CA2054834A1 - Boron carbide-reactive metal cermets having microstructure tailored by post-densification heat treatment - Google Patents
Boron carbide-reactive metal cermets having microstructure tailored by post-densification heat treatmentInfo
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- CA2054834A1 CA2054834A1 CA002054834A CA2054834A CA2054834A1 CA 2054834 A1 CA2054834 A1 CA 2054834A1 CA 002054834 A CA002054834 A CA 002054834A CA 2054834 A CA2054834 A CA 2054834A CA 2054834 A1 CA2054834 A1 CA 2054834A1
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- Prior art keywords
- phase
- reactive metal
- boron carbide
- aluminum
- densified composite
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Classifications
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- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C1/00—Making non-ferrous alloys
- C22C1/04—Making non-ferrous alloys by powder metallurgy
- C22C1/05—Mixtures of metal powder with non-metallic powder
- C22C1/058—Mixtures of metal powder with non-metallic powder by reaction sintering (i.e. gasless reaction starting from a mixture of solid metal compounds)
-
- C—CHEMISTRY; METALLURGY
- C22—METALLURGY; FERROUS OR NON-FERROUS ALLOYS; TREATMENT OF ALLOYS OR NON-FERROUS METALS
- C22C—ALLOYS
- C22C29/00—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides
- C22C29/02—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides
- C22C29/06—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides based on carbides, but not containing other metal compounds
- C22C29/062—Alloys based on carbides, oxides, nitrides, borides, or silicides, e.g. cermets, or other metal compounds, e.g. oxynitrides, sulfides based on carbides or carbonitrides based on carbides, but not containing other metal compounds based on B4C
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- Chemical & Material Sciences (AREA)
- Engineering & Computer Science (AREA)
- Materials Engineering (AREA)
- Mechanical Engineering (AREA)
- Metallurgy (AREA)
- Organic Chemistry (AREA)
- Chemical Kinetics & Catalysis (AREA)
- Powder Metallurgy (AREA)
- Manufacture Of Alloys Or Alloy Compounds (AREA)
Abstract
ABSTRACT
Boron carbide-reactive metal cermets can be prepared by consolidating a mixture of boron carbide powder and a particulate source of a metal that reacts with boron carbide to produce dense, soft, cermets in near net shape. The cermets can be hardened to produce a final multiphase microstructure having increased fracture toughness and hardness by a post-densification heat treatment.
Boron carbide-reactive metal cermets can be prepared by consolidating a mixture of boron carbide powder and a particulate source of a metal that reacts with boron carbide to produce dense, soft, cermets in near net shape. The cermets can be hardened to produce a final multiphase microstructure having increased fracture toughness and hardness by a post-densification heat treatment.
Description
~5~3~
BORON CARBIDE-~EACTIVE METAL CERMETS HAVING
MICROSTRUCTURE TAILORED BY POST-DENSIFICATION HEAT
T~EATMENT
This invention concerns ceramic-metal composites, also known as cermets. based upon boron carbide and a source of a metal whlch reacts with boron carbide.
Cermets have properties that differ from those of either the ceramic phase or the metal phase alone.
They are conventionally made by powder metallurgical methods, that iq, by preparing and mixing individual metal and ceramic powders, pres~ing the mixed powders into a required shape and subjecting the shape to a sintering heat treatment to bond the particles and develop the required structural integrity, often by direct ceramic-to-ceramic bonding.
Cermets are conventionally used to make engineering components, such as parts of ga~ turbine or diesel engines, that require mechanical property stability when exposed to rapid temperature changes.
strength at operating temperatures and creep resistance.
They tend to be less brittle and less prone to formation '5 38,885-F _1_ 2 ~ 3 ~
of extended defects within the material than conventional ceramics.
U.S. Patent 4,~6.424 discloses a method of improving the fracture toughness of a hard metal or metal-bound ceramic by exposing it to a heat treating process similar to the transformation toughening of steel. A ceramic composite, such as cobalt-bound tungsten carbide. is first cooled to transform the binder material from a first state to a second state and cause deformation of the binder material. The composite is then heated above ambient temperature to cause the binder material to revert to its first state while retaining some measure of the deformation. The heated composite material is then quenched or subjected to rapid cooling.
U.S. Patent Nos. 4,702,770 and 4,718,941 teach heat treating ceramic precursors infiltrated with a molten metal to tailor the microstructure of boron carbide-reactive metal cermets.
Figure 1 illu~trates the post-densification heat treatment of' boron carbide containing varying amounts of aluminum at var~ing temperatures. Figure 2 illustrates post-densification heat treatment of a boron carbide composite containing 30% by volume aluminum showing hardness as a function of heat treatment time and temperature. Figure 3 provides curves showing the effect of heat treatment time on fracture toughness of a boron carbide-aluminum composite which has been subjected to a post-densification heat treatment.
38.885-F -2-2 ~
One aspect of the present invention is a boron carbide-aluminum cermet characterized by a boron carbide phase, an aluminum phase and an AlB2 phase or an A148C
phase or an AlB2 phase and an A14BC phase.
A second aspect OI' the present invention is a boron carbide-reactive metal cermet characterized by a boron carbide phase, a reactive metal phase and a reactive metal oride phase, or a reactive metal boron carbide phase or a reactive metal boride phase and a reactive metal boron carbide p~lase. the reactive metal being arsenic, barium, beryllium. calcium, cobalt.
chromium. iron, hafnium, iridium, lanthanum, lithium.
magnesium, manganese, molybdenum, sodium, niobium, nickel, osmium, palladium. platinum, plutonium, rhenium, rhodium, ruthenium, scandium, silicon, strontium.
tantalum, technetium, thorium, titanium, uranium, vanadium, tung~ten, yttrium or zirconium.
A third aqpect i~ a proceYs for preparing a boron carbide-aluminum cermet comprising:
(a) den3ifying an admixture of boron carbide powder and aluminum metal powder at a temperature in the vicinity of the melting point of aluminum to produce a den~ified composite; and (b) heating the densified composite to a temperature within a range of 450C to 1000C for a period of time suf~icient to form, within the densified composite, an AlB2 phase or an A14BC phage or an AlB2 phase and an A14BC phase. The time period is within a range of one to fifty hours inclusive.
38,885-F _3_ 3 ~
A fourth aspect is a process for preparinF a boron carbide-reactive metal cermet comprising:
(a) densifying an admixture of boron carbide powder and a particulate source of a reactive metal at a temperature in the ~/icinity of the melting temperature of the reactive metal to produce a densified composite, the reactive metal being arsenic, barium, beryllium, calcium, cobalt, chromium, iron, hafnium, iridium.
lanthanum, lithium. magnesium, manganese, molybdenum.
sodium, niobium, nickel, osmium, palladium, platinum.
plutonium, rhenium. ~hodium. ~uthenium. scandium.
silicon. strontium, ,antalum. ~echnetium, thorium, titanium, uranium, /anadium, tungsten, ~ttrium or zirconium; and (b) heating the densified composite to a temperature within a range of 450C to 1000C for a period of time sufficient to form, within the densified composite, a reactive metal boride phase or a reactive metal boron carbide phase or a reactive metal phase and a reactive metal boron carbide phase. The time period is within a range of one to fifty hours inclusive.
Powder metallurgy procedures are preferred for preparing densified composites suitable for heat treatment. Powder mixtures may be densified as is, placed in a container before densification or converted to a preform, by a known procedure such as cold pressing, cold isostatic preqsing or cold isostatic pressing before densification. The powder mixture~
usually contain from 20 to 60 percent by volume of a reactive metal such as aluminum prior to densification.
After a post-densification heat treatment, the proportion of metal is from 2 to 12% by volume. ~he 38,885-~ _4_ post-densification heat treatment ?roduces a microstructure containing, in addition to the boron carbide and reactive metal phases. a metal boride ?hase such as AlB2, or a metal boron carbide phase such as A14BC or a metal bori~e phase and a metal boron carbide phase. Control of the ?ost-densification heat treatment temperature and time allows tailoring of the microstructure. Changes in microstructures vary physical properties of the heat-treated cermets.
0 The aluminum phase of boron carbide-aluminum cermets may be formed from aluminum metal. aluminum metal alloys or aluminum compounds that are reduced to their corresponding metal during ~ensification. aoron carbide-aluminum cermets have a number of potential applications or end uses. The applications include. but are not limited to, lightweight structures, cutting tools, spent nuclear fuel containers, radiation reqistant structures, hot and cool parts of turbine engines, impact resistant structures, abrasive and wear resistant materials, semiconducting devices, and structures requiring increased thermal shock resistance and a high degree of chemical stability.
Metals that react with boron carbide to form similar boron carbide-reactive metal cermets include arsenic, barium, beryllium, calcium, cobalt, chromium, iron, hafnium, iridium, lanthanum, lithium, magnesium.
manganese, molybdenum, sodium, niobium, nickel, osmium, 3 palladium, platinum, pluSonium, rhenium, rhodium, ruthenium, scandium, silicon, strontium, tantalum.
technetium, thorium, titanium9 uranium, vanadium.
tungsten, yttrium or zirconium. As with aluminum. the reactive metal phase may be formed from a reactive metal, a metal alloy containing the reactive metzl or a 38,885-F _5_ 2 ~
reactive metal compound that reduces to its corresponding metal or metal alloy during densification.
The cermets of the present invention result from a process which differs from prior art processes in two aspects. First, densification of the admixture of boron carbide and the source of aluminum or another reactive metal occurs at a temperature in the vicinity of the melting temperature of the metal. This temperature is believed to minimize the reaction between boron carbide and the aluminum or other reactive metal during densification. Second. the densified admixture or cermet undergoes a heat treatment to produce a microstructure that include~ one or more phases other than the boron carbide phase and the reactive metal phase. Variations in the microstructure lead to improvements in physical properties such as fracture toughness or impact strength.
Post-densification heat treatment occurs at a temperature within a range of 450C to 1000C. The range is desirably from 500C to 800C and preferably from 600C to 700C. Heat treatment times fall within a range of from one to 50 hours. The treatment t$me range is desirably from 1 to 30 hours and preferably from 10 to 20 hours.
Heat treatment temperatures within a range of 600C to 700C lead to formation of a reactive metal boride, ~uch as, AlB2. The boride enhances impact resistance over that of the densified composite prior to heat treatment. As the heat treatment temperature rises above 700C, a reactive metal boron carbide, such as, A14BC begins to form. As the temperature continues to increase, the amount of the reactive metal boron carbide 38,885-F -6-3 ~
relative to the reactile metal boride also increases.
Between 900C and 1000C. the reactive metal boron carbide is the dominant reaction product. Heat treatment temperatures in excess of 1000C cause the reactive metal boride 'o decompose and generate free reactive metal. ~ixtures of the reactive metal boride and the reactive metal boron carbide have fracture toughness and hardness ~alues greater than that of ~he densified composite prior to heat treatment.
The following exampies illustrate various aspects of the invention but are not intended to limit its scope. r~hen not otherwise specified throughout this specification and claims. temperatures are given in degrees centigrade ~nd ?arts. percentages. and proportions are by weight.
Ex_mDle 1 This example illustrateq that the properties and chemistry of the B4C/Al system can be tailored by changing proceqsing conditions. The L ollowing powders, proceqsing conditions and characterization methods were used:
The boron carbide was a powder with 21.27%
total carbon content, 0.4% free carbon, 1.27% oxygen and a surface area of 6.8 m2/g. The major impurities were 161 ppm Ca, 142 ppm Cr, 268 ppm Fe and 331 ppm Ni. The aluminum powder, Alcan 105 produced by Alcan-Toyo America, Inc., contained 0.8% Al203, 0.18% Fe and 0.12 Si and had a surface area of 0.5m2/g.
38,885-F _7_ A mixture of 70% by ~olume Doron carbide powder and 30% by volume aluminum powder was mixed and pressed into 24 mm diameter pellets. The pellets were heat-treated for one hour in a mullite tube furnace. in flowing argon, at a temper~ture within a range of 'IO0~C
to 1200C. The heat~trea~ed pellets were cooled to room temperature either at a rate of 10/minute or by quenching into liquid nitrogen.
Crystalline phases were ldentified by x-ray 0 iiffraction with a Phillips diffractometer using CuK
radiation and a scan rate o~ 2 ?er minute. The chemistry of all phases was determined from electron probe analysis of polished cross-sections using ~ CAMECA
CAMEBAX electron probe. The accura~y in the determination of elemental composition was better than 3% of the amount present.
The area of the aluminum melting endotherm in the high temperature DSC scan was used as a measure of the reactivity bstween a4C and Al at temperatures between 550C and 1200~C. The data were collected using a Perkin-Elmer DTA 1700 interfaced to a computer. The purge gas was ultra high purity argon flowing at 40 cc/min. The sample~ were heated in alumina crucibles at 20C/min and high purity aluminum (99.999%) was u~ed as a standard. The percent aluminum metal was given by A/B x 100, where A is the peak area in cal/g of the Ai melt endotherm in the sample and B is the same for the 3 Al standard. Precision and accuracy were 2 percent.
The results show that the reaction between boron carbide and aluminum starts at 450C with the formation of A14BC. The reaction rate is slow below oOO~C. rn the range of 550 to 600JC. 24% by ~/olume 38,885-F _~_ 2 ~ 3 ~
metal (80% of the original Al) can be recovered. Above 600C, AlB2 forms and aluminum is rapidly depleted. as illustrated in Figure 1. rhe open circles in Figure 1 represent the amount of unreacted Al metal retained in a B4C/Al powder mixture after heatin~ for one hour at temperatures between 450C and 1200~C. and cooling to room temperature at lO~minute. The open boxes indicate the amount of unreacted metal present after quenching in liquid nitrogen. 8etween 600 and 700C, AlB2 and B4C
are the predominant phases~ Above 700~C. AlB2 and A14BC
are present and. a.~ temper~ture increases. the relative amount of A14BC increases. Between 900~C and lOOO~C. 'he dominant reaction product is A14BC. .lt temperatures above 1000C, AlB2 decomposes and generates free aluminum. Heat treatment above lOOO~C produces mainly AlB24C4 and some A14C3. Phases formed below lOOOJC are aluminum rich and their formation leads to the rapid depletion of the metal. Phases formed above 1000C are boron and carbon rich resulting in B4C depletion and development composites with larger amounts of free metal and smaller amounts of boron carbide ~han the same starting powders heated below lOOO~C.
The major phases influencing the mechanical properties of B4C/Al based materials are A14BC. AlB2, AlB24C4 and A14C3. Because the formation of AlB24C4 i9 associated with the existence of undesirable A14C3, the heat treatment should be limited to temperatures of 1000C or lower where AlB2 and A14BC are the predominant new phases.
38.885-F _9_ Exam~le 2 Example 2 illustrates that hardness of 34C/Al cermets can be changed as a function of phase chemistry.
The same powders were used as in Example 1.
The boron carbide and aluminum powders were dry mixed in a rotary blender and then pressed into 75 mm diameter - discs using uniaxial compaction in a stainless steel die. No lubricants or binders were used. The samples io were placed into metal cans. The sealed cans were placed in fluid dies. The fluid dies with mixed B4C and Al powders were heated in the furnace of Example 1 to 640C. The fluid dies were then removed from the furnace, placed in a forging press, and compacted. The compacted dies were removed from the forging press and cooled to room temperature. The diqcs were separated from the cooled dies using conventional procedures and cut into various ~hapes for te~ting and character-ization.
The bulk hardness wa~ measured on surfacespolished successively with 45, 30, 16. 6 and 1 um diamond paste and finally finished using colloidaL
silica suspension on a LEC0 automatic poliqher. The Rockwell A hardness was measured using 2 13.3 kg load.
The Vicker9 microhardness of isolated phases was measured using a LEC0 tester and loads of 10 to 20 grams. The largest grain~ of a particular phase were examined in order to eliminate or minimize the contribution from adjacent or underlying material.
Generally, the distance from the center of the indent to the nearest grain boundary was over twice the indent 38.885~F _lo-liagonal dimension. The indent diagonals were measured ~sing a scanning electron microscope at 10000X.
After densification, the B4C/30% by volume Al material had a hardness o~ 81 in Rockwell A scale.
?ost-densi~ication heat treatment at 500C and 1000C
provided a maximum hardness value of 83, which remained stable for extended times as shown in Figure 2. At ~ 1000C, the hardness declined initially due to the decomposition of AlB2, but then increased again as Al4BC
was formed. Heat treatments conducted at 700C, 300C, ?00C and 1100C produced ~ maximum in the hardness versus time curve. At higher temperatures. less time ~as required to obtain maximum hardness.
The change in hardness is characteristic of the B4C/Al system and due to the kinetics and B-C-Al phase equilibrium. Below 1000C, in the initial stage of heat treatment, hard Al4BC is favored. With increased time, there is insufficlent aluminum available and softer AlB2 forms decreasing overall hardness. Between 600C and 1000C Al4BC and AlB2 are both present. Increases in the AlB2/Al4BC ratio lower the maximum hardness and maKe the tran~ition from rising to declining hardness more gradual.
After 20 hours of heat treatment, the AlB2/Al4BC ratio was 8, 0.7 and 0.4 at 700, 800 and 900C respectively. At 1100C, Al4RC and then AlB24C4 formed re~ulting in a high hardne3s composite. However.
after 8 to 10 hours of heat treatment, Al4C3 became the predominant new phase and the hardnes~ declined. ~he highest hardness values were achieved through heat 38,885-F
treatments of 10 hours at 1100C (HRA=89), 20 hours aoooC
(HRA=~8) and 20 hours at 800~C (HRA=88).
Example 3 Example 3 ilius~rates that fracture toughness of dense B4C/Al material can be increased. This increase is associated with densification of the metal - phase. While the formation of ceramic phases reduces toughness, the extent of this reduction depends on the type of phase formed. AlB2 is the 'east detrimental.
The same powders and processing conditions were used as in Examples 1 and 2. Fracture toughness was measured using the Chevron notch technique and standard 4 x 3 x 45 mm samples. The notch was produced with 250 um wide diamond blade and the notch depth to sample height ratio was 0.42. A cro~s head speed of 0.05mm/minute was used in a 3 point bend fixture. The average of 5 to 7 measurements was reported.
The values of the fracture toughness of many B-C-Al phaseq are unknown. However, the damage and cracking pattern in the indented phases indicates that AlB2 has a higher toughnes~ than A14BC or AlB24C4.
Aluminum containing small ceramic crystals deforms plastically. The damage in A182 represents shear deformation rather than brittle cracking, B4C and AlB24C4 usually behave similarly, even though, in some cases, the crack propagates through the AlB24C4 and stops at the boron carbide grain boundary. A14BC shows brittle behavior with several cracks running from the corners and ~ide9 of the indent.
38,885-F -12-The results in Figure 3 indicate that post-densification heat treatment can substantially improve fracture toughness. The KIC increases over the entire heat treatment range ~hen compared to the pressure densified material. ~eyond the maximum at 60noc. KIC
decreases. Analytical transmission electron microscopy did not reveal any differences (for example.
precipitation) between the pressure dersified and heat treated metal phases. However, a reduction in porosity was observed in the heat treated samples implying increased density and improved bonding between the ceramic and metal which would account for the increased fracture toughness. At 600C. where the highest fracture toughness was obtained. sintering of the metal takes place before significant quantities of new phases are formed. The materials containing AlB2 have higher KIC values than those with A14BC and AlB24C4.
ExamPle 4 Example 4 shows that post-densification heat treatment can be used to improve many properties of B4C~Al cermets by changing the cermet into a multi-phase ceramic material containing only a small amount of residual metal.
It is possible to produce B4C/Al composites with different phases, but similar amounts of residual unreacted aluminum. The following Table provides data comparing the mechanical properties of the pressure densified and heat treated B4C/Al compositeg. The highest hardness and modulus were obtained in samples containing A14BC . While these materials had improved toughness relative to the pressure densified materials.
it was only 8.2 MPa ml/2. On the other ~and. samples 38,885-F -13-AlB2 containing AlB2 exhibited increased fracture toughness (9.2 MPa m1/2), but only slight hardness (Prom 81 to 85) improvement.
Because the AlB2/Al4BC ratio can be changed and controlled by heat treatment at a temperature between 700C and 900C, the properties of B4C/Al composites can be selected for a specific application. Processing at 1000C or below allows the formation of large amounts of AlB2 and avoids the formation of Al4C3.
In heat treating at or below 600C. the new phases form in limited amounts and a multi-phase ceramic is not produced. The resultant B4C/Al cermets are characterized by a microstructure of isolated boron carbide grains in an aluminum matrix with improved fracture toughness and fracture strength.
TABLE - The effect of heat treatment on properties of B4C/Al materials with i~olated boron carbide in a metal matrix.
3o 38.885-F ~14-3 ~
After After Heating above pressure 600C to produce: After Property densi- Heatlng fication AlB2 Al4BC AlB24C4A
Density 2.57 2.63 2.70 2.62 2.58 g/cm3 Flexure 419 357 351 312 434 strength (MPa) Fracture 7.23 9.2 8.2 7.5 12.7 10 toughne/2 Young 254 290 310 280 260 modulus (GPa) ~ulk 138 167 175 156 140 modulus (GPa) Hardness 81 85 88 88 81 Rockwell A
Poissons's0.23 0.21 0.20 0.20 0.23 ratio While this invention has been deqcribed with reference to certain specific embodiments, it will be recognized by those skilled in the art that many variation~ are possible without departing from the scope and spirit of the invention and it will be understood that it is intended to cover all changes and modifications of the invention, discloqed herein for the purposes of illustration, which do not constitute 3 departures form the spirit and scope of the invention.
38,885-F _15_
BORON CARBIDE-~EACTIVE METAL CERMETS HAVING
MICROSTRUCTURE TAILORED BY POST-DENSIFICATION HEAT
T~EATMENT
This invention concerns ceramic-metal composites, also known as cermets. based upon boron carbide and a source of a metal whlch reacts with boron carbide.
Cermets have properties that differ from those of either the ceramic phase or the metal phase alone.
They are conventionally made by powder metallurgical methods, that iq, by preparing and mixing individual metal and ceramic powders, pres~ing the mixed powders into a required shape and subjecting the shape to a sintering heat treatment to bond the particles and develop the required structural integrity, often by direct ceramic-to-ceramic bonding.
Cermets are conventionally used to make engineering components, such as parts of ga~ turbine or diesel engines, that require mechanical property stability when exposed to rapid temperature changes.
strength at operating temperatures and creep resistance.
They tend to be less brittle and less prone to formation '5 38,885-F _1_ 2 ~ 3 ~
of extended defects within the material than conventional ceramics.
U.S. Patent 4,~6.424 discloses a method of improving the fracture toughness of a hard metal or metal-bound ceramic by exposing it to a heat treating process similar to the transformation toughening of steel. A ceramic composite, such as cobalt-bound tungsten carbide. is first cooled to transform the binder material from a first state to a second state and cause deformation of the binder material. The composite is then heated above ambient temperature to cause the binder material to revert to its first state while retaining some measure of the deformation. The heated composite material is then quenched or subjected to rapid cooling.
U.S. Patent Nos. 4,702,770 and 4,718,941 teach heat treating ceramic precursors infiltrated with a molten metal to tailor the microstructure of boron carbide-reactive metal cermets.
Figure 1 illu~trates the post-densification heat treatment of' boron carbide containing varying amounts of aluminum at var~ing temperatures. Figure 2 illustrates post-densification heat treatment of a boron carbide composite containing 30% by volume aluminum showing hardness as a function of heat treatment time and temperature. Figure 3 provides curves showing the effect of heat treatment time on fracture toughness of a boron carbide-aluminum composite which has been subjected to a post-densification heat treatment.
38.885-F -2-2 ~
One aspect of the present invention is a boron carbide-aluminum cermet characterized by a boron carbide phase, an aluminum phase and an AlB2 phase or an A148C
phase or an AlB2 phase and an A14BC phase.
A second aspect OI' the present invention is a boron carbide-reactive metal cermet characterized by a boron carbide phase, a reactive metal phase and a reactive metal oride phase, or a reactive metal boron carbide phase or a reactive metal boride phase and a reactive metal boron carbide p~lase. the reactive metal being arsenic, barium, beryllium. calcium, cobalt.
chromium. iron, hafnium, iridium, lanthanum, lithium.
magnesium, manganese, molybdenum, sodium, niobium, nickel, osmium, palladium. platinum, plutonium, rhenium, rhodium, ruthenium, scandium, silicon, strontium.
tantalum, technetium, thorium, titanium, uranium, vanadium, tung~ten, yttrium or zirconium.
A third aqpect i~ a proceYs for preparing a boron carbide-aluminum cermet comprising:
(a) den3ifying an admixture of boron carbide powder and aluminum metal powder at a temperature in the vicinity of the melting point of aluminum to produce a den~ified composite; and (b) heating the densified composite to a temperature within a range of 450C to 1000C for a period of time suf~icient to form, within the densified composite, an AlB2 phase or an A14BC phage or an AlB2 phase and an A14BC phase. The time period is within a range of one to fifty hours inclusive.
38,885-F _3_ 3 ~
A fourth aspect is a process for preparinF a boron carbide-reactive metal cermet comprising:
(a) densifying an admixture of boron carbide powder and a particulate source of a reactive metal at a temperature in the ~/icinity of the melting temperature of the reactive metal to produce a densified composite, the reactive metal being arsenic, barium, beryllium, calcium, cobalt, chromium, iron, hafnium, iridium.
lanthanum, lithium. magnesium, manganese, molybdenum.
sodium, niobium, nickel, osmium, palladium, platinum.
plutonium, rhenium. ~hodium. ~uthenium. scandium.
silicon. strontium, ,antalum. ~echnetium, thorium, titanium, uranium, /anadium, tungsten, ~ttrium or zirconium; and (b) heating the densified composite to a temperature within a range of 450C to 1000C for a period of time sufficient to form, within the densified composite, a reactive metal boride phase or a reactive metal boron carbide phase or a reactive metal phase and a reactive metal boron carbide phase. The time period is within a range of one to fifty hours inclusive.
Powder metallurgy procedures are preferred for preparing densified composites suitable for heat treatment. Powder mixtures may be densified as is, placed in a container before densification or converted to a preform, by a known procedure such as cold pressing, cold isostatic preqsing or cold isostatic pressing before densification. The powder mixture~
usually contain from 20 to 60 percent by volume of a reactive metal such as aluminum prior to densification.
After a post-densification heat treatment, the proportion of metal is from 2 to 12% by volume. ~he 38,885-~ _4_ post-densification heat treatment ?roduces a microstructure containing, in addition to the boron carbide and reactive metal phases. a metal boride ?hase such as AlB2, or a metal boron carbide phase such as A14BC or a metal bori~e phase and a metal boron carbide phase. Control of the ?ost-densification heat treatment temperature and time allows tailoring of the microstructure. Changes in microstructures vary physical properties of the heat-treated cermets.
0 The aluminum phase of boron carbide-aluminum cermets may be formed from aluminum metal. aluminum metal alloys or aluminum compounds that are reduced to their corresponding metal during ~ensification. aoron carbide-aluminum cermets have a number of potential applications or end uses. The applications include. but are not limited to, lightweight structures, cutting tools, spent nuclear fuel containers, radiation reqistant structures, hot and cool parts of turbine engines, impact resistant structures, abrasive and wear resistant materials, semiconducting devices, and structures requiring increased thermal shock resistance and a high degree of chemical stability.
Metals that react with boron carbide to form similar boron carbide-reactive metal cermets include arsenic, barium, beryllium, calcium, cobalt, chromium, iron, hafnium, iridium, lanthanum, lithium, magnesium.
manganese, molybdenum, sodium, niobium, nickel, osmium, 3 palladium, platinum, pluSonium, rhenium, rhodium, ruthenium, scandium, silicon, strontium, tantalum.
technetium, thorium, titanium9 uranium, vanadium.
tungsten, yttrium or zirconium. As with aluminum. the reactive metal phase may be formed from a reactive metal, a metal alloy containing the reactive metzl or a 38,885-F _5_ 2 ~
reactive metal compound that reduces to its corresponding metal or metal alloy during densification.
The cermets of the present invention result from a process which differs from prior art processes in two aspects. First, densification of the admixture of boron carbide and the source of aluminum or another reactive metal occurs at a temperature in the vicinity of the melting temperature of the metal. This temperature is believed to minimize the reaction between boron carbide and the aluminum or other reactive metal during densification. Second. the densified admixture or cermet undergoes a heat treatment to produce a microstructure that include~ one or more phases other than the boron carbide phase and the reactive metal phase. Variations in the microstructure lead to improvements in physical properties such as fracture toughness or impact strength.
Post-densification heat treatment occurs at a temperature within a range of 450C to 1000C. The range is desirably from 500C to 800C and preferably from 600C to 700C. Heat treatment times fall within a range of from one to 50 hours. The treatment t$me range is desirably from 1 to 30 hours and preferably from 10 to 20 hours.
Heat treatment temperatures within a range of 600C to 700C lead to formation of a reactive metal boride, ~uch as, AlB2. The boride enhances impact resistance over that of the densified composite prior to heat treatment. As the heat treatment temperature rises above 700C, a reactive metal boron carbide, such as, A14BC begins to form. As the temperature continues to increase, the amount of the reactive metal boron carbide 38,885-F -6-3 ~
relative to the reactile metal boride also increases.
Between 900C and 1000C. the reactive metal boron carbide is the dominant reaction product. Heat treatment temperatures in excess of 1000C cause the reactive metal boride 'o decompose and generate free reactive metal. ~ixtures of the reactive metal boride and the reactive metal boron carbide have fracture toughness and hardness ~alues greater than that of ~he densified composite prior to heat treatment.
The following exampies illustrate various aspects of the invention but are not intended to limit its scope. r~hen not otherwise specified throughout this specification and claims. temperatures are given in degrees centigrade ~nd ?arts. percentages. and proportions are by weight.
Ex_mDle 1 This example illustrateq that the properties and chemistry of the B4C/Al system can be tailored by changing proceqsing conditions. The L ollowing powders, proceqsing conditions and characterization methods were used:
The boron carbide was a powder with 21.27%
total carbon content, 0.4% free carbon, 1.27% oxygen and a surface area of 6.8 m2/g. The major impurities were 161 ppm Ca, 142 ppm Cr, 268 ppm Fe and 331 ppm Ni. The aluminum powder, Alcan 105 produced by Alcan-Toyo America, Inc., contained 0.8% Al203, 0.18% Fe and 0.12 Si and had a surface area of 0.5m2/g.
38,885-F _7_ A mixture of 70% by ~olume Doron carbide powder and 30% by volume aluminum powder was mixed and pressed into 24 mm diameter pellets. The pellets were heat-treated for one hour in a mullite tube furnace. in flowing argon, at a temper~ture within a range of 'IO0~C
to 1200C. The heat~trea~ed pellets were cooled to room temperature either at a rate of 10/minute or by quenching into liquid nitrogen.
Crystalline phases were ldentified by x-ray 0 iiffraction with a Phillips diffractometer using CuK
radiation and a scan rate o~ 2 ?er minute. The chemistry of all phases was determined from electron probe analysis of polished cross-sections using ~ CAMECA
CAMEBAX electron probe. The accura~y in the determination of elemental composition was better than 3% of the amount present.
The area of the aluminum melting endotherm in the high temperature DSC scan was used as a measure of the reactivity bstween a4C and Al at temperatures between 550C and 1200~C. The data were collected using a Perkin-Elmer DTA 1700 interfaced to a computer. The purge gas was ultra high purity argon flowing at 40 cc/min. The sample~ were heated in alumina crucibles at 20C/min and high purity aluminum (99.999%) was u~ed as a standard. The percent aluminum metal was given by A/B x 100, where A is the peak area in cal/g of the Ai melt endotherm in the sample and B is the same for the 3 Al standard. Precision and accuracy were 2 percent.
The results show that the reaction between boron carbide and aluminum starts at 450C with the formation of A14BC. The reaction rate is slow below oOO~C. rn the range of 550 to 600JC. 24% by ~/olume 38,885-F _~_ 2 ~ 3 ~
metal (80% of the original Al) can be recovered. Above 600C, AlB2 forms and aluminum is rapidly depleted. as illustrated in Figure 1. rhe open circles in Figure 1 represent the amount of unreacted Al metal retained in a B4C/Al powder mixture after heatin~ for one hour at temperatures between 450C and 1200~C. and cooling to room temperature at lO~minute. The open boxes indicate the amount of unreacted metal present after quenching in liquid nitrogen. 8etween 600 and 700C, AlB2 and B4C
are the predominant phases~ Above 700~C. AlB2 and A14BC
are present and. a.~ temper~ture increases. the relative amount of A14BC increases. Between 900~C and lOOO~C. 'he dominant reaction product is A14BC. .lt temperatures above 1000C, AlB2 decomposes and generates free aluminum. Heat treatment above lOOO~C produces mainly AlB24C4 and some A14C3. Phases formed below lOOOJC are aluminum rich and their formation leads to the rapid depletion of the metal. Phases formed above 1000C are boron and carbon rich resulting in B4C depletion and development composites with larger amounts of free metal and smaller amounts of boron carbide ~han the same starting powders heated below lOOO~C.
The major phases influencing the mechanical properties of B4C/Al based materials are A14BC. AlB2, AlB24C4 and A14C3. Because the formation of AlB24C4 i9 associated with the existence of undesirable A14C3, the heat treatment should be limited to temperatures of 1000C or lower where AlB2 and A14BC are the predominant new phases.
38.885-F _9_ Exam~le 2 Example 2 illustrates that hardness of 34C/Al cermets can be changed as a function of phase chemistry.
The same powders were used as in Example 1.
The boron carbide and aluminum powders were dry mixed in a rotary blender and then pressed into 75 mm diameter - discs using uniaxial compaction in a stainless steel die. No lubricants or binders were used. The samples io were placed into metal cans. The sealed cans were placed in fluid dies. The fluid dies with mixed B4C and Al powders were heated in the furnace of Example 1 to 640C. The fluid dies were then removed from the furnace, placed in a forging press, and compacted. The compacted dies were removed from the forging press and cooled to room temperature. The diqcs were separated from the cooled dies using conventional procedures and cut into various ~hapes for te~ting and character-ization.
The bulk hardness wa~ measured on surfacespolished successively with 45, 30, 16. 6 and 1 um diamond paste and finally finished using colloidaL
silica suspension on a LEC0 automatic poliqher. The Rockwell A hardness was measured using 2 13.3 kg load.
The Vicker9 microhardness of isolated phases was measured using a LEC0 tester and loads of 10 to 20 grams. The largest grain~ of a particular phase were examined in order to eliminate or minimize the contribution from adjacent or underlying material.
Generally, the distance from the center of the indent to the nearest grain boundary was over twice the indent 38.885~F _lo-liagonal dimension. The indent diagonals were measured ~sing a scanning electron microscope at 10000X.
After densification, the B4C/30% by volume Al material had a hardness o~ 81 in Rockwell A scale.
?ost-densi~ication heat treatment at 500C and 1000C
provided a maximum hardness value of 83, which remained stable for extended times as shown in Figure 2. At ~ 1000C, the hardness declined initially due to the decomposition of AlB2, but then increased again as Al4BC
was formed. Heat treatments conducted at 700C, 300C, ?00C and 1100C produced ~ maximum in the hardness versus time curve. At higher temperatures. less time ~as required to obtain maximum hardness.
The change in hardness is characteristic of the B4C/Al system and due to the kinetics and B-C-Al phase equilibrium. Below 1000C, in the initial stage of heat treatment, hard Al4BC is favored. With increased time, there is insufficlent aluminum available and softer AlB2 forms decreasing overall hardness. Between 600C and 1000C Al4BC and AlB2 are both present. Increases in the AlB2/Al4BC ratio lower the maximum hardness and maKe the tran~ition from rising to declining hardness more gradual.
After 20 hours of heat treatment, the AlB2/Al4BC ratio was 8, 0.7 and 0.4 at 700, 800 and 900C respectively. At 1100C, Al4RC and then AlB24C4 formed re~ulting in a high hardne3s composite. However.
after 8 to 10 hours of heat treatment, Al4C3 became the predominant new phase and the hardnes~ declined. ~he highest hardness values were achieved through heat 38,885-F
treatments of 10 hours at 1100C (HRA=89), 20 hours aoooC
(HRA=~8) and 20 hours at 800~C (HRA=88).
Example 3 Example 3 ilius~rates that fracture toughness of dense B4C/Al material can be increased. This increase is associated with densification of the metal - phase. While the formation of ceramic phases reduces toughness, the extent of this reduction depends on the type of phase formed. AlB2 is the 'east detrimental.
The same powders and processing conditions were used as in Examples 1 and 2. Fracture toughness was measured using the Chevron notch technique and standard 4 x 3 x 45 mm samples. The notch was produced with 250 um wide diamond blade and the notch depth to sample height ratio was 0.42. A cro~s head speed of 0.05mm/minute was used in a 3 point bend fixture. The average of 5 to 7 measurements was reported.
The values of the fracture toughness of many B-C-Al phaseq are unknown. However, the damage and cracking pattern in the indented phases indicates that AlB2 has a higher toughnes~ than A14BC or AlB24C4.
Aluminum containing small ceramic crystals deforms plastically. The damage in A182 represents shear deformation rather than brittle cracking, B4C and AlB24C4 usually behave similarly, even though, in some cases, the crack propagates through the AlB24C4 and stops at the boron carbide grain boundary. A14BC shows brittle behavior with several cracks running from the corners and ~ide9 of the indent.
38,885-F -12-The results in Figure 3 indicate that post-densification heat treatment can substantially improve fracture toughness. The KIC increases over the entire heat treatment range ~hen compared to the pressure densified material. ~eyond the maximum at 60noc. KIC
decreases. Analytical transmission electron microscopy did not reveal any differences (for example.
precipitation) between the pressure dersified and heat treated metal phases. However, a reduction in porosity was observed in the heat treated samples implying increased density and improved bonding between the ceramic and metal which would account for the increased fracture toughness. At 600C. where the highest fracture toughness was obtained. sintering of the metal takes place before significant quantities of new phases are formed. The materials containing AlB2 have higher KIC values than those with A14BC and AlB24C4.
ExamPle 4 Example 4 shows that post-densification heat treatment can be used to improve many properties of B4C~Al cermets by changing the cermet into a multi-phase ceramic material containing only a small amount of residual metal.
It is possible to produce B4C/Al composites with different phases, but similar amounts of residual unreacted aluminum. The following Table provides data comparing the mechanical properties of the pressure densified and heat treated B4C/Al compositeg. The highest hardness and modulus were obtained in samples containing A14BC . While these materials had improved toughness relative to the pressure densified materials.
it was only 8.2 MPa ml/2. On the other ~and. samples 38,885-F -13-AlB2 containing AlB2 exhibited increased fracture toughness (9.2 MPa m1/2), but only slight hardness (Prom 81 to 85) improvement.
Because the AlB2/Al4BC ratio can be changed and controlled by heat treatment at a temperature between 700C and 900C, the properties of B4C/Al composites can be selected for a specific application. Processing at 1000C or below allows the formation of large amounts of AlB2 and avoids the formation of Al4C3.
In heat treating at or below 600C. the new phases form in limited amounts and a multi-phase ceramic is not produced. The resultant B4C/Al cermets are characterized by a microstructure of isolated boron carbide grains in an aluminum matrix with improved fracture toughness and fracture strength.
TABLE - The effect of heat treatment on properties of B4C/Al materials with i~olated boron carbide in a metal matrix.
3o 38.885-F ~14-3 ~
After After Heating above pressure 600C to produce: After Property densi- Heatlng fication AlB2 Al4BC AlB24C4A
Density 2.57 2.63 2.70 2.62 2.58 g/cm3 Flexure 419 357 351 312 434 strength (MPa) Fracture 7.23 9.2 8.2 7.5 12.7 10 toughne/2 Young 254 290 310 280 260 modulus (GPa) ~ulk 138 167 175 156 140 modulus (GPa) Hardness 81 85 88 88 81 Rockwell A
Poissons's0.23 0.21 0.20 0.20 0.23 ratio While this invention has been deqcribed with reference to certain specific embodiments, it will be recognized by those skilled in the art that many variation~ are possible without departing from the scope and spirit of the invention and it will be understood that it is intended to cover all changes and modifications of the invention, discloqed herein for the purposes of illustration, which do not constitute 3 departures form the spirit and scope of the invention.
38,885-F _15_
Claims (12)
1. A boron carbide-aluminum cermet characterized by a boron carbide phase, an aluminum phase, and an AlB2 phase or an Al4BC phase or an AlB2 phase and an Al4BC phase.
2. The composition of Claim 1, wherein said aluminum phase is present in an amount of 2-12% by volume.
3. A boron carbide-reactive metal cermet characterized by a boron carbide phase, a reactive metal phase or a reactive metal boride phase and a reactive metal boron carbide phase, and a reactive metal boride phase or a reactive metal boron carbide phase, the reactive metal being arsenic, barium, beryllium, calcium, cobalt, chromium, iron, hafnium, iridium, lanthanum, lithium, magnesium, manganese, molybdenum, sodium, niobium, nickel, osmium, palladium, platinum, plutonium, rhenium, rhodium, ruthenium, scandium, silicon, strontium, tantalum, technetium, thorium, titanium, uranium, vanadium, tungsten, yttrium or zirconium.
38,885-F -16-
38,885-F -16-
4. The composition of Claim 3 wherein said reactive metal phase is present in the amount of 2-12%
by volume.
by volume.
5. A process for preparing a boron carbide-aluminum cermet comprising:
(a) densifying an admixture of boron carbide powder and aluminum metal powder at a temperature in the vicinity of the melting point of aluminum to produce a densified composite; and (b) heating the densified composite to a temperature within a range of 450°C to 1000°C for a period of time sufficient to form within the densified composite an AlB2 phase or an Al4BC phase or an AlB2 phase and an Al4BC phase.
(a) densifying an admixture of boron carbide powder and aluminum metal powder at a temperature in the vicinity of the melting point of aluminum to produce a densified composite; and (b) heating the densified composite to a temperature within a range of 450°C to 1000°C for a period of time sufficient to form within the densified composite an AlB2 phase or an Al4BC phase or an AlB2 phase and an Al4BC phase.
6.The process of Claim 5 wherein, prior to heating, the densified composite comprises from 20 to 60% by volume aluminum and 80 to 40% by volume boron carbide.
7.The process of Claim 5 wherein, after heating, the densified composite contains from 2 to 12%
by volume aluminum.
by volume aluminum.
8.A process for preparing a boron carbide-reactive metal cermet comprising:
(a) densifying an admixture of boron carbide powder and a particulate source of a reactive metal at a temperature in the vicinity of the melting temperature of the reactive metal to produce a densified composite, the reactive metal being arsenic, barium, beryllium, calcium, cobalt, chromium, iron, hafnium, iridium, 38,885-F -17-lanthanum, lithium, magnesium, manganese, molybdenum, sodium, niobium, nickel, osmium, palladium, platinum, plutonium, rhenium, rhodium, ruthenium, scandium, silicon, strontium, tantalum, technetium, thorium, titanium, uranium, vanadium, tungsten, yttrium or zirconium; and (b) heating the densified composite to a temperature within a range of 450°C to 1000°C for a period of time sufficient to form within the densified composite a reactive metal boride phase or a reactive metal boron carbide phase or a reactive metal phase and a reactive metal boron carbide phase.
(a) densifying an admixture of boron carbide powder and a particulate source of a reactive metal at a temperature in the vicinity of the melting temperature of the reactive metal to produce a densified composite, the reactive metal being arsenic, barium, beryllium, calcium, cobalt, chromium, iron, hafnium, iridium, 38,885-F -17-lanthanum, lithium, magnesium, manganese, molybdenum, sodium, niobium, nickel, osmium, palladium, platinum, plutonium, rhenium, rhodium, ruthenium, scandium, silicon, strontium, tantalum, technetium, thorium, titanium, uranium, vanadium, tungsten, yttrium or zirconium; and (b) heating the densified composite to a temperature within a range of 450°C to 1000°C for a period of time sufficient to form within the densified composite a reactive metal boride phase or a reactive metal boron carbide phase or a reactive metal phase and a reactive metal boron carbide phase.
9.The process of Claim 5 or Claim 8, wherein the densified composite is heated to a temperature of 500°C to 800°C for a period of 1 to 30 hours.
10.The process of Claim 5 or Claim 8 wherein the densified composite is heated to a temperature of 600°C to 700°C for a period of 10 to 20 hours.
11.The process of Claim 8, prior to heating, the densified composite comprises from 20 to 60% by volume of the source of reactive metal.
12.The process of Claim 8, wherein after heating, the densified composite contains 2 to 12% by volume of the source of reactive metal.
38,885-F -18-
38,885-F -18-
Applications Claiming Priority (2)
| Application Number | Priority Date | Filing Date | Title |
|---|---|---|---|
| US60932290A | 1990-11-02 | 1990-11-02 | |
| US609,322 | 1990-11-02 |
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|---|---|
| CA2054834A1 true CA2054834A1 (en) | 1992-05-03 |
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| Application Number | Title | Priority Date | Filing Date |
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| CA002054834A Abandoned CA2054834A1 (en) | 1990-11-02 | 1991-11-01 | Boron carbide-reactive metal cermets having microstructure tailored by post-densification heat treatment |
Country Status (6)
| Country | Link |
|---|---|
| EP (1) | EP0484014A1 (en) |
| JP (1) | JPH055150A (en) |
| CA (1) | CA2054834A1 (en) |
| FI (1) | FI915158A (en) |
| IL (1) | IL99944A0 (en) |
| NO (1) | NO914293L (en) |
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| US5780164A (en) * | 1994-12-12 | 1998-07-14 | The Dow Chemical Company | Computer disk substrate, the process for making same, and the material made therefrom |
| US5672435A (en) * | 1994-12-12 | 1997-09-30 | The Dow Chemical Company | Hard disk drive components and methods of making same |
| RU2578305C2 (en) * | 2011-03-08 | 2016-03-27 | Юбло Са, Женева | Composite material containing precious metal, method of producing and using said material |
| PT3650562T (en) * | 2017-05-11 | 2023-10-19 | Hyperion Materials & Tech Sweden Ab | An iron tungsten borocarbide body for nuclear shielding applications |
| CN115198211A (en) * | 2022-07-05 | 2022-10-18 | 贵州航天天马机电科技有限公司 | Toughening method of TiCx-Cu cermet |
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| US4605440A (en) * | 1985-05-06 | 1986-08-12 | The United States Of America As Represented By The United States Department Of Energy | Boron-carbide-aluminum and boron-carbide-reactive metal cermets |
| US4702770A (en) * | 1985-07-26 | 1987-10-27 | Washington Research Foundation | Multipurpose boron carbide-aluminum composite and its manufacture via the control of the microstructure |
| US4718941A (en) * | 1986-06-17 | 1988-01-12 | The Regents Of The University Of California | Infiltration processing of boron carbide-, boron-, and boride-reactive metal cermets |
| US5019539A (en) * | 1989-01-13 | 1991-05-28 | Lanxide Technology Company, Lp | Process for preparing self-supporting bodies having controlled porosity and graded properties and products produced thereby |
-
1991
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| FI915158A0 (en) | 1991-11-01 |
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| JPH055150A (en) | 1993-01-14 |
| NO914293L (en) | 1992-05-04 |
| IL99944A0 (en) | 1992-08-18 |
| EP0484014A1 (en) | 1992-05-06 |
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