Method and Apparatus for Forming Aluminum-Transition Metal Alloys Having High Strength at Elevated Temperatures

The invention relates to aluminum alloys having high strength at elevated temperatures, and relates to powder products produced from such alloys. More particularly, the invention relates to aluminum alloys having sufficient engineering tensile ductility for use in high temperatures structural applications which require ductility, toughness and tensile strength.

Methods for obtaining improved tensile strength at 350.degree. C. in aluminum based alloys have been described in U.S. Pat. No. 2,963,780 to Lyle, et al.: U.S. Pat. No. 2,967,248 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 had theretofore 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.6 .degree. to 10.sup.7 .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 % Ce or other rare earth metal from the Lathanum series.

U.S. Pat. No. 4,347,076 to Ray, et al. discusses high strength aluminum alloys have been produced by rapid solidification techniques. These alloys, however, have low engineering ductility 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), 1983, page 76.

Ray, et al. discusses a method for fabricating aluminum alloys containing a supersatured solid solution phase. The alloys were produced by melt spinning to form a brittle filament composed of a metastable, facecentered cubic, solid solution of the transition elements in the 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 of the alloys was not discussed in Ray, et al. However, it is known that many of the alloys taught by Ray, et al., when fabricated into engineering test bars, do not possess sufficient ductility for use in structural components.

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

The invention provides a method and apparatus for forming rapidly solidified metal, within an ambient atmosphere. Generally stated, the apparatus includes a moving casting surface which has a quenching region for solidifying thereon molten metal consisting essentially of the formula Al.sub.bal Fe.sub.a X.sub.b, wherein X is at least one element selected from the group consisting of Zn, Co, Ni, Cr, Mo, V, Zr, Ti, Y, Si and Ce, "a" ranges from about 7-15 wt %, "b" ranges from about 1.5-10 wt % and the balance is aluminum. A reservoir means holds the molten metal and has orifice means for depositing a stream of the molten metal onto the casting surface quenching region. Heating means heat the molten metal within the reservoir, and gas means provide a non-reactive gas atmosphere at the quenching region to minimize oxidation of the deposited metal. Conditioning means disrupt a moving gas boundary layer carried along by the moving casting surface to minimize disturbances of the molten metal stream that would inhibit quenching of the molten metal on the casting surface at a rate of at least about 10.sup.5 .degree. C./sec.

The apparatus of the invention is particularly useful for forming rapidly solidified alloys having a microstructure which is predominately microeutectic. The rapid movement of the casting surface in combination with the conditioning means for disrupting the high speed boundary layer carried along by the casting surface advantageously provides the conditions needed to produce the distinctive microeutectic microstructure within the alloy. Since the cast alloy has a microeutectic microstructure it can be processed to form particles that, in turn, can be compacted into consolidated articles having an advantageous combination of high strength and ductility at room temperature and elevated temperatures. Such consolidated articles can be effectively employed as structural members. X is at least one element selected from the group consisting of Zn, Co, Ni, Cr, Mo, V, Zr, Ti, Y, Si and Ce. "a" ranges from about 7-15 wt %, "b" ranges from about 1.5 wt % and the balance of the alloy is aluminum. The alloy particles have a microstructure which is at least about 70% microeutectic. The particles are heated in a vacuum during the compacting step to a pressing temperature ranging from about 300.degree. to 500.degree. C., which minimizes coarsening of the dispersed, intermetallic phases.

A consolidated metal article compacted from particles of the aluminum based alloy produced by the method of the invention is composed of an aluminum solid solution phase containing a substantially uniform distribution of dispersed, intermetallic phase precipitates therein. These precipitates are fine intermetallics measuring less than about 100 nm in all dimensions thereof. The consolidated article has a combination of an ultimate tensile strength of approximately 275 MPa (40 ksi) and sufficient ductility to provide an ultimate tensile strain of at least about 10% elongation when measured at a temperature of approximately 350.degree. C.

Thus, the invention provides a method and apparatus for producing alloys and consolidated articles which have a combination of high strength and good ductility at both room temperature and at elevated temperatures of about 350.degree. C. Such consolidated articles are stronger and tougher than conventional high temperature aluminum alloys, such as those taught by Ray, et al, and are more suitable for high temperature applications, such as structural members for gas turbine engines, missiles and air frames.

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

FIG. 1 shows a schematic representation of the casting apparatus of the invention:

FIG. 2 shows a photomicrograph of an alloy quenched in accordance with the method and apparatus of the invention:

FIG. 3 shows a photomicropraph of an alloy which has not been adequately quenched at a uniform rate:

FIG. 4 shows a transmission electron micrograph of an as-cast aluminum alloy having a microeutectic microstructure;

FIG. 5 (a), (b), (c) and (d) show transmission electron micrographs of aluminum alloy microstructures after annealing:

FIG. 6 shows plots of hardness versus isochronal annealing temperature for alloys of the invention;

FIG. 7 shows a plot of the hardness of an extruded bar composed of selected alloys as a function of extrusion temperature; and

FIG. 8 shows an election micrograph of the microstructure of a consolidated article produced using the method and apparatus of the invention.

FIG. 1 illustrates the apparatus of the invention. A moving casting surface 1 is adapted to quench and solidify molten metal thereon. Reservoir means, such as crucible 2, is located in a support 12 above casting surface 1 and has an orifice means 4 which is adapted to deposit a stream of molten metal onto a quenching region 6 of casting surface 1. Heating means, such as inductive heater 8, heats the molten metal contained within crucible 2. Gas means, comprised of gas supply 18 and housing 14 provides a non-reactive gas atmosphere to quenching region 6 which minimizes the oxidation of the deposited metal. Conditioning means, located upstream from crucible 2 in the direction counter to the direction of motion of the casting surface, disrupts the moving gas boundary layer carried along by moving casting surface 1 and minimizes disturbances of the molten metal stream that would inhibit the desired quenching rate of the molten metal on the casting surface.

Casting surface 1 is typically a peripheral surface of a rotatable chill roll or the surface of an endless chilled belt constructed of high thermal conductivity metal, such as steel or copper alloy. Preferably, the casting surface is composed of a Cu-Zr alloy.

To rapidly solidify molten metal alloy and produce a desired microstructure, the chill roll or chill belt should be constructed to move casting surface 1 at a speed of at least about 4000 ft/min (1200 m/min), and preferably at a speed ranging from about 6500 ft/min (2000 m/min) to about 9,000 ft/min (2750 m/min). This high speed is required to provide uniform quenching throughout a cast strip of metal, which is less than about 40 micrometers thick. This uniform quenching is required to provide the substantially uniform, microeutectic microstructure within the solidified metal alloy. If the speed of the casting surface is less than about 1200 m/min, the solidified alloy has a heavily dendritic morphology exhibiting large, coarse precipitates, as a representatively shown in FIG. 3.

Crucible 2 is composed of a refractory material, such as quartz, and has orifice means 4 through which molten metal is deposited onto casting surface 1. Suitable orifice means include a single, circular jet opening, multiple jet openings or a slot type opening, as desired. Where circular jets are employed, the preferred orifice size ranges from about 0.1-0.15 centimeters and the separation between multiple jets is at least about 0.64 centimeters. Thermocouple 24 extends inside crucible 2 through cap portion 28 to monitor the temperature of the molten metal contained therein. Crucible 2 is preferably located about 0.3-0.6 centimeters above casting surface 1, and is oriented to direct a molten metal stream that deposits onto casting surface 1 at an deposition approach angle that is generally perpendicular to the casting surface. The orifice pressure of the molten metal stream preferably ranges from about 1.0-1.5 psi (6.89-7.33 kPa).

It is important to minimize undesired oxidation of the molten metal stream and of the solidified metal alloy. To accomplish this, the apparatus of the invention provides an inert gas atmosphere or a vacuum within crucible 2 by way of counit 38. In addition, the apparatus employs a gas means which provides an atmosphere of non-reactive gas, such as argon gas, to quenching region 6 of casting surface 1. The gas means includes a housing 14 disposed substantially coaxially about crucible 2. Housing 14 has an inlet 16 for receiving gas directed from pressurized gas supply 18 through conduit 20. The received gas is directed through a generally annular outlet opening 22 at a pressure of about 30 psi (207 kPa) toward quenching region 6 and floods the quenching region with gas to provide the non-reactive atmosphere. Within this atmosphere, the quenching operation can proceed without undesired oxidation of the molten metal or of the solidified metal alloy.

Since casting surface 1 moves very rapidly at a speed of at least about 1200 to 2750 meters per minute, the casting surface carries along an adhering gas boundary layer and produces a velocity gradient within the atmosphere in the vicinity of the casting surface; at positions further from the casting surface, the gas velocity gradually decreases. This moving boundary layer can strike and destabilize the stream of molten metal coming from crucible 2. In sever cases, the boundary layer blows the molten metal stream apart and prevents the desired quenching of the molten metal. In addition, the boundary layer gas can become interposed between the casting surface and the molten metal to provide an insulating layer that prevents an adequate quenching rate. To disrupt the boundary layer, the apparatus of the invention employs conditioning means located upstream from crucible 2 in the direction counter to the direction of casting surface movement.

In a preferred embodiment of the invention, a conditioning means is comprised of a gas jet 36, as representatively shown in FIG. 1. In the shown embodiment, gas jet 36 has a slot orifice oriented approximately parallel to the transverse direction of casting surface 1 and perpendicular to the direction of casting surface motion. The gas jet is spaced upstream from crucible 2 and directed toward casting surface 1, preferably at a slight angle toward the direction of the oncoming boundary layer. A suitable gas, such as nitrogen gas, under a high pressure of about 800-900 psi (5500-6200 kPa) is forced through the jet orifice to form a high velocity gas "knife" 10 moving at a speed of about 300 m/sec that strikes and disperses the boundary layer before it can reach and disturb the stream of molten metal is uniformly quenched at the desired high quench rate of at least about 10.sup.6 .degree. C./sec, and preferably at a rate greater than 10.sup.6 .degree. C./sec to enhance the formation of the desired microeutectic microstructure.

The apparatus of the invention is particularly useful for producing high strength, aluminum-based alloys, particularly alloys consisting essentially of the formula Al.sub.bal Fe.sub.a S.sub.b, wherein X is at least one element selected from the group consisting of Zn, Co, Ni, Cr, Mo, V, Zr, Ti, Y, Si and Ce, "a" ranges from about 7-15 wt %, "b" ranges from about 1.5-10 wt % and the balance is aluminum. Such alloys have high strength and high hardness: the microVickers hardness is at least about 320 kg/mm.sup.2. To provide an especially desired combination of high strength and ductility at temperatures up to about 350.degree. C., "a" ranges from about 10-12 wt % and "b" ranges from about 1.5-8 wt %. In alloys cast by employing the apparatus and method of the invention, optical microscopy reveals a uniform featureless morphology when etched by the conventional Kellers etchant. See, for example, FIG. 2. However, alloys cast without employing the method and apparatus of the invention do not have a uniform morphology. Instead, as representatively shown in FIG. 3, the cast alloy contains a substantial amount of very brittle alloy having a heavily dendritic morphology with large coarse precipitates.

The inclusion of about 0.5-2 wt % Si in certain alloys of the invention can increase the ductility and yield strength of the as-consolidated alloy when those alloys are extruded in the temperature range of about 375.degree.-400.degree. C. For example, such increase in ductility and yield strength has been observed when Si was added to Al-Fe-V compositions and the resultant Al-Fe-V-Si, rapidly solidified alloy extruded within the 375.degree.-400.degree. C. temperature range.

Alloys produced by the method and apparatus of the invention have a distinctive, predominately microeutectic microstructure (at least about 70% microeutectic) which improves ductility, provides a microVickers hardness of at least about 320 kg/mm.sup.2 and makes them particularly useful for constructing structural members employing conventional powder metallurgy techniques. More specifically, the alloys have a hardness ranging from about 320-700 kg/mm.sup.2 and have the microeutectic microstructure representatively shown in FIG. 4.

This microeutectic microstructure is a substantially two-phase structure having no primary phases, but composed of a substantially uniform, cellular network (lighter colored regions) of a solid solution phase containing aluminum and transition metal elements, the cellular regions ranging from about 30 to 100 nanometers in size. The other, darker colored phase, located at the edges of the cellular regions, is comprised of extremely stable precipitates of very fine, binary or termary, intermetallic phases. These intermetallics are less than about 5 nanometers in their narrow width dimension and are composed of aluminum and transition metal elements (AlFe, AlFeX). The ultrafine, dispersed precipitates include, for example, metastable variants of AlFe with vanadium and zirconium in solid solution. The intermetallic phases are substantially uniformly dispersed within the microeutectic structure and intimately mixed with the aluminum solid solution phase, having resulted from a eutectic-like solidification. To provide improved strength, ductility and toughness, the alloy preferably has a microstructure that is at least 90% microeutectic. Even more preferably, the alloy is approximately 100% microeutectic.

This microeutectic microstructure is retained by alloys produced in accordance with the invention after annealing for one hour at temperatures up to about 350.degree. C. (660.degree. F.) without significant structural coarsening, as representatively shown in FIG. 5(a), (b). At temperatures greater than about 400.degree. C. (750.degree. F.), the microeutectic microstructure decomposes to the aluminum alloy matrix plus fine (0.005 to 0.05 micrometer) intermetallics, as representatively shown in FIG. 5(c), the exact temperature of the decomposition depending upon the alloy composition and the time of exposure. At longer times and/or higher temperatures, these intermetallics coarsen into spherical or polygonal shaped dispersoids typically ranging from about 0.1-0.05 micrometers in diameter, as representatively shown in FIG. 5(d). The microeutectic microstructure is very important because the very small size and homogeneous dispersion of the inter-metallic phase regions within the aluminum solid solution phase, allow the alloys to tolerate the heat and pressure of conventional powder metallurgy techniques without developing very coarse intermetallic phases that would reduce the strength and ductility of the consolidated article to unacceptably low levels.

As a result, alloys produced by the method and apparatus of the invention are useful for forming consolidated aluminum alloy articles. The alloys, however, are particularly advantageous because they can be compacted over a broad advantageous because they can be compacted over a broad range of pressing temperatures and still provide the desired combination of strength and ductility in the compacted article. For examples, one of the preferred alloys, nominal composition Al-12Fe-2V, can be compacted into a consolidated article having a hardness of at least 92 R.sub.B even when extruded at temperatures up to approximately 490.degree. C. See FIG. 7.

Rapidly solidified alloys having the Al.sub.bal Fe.sub.a X.sub.b composition described above can be processed 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 -60 to 200 mesh.

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. C.-500.degree. C., preferably ranging from about 325.degree. C.-450.degree. C., to preserve the microeutectic microstructure and minimize 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 rolling, vacuum hot compaction, blind die compaction in an extrusion press or forging press, direct and indirect extrusion, impact forging, impact extrusion and combinations of the above.

As representatively shown in FIG. 8, the compacted consolidated article of the invention is composed of an aluminum solid solution phase containing a substantially uniform distribution of dispersed, intermetallic phase precipitates therein. The precipitates are fine, irregularly shaped intermetallics measuring less than about 100 nm in all linear dimensions thereof: the volume fraction of these fine intermetallics ranges from about 25 to 45%. Preferably, each of the fine intermetallics has a largest dimension measuring not more than about 20 nm, and the volume fraction of coarse intermetallic precipitates (i.e. precipitates measuring more than about 100 nm in the largest dimension thereof) is not more than about 1%.

At room temperature (about 20.degree. C.), the compacted, consolidated article of the invention has a Rockwell B hardness (R.sub.B) of at least about 80. Additionally, the ultimate tensile strength of the consolidated article is at least about 550 MPa (80 ksi), and the ductility of the article is sufficient to provide an ultimate tensile strain of at least about 3% elongation. At approximately 350.degree. C., the consolidated article has an ultimate tensile strength of at least about 240 MPa (35 ksi) and has a ductility of at least about 10% elongation.

Preferred consolidated articles of the invention have an ultimate tensile strength ranging from about 550 to 620 MPa (80 to 90 ksi) and a ductility ranging from about 4 to 10% elongation, when measured at room temperature. At a temperature of approximately 350.degree. C., these preferred articles have an ultimate tensile strength ranging from about 240 to 310 MPa (35 to 45 ksi) and a ductility ranging from about 10 to 15% elongation.

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 and practice of the invention are exemplary and should not be construed as limiting the scope of the invention. All alloy compositions described in the examples are nominal compositions.

EXAMPLES 1 to 65

Alloys were cast with the method and apparatus of the invention. The alloys had an almost totally microeutectic microstructure, and had the microhardness values as indicated in the following Table 1.

Alloys outside the scope of the invention were cast, and had corresponding microhardness values as indicated in Table 2 below. These alloys were largely composed of a primarily dendritic solidification structure with clearly defined dendritic arms. The dendritic intermetallics were coarse, measuring about 100 nm in the smallest linear dimensions thereof.

FIG. 5, along with Table 3 below, summarizes the results of isochronal annealing experiments on (a) ascast strips having approximately 100% microeutectic structure and (b) as-cast strips having a dendritic structure. The Figure and Table show the variation of microVickers hardness of the ribbon after annealing for 1 hour at various temperatures. In particular, FIG. 6 illustrate that alloys having a microeutectic structure are generally harder after annealing, than alloys having a primarily dendritic structure. The microeutectic alloys are harder at all temperatures up to about 500.degree. C.; and are significantly harder, and therefore stronger, at temperatures ranging from about 300.degree. to 400.degree. C. at which the alloys are typically consolidated.

Alloys containing 8Fe-2Mo and 12Fe-2V, when produced with a dendritic structure, have room temperature microhardness values of 200-300 kg/m.sup.2 and retain their hardness levels at about 200 kg/mm.sup.2 up to 400.degree. C. An alloy containing 8Fe-2Cr decreased in hardness rather sharply on annealing, from 450 kg/mm.sup.2 at room temperature to about 220 kg/mm.sup.2 (which is equivalent in hardness to those of Al-1.35Cr-11.59Fe and Al-1.33Cr-13Fe claimed by Ray et al.).

On the other hand, the alloys containing 7Fe-4.6Y, and 12Fe-2V went through a hardness peak approximately at 300.degree. C. and then decreased down to the hardness level of about 300 kg/mm.sup.2 (at least 100 kg/mm.sup.2 higher than those for dendritic Al-8Fe-2Cr, Al-8Fe-2Mo and Al-8Fe-2V, and alloys taught by Ray et al.). Also, the alloy containing 8Fe-4Ce started at about 600 kg/mm.sup.2 at 250.degree. C. and decreased down to 300 kg/mm.sup.2 at 400.degree. C.

FIG. 6 also shows the microVickers hardness change associated with annealing Al-Fe-V alloy for 1 hour at the temperatures indicated. An alloy with 12Fe and 2V exhibits steady and sharp decrease in hardness and high temperature of at least about 600 kg/mm.sup.2 when cast in accordance with the invention. The present experiment also shows that for high temperature stability, about 1.5 to 5 wt % addition of a rare earth element; which has the advantageous valancy, size and mass effect over other transition elements; and the presence of more than 10 wt % Fe, preferably 12 wt % Fe, are important.

Transmission electron microstructures of alloys of the invention, containing rare earth elements, which had been heated to 300.degree. C., exhibit a very fine and homogeneous distribution of dispersoids inherited from the "microeutectic" morphology cast structure, as shown in FIG. 5(a). Development of this fine microstructure is responsible for the high hardness in these alloys. Upon heating at 450.degree. C. for 1 hour, it is clearly seen that dispersoids dramatically coarsen to a few microns sizes (FIG. 5(d)) which was responsible for a decrease in hardness by about 200 kg/mm.sup.2. Therefore, these alloy powders are preferably consolidated (e.g., via vacuum hot pressing and extrusion) at or below 450.degree. C. to be able to take advantage of the unique alloy microstructure presently obtained by the method and apparatus of the invention.

Table 4A and 4B shows the mechanical properties measured in uniaxial tension at a strain rate of about 10.sup.-4 /sec for the alloy containing Al-12Fe-2V at various elevated temperatures. The cast ribbons were subject first to knife milling and then to hammer milling to produce -60 mesh powders. The yield of -60 mesh powders was about 98%. The powders were vacuum hot pressed at 350.degree. C. for 1 hour to produce a 95 to 100% density preform slug, which was extruded to form a rectangular bar with an extrusion ratio of about 18 to 1 at 385.degree. C. after holding for 1 hour.

Table 5 below shows the mechanical properties of specific alloys measured in uniaxial tension at a strain rate of approximately 10.sup.-4 /sec and at various elevated temperatures. A selected alloy powder was vacuum hot pressed at a temperature of 350.degree. C. for 1 hour to produce a 95-100% density, preform slug. The slug was extruded into a rectangular bar with an extrusion ratio of 18 to 1 at 385.degree. C. after holding for 1 hour.

Important parameters that affect the mechanical properties of the final consolidated article include the composition, the specific powder consolidation method, (extrusion, for example,) and the consolidation temperature. To illustrate the selection of both extrusion temperature and composition, FIG. 7, shows the relationship between extrusion temperature and the hardness (strength) of the extruded alloy being investigated. In general, the alloys extruded at 315.degree. C. (600.degree. F.) all show adequate hardness (tensile strength): however, all have low ductility under these consolidation conditions, some alloy having less than 2% tensile elongation to failure, as shown in Table 6 below. Extrusion at higher temperatures: e.g. 385.degree. C. (725.degree. F.) and 485C. (900.degree. F.): produces alloys of higher ductility. However, only an optimization of the extrusion temperature (e.g. about 385.degree. C.) for the alloys, e.g. Al-12Fe-2V and Al-8Fe-3Zr, provides adequate room temperature hardness and strength as well as adequate room temperature ductility after extrusion. Thus, at an optimized extrusion temperature, the alloys of the invention advantageously retain high hardness and tensile strength after compaction at the optimum temperatures needed to produce the desired amount of ductility in the consolidated articles. Optimum extrusion temperatures range from about 325.degree. to 450.degree. C.

The alloys produced by the method and apparatus of the invention are capable of producing consolidated articles which have a high elastic modulus at room temperature and retain the high elastic modulus at elevated temperatures. Preferred alloys are capable of producing consolidated articles which have an elastic modulus ranging from approximately 100 to 70 GPa (10 to 15.times.10.sup.3 KSI) at temperatures ranging from about 20.degree. to 400.degree. C.

Table 7 below shows the elastic modulus of an Al-12Fe-2V alloy article consolidated by hot vacuum compaction at 350.degree. C., and subsequently extruded at 385.degree. C. at an extrusion ratio of 18:1. This alloy had an elastic modulus at room temperature which was approximately 40% higher than that of conventional aluminum alloys. In addition, this alloy retained its high elastic modulus at elevated temperatures.

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 subjoined claims.