Masahiko Demura

Fabrication of Ni3Al thin foil by cold-rolling

Abstract Thin foils of stoichiometric Ni 3 Al with thicknesses ranging from 57 to 315 μm were successfully fabricated by heavily cold-rolling without intermediate annealing. Starting materials were produced by directional solidification using the floating zone method. The total reduction in thickness obtained was as much as 95.5%. This high rolling ductility is considered to be due to the monocrystalline or near monocrystalline form of the starting materials. X-ray pole figures showed the formation of {110} rolling texture. This {110} texture is considered to develop mainly as a result of compressive deformation normal to the rolling plane. The foils recrystallized at temperatures over 1273 K had some tensile ductility (3.0–14.6%) at room temperature in air, in contrast to the usual brittleness of polycrystalline Ni 3 Al. Electron back scatter diffraction measurements revealed that low angle and Σ3 coincidence site lattice boundaries, which are considered to be crack-resistant, comprised 41–84% of the total grain boundary area in the recrystallized foils. This large fraction is probably a chief cause of the observed ductility. These results demonstrate that it may be possible to utilize Ni 3 Al thin foils as lightweight, high-temperature structural materials, e.g. honeycomb structures.

Mechanical behaviour of Σ3 boundaries in Ni3Al

Abstract The mechanical properties of Σ 3 boundaries in binary stoichiometric Ni 3 Al were studied by performing tensile tests on miniature bicrystal specimens. It was found that Σ 3 boundaries do not fracture in air. They are intrinsically strong and show no environmental embrittlement, independent of the grain boundary plane. An upper angular deviation limit, Δθ c = 15 Σ -1 ~ 15 Σ -2/3 , is suggested for the Σ 3 boundary depending on the fracture behaviour. The boundaries lying within Δθ c do not fracture, whereas the boundary with an angular deviation larger than Δθ c exhibits brittle fracture. The macroscopic slip behaviour at the grain boundary was found to be related to specimen geometry. when the common slip system in both halves of the bicrystal is the primary slip system, slip can transfer readily across the grain boundary, leading to uniform deformation. Otherwise, slip is impeded at the grain boundary.

Stress response by the strain-rate change in a binary stoichiometric Ni3Al single crystal

The strain-rate dependence of flow stress in single crystals of binary stoichiometric Ni Al has been studied in the temperature region of the yield3 stress anomaly. Below 400 K, the flow stress was found to be independent of strain rate, although it changed temporarily when the strain rate was changed. The strain-rate insensitivity can be explained by assuming that the flow stress is controlled by the multiplication and immobilization of mobile dislocations.

Grain-boundary fracture strength in Ni3Al bicrystals

Abstract The fracture properties of grain boundaries in binary stoichiometric Ni3Al were studied by performing tensile tests at room temperature in air on miniature bicrystal specimens with various grain-boundary types, namely. ∑ = 1, ∑ = 3, ∑ = 5, ∑ = 1, ∑ = 9, ∑ = 13a and random boundaries (RBs). All the specimens exhibited some tensile ductility prior to fracture. A fairly good relationship was found between fracture mode, fracture strength and ∑ value. The ∑ = 1, ∑ = 3 and ∑ = 9 boundaries were strongly crack resistant and the adjacent bulk fractured instead. In contrast, the ∑ = 5, ∑ = 7 and ∑ = 13a boundaries and RBs were less crack resistant, showing a typical intergranular fracture. The fracture strength of these boundaries was low and widely varied with the orientation of the component grains and the grain-boundary plane. The cohesive strengths of ∑ = 1, ∑ = 3 and ∑ = 9boundaries were estimated to be high in comparison with that in bulk Ni3Al, that of RBs much smaller and those of ∑ = 5, ∑ = 7 and ∑ = 13a boundaries slightly larger than that of RBs.

Fabrication of thin foils of binary Ni–Al γ/γ′ two-phase alloys by cold rolling

Abstract Recently we have successfully fabricated thin foils (57 μm in thickness with 94% reduction) of boron-free binary stoichiometric Ni 3 Al by using directional solidification and cold rolling [1] . In this study the same process was applied to binary Ni–Al γ/γ′ two-phase alloys. Single crystals were grown for Ni–16 and Ni–18 at.% Al by a floating zone method, while columnar-grained polycrystals for Ni–20 and Ni–22 at.% Al. It turned out to be possible to cold-roll to 200 μm-thick foils with 80% reduction without any cracks for Ni–16 and Ni–18 at.% Al similar to the case of the stoichiometric Ni 3 Al, while some cracking occurred up to the same reduction for Ni–20 and Ni–22 at.% Al. Strong {110} rolling texture evolved on all the foils. The tensile properties of the rolled foils were measured at room temperature.

Orientation dependence of texture evolution in cold-rolled Ni3Al single crystals

Texture evolutions of cold-rolled thin foils of binary stoichiometric Ni3Al single crystals were examined as a function of the initial crystal orientation. In the cases of the initial rolling direction (RD) close to ⟨001⟩, a double {110} {112} texture associated with a banded structure tends to develop irrespective of the initial foil normal direction (ND). The macroscopic shape of these foils with the banded structure is straight and simply elongated along RD, keeping their rectangular shape. In contrast, when the initial RD is close to {112}, the texture and the microstructure are rather uniform without the texture splitting and the banded structure formation. The foils of group B are macroscopically curved, twisted and eventually cracked from the side edge of the samples. Tendencies of the lattice rotations vary with the initial ND in the latter case. According to the plastic strain analysis, the formation of the banded structure is found to be essential for fabricating the thin straight foils in the case of the cold rolling of Ni3Al single crystals.

Compliance to Schmid's law in the stress anomaly regime of binary stoichiometric Ni3Al

Using single crystals of binary stoichiometric Ni3Al, the orientation dependence of the critical resolved shear stress (CRSS) for {111}〈101〉 slip and the tension/compression (T/C) asymmetry were studied in the yield stress anomaly regime. Unlike the case in ternary alloys, it was found that Schmids law holds and the T/C asymmetry vanishes in stoichiometric Ni3Al. This indicates that plastic deformation occurs according to Schmids law in the highly ordered state. Also, the violation of Schmids law and the T/C asymmetry, which were believed to be inherent to the stress anomaly so far, are considered to be additional effects due to the anti-site defects or specific ternary elements. The results are consistent with the idea that the CRSS is controlled by the dislocation multiplication in (111).

Ductility of cold-rolled and recrystallized Ni3Al foils

The room-temperature ductility of 95% cold-rolled and recrystallized Ni 3 Al(Ni–24.0 at.% Al) foils was examined as a function of heat-treatment conditions. The cold-rolled, diffused Goss texture changed to a complicated, transitional texture in the early stage of grain growth and then returned to a similar diffused Goss texture in the late stage. With the texture evolution, the total area fraction of the tough grain boundaries (GBs) such as Σ1, Σ3, and Σ9 increased from 0.23–0.38 in the early stage to 0.56–0.73 in the late stage. Tensile and bending tests revealed that the ductility was drastically improved with the grain growth. The foils in the early stage fractured without showing yielding. In contrast, the foils in the late stage were very ductile, and the tensile elongation increased to 10% with the grain growth. It was confirmed that there was no in-plane anisotropy in ductility. The ductility improvement with the grain growth was ascribed to the increase in the area fraction of the tough GBs.

Compressive flow stress of a binary stoichiometric Ni3Al single crystal

Ni{sub 3}Al exhibits the yield stress anomaly, i.e. its flow stress increases with increasing temperature. The yield stress anomaly has been extensively studied using single crystals. These studies have been carried out on Ni-rich or ternary Ni{sub 3}Al whereas the mechanical properties of a binary stoichiometric Ni{sub 3}Al single crystal that do not contain ternary additions have never been evaluated. Since deviations from stoichiometry or ternary elements essentially produce point defects, the nature of the yield stress anomaly can be affected by these. Therefore it is worth studying Ni{sub 3}Al deformation mechanisms using a binary stoichiometric single crystal. Very recently the authors have found that large binary stoichiometric Ni{sub 3}Al single crystals can occasionally be grown using a floating zone (FZ) method. The present paper reports the orientation dependence of the compressive flow stress of such crystals at 293--1,273 K for testing axis between [001] and [011].

Effect of directional growth-rate on the mechanical properties of Ni3Al

Abstract Polycrystalline Ni 3 Al were grown unidirectionally at various growth rates using a floating zone method. The as-grown alloys were heat-treated just below solidification temperature. Room-temperature tensile properties of the as-grown and heat-treated alloys were examined and the cause of the growth-rate dependence of the mechanical properties in the as-grown alloys was studied. Yield stress and work-hardening rate decreased and tensile elongation increased by the heat treatment. The heat-treated alloys fractured transgranularly similar to the as-grown single-phase Ni 3 Al. This proves that the major cause of the growth rate dependence of the mechanical properties is the presence of the NiAl in the Ni 3 Al matrix. The growth rate dependence on the ductility still remained in the heat-treated alloys. This suggests that the bulk Ni 3 Al properties are dependent on the growth rate.