Self-propagating high-temperature synthesis (SHS) of intermetallic matrix composites

Self-propagating high-temperature synthesis (SHS) of intermetallic matrix composites

8

K. Naplocha Wroclaw University of Science and Technology, Wrocław, Poland

8.1 Fundamentals of SHS technique Self-propagating high-temperature synthesis (SHS) method with many types and variety of components allows to produce the materials of any shape, morphology, chemical complexity, and unique properties. So far, the industrial products produced by SHS include cutting tools, abrasives, heaters, insulation, anticorrosion coatings, or composite materials. Developed processes based on SHS method enable the production of complex materials with advanced properties, which fulfilled almost every specialist demand. Frequently, these processes were limited to the systems with high enthalpy reactions. Moreover, the obtained materials were characterized by a large porosity content. Currently, there are many methods: compression of the products during the synthesis or the application of vacuum or high pressure in the chamber, which promotes densification. The low exothermic systems, like Al-Cr with enthalpy less than 20 kJ/ mol at, can be supplemented with additional, highly reactive components; the reagents can be preheated, or as it was done in Ref. [1], the synthesis ignited and maintained by additional couple of materials with the high enthalpy (thermally coupled TC). In the classical synthesis variant, a mixture of starting materials (powders) is compressed in order to produce a compact and achieve a better contact between the particles of reactive constituents. Subsequently, a compact is placed in the processing chamber, wherein the synthesis is carried out. In contrast to the sintering technology or other methods requiring heating from the outside, here, the internal chemical energy drives the process. Usually, to start the reaction, a warmed-up tungsten wire ignites the material at a point, and then, due to the release of significant amounts of energy, the propagation of the reaction front moves through its entire volume (see Fig. 8.1). When the amount of heat released from the reaction is greater than the amount of heat dissipated to the surroundings, the ignition of material and self-sustaining synthesis are not possible. On the other hand, heating the successive layers of material requires adequate thermal conductivity and therefore temperature gradient. Examination of a possibility to initiate the reaction in terms of particle size and thermal conductivity is presented in Ref. [2]. This formula identifies a prerequisite for ignition: 1 ∂   2 ∂T  r λe   ∂r  ti r ∂r 

( −∆H ) ki (Tig ,κ ) >  2

Intermetallic Matrix Composites. http://dx.doi.org/10.1016/B978-0-85709-346-2.00008-X © 2018 Elsevier Ltd. All rights reserved.

(8.1)

204

Intermetallic Matrix Composites Tm

Temperature Degree of chemical conversion

Direction of wave propagation Cooling zone Tig Rate of heat releasing

T0

x Phenomena affected the wave speed

Self-annealing and forming of the final structure

Fig. 8.1  Wave propagation characteristics of SHS method along x coordinate including the temperature variations, the chemical conversion, and the heat release rate.

where Tig and ti are, respectively, the temperature and time of ignition, k is the degree of reaction progress, ΔH is the reaction enthalpy, k is the constant of reaction rate at ignition temperature, λe is an effective thermal conductivity of the starting mix ture, and r is the position vector. It indicates that the favorable exothermic heat of the synthesis must be at least 100 kJ/mol; a small temperature gradient is favorable, and the reaction rate constant k at the moment of ignition must be large, which means thin layer of products through which atoms diffuse.

8.1.1 Synthesis parameters In the typical self-propagation high-temperature synthesis, the mixture of reactants is prepared from powders of granularity, which ranges usually from 10 to 100 μm. Depending on the planned composition of the final products, the adequate stoichiometric ratio is selected from the elemental powders (e.g., Al, Ni, Ti, Si, C), the ­oxides (TiO2, B2O3, BaO2), the compounds, and, in some systems with suitable gases (N2), flow rate and pressure. The mixed powders are compacted under the pressure (20–500 MPa), usually uniaxial cold pressing, to produce pellet of a certain green density and porosity. With increasing compaction load, powder particles are plastically deformed, thin layer of oxides are broken and moved, and contact areas between particles are enlarged. This plays an important role in the initiation and development of reaction because atomic or ionic diffusion between particles can take place freely, gradually forming the planned compounds. The released heat from exothermic reaction in local volume preheats and ignites the mixture layer by layer, and then the reaction front propagates with velocity of 0.1–10 cm/s. The combustion synthesis and related structure formation involve melting, spreading of the melt, and liquid droplet coalescences. The produced liquid phases and the gaseous species are subjected to gravity-driven flow, infiltration, and convection.

Self-propagating high-temperature synthesis

205

Therefore, the mass transfer caused by the effect of gravity affects reaction, product's morphology, and its properties [3]. This phenomenon could be altered by mixing of constituents through convection or externally applied pressure. Concluding, the reaction parameters that significantly affect SHS technique characteristic are - a reactant particle size - a mixture green density/porosity, its conductivity, and possible gas filtration - the stoichiometry of reactants, amount of diluents (products) to moderate process, and active/inert gases - the ignition temperature, in system with metal powders corresponding approximately to melting point of one of the reagents - the combustion temperature and adiabatic temperature in perfectly insulated system - the preheating temperature, heating, and cooling rate - the gravitational field

8.1.2 Physicochemical reaction mechanism The reaction of the mixture components involves the physicochemical process in which the temperature and the velocity of the front propagation significantly affect the structure and properties of the product. The synthesis front passing through the sample leaves behind a layer of reacting substrates from which the released energy preheats next layers of unprocessed material. Thus, reaction enthalpy is split into the part to ignite (ΔHS) and preheat substrates (ΔHP) beyond the front and the part related to complete transformation of the reactants to products. Hence, the reaction enthalpy of SHS at the ignition temperature Tig is described by the following equation: − ( ∆H S + ∆H P ) = ∆Hig

(8.2)

The reaction enthalpy at the point ΔHig, as the amount of heat generated through adiabatic heating, raises the temperature of adjacent layers of substrates from T0 to Tig, whereas the remaining part of the energy absorbed by the products (ΔHP) preheats them to temperature Tc. Using the data available in Refs. [4–6], a sample enthalpy diagram for NiAl compound was constructed (see Fig. 8.2). A preliminary preheating of substrates from T0 to Tig reduces the value (ΔHS) to 0 J/mol, and the whole heat from the reaction can be adsorbed by the products resulting in higher synthesis temperature. In practice, the reaction is conducted in conditions that differ from the adiabatic and heat dissipates into the environment. Therefore, the heat loss (ΔQ) reducing the temperature of the synthesis disrupts the stability of the front propagation and can even halt it. At the adiabatic conditions, the reaction enthalpy (ΔHig) at Tig temperature is as follows: Tig

∆Hig = ∆H 298 +

∫ C ( product ) − C ( substrate ) dT p

s

298

where Cp and Cs are, respectively, the specific heat of reactants and products.

(8.3)

206

Intermetallic Matrix Composites 140

1728 K

Enthalpy (kJ/mol at.)

120

Al+Ni

933 K

1912 K NiAl

80

∆HS ∆Q

40

∆HP 0

T0

−40

1000

500

TC

Tign 1500

Tad 2000

2500

T (K)

Fig. 8.2  Enthalpy changes for the formation of NiAl compound vs temperature relating to preheating of a product (ΔHP) and the substrates (ΔHS).

Considering the simplest two-component A–B system, the reaction with x stoichiometric fraction of A component is then xA + (1 − x ) B → A x B1− x

(8.4)

and includes the maximum temperature of the synthesis Tc with the reaction enthalpy from Equations (8.2) and (8.3), and then relationship for the A–B compounds is as follows: Tig

−

∫

298

Tc

C p ( xA + (1 − x ) B) dT − y∆H Al −l − ∫ C p ( A x B1− x ) dT − ∆Q = ∆H( 298) Tig

Tig

+

∫ C

298

p

(8.5)

(A x B 1− x ) − C p ( xA + (1 − x ) B  dT − y∆H A −l

where Cs(xA+(1 − x)B) and Cp(AxB1−x) are the specific heat of reactants and products; y is a fraction of the liquid component, usually Al, Mg, or Ti; ΔHA−1 and ΔH298 are, respectively, the melting enthalpy of A component and reaction enthalpy at 298 K temperature. Finally, the reaction enthalpy at room temperature can be expressed by the equation: Tc

−∆H 298 =

∫C

p

(A x B 1− x )dT + ∆Q

(8.6)

298

where Cp(AxB1−x) is the specific heat capacity, values of which as a function of temperature are available in the thermodynamic databanks (Thermo-Calc, SGTE, etc.).

Self-propagating high-temperature synthesis

207

On this basis, the maximum adiabatic temperature of the synthesis can be determined, or when the value of temperature Tc is measured experimentally, the heat loss can be evaluated and then, if possible, reduced. Nowadays, there is a software (THERMO software) to facilitate a comprehensive analysis of the SHS synthesis. Especially, it has been developed by the leader in this field, the scientific center ISMAN (Russia). Deviations from the adiabatic conditions and the heat loss affect the important period of synthesis—the cooling time tc occurs after reaching the maximum temperature Tm. It comprises the self-annealing and the formation of an equilibrium structure. The thermal relaxation time—sample cooling can be determined by the temperature curve of the synthesis, which is given by the equation [7]:  T −T tc = t  m 0  e

  − t ( Tm ) 

(8.7)

where T0 is the initial temperature of the sample and t time. Therefore, it is important to control the temperature and front velocity, which above all depend on the reaction enthalpy, thermal conductivity, and general process ­parameters such as the stoichiometry of the substrates, their granularity, the degree of compaction, the pressure, and the type of shielding gas. The analysis of SHS technique should involve at least two reaction mechanisms. Depending on the particle size of the substrates and their physical state in the reaction zone, the capillary or the diffusion reaction mechanism can be found. In the case of smaller particles in the solid state, the atoms (usually one of the substrates) diffuse through the forming layer of products until the reaction is completed. The occurrence of such mechanism can be determined by the particle size criterion, defined in Ref. [8]: T − T  r02  γλt rr µ m v 2 ln  c 0  (8.8)  Tt − T0  where r0 is the initial radius of the particle, rr is the radius of undissolved particle, lt is the coefficient of thermal diffusion, γ is the surface tension of the liquid, μm is the viscosity of the fluid, v is the velocity of the propagation front, Tc is the synthesis temperature, T0 is the initial preheating temperature of the mixture, and Tt is the melting temperature for the formation of liquid component. If one of the components of lower melting point flows over the surface of the solid particles, the process is determined by the kinetics on the surface. Then, the reaction occurs directly between a liquid and a solid component. In this case, the particle size undergoes the synthesis determined by the relation [8]: r02 

γ ⋅ rr µD

(8.9)

where D is the diffusion coefficient. The initial synthesis course may transform depending on the temperature and physical state of the components. In Si-C system [2], the initially formed layer of SiC can block synthesis progress, but when the temperature exceeds 2150°C, it is dissolved, and carbide precipitates directly from a supersaturated Si solution. This phenomenon

208

Intermetallic Matrix Composites

also allows to introduce other components (B and Ti) and to form the double interpenetrating phases. In multicomponent systems, the formation of low-melting eutectics may occur first, and, e.g., in the system Ti-Si-C, fine SiC carbide phases precipitated from supersaturated solution Si(C) form eutectic composition [2].

8.1.3 Substrates The particle size significantly affects the heat balance of the synthesis. The initial ­porous structure with fine particles quickly transfers heat into the environment, and at a reduced thermal conductivity and high-temperature gradient, the reaction cannot progress to adjacent layers. On the other hand, too high conductivity leads to the spreading of the reaction zone and its cooling down. In the mixture of larger granularity, number joints between particles and potential reaction places are smaller. It reduces the wave propagation velocity and temperature of synthesis and consequently leads to the formation of nonequilibrium structures. Equally important are the degree of powder compaction and the porosity of the compact. On one hand, better adherence between particles facilitates the reaction. On the other, it increases the conductivity and heat dissipation. Additionally, in the case of the synthesis with filtration, the volume of molten metal should correspond to the pore volume and then should completely fill them, maximizing the contact surface. In turn, in the synthesis with reactive gas, the liquid metal may intensively coat the particles and separate the reagents. Therefore, other selection criteria for synthesis parameters should be defined. Another important factor that affects the reaction kinetics is the stoichiometry of the components in the starting mixture. An excess of one reagent products usually contains unreacted starting materials or unstable compounds. Sometimes to prepare mixture, produced in a separate process, such compounds are deliberately added. It reduces the contact surface between substrates along with involved enthalpy and moderates the reaction. Generally, the deviation from the stoichiometry of the system lowers the amount of released heat and the synthesis adiabatic temperature. Other factors that affect the synthesis course and form the specific structure of the product are the pressure and the type of atmosphere in the process chamber. The use of argon generally prevents the undesired reactions with the surrounding atmosphere, while pressurized vapor reduces the volatility of reactive elements. In turn, the vacuum removing gases and venting the chamber may prevent bursting and deformation of the material, especially when reactants are preheated and the thermal explosion mode of SHS occurs [9]. Usually, the powder surface is coated with oxide, and in the case of porous materials (graphite), the high water absorption is observed. The surface of Ti powder particles is coated with TiH2—a hydride, which at high temperature decomposes and can be treated as a source of highly gasification hydrogen. This compound is widely used in the manufacture of metal foam, a very promising lightweight and durable material. Generally, the compounds in powder surface at high temperature decompose, releasing large quantities of gases usually in the form of CO or H2 [10,11]. The pressurized vapors cause the formation of gas bubbles or gaps through which the gas is released outside a sample. In most cases, the undesired porosity of the ­product is reduced by grinding and compaction of the reactive powders or selection of

Self-propagating high-temperature synthesis

209

proper composition of the starting mixture. However, applications of porous products, which exhibit interesting and unique properties, have been previously elaborated [12]. Compared to the classical solid materials, they have greater specific strength [13] and can be produced with a very low density. Gaseous components fed under pressure during synthesis play basically different roles. Depending on the pressure and porosity of the sample, the gas flows, and the reaction rate is controlled. Typically, at high pressure (100–500 MPa [8]), gas completely penetrates the sample, reacts, and forms a product layer by layer. In addition, the intensive filtration of material allows to extend the cooling time tc, to complete conversion and finally to produce stable structures. Under lower pressure, gas distribution within the sample is too small, and the synthesis can be carried out only on the surfaces from the outside to the center of a sample. Moreover, at high porosity, the natural filtration induced by the gas consumption and the pressure gradient inside the sample can occur. Then, the gases from the outside will pass to the inside and react gradually. There are methods to control the gas flow by regulating its pressure along the direction of the front propagation or in the opposite.

8.1.4 Activation methods Using the conventional powder metallurgical methods and high-energy ball milling of the starting mixture, fragmentation and better adhering of the reagents can be achieved. As a result of the intensive deformation of grains, there is a significant increase in density of structural defects and surface area, which leads to increase in enhancing the diffusion kinetics of reactive constituents. In addition, the strain energy and the modified surface between particles lower the activation barrier for the reaction. Using these phenomena, the synthesis can be carried out directly in the ball mill (mechanically induced self-propagating reaction—MRS) or mixture can be activated (mechanically activated SHS (MASHS)), and then the standard treatments for SHS can be applied. In the case of MASHS method, the ignition temperature can be reduced, and due to the higher number of interparticle contacts, the synthesis propagation is significantly increased. Maintaining of the reaction and overcoming of the thermodynamic limitations for the system, which is characterized by the low level of released heat, can be realized by preheating the mixture (external power source). Besides the conventional heating methods, an interesting technique is to subject the material to an electric field (field-activated SHS—FACS) [8,14,15]. Because after the initiation of the synthesis front propagation can expire, the current flowing through the sample generates heat, and by the local electric discharges between particulate reactants, the process is enhanced. By controlling the current, voltage, and power, the synthesis can be carried out in the system with low or high enthalpy. Similar opportunities present microwave heating. It combines the advantages of heating the entire volume and SHS reaction. The used method allows both to ignite the synthesis and to support its progress. Mostly, microwave heating is used for ­synthesis involving thermoreduction of TiO2 with Al powder to produce TiC and Al2O3 [16,17]. The microwave ignition of synthesis is possible, though the influence of the ­magnetron

210

Intermetallic Matrix Composites

power on synthesis temperature and structure of the material is ambiguous. Supporting the synthesis of Ti-Al intermetallic compounds elaborated in Ref. [18] was defined as new method MACS—microwave-activated combustion synthesis. In [19], SHS reaction is initiated by high-current electric impulses generated periodically from a battery of capacitors. The powder mixture placed between two punches as electrodes was heated by the Joule heating, and the spark discharges occurred between the particles. This technique combined with pressure exerted upon the powder mixture allows to produce NiAl-Al2O3 composites with relative density exceeding 99%, high hardness, and fracture toughness.

8.2 Densification of combustion synthesis products The relatively high level of porosity formed by the gases, which are released during the high-temperature reaction, is the characteristic of typical combustion materials. Any contaminations or absorbed gases on the reactant powders and gases evolved from low boiling point reactants and products should be removed either during or immediately after the synthesis is completed. At high temperature, the material is still in a plastic state, and then under pressure, gases are forced out. Therefore, the moment of material squeezing is critical. When load is applied too early, at time when gases are partly developed and the temperature is low, the solid structure can be crushed. Moreover, squeezing of gases from solid may lead to cracking. Conventional hot uniaxial pressing involves heating of the powder mixture in the die with vents, under vacuum, with applied pressure during the entire process or for a few seconds after the reaction. Such compaction generates inhomogeneous density distribution, inhomogeneous reaction characteristics and finally specific structure of products [8]. Consolidation can be enhanced by explosive compaction, which induces plastic flow under impulse (0.1 s) of very high pressure. Furthermore, to improve the effectiveness of degassing reactant powders, vacuum sealed containers can be used, and then press formed. Sheng et al. [20,21] have elaborated productive densification method based on hot extrusion. Besides reducing porosities, the composite structures were comminuted, the reinforcement was uniformly distributed, and the mechanical properties were significantly improved, especially the compressive ductility. In another approach to improve the density, the combination of hot rolling and combustion synthesis was developed [22]. It was found out that rolling speed and position of rolls should be precisely controlled and located in the vicinity of the reaction zone.

8.3 Microstructural evolution during synthesis The SHS technique of intermetallics from metal powders proceeds rather at low combustion temperature (1000–1500°C), sometimes with partial conversion. Therefore, preheating of reagents or adding another reacting component can help to support the

Self-propagating high-temperature synthesis

211

wave propagation. The ignition and local preheating of starting mixture to produce most popular aluminides or titanium nickelides [23–25] involve melting of particles. Low-melting-point Al spreads, and depending on synthesis parameters, i.e., stoichiometry, granularity, and heating rate, the intermediate aluminides are formed. Through exothermic reactions of Al with Ni, the first intermetallic phase to form is Ni3Al, which is followed by that of Al(Ni) + Al3Ni mixture through eutectic reaction at 640°C [23,26]. Finally, depending on the composition of initial mixture, NiAl or other compounds corresponding to stoichiometric ratio are formed. In system with Ti, thermodynamically favorable phases formed between Al and Ti particles is Al3Ti. With progress of reaction and metal diffusion, next layers of Al2Ti, AlTi, and Ti3Al are formed, as transitional phases, until the synthesis is completed and the final compound corresponding to stoichiometric ratio is formed [27]. Another frequently studied system, Ni-Ti with shape memory compound NiTi, can start from the formation of the eutectic liquid at 942°C [24], which then transforms into Ni3Ti and NiTi. To predict first phase formation and development of synthesis in binary system, the effective heat formation model [28,29] based on minimum reaction enthalpy and the lowest liquidus temperature can be used. The determined effective heat ­formation ΔHʹ is defined as ΔHʹ = ΔH0 × Cʹ/C0 where Cʹ is the effective concentration of limiting element and C0 is compound concentration of this element. In the analysed binary-system, the heat formation, ΔH0, expressed in kJ/mol of each compound is plotted against its composition expressed in terms of concentration of constituent elements. The example of such graph constructed for Al-Cr system is presented in Fig. 8.3. At liquidus temperature, when the greatest mobility and most effective mixing occur, it corresponds in this system to 2 at% of Cr. Because Al7Cr has the most negative formation enthalpy ΔHʹ = −2.14 kJ/mol at (ΔH0 = 13.4 kJ/ mol at, Cʹ = 2% at ~660°C and C = 1/8), it is expected to be the first phase formed in the diffusion zone (Fig. 8.3). The synthesis of more complex system to produce composite materials requires almost individual analysis and insightful investigation. The combustion synthesis of TiC-AlNi composites starts with the formation of TiC followed by the melting of Al and subsequent formation of NiAl during the cooling [25]. A slightly opposite conception is presented in Ref. [31] when first intermetallic Al3Ti compound is formed, and then with increasing temperature, C reacts with Al3Ti, formed in the melt, and finally, TiC precipitates out. A similar when first intermetallic NiTi is formed, which then reacts with B to produce reinforcement particle TiB2, as reported earlier in Ref. [24]. The thermite-type reaction systems that involve reduction of an oxide to the element and next reaction of this element to form the reinforcement oxide and/or intermetallic compound could be very complex and need wide examinations. Kunrath et al. [32] investigated synthesis of composites described by the reaction: xTi + 3TiO2 + 5Al = 2Al 2 O3 + iTiAl + jAlTi 3 + kTi It was found out that the excess of Ti acts as both reactants and diluent. As a reactant, it combines with Al and lowers the ignition temperature. A diluent decreases the combustion temperature so that the alumina does not melt and, thereby, provide finer grains.

212

Intermetallic Matrix Composites

0 −4

−28 0,0

0,2

0,4

0,6

0,8

Al7Cr

−24

Al4Cr

−20

Al7Cr Al11Cr2 Al4Cr Al9Cr4 Al8Cr5 AlCr2

Al9Cr4

−16

lowest liquidus (Al0.98Cr0.02)

Al11Cr2

−12

Al8Cr5

∆H’ (kJ/mol.at.)

−8

1,0

Atomic % Al

Fig. 8.3  Effective heat formation model for Al-Cr system. The point of intersection of dotted liquidus line and Al7Cr line in top right corner represents the most negative formation enthalpy ΔHʹ [30].

8.4 Some intermetallic composite systems 8.4.1 Particulate reinforced The intermetallic compounds have gained considerable interest due to their high melting point, low density, excellent corrosion resistance, and high specific strength. However, as single phase materials, they exhibit low ductility at ambient temperature, low creep resistance, and high brittleness. To enhance mechanical properties and promote the application of intermetallic materials, many investigations on strengthening methods have been done. Usually, these processes involved solid solution, precipitation, or dispersion strengthening. Among them, the SHS technique is considered as an economic and convenient technique. The obtained composites reinforced with boride, carbide, or oxide are manufactured in one step, involving the formation of both matrix and reinforcement phases, and possess combination of high stiffness and toughness. The elemental powders or inexpensive widespread compounds are mixed in the stoichiometric molar ratio and then compressed to appropriate green density of compact. After ignition, the components react, producing the intermetallic matrix with uniformly distributed reinforcing particles. An example of such synthesis can be the formation of alumina precipitation in titanium aluminide matrix: 3TiO2 + 2 Al → 2 Al 2 O3 + Ti x Al y or

Self-propagating high-temperature synthesis

213

Ti + 3B + x ( 3Ni + Ti ) → TiB2 + xNi3 Ti which in elaborated process [24] proceeds gradually, first forming the intermetallic matrix and then precipitating the TiB2 phase from reaction of NiTi with B. Obviously, for designing the required product, first the thermodynamic possibilities and synthesis parameters are estimated. Although the process itself is simple, effective, and cheap, it requires several tests to establish suitable process conditions and to avoid the formation of unfavorable phases or porosity. The starting powders should be dried and treated to remove any contaminations on their surface. Also, in order to reduce the volatile gases, the compaction should be performed gradually in the vacuum [21]. But even at well-selected parameters, the reaction products usually contain a small amount of undesired phases or porosities. Presented in Refs. [33,34], microstructure analysis has confirmed the presence, apart from TiAl matrix, of a minor amount of Ti3Al. An incomplete phase conversion can be caused by the low reaction temperature or by using coarse Al particle. This phenomenon is often reduced by increasing reaction temperature, e.g., by preheating of starting mixture or adding other reagents that react, releasing more energy. Fortunately, during the synthesis, the reinforcement phases generally precipitate in highly exothermic reactions. In synthesis of TiAl matrix composites, more heat is liberated from formation of Ti2AlC [34] than from TiAl. Thus, at 30% Ti2AlC content, flame-front propagation and combustion temperature increase, and finally, no secondary aluminide phases are detected. Similarly, the formation of TiB2 in TiAl matrix reduces the amount of Al3Ti secondary phase and, additionally lower volume expansion during the synthesis is responsible for less amount of residual porosity [33]. Another important factor that affects the phase composition, the morphology, and the quantity of microstructure defect is the stoichiometric ratio. Small changes of this factor result in completely different products. The combustion synthesis of Al + Ti + C mixture having the ratio of 1:2:2 leads to the formation of globular TixC reinforcement embedded in α-Al + Al3Ti matrix (see Fig. 8.4A). Besides, large interconnected pores

(A)

(B)

Fig. 8.4  Microstructure of the composites fabricated from a starting mixture of Al + Ti + C with stoichiometric ratio of 1:2:2, interconnected porosity (A) and 1:1:1, platelike precipitates in Al3Ti (B).

214

Intermetallic Matrix Composites

are formed in the entire structure. The materials characterized by high ductility and refractory type properties can be applied in filter and catalyst production. Using the Al:Ti:C ratio = 1:1:1, occasional pores with rough surface are closed, and the microstructure of the composite product contains Ti8Al6C matrix reinforced with plate like Al3Ti phases (see Fig. 8.4B). The intermetallic composite materials fabricated by the combustion synthesis are novel materials, and though intensively tested, the available mechanical properties data are very limited. Presented in Ref. [35], examination of NiAl/TiC composites shows that at 980°C, the tensile yield strength is approximately three times higher than that of the monolithic NiAl. Moreover, compressive yield strength is significantly better, and the ambient temperature fracture toughness of the composite increases by 50%. Sheng et al. [21] investigated the effect of TiC and Al2O3 particle reinforcement on the strength of NiAl-based composite. Processing technology combining SHS and hot extrusion method allows to significantly increase the yield strength to 1370 MPa simultaneously with 18% of compressive strain. Undoubtedly, the reinforcing effect of intermetallic phases can be achieved, and future investigations will find out more convenient and effective technologies.

8.4.2 Laminates The metal-intermetallic laminated composites (MIL) offer attractive mechanical properties, which are distinct from the monolithic intermetallic materials. The brittle matrix reinforced with numerous ductile metal phase exhibits high oxidation resistance combined with toughness of metal, high modulus, crack propagation resistance, and relatively low density. The laminar composites are being considered for high-temperature structural components, and armor applications, as well as for vibration damping or heat exchanger. The multilayered structures can be produced by diffusion bonding, vapor deposition, or plasma spraying. These methods require sophisticated processing equipment and limit the size and shape of products. The fabrication of MIL composites that utilizes SHS technique begins at lower temperature, and it is accomplished in a short time, at low cost and energy consumption. Moreover, the prepared layered stack prior to the reaction can be performed into desired final shape. This technology, sometimes called the reaction sintering or hot pressing, involves compressing metal foil stack and heating them until SHS technique is initiated at metal/metal interface. Usually, the elemental foils of 10–400 μm thickness are polished, cleaned, and etched to remove oxides from their surfaces. Then after drying, they are placed alternately to form a multilayered sandwich. Sometimes to reduce the layer thickness, the stack is cold-rolled [36]. The fabrication process basically consists of three steps: first, diffusion bonding under mild pressure at high temperature, performed, e.g., in the vacuum hot press; second, with increase of temperature, SHS technique is ignited; and the final step, thermal aging treatment under high pressure is performed. The reaction synthesis can proceed after ignition from one end of sample with a typical combustion wave, or the whole reactant sample is heated in the furnace and thermal explosion (TE) mode occurs. In the first case, the reaction front

Self-propagating high-temperature synthesis

215

can spread along the foil at relatively low rate and temperature. Then, during the second period, the reaction front propagates rapidly in the direction perpendicular to the interfacial plane until at least one reactant is consumed. Depending on the initial foil thickness and its chemical composition, the synthesis consumes or not the entire foil. Usually, after the synthesis, the microstructure consists of unreacted metal layer, or its solid solution, and a zone with several intermetallic compounds. For example, in Ti-Al system, starting from Al-rich area, it will be TiAl3, TiAl2, TiAl, Ti3Al, α-Ti, and finally Ti [37]. Less compounds were detected after synthesis with Ni, when between Ni and Al foil, only Ni2Al and NiAl3 intermetallic existed [38]. Undesirable features observed in the microstructure of the composite are voids and even cracks. They were mainly caused by the melting of the reagents (usually Al), difference in molar volumes between reactants and products, and Kirkendall effect [39]. Therefore, applied in the end, hot press and heat treatment allows to reduce the number of intermetallic unstable phases and to produce a dense structure. An exemplary formation process with the transformation sequence of the microstructure is schematically illustrated in Fig. 8.5. The produced multilayered structures with fine interlamellar spacing are designed to improve the impact and fracture resistance. Materials are insensitive to the original cracks, and in comparison with powder metallurgy technology, the oxygen content of the intermetallics is limited. Generally, MIL composites are characterized by higher elastic modulus and lower ductility. Under the load and high stresses, the cracks develop first in intermetallic layers. The metal layer undergoes plastic deformation. Additionally, due to the differences in thermal expansion and elastic modulus between intermetallic and metal layers, after the synthesis with solid-liquid transformation, the high residual stresses stay. The failure of the composite considerably depends on the foil thickness as well as on the metal-to-intermetallic volume ratio. For high values, the cracks that are propagating through the intermetallic layer can be absorbed by relatively thick metal layer. As the damage in the intermetallic layers can be arrested, tensile strength and ductility of composite increases [40].

Stacking A

B

Local SHS reaction Layer SHS reaction A

Heat treatment A

BxAy

Fig. 8.5  Schematic illustration of the microstructure evolution during a typical reactive sintering of multilayered composite. Metal foils A and B are stacked and then heated to ignite SHS reaction that spreads and consumes the foil B almost completely. Annealing at high temperature and pressure transforms the intermetallics and reduces the porosity.

216

Intermetallic Matrix Composites

Recently, an innovative processing technique has been developed for fabricating multilayered composites additionally reinforced with ceramic particles. Alternating TiB whisker-rich layer and Ti3Al matrix have been successfully produced [41], employing a solid-liquid reaction between foils, though one of them was TiB/Ti composite foil. Such composite sheets show much finer equiaxed grains of the intermetallic phase, significant strength at high temperature and remarkably enhanced plasticity.

8.4.3 Interpenetrating phase composites The composites with interpenetrating phases, i.e., continuous reinforcement network penetrated by the matrix, are intensively studied and developed to utilize their favorable mechanical properties and unique morphology. The synthesis proceeds with excess of liquid phase, which simultaneously infiltrates the pores of the solid structure of reinforcement. Horovitz and Gotman [42] fabricated TiAl-based composites with magnesium aluminate spinel MgAl2O4 by applying an explosion mode of SHS and uniaxial pressing. During the combustion, reactively formed spinel remains solid, while liquid TiAl under pressure flows and penetrates the porosity. The external pressure (20–80 MPa) should be controlled and adjusted to meet the requirements of reaction stage and the state of components. Then, avoiding cracking or squeezing of liquid, a fully dense (~98%) near-net-shape part can be produced. In some systems, during synthesis, penetration of the solid structure by liquid phase succeeds freely. To improve synthesis and microstructure of TiC-TiB2, the composite mixture of Ni + Al was added [43]. Formed liquid NiAl phase fills the gaps between solid particles, which enhances diffusion, synthesis progress, and, due to high thermal conductivity of NiAl, the cooling rate of the product. As a result, the fabricated composite was found to be dense, with fine microstructure, and its compressive strength is being increased by 50%. In another fabrication method, infiltration of intermetallic skeleton was performed in a separate step by squeeze-casting of Al or Cu alloy [44]. Prepared by combustion synthesis, the preforms should exhibit open porosity structure and sufficient strength to resist pressure infiltration. Under optimized condition of squeeze-casting, the molten metal can saturate the skeleton and simultaneously react to form a new material and with tailored properties. On applying squeeze casting under pressure 90 MPa, molten Cu infiltrates porous structure of Al9Cr4 preform and simultaneously diffuses and changes its chemical composition [30]. The porous preform reacting with Cu releases Al to the matrix. The morphology and the shape of the preform stay unchanged, whereas the phase composition transforms into the globular precipitates of Cr52Al35Cu13 embedded in Cu47Al41Cr12 phase. At the same time, the Al-Cr alloy melt outflowed from the preform to the matrix. As a result, the matrix was converted into Cu9Al4(Cr). The obtained composite exhibited high hardness and resistance to high-temperature oxidation. By developing a compact layer of Al2O3 and (AlCu)2O3 (see Fig. 8.6), growth of the oxide scale is hindered. The parabolic kinetics of oxidation proceeded at constant rate (kp) of 1.9 × 10−6 g2/m4/s.

Self-propagating high-temperature synthesis

217

Reacting front

preform composite matrix

(A)

(B)

Fig. 8.6  SEM micrographs showing reaction front and partly converted composite (A), view of casting, locally with composite, subjected to oxidization in air for 24 h at 800°C (B).

8.5 Summary The combustion synthesis with its variant SHS enables the production of intermetallic materials at low energy consumption, without specialist equipment, relatively quickly and effectively. The produced materials of any size and shape exhibit complex morphology, phase composition, high purity, and unique properties. The synthesis course is usually affected by the stoichiometry of reactants, their granularity, the compaction pressure that improves contact areas between particles, the preheating temperature, and the amount of energy released during the exothermic reaction. The compacted mixture of powders is locally preheated to initiate ignition temperature, and then the reaction develops in entire volume of sample. The released heat corresponding to the reaction enthalpy is split into a part for preheating substrates beyond the front and a part responsible for increasing the temperature of products and the conversion rate of structure. Then, due to this, the self-annealing formation of an equilibrium structure is observed. The reaction can proceed in solid state, when atoms of one of the substrates diffuse through a forming layer of products or directly between a liquid and a solid component, which is determined by the capillary pressure in the penetrated porosity. During the high-temperature reaction, the gases evolved from contaminations, and the compounds absorbed on the reactant powders form pores in the metal structure. Therefore, to force them out and to produce a dense structure, the uniaxial compaction or extrusion is applied to the material, which is in a plastic state. The synthesis of intermetallics from metal powders usually starts from melting the particles. In systems with Al, intermediate, and thermodynamically favorable ­aluminides are formed, e.g., Al3Ni and Al3Ti close to the melting point of 660​°C. In some systems like Ni-Ti, the reaction starts from the formation of a eutectic liquid, which then transforms into Ni3Ti and NiTi. Generally, the complex reaction occurs in the composite synthesis and needs individual analysis and investigation. Among the SHS processed intermetallic matrix composites, the particle reinforced, laminated, and interpenetrating type composites can be found. The prepared mixture

218

Intermetallic Matrix Composites

of elemental powders or compounds is ignited to produce in a single step intermetallic matrix with uniformly distributed reinforcing particles like borides, carbides, and oxides. Such materials may exhibit much higher yield strength or fracture toughness than monolithic alloy. Manufacturing of laminates covers the compressing metal foil stack and heating them to ignite the synthesis at metal/metal interface. Usually, the microstructure consists of unreacted metal layer and a zone with several intermetallic compounds. Thus, in relatively brittle intermetallic matrix, the possible cracks can be absorbed by the metal layer. It results in significant strength and modulus at high temperature and good oxidation resistance combined with reasonable amount of toughness. The synthesis of the composites with interpenetrating phases proceeds with the excess of liquid phase, which simultaneously infiltrates the pores of the solid reinforcement structure. Formed during the synthesis, the liquid phase, e.g., TiAl and NiAl, flows and fills the gaps freely or under the applied external pressure. It has been found that the SHS-produced fully dense materials are characterized by high compressive strength, hardness, or oxidation resistance.

References [1] Merzhanov AG. Thermally coupled processes of self-propagating high-temperature synthesis. Doklady Phys Chem 2010;434:159–62. [2] Pampuch  R. Some fundamental versus practical aspects of self propagating high-­ temperature synthesis. Solid State Ion 1997;101–103(2):899–907. [3] Orru R, Licheri R, Locci AM, Cao G. Self-propagating combustion synthesis of intermetallic matrix composites in the ISS. Z-Tec Publishing, Bremen Microgravity Sci Technol; 2007. XIX-5/6. [4] Morsi K. Review: reaction synthesis processing of Ni-Al intermetallic materials. Mater Sci Eng A 2001;299:1–15. [5] Zhu P, Li JCM, Liu CT. Adiabatic temperature of combustion synthesis of Al-Ni systems. Mater Sci Eng A 2003;357:248–57. [6] Zhou S, Hawley MC. A study of microwave reaction rate enhancement effect in adhesive bonding of polymers and composites. Compos Struct 2003;61(4):303–9. [7] Merzhanov AG, Kovalev DYu, Shkiro VM, Ponomarev VI. Equilibrium of products of self-propagating high-temperature synthesis. Doklady Phys Chem 2004;394:34–8. [8] Mossino  P. Some aspects in self-propagating high-temperature synthesis. Ceram Int 2004;30(3):311–32. [9] Merzhanov AG. Combustion processes that synthesize materials. J Mater Process Technol 1996;56(1–4):222–41. [10] Dunmead SD, Munir ZA, Birch Holt J. Gas-solid reactions under a self-propagating combustion mode. Solid State Ion 1989;32–33:474–81. [11] Eslamloo-Grami M, Munir ZA. Effect of porosity on the combustion synthesis of titanium nitride. J Am Ceram Soc 1990;73:1235–9. [12] Karczewski  K, Jóźwiak  S, Karczewski  K, Bojar  Z. Porous Fe-Al intermetallics fabricated by SHS reaction in volume control environmental reactor. Inzynieria Materialowa 2010;31(3):642–5. [13] Efimov OYu, Zaripov NG, Bloshenko VN, Bokij VA, Kashtanova AA, Belinskaya UL. The effect of residual pressure on the structure and the strength properties of porous titanium carbide obtained by the SPHTS method in vacuum. Fizika Goreniya i Vzryva 1992;28:50–3.

Self-propagating high-temperature synthesis

219

[14] Hu Q, Luo P, Yan Y. Influence of an electric field on combustion synthesis process and microstructures of TiC–Al2O3–Al composites. J Alloys Compd 2007;439(1-2):132–6. [15] Mei Z, Yu P, Tjong SC. Structure, thermal and mechanical properties of in situ Al-based metal matrix composite reinforced with Al2O3 and TiC submicron particles. Mater Chem Phys 2005;93(1):109–16. [16] Atong D, Clark DE. Ignition behavior and characteristics of microwave-combustion synthesized Al2O3–TiC powders. Ceram Int 2004;30(7):1909–12. [17] Golkar  G, Zebarjad  SM, Vahdatikhaki  J. Optimizing the ignition behavior of ­microwave-combustion synthesized Al2O3/TiC composite using Taguchi robust design method. J Alloys Compd 2009;487(1-2):751–7. [18] Jokisaari JR, Bhaduri S, Bhaduri SB. Microwave activated combustion synthesis of titanium aluminides. Mater Sci Eng A 2005;394:385–92. [19] Michalski A, Jaroszewicz J, Rosinski M, Siemiaszko D. NiAl–Al2O3 composites produced by pulse plasma sintering with the participation of the SHS reaction. Intermetallics 2006;14:603–6. [20] Sheng LY, Yang F, Xi TF, Guo JT, Ye HQ. Microstructure evolution and mechanical properties of Ni3Al/Al2O3 composite during self-propagation high-temperature synthesis and hot extrusion. Mater Sci Eng A 2012;555:131–8. [21] Sheng LY, Yang F, Guo JT, Xi TF, Ye HQ. Investigation on NiAl–TiC–Al2O3 composite prepared by self-propagation high temperature synthesis with hot extrusion. Compos B 2013;45:785–91. [22] Rice RW. Hot rolling of ceramics by the use of self propagating synthesis. Patent number: 4642218, USA; 1987. [23] Moo JJ, Feng HJ. Combustion synthesis of advanced materials: part I. Reaction parameters. Prog Mater Sci 1995;39:243–73. [24] Yi HC, Woodger TC, Guigne JY, Moore JJ. Combustion characteristics of the Ni3Ti-TiB2 intermetallic matrix composite. Metall Mater Trans B 1998;29:867–75. [25] Curfs  C, Cano  IG, Vaughan  GBM, Turrillas  X, Kvick  A, Rodrıguez  MA. TiC– NiAl composites obtained by SHS: a time-resolved XRD study. J Eur Ceram Soc 2002;22:1039–44. [26] Moor JJ, Feng HJ. Combustion synthesis of advanced materials: part II. Classification, applications and modeling. Prog Mater Sci 1995;39:275–316. [27] Naplocha K, Granat K. Microwave activated combustion synthesis of porous Al-Ti structures for composite reinforcing. J Alloys Compd 2009;486(1/2):178–84. [28] Mogilevsky P, Gutmanas EY. On thermodynamics of first-phase formation during interfacial reactions. Mater Sci Eng A 1996;221(1-2):76–84. [29] Theron CC, Pretorius R, Ndwandwe OM, Lombaard JC. First phase formation at interfaces: comparison between Walser-Bené and effective heat of formation model. Mater Chem Phys 1996;46(2–3):238–47. [30] Naplocha K, Granat K. Combustion synthesis of Al-Cr preforms activated in microwave field. J Alloys Compd 2009;480(2):369–75. [31] Nath H, Amosov AP. SHS amidst other new processes for in-situ synthesis of Al-matrix composites: a review. Int J Self Propagating High Temp Synt 2016;25(1):50–8. [32] Kunrath AO, Strohaecker TR, Moore JJ. Combustion synthesis of metal-matrix composites: part II, the Ti-TixAly-Al2O3 system. Scr Mater 1996;34(2):183–8. [33] Yeh  CL, Su  SH. In situ formation of TiAl–TiB2 composite by SHS. J Alloys Compd 2006;407:150–6. [34] Yeh CL, Li RF, Shen YG. Formation of Ti3SiC2–Al2O3 in situ composites by SHS involving thermite reactions. J Alloys Compd 2009;478:699–704.

220

Intermetallic Matrix Composites

[35] Xing ZP, Han YF, Guo JT, Yu LG. Microstructure and mechanical behavior of the NiAlTiC In situ composite. Metall Mater Trans A 1997;28(4):1079–87. [36] Qiu X, Wang J. Experimental evidence of two-stage formation of Al3Ni in reactive Ni/Al multilayer foils. Scr Mater 2007;56:1055–8. [37] Sun Y-B, Zhao Y-Q, Zhang D, Liu C-Y, Diao H-Y, Ma C-L. Multilayered Ti-Al intermetallic sheets fabricated by cold rolling and annealing of titanium and aluminum foils. Trans Nonferrous Met Soc China 2011;21:1722–7. [38] Kim  HY, Chung  DS, Hong  SH. Reaction synthesis and microstructures of NiAl/Ni ­micro-laminated composites. Mater Sci Eng A 2005;396:376–84. [39] Alman  DE, Dogan  CP, Hawk  JA, Rawers  JC. Processing, structure and properties of ­metal-intermetallic layered composites. Mater Sci Eng Al 1995;92(l):624–32. [40] Rawers JC, Alman DE. Fracture characteristics of metal/intermetallic laminar composites produced by reaction sintering and hot pressing. Compos Sci Technol 1995;54:379–84. [41] Cui X, Fan G, Huang L, Gong J, Wu H, Zhang T, Geng L, Meng S. Preparation of a novel layer-structured Ti3Al matrix composite sheet by liquid–solid reaction between Al foils and TiB/Ti composite foils. Mater Des 2016;101:181–7. [42] Horvitz D, Gotman I. Pressure-assisted SHS synthesis of MgAl2O4–TiAl in situ composites with interpenetrating networks. Acta Mater 2002;50:1961–71. [43] Cui H-Z, Ma L, Cao L-L, Teng F-L, Cui N. Effect of NiAl content on phases and microstructures of TiC-TiB2-NiAl composites fabricated by reaction synthesis. Trans Nonferrous Met Soc China 2014;24:346–53. [44] Naplocha K, Granat K, Kaczmar J. Reactive melt infiltration of copper in Al-Cr preforms produced through combustion synthesis. J Alloys Compd 2014;613(15):193–8.