Tkachenko S. Regularities of phase and structural transformations in Ti-Si based alloys during quenching and ageing

In this thesis the results of experimental researches and analysis of influence of heat treatment and alloying on phase composition, structure and mechanical properties of Ti-Si system based alloys are presented. It was shown that water quenching of model alloys of the system Ti -Si-X (where X - Al, Zr, Nb, Mo) allowed to get the wide spectrum of supersaturated solid solutions with the different structural-phase state and mechanical properties. Influence of alloying elements on the parameters of crystal lattices, crystalline symmetry and also morphology and substructure of titanium martensite is detected. The conjoint flowing of processes of ordering and change of symmetry of lattice from HCP to orthorhombic symmetry is revealed in the alloys Ti-4Si-4Hf and Ti-4Si-4Zr (at. %). The substantial increase of hardness and strength after quenching of alloys of Ti-Si-X is shown. Also it is established that the module of elasticity of all alloys goes down after tempering. It was identified that the size of grain, mainly, determines plastic properties of quenched alloys, that is why in practical aims more expedient to quench the Ti-Si alloys from dual-phase (?+silicide)-region. For determination of influence of alloying and cooling rate on the structurally-phase state and mechanical properties of alloys of Ti-Si surface quenching is conducted. It was shown that such a treatment allowed to get to get a gradient structure with mechanical properties which fluently change on length of specimen. It is shown that zirconium substantially promotes hardenability and hardening characteristics of Ti-Si alloys. Correlation analysis was conducted between wearproofness of Ti-4,3Si-2,6Zr alloy and its hardness, modulus of elasticity and H3/E2 (Johnson's criteria). It showed that the best correlation is observed between wearproofness and Johnson's criteria. Structural evolution, phase composition and mechanical properties of quenched Ti-Si alloys during ageing in temperature region 500-600 °C were investigated. It was determined that optimal ageing temperature of quenched Ti-Si that is 540 °С. An estimation of activation energy of supersaturated solid solution decay showed that its value U=96-120 kJ/mol is close to activation diffusion energy for binary titanium alloys Ti-Nb, Ti-Mo, Ti-Zr, although it much more greater than its value for iron-carbon alloys.