity was observed most strongly at 673 K and at a strain rate of 3.2 × 10−4 s−1. The serrated plastic flow became more significant as the alloy departed from a stoichiometric composition. The static strain aging at 673 K resulted in a reduced flow strength. The activation energy of the serrated plastic flow was estimated to be about 66 kJ/mol, which suggests that it is smaller than that for lattice diffusion of solutes in L12 lattices. The serrated plastic flow behavior of the Ni3Al alloys was compared with that of the L12 Co3Ti and Ni3(Si,Ti) alloys, and is qualitatively explained on the basis of the dynamics of solutes in the core of dissociated screw dislocations.

Alloying effect on microstructure and mechanical properties of thermomechanically processed Ni3(Si,Ti) alloys

Abstract Four different alloys of L12-type Ni3(Si,Ti,X), where X = Al, Cr, Co and Mo were prepared in thin sheet form by thermomechanical processing. The effect of alloying elements on microstructure and tensile properties was investigated using recrystallized Ni3(Si,Ti) alloys. 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 A1(fcc Ni solid solution). Room-temperature strength of the Ni3(Si,Ti) alloy was enhanced by the addition of all quaternary elements, whereas high-temperature strength was enhanced especially by the addition of Mo and Co. High-temperature tensile elongation was remarkably improved by the addition of all quaternary elements via suppression of intergarnular fracture. Also, the oxidation resistance of the Ni3(Si,Ti) alloy was improved particularly by the addition of Al and Cr. The observed mechanical properties are discussed from the microstructural as w...

The Effects of Nb and Cr Addition on Mechanical and Chemical Properties of Cold-Rolled Ni3(Si,Ti) Intermetallic Foils

Nb and/or Cr added Ni3(Si,Ti) as well as unalloyed Ni3(Si,Ti) intermetallic thin foils (i.e., Ni3(Si,Ti), Ni3(Si,Ti)+Nb, Ni3(Si,Ti)+Cr and Ni3(Si,Ti)+Nb,Cr) were fabricated from arc-melted polycrystalline ingots by thermomechanical process and subsequent heavy cold-rolling. Tensile property at room temperature as well as at high temperature and oxidization behavior of the cold-rolled foils with a thickness of ~200μm were investigated. The Ni3(Si,Ti) and Ni3(Si,Ti)+Nb alloys showed a single-phase microstructure consisting of L12 phase, while the Ni3(Si,Ti)+Cr and Ni3(Si,Ti)+Nb,Cr alloys exhibited a two-phase microstructure with A1 (fcc) Ni solid solution phase within the L12 grains. All the cold-rolled foils showed high tensile strength (over 2GPa) at room temperature although no plastic elongation was observed. The addition of Nb and/or Cr slightly enhanced the room-temperature tensile strength of the Ni3(Si,Ti) alloy. On the other hand, the addition of Nb and/or Cr prominently enhanced high-temperature tensile strength as well as oxidization resistance, while the addition of Cr improved high-temperature elongation.