AU2010284750B9

AU2010284750B9 - A process for producing a metal-matrix composite of significant deltaCTE between the hard base-metal and the soft matrix

Info

Publication number: AU2010284750B9
Application number: AU2010284750A
Authority: AU
Inventors: Azizan Aziz; Azmi Rahmat; Kahtan S. Mohammed
Original assignee: Universiti Sains Malaysia (USM)
Current assignee: Universiti Sains Malaysia (USM)
Priority date: 2009-08-17
Filing date: 2010-07-05
Publication date: 2014-07-24
Anticipated expiration: 2030-07-05
Also published as: CA2770464A1; WO2011021923A1; AU2010284750B2; MY153686A; EP2467223A1; AU2010284750A1

Description

WO 20111021923 PCT/MY2010/000115 A PROCESS FOR PRODUCING A METAL-MATRIX COMPOSITE OF SIGNIFICANT ACTE BETWEEN THE HARD BASE-METAL AND THE SOFT MATRIX FIELD OF INVENTION 5 The present invention relates to a process for producing a metal-matrix composite of significant ACTE between the hard base-metal and the soft matrix. BACKGROUND ART 10 Recently metal matrix composites (MMCs) have received much attention due to their electrical and mechanical properties. Tungsten composite (W-bronze and W-Cu) is an example of MMC's materials that has gained great importance in many applications. Characterized by its high density, high strength, adequate fracture toughness, 15 hardness, high wear resistance and low thermal expansion, make it very a good candidate as lead replacement materials in many military and industrial applications. They are suitable for fabrication of ammunition, center of gravity (CG) adjusters, gyroscope rotors and radiation shelters. Other applications are irrelevant to lead replacement like kinetic energy penetrators and jet vanes. Additionally for W-Cu 20 composites and due to their excellent thermal conductivity, they found their way to various electrical and electronic applications, these include, electrical contacts, resistance welding electrodes, electro-discharge machining electrodes, heat sinkers and power packaging for microelectronic and optoelectronics applications. 25 Alloying of W with bronze alloy is difficult to cast. Consulting W-Cu equilibrium phase diagram shows that W metal and Cu are almost completely immiscible in both solid WO 2011/021923 PCT/MY2010/000115 2 and liquid phase. Cu heat of mixing with W is positive i.e. 35.5kJ/mole. Energy of formation of W-Sn solid solution is positive as well i.e. 20kJ/mole. Accordingly attaining fully dense sintered compacts of these systems is not easy to handle. To tackle this problem the researchers in the recent three decades tried different techniques and 5 approaches. These techniques comprise liquid phase sintering, sintering at higher temperatures, using finer elemental powders and incorporating sintering activators (Fe, Co, Ni and Pd). Other more sophisticated technique is to change the surface morphology and the wettability of the W-base metal by thin film Ni coatings on the W particles prior to sintering and mechanical alloying (MA) of the W composite elemental tO powders to reduce the W-W spacing and to mechanically super induce inter diffusion. layer on the elemental powders. So far all these techniques were not sufficient to attain fully dense net shape compacts out of these composite systems. Despite the success of all these approaches and 15 techniques, they bring along negative impacts to at least one or two of the physical and mechanical properties of the sintered compacts like. conductivity, thermal expansion and strength. Another method and the most widely used are to infiltrate the sintered and porous W skeleton preforms by the matrix melt (bronze or copper). The object of all these processes is to achieve homogeneous pore free MMC material with 20 even distribution of the Cu or bronze phase in the W composite structure. Porosity is deleterious to flexural strength, electrical and thermal conductivity of the composite. The pores and voids act as points of stress concentrations and reduce the cross sectional area across which a load is applied and lead to a tremendous fall of the flexural strength. Air that is present in the pores has poor thermal and electrical 25 conductivity and thus it deteriorates the overall thermal and electrical properties of the composite. Therefore, it is essential to avoid pores formation in the composite during WO 2011/021923 PCT/MY2010/000115 3 its manufacturing process. Currently there are many infiltration techniques available for producing metal or metal-ceramics composite materials in particular tungsten-copper or tungsten-bronze composites. 5 The main steps of any conventional infiltration process are as follow. e Tungsten powder preparation with average size of 1-5pm. Optional step: coating the powder with nickel. Total nickel content is about0.04%. e Mixing the tungsten powder with a polymer binder. 10 e Compacting the powder by a modeling method (metal injection molding, die pressing, isostatic pressing). Compaction should provide the predetermined porosity level (apparent density) of the tungsten structure. * Solvent debinding. * Sintering the green compact at 1200OC-13OOcC in hydrogen atmosphere for 15 2hrs. e Placing the sintered part on copper plate in the infiltration/sintering furnace. " Infiltration of the sintered tungsten skeleton porous structure with copper at 1110 -1260*C in either hydrogen atmosphere or vacuum for hour. Infiltration of the sintered tungsten skeleton porous structure by the second metal, 20 having a lower melting point, can be conducted under gas pressure (gas pressure infiltration), under the pressure of mold movable part i.e. ramming (squeeze casting infiltration) and under the die pressure (pressure die casting). US Patent No 5963773 disclosed a method of fabricating tungsten skeleton structure 25 comprising the step of forming a source powder by coating a tungsten powder with nickel Then admixing the source powder and a polymer binder, performing powder WO 2011/021923 PCT/MY2010/000115 4 injection molding and obtaining a tungsten skeleton structure by removing the polymer binder. A copper plate is then placed beneath the tungsten skeleton structure and infiltration is carried out at temperature between 11509C and1250QC. This method is not viable for producing complicated shapes. 5 US Patent No. 5413751 describes a process for forming heat sinks and other heat dissipating elements by press-forming composite powders for metal components, for example tungsten and copper, to form pressed compacts and then sintering the pressed compacts to achieve a homogenous distribution of the copper throughout the 10 tungsten-copper structure. US Patents No. 4942076, 4988386, 5563101, disclose procedures of improving heat sink properties of W-Cu composite material utilized in microwave devices by maintaining even dispersion of tungsten particles having low thermal expansion 15 coefficient within a copper matrix having high thermal conductivity. This process improves the thermal conductivity of the W-Cu composite and modifies the thermal expansion coefficient to be correspondent to that of Gallium Arsenide (Ga. As.) Substrate utilized in microwave devices. 20 In fact almost all previous infiltration techniques utilized so far to infiltrate the W porous structure preforms, have been yielding composite structures which more or less have some weak points in their mechanical, electrical and micro structural properties. These points can be summarized as follow. 25 e The percentage of the infiltrated component cannot be easily controlled through infiltration process and going for higher infiltrant percentages is difficult.

5 * Liquid copper is known to have high, contact angles with several oxides which impede the infiltration process and accordingly control of furnace atmosphere is very essential for successful infiltration process. * In the thermal debinding cycle of the tungsten green compact to produce porous 5 tungsten structure preform, formation of tungsten oxide (W0 3 ) is very common. This oxide is known to be very volatile and hence it is very important that it is to be avoided to prevent material loss. * For optimum and maximum infiltration, specific pore length and pore diameter (L/D) are recommended and maintaining these recommendations in the 10 tungsten porous preform is not easy. * Mismatch between W skeleton and the matrix increases resistivity and deteriorates electrical specifications. * Lakes of matrix solidified melts within the W skeleton lead to non-consistent mechanical properties. 15 Any discussion of the prior art throughout the specification should in no way be considered as an admission that such prior art is widely known or forms part of common general knowledge in the field. 20 It is an object of the present invention to overcome or ameliorate at least one of the disadvantages of the prior art, or to provide a useful alternative. SUMMARY OF THE INVENTION According to a first aspect, the present invention provides a process for producing a 25 metal-matrix composite made of a matrix phase and a dispersed phase materials, the process includes the steps of: (a) preparing of composite powder components comprising the hard base material and a soft metal matrix by mixing or ball milling; (b) pressing the composite powder components as obtained from step (a) 30 under a preferred pressure; (c) sintering the composite powder components as obtained from step (b) at a preferred heating rate, temperature and time; and (d) cooling composite powder components as obtained from step (c) at a preferred cooling down rate; 35 6 wherein the process is driven by differential thermal expansion coefficients between shell and core materials. According to a second aspect, the present invention provides a metal-matrix composite 5 made of a matrix phase and a dispersed phase materials produced by the process of the invention. Unless the context clearly requires otherwise, throughout the description and the claims, the words "comprise", "comprising", and the like are to be construed in an inclusive sense as opposed to an exclusive or exhaustive sense; that is to say, in the 10 sense of "including, but not limited to". Accordingly, the present invention provides a process for producing a metal-matrix composite of significant ACTE between the hard base-material and any element type of the soft metal matrix. The process includes the steps of sintering, pressing and 15 enforced infiltration of the sintered compact wherein the process is driven by differential thermal expansion coefficients between the shell and the core materials. The present invention consists of several novel features and a combination of parts hereinafter fully described and illustrated in the accompanying description and 20 drawings, it being understood that various changes in the details may be made without departing from the scope of the invention or sacrificing any of the advantages of the present invention.

WO 2011/021923 PCT/MY2010/000115 7 BRIEF DESCRIPTION OF THE DRAWINGS The present invention will be fully understood from the detailed description given herein below and the accompanying drawings which are given by way of illustration 5 only, and thus are not limitative of the present invention, wherein: Fig. 1 is a schematic representation of the process for producing a metal-matrix composite of significant ACTE between the hard base-metal and the soft matrix according to the preferred embodiments of the present invention; 10 Fig. 2 is a 3D schematic diagram of the process of the present invention; Fig. 3 (a) is an optical graph of W80wt.%-Cu18-Sn shell-on-core sintered compact cross section of as received elemental powder, the shell is bronze admix. The compact 15 sintered at 1150C for 3 hours under H 2

/N

2 20/80 wt. ratio as protective gas utilizing the process of the present invention. Fig. 3 (b) is an optical graph of shell-on-core sintered compact cross section of W50wt.%-Cu45-Sn sintered compact of two-steps ball milled powder. The shell is 20 copper wherein the compact is sintered at 1150*C for 3 hours under H 2

/N

2 20/80 wt. ratio as protective gas utilizing the process of the present invention; Fig. 3 (c) is an optical graph of shell-on-core sintered compact cross section of W80wt.%-Cu18-Sn sintered compact. The compact is sintered at 1150*C for 3 hours 25 under H 2

/N

2 20/80 wt. ratio as protective gas utilizing the process of the present invention WO 2011/021923 PCT/MY2010/000115 8 Fig. 3 (d) is an optical graph shell-on-core sintered compact cross section of W40wt% pre-alloyed bronze sintered compact. The compact is sintered at 1150 0 C for 3 hours under. H 2

/N

2 20/80 wt ratio protective gas utilizing the process of the present invention; 5 Fig. 4 is the micro hardness profile across the solidified Cu-Sn shell and the W80wt.% Cui 8-Sn sintered compact of 99% of its theoretical density. Fig. 5 shows SEM micrographs of W 80wt %-Cul8-Sn sintered compacts of as received elemental powder sintered and densified by this invention, The last 10 micrograph represents sintered compact of similar composition sintered by the conventional method of uniaxial compaction and sintering. Fig. 6 (a) shows the optical micrographs of W80wt.%-Cul8-Sn compacts of as received powder sintered conventionally at 1150"C for 3 hours; 15 Fig. 6 (b) shows the optical micrographs of W80wt.%-Cul8-Sn compacts of as received powder sintered by the process of the present invention under similar sintering conditions as in Fig. 6 (a). 20 Fig. 6 (c) shows the optical micrographs of W90wt.%-Cu18-Sn compacts of as received elemental powder sintered conventionally at 11500C for 3 hours. Fig. 6 (d) shows the optical micrographs of W90wt.%-Cul8-Sn compacts of similar composition to Fig. 6 (c) sintered by the process of the present invention under similar 25 sintering conditions as in Fig. 6 (c).

WO 2011/021923 PCT/MY2010/000115 9 Fig. 7 shows a schematic representation of the process of the present invention at its final stage. Fig. 8 (a) shows a SEM micrograph of W50wt.%-pre-alloy bronze compact of ball 5 milled powder sintered conventionally. Fig. 8 (b) shows a SEM micrograph of W50wt.%-pre-alloy bronze compact of ball milled powder sintered by the present invention wherein sintered density of 99% theoretical density is achieved. 10 Fig. 8 (c) shows a SEM micrograph of W50wt.%-Cu45-Sn compact of two-step ball milled powder sintered conventionally. Fig. 8 (d) shows a SEM micrograph of W50wt.%-Cu45-Sn compact of two-step ball 15 milled powder sintered by the present invention technique wherein sintered density of 98% theoretical density is achieved. Fig. 9 (a) shows the EDX line scan across the shell/core boarder of W50wt/.-Cu45-Sn sintered compact of ball milled powder sintered by the process of the present 20 invention. Fig. 9 (b) shows the EDX line scan of sintered compact of similar composition of W50wt%-Cu45-Sn sintered compact of ball milled powder sintered by the present invention technique but with pre-alloy bronze matrix. The shell is of Cu element. 25 WO 2011/021923 PCT/MY2010/000115 10 Fig. 10 shows the alumina ceramics mould shows the cavity in where the W-bronze green compact placed and covered all around by the Cu-Sn powder mix shell before sintering. 5 Fig.1 1 shows a typical thermal and infiltration sintering cycle program of the present invention wherein the sintering, heating and cooling temperature rates can be altered according to the sintered material specifications.

WO 2011/021923 PCT/MY2010/000115 11 DESCRIPTION OF THE PREFERRED EMBODIMENTS The present invention relates to a process for producing a metal-matrix composite of significant ACTE between the hard base-metal and the soft matrix. Hereinafter, this 5 specification will describe the present invention according to the preferred embodiments of the present invention. However, it is to be understood that limiting the description to the preferred embodiments of the invention is merely to facilitate discussion of the present invention and it is envisioned that those skilled in the art may devise various modifications and equivalents without departing from the scope of the 10 appended claims. The present invention is generally related to a method for producing a composite material comprising a matrix phase and a dispersed phase, in particular metal-metal, metal-ceramics/carbide or composite material, such as tungsten-bronze, tungsten 15 copper, Al-SiC and Al-A1 2 0 3 . The hard base component could be particulates, dispersoids, cermets, short or continuous fibers, monofilament or whiskers reinforcements of any material type having significantly lower CTE value than that of the matrix. The process includes hot pressing, sintering and infiltration, acting simultaneously in one stage to yield microstructure of even distribution of the 20 dispersed phase in the metal matrix. The "three in one densification process" relates to the sintering/infiltration of metal metal or metal-ceramics/carbide composite material by one process comprises sintering, hot pressing and enforced infiltration acting simultaneously in one step. The objective of this invention is to produce, pore free, homogeneous sintered compact 25 structure with minimum defects at its near theoretical density. The hard metal or WO 2011/021923 PCT/MY2010/000115 12 ceramics reinforcements uniformly distributed in a softer metal matrix. The first stage is the preparation of the as received composite powder components, either by mixing or ball milling. Usually the composite consists of two or more components, i.e. the hard base metal component of higher melting point like tungsten and the matrix soft 5 component having lower melting point, like bronze or copper. The powders mixture consist of W base metal ranges between 50-90 wt percent and the balance is pre alloyed bronze or admixture of Cu-SnlOwt%. The ball milled and the as-received admixed powders are die-pressed separately under uniaxial pressure ranges between 360-720 MPa preferably around 400 MPa to yield compact discs of 13mm diameter 10 and around 4 mm thickness. In case of utilizing the as received hard pre-alloyed bronze powder as the compact soft component i.e. the matrix, it is necessary to add 0.01 of zinc stearate as a binder to enhance the green compaction process, while it is not important to add any sort of binder if the compact soft component is Cu or Cu SnlOwt% admixture. The green compact is then placed in a ceramic mold of suitable 15 cavity, large enough to accommodate the green compact and then covered all around by Cu or bronze powder called here as the outer shell. In case of green compact being fabricated from ball milled and mechanically alloyed powders, the outer covering shell should be a single phase elemental powder; otherwise the sintered. compact suffers severe swelling induced by the divergency in diffusion pathways and powder particle 20 size between the core and the covering shell. The composite system i.e. the green compact as a core and the covering shell is then sintered in a furnace following a pre designed sintering program comprises heating up stage at heating rate of 5-8 'C/min and isothermal sintering stage at a temperature of 1150*C- 1300*C for 2-3 hours. The final stage is the cooling down stage. It is essential to pay great attention to the cooling 25 down rate. Usually 4-8*C/min is suitable for the W composite system.

WO 2011/021923 PCT/MY2010/000115 13 Hereinafter the detailed description of the invention will be described in accordance with the accompanying drawings which will be referred either individually or collectively, in any combination thereof, wherein: Fig. 1 is a schematic representation of the process for producing a metal-matrix 5 composite of significant ACTE between the hard base-metal and the soft matrix according to the preferred embodiments of the present invention. This figure shows that the process which is a "three in one densification process" on its ongoing action. As the skin of the outer shell starts to solidify and as the solidification process proceeds the outer shell shrinks and compresses continuously towards the center, the 10 compression stress builds up gradually driven by the differential thermal expansion coefficients ACTE of the shell and the compact materials at the core. Fig. 2 shows a 3D schematic diagram of the present invention technique wherein the core is representing the W-Cu-Sn compact encircled or surrounded by Cu-Sn shell. As 15 the shell solidifies, it shrinks and exerts compression stress on the core which enhances the core densification process. Fig. 3 (a) shows an optical graph of W80wt.%-Cul8-Sn shell-on-core sintered compact cross section of as received elemental powder, the shell is bronze admix. The compact 20 sintered at 1150*C for 3 hours under H 2

/N

2 20/80 wt. ratio as protective gas utilizing the process of the present invention. Fig. 3 (b) shows an optical graph of shell-on-core sintered compact cross section of W50wt.%-Cu45-Sn sintered compact of two-steps ball milled powder. The shell is WO 2011/021923 PCT/MY2010/000115 14 copper wherein the compact is sintered at 1150*C for 3 hours under H 2

/N

2 20/80 wt. ratio as protective gas utilizing the process of the present invention; Fig. 3 (c) shows an optical graph of shell-on-core sintered compact cross section of 5 W80wt.%-Cul8-Sn sintered compact. The compact is sintered at 1150'C for 3 hours under H 2

/N

2 20/80 wt. ratio as protective gas utilizing the process of the present invention Fig. 3 (d) shows an optical graph shell-on-core sintered compact cross section of 10 W40wt%-pre-alloyed bronze sintered compact. The compact is sintered at 1150*C for 3 hours under H 2

/N

2 20/80 wt ratio protective gas utilizing the process of the present invention Fig. 4 shows the micro hardness profile across the solidified Cu-Sn shell and the 15 W80wt.%-Cul 8-Sn sintered compact of 99% of its theoretical density. Fig. 5 shows the SEM micrographs of W 80wt.%-Cul8-Sn sintered compacts of as received elemental powder densified by the process, in which hot pressing, sintering and infiltration process acting together. The micrographs reveal sintered compacts 20 having 99% theoretical density. The last micrograph represents sintered compact of similar composition sintered by the conventional method of uniaxial compaction and sintering. Fig. 6 (a) shows the optical micrographs of W80wt.%-Cul8-Sn compacts of as 25 received powder sintered conventionally at 1150 0 C for 3 hours; WO 2011/021923 PCT/MY2010/000115 15 Fig. 6 (b) shows the optical micrographs of W80wt.%-Cu18-Sn compacts of as received powder sintered by the process of the present invention under similar sintering conditions as in Fig. 6 (a). 5 Fig. 6 (c) shows the optical micrographs of W90wt.%-Cu18-Sn compacts of as received elemental powder sintered conventionally at 1150*C for 3 hours. Fig. 6 (d) shows the optical micrographs of W90wt.%-Cu18-Sn compacts of similar composition to Fig. 6 (c) sintered by the process of the present invention under similar 10 sintering conditions as in Fig. 6 (c). Fig. 7 shows a schematic representation of the process of the present invention at its final stage. As the sintering process elapsed and the covering shell melt cools down and gradually solidifies, it undergoes a substantial contraction driven by its high 15 thermal expansion coefficient and step by step it commences compressing and tightening firmly the core which has less thermal expansion coefficient and less contraction. This action is hot-isostatic-pressing-like action and leads to denser sintered compacts of near its theoretical density. 20 Fig. 8 (a) shows a SEM micrograph of W50wt.%-pre-alloy bronze compact of ball milled powder sintered conventionally. Fig. 8 (b) shows a SEM micrograph of W50wt.%-pre-alloy bronze compact of ball milled powder sintered by the present invention wherein sintered density of 99% 25 theoretical density is achieved.

WO 2011/021923 PCT/MY2010/000115 16 Fig. 8 (c) shows a SEM micrograph of W50wt.%-Cu45-Sn compact of two-step ball milled powder sintered conventionally. Fig. 8 (d) shows a SEM micrograph of W50wt.%-Cu45-Sn compact of two-step ball 5 milled powder sintered by the present invention technique wherein sintered density of 98% theoretical density is achieved. Fig. 9 (a) shows the EDX line scan across the shell/core boarder of W50wt%/.-Cu45-Sn sintered compact of ball milled powder sintered by the process of the present 10 invention.. Fig. 9 (b) .shows the EDX line scan of sintered compact of similar composition of W50wt%-Cu45-Sn sintered compact of ball milled powder sintered by the present invention technique but with pre-alloy bronze matrix. The shell is of Cu element. 15 Fig. 10 shows the alumina ceramics mould shows the cavity in where the W-bronze green compact placed and covered all around by the Cu-Sn powder mix shell before sintering. 20 Fig. 11 shows a typical thermal and infiltration sintering cycle program of the present invention wherein the sintering, heating and cooling temperature rates can be altered according to the sintered material specifications. So far, the most widely used method to attain fully dense W-bronze or W-Cu composite compacts, is to infiltrate pre sintered, porous W-base metal preforms by 25 bronze or Cu melts. This technique proves to be costly and time consuming as it comprises many stages. In this investigation, manipulation the powder constituent WO 2011/021923 PCT/MY2010/000115 17 physical properties of W composites has led to the implementation of exclusively novel "three in one densification invention" process includes sintering, in-situ hot isostatic pressing (HIP) and infiltration acting simultaneously at relatively low densification temperature of 1150cC under H 2

/N

2 20/80 wt. ratio as protective gas. Pilot 5 sintered/infiltrated compacts of = 99% theoretical density of different W (50, 80 and 90) wt.% balance is Cu-10wt.%Sn compacts of as received W, Cu and Sn metal powder precursors were produced. Other. sintered/infiltrated compact sets of W50wt.% and W80wt.%, balance is bronze 10wt.%Sn compacts of ball milled powder mixes gave sintered density 95% of theoretical density. The compacts were subjected to density 10 measurements, shrinkage and porosity characterization. Microstructure, hardness and densification mechanisms of the sintered/infiltrated compacts were evaluated and examined using scanning electron microscopy (SEM), energy dispersive x-ray analysis (EDX) and x-ray diffraction analysis (XRD). During the heating up stage and the isothermal stage, sintering of the compact at the 15 core is proceeding. The main sintering mechanisms at those two stages are, firstly by solid state diffusion and as the liquid phase forms particles rearrangement becomes the dominant sintering mechanism.- As the temperature exceeds the shell's powder melting temperature, the powder starts to melt and wet the compact surfaces which now become a concentric core within the covering shell and forming the so called 20 shell-core system. The melted covering shell usually assumes spherical shape under its surface tension. At this stage, the melted. shell enhances the sintering process of the core. It improves core protection from furnace environment, reduces oxidation and prevents contamination. As the heating-up stage and the isothermal stage are elapsed. The melted shell cools down and eventually starts to solidify. The 25 solidification process proceeds gradually from the outside towards the center. Now if WO 2011/021923 PCT/MY2010/000115 18 the density of the solidified shell is greater than that of its liquid, the new solidified thin shell will occupies less volume than the original liquid from which it solidifies; accordingly negative internal pressure builds up in the not-yet-solidified metal melt around the core. The magnitude of this negative internal pressure depends on the 5 cooling rate, the solidified sphere diameter and its material physical properties (CTE of metal, metal thermal conductivity and the ratio of solid/liquid density). As far as this internal negative pressure is not counter balanced by an external force or by outer sphere surface buckling, it will experience vacuum-sintering-like effects and leads to the elimination of some voids and porosities from the core. As the solidification of the 10 outer shell terminates, it undergoes a substantial thermal contraction due to its high CTE and starts compressing firmly and isostaticly the compact at the core which is not solidified yet, firstly because of the heat transfer sequence and secondly because it contains tungsten particles having low heat dissipation rate and act as a thermal source. At this stage the strength modulus of the solidified covering shell is 15 substantially higher than that of the not-yet-solidified soft component (mushy matrix) within the compact at the core. The tensile strength of bronzel0%Sn is temperature dependant. As the temperature increases from room temperature to 300*Cthe bronze metal looses around 80% of its original ultimate tensile strength. Accordingly, the matrix component of lower strength within the core, yields under the applied external 20 isostatic pressure and bring the W particles together and expel all voids and residual porosities out to the compact/covering shell interface leaving high dense sintered core. The sintered compact i.e. the core remains under compressive stress even after its own matrix entirely turns to solid and its temperature reaches room temperature. 25 The main features of the "three in one densification invention" are. It is very simple and cost effective.

WO 2011/021923 PCT/MY2010/000115 19 0 The weight percentages of the sintered compact components can be preliminary controlled. It is easy to increase the matrix weight percentage of the compact which is important in improving the thermal conductivity of the product. 0 No need for any sort of sintering additives which can lead to non controllable 5 shrinkage during liquid phase sintering and have negative effects on thermal conductivity of the sintered part. e The covering shell shields the composite compact during sintering process and prevents compact oxidation and contamination. & Post sintering/infiltration process, it is easy to salvage the outer shell machined 10 metal for the next production batch usages. An aspect of the present invention specifies a method for producing a composite material having matrix and dispesoid phase like W-bronze, where W is the dispesoid phase and the bronze is the matrix. The method comprises die pressing of the 15 composite elemental powders to yield a green compact and then to place this compact in a ceramics mold of suitable cavity. The green compact is adjusted and placed in the cavity as a concentric core surrounded by the covering shell powder. The weight of the loose covering shell powder encircled the green compact is at least equal to its weight. The ceramics mold and its charge are then introduced into a furnace and the green 20 compact with the covering shell powder sinter at temperature above the shell powder melting temperature for a certain time under protective gas, hydrogen or inert gas to prevent oxidation. The compact soft component metal, called here the matrix, is usually similar to the covering shell metal unless specified otherwise. Now as the core i.e. the composite green compact gets sintered and as the temperature exceeds the 25 matrix melting temperature the liquid phase forms inside and outside the core and enhance the densification process. When the isothermal sintering stage entirely WO 2011/021923 PCT/MY2010/000115 20 elapsed, the charge starts to cool down. The differential thermal expansion between the core material and the covering shell material leads to a different degree of contraction depending on their materials thermal expansion coefficients. At certain stage, the completely solidified shell starts contracting and exerting isostatic pressure 5 on the semi solidified core. This induced pressure and stress/strain reaches its maximum degree as the temperature drops to room temperature. The amount of the exerted stress/strain on the sintered compact at the core depends on the temperature gradient, the volume fraction of the W hard component owing to the lower thermal expansion coefficient and the bulk volume of the core and that of the outer shell. The 10 coefficients of thermal expansion (commonly referred to as ACTE) of the W-bronze core are. a.= Vfb 4.5*10-6 +VtlA8.4*1b 6 1 am = 18.4*106 a. is the thermal expansion coefficient of the sintered compact named here as 15 the core am is the thermal expansion coefficient of the compact matrix and frequently of the covering shell as well. Vib and Vfm are the volume fraction of the W-base metal and the bronze matrix respectively. 20 During temperature drop from 800*C (the approximate lower solidification temperature of bronze) to 25"C (the room temperature), the covering shell metal shrinks and tightly hold around the core compact, which its metal matrix and despite of its similarity to the covering shell metal, is still not yet entirely solidified, exerts compressive stress/strain 25 on it resulting in densification and pore elimination. This process incorporates, hot WO 2011/021923 PCT/MY2010/000115 21 pressing, sintering and infiltration, acting simultaneously and resulting in homogeneous pore free sintered composite structure. The process of the present invention generally relates.to a process for producing a 5 metal-matrix composite of significant ACTE between the hard base-metal and the soft matrix, the process includes the steps of sintering, pressing and enforced infiltration of the metal-matrix composite compact wherein the process is driven by differential thermal expansion coefficients between shell and core materials. 10 The materials are particulate composite materials or fiber composite materials and the metal-matrix composite is composite comprising tungsten, bronze and zinc stearate binder. The composite preferably includes 50 wt% of tungsten, 49 wt% of bronze and 1 wt% of zinc stearate. 15 The tungsten component of the composite. is ranging from 50 wt% to 90wt%. The bronze component is ranging from 10 wt% to 50 wt% wherein the zinc stearate. binder of 1 wt% is added to the composite having pre-ally bronze component only. The zinc stearate binder is intimately mixed with the tungsten and bronze whereas the 20 bronze component of the composite can be pre alloyed bronze or Cu-Sn admixed bronze. The weight of the Sn element in bronze is 10 % whether it is in the pre alloyed bronze or in the Cu-Sn admixed bronze. During pre sintering of the tungsten-bronze, the green compact is set as a core 25 covered by bronze powder shell whereas post sintering, the tungsten-bronze composite compact becomes a concentric core in bronze solidified sphere shell.

WO 2011/021923 PCT/MY2010/000115 22 The composite material can be a ball milled tungsten-Cu-Sn powder or as received tungsten-Cu-Sn elemental powder. The composite could be a ball milled tungsten-pre alloy bronze as well. 5 The material of the covering sphere shell can be as received pre-alloy bronze powder or as received Cu-Sn admix powder or as received Cu elemental powder. The weight of the covering shell should be at least equivalent to the weight of the 10 compact. The densification of the tungsten-bronze compacts is conducted at a temperature ranging from 11 50OC-1400*C, preferably at 1200*C under H 2 or H 2

/N

2 protective gas.

WO 2011/021923 PCT/MY2010/000115 23 Example Tungsten-Cu-Sn composites 5 In order to apply the "three in one densification invention" for the production of W80wt%-Cul 8-Sn sintered compacts, a suitable ceramic mold with a certain cavity of required shape was fabricated. As received tungsten powder of 12pm particle size and 99.9 purity was admixed with 10 copper powder of <45pm particle size, 99.5 purity and tin powder of <45pm particle size and 99.8 purity. The amount of tungsten was 80wt% and the balance was Cu SnlOwt. mix, no binder was used. The admixture was mixed manually in small glass container for 30 minutes to avoid any sort of segregation induced by the variations in particle size and density of the mixture components. Then admixture was die pressed 15 uniaxialy under 360 MPa. The green compact disc produced was 5 gram weight of 13mm diameter and nearly 4 mm thickness and having 70% of its theoretical density.. The green compact was then placed in the ceramic mold cavity and covered all around by 5 g weight as received Cu-Sn10 wt% powder mix. The covered green compact was sintered in alumina tube furnace. The heating up rate was 84C/min, the isothermal 20 sintering temperature was 11500 for 3 hours and the cooling down rate was 4 0 C/min. The "three in one densification invention" sintering process was conducted under H2/N2 gas of 20/80 wt ratio. Post sintering, the mold charge was dismantled and. the solidified covering shell was machined and grinded away to extract the sintered compact. The invention yielded sintered compact of 99% theoretical density with even 25 dispersion of the W phase in Cu-Sn matrix, homogeneous, voids and cracks free structure. The sintered compact had an average micro hardness value of 250 (Hv).

WO 2011/021923 PCT/MY2010/000115 24 The Cu-Sn volume fraction in the sintered compact was of 36% which is very difficult to be attained by the existing conventional infiltration technology. However, by this invention, controlling the volume fraction of the soft component in the compact, which is very important-factor in electronic industry applications, is not a real problem as it 5 can be designed prior to green compact production. The three main mechanical and physical parameters governing the "three in one densification invention" are: e The ACTE between the compact at the core and the covering shell. 10 e The differential amount of the radial displacement (Ur) of the solidified compact and the surround covering shell. " The compression stress or the radial stress (aCr) induced by the differential radial displacement and becomes responsible for the compact densification. Sintered compacts of 50-90 W wt/ 0 were produced successfully by this invention. 15 Sintered compacts of 80-90 W wt% showed the best results. Besides these parameters, other factors like, connectivity, contiguity and the particle size of the hard component have great effects on "the three in one densification invention" action.

Claims

1. A process for producing a metal-matrix composite made of a matrix phase and a dispersed phase materials, the process includes the steps of: 5 (a) preparing of composite powder components comprising the hard base material and a soft metal matrix by mixing or ball milling; (b) pressing the composite powder components as obtained from step (a) under a preferred pressure; (c) sintering the composite powder components as obtained from step (b) at 10 a preferred heating rate, temperature and time; and (d) cooling composite powder components as obtained from step (c) at a preferred cooling down rate; wherein the process is driven by differential thermal expansion coefficients between shell and core materials. 15

2. The process as claimed in claim 1, wherein the preferred pressure applied in step (b) is 360-720 MPa.

3. The process as claimed in claim 1 or claim 2, wherein the preferred pressure applied in step (b) is around 400 MPa.

4. The process as claimed in any one of the preceding claims, wherein the 20 preferred heating rate applied in step (c) is of 5-8*C/min, temperature of 1 150*C-1300*C for a period of 2-3 hours.

5. The process as claimed in any one of the preceding claims, wherein the preferred cooling down rate is 4-8*C/min.

6. The process as claimed in any one of the preceding claims, wherein the 25 materials are particulate dispersoids, cermets, short or continuous fiber, whiskers or monofilament composite materials.

7. The process as claimed in any one of the preceding claims, wherein the metal matrix composite is composite comprising tungsten, bronze and zinc stearate binder.

8. The process as claimed in claim 7, wherein the tungsten component of the 30 composite is ranging from 50 wt% to 90 wt%. 26

9. The process as claimed in claim 7 or claim 8, wherein the bronze component is ranging from 10 wt% to 50 wt% wherein the zinc stearate binder of 1 wt% is added to the composite having pre-alloy bronze component only.

10. The process as claimed in claim 9, wherein the composite includes 50 wt% of 5 tungsten, 49 wt% of bronze and 1 wt% of zinc stearate binder.

11. The process as claimed in claim 10, wherein densification of the tungsten bronze compacts is conducted at a temperature ranging from 1150*C-1400 0 C under H 2 or H 2 /N 2 protective gas.

12. The process as claimed in claim 10 or claim 11, wherein densification of the 10 tungsten-bronze compacts is conducted at a temperature of 1200*C under H 2 or H 2 /N 2 protective gas.

13. The process as claimed in any one of claims 7 to 12, wherein the bronze component of the composite is pre-alloyed bronze or Cu-Sn admixed bronze.

14. The process as claimed in claim 13, wherein % weight of Sn element in bronze 15 is 10% whether it is in pre-alloyed bronze or in Cu-Sn admixed bronze.

15. The process as claimed in claim 7, wherein the composite is a ball milled tungsten-Cu-Sn powder or as received tungsten-Cu-Sn elemental powder.

16. The process as claimed in claim 7, wherein the composite could be a ball milled tungsten-pre-alloy bronze. 20

17. The process as claimed in claim 1, wherein material of a covering sphere shell is as received pre-alloy bronze powder or as received Cu-Sn admix powder or as received Cu elemental powder.

18. The process as claimed in claim 17, wherein the weight of the covering shell is at least equivalent to the weight of the compact. 25

19. A metal-matrix composite made of a matrix phase and a dispersed phase materials produced by the process as claimed in claims 1 to 18.

20. The metal-matrix composite as claimed in claim 19, wherein the metal-matrix composite is composite comprising tungsten, bronze and zinc stearate binder. 27

21. The metal-matrix composite as claimed in claim 19 or claim 20, wherein the tungsten component of the composite is ranging from 50 wt% to 90 wt%.

22. The metal-matrix composite as claimed in any one of claims 19 to 21, wherein the bronze component is ranging from 10 wt% to 50 wt% wherein the zinc stearate 5 binder of 1 wt% is added to the composite having pre-alloy bronze component only.

23. The metal-matrix composite as claimed in claim 22, wherein the composite includes 50 wt% of tungsten, 49 wt% of bronze and 1 wt% of zinc stearate binder.

24. A process for producing a metal-matrix composite made of a matrix phase and a dispersed phase materials; a metal-matrix composite made of a matrix phase and a 10 dispersed phase materials produced by the process substantially as herein described with reference to any one or more of the examples but excluding comparative examples.