Research relating to amorphous alloys in recent years have concentrated on searches for obtaining amorphous structures even with small cooling rates, that is, so-called bulk metallic glasses. Up until now, alloy compositions giving bulk metallic glasses have been discovered by numerous systems of components.
In Japan, Tohoku University's Inoue et al. have been engaged in cutting edge research. The fact that since 1988, Mg—La—(Ni,Cu)-based alloys, lanthanide-Al-transition metal-based alloys, Zr—Al-transition metal-based alloys, and Pd—Cu—Ni—P-based alloys giving bulk metallic glasses have been discovered is explained in Akihisa Inoue, Akira Takeuchi, Material Science and Engineering A, Vol. 375-377 (2004) p. 16-30.
Outside Japan, the fact that Hf—Cu—Ni—Al-based alloys, Ti—Ni—Cu-based alloys, and Ca—Mg—Ag-based alloys giving bulk metallic glasses have been discovered is explained in A. Revesez, J-L. Uriarte, D. Louzguine, A. Inoue, S. Surinach, M. D. Baro, A. R. Yavari, Materials Science and Engineering A, Vol. 375-377 (2004) p. 381-384, Tao Zhang, Akihisa Inoue, and Tsuyoshi Masumoto, Materials Science and Engineering A, Vol. 181/182 (1994) p. 1423-1426, and Oleg N. Senkov and J. Mike Scott, Materials Research Society Symposium Proceedings, v806, Amorphous and Nanocrystalline Metals (2003) p. 145-150. Further, almost all of the bulk metallic glasses currently reported fall under one of these systems of components.
The features common to these alloys are that the element with the highest concentration among the elements forming the alloy has the greatest atomic radius, the element having the next highest concentration has the smallest atomic radius, and the remaining components are made of elements having intermediate atomic radii, that is, the relationship between the atomic radii and concentrations of the component elements.
The relationship between the atomic radii and concentrations of component elements is disclosed in U.S. Pat. No. 6,623,566 as the rule for selection of elements with a high glass forming ability.
That is, the reported amorphous alloys are alloys using the known discovery of using atoms having giant atomic radii (giant atoms) to increase the difference in atomic radii between elements forming the alloys and thereby improve the glass forming ability. Lanthanide atoms, Ca, etc. are typical examples of giant atoms.
Bulk metallic glass-es which do not fit into this relationship of atomic radii and concentrations of the component elements have been discovered in Fe—B—Si—Nb-based alloys, Ni—Cr—P-B-based alloys, (Co,Cr,Ni)-(Mo,Nb)-(B,P)-based alloys, etc.
However, these alloys use metalloid elements such as B, Si, and P. As metalloid-metal alloys, these can be classified as alloys different from metal-metal alloys.
Currently, the alloys utilizing the high glass forming ability of the metalloid elements of B, Si, or P to obtain bulk metallic glasses are limited to alloys based on the iron-group elements of Fe, Co, and Ni.
Further, on the other hand, as exceptions to the rule for selection of elements disclosed in U.S. Pat. No. 6,623,566, Japanese Patent Publication (A) No. 2002-256401 discloses Cu-based amorphous alloys. Cu has a relatively small atomic radius (0.12780 nm) even among the group of metal elements having small atomic radii, has a large difference in atomic radius from other elements, and enables easy design of an alloy with a high glass forming ability.
Therefore, Cu can be said to be an element relatively easily giving a bulk metallic glasses. However, the Cu-based bulk metallic glasses up to now, as described in Japanese Patent Publication (A) No. 2002-256401, are systems of components using Zr, Hf, or other expensive elements. Amorphous systems of components using less expensive component element are desired.
If judged from the combinations of elements of amorphous alloys discovered up to now, the elements particularly difficult to obtain bulk metallic glasses from as base-elements are metal elements which, while belonging to the group of elements with small atomic radii, have relatively large atomic radii among the group of elements with small atomic radii. Al and Zn correspond to such elements.
Regarding Al-based alloys, Al—Y—Ni-based alloys, Al—, Zr— (Fe,Co,Ni)-based alloys, etc. are described as amorphous alloys in M. Gogebakan, Journal of Light Metals, Vol. 2 (2002), p. 271-275 and Limin Wang, Liqun Ma, Hisamichi Kimura, Akihisa Inoue, Materials Letters, Vol. 52 (2002), p. 47-52.
However, these alloys cannot be said to be high in glass forming ability. Bulk metallic glasses still cannot be obtained. Further, regarding Zn-based alloys, in the past, amorphous alloy have rarely been reported.
The two elements of Al and Zn have the common points that they have large atomic radii in the group of elements of small atomic radii and also have relatively low melting points among metals.
There is a conventional discovery that “in a composition near the eutectic point with a deep drop, the glass forming ability becomes higher”. If the melting point of the base element is low, in a composition with a high concentration of the low melting point element, it is difficult to form a deep eutectic point.
In actuality, in compositions with high Al concentrations or Zn concentrations, there are almost no eutectic compositions with deep drops. This is also a reason why it is difficult to improve the glass forming ability in Al-based alloys and Zn-based alloys.
For example, Japanese Patent Publication (A) No. 5-70877 discloses a high strength, high toughness aluminum alloy material and method of production of the same, but C the aluminum alloy disclosed in this Patent Document has a low glass forming ability. Even if using a copper casting mold for high pressure die-casting, an amorphous phase can only be obtained at the surface layer part.
That is, the aluminum alloy disclosed in the above Patent Document cannot be said to be a bulk metallic glass.
Japanese Patent Publication (A) No. 7-113101 discloses a method of producing an extruded material from an Al-based amorphous alloy powder produced by mechanical ironing. In the case of this method, at the time of hot extrusion, the working temperature ends up exceeding the crystallization temperature, so this method cannot be used to produce an Al-based bulk metallic glass.
Japanese Patent Publication (A) No. 7-216407 discloses a method of using the gas atomizer method to fabricate an Al-based alloy powder including an amorphous phase, filling the powder in a mold, then raising the temperature to the crystallization temperature to obtain a fine crystalline plastically worked material.
Even if trying to improve this technique and produce a bulk metallic glass by raising the temperature to the crystallization temperature or less, it is difficult to believe that the powder particles filled in the mold would adhere and bind at a temperature of the crystallization temperature or less.
In this way, up to now, in Al-based alloys, compositions with a high glass forming ability could not be obtained, so Al-based amorphous alloys could only be obtained as powders or surface layer parts of castings.
On the other hand regarding Zn-based amorphous alloys, Japanese Patent Publication (A) No. 2005-126795 discloses a method of fabrication of a Zn-based amorphous coating film by flame spraying.
This method uses a Zn-based alloy containing 2 to 5 mass % of Mg and rapidly cools it by a 105° C./sec or more cooling rate to obtain a Zn-based amorphous coating film.
This method is an invention making up for the low level of glass forming ability of an Zn-based alloy by the large cooling rate process called “flame spraying”.
The flame spraying method is utilized for the formation of local coating films or the formation of coating films of small objects, but the productivity is poor, so this method of production is not suited for mass production or production of bulk parts.
Japanese Patent Publication (A) No. 2005′-60805 discloses amorphous alloys comprised of Fe-based alloys, Co-based alloys, and Ni-based alloys including, as a selectively added element, Zn in an amount of up to 20 atm %.
Said amorphous alloy is a film-like alloy member including an amorphous phase fabricated by making amorphous alloy particles having a volume fraction of amorphous phase of 50% or more strike a substrate at a high speed. The Zn concentration of the amorphous alloy particles necessary as a material should again be kept down to within 20 atm %.
Further, Japanese Patent Publication (A) No. 2006-2252 discloses as a magnesium-based amorphous alloy an alloy containing Zn up to 30 atm %. Japanese Patent Publication (A) No. 2004-149914 discloses an alloy comprised of a Zr/Hf-based bulk metallic glass etc. including Zn as a selective element in an amount of 5 to 15 atm %.
However, all of these amorphous alloys are low in Zn concentration. There has never been a bulk metallic glass which could be said to be Zn-based.
At the present time, the issue in the fabrication of Al-based bulk metallic glasses and Zn-based amorphous alloys is that the method for designing an alloy composition with a high glass forming ability when using Al and/or Zn as the base has not yet been elucidated.
If an alloy composition with a high glass forming ability can be obtained, a bulk metallic glass can be obtained in an Al-based amorphous alloy from which a bulk metallic glass could not be obtained in the past and further progress can be expected in the utilization of amorphous alloys.
Further, if Zn-based amorphous alloys never obtained before can be obtained, not only use for hot dip plating materials, but also expanded-new applications of amorphous alloys can be expected.