Michail A. Korchagin

Microstructure changes in TiB2-Cu nanocomposite under sintering

Stability and growth of nanoparticulate reinforcements in metal matrix composites during heating are widely studied for dispersion-strengthened alloys, which contain several volume percent of reinforcing phase. When high volume content of nanoparticles distributed within a matrix is concerned results of particles aggregation and growth as well as crystallization mechanisms are not so evident. In this work microstructural evolution under sintering in metal-matrix composite TiB2-Cu with high volume content (up to 57%) of titanium diboride nanoparticles 30-50 nm in size was investigated. The nanocomposite powders were produced through synthetic method combining preliminary mechanical treatment of initial powder mixtures in high-energy ball mill, self-propagating exothermic reaction and subsequent mechanical treatment of the product. We focused on microstructure changes in TiB2-Cu nanocomposite consolidated by Spark-Plasma Sintering and conventional sintering and showed that in the former case fine-grained skeleton of titanium diboride is formed with connectivity between particles well established. In the latter case behavior of nanoparticles is surprising: at low temperatures fiber-like structures are formed while increasing temperature causes appearance of faceted crystals. These unusual results allow us to propose the direct involvement of nanoparticles in the processes of crystallization by moving as a whole in the matrix.

Ti3SiC2-Cu composites by mechanical milling and spark plasma sintering: Possible microstructure formation scenarios

We present several possible microstructure development scenarios in Ti3SiC2-Cu composites during mechanical milling and Spark Plasma Sintering (SPS). We have studied the effect of in situ consolidation during milling of Ti3SiC2 and Cu powders and melting of the Cu matrix during the SPS on the hardness and electrical conductivity of the sintered materials. Under low-energy milling, (3–5) vol.%Ti3SiC2-Cu composite particles of platelet morphology formed, which could be easily SPS-ed to 92–95% relative density. Under high-energy milling, millimeter-scale (3–5) vol.%Ti3SiC2-Cu granules formed as a result of in situ consolidation and presented a challenge to be sintered into a bulk fully dense sample; the corresponding SPS-ed compacts demonstrated a finer-grained Cu matrix and more significant levels of hardening compared to composites of the same composition processed by low-energy milling. The 3 vol.% Ti3SiC2-Cu in situ consolidated and Spark Plasma Sintered granules showed an extremely high hardness of 227 HV. High electrical conductivity of the Ti3SiC2-Cu composites sintered from the granules was an indication of efficient sintering of the granules to each other. Partial melting of the Cu matrix, if induced during the SPS, compromised the phase stability and uniformity of the microstructure of the Ti3SiC2-Cu composites and thus it is not to be suggested as a pathway to enhanced densification in this system.

Detonation Spraying of Ti–Al Intermetallics: Phase and Microstructure Development of the Coatings

The goal of this work was to study the phase and microstructure changes involved in the process of coating formation by detonation spraying of Ti3Al, TiAl, and TiAl3 intermetallics. The O2/C2H2 ratio was varied between 1.1 and 2.0, and the explosive charge was 30–40% of the barrel volume. In most experiments air was used as a carrier gas; selected experiments were performed with argon. We found that depending on the spraying parameters, TiAl3 essentially retains in the coatings or partially decomposes forming TiAl and Ti3Al as minor phases. Detonation sprayed Ti3Al reacts with nitrogen and oxygen partially transforming into titanium nitrides TiN/Ti2N and titanium oxynitrides TiNxOy. TiAl partially decomposes forming Ti3Al, which further reacts with oxygen and nitrogen as the particle temperature and the content of oxygen in the explosive mixture increase. The in situ formed titanium nitrides and oxynitrides show a reinforcing effect increasing the hardness of the coatings.

Control of interfacial interaction during detonation spraying of Ti 3 SiC 2 -Cu composites

Changes in the phase composition of the 20 vol % Ti3SiC2-Cu composite during detonation spraying as well as corresponding microstructure formation processes in the sprayed coatings have been studied. It was demonstrated that when the amount of the explosive acetylene+oxygen mixture is kept constant (under the constant filled volume fraction of the barrel of the detonation gun of a CCDS2000 facility), the phase composition of the coating depends on the composition of the explosive mixture. The Ti3SiC2-Cu system is prone to interfacial interaction; therefore, in order to produce a dense coating preserving the phase composition, oxygen-depleted explosive mixtures should be used and small filled fractions of the barrel. As the temperature of the sprayed particles increases with increasing oxygen content in the explosive mixture, titanium silicon carbide reacts with copper, which results in the formation of the titanium carbide phase and dissolution of the de-intercalated silicon in the copper matrix leading to the formation of TiCx-Cu〈Si〉 coatings.

Crystallization of Ti33Cu67 metallic glass under high-current density electrical pulses

AbstractWe have studied the phase and structure evolution of the Ti33Cu67 amorphous alloy subjected to electrical pulses of high current density. By varying the pulse parameters, different stages of crystallization could be observed in the samples. Partial polymorphic nanocrystallization resulting in the formation of 5- to 8-nm crystallites of the TiCu2 intermetallic in the residual amorphous matrix occurred when the maximum current density reached 9.7·108 A m-2 and the pulse duration was 140 μs, though the calculated temperature increase due to Joule heating was not enough to reach the crystallization temperature of the alloy. Samples subjected to higher current densities and higher values of the evolved Joule heat per unit mass fully crystallized and contained the Ti2Cu3 and TiCu3 phases. A common feature of the crystallized ribbons was their non-uniform microstructure with regions that experienced local melting and rapid solidification. PACS: 81; 81.05.Bx; 81.05.Kf.

TiB 2 -Cu Interpenetrating Phase Composites Produced by Spark-plasma Sintering

Interpenetrating phase composites of -Cu system were produced via Spark-Plasma Sintering (SPS) oi nanocomposite powders. Under simultaneous action of pressure, temperature and electric current titanium diboride nanoparticles distributed in copper matrix move, agglomerate and form a fine-grained skeleton. Increasing SPS-temperature and he]ding time promote densification due to local melting of copper matrix When copper melting is avoided the compacts contain 17-20% porosity but titanium diboride skeleton is still formed representing the feature of SPS . High degree of densification and formation of titanium diboride network result in increased hardness of high-temperature SPS-compacts.

In situ formation of metal-ceramic composite coatings by detonation spraying of titanium

We have studied the phase formation and concomitant microstructure evolution in the coatings deposited by detonation spraying of a titanium powder. By varying the O2/C2H2 ratio, detonation products of different compositions were produced, which determined the course of the phase evolution. The carrier gas also played an important role in the phase formation of the coatings. It was found that the phase composition of the coatings forms as a result of nitridation, oxidation and carbon capture depending on the O2/C2H2 ratio and the nature of the carrier gas. The in situ formed metal-ceramic coatings possess unique microstructures and present interesting objects for studying their mechanical behavior. Detonation spraying of titanium can be suggested as a convenient way of forming metal-ceramic coatings with a controlled phase composition resulting from the reactions of titanium with the spraying environment of variable chemistry.

Preparation and electrical erosion resistance of TiB2/Cu nanocomposites

A procedure is described for producing nanocomposite TiB2/Cu powders containing up to 57 vol % TiB2. Using shock compression of composite powders, we have prepared electrode materials offering enhanced electrical erosion resistance at high arc discharge currents. The effect of titanium diboride nanoparticles embedded in the copper matrix on the erosion behavior of the nanocomposites is examined. The nanoparticles are shown to suppress the copper droplet entrainment during the service of the electrode. TiB2/Cu nanocomposite electrodes containing more than 10 vol % TiB2 retain their shape and dimensions in the course of electrical erosion tests and offer enhanced service life.

Shock-Wave Synthesis of Titanium Diboride in Copper Matrix and Compaction of TiB2-Cu Nanocomposites

TiB2-Cu composites in a nanostructured state are candidates for high-strength conductive and erosion-resistant materials. In this work, we studied formation of nanostructured TiB2-Cu composites under shock wave conditions. We investigated the influence of preliminary mechanical activation (MA) of Ti-B-Cu powder mixtures on the peculiarities of the reaction between Ti and B under shock wave. In the MA-ed mixture the reaction proceeded completely while in the nonactivated mixture the reagents remained along with the product – titanium diboride. The size of titanium diboride particles in the central part of the compact was 100-300 nm. This research shows that shock wave synthesis in mechanically activated powder mixtures with simultaneous compaction of the composite is a promising way to materials with submicron and nanostructures.