Investigation Of Phase Transformation Kinetics And Microstructure Evolution In Ti60 Alloy

Ti60 titanium alloy was developed by our country for the application at the temperature up to 600oC with independent intellectual property. The Ti60 alloy needs to be processed by considerable heat treatments to achieve the comprehensively matched performances. In order to optimize the heat treatment processes, and investigate the stability of the microstructure under the service condition, it requires a systematic investigation of the microstructure evolutions during the heat treatment process and long term service period, and then discusses the mechanisms of phase transformation. Therefore, the microstructure evolutions during the solution and aging process, continuous cooling, isothermal process and long term exposure are investigated through the dilatometric test associated by scanning electron microscopy(SEM) and transmission electron mcroscopy(TEM). The nucleation and growth mechanisms of α phase are discussed, while the decomposition mechanisms of α’ phase is also clarified. In addition, the precipitation behavior of the ordered α2 phase and the silicide is analyzed. The further effect of the thermal exposure on the mechanical properties is studied. The main contents and conclusions include:(1) The volume fraction of α phase keeps linear relationship with the solution temperature. When the solution temperature increases, the concentration of Al in the α phase also increases. During the solution treatment, the αâ†'β phase transformation happens. Not only the width of the α plate decreases, but the ends of the long axis also solve to form the short plate, when the solution temperature exceeds 1020oC.(2) When the alloy is cooled down from β phase field in water or air, the βâ†'α’ martensite transformation happens, and the dislocations and stacking faults form inside the α’ martensite. In addition, the αâ†'α2 ordering transformation happens during the furnace cooling. The massive α’ martensite has been found in the α+β solution treated microstructures. It grows around and keeps the same orientation with the primary α phase, but its concentration of solute atoms approximates to the β transformed microstructure, moreover, it is more suitable for the formation of massive α’ phase during the short solution treatments at higher temperature.(3) The phase transformation kinetics during continuous cooling is studied by the dilatometric method. The results indicate: the βâ†'α’ transformation happens when the cooling rate exceeds 50°C?s-1, and the martensite transformation starts at about 900°C(Ms). In the range of 20°C?s-1~5°C?s-1, the βâ†'α transformation happens at about 920°C, and the α phase nucleate along the grain boundary prior to other positions, then the rest of β phase transforms to martensite below the Ms. When the cooling rate decreases to 3°C?s-1, the α phase nucleates inside the grain and along the grain boundary. Until the cooling rate decreases to 1°C?s-1, the α phase just nucleate along the grain boundary. Based on the results mentioned above, the continuous cooling transformation diagram(CCT) is built.(4) The isothermal kinetics of βâ†'α phase transformation is investigated by the dilatometric menthod. The nucleation and growth mechanisms of α phase are described by JMA model. When the alloy is treated at 1030°C, the α phase mainly nucleates and grows along grain boundary, corresponding to the Avrami index n approximating to 1.0. As the temperature decreases to 950°C, most of the α phase nucleates grow from grain boundary to the inside, and the n value increases to 1.31.(5) The α’â†'α+β transformation happens during the isothermal treatment when the initial microstructure is consisted of α’ phase. The α phase nucleates inside the α’ phase accompanying with the precipitation of the small β phase, which results to a refined microstructure. As the isothermal temperature decreases, the size of the α phase decreases. Based on above, the microstructure can be refined by using the decomposition of α’ phase during the isothermal treatment.(6) When the Ti60 alloy is exposed at 650°C, 600°C and 550°C for long terms, the retained β phase diminishes gradually or even disappears, which induces the mergence of secondary α phase. In addition, the αâ†'α2 ordering transformation happens inside the α phase to form dispersed α2 particles with size in nanometer. After the exposure at 550°C, the silicide only nucleates along the boundary of α phase, and grows up to the S2 type ellipsoid particle with about 150 nm long axis. After the exposure at 650°C, the silicide nucleates along the boundary and the defects inside the α phase, including dislocations and stacking faults. The precipitation of such nano-phases leads to the embrittlement of the alloy, and the tensile elongation at room temperature is less than 5%.