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In order to study the influence of the carbon nanotubes (CNTs) as a source of carbon on the microstructure of in-situ synthesized TiC in Cu melts, CNTs and Ti powders were introduced into melted Cu to prepare TiC-reinforced Cu matrix composites. The influence of Ti/C ratio and Si on the microstructures and properties of the composites were also examined. It is found that CNTs can be effectively wetted through the Ti-C reaction and successfully introduced into Cu melt to synthesize TiC. In examining the changes in Ti/C ratio, it was found that an increase in the Ti content may result in the decrease of TiC agglomeration and improvement of TiC dispersion, while simultaneously causing an increase in the TiC particle size. Besides, while the addition of Si into Ti-CNTs mixture can also improve the distribution of TiC, the effect is weak compared with that of increasing the content of Ti. It was also found that the highest hardness (238.8 HV) is achieved by the Cu-Ti-C composite with the highest Ti/C ratio, while the electrical conductivities of all the prepared composites are relatively low, which should be due to the insufficient reaction between Ti and CNTs.

An innovative method for the preparation of TiN–Ti5Si3 hybrid reinforced copper matrix composites by the reaction and spontaneous dispersion of Ti–Si3N4 mixture in Cu melts is proposed in this study. The microstructures of the composites and the reaction process of Ti and Si3N4 with different sizes in Cu melts were analyzed. It is found that the Si3N4 with small size, such as 2 μm, can easily react completely with Ti to form TiN and Ti5Si3. The formed TiN are particle-like and their size ranges from 0.5 μm to 3 μm, while Ti5Si3 are rod-like with a certain length to diameter ratio. In addition, many Ti5Si3 rods are combined with Cu cores to form core–shell structure. It is considered that these microstructure characteristics are helpful for TiN and Ti5Si3 to form a good hybrid enhancement effect. Although the Si3N4 with large size, such as 50 μm, can also be effectively added into Cu melts by Ti–Si3N4 reaction. However, unlike the composite prepared by 2 μm Si3N4, besides the forming TiN and Ti5Si3, there are other kinds of core–shell particles present in the matrix, including Si3N4@TiN, Cu@TiN and Si3N4@TiN@Cu@TiN.

The Cu-Ti-Si alloys containing in-situ formed Ti5Si3 are prepared. In order to clarify the Ti5Si3 formation processes and its microstructure characteristics, the as-cast and deeply etched Cu-Ti-Si alloys with different compositions and cooling rates were investigated using scanning electron microscopy (SEM), transmission electron microscopy (TEM) and X-ray diffraction (XRD). It is found that the eutectic Ti5Si3 phases in Cu-Ti-Si alloys are rod-like with hexagonal cross section which tend to intertwine with each other to form a firm skeleton like a bird nest structure which can make the alloys keep their original shape even after etching off the Cu matrix. In addition, there is Cu in the center of many Ti5Si3 rods, resulting in a core-shell structure. With the increase of the cooling rate, Ti5Si3 distributes more uniformly, and the diameter of Ti5Si3 significantly decreases, with a minimum size of less than 100 nm, while the aspect ratio of Ti5Si3 increases.

The influence of Al addition on the microstructure of Cu−B alloys and Cu−ZrB2 composites was investigated using scanning electron microscopy, X-ray diffraction and first-principles calculation. The results show that the eutectic B in Cu−B alloys can be modified by Al from coarse needles to fine fibrous structure and primary B will form in hypoeutectic Cu−B alloys. As for Cu−ZrB2 composites, Al can significantly refine and modify the morphology of ZrB2 as well as improve its distribution, which should be due to its selective adsorption on ZrB2 surfaces. The first-principles calculation results indicate that Al is preferentially adsobed on ZrB2 (12(__)10), then on ZrB2 (101(__)0), and finally on ZrB2 (0001). As a result, smaller sized ZrB2 with a polyhedron-like, even nearly sphere-like morphology, can form. Due to Al addition, the hardness of Cu−ZrB2 composites is greatly enhanced, but the electrical conductivity of the composites is seriously reduced.

In this work, ZrC particles have been successfully synthesized in the Cu melts using graphite as the source of C. It is considered that the instantaneous reaction between Zr and C which can realize the reactive wetting of Cu and graphite is attributed to the effective introduction of graphite into the melts. However, because it is impossible that all the graphite can contact with Zr in the mixture by the present method, only a fraction of graphite can be reactive wetted and the others will be burned out. As a result, when the Zr/C atomic ratio is 1:1 in the raw Zr-G mixture, Zr will ultimately be excessive for the formation of ZrC. Increasing the Zr/C ratio in the mixture is one of the effective methods to introducing more graphite into the melts. It is found that, besides the synthesis of more ZrC particles, the morphology, size and distribution of ZrC can also be remarkably improved with the Zr/C ratio increasing.

In this work, the aluminum conductors containing different trace elements have been prepared. And the influence of the trace elements on the microstructures and properties of aluminum conductor has been examined. It is found that the higher content of Fe and Si as well as higher Fe/Si ratio is helpful for improving the hardness, but it is harmful for the electrical conductivity. The boron addition can make Ti and V transform into borides. Ti element tend to react with B to form TiB2 particles, but when there is both Ti and V, (V, Ti)B2 prefers to be formed. Although borides can be the nucleate substrate for a-Al and refine the grain size of the as-cast Al, the grain size of aluminum conductor containing higher Ti and V is not always the smaller, which is due to the agglomeration and settling down of the borides during the holding. Keywords: Aluminum conductors, Trace elements, Electrical conductivity, Hardness

In this work, SiC particles with different sizes have been effectively introduced into Cu melts based on Ti–SiC system, and the types and microstructures of Ti–SiC reaction products are varied with the SiC size change. With small size, SiC can absolutely react with Ti to form TiC particles and Ti5Si3, while large SiC cannot react completely. In the latter case, besides TiC particles and Ti5Si3, the core–shell particles, including Cu@TiC, SiC@TiC, carbon derived from silicon carbide (SiC-CDC) @TiC and ternary layers SiC@SiC-CDC@TiC, can be formed. It is considered that the reaction of Ti–SiC in Cu melts not only has two alternative reaction path ways but also has two stages. The first stage is the instantaneous reaction between Ti and SiC during the addition and start with TiC formation, which realizes the reactive wetting of SiC by Cu melts and leads to the eventual formation of TiC or SiC@TiC core–shell particles. The second stage is the reaction between Ti and residual SiC in Cu melts which starts with the transformation of SiC into SiC-CDC and leads to the eventual formation of Cu@TiC, SiC-CDC@TiC or SiC@SiC-CDC@TiC core–shell particles.

The distribution of stress and strain between adjacent particles in particulate reinforced metal matrix composites was investigated using cohesive zone models. It is found that the strain of the composite is concentrated in the matrix, and there is a region with higher strain along the loading path, which can promote the formation of a void near the particles pole. The stress and strain in matrix near the particles gradually decrease with the increase of the distance between particles. And it is calculated that there is a critical distance within which the stress and strain fields of the neighboring particles can influence with each other. This critical distance increases with the increase of particle size. It is also found that the angle between the tensile direction and the center line of particles plays an important role in the stress and strain distribution. The model with the angle of 0° has the greatest influence on the distribution of stress and strain in the matrix, while the model with the angle of 45° has the least influence on the distribution of stress and strain in the matrix.

In this work, the degree of stress concentration in particulate reinforced metal matrix composites (PRMMCs) is investigated by finite element analysis method. It is found that, besides the particle morphology, particle size and its distribution can also remarkably influence the stress distribution of the composites, especially that in the matrix. The results indicate that the particles will bear more loads in the composites reinforced by irregular particles than those reinforced by spherical ones and the stress concentration in the matrix near the particle angularity is more serious. As to the particle size, the degree of stress concentration in the matrix will be reduced with the decrease of the size, while the size distribution has greater influence on that than the size itself. The calculated standard deviation of the stress indicates that the degree of stress concentration in matrix in multi-size particles reinforced models is obviously weaker than that in single-size particles reinforced models. Therefore, changing the size distribution is more effective to reduce the degree of stress concentration in the matrix and can be used to improve the fracture toughness of PRMMCs. It is considered that the results could be helpful for design better performance PRMMCs by controlling the ratio of particles with different sizes.

The Cu-TiC composites with non-stoichiometric TiC have been tried to prepare by melt reaction method in this work. It is confirmed that the TiC with different stoichiometries can be successfully prepared by changing the Ti/C ratio in raw material. When there is much excessive Ti in Cu melts, sub-stoichiometric TiC, such as TiC0.6, will be synthesized. However, the sub-stoichiometric TiCx, is not formed by direct reaction between Ti and graphite. In Cu-TiC composites prepared by melt reaction method, near stoichiometric TiCy prefer to be firstly synthesized by reaction between Ti and graphite. Then near stoichiometric TiCy will further react with Ti in the melts to form sub-stoichiometric TiCx (x

In this work, Cu-TiC composites have been successfully prepared by reaction of soluble Ti and carbon coating TiC. Firstly, the ball milling of graphite and TiC mixtures is used to obtain the carbon coating TiC which has fine size and improved reaction activity. After adding the ball milled carbon coating TiC into Cu–Ti melts, the soluble Ti will easily react with the carbon coating to form TiC. This process will also improve the wettability between Cu melts and TiC core. As a result, besides the TiC prepared by reaction of soluble Ti and carbon coating, the ball milled TiC will also be brought into the melts. Some of these ball-milled TiC particles will go on being coated by the formed TiC from the reaction of Ti and the coating carbon and left behind in the composites. However, most of TiC core will be further reacted with the excessive Ti and be transformed into the newly formed TiC with different stoichiometry. The results indicate that it is a feasible method to synthesize TiC in Cu melts by reaction of soluble Ti and ball-milled carbon coating TiC. Keywords: Composites, carbon coating TiC, Cu alloys, Ball-milling

In this work, the stability and transformation of two kinds of TiC with different stoichiometries in Cu–Si melts are studied, respectively. It is found that the near-stoichiometirc TiC with fewer carbon vacancies (designated as TiCx), such as TiC0.9, is relatively stable in Cu–Si melts, while the non-stoichiometric TiC with more carbon vacancies (designated as TiCy), such as TiC0.6, is unstable. And TiCy tends to transform into TiCx. Meanwhile, some Ti element from TiCy will be dissolved into the melts. It is considered that the addition of Si influences the equilibrium between TiC with different stoichiometries and soluble Ti in Cu melts, which results in the transformation of TiC. According to the results, the stoichiometry of TiC must be controlled when it is used as the reinforced phase in Cu–Si alloys. Keywords: Composites, TiC, Cu–Si alloys, Microstructures

The stress and strain distribution around the reinforced particles in Al/TiC composites is investigated in this work. It is found that there is a region with higher strain in the matrix along the loading path under the tensile load, which can promote the formation of a void near the particles. And the location of the highest strain is not in but away from the interface in the models with the perfect interface. The distance of the location of the strain peak from the interface is relevant to the size of the particles and the ratio between the distance of the location of the strain peak from the interface and the radius of the particle is close to 0.3. The influence of volume fraction and load on distribution of strain and stress around the particles is diverse in different regions of the matrix. The average strain of the matrix region (Matrix A) which is under the particle mapping along the tensile direction tends to increase with the increasing of the volume fraction, so is the load. Besides that, the main strain bearing part in matrix is gradually transferred from Matrix A to the other region near the particles when the interface becomes weaker. And the location of strain peak along the loading path will be much closer to the interface with the decrease of the interface strength.

In this work, in situ TiC particles reinforced Cu matrix composites were fabricated by melting-casting method. The synthesized mechanism of TiC in Cu melts and microstructures of prepared composites were investigated. It is found that TiC are synthesized in Cu–Ti melts by the reaction of graphite and soluble Ti. After adding graphite into Cu–Ti melts, Ti will firstly react with graphite on the surface to form small TiC nucleus and then the TiC will further grow by the continuous diffusion of Ti. During the formation process, most of the prepared TiC particles will be separated with graphite and incorporated into the melts. Once TiC particles into melts, they will hardly grow further due to the lack of C element in Cu melts. Since TiC particles will be separated with graphite at different growth stage, TiC with a wide range of size and various morphologies can be obtained. At the same time, some TiC particle may never be broken with the graphite and will grow into different agglomerations or integrate into larger plate-like TiC.

Introduction of carbon into metal melts, especially those with limited carbon solubility, for composites preparation by casting method is very difficult due to the poor wettability between carbon and melts. In this work, carbon in various forms has been effectively introduced into Cu melts and TiC has been in-situ synthesized based on Ti–C–Si system, and its mechanism has been investigated. It is found that, after the addition of TiC mixture into Cu melts, carbon can be wetted by Cu through TiC reaction. However, because of the TiC interconnected network formation, the mixture tends to become the block which cannot be dispersed into the melts. Using Ti–C–Si instead of TiC system is an effective way to wet carbon and in-situ synthesize TiC while prevent the formation of TiC network. It is considered that Si changes the reaction pathway of the system. The Ti5Si3 which is prior to be formed in Ti–C–Si system can prevent the following formed TiC from interconnecting and its subsequently dissolution due to TiC formation can further help the products to distribute more uniformly in Cu melts. Keywords: Metal-matrix composites, Carbon, Cu alloys, Casting

In this work, ZrB2 particles have been in-situ synthesized in the Cu melts using Cu-B master alloy as the starting material. It is found that the non-uniformly distributed ZrB2 particles in the prepared as-cast composites tend to aggregate into agglomerations and this trend is more pronounced with the ZrB2 content increasing.In addition, the size of hexagonal plate-like ZrB2 whose diameter ranges from less than 1 μm to more than 10 μm accompanied by the thickness ranging from about 0.5 μm to 2 μm shows an uneven appearance. The increasing ZrB2 contents can give rise to the higher hardness values of 160–205% of the original pure Cu. However, the electrical conductivity which is about 52.9% IACS to 76.8% IACS will be severely affected. Besides, the decreased friction coefficient from 0.6 to 0.4 and the gradually declining lost mass of the samples indicates the better wear resistance property with increasing ZrB2 contents. Keywords: Composites, ZrB2, Microstructures, Cu melts, Morphologies

The excellent grain refinement effectiveness and good electrical conductivity is difficult to be simultaneously obtained due to the influence of both the grain boundary and excessive Ti in the usually used refiners, such as Al-5Ti-1B master alloy. In order to solve this problem, in this work, Al-Ti-B mater alloys with less or without excessive Ti but with high refinement potency TiB2 have been tried to prepare, and the grain refinement effectiveness of them and its influence on the electrical conductivity of the refined Al have been investigated. It is found that the grain refinement effectiveness of the Al-Ti-B master alloys prepared using the Al-5Ti-1B and Al-B master alloys as the starting materials at 720 °C is much better than those prepared using the Al-10Ti and Al-B master alloys, which is because that the high refinement potency TiB2 with TiAl3 2DC in the Al-5Ti-B can be kept in the prepared master alloys. As a result, using the prepared Al-Ti-B master alloys as the refiners, both the good grain refinement effectiveness and improved electrical conductivity of Al can be realized. Keywords: Grain refinement, Electrical conductivity, Al, Master alloys

In this work, Cu-TiC composites have been successfully prepared by adding Ti and carbon black powder mixture into Cu melts. The microstructure evolution of the composites with the change of Ti/C ratio and its mechanism has been investigated. It is found that when Ti and carbon black powder mixture is added into Cu melts, Ti will react with carbon black to form TiC, which makes the carbon black be wetted. Then the unreacted Ti will be dissolved and further reacted with the wetted carbon black to form more TiC. However, the formed TiC particles are dispersed difficultly and tend to aggregate into agglomerations. Increasing the Ti/C ratio is an effective method to improve the distribution of TiC. It is found that, comparing with the Cu-4Ti-1C, non-stoichiometric TiCx with smaller x which has the better wettability with Cu melts can be synthesized in the Cu-5Ti-1C and Cu-6.67Ti-1C. As a result, TiC particles are more uniformly distributed in the prepared Cu-Ti-C composites with higher Ti/C ratio. Keywords: Metal matrix composites, Carbides, Microstructure, Cu alloys

International audience; First-principles calculations have been performed to study hydrogen incorporation into the MAX phase Ti3AlC2. It is found that H atoms are thermally favorable to be incorporated into the interstitial sites of Ti-Al layers in stoichiometric Ti3AlC2. Only when C vacancies exist, H atoms can be incorporated into Ti-C layers. In Ti-Al layers, the hexagonal interstitial site (Ihexa) consisting of three Al atoms and two Ti atoms, the tetrahedral interstitial site (Itetr-2) consisting of one Al atom and three Ti atoms, and the octahedral interstitial site (Ioct-3) consisting of three Ti atoms and three Al atoms are all possible sites for H incorporation. Among them, Itetr-2 is the most stable one, followed by Ioct-3, and the last one is Ihexa sites. The linear synchronous transit optimization study shows that the diffusion of H in the Ti-Al layers is feasible which could be helpful for the continuous insertion and the following extraction of H in Ti3AlC2.

The stress distribution in microregion close to the shell of core-shell particle reinforced metal matrix composites is studied by the finite element analysis in this work. It is found that the stress distribution of the composites is dependent on the elastic modulus of both shell and core phases as well as the difference between them. If the elastic modulus of the formed shell is larger than that of the core, the load borne by the shell will be increased, otherwise, the load borne by the matrix and the core will be increased. The formation of shell and its nature can also significantly change the position of the maximum stress on the reinforced particles but have little influence on that in the matrix. When the elastic modulus of the shell is smaller than that of the core, the maximum stress position in the core-shell particle will be at the core and close to the interface between core and shell, otherwise, it is shift to the shell and also close to the interface between core and shell. The degree of the stress concentration in Al matrix becomes more serious after the formation of the shell in the reinforced particles, while that in shell of core-shell particles mainly depends on the modulus difference of core and shell.

In this work, the initial stage of Ti–SiC reaction in the Cu melts has been verified. It is proved that there are two alternative Ti–SiC reaction path ways which will make SiC firstly transform into TiC or carbon derived from silicon carbide (CDC), respectively. Although TiC is more thermally favorable than both SiC and CDC in Cu melts, which means that TiC should be the ultimate C-containing product of Ti–SiC reaction of both reaction path ways, the presence of TiC shell can prevent the SiC or CDC from further transformation. As a result, SiC@TiC or CDC@TiC core-shell particles can be obtained. The synthesized CDC is composed of polycrystalline graphite and a hexagonal carbon (PDF 26-1083), and with the former one dominated.

In situ TiC particles reinforced Cu matrix composites have been prepared by melt reaction method in this work. It is found that, during the preparation, not all the added graphite can react with soluble Ti to form TiC due to its poor wetting and very limited solubility in Cu melt, which will lead to residue of both graphite and Ti in the composites. Because the residual Ti will dissolve in the Cu matrix, it will seriously degrade the conductivity of the composites. In order to improve the electrical conductivity of Cu Ti C composites prepared by the melt reaction method, boron can be added to the composites to consume the residual Ti by the formation of TiB2. It is confirmed that the addition of B will make the distribution of the reinforced particles more homogeneous, most of all, can remarkably improve both the hardness and electrical conductivity of the Cu Ti C composites.