Titanium as a structural material has unique physical and mechanical properties. However, to increase its performance, titanium alloys are alloyed with various elements, including boron, which forms titanium-matrix composites.
The method of melting metallurgy is rarely used, but it is interesting because it allows you to get high-performance materials in the traditional and therefore not too expensive way.
To increase the strength characteristics of cast composites, hard boride hardening should be combined with solid-soluble titanium matrix hardening.
In order to improve the physical and mechanical properties of the Ti-TiB alloy, a technological scheme for obtaining a directional-crystallized alloy by the method of EPP and BZP was developed. Based on the Ti-B state diagram, the starting materials were taken in quantities corresponding to the eutectic composition: 94.7% Ti and 5.3% TiB2.
The method of micro-X-ray spectral analysis showed that the chemical composition of the inclusions of the extracted darker phase corresponds to the equimolar ratio, which confirms the full course of the reaction of the interaction of Ti with TiB2 and the formation of TiB.
The microstructure of the alloy in the plane of the transverse and longitudinal section is a matrix of Ti with inclusions of diboride phase. The length of the diboride phase in the longitudinal direction is 50–100 times longer than the length of the boride phase in the cross section of the ingot. It is established that the microstructure of the central part of the ingot is characterized by much larger inclusions of the boride phase, both in longitudinal and transverse sections.
Quantitative metallography shows how the number of boride inclusions changes in the plane per 100 μm2. It is established that the number of boride phase inclusions increases from the center to the edge of the casting.
It is established that in the plane of longitudinal section of the crystal with increasing crystallization rate the number of fibers decreases and their size increases. In the central part, the size of the inclusions naturally decreases, and their number increases, which may be due to more intensive mixing of the melt in a thin layer due to induction heating.
To significantly increase the temperature gradient at the crystallization front, the fins were subjected to a 2 mm thick plate of directionally crystallized from the melt and rolled crystal eutectic alloy and heated with a beam 200 μm in diameter. This remelting allowed to increase the temperature gradient by about 2 times compared to the zone induction melting.
It was found that the integral microhardness of Ti-TiB composite practically does not depend on the size of boride inclusions, but is mainly determined by their volume fraction in the composite crystal and the load on the indenter.
It is established that with increasing deformation rate with the same structural and geometrical characteristics of the composite, the strength increases. The stress-strain diagram has almost the same appearance and there are four characteristic areas.
In order to determine the mechanical characteristics of the alloy, in which the fibers are oriented mainly in one direction, the workpiece obtained by the EPP method was subjected to rolling. The microstructure of the alloy remains fibrous, only in contrast to the crystallized, in which the fibers are arranged chaotically, during rolling the fibers are rotated mainly in the rolling direction. It is established that the maximum strength values are 150–200 MPa higher in comparison with the unrolled ones with the smallest boride inclusions. During the strength test, the maximum value of strength increases by 2 times or more compared to pure non-reinforced titanium and reaches 840-910 MPa.
The influence of the nature of the matrix phase and the rate of crystallization on the size and number of fibers during solidification of the melt of the quasi-binary eutectic alloy Ti-TiB is established. The investigated crystals were crystallized in the conditions of crucible-free zone melting with a cooling rate of 103 ° / s and centrifugal-spray powders, the cooling rate of the melt in the production of which was 105 ° / s. In fact, in the physical experiment of spraying melts of eutectic alloys LaB6-TiB2 and Ti-TiB, the similarity of heat transfer processes from the center of the drop to its surface is realized, which allows to establish the predominant influence of matrix phase nature on crystallization processes. Analysis of the microstructure of the powders obtained by centrifugal spraying showed that the structure is a gray matrix, and densely distributed throughout the volume of the powder is much more dispersed, compared with directional crystallized alloy and chaotically oriented inclusions of dark and light gray.
