Estudo da cinética de precipitação do Ti3Ni4 e sua influência nas propriedades mecânicas e funcionais de uma liga Ti-51%Ni

• Ano de defesa: 2020
• Autor(a) principal: Guilherme Simões Pandolfi
• Tipo de documento: Tese
• Tipo de acesso: Acesso aberto
• Idioma: por
• Instituição de defesa: Universidade Federal de Minas Gerais; Brasil; ENG - DEPARTAMENTO DE ENGENHARIA METALÚRGICA; Programa de Pós-Graduação em Engenharia Metalúrgica, Materiais e de Minas; UFMG
• Palavras-chave em Português: NiTi; Ti3Ni4; Envelhecimento; Cinética de precipitação; Fração volumétrica; Comportamento mecânico; Engenharia metalúrgica; Metalurgia física; Metais - Propriedades mecânicas; Precipitação (Química)
• Link de acesso: http://hdl.handle.net/1843/46758

This study analyzed the influence of aging temperature and aging time on Ti3Ni4 precipitation kinetics, besides its impact on the mechanical and functional behavior of a commercial Ti-51.0 at.% Ni alloy. It investigated Ti3Ni4 precipitation kinetics through a proposed numerical parameter based on the phase transformations temperatures, called "M" _"p" ^" global" , which allowed to estimate Ti3Ni4 reacted fraction for each aging condition. The Johnson-Mehl-Avrami-Erofeyev-Kolmogorov model proved to be adequate in describing the Ti3Ni4 precipitation kinetics due to the excellent correlation between the model and the experimental data. The Avrami exponents obtained for the Ti-51.0% Ni alloy indicate that the Ti3Ni4 precipitation is controlled by diffusion, the growth of the precipitate is one-dimensional, and the probable occurrence of saturation of sites for nucleation during the precipitation process. The apparent activation energy estimated for Ti3Ni4 precipitation in the Ti-51% Ni alloy was 108 (± 8) kJ/mol. It estimated Ti3Ni4 volumetric fraction for several aging conditions using the parameter "M" _"p" ^" global" . Considering aging for 1h, the samples aged at 400 and 450°C exhibited the highest values for the martensite apparent yield strength, the lowest stress values for martensite induction, and changed the functional behavior from superelasticity to shape memory effect. These results are associated with the higher volumetric fractions of Ti3Ni4 in these samples compared to the others. Regarding the influence of aging time in samples aged at 350, 400, and 450°C, it was possible to correlate Ti3Ni4 precipitation kinetics with the changes observed in the martensite apparent yield strength, the tension required to induce martensite, the plastic deformation after the 6% deformation cycle, and the functional behavior. The highest levels of precipitation hardening were achieved in samples aged at 350°C, followed by those aged at 400 and 450°C. The stress required for martensite induction exhibited the lowest values in samples aged at 450°C, while those aged at 350°C exhibited the highest values. Aging for up to 2h presented the smallest plastic deformations in samples aged at 350, 400, and 450°C, being conditions that probably provide better resistance to functional fatigue. Concerning the resistance to structural fatigue, the aging conditions 400°C/8h, 400°C/4h, and 450°C/2h promoted the best results. In aged samples, this property depends on the amount of R-phase in the matrix, the level of stress during loading, and the extent of the stage III deformation occasioned.