The Al2014 composites reinforced with uncoated and Ti5Si3-coated SiCP were fabricated by stir-casting. The results suggested that the Ti5Si3-coated SiCP/Al2014 composites exhibited more uniform dispersion of SiCP, higher elastic modulus and tensile properties than uncoated ones. Additionally, the nano-sized Ti3AlC2 interfacial layer was detected between the Ti5Si3-coated SiCP and α-Al.

•The abrasive wear behavior of Ti5Si3-coated SiCP/Al2014 has been firstly investigated.•Ti5Si3-coated SiCP/Al2014 showed better abrasive wear resistance than uncoated one.•The better wear resistance was mainly attributed to the enhanced interfacial bonding.This work described the differences of abrasive wear behavior and wear mechanism between 5 and 20 vol% salt-bath plated Ti5Si3-coated and uncoated SiCP/Al2014 composites. The composites were prepared by the semisolid sintering, and the wear experiment was conducted on a pin-on-disk tester under 5–20  N. The results suggested that Ti5Si3-coated SiCP/Al2014 composites exhibited better abrasion resistance than uncoated ones when tested under same condition. The enhanced abrasive wear resistance was benefited from the improved interfacial combination between Ti5Si3-coated SiCP and Al2014, which could prevent the SiCP from “being pulled out” and abrasive particle penetrating deeply into matrix. Besides, lower porosity and higher hardness also contributed to part of enhanced wear resistance for Ti5Si3-coated SiCP/Al2014 composites.

The creep resistance of Al-Cu alloys deteriorates significantly at high temperatures owing to the coarsening of θ′ precipitates during creep, which limits their applications at elevated temperatures. We fabricated a cast Al-Cu matrix composite containing finer and denser θ′ precipitates by adding 0.3 wt% in situ nano-sized TiCp into an Al-Cu alloy. The steady creep rates of the nano-sized TiCp/Al-Cu composite were 3–17 times lower than those of the Al-Cu matrix alloy at 453–493 K under applied stresses of 120–200 MPa, respectively, which was attributed to the higher threshold stresses of the composite due to the strengthening effect of the nano-sized TiCp and the larger number of θ′ precipitates with smaller diameters. The accelerated coarsening of θ′ precipitates after higher temperature creep could contribute to the decrease of threshold stress with increasing temperature. The analysis of the true stress exponent indicates that the dislocation climb mechanism is dominant in both the matrix alloy and the composite during creep.

The composites reinforced with Ti5Si3-coated SiCP showed higher relative densities and mechanical properties than those of uncoated ones. It is mainly due to the translation of Ti5Si3 coating layer into Al3Ti interfacial layer during the sintering process, increasing the interfacial bonding strength between α-Al and SiCP.

The (micron+nano) bimodal sized SiCp/Al2014 composites fabricated by semi-solid stirring exhibited fine α-Al grains, well-dispersed bimodal sized SiCp and well-bonded interface between SiCp and matrix. The yield and ultimate tensile strength of the extruded bimodal sized SiCp/Al2014 composites were significantly enhanced, compared to the extruded single-sized SiCp/Al2014 composites.

•Effect of Fe/Co/Ni on the ductility of TiAl was studied by theory and experiments.•Ni exists in the form of NiTi, which is detrimental to the ductility of TiAl.•Fe Co change electronic and elastic properties to improve ductility of TiAl.The Ni atom is difficult to occupy the Ti or Al site in TiAl, it exists in the form of NiTi phase at the grain boundary of TiAl alloy, which is detrimental to the ductility of the TiAl alloy. The Fe and Co atoms preferentially occupy the Al sites and can improve the electronic structures and elastic properties of TiAl, leading to the improvement of the ductility of TiAl alloy. With the addition of 3 at.% Fe and Co, the tested average fracture strain of TiAl alloy increases from 17.3% to 19.1% and 18.0%, respectively.

Compared to the micro-sized particle-reinforced metal matrix composites, the nano-sized particle-reinforced metal matrix composites possess superior strength, ductility, and wear resistance, and they also exhibit good elevated temperature properties. Therefore, the nano-sized particle-reinforced metal matrix composites are the new potential material which could be applied in many industry fields. At present, the nano-sized particle-reinforced metal matrix composites could be manufactured by many methods. Different kinds of metals, predominantly Al, Mg, and Cu, have been employed for the production of composites reinforced by nano-sized ceramic particles such as carbides, nitrides, and oxides. The main drawbacks of these synthesis methods are the agglomeration of the nano-sized particles and the poor interface between the particles and the metal matrix. This work is aimed at reviewing the ex situ and in situ manufacturing techniques. Moreover, the distinction between the two methods is discussed in some detail. It was agreed that the in situ manufacturing technique is a promising method to fabricate the nano-sized particle-reinforced metal matrix composites.

The microstructures and tensile properties of Ni coated nano-sized TiN particle reinforced Al–Cu matrix composites via casting were studied. It was found that with the increase of the TiN particle addition, the average size of the α-Al grains decreases. The nano-sized TiN particles enclosed inside the α-Al grains provide some heterogeneous nucleation sites during solidification, resulting in a more refined microstructure. Moreover, the θ' precipitates in the composites matrix were much finer and more uniformly distributed. After T6 heat treatment, the strength and ductility of the composites increase simultaneously. The yield strength, ultimate strength and elongation of the 2.0 wt% nano-sized TiNp/Al–Cu composite can reach to 365 MPa, 594 MPa and 13.5%, increased by 20.5%, 22.5% and 104.5% respectively from those of the Al–Cu matrix alloy (303 MPa, 485 MPa and 6.6%).

The compression yield strength and hardness of the TiB2/Al composites increase with the increase in the TiB2 content, while the fracture strain decreases, and the ultimate compression strength first increases and then decreases. The addition of Mg improves the ductility of the 50 vol% TiB2/Al composite along with maintaining the ultimate compression strength. The fracture strain of the TiB2/Al–Mg composite increases from 8.18% to 10.68%. The addition of Mo significantly improves the compression yield strength and hardness of the composite while sacrificing the ductility. The compression yield strength and the hardness of the TiB2/Al–Mo composite are, respectively, 296 MPa and 107.5 Hv higher than that of the TiB2/Al composite, but the fracture strain decreases to 4.41%. With the addition of V, the TiB2/Al–V composite possesses the highest ultimate compression strength and relatively good ductility. The ultimate compression strength of the TiB2/Al–V composite is 218 MPa higher than that of the TiB2/Al composite, and the fracture strain is 6.73%.

•Pr addition enhances the hardness and creep resistance of the Al–Cu alloy.•Pr addition facilitates the formation of the θ′ precipitates.•Pr addition results in the formation of the Al11Pr3 phase in the Al–Cu alloy.The effects of praseodymium on age hardening behavior and creep resistance of cast Al–Cu alloy were investigated. The results indicated that praseodymium facilitated the formation of the θ′ precipitates during the age process and improved the hardness of the Al–Cu alloy. Besides, praseodymium resulted in the formation of the Al11Pr3 phase in the grain boundaries and among the dendrites of the modified alloy. Because of the good thermal stability of Al11Pr3 phase, it inhibits grain boundary migration and dislocation movement during the creep process, which contributes to the improvement in the creep resistance of the modified alloy at elevated temperatures.

The morphologies of the transition metal carbide (TMC) (ZrCx, NbCx, and TaCx), transition metal nitride (TMN) (TiNx), and transition metal diboride (TMD) (NbB2x and TaB2x) particles formed during the self-propagating high-temperature synthesis (SHS) were investigated. The results indicate that the ceramics with wide stoichiometric ranges all show a stoichiometry-induced morphology evolution, i.e., octahedron → truncated-octahedron → spherelike → sphere, for TMCs and TMNs, and hexagonal prism → polyhedron → spherelike, for TMDs. For TMCs and TMNs, the increase in the stoichiometry leads to the increase in the growth rate in the ⟨111⟩ crystalline direction. Hence, their morphologies show an evolution process of gradual exposure of the {100} surfaces and shrinkage of the {111} surfaces. When the exposed {100} surfaces are roughed because of the extremely high combustion temperatures during the SHS and thus turn round, the growth shapes of the TMC and TMN crystals change to spherelike. On the other hand, when the TMCs and TMNs are stoichiometric or near stoichiometric, the critical transition temperature for thermodynamic roughening of the {100} surfaces could be very high. Then, the rounded {100} will restore to the flat surfaces, and the cubic and truncated-cubic TMCs and TMNs particles are formed. For TMDs, the morphology evolution could be caused by the decrease in the stability of the {0001} and {101̅0} surfaces at high stoichiometries. With the increase in the stoichiometry, these two surfaces are less-exposed gradually while the {11̅01} surfaces are exposed and expand. The growth shapes of TMDs change from regular hexagonal prism to polyhedron. With the rounding (roughening) transition of the {11̅01} surfaces at high temperatures, the TMDs particles become spherical.

Spherical NbB2−x particles were prepared by increasing the intrinsic B/Nb stoichiometry (x) during self-propagating high-temperature synthesis. Because of the strong anisotropy of the chemical bonding, the shapes of the hexagonal-structured transition metal diborides are generally reported as a hexagonal plate and prism, and this is the first time for the hexagonal-structured transition metal diborides to display the spherical morphology. Moreover, this paper suggested a technical and theoretical approach to synthesize the spherical particles for the compounds with wide stoichiometric ranges.

TiC–TiB2 particulate locally reinforced steel matrix composites were fabricated by a novel TE-casting route from an Al–Ti–B4C system with various B4C particle sizes. The formation mechanism of TiC and TiB2 in the locally reinforced regions was investigated. The results showed that TiC and TiB2 are formed and precipitated from Al–Ti–B–C melt resulting from the dissociation of B4C into Al–Ti melt when the concentrations of B and C atoms in the Al–Ti–B–C melt become saturated. However, in the case of coarse B4C powders (≥40 μm) used, the primary reaction in the Al–Ti–B–C melt is quite limited due to the poor dissociation of B4C. The poured steel melt infiltrates into the primary reaction product and thus leads to the formation of Al–Fe–Ti–B–C melt, thanks to the favorable reaction of molten Fe with remnant B4C, and then TiC and TiB2 are further formed and precipitated from the saturated Al–Fe–Ti–B–C melt. The relationship between the mechanisms of thermal explosion (TE) synthesis of TiC and TiB2 in the electric resistance furnace and during casting was proposed.Highlights► We successfully fabricated TiC–TiB2 locally reinforced steel matrix composites. ► We investigated the product microstructure and formation mechanism of TiC and TiB2. ► The formation mechanism during casting is dependent on B4C particle size. ► Fe melt promotes the full dissociation of remnant B4C particles during casting. ► The relationship between the mechanisms in the ERF and during casting was proposed.

Through multi–modification with PrxOy and LaxOy, the tensile strength and elongation of casting Al−Cu alloy can reach to 580 MPa and 10.5%, increased by 26% and 50% respectively than those of the unmodified alloy. Simultaneous increase in the tensile strength and elongation of the multi-modified alloys is suggested to be induced by the refined α-Al dendrites, the formation of Al−RE intermetallic compounds at the grain boundaries, and the most important, more homogeneously distributed nano-scale θ′ precipitates with higher diameter/thickness ratio.

The phase transitions in the Ti–Al and the Ti–Al–C systems are investigated by the differential thermal analysis (DTA) and X-ray diffraction (XRD). The results reveal that the addition of C not only leads to the formation of the Ti2AlC phase but also lowers the formation temperatures of the TiAl and Ti3Al phases. In the both systems, the combustion synthesis reactions are ignited at the melting point of Al, so the TiAl alloy and the Ti2AlC/TiAl composites are successfully fabricated at a lower temperature (about 660 °C) with an applied pressure. The yield strength, the ultimate compression strength and the work hardening rate of the Ti2AlC/TiAl composites are all higher than those of the TiAl alloy and increase with the increase in the Ti2AlC content. The average ultimate compression strength of the 6 vol.% Ti2AlC/TiAl composite is 179 MPa higher than that of the TiAl alloy without sacrificing ductility.Highlights► Phase transitions in the Ti–Al–C system during combustion synthesis were studied. ► The Ti2AlC/TiAl composites were successfully fabricated in the Ti–Al–C system. ► The addition of the C improves the compression properties of the TiAl alloy. ► The work-hardening behavior of the Ti2AlC/TiAl composites was discussed in detail.

The microstructure, compression property and work-hardening effect of the NiAl-matrix composite with 1.7 wt.% NbB2 and NbxC (x = 1 or 2) fabricated by the combustion synthesis and hot pressing technique have been investigated in the study. The NiAl-matrix composite has strong strain rate sensitivity. The yield strength of the NbB2–NbxC/NiAl composite decreases while the fracture strain and the work-hardening capacity (Hc) increase with decreasing strain rate. A high true ultimate strength of 1472 MPa, a fracture strain of 20.1% and Hc = 1.41 were obtained under the strain rate of 2.0 × 10−5 s−1.Highlights► The NiAl-matrix composites exhibits high room-temperature mechanical properties. ► The NiAl-matrix composite has strong strain rate sensitivity. ► The better mechanical properties were obtained at the strain rate of 2.0 × 10−5 s−1.

Compression tests of the 50 vol% TiCx/Al and the 50 vol% TiB2/Al composites have been performed in the strain rate range from 1 × 10−4 s−1 to 1 × 10−1 s−1, and the effect of strain rate on the compressive behavior and the work-hardening capacity (Hc) of them have been investigated. The yield strength (σtruey) and the Hc of both the composites are strain rate sensitive, while the ductility is insensitive to the strain rate. The ultimate compressive strength (σtrueUCS) of the TiB2/Al composites is not significantly influenced by the strain rate, while that of the TiCx/Al composite increases from 615 MPa to 826 MPa with the strain rate increasing. Under various strain rates, the σtrueUCS of the TiB2/Al composite is significantly better than that of the TiCx/Al composite, while its ductility and Hc are lower than that of the TiCx/Al composite. At the strain rate of 1 × 10−4 s−1, the σtrueUCS of the TiB2/Al composite and the TiCx/Al composite are 1071 MPa and 615 MPa, respectively; while the Hc of them are 0.87 and 0.99, respectively.Highlights► 50 vol% TiCx/Al and 50 vol% TiB2/Al were fabricated by the in situ method. ► The compression behavior of two composites was studied at different strain rates. ► Strength of TiB2/Al is higher than that of TiCx/Al, while ductility is lower. ► The strain rate sensitivity of these two composites was compared.

The microstructure, compression property and work-hardening effect of the NiAl-matrix composite reinforced by 5 vol.% ceramic particulates (TaB and TaB2) were investigated. The compression properties and work-hardening capacity (Hc) under the different strain rates were also investigated. The NiAl-matrix composite has strong strain rate sensitivity. A high true ultimate strength of 1399 MPa, a true fracture strain of 18.7% and Hc = 1.84 were obtained under the strain rate of 1.0 × 10−4 s−1. The good compression properties may be attributed to the refinement of the grain size and the good combination between the ceramic particulates and the NiAl-matrix.

The effect of Mn on the mechanical properties of the Ti2AlC/TiAl composites is investigated by experiments and first principles calculations. The Mn atoms preferentially occupy the Al sublattice sites and can cause the lattice contraction of TiAl along its c-axes. As a result, the lattice tetragonality of TiAl, which is related with its intrinsic brittleness, could be reduced. The calculation results are consistent with the experimental observations. The addition of Mn remarkably improves the ductility of the composites. Moreover, the strength and work-hardening rate are also enhanced due to the solid solution of Mn and the grain refinement of the TiAl matrix. The strength and the work-hardening rate increase with the increase in the Mn content, while the ductility increases firstly and then decreases. The composite with the addition of 2 at.% Mn possesses the best compression properties, of which the average compression strength is 256 MPa higher than that of Mn-free sample, and the average compression fracture strain increases from 16.4% to 19.9%.Highlights► Effect of Mn on the mechanical properties of the Ti2AlC/TiAl composites is studied. ► Mn addition improves the compression properties of the Ti2AlC/TiAl composites. ► The work-hardening behavior of the Ti2AlC/TiAl composites was discussed in detail.

The combustion reaction mechanism of the Ni-Ti-C-BN system was investigated by a combustion front quenching method. The results showed that the SHS reaction in the Ni-Ti-C-BN system starts with the formation of TiNx and TiB from the solid-state reaction between Ti and BN. Meanwhile, the solid-state reaction between Ni and Ti occurs at some locations to form Ni3Ti. Subsequently, the resultant TiNx reacts with Ni to form the Ni-Ti compounds such as NiTi and Ni3Ti. As the temperature increases further, Ni-Ti liquid appears between NiTi and Ni3Ti, and some B atoms that diffused away from BN could dissolve into the Ni-Ti liquid to form TiB2. The heat generated from these reactions promotes the further dissolution of the C, B and N atoms in the Ni-Ti liquid to form the Ni-Ti-B-N-C liquid. As the B, N and C atoms in the liquid become sufficiently supersaturated, precipitation of plenty of TiB2 and TiCxNy grains occurs. In addition, the reasons for the prior formation of TiB2 over TiCxNy and separated residence of the TiB2 and TiCxNy grains in the microstructure were discussed.In this work, the combustion reaction mechanism of the Ni-Ti-C-BN system was investigated by a combustion front quenching method. The results showed that the ceramic phases of TiCxNy and TiB2 were formed through the dissolution–reaction–precipitation mechanism.

Effect of C particle size on the mechanism of self-propagation high-temperature synthesis (SHS) in the Ni–Ti–C system was investigated. Fine C particle resulted in a traditional mechanism of dissolution-precipitation while coarse C particle made the reaction be controlled by a mechanism of the diffusion of C through the TiCx layer. The whole process can be described: C atoms diffusing through the TiCx layer dissolved into the Ni–Ti liquid and TiC were formed once the liquid became supersaturated. Simultaneously, the heat generated from the TiC formation made the unstable TiCx layer break up. However, with the spread of Ti–Ni liquid, a new TiCx layer was formed again at the interface between spreading liquid and C particle. This process cannot stop until all the C particles are consumed completely.Highlights► We investigated the effect of C particle size on the self-propagating high temperature reaction mechanism. ► Coarse C particle size (>38 μm) resulted in the formation of prior TiCx layer between Ti and C. ► Prior TiCx layer control the whole reaction of Ni-Ti-C and domain the reaction kinetics. ► The selection of C particle size is the most important factor to fabricate TiC/Ni composite using Ti, C and Ni mixtures.

The microstructures, interfaces, compression properties and work-hardening effect of the NiAl-matrix composites reinforced by 5–20 wt.% ceramic particulates (Nb2C, NbC and NbB2) fabricated by combustion synthesis and hot pressing (CSHP) have been investigated. The ultimate compression strength and yield strength increase with the increasing content of the ceramic particulates, while the fracture strain and work-hardening capacity (Hc) decrease. The NiAl-matrix composite with 5 wt.% ceramic particulates has a high true ultimate strength of 1497 MPa, a fracture strain of 18.3%, and work-hardening capacity Hc = 1.29. The good interface bonding between ceramic particulate and matrix, the high density dislocation in the NiAl matrix, the seriously distorted lattices and the intense interactions between dislocations and other crystal defects contribute to its prominent mechanical properties.Research highlights► The NiAl-matrix composite with 5 wt.% ceramic particulates (Nb2C, NbB2 and NbC) has a high true ultimate strength of 1497 MPa, fracture strain of 18.3%, and work-hardening capacity Hc = 1.29. ► The microstructures and the interface between the NiAl and the ceramic particulates are studied. ► The lattice misfit of the NiAl and the ceramic particulates are calculated.

Differential thermal analysis was undertaken to determine the reaction path of the synthesis of α-Al2O3-TiC-TiB2 in an Al-TiO2-B4C system under argon. The Al content plays a significant role in controlling the reaction path and product. When the Al content is no more than 26.7 wt.%, TiO2 first reacts with B4C to yield TiB2 with TiBO3 being the intermediate phase, and then increasing temperature leads to the subsequent reactions between Al and TiO2 or its sub-oxides to yield α-Al2O3 and Al3Ti, and the resultant Al3Ti then reacts with B4C to produce TiC and TiB2. When the Al content is high (e.g. ≥ 34 wt.%), the reaction between Al and TiO2 for the formation of α-Al2O3 and Al3Ti occurs initially, and then the Al3Ti reacts with B4C. With the increasing Al content, the onset of the exothermic reaction in the Al-TiO2-B4C system shifts to lower temperature and the degree of reaction conversion is enhanced.Highlights► We investigated the reaction path in an Al-TiO2-B4Csystem by DTA and XRD. ► Al content plays a significant role in controlling the reaction path and products. ► The reaction of TiO2 with B4C occurs first for Al content no more than 26.7wt%. ► With an increase in Al, the prior reaction of TiO2 with Al becomes more favorable. ► Ideal products could be obtained for high Al content even under DTA condition.

The different shapes of titanium carbide (TiCx) grains formed at different growth stages of self-propagating high-temperature synthesis (SHS) were obtained in the quenched sample. The shape evolution of the TiCx grains and the growth mechanism are discussed. As the highly substoichiometric TiCx nucleates at the initial stage of the SHS, the (111) surfaces become stable and the TiCx grains nucleate as octahedra. With an increase of the TiCx stoichiometry, the free energy of the (111) surfaces increases. Hence, the area of the (111) surfaces on the TiCx grains decreases gradually while the (100) surfaces are exposed. The growth shape of the TiCx grains turns to truncated-octahedron. Moreover, when the combustion temperature during the SHS exceeds a certain value (about 1800 °C), the (100) surfaces of the TiCx grains turn round and these rounded (100) regions grow and coalesce with further increasing of the TiCx stoichiometry. The growth shape of the TiCx grains then turns to close-to-sphere.

The morphologies of the TiCx grains extracted from the self-propagating high-temperature synthesis (SHS) products in an Al−Ti−C system were examined by field-emission scanning electron microscope (FESEM) and their lattice parameters were calculated from the XRD results. It is believed that the combustion temperature plays an important role in determining their morphologies, which may evolve from octahedron to truncated octahedron and finally to sphere with the increase in the temperature.

Excellent wettability of SiC and TiC was obtained with a molten Cu47Ti34Zr11Ni8 alloy with final contact angles of ∼0 and ∼5°, respectively. Cu-based glassy composites, reinforced by in situ TiC particles produced by the reaction between Ti and SiC, were then fabricated by copper mould casting. Uniaxial compression tests indicate that the fracture strength increases from 2000 MPa in the monolithic glass to 2159 MPa in the glassy composite, and the plastic strain increases from 0 to 2.5%.

The ignition temperature of the thermal explosion reaction from the Ti-C system under air is much lower than that under Ar atmosphere. The ignition mechanism for the Ti-C system under air is determined to be a mechanism of chemical oven, and the reaction mechanism is dissolution, reaction, and precipitation. Namely, the heat generated from the oxidation and nitrification of Ti and C can promote the melting of Ti inside the compact; subsequently, the carbon atoms dissolve into the Ti melt and TiC precipitate.

Creep behavior of the unmodified and modified cast Al–Cu alloys was investigated at temperatures from 393 to 483 K in the tension test. The creep resistance ability of the Al–Cu alloy modified by PrxOy is almost 3–5 times as high as that of the unmodified Al–Cu alloy, which is attributed to a large number of nano-scale θ′ precipitates with high thermal stability in the modified Al–Cu alloy restricting and impeding the dislocation movement during the creep. The induction of a threshold stress in the analysis leads to a stress exponent of 5, which suggests that the creep behavior of both the present alloys is associated with the lattice diffusion-controlled dislocation climb (n = 5).

The effects of different as-cast microstructures which were initially cast in graphite, metal, sand and firebrick moulds, respectively on the semisolid microstructure of AZ91D alloy, have been investigated during the strain-induced melt activation (SIMA) process. The experimental results showed that the moulds with high cooling capacity could produce the fine-grained as-cast microstructure in which the fine α-Mg dendrites were surrounded by a narrow layer of eutectic mixtures. After compressive deformation, in the fine-grained as-cast microstructure, the more systemic strain energy would be gradually accumulated and abundantly stored due to uniform inner crystal lattice distortion, so the recrystallization was easily induced by the stored strain energy at the elevated temperature. As a channel for the diffusion of atoms, the subgrain boundary along which Al element was enriched, foremost melted above the eutectic temperature and resulted in the separation of neighboring subgrains from primary dendrites. Therefore, the refining role of recrystallization on the microstructural evolution from dendrite to globular particles in morphology was easier to play in the fine-grained as-cast microstructure, which was advantageous for the production of fine-grained semisolid microstructure. Additionally, in the fine-grained as-cast microstructure, the melting fracture of narrow secondary dendritic arms was easy to occur in their roots, which also attributed to the production of fine globular grains in semisolid microstructure from primary dendrites. The finer dendrites in the initial as-cast alloy could evolve into the finer globular grains with relatively small grain size distribution range in the semisolid microstructure during partial remelting; therefore, the finer the dendrites in the initial as-cast microstructure, the better were the tensile properties of the evolved semisolid microstructure.

The reaction behavior and mechanism during self-propagating high-temperature synthesis (SHS) in an Al-TiO2-B4C system were investigated. The result shows that the reaction could be self-sustaining for the Al content no more than 58.4 wt pct in the reactants. With the increase in Al, the combustion temperature decreases monotonically, the ignition delay time first decreases and then increases, while the combustion wave velocity displays an opposite behavior. The reaction mechanisms are somewhat dependent on the Al content, while the resultant types of the final products are independent. The desired reaction products make the Al-TiO2-B4C system ideal for producing the (α-Al2O3-TiC-TiB2)/Al composites using the SHS technique.

A method is proposed to estimate quantitatively the shifting distance of the eutectic point in a horizontal direction (under non-equilibrium solidification condition). The shifting distances of the eutectic point in Al–20 wt.%Si alloys under the non-equilibrium solidification condition are 0.4 and 0.9 wt.% in a horizontal direction towards higher silicon contents with the addition of 0 and 3 wt.% master alloys (Al–20 wt.%Y2O3), respectively.

The reaction sequence and phase formation mechanism in self-propagating high temperature synthesis (SHS) processing of the Al–Ti–B4C compacts were explored through a delicate microstructure and phase analysis on the combustion-wave quenched samples in combination with differential thermal analysis. The reaction sequence could be described as Al +Ti + B4C → TiAl3 + Ti + B4C → TiAln + B4C → TiAln + B13C2 + TiCx + TiaAlbCc → TiAln + TiCx + TiB2 + TiaAlbCc → Al + TiCx + TiB2. The phase formation mechanism could be ascribed to the displacive reaction between B4C and the Ti–Al melt and is essentially dependent on the dissociation and diffusion rates of carbon and boron from the B4C crystal. The reasons for the prior formation of TiCx over TiB2 and the separated residence of the TiCx and TiB2 grains in the microstructure were addressed.

The TiC–TiB2/Al composites were fabricated by self-propagating high-temperature synthesis (SHS) from Al–Ti–B4C compacts. The addition of Al to the Ti–B4C reactants facilitates the ignition occurrence, lowers the reaction exothermicity, and modifies the resultant microstructure. The maximum combustion temperature and combustion wave velocity decrease with the increase in the Al amount. The B4C particle size exerts a significant effect on the combustion wave velocity and the extent of the reaction, while that of Ti has only a limited influence. The reaction products are primarily dependent on the B4C particle size and the Al content in the reactants. Desired products consisting of only the TiC, TiB2, and Al phases could be obtained by a cooperative control of the B4C particle size and the Al content.

Steel matrix composites locally reinforced with different molar ratios of in situ TiC/TiB2 particulates (2:1, 1:1 and 1:2, respectively) have been fabricated successfully utilizing the self-propagating high-temperature synthesis (SHS) reactions of Ni–Ti–B4C and Ni–Ti–B4C–C systems during casting. Differential thermal analysis (DTA) and X-ray diffraction (XRD) results reveal that the exothermic reactions of the Ni–Ti–B4C and Ni–Ti–B4C–C systems proceed in such a way that Ni initially reacts with B4C and Ti to form Ni2B and Ti2Ni compounds, respectively, with heat evolution at 1037 °C; Subsequently, the external heat and the evolved heat from these exothermic reactions promote the reactions forming TiC and TiB2 at 1133 °C. In the composites reinforced with 1:2 molar ratio of TiC/TiB2, almost all TiB2 grains have clubbed structures, while TiC grains exhibit near-spherical morphologies. Furthermore, TiB2 grain sizes decrease, with the increase of TiC content. In particular, in the composites reinforced with 2:1 molar ratio of TiC/TiB2, it is difficult to find the clubbed TiB2 grains. Macro-pores and blowholes are absent in the local reinforcing region of the composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2, while a few macro-pores can be observed in the composite reinforced with 2:1 molar ratio of TiC/TiB2. Moreover, the densities of the composites reinforced with 1:1 and 1:2 molar ratios of TiC/TiB2 are higher than that of the composite reinforced with 2:1 molar ratio of TiC/TiB2. The composite reinforced with 1:2 molar ratio of TiC/TiB2 has the highest hardness and the best wear resistance.