Tough and heat resisting aluminum alloy

Details Tough and heat resisting aluminum alloy Tough and heat resisting aluminum alloy

1998-04-29

2001-05-15

King, Roy (Department: 1742)

Alloys or metallic compositions

Aluminum base

C420S552000

Reexamination Certificate

active

06231808

ABSTRACT:

FIELD OF THE INVENTION
This invention relates to an aluminum alloy having high toughness and excellent heat resistance which can be used as a part or a structural material required to have high toughness.
BACKGROUND OF THE INVENTION
Various studies have been given to high strength aluminum alloys obtained from an alloy containing amorphous metal, a supersaturated solid solution, and microcrystalline metal which is obtained by rapid quenching. For example, JP-B-6-21326 (the term “JP-B” as used herein means an “examined published Japanese patent application”) discloses that a rapid quenching and solidification of a ternary alloy represented by the formula Al
a
M
b
X
C
(wherein M represents at least one element selected from Cr, Mn, Fe, Co, Ni, Cu, Zr, Ti, Mg and Si; X represents at least one element selected from Y, La, Ce, Sm, Nd, Nb and Mm (mish metal); a, b, and c are atomic percentages, in which a is from 50 to 95, b is from 0.5 to 35 and c is from 0.5 to 25) yields an amorphous alloy or a composite of amorphous matter and microcrystalline matter, each having a tensile-strength of from 853 to 1010 MPa (from 87 to 103 kgf/mm
2
) and a yield strength of from 804 to 941 MPa (from 82 to 96 kgf/mm
2
).
The resulting aluminum alloy has a high tensile strength which is twice or more that of conventional crystalline aluminum alloys, but its Charpy impact strength is less than about one fifth of that of conventional ingot aluminum.
JP-A-5-1346 (the term “JP-A” as used herein means an “unexamined published Japanese patent application) discloses that an aluminum alloy having a tensile strength of from 875 to 945 MPa (from 89.2 to 96.3 kgf/mm
2
) and an elongation in tensile test of from 1.7 to 2.9% is obtained by rapid quenching and solidifying an alloy system represented by the formula Al
a
M
b
Ln
c
or Al
a
M
b
X
d
Ln
c
(wherein M is at least one element selected from Co, Ni and Cu; Ln is at least one element selected from Y, rare earth elements and Mm; and X is at least one element selected from V, Mn, Fe, Mo, Ti and Zr). The metallographic structure of the alloy has an average grain size of from 0.1 to 80 &mgr;m. The matrix is aluminum or a supersaturated solid solution of aluminum, and fine particles of an intermetallic compound in a stable or metastable phase having a particle size of 10 to 500 nm are distributed in the matrix. The term “matrix” as used in the present invention means the host phase which encloses the other phase therewith.
In the case of the alloy disclosed in JP-A-5-1346-in which fine intermetallic compound particles at- the order of nanometers are dispersed in the supersaturated solid solution matrix, the finely dispersed intermetallic compound particles expand upon application of heat. Therefore, the toughness of the aluminum alloy is considerably reduced at a certain temperature or higher.
Therefore, the aluminum alloys described in JP-B-5-21326 and JP-A-5-1346 are both unsuitable for use as a material for machine parts and automotive parts that are required to have high reliability.
In order to overcome the above problems, the present inventors have studied the microstructures of aluminum alloys in the order of nanometers and their mechanical characteristics. They have found that, when a conventional supersaturated solid solution is heat-treated, there is produced a clear crystalline grain boundary between a precipitated intermetallic compound and the Al matrix, and the anchoring of dislocation upon plastic deformation concentrates at the grain boundary. This interferes the attempt to increase the toughness.
The inventors considered that concentration of dislocation anchoring might be prevented by using a modulated structure (a microstructure having regular fluctuations in concentration) having no clear boundaries between an intermetallic compound and an Al matrix. It was revealed that such a modulated structure exhibits high toughness while the intermetallic compound is precipitating, but the toughness is considerably reduced with the progress of precipitation till complete precipitation. This is because clear crystalline grain boundaries are formed between the Al matrix and the precipitate at the completion of precipitation, and dislocations upon plastic deformation are concentrated at the grain boundaries.
SUMMARY OF THE INVENTION
An object of the present invention is to solve the above-described problems by providing an aluminum alloy which has improved toughness and improved heat resistance as compared to conventional aluminum alloys and which can be produced on an industrial scale.
Another object of the present invention is to provide a process for producing such a tough and heat resisting aluminum alloy.
Other objects and effects of the present invention will be apparent from the following description.
The above objectives of the resent invention have been achieved by providing a tough and heat resisting aluminum alloy comprising aluminum, a transition metal element and a rare earth element, and having a modulated structure which comprises an aluminum matrix and an intermetallic compound precipitated to form a network in said aluminum matrix.
The aluminum alloy according to the present invention is generally obtained by heat treating an aluminum-based supersaturated solid solution containing a transition metal element and a rare earth element.
In order to retard the precipitation of the intermetallic compound, a metal element that has a high melting point and is slow in diffusing in an Al matrix is generally selected as one of the constituent elements. In the modulated structure of the aluminum alloy according to the present invention, the network preferably comprises intermetallic compound bands each having a width of 10 to 500 nm and being located at a spacing with neighboring bands of from 10 to 100 nm.
If the network width and spacing are out of the above respective ranges, the toughness tends to largely reduced. That is, if the width and spacing are both smaller than 10 nm, the Al alloy has sufficient strength, but may has poor ductility. If the width and spacing are greater than 500 nm and 100 nm, respectively, both ductility and strength may be greatly reduced. Also, if either one of the width and the spacing fails to meet the respective condition, both ductility and strength may be reduced.
It seems that the modulated structure is formed by spinodal decomposition in the course of precipitation or the initial stage of nucleation in the course of the precipitation. In the network structure, the interface between the Al matrix and the precipitate is coherent, and aluminum and the constituent elements of the intermetallic compound continuously change their concentrations around the coherent interface therebetween. This is because the concentration fluctuation becomes larger to induce precipitation without requiring nucleation so that there is no incubation period in the precipitation and also because the supersaturated solid solution decomposes while keeping perfect coherency with the Al matrix. Since there is no distinct interface (crystalline grain boundary) between the Al matrix and the precipitate, the anchoring of dislocations hardly concentrates at one site, and high toughness can thus be exhibited.
In selecting the combination of metal elements for forming the modulated structure, it is important that the metal elements be capable of forming a supersaturated solid solution with an aluminum matrix and be separated into two phases. The first requirement can be met by selecting an element that has an atomic radius close to that of Al. The second requirement can be fulfilled by selecting an element which is incapable of forming a solid solution or intermetallic compound with the element meeting the first requirement.
The binary state diagram of the thus selected elements is preferably of a two-phase separation type.
The aluminum alloy according to the present invention can be produced by a process which comprises the steps of:
rapid quenching and solidifying a liquid aluminum alloy containing a transition metal element and a rare earth eleme

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