Ni$_3$Al thin foil

Ni3Al thin foil

5th subgroup, Mechanical Properties Research Group NIMS
Japanese

Cold-rolled thin foil of Ni3Al
Figure 1: 100 micron thick foil of Ni3Al fabricated by cold-rolling.

Ni3Al has attractive high-temperature properties. Very recently, we have succeeded in fabricating Ni3Al thin foils by cold-rolling, which is based on the ductility improvement by directional solidification technique. Ni3Al foils can be used for lightweight, high-temperature structural materials, having honeycomb structure.

Contents

1  Why Ni3Al foil?
2  Fabrication of Ni3Al thin foil by cold-rolling
3  Mechanical properties of cold-rolled foils
4  Ductility of recrystallized foils
5  Related publications

1  Why Ni3Al foil?

Heat-resistant metallic foils are promising materials for lightweight, high-temperature structural application, having honeycomb structure. At present thin metallic foils below 100 micron in thickness are limited to ductile common metals, i.e. aluminum, copper, and stainless steel. Unfortunately, high-temperature strength of these metals is low and corrosion and oxidation resistances are not good enough. Thin foils are desirable for intermetallic compound with excellent high temperature properties. For example, Ni3Al is have attractive high temperature properties such as anomalous strengthening with increasing temperature (Fig. 2) and excellent oxidation and corrosion resistances (see the review by Stoloff [1]). Thus, Ni3Al foils could be applied for high-temperature structural materials.

Yield stress anomaly in Ni3Al
Figure 2: Temperature dependence of yield stress for Ni3Al and 304 stainless steel.

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2  Fabrication of Ni3Al thin foil by cold-rolling

Thin foils of brittle intermetallic compounds have been unrealistic so far. In the case of Ni3Al, the brittleness arises from intergranular fracture [2]. It is well known that the ductility of Ni3Al can be improve by micro alloying with boron additions [3,4], but with all this beneficial effect, the ductility is not sufficient to fabricate thin foils on engineering scale by cold rolling [5].

Alternatively, we found that the directional solidification with a floating zone method is more effective in improving the ductility of Ni3Al without any ternary additions  [6,7]. The high ductility of directionally solidified (DS) polycrystals was ascribed to the large fraction of low angle and low S-value coincidence site lattice boundaries in the columnar-grained structure [8]. Using the same technique we recently have succeeded in growing single crystals of stoichiometric Ni3Al [9,10]; single-crystalline Ni3Al is well known to have substantial ductility. Taking advantage of the high ductility of these DS materials, we fabricated thin foils with thicknesses ranging from 57 to 315 micron by cold rolling [11].

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3  Mechanical properties of cold-rolled foils

Cold -rolled foils are heavily work-hardened. The Vickers hardness number of the cold-rolled foils reaches to 600 in 83% cold reduction, while that of as-grown Ni3Al is about 260.

Figure 3 plots the room-temperature tensile stress-strain curve of cold-rolled foil (83% reduction in thickness). Ultimate tensile strength of the foil is 1.9 GPa, which is much high than those of as grown Ni3Al. Because of these heavy work hardening, the cold-rolled foil fractures with almost no tensile elongation. However, it can be bent plastically as demonstrated in Fig. 4. with this ductility of cold-rolled foils, honeycomb structure can be fabricated by pressing.

Tensile stress-strain curve of cold-rolled foil
Figure 3: Tensile stress-strain curve for 300 micron thick foil cold-rolled to 83% reduction (sample no. 47-1) at room temperature.

A photograph, showing the ability of plastic bending
Figure 4: A photograph, showing the ability of plastic bending in cold-rolled foil.

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4  Ductility of recrystallized foils

Cold-rolled foils are recrystallized in high-temperature. Since polycrystalline Ni3Al is believed to be brittle due to intergranular fracture, the ductility loss due to recrystallization is of great concern. Thus, we examined the ductility of recrystallized foils by tensile tests at room temperature.

Figure 5 shows the stress-strain curves for the recrystallized foils as a function of recrystallization temperature and foil thickness. The recrystallized foils have some tensile ductility (3.0-14.6%). That is, the ductility loss due to recrystallization is not a serious problem for our foils. Based on the measurement of grain boundary character distribution, we concluded that the ductility of our recrystallized foils are ascribed to the high fraction of crack-resistant boundaries such as low angle and S 3 boundaries [11].

Stress strain curves of recrystallized foils
Figure 5: Stress-strain curves at room temperature for recrystallized foils of Ni3Al as a function of recrystallization temperature foil thickness.

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5  Related publications

References

[1]
Stoloff N. S, Int. Mater. Rev., 1989, 34, 153.

[2]
Aoki K, Izumi O, Trans. JIM, 1978, 19, 203.

[3]
Aoki K, Izumi O, Nihon Kinzoku Gakkai shi, 1979, 43, 1190.

[4]
Liu C. T, White C. L,, Horton J. A, Acta metall., 1985, 33, 213.

[5]
Liu C. T, Sikka V. K, J. Metals, 1986, 38,  19.

[6]
Hirano T, Acta metall. mater., 1990, 38, 2667.

[7]
Hirano T, Scripta Metall. Mater., 1991, 25, 1747.

[8]
Watanabe T, Hirano T, Ochiai T,, Oikawa H, Materials Science Forum, 1994, 157-162, 1103.

[9]
Demura M, Hirano T, Phil. Mag. Letters, 1997, 75, 143.

[10]
Golberg D, Demura M,, Hirano T, J. Crys. Growth, 1998, 186, 624.

[11]
Demura M, Suga Y, Umezawa O, Kishida K, George E. P,, Hirano T, Intermetallics, 2000, ?,  ?

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File translated from TEX by TTH, version 2.21.
On 21 Dec 2000, 09:53.