Abstract
The microstructural evolution and phase transformation of a Ti-5Nb-5Al alloy during isothermal annealing treatment were studied in this paper. The microstructural evolution was analyzed by X-ray diffraction (XRD), scanning electron microscope (SEM) equipped with X-ray energy dispersive spectroscopy (EDS) and electron backscattering diffraction (EBSD). The first-principle calculation by density functional theory method was performed to analyze the effect of niobium diffusion on the α to β transformation. The results showed that the α phase initially grew by merging neighboring grains and then by boundary splitting with increasing the solution time below the β-transus temperature. However, the α phase disappeared absolutely above the β-transus temperature. According to the thermodynamic analysis by DFT calculations, the diffusion of niobium from β to α phase can promote the α to β transformation. After quenching in water, the acicular α’ phase precipitated from β matrix with an orientation relationship of {110}bcc||{0001}hcp and bcc|| hcp, and thus increased the hardness of the alloy.
Highlights
Over the past decades, titanium alloys have been widely used in various fields, such as aerospace, biomedical and chemical industries, due to their high strength, low Young’s modulus, good biocompatibility and excellent corrosion resistance[1,2,3,4]
SHI et al.[7] investigated microstructural evolution and mechanical properties of TC21 titanium alloy under different heat treatment conditions and found that the existence of crisscross α plates with large thickness contributed to the improvement of the fracture toughness
It can be seen that the original sample without heat treatment has the lowest hardness value of 283 HV and the 1000-30 sample has the highest hardness value of 328 HV
Summary
Titanium alloys have been widely used in various fields, such as aerospace, biomedical and chemical industries, due to their high strength, low Young’s modulus, good biocompatibility and excellent corrosion resistance[1,2,3,4]. Previous investigations[5,6,7] have been focusing on the microstructural evolution of titanium alloys in order to improve the mechanical properties. HUANG et al.[6] revealed the effects of microstructures on the strength and plasticity of a metastable β-βTi-5Al-5Mo-5V-3Cr-1Zr alloy (Ti-55531). They found that the existence of twinning structure can greatly enhance the ductility of lamellar microstructure during plastic deformation. SHI et al.[7] investigated microstructural evolution and mechanical properties of TC21 titanium alloy under different heat treatment conditions and found that the existence of crisscross α plates with large thickness contributed to the improvement of the fracture toughness
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