Vyacheslav I. Mali

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.

Formation and structure of vortex zones arising upon explosion welding of carbon steels

Presented are the results of investigation of vortex zones arising upon explosion welding of thin plates of steel 20. Specific features of the structure of the vortices and zones of the deformed material adjacent to them have been revealed by methods of structure analysis. It has been shown that in the process of explosive loading the central regions of the vortices characterized by an enhanced carbon content were in the molten state. The microhardness in the region of vortex zones reaches 5700 MPa. The character of the arrangement of ferrite grains and martensite microvolumes in peripheral regions of vortices is caused by intense rotation of the material. The intense intermixing of materials in different states of aggregation in vortex zones is one of the factors responsible for the formation of cavities, whose volume exceeds the volume shrinkage occurring upon casting of carbon steels. It has been established that traces of vortex zones are retained even after one-hour annealing of welded packets at 800°C.

Nucleation and growth of titanium aluminide in an explosion-welded laminate composite

Processes of nucleation and growth of titanium aluminide in a 23-layer aluminum-titanium composite produced by explosion welding have been studied. In the vortex zones of seven upper welds, microvolumes of melted metal whose microhardness is ∼5500 MPa have been revealed, which corresponds to the microhardness of the intermetallic compound Al3Ti. No formation of titanium aluminide in welded junctions that were not subjected to additional heat treatment has been revealed by X-ray diffraction. The holding of the composites at 630°C is accompanied by the formation of interlayers of intermetallic compounds of the Al3Ti type. Intermetallic compounds of two morphological types are formed in the welds. In the regions of vortex zones, compact precipitates of Al3Ti are formed; in the other regions of the welds, intermetallic compounds in the form of a film are precipitated. The intermetallic compounds of the first type grow more rapidly and in final account absorb the precipitates of the film type. The activation of diffusion in the upper junctions that occurs upon heating of the welded composites is favored by the nonequilibrium state of the material caused by the strain hardening of the initial samples. In the welds located deeper than the 13th layer, no signs of the formation of compact intermetallic compounds have been revealed upon the annealing for 5 h and less.

Influence of the explosively welded composites structure on the diffusion processes occurring during annealing

Currently layered composites with intermetallic interlayers are promising materials for many industries. Materials of this type involve composites consisting of titanium and titanium aluminides layers. One of the methods of such composites fabrication is explosive welding and annealing. In this paper the structure and properties of explosively welded multilayered composites is described. Structural studies were carried out by the methods of optical metallography, scanning and transmission electron microscopy. The properties of local zones of material were estimated by the measuring of a microhardness level. The explosive welding process accompanied by the structural changes of material adjacent to “Al-Ti” interfaces. The structural investigation revealed the formation of several areas: vortex zones, which are characterized by aluminum and titanium mixing, and severely deformed areas. These structural features have the strong influence on the diffusion processes occurring during annealing of composites. The most intensive diffusion was observed in the welded joints where the most significant structural changes occurred. It was identified that Al3Ti is only one phase which forms during heat treatment of “Ti-Al” composites. The main conditions of “Ti-Al3Ti” composites fabrication are the correct choice of aluminum and titanium plates thickness and optimal conditions of heat treatment.

Structure and Microhardness of Cu-Ta Joints Produced by Explosive Welding

The structure and microhardness of Cu-Ta joints produced by explosive welding were studied. It was found that, during explosive welding, an intermediate layer 20⋯40 μm thick with a finely dispersed heterophase structure, formed between the welded copper and tantalum plates. The structure of the layer was studied by scanning and transmission electron microscopy. Microvolumes with tantalum particles distributed in a copper matrix and microvolumes of copper particles in a tantalum matrix were detected. The tantalum particles in copper have a size of 5⋯500 nm, with a predominance of 5⋯50 nm particles. A mechanism for the formation of the finely dispersed heterophase structure in explosive welding is proposed. The microhardness of interlayers with the heterophase structure reaches 280 HV, which far exceeds the microhardness of copper (~130 HV) and tantalum (~160 HV). Many twins of deformation origin were found in the structure of the copper plate. The effect of heating temperature in the range from 100 to 900°C on the microhardness of copper, tantalum, and the Cu-Ta welded joint was studied. Upon heating to 900°C, the microhardness of the intermediate layer decreases from 280 to 150 HV. The reduction in the strength properties of the weld material is mainly due to structural transformations in copper.

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.

Inter-particle interactions in partially densified compacts of electrically conductive materials during spark plasma sintering

Spark Plasma Sintering (SPS) is normally carried out to obtain consolidated materials of low residual porosity. SPS can also be used for the preparation of partially densified (porous) materials. Partial densification is achieved during SPS by using relatively low sintering temperatures or pressureless conditions. For a pressureless assembly, short punches are used; in addition, sintering without the upper punch is possible. It was shown that the number of contacts between the nickel and synthetic diamond particles was higher in the compacts partially sintered by SPS than in those cold-pressed and vacuum-annealed at the same temperature. During pressureless SPS, conditions for non-uniform current distribution can be realized causing non-uniformities in the microstructure of the compacts. It was shown that SPS of metals without the upper punch produces porous gradient structures. Electric current passing through the porous compacts and interfaces between the compacts and the punches/foils under pressureless conditions induces specific local effects evidenced by the morphology evolution of the particles and microstructure of the sintered material in those areas of the compacts.

Explosive welding of titanium with stainless steel using bronze — Tantalum as interlayer

Explosive welding was used to form reliable joining between commercially pure titanium and stainless steel. To prevent cracking at the interface induced by brittle intermetallic bronze - tantalum interlayer was used. Microstructural characterization of produced composite was analyzed using optical and scanning electron microscopy as well as energy dispersive analysis. Determination of structure has revealed no micro defects such as cracks in the composite. Intermediate layer of 5 μm thick at the bronze - tantalum interface was formed. Structure of intermediate layer corresponds to highly dispersed mixture of tantalum and copper alloy particles. Ultimate strength of 4 layered composite is 1000 MPa that exceeds of ultimate strength of composite produced without interlayer. Analysis of fractograph indicates a high adhesion level between interactive materials.

Formation of Metal-Intermetallic Laminate Composites by Spark Plasma Sintering of Metal Plates and Powder Work Pieces

Laminate composites with an intermetallic component are some of the most prospective constructional and functional materials. The basic formation method of such materials consists in heating a stack composed of metallic plates reacting at elevated temperatures to form intermetallic phases. The temperature of the process is usually approximately equal to a melting point of a more easily fusible component. In this study, an alternative technology of producing a titanium – titanium aluminide composite with a laminate structure is suggested. It consists in combining metallic (titanium and aluminum) powder mixtures pre-sintered at 400 оС with titanium plates, alternate stacking of these components and subsequent spark plasma sintering (SPS) of the fabricated workpieces. Applying this technology allowed for the fabrication of metal-intermetallic laminate (MIL) materials with an inhomogeneous structure of intermetallic interlayers. The phases revealed in the composite by X-Ray diffraction (XRD) were α-Ti, Al, Al3Ti and Al2Ti. Moreover, the results of the energy-dispersive analysis gave the evidence of the formation of Ti-enriched phases in powder layers after SPS. A small number of voids were observed between the structural components of the intermetallic layers. Voids were also detected at “metal-intermetallic” interfaces; however, the quality of connection between different layers in the composite was very high. The microhardness of an intermetallic layer formed in the composite was comparable to the microhardness of the Al3Ti compound. The microhardness of titanium was equal to 1600 MPa.

Microstructure and mechanical properties of copper-tantalum joints produced by explosive welding

Microstructure and mechanical properties of copper and tantalum joints produced by explosive welding technology were characterized. It was established by structural analysis technique that mixing of copper and tantalum at the interface occurs due to a nonequilibrium process. The mixing zone consists of tantalum globular particles which sizes vary from 8 to 40 nm. In the close vicinity to the interface copper grains contains a lot of microtwins bundles. Their formation plays a crucial role both in the microstructure evolution and fragmentation process of copper grains. Microhardness of copper-tantalum mixing zone is 2800 MPa which is 2 times higher than that of initial materials. The tensile test has shown the significant growth of composite yield strength in comparison with that of initial materials.