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ResumoA liga Ti-6Al-4V apresenta propriedades importantes como alta resistência específica, boa resistência à corrosão e à fluência e estabilidade metalúrgica e tem sido bastante utilizada nas indústrias aeronáutica e aeroespacial, em algumas aplicações que requerem resistência em altas temperaturas. Assim, é importante entender seu comportamento em deformação, sob temperaturas elevadas. Um método de se aumentar a resistência de um material é a realização de tratamentos térmicos que alterem sua microestrutura. Visando melhorar a resistência à fluência da liga Ti-6Al-4V foi realizado um tratamento térmico específico para obter uma microestrutura martensítica onde o material foi aquecido a 1.050°C por 30 minutos e resfriado em água até a temperatura ambiente. O objetivo deste trabalho é avaliar o comportamento em fluência da liga Ti-6Al-4V com estrutura equiaxial, e após tratamento térmico, estrutura martensítica na temperatura de 600°C e nas tensões de 125 MPa, 250 MPa e 319 MPa. Os ensaios de fluência foram realizados à carga constante e a liga com estrutura martensítica apresenta maior resistência à fluência com maior tempo de vida em fluência e redução da taxa de fluência estacionária. Palavras-chave: Ti-6Al-4V; Tratamento térmico; Fluência. CREEP BEHAVIOR OF TI-6AL-4V WITH MARTENSITIC AND EQUIAXED STRUCTURES AT 600°C AbstractTi-6Al-4V presents important properties as high specific strength, corrosion and creep resistance and metallurgical stability and it has been used in aerospace and aeronautical industries in some applications that requires high temperatures resistance. For these reasons is important understand Ti-6Al-4V deformation at high temperatures. A method of increasing the resistance of a material is heat treatments which can modify its microstructure. Aiming the improvement of Ti-6Al-4V creep resistance it was performed a specific heat treatment in this alloy to obtain a martensitic microstructure. The material was heat-treated at 1,050°C for 30 minutes and cooled in water until room temperature. The aim of this work is to evaluate Ti-6Al-4V creep behavior with equiaxed and martensitic microstructure at 600°C and stress conditions of 125 MPa, 250 MPa e 319 MPa at constant load. The alloy with martensitic structure showed higher creep resistance with a longer time in creep and lower steady-state creep rate.

Ti-6Al-4V alloy has been considered in applications of aeronautical and aerospace industries, due to its properties such as high specific resistance, good creep resistance and metallurgical stability. However, its use in applications for high temperatures is restricted due to its great affinity with the oxygen, which results in the formation of oxide layers and limits its mechanical resistance at these conditions. Thus, specific treatments have been employed in the material to work as surface barriers to avoid the oxygen diffusion in the alloy under high temperature conditions. One surface treatment that can be used is laser nitriding. In the present work, the surface of Ti-6Al-4V alloy with Widmanstätten microstructure was nitrided by applying Nd:YAG laser focal with 0.6 mm diameter, at laser power of 700, 750 and 800 W, process speed of 100 mm/s and 20 L/min of N2 flow. Creep tests were performed at constant load at 600 °C and 125 MPa, to verify the influence of treatment on the Ti-6Al-4V alloy. Results have indicated a lower stationary creep rate for the titanium alloy with Widmanstätten laser-nitrided structure when compared to the non-nitriding material. Besides that, the surface hardness increased from 368 HV of base material to 1000 HV after laser nitriding.

Titanium and its alloys are largely used for many industrial applications due to their high mechanical and corrosion resistance and low specific mass. Ti-6Al-4V alloy is the most used in aerospace industry and it is applied for the manufacturing of aircraft blades and steam turbines. This alloy has high affinity with oxygen which constrains its application at high temperatures due to creep resistance reduction. Methods to increase creep resistance of Ti-6Al-4V alloys includes the use of metallic coatings and its combination with ceramic thermal barrier coating deposition on the material surface. The aim of this work is to compare the creep behavior of Ti-6Al-4V alloy in three different conditions: uncoated, metallic coated, metallic + ceramic coated specimens. The metallic coating layer (CoNiCrAlY) and the ceramic coating (ZrO2+ 8wt%Y2O3) are both applied by plasma spray deposition technique. The specimens were submitted to constant load creep tests at 600oC and stress conditions of 125, 250 and 319MPa. Specimens of metallic + ceramic coating have presented higher creep resistance, longer lifetime and lower stationary creep rate when compared to uncoated and metallic coated specimens.

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