The dissertation is devoted to the study of directionally crystallized alloys of the B4C-NbB2-SiC and B4C-TaB2-SiC systems, in particular to the study of their structure, phase composition, physical-mechanical and high-temperature properties, as well as regularities of structure formation of quasi-triple eutectic alloys.
The author analyzes the properties of studied systems' components, considers the methods of obtaining directionally crystallized eutectic alloys, reviews double eutectic systems such as B4C-MeB2, SiC-B4C, and SiC-MeB2 and ternary eutectic systems B4C-MedB2-SiCd. Features of structure formation in eutectic systems were considered, mechanical properties of directionally crystallized alloys were analyzed.
The eutectic composition of alloys of the B4C-NbB2-SiC and B4C-TaB2-SiC systems was determined by metallographic analysis: B4C-(15-20)NbB2-(33-40)SiC and B4C-(8-12)TaB2-(38-42)SiC (mol.%), respectively.
Ternary eutectic composites B4C-NbB2-SiC and B4C-TaB2-SiC with a uniform fine structure along the entire sample were experimentally obtained.
It is established that the eutectic structure of the B4C-NbB2-SiC system contains three phases: B4C, NbB2, SiC. The ternary eutectic (B4C-15NbB2-35SiC mol.%) has a lamellar structure in which boron carbide acts as a matrix evenly distributed throughout the volume inclusions of silicon carbide and niobium diboride. For composites in the transverse direction, there is a directed texture of phases of the NbB2 in the direction (100), SiC in the direction (111), and B4C in the direction (104). The most directional phase is the SiC phase in the direction (111), both in the longitudinal and in the transverse directions, for which the calculated Lotgering factor is in the range of 0.8−0.9. The structural components in the ternary eutectic have the following sizes: B4C − 4−6 μm, NbB2 − 1−3 μm, SiC − 2−3 μm.
The following mechanical properties of the eutectic composite B4C-15NbB2-35SiC were determined: Vickers hardness (35.8 GPa), fracture toughness under load 9.8 N (6.4 MPa∙m1/2), Young's modulus by the impulse method (462 GPa), electrical conductivity (2.85−5.36×104 Cm/m); the сoefficient of thermal expansion in the range of 22−1600 °С, flexural strength at the room temperature (220 MPa) and 1600 °C (395 MPa) were investigated.
The structure of the ternary eutectic composite B4C-8TaB2-40SiC (mol.%) is a uniform ternary eutectic system B4C-TaB2-SiC of a lamellar type throughout the volume of the sample, in which boron carbide is the matrix, and SiC and TaB2 act as reinforcing phases. In general, the systems B4C-TaB2-SiC and B4C-NbB2-SiC have a similar eutectic structure. The X-ray phase analysis showed the presence of only the following phases: silicon carbide, boron carbide, and tantalum diboride; no other phases were detected. The correlation between the crystallization rate and the linear parameter of the eutectic structure was established.
The following mechanical properties of the eutectic composite B4C-8TaB2-40SiC were determined: Vickers hardness (33−34 GPa), fracture toughness under a load of 9.8 N (3.9 MPa∙m1/2), the coefficient of thermal expansion in the range of 22−1600 °С was investigated.
The influence of crystallization rate on the structure and properties of the obtained composites has been investigated. Increasing the crystallization rate leads to a change in the morphology of inclusions from the plate-like one to more lamellar, as well as to a natural size reduction of the structural components. Experimental studies of micromechanical properties have shown that Vickers hardness and fracture toughness increase with increasing crystallization rate in both the longitudinal and transverse to the growth directions. The increase in hardness with increasing the rate of crystallization of the directionally crystallized alloys B4C-NbB2-SiC and B4C-TaB2-SiC occurs similarly to the Hall−Patch rule.
The analysis of crack propagation after indentation showed that the crack most easily passes through the areas of the matrix phase of boron carbide or silicon carbide inclusions.However, the presence of silicon carbide and diborides inclusions in the structure leads either to a change in the direction of motion, ie deviation of the crack, or to its cessation in general, which, in turn, increases the fracture energy and, consequently, fracture toughness of the composite.
Peculiarities of structure formation were analyzed and the mechanism of growth of the ternary four-component eutectic in the B4C-NbB2-SiC system was established. According to this mechanism, crystallization of ternary eutectic (В4С-NbB2-SiC) occurs as continuous compatible growth of dendritic phases, while the two-phase structural component (SiC-NbB2) increases in the cooperative mode, and the third phase В4С grows synchronously in the autonomous mode.
A spatial model of an eutectic cell for the В4С-NbB2-SiC system was constructed.
