Abstract
The law of microstructure evolution and transformation mechanism of the α′ martensite decomposition during 400–600 °C were studied by the isothermal dilatometry. The transformation process of α′ martensite was quantitatively characterized based on Johnson–Mehl–Avrami (JMA) model, and the microstructure evolution under different aging processes was observed and compared on Scanning Electron Microscopy (SEM) and Transmission Electron Microscopy (TEM). The results showed that α′ → α + β is the elemental diffusion transformation, the position and shape of the precipitate gradually change with the holding time and temperature. The decomposition rate of α′ martensite was positively correlated with the aging temperature. The whole transformation process was divided into two stages based on the value of the Avrami exponent n, and the corresponding average values of the transformation activation energies Q are 46.1 kJ/mol and 116.8 kJ/mol, respectively. The calculated model had good agreement with the experimental data, and the transformation curve of α′ martensite with time and temperature during the isothermal aging at 400–600 °C was drawn.
Highlights
TC4 alloy is broadly used in aerospace, marine engineering and biomedical fields due to their good corrosion resistance, thermal stability, high specific strength and formability [1,2]
TC4 alloy, and the hardness value increased from 350 HV to 410 HV with the increase of decomposition degree
The decomposition process of martensite in the aging process is systematically studied by the isothermal expansion method and microstructure observation, and the kinetic information of martensite transformation is obtained based on the JMA model
Summary
TC4 alloy is broadly used in aerospace, marine engineering and biomedical fields due to their good corrosion resistance, thermal stability, high specific strength and formability [1,2]. The addition of the alloying elements achieves the coexistence of α and β phases at room temperature, which makes alloy achieve the ability to solid solution aging strengthening. The microstructure and mechanical properties of the alloy often have significant differences due to the influence of alloy composition and external conditions (stress state, temperature, cooling rate, etc.) [3,4,5]. The microstructure and mechanical behavior of TC4 alloy are very sensitive to the history of the thermomechanical deformation and thermochemical treatment [6,7,8,9]. Ahmed [10] found that the transformation mechanism of TC4 alloy is complete martensite, massive transformation and diffusion transformation with the cooling rate decreasing from 525 ◦ C/s to 1.5 ◦ C/s. Gupta [12] found that in addition to the effect of aging temperature and aging time on the Crystals 2020, 10, 229; doi:10.3390/cryst10030229 www.mdpi.com/journal/crystals
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