Rapidly Solidified Aluminum Based Alloys Containing Silicon for Elevated Temperature Applications

1. Field of the Invention

The invention relates to aluminum based, silicon containing, alloys having strength, ductility and toughness at ambient and elevated temperatures and relates to powder products produced from such alloys. More particularly, the invention relates to Al-Fe-Si alloys that have been rapidly solidified from the melt and thermomechanically processed into structural components having a combination of high strength, ductility and fracture toughness.

2. Brief Description of the Prior Art

Methods for obtaining improved tensile strength at 350.degree. C. in aluminum based alloys have been described in U.S. Pat. Nos. 2,963,780 to Lyle, et al.; U.S. Pat. No. 2,967,351 to Roberts, et al.; and U.S. Pat. No. 3,462,248 to Roberts, et al. The alloys taught by Lyle, et al. and by Roberts, et al. were produced by atomizing liquid metals into finely divided droplets by high velocity gas streams. The droplets were cooled by convective cooling at a rate of approximately 10.sup.4 .degree. C./sec. As a result of this rapid cooling, Lyle, et al. and Roberts, et al. were able to produce alloys containing substantially higher quantities of transition elements than has hither to been possible.

Higher cooling rates using conductive cooling, such as splat quenching and melt spinning, have been employed to produce cooling rates of about 10.sup.5 .degree. to 10.sup.6 .degree. C./sec. Such cooling rates minimize the formation of intermetallic precipitates during the solidification of the molten aluminum alloy. Such intermetallic precipitates are responsible for premature tensile instability. U.S. Pat. No. 4,379,719 to Hildeman, et al. discusses rapidly quenched aluminum alloy powder containing 4 to 12 wt % iron and 1 to 7 wt % cerium or other rare earth metal from the lanthanum series.

U.S. Pat. No. 4,347,076 to Ray, et al. discusses high strength aluminum alloys for use at temperatures of about 350.degree. C. that have been produced by rapid solidification techniques. These alloys, however, have low engineering ductility and fracture toughness at room temperature which precludes their employment in structural applications where a minimum tensile elongation of about 3% is required. An example of such an application would be in small gas turbine engines discussed by P. T. Millan, Jr.; Journal of Metals, Volume 35(3), page 76, 1983.

Ray, et al. discusses aluminum alloys composed of a metastable, face-centered cubic, solid solution of transition metal elements with aluminum. The as cast ribbons were brittle on bending and were easily comminuted into powder. The powder was compacted into consolidated articles having tensile strengths of up to 76 ksi at room temperature. The tensile ductility or fracture toughness of these alloys was not discussed in detail in Ray, et al. However, it is known (NASA REPORT NASI-17578 May 1984) that many of the alloys taught by Ray, et al., when fabricated into engineering test bars do not posses sufficient room temperature ductility or fracture toughness for use in structural components.

Thus, conventional aluminum alloys, such as those taught by Ray, et al. have lacked sufficient engineering toughness. As a result, these conventional alloys have not been suitable for use in structural components.

The invention provides an aluminum based alloy consisting essentially of the formula Al.sub.bal Fe.sub.a Si.sub.b X.sub.c, wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta, "a" ranges from 1.5 to 7.5 at %, "b" ranges from 0.75 to 9.0 at %, "c" ranges from 0.25 to 4.5 at % and the balance is aluminum plus incidental impurities, with the proviso that the ratio [Fe+X]:Si ranges from about 2.01:1 to 1.0:1.

To provide the desired levels of ductility, toughness and strength needed for commercially useful applications, the alloys of the invention are subjected to rapid solidification processing, which modifies the alloy microstructure. The rapid solidification processing method is one wherein the alloy is placed into the molten state and then cooled at a quench rate of at least about 10.sup.5 .degree. to 10.sup.7 .degree. C./sec. to form a solid substance. Preferably this method should cool the molten metal at a rate of greater than about 10.sup.6 .degree. C./sec, i.e. via melt spinning, spat cooling or planar flow casting which forms a solid ribbon or sheet. These alloys have an as cast microstructure which varies from a microeutectic to a microcellular structure, depending on the specific alloy chemistry. In alloys of the invention the relative proportion of these structures is not critical.

Consolidated articles are produced by compacting particles composed of an aluminum based alloy consisting essentially of the formula Al.sub.bal Fe.sub.a Si.sub.b X.sub.c, wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta, "a" ranges from 1.5 to 7.5 at %, "b" ranges from 0.75 to 9.0 at %, "c" ranges from 0.25 to 4.5 at % and the balance is aluminum plus incidental impurities, with the proviso that the ratio [Fe+X]:Si ranges from about 2.01:1 to 1.0:1. The particles are heated in a vacuum during the compacting step to a pressing temperature varying from about 300.degree. to 500.degree. C., which minimizes coarsening of the dispersed, intermetallic phases. Alternatively, the particles are put in a can which is then evacuated, heated to between 300.degree. C. and 500.degree. C., and then sealed. The sealed can is heated to between 300.degree. C. and 500.degree. C. in ambient atomosphere and compacted. The compacted article is further consolidated by conventionally practiced methods such as extrusion, rolling or forging.

The consolidated article of the invention is composed of an aluminum solid solution phase containing a substantially uniform distribution of dispersoid intermetallic phase precipitates of approximate composition Al.sub.15 (Fe, X).sub.3 Si.sub.2. These precipitates are fine intermetallics measuring less than 100 nm. in all linear dimensions thereof. Alloys of the invention, containing these fine dispersed intermetallics are able to tolerate the heat and pressure associated with conventional consolidation and forming techniques such as forging, rolling, and extrusion without substantial growth or coarsening of these intermetallics that would otherwise reduce the strength and ductility of the consolidated article to unacceptably low levels. Because of the thermal stability of the dispersoids in the alloys of the invention, the alloys can be used to produce near net shape articles, such as wheels, by forging, semi-finished articles, such as T-sections, by extrusion, and plate or sheet products by rolling that have a combination of strength and good ductility both at ambient temperature and at elevated temperatures of about 350.degree. C.

Thus, the articles of the invention are especially suitable for high temperature structural applications such as gas turbine engines, missiles, airframes, landing wheels, etc.

The invention will be more fully understood and further advantages will become apparent when reference is made to the following detailed description of the prefered embodiment of the invention and the accompanying drawings in which:

FIG. 1 shows a transmission electron micrograph of an as-cast alloy of the invention; and

FIG. 2 shows a transmission electron micrograph of a consolidated article of the invention.

To provide the desired levels of strength, ductility and toughness needed for commercially useful applications, rapid solidification from the melt is particularly useful for producing these aluminum based alloys. The alloys of the invention consist essentially of the formula Al.sub.bal Fe.sub.a Si.sub.b X.sub.c, wherein X is at least one element selected from the group consisting of Mn, V, Cr, Mo, W, Nb, Ta, "a" ranges from 1.5 to 7.5 at %, "b" ranges from 0.75 to 9.0 at %, "c" ranges from 0.25 to 4.5 at % and the balance is aluminum plus incidental impurities, with the proviso that the ratio [Fe+X]:Si ranges from about 2.01:1 to 1.00:1. The rapid solidification processing typically employs a casting method wherein the alloy is placed into a molten state and then cooled at a quench rate of at least about 10.sup.5 .degree. to 10.sup.7 .degree. C./sec. on a rapidly moving casting substrate to form a solid ribbon or sheet. This process should provide provisos for protecting the melt puddle from burning, excessive oxidation and physical disturbances by the air boundary layer carried along with a moving casting surface. For example, this protection can be provided by a shrouding apparatus which contains a protective gas, such as a mixture of air or CO.sub.2 and SF.sub.6, a reducing gas, such as CO or an inert gas; around the nozzle. In addition, the shrouding apparatus excludes extraneous wind currents which might disturb the melt puddle.

As representatively shown in FIG. 1, the as-cast alloy of the present invention may have a microeutectic microstructure or a microcellular microstructure.

Rapidly solidified alloys having the Al.sub.bal Fe.sub.a Si.sub.b X.sub.c compositions (with the [Fe+X]:Si ratio proviso) described above have been processed into ribbons and then formed into particles by conventional comminution devices such as pulverizers, knife mills, rotating hammer mills and the like. Preferably, the comminuted powder particles have a size ranging from about -40 to +200 mesh, U.S. standard sieve size.

The particles are placed in a vacuum of less than 10.sup.-4 torr (1.33.times.10.sup.-2 Pa.) preferably less than 10.sup.-5 torr (1.33.times.10.sup.-3 Pa.), and then compacted by conventional powder metallurgy techniques. In addition the particles are heated at a temperature ranging from about 300.degree. to 550.degree. C., preferably ranging from about 325.degree. to 450.degree. C., minimizing the growth or coarsening of the intermetallic phases therein. The heating of the powder particles preferably occurs during the compacting step. Suitable powder metallurgy techniques include direct powder extrusion by putting the powder in a can which has been evacuated and sealed under vacuum, vacuum hot compaction, blind die compaction in an extrusion or forging press, direct and indirect extrusion, conventional and impact forging, impact extrusion and combinations of the above.

As representatively shown in FIG. 2, the compacted consolidated article of the invention is composed of a substantially homogeneous dispersion of very small intermetallic phase precipitates within the aluminum solid solution matrix. With appropriate thermo-mechanical processing these intermetallic precipitates can be provided with optimized combinations of size, e.g. diameter, and interparticle spacing. These characteristics afford the desired combination of high strength and ductility. The precipitates are fine, usually sperical in shape, measuring less than about 100 nm. in all linear dimentions thereof. The volume fraction of these fine intermetallic precipitates ranges from about 10 to 50%, and preferably, ranges from about 20 to 35% to provide improved properties. Volume fractions of coarse intermetallic precipitates (i.e. precipitates measuring more than about 100 nm. in the largest dimention thereof) is not more than about 1%.

Composition of fine intermetallic precipitates found in the consolidated article of the invention is approximately Al.sub.15 (Fe, X).sub.3 Si.sub.2. For alloys of the invention this intermetallic composition represents about 80% of the fine dispersed intermetallic precipitates found in the consolidated article. The addition of one or more of the elements listed as X when describing the alloy composition as the formula Al.sub.bal Fe.sub.a Si.sub.b X.sub.c (with the [Fe+X]:Si ratio of 2.01:1 to 1.0:1) stabilize this metastable ternary intermetallic precipitate resulting in a general composition of about Al.sub.15 (Fe, X).sub.3 Si.sub.2. X-ray diffraction traces made from consolided articles according to this invention reveal the structure and lattice parameter of the intermetallic phase precipitate and of the aluminum matrix. The prefered stabilized intermetallic precipitate has a structure that is primative cubic and a lattice parameter that is about 1.25 to 1.28 nm.

Alloys of the invention, containing this fine dispersed intermetallic precipitate, are able to tolerate the heat and pressure of conventional powder metallurgy techniques without excessive growth or coarsening of the intermetallics that would otherwise reduce the strength and ducility of the consolidated article to unacceptably low levels. In addition, alloys of the invention are able to withstand unconventionally high processing temperatures and withstand long exposure times at high temperatures during processing. Such temperatures and times are encountered during the production of near net-shape articles by forging and sheet or plate by rolling, for example. As a result, alloys of the invention are particularly useful for forming high strength consolidated aluminum alloy articles. The alloys are particularly advantageous because they can be compacted over a broad range of consolidation temperatures and still provide the desired combinations of strength and ductility in the compacted article.

The following examples are presented to provide a more complete understanding of the invention. The specific techniques, conditions, materials, proportions and reported data set forth to illustrate the principles of the invention are exemplary and should not be construed as limiting the scope to the invention.

Alloys of the invention were cast according to the formula and method of the invention and are listed in Table 1.

1. Al.sub.93.55 Fe.sub.3.23 V.sub.0.8 Si.sub.2.42

2. Al.sub.93.55 Fe.sub.2.97 V.sub.1.06 Si.sub.2.42

3. Al.sub.93.55 Fe.sub.2.74 V.sub.1.29 Si.sub.2.42

Table 2 below shows the mechanical properties of specific alloys measured in uniaxial tension at a strain rate of approximately 5.times.10.sup.-4 S.sup.-1 and at various elevated temperatures. Each selected alloy powder was vacuum hot pressed at a temperature of 350.degree. C. for 1 hour to produce a 95 to 100% density preform slug. These slugs were extruded into rectangular bars with an extrusion ratio of 18:1 at 385.degree. to 400.degree. C. after holding at that temperature for 1 hour.

The alloys of the invention are capable of producing consolidation articles which have high fracture toughness when measured at room temperature. Table 3 below shows the fracture toughness for selected consolidation articles of the invention. Each of the powder articles were consolidated by vacuum hot compaction at 350.degree. C. and subsequently extruded at 385.degree. C. at an extrusion ratio of 18:1. Fracture toughness measurements were made on compact tension (CT) specimens of the consolidated articles of the invention under the ASTM E399 standard.

Having thus described the invention in rather full detail, it will be understood that these details need not be strictly adhered to but that various changes and modifications may suggest themselves to one skilled in the art, all falling within the scope of the invention as defined by the subjoining claims.