Ceramic-metallic composites devoid of porosity and their methods of manufacture

Abstract

Ceramic-metallic composites are disclosed along with the equipment and processes for their manufacture. The present invention improves the densities of these composites by eliminating porosity through the use of a unique furnace system that applies vacuum and positive gas pressure during specific stages of processing. In the fabrication of Al.sub.2O.sub.3—Al composites, each process commences with a preform initially composed of at least 5% by weight silicon dioxide, and the finished product includes aluminum oxide and aluminum, and possibly other substances.

Claims

1. A method of manufacturing ceramic-metallic composites substantially devoid of centerline porosity, including the steps of: a) providing a vacuum-pressure-transformation (VPT) furnace comprising: i) a chamber having an access door; ii) a crucible within said chamber; iii) a source of inert gas connected to said chamber and a pressurization system for selectively pressurizing said chamber with said inert gas; iv) a vacuum pumping system connected to said chamber and selectively operable to evacuate said chamber; and v) a heating system operable to selectively heat said chamber; b) adding a charge of aluminum or an aluminum alloy into said crucible; c) placing a preform into said chamber above said crucible, said preform having unenclosed sides directly exposed to an interior of said chamber; d) operating said vacuum pumping system to establish a partial vacuum within said chamber; e) during partial vacuum status of said chamber, operating said heating system to raise chamber temperature above a melting point of said charge, whereby said charge is melted with said preform being heated to substantially the same temperature as said charge; f) immersing said preform into said melted charge within said crucible; g) operating said pressurization system to pressurize said chamber with said inert gas to a pressure greater than 50 psi; h) maintaining temperature and pressure constant until a displacement reaction has been completed; i) reducing temperature in said chamber while maintaining positive pressure within said chamber and extracting a now ceramic-metallic composite shape from said crucible; and j) allowing said composite shape to cool under positive pressure; and k) removing said ceramic-metallic composite shape from said chamber, said composite shape being substantially devoid of centerline porosity.

2. The method of claim 1, wherein said placing step includes the step of mounting said preform in a fixture.

3. The method of claim 1, wherein said inert gas is chosen from the group consisting of argon and helium.

4. The method of claim 2, wherein said fixture is placed on an actuator.

5. The method of claim 4, wherein said immersing step includes the step of operating said actuator to lower said preform into said crucible.

6. The method of claim 1, wherein said immersing step comprises lowering said preform into said crucible by force of gravity.

7. The method of claim 1, wherein said step of operating said vacuum pumping system comprises first evacuating said chamber to a pressure of 1×10.sup.−3 to 1×10.sup.−9 torr until said charge within said crucible is melted, then increasing pressure to about 0.001 to 25 torr.

8. The method of claim 1, wherein said step of operating said pressurization system comprises pressurizing said chamber to a pressure of 50 to 1000 psi.

9. The method of claim 1, wherein said method being conducted at a reaction rate of at least 8.5 mm/hr.

10. The method of claim 1, wherein said heating system is operated to raise temperature of said chamber to above 900° C.

11. The method of claim 1, wherein said ceramic-metallic composite shape is extracted from said crucible above a melting point of aluminum.

12. The method of claim 11, wherein: a) said crucible is removed from said chamber and said ceramic-metallic composite shape is re-inserted into said chamber; b) said vacuum pumping system is activated to reduce chamber pressure to no greater than 25 torr; c) heating said chamber to just below a melting point of said shape and activating said pressurization system to flow inert gas into said chamber to a pressure of at least 50 psi; d) increasing chamber temperature to above a melting temperature of metal in said shape; e) maintaining substantially constant temperature until substantially all porosity is removed from said shape; f) cooling said shape within said chamber under positive pressure; and g) removing said shape from said chamber.

13. The method of claim 12, wherein said inert gas is chosen from the group consisting of argon and helium.

14. The method of claim 12, wherein said pressure is 50 to 1000 psi.

15. The method of claim 12, wherein prior to said removing step recited in claim 12, subparagraph g), temperature within said chamber is maintained above aluminum solidification temperature.

16. The method of claim 15, wherein prior to said removing step recited in claim 12, subparagraph g), pressure in said chamber is reduced to ambient atmospheric pressure.

17. A method of manufacturing ceramic-metallic composites substantially devoid of centerline porosity, including the steps of: a) providing a vacuum-pressure-transformation (VPT) furnace comprising: i) a chamber having an access door; ii) a crucible within said chamber; iii) a source of inert gas connected to said chamber and a pressurization system for selectively pressurizing said chamber with said inert gas; iv) a vacuum pumping system connected to said chamber and selectively operable to evacuate said chamber; and v) a heating system operable to selectively heat said chamber; b) adding an aluminum charge into said crucible; c) placing a preform into said chamber above said crucible in a fixture on an actuator, said preform having unenclosed sides directly exposed to an interior of said chamber; d) operating said vacuum pumping system to establish a partial vacuum within said chamber; e) during partial vacuum status of said chamber, operating said heating system to raise chamber temperature above 900° C., whereby said aluminum charge is melted with said preform being heated to substantially the same temperature as said charge; f) operating said actuator to immerse said preform into liquid aluminum within said crucible; g) operating said pressurization system to pressurize said chamber with said inert gas chosen from the group consisting of argon and helium, to a pressure greater than 50 psi; h) maintaining temperature and pressure constant until a displacement reaction has been completed; i) reducing temperature in said chamber while maintaining positive pressure within said chamber and operating said actuator to extract a now ceramic-metallic composite shape from said crucible; j) allowing said composite shape to cool under positive pressure; and k) removing said ceramic-metallic composite shape from said chamber, said composite shape being substantially devoid of centerline porosity.

18. The method of claim 17, wherein said immersing step includes the step of operating said actuator to lower said preform into said crucible.

19. The method of claim 17, wherein said step of operating said vacuum pumping system comprises first evacuating said chamber to a pressure of 1×10.sup.−3 to 1×10.sup.−9 torr until said aluminum within said crucible is melted, then increasing pressure to about 0.001 to 25 torr.

20. The method of claim 17, wherein said method being conducted at a reaction rate of at least 8.5 mm/hr.

21. A method of manufacturing ceramic-metallic composites substantially devoid of porosity, including the steps of: a) providing a vacuum-pressure-transformation (VPT) furnace comprising: i) a chamber having an access door; ii) a crucible within said chamber; iii) a source of inert gas connected to said chamber and a pressurization system for selectively pressurizing said chamber with said inert gas; iv) a vacuum pumping system connected to said chamber and selectively operable to evacuate said chamber; and v) a heating system operable to selectively heat said chamber; b) adding a charge of aluminum or an aluminum alloy into said crucible; c) placing a preform into said chamber above said crucible; d) operating said vacuum pumping system to establish a partial vacuum within said chamber; e) operating said heating system to raise chamber temperature above a melting point of said charge, whereby said charge is melted; f) immersing said preform into said melted charge within said crucible; g) operating said pressurization system to pressurize said chamber with said inert gas to a pressure greater than 50 psi; h) maintaining temperature and pressure constant until a displacement reaction has been completed; i) reducing temperature in said chamber while maintaining positive pressure within said chamber and extracting a now ceramic-metallic composite shape from said crucible above a melting point of aluminum; j) allowing said composite shape to cool under positive pressure; k) removing said ceramic-metallic composite shape from said chamber; l) said crucible being removed from said chamber and said ceramic-metallic composite shape being re-inserted into said chamber; m) said vacuum pumping system being activated to reduce chamber pressure to no greater than 25 torr; n) heating said chamber to just below a melting point of said shape and activating said pressurization system to flow inert gas into said chamber to a pressure of at least 50 psi; o) increasing chamber temperature to above a melting temperature of metal in said shape; p) maintaining substantially constant temperature until substantially all porosity is removed from said shape; q) cooling said shape within said chamber under positive pressure; and r) removing said shape from said chamber.

Description

BRIEF DESCRIPTION OF THE DRAWINGS

(1) FIG. 1 is an optical photograph at 5× magnification of a prior art cross-sectioned test bar showing centerline porosity.

(2) FIG. 2 is an optical photograph at 5× magnification of a cross-sectioned test bar made in accordance with the teachings of the present invention showing no centerline porosity.

(3) FIG. 3 is an optical photograph at 5× magnification of a prior art broken test bar showing centerline porosity.

(4) FIG. 4 is an optical photograph at 5× magnification of a broken test bar made in accordance with the teachings of the present invention showing no centerline porosity.

(5) FIG. 5 is a cross-sectional drawing of a Vacuum-Pressure-Transformation furnace with a shape actuator, with the shape in the preheating position.

(6) FIG. 6 is a cross-sectional drawing of a Vacuum-Pressure-Transformation furnace with an actuator, with a shape immersed in liquid metal.

(7) FIG. 7 is a cross-sectional drawing of a Vacuum-Pressure-Transformation furnace without an actuator, with a shape in the preheating position on top of solid metal.

(8) FIG. 8 is a cross-sectional drawing of a Vacuum-Pressure-Transformation furnace without an actuator, with a shape immersed in liquid metal.

(9) FIG. 9 is a cross-sectional drawing of a Vacuum-Pressure-Transformation furnace without an actuator, with a shape going through a remelting-repressurization treatment cycle.

SPECIFIC DESCRIPTION OF THE PREFERRED EMBODIMENTS

(10) The Vacuum-Pressure-Transformation furnace is designed to carry out the transformation process and operates in two primary pressure conditions, either under negative pressure (i.e., vacuum) or positive pressure, and both modes are utilized during a complete furnace cycle. At the beginning as well as at the end of each furnace cycle the furnace chamber is allowed to reach ambient atmospheric pressure when opened up for loading and unloading shapes. An example of a Vacuum-Pressure-Transformation furnace is shown in FIG. 5, with the subcomponents individually labeled:

(11) A chamber system 10 capable of withstanding negative pressures and positive inert gas pressures, both while operating at elevated temperatures;

(12) A chamber lid 11 or door that allows the crucible, metal charge, fixture, and preform or composite shapes to be inserted into or removed out of the chamber;

(13) A heating system 13 that is integrated with the chamber, capable of heating metal to a liquid state and then to temperatures of at least 900° C. and higher, plus capable of preheating the preform shapes before being immersed into the liquid metal bath;

(14) A crucible 15, containing the metal 17 for the transformation process;

(15) A vacuum pumping system 19;

(16) An inert gas source and pressurization system 21;

(17) A fixture 23 used to secure the preform/composite shape 25 during processing.

(18) Finally, an optional actuator mechanism 27 used to fully immerse the preform shape into the liquid metal bath and to extract the ceramic-metallic composite shapes out of the liquid metal bath.

(19) Even though the exact configuration of the VPT furnace may be varied, it is the combination of these characteristics that will result in the successful fabrication of ceramic-metallic composites without porosity.

(20) The Vacuum-Pressure-Transformation furnace is designed to achieve specified levels of vacuum pressure (as low as possible) and positive inert gas pressure (as high as possible), but the exact values utilized during processing will be determined by a number of factors, as described below.

(21) While a perfect vacuum is defined as a volume of space entirely devoid of matter, industrial applications utilize partial vacuums whereby some matter is still present. Furthermore, it is more meaningful to define the quality of a vacuum by how closely it approaches a perfect vacuum as measured by partial pressure (less the atmospheric pressure) in units such as torr. With standard atmospheric pressure at 760 torr, the quality of a vacuum is typically grouped as follows: low vacuum (760 to 25 torr), medium vacuum (25 to 0.001 torr), high vacuum (1×10.sup.−3 to 1×10.sup.−9 torr), and ultra-high vacuum (1×10.sup.−9 to 1×10.sup.−12 torr). There are three benefits of applying a vacuum during the initial part of a Vacuum-Pressure-Transformation furnace cycle. The first is the removal of moisture from the chamber, which significantly reduces the amount of dissolved hydrogen in the liquid aluminum. The second is the removal of oxygen from the chamber; oxygen from air and moisture causes the top surface of the liquid aluminum bath to form an oxide layer. This oxide layer, which is also called dross in the prior art, forms a tenacious skin on the top surface of the melt and this skin will increase in thickness with increasing temperature and exposure time to oxygen. The oxide layer is broken up while immersing the preform shape into the liquid bath and fragments can cling to the outside of the shape or infiltrate into the shape, both of which cause potential quality issues in the final product. The use of a vacuum reduces this oxide layer to a minimal thickness. The third benefit is the evacuation of oxygen and moisture from the surfaces of the preform shape as well as the pores within the material, which removes another potential source of hydrogen gas or oxide defects. Therefore, optimizing the use of a vacuum during the initial part of a Vacuum-Pressure-Transformation furnace cycle essentially eliminates the formation of hydrogen gas porosity and dross defects in the final ceramic-metallic composite, and these results are typically achieved by utilizing a medium (25 to 0.001 torr) or high vacuum (1×10.sup.−3 1×10.sup.−9 torr). The three benefits outlined above can be realized while applying a vacuum in the VPT furnace while the system is at lower temperatures, but as the temperature is raised to 900° C. and above there is the risk that the boiling points of materials inside the VPT furnace are exceeded. For instance, the boiling point of liquid aluminum at 760 torr is 2327° C. but as the vacuum pressure is dropped to 0.1 torr the boiling point drops to 1123° C. In this example, a VPT furnace cycle may utilize a high vacuum such as 1×10.sup.4 torr at temperatures below 900° C. but then reduce to a medium vacuum such as 5 torr as the temperature is increased beyond 900° C. Therefore, careful consideration must be given to the boiling points of the materials that are contained within the VPT chamber, as well as the vacuum levels achieved during a VPT furnace cycle.

(22) Finally, the benefit of applying a positive inert gas pressure during and after the shape is extracted from the liquid metal bath in the VPT furnace is to eliminate the formation of centerline porosity during cooling of the shape. When the composite shape is extracted from the liquid metal bath the outside surfaces cool first and possibly faster than the interior region. Therefore the liquid metal located near the outside surfaces inside the composite shape starts solidifying first. Centerline porosity forms because the center region in the composite shape is the last to solidify and there is no ability to backfill this porosity with additional liquid metal. By applying positive inert gas pressure and controlling the rate of cooling, the liquid metal inside the composite shape is allowed to solidify more uniformly while being pushed to the center regions, thereby eliminating the possibility of having centerline porosity form. The present application has found that this benefit can be achieved at positive pressures as low at 0.3 MPa (50 psi) to 6.9 MPa (1000 psi) and higher.

(23) A Vacuum-Pressure-Transformation furnace can be configured to run in two operating modes, as seen in FIGS. 5 through 9.

(24) The first mode, shown in FIGS. 5 and 6, carries out transformation of a preform shape by utilizing an actuator 27. With this mode the entire process is carried out in one furnace cycle, resulting in ceramic-metallic composites without centerline porosity.

(25) The second mode, shown in FIGS. 7, 8, and 9, must be carried out in two furnace cycles. In the first cycle transformation of a preform shape 25 is carried without an actuator, as shown in FIGS. 7 and 8. This is followed by the second cycle with a remelting-repressurization treatment on that same composite shape 25, as shown in FIG. 9; the crucible 15 and metal bath 17 are removed and the composite shape 25 is placed back into the furnace chamber.

(26) Both modes will result in a composite shape that does not contain centerline porosity. While the first Vacuum-Pressure-Transformation furnace configuration mode appears to be the most expedient by carrying out the entire process in one furnace cycle, incorporating an actuator in the system presents design and operational challenges since it must withstand both vacuum and high positive pressures while under high temperature conditions. Therefore, operating the VPT furnace in the second configuration mode with two furnace cycles is an alternative.

(27) In the case of the second configuration mode, the first furnace cycle is used to transform the preform material into the ceramic-metallic composite without the use of an actuator. The composite shape stays in the liquid metal bath at the end of the transformation cycle and then removed from the liquid under ambient pressure conditions. Even though the furnace cycle eliminates dissolved hydrogen gas through the use of vacuum pressure, centerline porosity is formed due to the shrinkage of liquid aluminum during solidification and because the center region of the shape is the last to cool. Therefore the second furnace cycle to carry out a remelting-repressurization treatment is necessary in order to eliminate the centerline porosity in the final composite shape when the VPT furnace does not have an actuator.

(28) During the second furnace cycle, the ceramic-metallic composite shape is heated to a temperature just above the melting point of the metal constituent, with the furnace cycle starting off with a negative pressure in order to remove oxygen and moisture from the chamber and then switched over to a positive inert gas pressure in order to complete the cycle. The shape is heated to a high enough temperature and for a long enough time to melt the metal constituent in the composite. By applying positive inert gas pressure and controlling the rate of cooling, the liquid metal inside the composite shape is allowed to solidify more uniformly while being pushed to the center regions, thereby eliminating the possibility of having centerline porosity forming again. After the remelting-repressurization treatment the ceramic-metallic composite material will have improved properties.

(29) As defined by the Metals Handbook Desk Edition, the prior art definition for the term “heat treatment” is “heating and cooling a solid metal or alloy in such a way as to obtain desired conditions or properties”. Also, as defined by Metals Handbook Ninth Edition, Volume 15: Casting, the prior art process of “hot isostatic pressing” of solid aluminum alloys is carried out at high inert gas pressures but at temperatures below were incipient melting occurs. In both cases, these prior art processes require the metal shape being processed to remain solid and below the melting point. In contrast, the present invention's use of a remelting-repressurization treatment is unique in that the process is specifically used to eliminate porosity in ceramic-metallic composites by heating the material to a point where the metal constituent in the composite shape is melted as well as through the application of negative and positive pressures.

(30) The minimum processing temperatures utilized in a remelting-repressurization treatment furnace cycle will depend upon the composition of the metal in the ceramic-metallic composite, more specifically the melting point of that metal or metal alloy. For instance, pure aluminum melts at 660° C., when aluminum is alloyed with magnesium the initial melting point is 450° C., and when aluminum is alloyed with silicon the initial melting point is 577° C. Therefore careful consideration needs to be given of the metal alloy composition in the ceramic-metallic composite and the associated liquidus temperature.

(31) None of the benefits discovered by using the Vacuum-Pressure-Transformation furnace are possible with the conventional transformation furnace from the prior art. Through the utilization of the VPT furnace the elimination of porosity in ceramic-metallic composites has a very significant effect on the final material properties. For example, a test bar similar to that shown in FIG. 1 had a three-point flexural strength of 275 MPa or 39,900 psi (per ASTM procedure C 1161) and 1.5 volume % porosity as measured using the Archimedes' principle (per ASTM procedure C 20), while a test bar similar to that shown in FIG. 2 had a three-point flexural strength of 423 MPa or 61,400 psi and no measurable porosity. By eliminating centerline porosity through the use of this unique furnace system, the increased density maximizes the strength of the composites.

(32) The following two embodiments for this invention make significant changes to the final density of the ceramic-metallic composites in the prior art embodiment, resulting in unique composites with lower amounts of porosity and subsequently improved properties over those resulting from the prior art embodiment.

(33) In one preferred embodiment the Vacuum-Pressure-Transformation furnace incorporates an actuator to handle the preform shape during controlled immersion into the liquid metal bath as well as extract the part out of the bath upon completion of the transformation reaction. The configuration of the actuator will vary depending upon a combination of factors including desired complexity and cost, but its function will always be to constrain the shape during immersion and extraction from the liquid metal bath. When using this actuator the VPT furnace cycle is comprised of essentially seven stages.

(34) Stage 1: The preform shape is mounted in a fixture, a crucible containing an aluminum charge is prepared, all are loaded into the VPT furnace while at ambient atmospheric pressure, and the chamber is then closed.

(35) Stage 2: A vacuum is pulled on the chamber while the preform is held by the actuator above the crucible and the heating system preheats the preform shape to a temperature above 900° C. as well as melts the aluminum. Initially a high vacuum (1×10.sup.−3 to 1×10.sup.−9 torr) may be utilized but once the metal is melted and reaches 900° C., the vacuum quality will need to be lowered to a medium vacuum (25 to 0.001 torr), with the final value dependent upon the final hold temperature and the boiling point of the constituents in the melt.

(36) Stage 3: While the chamber is under a vacuum, and the preform shape and aluminum bath are both above 900° C., the preform is fully immersed into the liquid aluminum bath by the actuator.

(37) Stage 4: Shortly after the shape is fully immersed into the liquid aluminum bath, positive inert gas pressure is applied, and the temperature and pressure are held constant until the displacement reaction is fully completed. Argon or helium gas may be utilized at pressure ranging from 0.3 MPa (50 psi) to 6.9 MPa (1000 psi) or higher.

(38) Stage 5: While the chamber is under positive pressure, the shape is fully extracted out of the liquid aluminum bath by the actuator.

(39) Stage 6: While the chamber is under positive pressure, the shape is allowed to cool.

(40) Stage 7: Once the temperature of the shape has dropped below the aluminum solidification temperature, the pressure in the VPT furnace is reduced to the ambient atmospheric pressure, the chamber is opened up, and the ceramic-metallic composite shape is removed. This temperature is 660° C. for pure aluminum or at lower temperatures for aluminum alloys, dependent upon the alloying elements. The final composite material has no centerline porosity and improved properties.

(41) In a second preferred embodiment the Vacuum-Pressure-Transformation furnace does not incorporate an actuator to handle the preform shape into or out of the liquid metal bath. Because this prevents the application of pressure after the shape is extracted from the bath, the shape must be run again through a second VPT furnace cycle. In this embodiment the first VPT furnace cycle is comprised of essentially six stages.

(42) Stage 1: A solid aluminum charge is loaded into a crucible, the preform shape is mounted in a fixture and then placed on top of the solid aluminum, the entire assembly is loaded into the VPT furnace while at ambient atmospheric pressure, and then the chamber is closed.

(43) Stage 2: A vacuum is pulled on the chamber and the heating system heats the preform shape and aluminum to a temperature above 900° C. Initially a high vacuum (1×10.sup.−3 to 1×10.sup.−9 torr) may be utilized but once the metal is melted and reaches 900° C., the vacuum quality will need to be lowered to a medium vacuum (25 to 0.001 torr), with the final value dependent upon the final hold temperature and the boiling point of the constituents in the melt.

(44) Stage 3: While the chamber is under a vacuum and when the aluminum has melted, the fixtured preform fully immerses itself into the liquid aluminum bath by the force of gravity. The melting temperature of the metal is 660° C. for pure aluminum or at lower temperatures for aluminum alloys, dependent upon the alloying elements.

(45) Stage 4: Once the VPT furnace has reached a temperature above 900° C., positive inert gas pressure is applied, and the temperature and pressure are held constant until the displacement reaction is fully completed. Argon or helium gas may be utilized at pressure ranging from 0.3 MPa (50 psi) to 6.9 MPa (1000 psi) or higher.

(46) Stage 5: While the chamber is under positive pressure, the shape is allowed to cool to a temperature just above the aluminum solidification temperature. Again, this temperature is above 660° C. for pure aluminum or at lower temperatures for aluminum alloys.

(47) Stage 6: While holding the VPT furnace temperature just above the aluminum solidification temperature, the pressure in the chamber is reduced to the ambient atmospheric pressure, the chamber is opened up, the ceramic-metallic composite shape is extracted from the liquid metal bath, and the crucible is removed from the furnace.

(48) After the first VPT furnace cycle, the ceramic-metallic shape must be run through a second VPT furnace cycle in order to eliminate the centerline porosity through a remelting-repressurization treatment. This second VPT furnace cycle is comprised of essentially seven stages.

(49) Stage 1: The composite shape is loaded into the empty VPT furnace while at ambient atmospheric pressure and the chamber is then closed.

(50) Stage 2: A vacuum is pulled on the chamber and then the heating system begins to heat the composite shape. A medium vacuum (25 to 0.001 torr) or high vacuum (1×10.sup.−3 to 1×10.sup.−9 torr) may be utilized.

(51) Stage 3: Before the metal in the composite reaches its melting temperature, a positive pressure is applied by utilizing an inert gas such as argon or helium. This temperature is 660° C. for pure aluminum or at lower temperatures for aluminum alloys, dependent upon the alloying elements. The inert gas may be utilized at pressure ranging from 0.3 MPa (50 psi) to 6.9 MPa (1000 psi) or higher.

(52) Stage 4: While the positive inert gas pressure is applied, the heating system increases the temperature of the shape to a point above the melting temperature of the metal in the composite.

(53) Stage 5: While the chamber is under positive pressure, the temperature and pressure are held constant until all the porosity is removed from the composite shape.

(54) Stage 6: The shape is allowed to cool while the chamber is under positive pressure.

(55) Stage 7: Once the temperature of the shape has dropped below the aluminum solidification temperature, the pressure in the VPT furnace is reduced to the ambient atmospheric pressure, the chamber is opened up, and the final ceramic-metallic composite shape is removed. This temperature is 660° C. for pure aluminum or at lower temperatures for aluminum alloys, dependent upon the alloying elements. The final composite material has no centerline porosity and improved properties.

(56) The following are examples of these preferred embodiments of the present invention.

Example 1

(57) A preform test bar shape containing 36 weight % silicon dioxide (SiO.sub.2) and 64 weight % silicon carbide (SiC) was conventionally fabricated. The preform shape and a crucible containing an alloy of 80 weight % aluminum and 20 weight % silicon were loaded into the Vacuum-Pressure-Transformation furnace. The VPT furnace was then closed and a high vacuum of 1×10.sup.−4 torr was pulled on the chamber. While under this negative pressure the preform was held by an actuator above the crucible, and the heating system preheated the preform shape to a temperature of 900° C. and melted the Al—Si alloy. When 900° C. was reached, vacuum was reduced to a medium vacuum of 5 torr. Once the preform shape and aluminum alloy were both heated to 1200° C. the preform was fully immersed into the liquid Al—Si bath by the actuator while under this negative pressure. Shortly after the shape was fully immersed into the liquid metal bath, a positive inert argon gas pressure was applied. The temperature was held at 1200° C. and the pressure was held at 5.0 MPa (725 psi) for 90 minutes. Transformation was completed in that time, achieving a rate of 8.5 mm per hour. While the chamber was kept at this positive pressure the shape was fully extracted out of the liquid aluminum bath by the actuator and then allowed to cool to below 450° C. After the pressure in the VPT furnace was reduced to ambient atmospheric pressure, the chamber was opened up and the SiC—Al.sub.2O.sub.3—Al ceramic-metallic composite shape was removed. The test bar was measured for flexural strength and apparent porosity per ASTM standards C1161 and C20: the three-point flexural strength was 423 MPa (61,400 psi) and the apparent porosity was zero. The final composite material had no centerline porosity as determined by measuring its density and a structure similar to that seen in FIGS. 2 and 4.

Example 2

(58) Two preform test bar shapes containing 36 weight % SiO.sub.2 and 64 weight % SiC were conventionally fabricated. The preform shapes were mounted in a fixture and placed on top of a crucible containing a solid alloy of 80 weight % Al and 20 weight % Si, all at room temperature. The crucible and fixture assembly were loaded into the Vacuum-Pressure-Transformation furnace. The VPT furnace was closed and a high vacuum of 1×10.sup.−4 torr was pulled on the chamber. While under this negative pressure the preform shapes and Al—Si alloy were heated to a temperature of 900° C. At some point above 577° C., the Al alloy melted and the fixture and two preform shapes were fully immersed into the liquid metal bath by the force of gravity. When 900° C. was reached, vacuum was reduced to a medium vacuum of 5 torr. Once the alloy bath reached 1200° C., a positive inert argon gas pressure of 7.0 MPa (1015 psi) was applied for 90 minutes. While the chamber was kept at this positive pressure the liquid metal was then allowed to cool to 700° C. or above its temperature of solidification. At that point the pressure in the VPT furnace was reduced to ambient atmospheric pressure, the chamber was opened up, the fixture was extracted from the liquid metal bath, the SiC—Al.sub.2O.sub.3—Al ceramic-metallic composite shapes were extracted from the fixture, and the crucible was removed from the furnace. One test bar was set aside and left as is, while the second test bar was run through a second VPT furnace cycle furnace cycle for a remelting-repressurization treatment. That composite shape was loaded into the VPT furnace (after removing the crucible and metal) while at ambient atmospheric pressure and the chamber was then closed. A high vacuum of 1×10.sup.−4 torr was pulled on the chamber and then the furnace began heating the composite shape. The negative pressure was held until the shape reached 400° C. and then a positive inert argon gas pressure of 7.0 MPa (1015 psi) was applied. Heating continued until the shape reached 700° C. and then the test bar was held at that temperature for 90 minutes. While still being held under the positive pressure, the temperature of the shape was allowed to drop to 400° C. At that point the pressure in the VPT furnace was reduced to the ambient atmospheric pressure, the chamber was opened up, and the final ceramic-metallic composite shape is removed. Both test bars were measured for flexural strength and apparent porosity per ASTM standards C1161 and C20. The test bar that was only run through the VPT furnace had a three-point flexural strength of 275 MPa (39,900 psi), 1.5 volume % apparent porosity, and centerline porosity similar to that seen in FIGS. 1 and 3. In contrast, the test bar that was run through the VPT furnace twice had a three-point flexural strength of 420 MPa (61,000 psi), an apparent porosity of zero as determined by measuring density, and no centerline porosity with a structure similar to that seen in FIGS. 2 and 4.

Example 3

(59) A preform rod shape containing silicon dioxide (SiO.sub.2) was conventionally fabricated. The preform shape and a crucible containing commercially pure aluminum were loaded into the Vacuum-Pressure-Transformation furnace. The VPT furnace was then closed and a high vacuum of 1×10.sup.−4 torr was pulled on the chamber. While under this negative pressure the preform was held by an actuator above the crucible, and the heating system preheated the preform shape to a temperature of 900° C. and melted the aluminum. When 900° C. was reached, vacuum was reduced to a medium vacuum of 5 torr. Once the preform shape and aluminum alloy were both heated to 1200° C. the preform was fully immersed into the liquid Al bath by the actuator while under this negative pressure. Shortly after the shape was fully immersed into the liquid metal bath, a positive inert argon gas pressure was applied. The temperature was held at 1200° C. and the pressure was held at 5.0 MPa (725 psi) for 90 minutes. While the chamber was kept at this positive pressure the shape was fully extracted out of the liquid aluminum bath by the actuator and then allowed to cool to below 600° C. After the pressure in the VPT furnace was reduced to ambient atmospheric pressure, the chamber was opened up and the Al.sub.2O.sub.3—Al ceramic-metallic composite shape was removed. The final composite material had no centerline porosity as determined by measuring density.

Example 4

(60) A preform rod shape containing SiO.sub.2 was conventionally fabricated. The preform shape and a crucible containing an alloy of 60 weight % aluminum and 40 weight % silver (Ag) were loaded into the Vacuum-Pressure-Transformation furnace. The VPT furnace was then closed and a high vacuum of 1×10.sup.−4 torr was pulled on the chamber. While under this negative pressure the preform was held by an actuator above the crucible, and the heating system preheated the preform shape to a temperature of 900° C. and melted the aluminum alloy. When 900° C. was reached, vacuum was reduced to a medium vacuum of 10 torr. Once the preform shape and alloy were both heated to 1200° C. the preform was fully immersed into the liquid Al—Ag bath by the actuator while under this negative pressure. Shortly after the shape was fully immersed into the liquid metal bath, a positive inert argon gas pressure was applied. The temperature was held at 1200° C. and the pressure was held at 6.9 MPa (1000 psi) for 90 minutes. While the chamber was kept at this positive pressure the shape was fully extracted out of the liquid aluminum alloy bath by the actuator and then allowed to cool to below 550° C. After the pressure in the VPT furnace was reduced to ambient atmospheric pressure, the chamber was opened up and the Al.sub.2O.sub.3—Al—Ag ceramic-metallic composite shape was removed. The final composite material had no centerline porosity as determined by measuring density.

Example 5

(61) A preform rod shape containing SiO.sub.2 was conventionally fabricated. The preform shape and a crucible containing an alloy of 85 weight % aluminum and 15 weight % iron (Fe) were loaded into the Vacuum-Pressure-Transformation furnace. The VPT furnace was then closed and a high vacuum of 1×10.sup.−4 torr was pulled on the chamber. While under this negative pressure the preform was held by an actuator above the crucible, and the heating system preheated the preform shape to a temperature of 900° C. and melted the aluminum alloy. When 900° C. was reached, vacuum was reduced to a medium vacuum of 7.5 torr. Once the preform shape and alloy were both heated to 1200° C. the preform was fully immersed into the liquid Al—Fe bath by the actuator while under this negative pressure. Shortly after the shape was fully immersed into the liquid metal bath, a positive inert argon gas pressure was applied. The temperature was held at 1200° C. and the pressure was held at 5.0 MPa (725 psi) for 90 minutes. While the chamber was kept at this positive pressure the shape was fully extracted out of the liquid aluminum alloy bath by the actuator and then allowed to cool to below 650° C. After the pressure in the VPT furnace was reduced to ambient atmospheric pressure, the chamber was opened up and the Al.sub.2O.sub.3—Al—Fe ceramic-metallic-intermetallic composite shape was removed. The final composite material had no centerline porosity as determined by measuring density.