WO2012132280A2

WO2012132280A2 - Heat resistant and high strength aluminum alloy and method for producing the same

Info

Publication number: WO2012132280A2
Application number: PCT/JP2012/001742
Authority: WO
Inventors: Hideaki Matsuoka; Yuka Yamada
Original assignee: Kabushiki Kaisha Toyota Chuo Kenkyusho
Priority date: 2011-03-30
Filing date: 2012-03-13
Publication date: 2012-10-04
Also published as: JP5556723B2; WO2012132280A3; JP2012207283A; EP2646585B1; EP2646585A2

Abstract

A heat resistant and high strength aluminum alloy is provided to have significantly excellent high temperature strength and other properties. The heat resistant and high strength aluminum alloy according to the present invention is characterized by having an alloy composition, with the whole 100 mass% (referred simply to as "%" hereinafter), of 3% to 6% iron (Fe), 0.66% to 1.5% zirconium (Zr), 0.6% to 1% titanium (Ti), and the balance aluminum (Al) with inevitable impurities and/or modifying element, the mass ratio of Zr to Ti (Zr/Ti) being 1.1 to 1.5. The heat resistant and high strength aluminum alloy according to the present invention is comprised primarily of a matrix phase and an Al-Fe series intermetallic compound phase (first compound phase), and an Al-(Zr, Ti) series intermetallic compound of L1₂ structure (second compound phase) is possible to precipitate in the matrix phase in the vicinity of the boundary with the first compound phase so as to be in a matching manner. This second compound phase is stable even in a high temperature environment, and it is considered that the second compound phase inhibits the coarsening etc. of the first compound phase associated with the high temperature strength etc, thereby allowing the heat resistant and high strength aluminum alloy according to the present invention to achieve an excellent heat resistance.

Description

HEAT RESISTANT AND HIGH STRENGTH ALUMINUM ALLOY AND METHOD FOR PRODUCING THE SAME

The present invention relates to a heat resistant and high strength aluminum alloy suitable for a member to be exposed to a high temperature environment and a method for producing the same.

With the raised environmental awareness, the field of transporters, such as automobiles, motorcycles and aircrafts, strongly requires to improve environmental properties which affect fuel consumption or CO₂ emission, etc. Effective actions for this include weight saving and enhancement in properties of various members. To this end, even for members to be used in a severe environment such as high temperature ambient, lightweight aluminum alloys with excellent practical strength is being used and substituted for conventional iron and steel materials and cast iron materials.

The melting point of aluminum, however, is low in general and typically the heat resistance is not necessarily sufficient. As such, in order to conduct the expanding use as described above, the heat resistance of aluminum alloy must be improved. From such viewpoints, various proposals for aluminum alloys have been made, which include the descriptions relevant to the following patent literature, for example.

Japanese Unexamined Patent Application Publication No. 2011-42861

Patent Literature (PTL) 1 proposes, for example, an aluminum alloy comprising 5 to 10 mass% (hereinafter referred simply to as "%") Fe, 0.05% to 3% Zr and Ti, and 0.1% to 2% Mg. As an example thereof, an aluminum alloy material is disclosed which is obtained by solidifying a molten alloy comprising a certain composition, such as Al-5%Fe-1%Zr-0.5%Ti-1%Mg, at a cooling rate of 200 to 500 degrees C per second to form a cast plate material and subjecting the cast plate material to a heat treatment at 400 degrees C during one hour.

Indeed, the cast plate material of aluminum alloy described in PTL 1 has more significant advantages in heat resistance than the conventional aluminum alloy material. However, further enhanced heat resistance is being required than that of the disclosure in PLT 1. Moreover, it has also been found out that the aluminum alloy described in PLT 1 leaves much to be improved with respect to its alloy composition, cooling rate, and other factors.

The present invention has been created in view of such circumstances, and objects of the present invention include providing a heat resistant and high strength aluminum alloy which is further excellent in high temperature strength and other properties than the conventional heat resistant aluminum alloy and also providing a method for producing the same.

As a result of intensive studies to solve such problems and contemplating in detail the alloy composition of aluminum alloy, the present inventors have found out that a specified composition range of Fe, Zr and Ti enables an aluminum alloy to exhibit a significant heat resistance compared to the conventional technique. Developing this achievement, the present invention has been accomplished as will be described hereinafter.

<<Heat Resistant and High Strength Aluminum Alloy>>
(1) The heat resistant and high strength aluminum alloy (hereinafter referred arbitrarily to as "the aluminum alloy") according to the present invention is characterized by having an alloy composition, with the whole 100%, of 3% to 6% iron (Fe), 0.66% to 1.5% zirconium (Zr), 0.6% to 1% titanium (Ti), and the balance aluminum (Al) with inevitable impurities and/or modifying element, wherein the mass ratio of Zr to Ti (Zr/Ti) is 1.1 to 1.5.

(2) The aluminum alloy according to the present invention exhibits an excellent strength, hardness and other advantageous properties even if being exposed during a long time to a high temperature ambient, such as for example 300 degrees C or more, and further 400 degrees C or more, and deterioration in the strength or hardness due to heat history is thus hard to occur. Rather, the aluminum alloy according to the present invention may possibly improve its strength or hardness owing to heating. The aluminum alloy according to the present invention, which can stably exhibit such a high heat resistance (high temperature strength, softening resistance or thermal stability, etc), may be enough to substitute for conventional heat resistant aluminum alloys as well as conventional iron and steel materials, titanium materials and other materials having been used as heat resistant materials.

(3) Meanwhile, although the mechanism that the aluminum alloy according to the present invention develops such an excellent heat resistance as described above is not necessarily sure, it may be considered at present as follows. First, the aluminum alloy according to the present invention contains an appropriate amount of Fe thereby being formed with an intermetallic compound of Al and Fe (Al-Fe series intermetallic compound: first compound phase) in the matrix phase (alpha-Al phase). This first compound phase enhances the strength and/or hardness of the aluminum alloy. This first compound phase is, however, not necessarily thermally stable, and if exposed to a high temperature ambient during long time, the first compound phase may cause phase transformation, form deformation (coarsening) and other changes.

As such, the aluminum alloy according to the present invention further contains an appropriate amount of Zr and Ti, and these elements form with Al an Al-(Zr, Ti) series intermetallic compound of L1₂ structure. This intermetallic compound is formed in the matrix phase such that Zr and Ti having been solid-solved in supersaturated state in the matrix phase precipitate with ultrafine form (e.g. average size of approximately 1 to 30 nm). In the present description, this Al-(Zr, Ti) series intermetallic compound is referred to as a second compound phase, which may also be referred to as a matching phase or a precipitation phase as the case may be.

This second compound phase is matching the matrix phase and appears in the vicinity of the boundary (interface) between the Al-Fe series intermetallic compound and the matrix phase thereby to be stable even in a high temperature region. More specifically, the second compound phase scarcely leads to phase transformation or coarsening at a temperature lower than or equal to the temperature where the precipitation thereof has started.

Thus, the first compound phase is associated with the strength and hardness of the aluminum alloy, while the second compound phase, which exists in the vicinity of contact area between the first compound phase and the matrix phase, acts to suppress the phase transformation and coarsening etc. of the first compound phase at a high temperature (so-called pinning). That is, the strength and other properties achieved by the first compound phase are kept even in a high temperature region owing to the second compound phase. It is thus considered that the first compound phase and the second compound phase act synergistically with each other thereby allowing the aluminum alloy according to the present invention to achieve an excellent heat resistance compared to the prior art.

Incidentally, it has also been understood that the second compound phase is in a form of nanoparticle of which the center area is rich with the Zr concentration whereas the outer area is rich with Ti concentration. In other words, respective concentrations of Zr and Ti in the Al₃(Zr, Ti) appear to be inclined from the center to the outer. The formation of such a second compound phase requires as an importance that Zr exists more than Ti and the mass ratio of Zr to Ti (Zr/Ti) is within a predetermined range.

Further, in order for the second compound phase to be finely dispersed in a part of the matrix in the vicinity of the boundary with the first compound phase, it is also important that Zr and Ti are caused to precipitate after having been sufficiently solid-solved (solid-solved in supersaturated state) in the matrix base. Specifically, after causing an appropriate amount of Zr and Ti to be solid-solved in supersaturated state by rapid cooling and solidifying, some imparting of energy is required for the driving force to facilitate the precipitation. Examples of such energy include thermal energy applied by heat treatment, hot working or other processes, strain energy applied by plastic working or the like, and other appropriate energies. Thermal energy may be applied alone by heating process, or alternatively thermal energy and strain energy may be applied in combination at the same time by hot working or other appropriate means. Furthermore, thermal energy may be applied after having introduced strain energy, such as by performing heating process after cold working or warm working. Strain energy is added to thermal energy thereby accelerating the precipitation of the second compound phase, and the heat resistant and high strength aluminum alloy may thus be efficiently obtained for a short period of time.

<<Method for Producing Aluminum Alloy>>
(1) The present invention may be understood as not only being an aluminum alloy but being a method for producing the same. Considering the above circumstances, it is preferred that this method for producing is characterized by comprising a working step for obtaining a worked material by subjecting a raw material to a hot plastic working, wherein the raw material comprises a solidified material obtained by rapidly cooling and solidifying a molten alloy comprising the above alloy composition at a cooling rate of 300 degrees C per second or more.

(2) In this method for producing, first of all, the raw material comprising the solidified material rapidly cooled and solidified is to be a workpiece material. Consequently, the solidified material as the raw material is in a condition where Zr and Ti are solid-solved in supersaturated state in the matrix base. If the raw material is subjected to hot plastic working, then not only that the worked material is obtained as being created in a desirable shape, but thermal energy and strain energy are sequentially or concurrently applied to the raw material thereby to facilitate the precipitation of the second compound phase. In this manner, the worked material (aluminum alloy) with excellent heat resistance is easily obtained in which, in addition to the first compound phase, the second compound phase precipitates as a considerable number of ultrafine forms in the matrix phase. Moreover, the aluminum alloy is allowed to be efficiently obtained at low cost without the necessity of performing an aging treatment or other treatment, which requires long time for the precipitation of the second compound phase.

Note, however, that the present invention is to be understood as not excluding the case of precipitating the second compound phase by heat treatment (e.g. aging process) and/or one or more appropriate processes.

<<Others>>
(1) The "aluminum alloy" as used herein is enough if it has the above composition, and therefore the form, metallographic structure, and working stage thereof etc. are not limited. For example, embodiments of the aluminum alloy according to the present invention include rapidly cooled and solidified powder, thin strips and crushed powder thereof, compacts and billets as well as sintered materials, expanded materials (extruded materials etc) and other appropriate materials. Further, the aluminum alloy according to the present invention may be in any form, such as in a base material, an interim product, or a finished product.

(2) While the "heat resistance" as used herein involves various properties, the aluminum alloy according to the present invention is enough if it has at least one excellent property. Note that the aluminum alloy according to the present invention with excellent high temperature strength also necessarily has excellent room temperature strength.

(3) The "matching" as used herein is intended to mean that the crystal basic structure of the second compound phase is identical with that of the matrix phase and the atomic planes or the atomic rows are aligned in just proportion at the boundary (interface) between the second compound phase and the matrix phase. Note, however, that dislocations introduced by working or other necessary processes may cause disorders of atom rows, point defects and other defects, but such defects may be counted out from consideration. That is, the "matching" as used herein includes even such cases where disorders of atom rows, point defects or other defects are present.

(4) The "modifying element" as used herein is an element other than Al, Fe, Zr, Ti, and Mg, which is effective for improving properties of the aluminum alloy. Properties to be improved are such as, but not limited to, strength, hardness, toughness, ductility, and dimensional stability in high temperature region or room temperature region. Specific examples of such a modifying element include chrome (Cr), manganese (Mn), cobalt (Co), nickel (Ni), scandium (Sc), yttrium (Y), lanthanum (La), vanadium (V), hafnium (Hf), niobium (Nb), and other appropriate elements. While compounding of each element is arbitrary, the contained amount thereof is usually extremely small.

The "inevitable impurities" are intended to mean impurities having been contained in the molten raw material, impurities to be mixed or the like during each process, and/or other impurities, which are elements difficult to be removed for the reason of cost, technical reason, or other reasons. In the case of the aluminum alloy according to the present invention, silicon (Si) is relevant, for example.

(5) When there is no particular explanation, "x to y" as used herein includes the lower limit value x and the upper limit value y. Using various numerical values or any numerical values included in a numerical value range described herein as a new lower limit value or upper limit value, a newly-created range such as "a to b" may be possible.

Fig. 1 is a graph illustrating the relationship between the Vickers hardness and the heating time at 400 degrees C. Fig. 2A is a microscope photograph taken for observation in a metallographic structure of an aluminum alloy. Fig. 2B is a microscope photograph in which the vicinity of an interface between a matrix phase and a first compound phase in the metallographic structure is observed Fig. 2C is a microscope photograph in which the circumference of second compound phases having precipitated in the vicinity of the interface is observed. Fig. 3A is a microscope photograph in which the second compound phases are observed after enlarged. Fig. 3B is a diagram schematically illustrating the concentration distributions of Al, Zr, and Ti in the second compound phase.

The present invention will be described in more detail by exemplifying embodiments according to the invention. The contents described herein are to be applicable to an aluminum alloy as well as a method for producing the same. Entities regarding the method for producing are, when understood as a product-by-process, also to be entities regarding a product. Further, one or more constituent elements arbitrarily selected from the present description may be added to as one or more constituent elements of the above present invention. Note that whether or not either embodiment is the best is different according to the required properties and other factors.

<<Composition of Aluminum Alloy>>
(1) Fe
Fe is an element which enhances the strength and/or the hardness of an aluminum alloy. When whole of the aluminum alloy is 100 mass% (this recitation will be hereinafter omitted), it is preferred that Fe is of 3% to 6%, 4% to 6%, and more preferred is 4.5% to 5.5%. If Fe is unduly low, then sufficient strength and hardness are hardly obtained, whereas if Fe is unduly high, then the ductility is reduced and the strength is undesirably increased thereby to deteriorate the formability and workability.

(2) Zr and Ti
Zr and Ti are important elements which form in combination with Al a second compound phase to enhance the heat resistance of the aluminum alloy. Zr is preferably of 0.66% to 1.5%, more preferably of 0.7% to 1.3%, and further preferably of 0.8% to 1.2%. Ti is also preferably of 0.6% to 1%, and more preferably of 0.7% to 0.9%. On this occasion, the mass ratio of them (Zr/Ti) is preferably 1.1 to 1.5 and more preferably 1.15 to 1.4 because in such ranges the second compound phase is formed to be stable even in a high temperature region. Note that, converting this mass ratio into atomic ratio of Zr (atomic%) to Ti (atomic%) (Zr/Ti)a, preferred is 0.57 to 0.79, and more preferred is 0.6 to 0.7.

If Zr or Ti is unduly low, there is obtained no advantageous effect. If Zr or Ti is unduly high, then the melting temperature extremely increases, thereby to increase the production cost and to form coarse crystallized particles or precipitations with Al, or possibly to deteriorate the workability and formability of the aluminum alloy. Thus, unduly low or high Zr/Ti causes the formation of a desired second compound phase to be difficult.

(3) Mg
Mg is an effective element for improving the strength (particularly room temperature strength) of the aluminum alloy. Mg is preferably of 0.6% to 2.2%, more preferably of 1% to 2%, and further preferably of 1.2% to 1.8%. If Mg is unduly low, there is obtained no advantageous effect, whereas unduly high Mg leads to deterioration of the workability and formability of the aluminum alloy material.

<<Metallographic Structure of Aluminum Alloy>>
(1) The aluminum alloy according to the present invention comprises a composite structure having at least an aluminum matrix phase (alpha-phase), an Al-Fe series intermetallic compound phase (first compound phase), and an Al-(Zr, Ti) series intermetallic compound phase (second compound phase). Such a metallographic structure allows the aluminum alloy according to the present invention to have an excellent heat resistance.

(2) Average size of the second compound phase is preferably 1 to 30 nm, more preferably 2 to 20 nm, and further preferably 3 to 15 nm. If this average size is unduly low or high, then the effect of improving the heat resistance of the aluminum alloy by the second compound phase may be reduced. Note that the average size used herein is a value obtained by observing samples randomly selected in the aluminum alloy using a transmission electron microscope (TEM) and analyzing the average diameter of 30 or more dispersed second compound phases using an image processing method.

<<Method for Producing Aluminum Alloy>>
Various methods are possible as the method for producing an aluminum alloy according to the present invention. Among them, in order to obtain a metallographic structure in which the second compound phase is homogeneously dispersed with ultrafine form in the matrix base, a producing method is preferred as previously described where a raw material comprising a solidified material obtained by rapidly cooling and solidifying is subjected to a hot plastic working.

Through using such a solidified material having been rapidly cooled and solidified, a raw material may readily be obtained in which Zr and Ti necessary for generating the second compound phase are solid-solved in supersaturated state. Higher cooling rate for the solidified material is preferred, and it is preferably 300 degrees C per second or higher, more preferably 1,000 degrees C per second or higher, further preferably 5,000 degrees C per second or higher, and still further preferably 10,000 degrees C per second or higher, for example.

Thus, such rapidly cooling and solidifying are performed, for example, by atomizing method, spray forming method, strip cast method (roll cast method etc), or other appropriate method. The atomizing method allows for obtaining powdered solidified material (atomized powder where atomized particles are collected). The spray forming method allows for obtaining block-like solidified materials. The continuous cast method allows for obtaining a solidified material in a thin strip form.

Although the dimensions of the solidified materials are not particularly limited, preferred for atomized particles is an average particle diameter of 50 to 300 micrometers, for example, and preferred for thin pieces is a thickness of 0.05 to 1.5 mm and 5 to 8 mm square, for example.

The raw material may be such a solidified material in itself. Alternatively, it is preferred to use compacts or billets as the raw material for the productivity or other reasons, wherein the compacts or billets are formed by compressively molding the atomized powder (water atomized powder, gas atomized powder, or water-gas atomized powder), crushed powder comprising thin pieces obtained by crushing or smashing thin strips, or other powder.

Such a raw material is subjected to heat treatment, plastic working, hot plastic working or other appropriate treatment thereby to result in that Zr and Ti having been solid-solved in supersaturated state finely precipitate as an Al-(Zr, Ti) series intermetallic compound phase. In particular, hot plastic working (working process) is efficiently performed because capable of both the feature creating and precipitating the second compound phase at the same time.

Examples of the hot plastic working include extrusion processing, forging processing, rolling processing, and other processing. For example, in the case of extrusion processing where billets are hot extrusion formed to obtain an extruded material (worked material), the extrusion temperature for billets is preferably 350 to 500 degrees C, and more preferably 400 to 480 degrees C. If the extrusion temperature is unduly low, then the precipitation of the second compound phase or the heat resistant temperature of the aluminum alloy comes to be insufficient. In addition, the working force also increases thus being undesirable. If, on the other hand, the extrusion temperature is unduly high, then the coarsening of the metallographic structure progresses thereby to adversely deteriorate the heat resistance of the aluminum alloy.

The extrusion ratio for billets is preferably 5 to 30, and more preferably 10 to 20. If the extrusion ratio is unduly low, then the pressurized contact between the powder particles or between the crushed pieces comes to be insufficient thereby not capable of achieving a desired strength or ductility, whereas if the extrusion ratio is unduly high, then the increased working force leads to forming difficulty.

Note that, although the relative density (bulk density/true density) of billets used for the extrusion forming or other use is not particularly limited, it is preferably 60% or more, 70% or more, 80% or more, and 85% or more in this order, and further preferably 90% or more. If the relative density is unduly low, then the shape retaining ability or the handling ability of billets deteriorates. While the upper limit of the relative density is not particularly limited, it is preferably 95% or less in consideration of the productivity.

<<Applications>>
While the intended use and the usage environment are not particularly limited, the aluminum alloy according to the present invention, which is excellent in heat resistance, is preferred to be used as a high strength member for high temperature environment, such as, but not limited to, piston, inlet valve, and con rod for internal-combustion engine, rotor for supercharger, and impeller for compression machine. Note that conditions for the heat treatment, working or processing to be applied to the aluminum alloy may be appropriately adjusted in response to the required specifications for products. Note also that the aluminum alloy according to the present invention exhibits high strength properties not only in a high temperature region but also in a room temperature region. Consequently, the aluminum alloy according to the present invention is widely applicable even to high strength members required to be lightweight as well as members to be used in a high temperature region.

The present invention will be specifically described with reference to examples.
<<Production of Samples>>
Molten aluminum alloys were prepared to have respective compositions as shown in Table 1 (step for preparing molten alloy). Each molten alloy was atomized in a vacuum atmosphere to obtain air atomized powder (solidified material) (step for solidifying). The obtained particles of air atomized powder (atomized particles) were classified to prepare atomized powder having particle diameter of 150 micrometers or less. Note that the relationship between the size of the obtained powder particles by air atomizing and the cooling rate is known in the art. This supports that the above atomized powder would comprise particles having been rapidly cooled and solidified at a cooling rate of 10⁴ degrees C per second or more.

The atomized powder was subjected to cold isostatic press (CIP) to obtain each extrusion billet (raw material) of 40 mm x 40 mm (diameter x length) and relative density 85%.

This extrusion billet was loaded into a container (not shown) of the extrusion machine. Thereafter, the extrusion billet was heated to 430 degrees C by a heating apparatus provided with the container, and extruded to obtain a rod (solid rod) material (worked material) of 12 mm x 400 mm (diameter x length) (step for hot plastic working/working). At that time, the extrusion ratio (cross-sectional area of the raw material/cross-sectional area of the worked material) was set as being 11.1. Using samples collected from these aluminum alloy rod materials obtained in such a manner, the following measurements were performed.

<<Measurements for Samples>>
(1) Strength and ductility
Tensile test was performed for a test piece cutout from each sample, and strength and ductility at room temperature and strength at 300 degrees C (no pre-heating) were measured. Results thereof are shown together in Table 1. Note that the tensile test was performed in compliance with JIS Z2241, each strength shown in Table 1 is a breaking strength, and each ductility is a percentage elongation of distance between reference points from the time of starting test to the time of breaking.

(2) Measurements for residual hardness (softening-resistance)
Residual hardness in each sample (hardness at room temperature after heating each sample at a high temperature) was also measured. Specifically, Vickers hardness was measured for each sample after holding it in atmosphere of 400 degrees C during 10 hours and then getting it back to the room temperature condition. The measurement of Vickers hardness was performed with 0.49N load and 15 seconds holding time using a Vickers hardness tester.

Moreover, some of samples shown in Tale 1 and a conventional material commercially available as a heat resistant aluminum alloy (JIS A2618) were held in atmosphere of 400 degrees C each during a predetermined time, and changes in high temperature hardness (Vickers hardness) were measured in respective cases. The results thereof are shown in Fig. 1.

<<Observation for Samples>>
Fig. 2A to Fig. 2C and Fig. 3A are photographs taken for observation in the metallographic structure of Sample No. 11 shown in Table 1. Fig. 2B is a photograph in which the vicinity of the interface between the matrix phase in Fig. 2A and the first compound phase is observed, Fig. 2C is a photograph in which the second compound phases (precipitation phases or matching phases) in the matrix phase are observed, and Fig. 3A is a photograph in which these second compound phases are observed after further enlarged. Note that Fig. 2A was observed using a scanning electron microscope (SEM) while Fig. 2B to Fig. 3A were observed using a transmission electron microscope (TEM).

For the second compound phases and the vicinities thereof shown in Fig. 3A, concentration distributions of constituent elements were analyzed using a three-dimensional atom probe, and the results thereof are schematically illustrated in Fig. 3B.

<<Evaluation for Samples>>
(1) Initial properties
As apparent from Table 1, any samples falling within the composition range according to the present invention exhibit excellent initial properties at room temperature. This is obvious from the comparison with Sample No. C1 (T6 processed material of A2618/JIS) and Sample No. C2 (AC8A/JIS) as conventional heat resistant aluminum alloys. In particular, samples according to the present invention have higher strength as the amount of Fe and the amount of Mg increase. Conversely, if the amount of Fe is unduly low, then the strength comes to be insufficient even within the room temperature region. On the other hand, if the amount of Fe is unduly high, then higher working force is required during extrusion working and the ductility deteriorates as well.

(2) High temperature properties
As apparent from Table 1, any samples falling within the composition range according to the present invention also exhibit excellent high temperature properties. This aspect is also obvious from the comparison with Sample No. C1 and Sample No. C2. In addition, samples according to the present invention tend to be enhanced in their high temperature strengths as the amount of Fe increases, but if the amount of Zr or the amount of Ti is not appropriate, then sufficient high temperature strength is not obtained.

Moreover, as apparent from Fig. 1, any samples according to the present invention exhibit properties that the hardness substantially reaches a peak after heating during approximately one hour and that the hardness is stably maintained even in the high temperature environment of 400 degrees C. In this respect, they are significantly different from the conventional heat resistant aluminum alloy (Sample No. C1) in which the initial hardness is sufficiently high while the hardness decreases with the increase of heating time.

(3) Metallographic structure
First, from Fig. 2A, it is understood that the aluminum alloy according to the present invention is primarily comprised of matrix phases (gray portions in the photograph) and first compound phases (white portions in the photograph) comprising Al-Fe series intermetallic compound phases.

Next, from Fig. 2B and Fig. 2C, it is understood that the second compound phases matching the matrix phase finely precipitate from the internal of the matrix phase. It is also understood that the second compound phases precipitate at least in the vicinities of the interface between the matrix phase and the first compound phase.

Further, from Fig. 3A and Fig. 3B, it is understood that the center area of each second compound phase is rich with Zr concentration whereas the outer area surrounding the center area is rich with Ti concentration. More specifically, it is understood that Zr or Al-Zr is to be a nucleus of the second compound phase and that the amount of Zr decreases as departing from there while the fraction of Ti or Al-Ti increases. Thus, it appears that the presence of concentration distributions of Al, Zr and Ti in the ultrafine second compound phases of approximately 1 to 30 nm also causes the second compound phases to exhibit the high temperature stability.

Claims

A heat resistant and high strength aluminum alloy having an alloy composition, with the whole 100 mass% (referred simply to as "%" hereinafter), of 3% to 6% iron (Fe), 0.66% to 1.5% zirconium (Zr), 0.6% to 1% titanium (Ti), and the balance aluminum (Al) with inevitable impurities and/or modifying element, the mass ratio of Zr to Ti (Zr/Ti) being 1.1 to 1.5.
The heat resistant and high strength aluminum alloy as recited in claim 1, comprising a worked material obtained by subjecting a raw material to a hot plastic working, the raw material comprising a solidified material obtained by rapidly cooling and solidifying a molten alloy comprising the alloy composition at a cooling rate of 300 degrees C per second or more.
The heat resistant and high strength aluminum alloy as recited in claim 2, wherein the raw material is a billet obtained by compressively molding atomized particles or thin pieces, and the worked material is an extruded material obtained by hot extrusion forming the billet.
The heat resistant and high strength aluminum alloy as recited in either one of claims 1 to 3, wherein the alloy composition further includes 0.6% to 2.2% magnesium (Mg).
A method for producing a heat resistant and high strength aluminum alloy, comprising a working step for obtaining a worked material by subjecting a raw material to a hot plastic working, the raw material comprising a solidified material obtained by rapidly cooling and solidifying a molten alloy comprising the alloy composition as recited in claim 1 or 4 at a cooling rate of 300 degrees C per second or more.
The method for producing a heat resistant and high strength aluminum alloy as recited in claim 5, wherein the raw material is a billet obtained by compressively molding atomized particles or thin pieces, and the working step is an extrusion step for obtaining an extruded material by extrusion forming the billet with an extrusion ratio of 5 to 30 after heating the billet to 350 to 500 degrees C.