Naoya Masahashi

Electronic and structural studies of grain boundary strength and fracture in Ll2 ordered alloys—II. On the effect of third elements in Ni3Al alloy

Abstract It was proposed in the Paper I that the grain boundary of the polycrystalline Ni 3 Al compound is intrinsically weak due to the heterogeneous bonding environment at the grain boundary region. In this paper, the effect of the substitutional third elements on the mergranular embrittlement of the Ni 3 Al compound was systematically investigated at room temperature. Various kinds of third elements ranging from groups IIIa to V b in the periodic table were selected and alloyed in the Ll 2 structure range. First the phenomenological aspects such as the mechanical tests, metallographic and fractographic observations were described. It was shown that the third elements (Mn and Fe) which have the similar electronic chemical bonding nature with the Ni atom prohibited the grain boundary fracture when they substitute for the Al site. Several factors responsible for these phenomena were considered. As a consequence, the value of the valency difference between the third element and constitutive solvent atom substituted by the third element seems to control the grain boundary strength of the ternary Ni 3 Al compound. The grain boundary is strengthened when the value of the valency difference is positive. The possible fracture mechanism was proposed, based on the crystal structures connected with the electronic chemical bonding nature at the grain boundary region. The modification in that the electronic chemical bonding environment of the grain boundary region becomes more homogeneous by the alloying makes the grain boundary stronger.

Mechanical properties of Ni3Al containing C, B and Be

The mechanical properties of the flow strength, ductility and fracturing in the Ni3Al polycrystals containing C, B and Be were extensively investigated by tension and compression tests. The strengthening by the dopant was very significant at ambient temperatures as well as at temperatures showing the anomalous temperature dependence of the yield stress while was almost negligible at temperatures above the peak for every ternary alloy. The activation energy for the thermal stress term producing the anomalous positive temperature dependence was, by the addition of every dopant, enhanced in the just-stoichiometric (25 at.%Al) alloys while reduced in the off-stoichiometric (24 at.%Al) alloys. The yield stress at 77 K implying the athermal term (solid solution strengthening) increased linearly with increasing concentration of each ternary atom, the slope of which was very remarkable for the addition of C and B atoms. The evaluation in the correlation between the increment of the yield stress ΔσyΔC and the increment of lattice strain ΔϵΔC, per atom portion of the ternary addition indicated that the solid solution strengthening by C, B and Be atoms in Ni3Al alloy was not fully expained only by the size effect in the elastic interaction between solute atoms and dislocations. The ductilization was found for the addition of B and Be atoms but not for the addition of C atom, and then disappeared above 1000 K for the addition of B atom and above 800 K for the addition of Be atoms. These ductilization behavior was discussed in terms of the species and concentration of the dopant, alloys stoichiometry and test environment.

Development of preferred orientation in annealing of Fe–3.25%Si in a high magnetic field

Annealing of a cold-rolled Fe–3.25%Si sheet having {111} 〈112〉 preferred orientation is performed in a high magnetic field in order to control microstructure orientation. Magnetic field (10 T) was applied in a direction parallel to the rolling direction. Distributions of orientation and misorientation of primary recrystallization grains in the magnetically annealed specimens are characterized with electron backscattering pattern analysis. Magnetic annealing is found to enhance the selection of ?001? axis alignment parallel to the rolling direction in the { hk 0} 〈001〉 recrystallization texture and to favor the occurrence of low energy grain boundaries in the recrystallized microstructure. The high frequency of low angle grain boundaries results in the appearance of coarse grains with traces of faint prior grain boundaries, suggesting extensive operation of the mechanism of grain coalescence. As a cause of selective formation of 〈100〉 grains in recrystallization, magnetostriction induced by applying a magnetic field is suggested.

Hydrogen embrittlement of pseudobinary l12-type Ni3(Alo.4Mno.6) intermetallic compound

The mechanical properties of the Ll2-type intermetallic compound Ni2(Al0.4,Mn0.6), which exhibited enough deformability accompanied with the positive temperature dependence of yield stress, were examined from the point of view of its susceptibility to hydrogen embrittlement. Remarkable and moderate increases of elongation and ultimate tensile strength were observed by evacuating and with increasing strain rate, respectively, although the yield stress was almost constant independent of not only testing environment but also strain rate. On the other hand, the cathodically precharged hydrogen showed deteriorated elongations while the baking treatment resulted in restoration of elongation. Fractographic observation revealed the expected correlation between elongation and fracture mode; with increasing elongation the transgranularly fractured region increased. From this result, it is suggested that hydrogen can be removed from the specimen interior by evacuating or baking, and also that hydrogen can be injected from the sample surface by hydrogen charging; that is to say, hydrogen permeation is almost reversible for both processes. It is thus concluded that the present alloy has severe hydrogen embrittlement susceptibility, both from the residual hydrogen in the specimens as well as from that penetrated from the environment.

Atomistic defect structures of Ni3Al containing C, B and Be

Abstract The atomistic defect structures of Ni 3 Al containing C, B and Be atoms were investigated using Debye-Scherrer technique and X-ray diffractometer. It was proposed that the elements of C and B occupy on the interstitial site of the body centered position of the Ll 2 structure while the elements of Be substitute on the Al site of the Ll 2 structure. Also, it was observed that the additions of C and B atoms into the Ni 3 Al induced the further ordering of the constituent atoms of Ni and Al. This was more significant at the off-stoichiometric compositions of the Ni 3 Al. The energetic consideration, involving the nearest neighbor interactions between the constituent atoms, and between the interstitial atom and the constituent atom, was presented in order to explain the further ordering of the constituent atoms.

Electronic and structural studies of grain boundary strength and fracture in L12 ordered alloys—III. On the effect of stoichiometry

Abstract In order to clarify the effect of alloy stoichiometry on the grain boundary strength and fracture behavior of L12-type A3B compounds, the mechanical properties of Ni3Al, Ni3Ga, Co3Ti and Ni3(Al0.52Mn0.48) compounds were investigated at room temperature. Also, the effect of interstitial atoms of boron and hydrogen and the interaction of its effect with alloy stoichiometry were observed. The result indicated that the magnitude of compositional dependence of alloy stoichiometry on the elongation, the ultimate tensile strength and fracture behavior varied as Ni3Al > Ni3Ga > Co3Ti > Ni3(Al0.52Mn0.48). It was also revealed that the effect of interstitial atoms on the grain boundary cohesive strength is independent on and then additive to the effect of alloy stoichiometry. The significance of the grain boundary structure and related electronic environment in the grain boundary region was, in the geometrical frame, emphasized to interpret the present behavior.

Composition dependence of young’s modulus in Ti-V, Ti-Nb, and Ti-V-Sn alloys

The Young’s modulus of Ti-V and Ti-V-Sn alloys quenched from the β-phase region after solution treatment and cold rolling was investigated in relation to alloy compositions, microstructures, and constituent phases. The composition dependence of the Young’s modulus for quenched Ti-V binary alloys shows two minima of 69 GPa at Ti-10 mass pct V and 72 GPa at Ti-26 mass pct V. Between the two compositions, athermalω or stress-induced ω is introduced in retainedβ phase and increases Young’s modulus. That is, a low Young’s modulus is attained unless alloys undergoω transformation. In Ti-5 and -8 mass pct V, which under goα′ (hcp) martensitic transformation on quenching, the Young’s modulus further decreases by cold rolling, which can be reasonably explained by the formation ofα′ rolling texture. Comparing Young’s modulus in Ti-V binary alloy with that in Ti-Nb binary alloy, it is found that Young’s modulus is remarkably increased by athermal- or stress inducedω phase, and it shows a minimum when both martensitic andω transformation are suppressed during quenching in metastableβ alloys. The Sn addition to Ti-V binary alloy retards or suppresses athermal and stress-inducedω transformation, thereby decreasing Young’s modulus. Young’s modulus exhibits minimum values of 51 GPa in quenched (Ti-12 pct V)-2 pct Sn and of 57 GPa in cold-rolled (Ti-12 pct V)-6 pct Sn.

Preparation of superparamagnetic magnetite nanoparticles by reverse precipitation method: contribution of sonochemically generated oxidants.

Magnetic iron oxide nanoparticles were successfully prepared by a novel reverse precipitation method with the irradiation of ultrasound. TEM, XRD and SQUID analyses showed that the formed particles were magnetite (Fe(3)O(4)) with about 10nm in their diameter. The magnetite nanoparticles exhibited superparamagnetism above 200K, and the saturation magnetization was 32.8 emu/g at 300 K. The sizes and size distributions could be controlled by the feeding conditions of FeSO(4).7H(2)O aqueous solution, and slower feeding rate and lower concentration lead to smaller and more uniform magnetite nanoparticles. The mechanisms of sonochemical oxidation were also discussed. The analyses of sonochemically produced oxidants in the presence of various gases suggested that besides sonochemically formed hydrogen peroxide, nitrite and nitrate ions contributed to Fe(II) ion oxidation.

Degradation of hydrogen absorbing capacity in cyclically hydrogenated TiMn2

Abstract Degradation of the hydrogen absorbing capacity in cyclically hydrogenated TiMn2 (Ti–60 at.% Mn) with Laves structure was studied by measurement of pressure–composition–isotherm (PCT) curves, X-ray diffraction and microstructure observation. Characteristic changes with cyclic hydrogenation are observed as follows. The hydrogen absorbing capacity remarkably decreases, the lattice constant increases, the amount of hydrogen retained in TiMn2 increases and it approaches almost zero by dehydrogenation at 673 K in vacuum. Nano-sized regions with Moire patterns are produced in TiMn2 with hydrogenation and Debye rings corresponding to titanium hydride δ-TiH appear in the diffraction pattern. Based on these observations it is concluded that the degradation of hydrogen absorbing capacity after cyclic hydrogenation is attributable to the introduction of retained hydrogen, heterogeneous strain and/or nano-sized regions.

Mechanical properties and microstructures of β Ti–25Nb–11Sn ternary alloy for biomedical applications

The mechanical properties and microstructures of β Ti-25%Nb-11%Sn ternary alloy rods were investigated for biomedical applications as a function of heat treatment temperature after swaging by an 86% reduction in cross-section area. An as-swaged rod consisting of a β (bcc) single phase shows a low Youngs modulus of 53 GPa, which is interpreted in terms of both the metastable composition of the β alloy undergoing neither an athermal ω transformation nor a deformation-induced ω transformation and <110>texture development during swaging. Heat treatment at 673 K (400 °C) for 2h leads to a high strength of approximately 1330 MPa and a high spring-back ratio of yield stress to Youngs modulus over 15×10(-3), with acceptable elongation. This high strength is attributable to needle-like α precipitates, which are identified by high-angle annular dark-field scanning transmission electron microscopy (HAADF-STEM) and high-resolution electron microscopy (HREM).