| 其他摘要 | The phase transformations of Ti-5Al-5Mo-5V-1Cr-1Fe(TC-18) have been studied during isothermal holding and continuous cooling after β-solution treatment. For this purpose, dilatometer measurements and metallographical examinations have been carried out, and the temperature time transformation diagram and continuous cooling transformation diagram of TC18 alloy have been plotted.
The Ti-5Al-5Mo-5V-1Cr-1Fe(TC18) alloy is a typical metastable α+β alloy, which is designed to provide a wide variety of mechanical properties and microstructures, depending on thermomechanical processing. The dilatometric analysis of TC18 titanium alloy was tested on Gleebe-3800, according to which, the transformation of metastable β phase was studied, the temperature time transformation diagram and continuous cooling transformation diagram of TC18 alloy have also been plotted. X-ray is used to analyze the phase composition. SEM and TEM techniques are applied to observe the microstructures and transformation characters.
In the study of TTT diagram, the results show that single metastable β phase is obtained after quenching from β phase region. But the metastable β phase is quite unstable under the β transus. The temperature of α phase precipitation from β matrix consists of three parts. At high holding temperature (from 500℃ to the β transus), α phase directly precipitates form β matrix as lamellar structure. At lower aging temperatures (from 350℃ to 450℃), metastable β transform into ω phase firstly, and then fine α precipitates from ω phase. Below 300℃, no α phase was observed even after isothermal holding for 100h. The percentage of α phase firstly increases with aging temperature dropping, and reaches 70% at 550℃, and then decreases. Microstructure does not change with increasing isothermal holding times after phase gets stable. The dilatometer-time curve presents that the nose of TTT curve is at 600℃, the β phase is mostly unstable under this temperature, phase transformation starts at 16s, and end at 16min.
The different kinds of β-phase decomposition schemes in TC18 alloy during continuous cooling have been investigated in detail. Two main morphological features of the α/β structure are involved, depending on the cooling rate, the lamellar structure and the colony structures are observed for low and high cooling rates. Finally, a generalized scheme of the β→β+α transformation sequences during continuous cooling is presented. α phase nucleates at grain boundaries and develops in a continuous layer or in allotriomorph precipitates at the grain boundaries(termed αGB). From these intergranular nucleation sites, Widmanstatten α phase grows as laths toward the inside of the β matrix, resulting in a colony structure(termed αWGB). Then fine plates of α phase precipitates in intragranular area because the intergranular precipitation sites being already occupied by αGB and αWGB. Of all cooling conditions in the experiments, only α phase precipitates from the β matrix, the percentage of α phase increases with cooling speed dropping, reaches 10% at 1℃/s cooling. When the cooling speed drops from 0.1℃/s to 0.7℃/s, the phase transformation starts temperature drops from 650℃ to 615℃, and the phase transformation ends temperature drops from 560℃ to 400℃. |
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