Tensile Deformation Behavior and Microstructure Evolution Mechanism of TC4 Alloy at High Temperature AITranslate

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Basic Information:

DOI：10.13373/j.cnki.cjrm.XY24070009

Chinese Library Classification Number:TG146.2

Citation Information:

Ti-6Al-4V（TC4）, as an excellent dual-phase titanium alloy, exhibits good thermal stability at high temperatures and possesses superior overall performance, leading to its extensive use in the aerospace industry. Due to the complex geometries and poor room-temperature formability of titanium alloys, hot forming processes are typically employed during manufacturing. However, TC4 alloy has a narrow hot working window, where the microstructure is highly sensitive to deformation parameters. The competition among deformation mechanisms such as dynamic recrystallization（DRX）, work hardening（WH）, and dynamic recovery（DRV）complicates microstructural evolution during high-temperature deformation. Therefore, investigating the effects of deformation parameters on microstructural evolution and corresponding deformation mechanisms is crucial. Traditionally, hot compression tests are used to evaluate alloy forming processes. However, in the complex process of hot stamping of intricate parts, tensile stresses may lead to accumulated damage, significantly reducing final mechanical performance. High-temperature tensile tests accurately reflect the stress state during material deformation. The tensile deformation behavior of titanium alloys differs in mechanism from hot compression, making it essential to elucidate the evolution of microstructure during high-temperature tensile deformation for a better understanding of mechanical property changes. Previous studies have indicated that high-temperature deformation of TC4 alloy involves complex microstructural changes such as processing hardening due to dynamic grain growth, deformation and high-temperature movement of intragranular dislocations, α→β phase transformation, globularization of lamellar α phase, dynamic recovery, and dynamic recrystallization. Consequently, material mechanical properties undergo intricate changes, necessitating exploration of TC4 titanium alloy's mechanical characteristics under different high-temperature conditions. The plastic deformation capability of the alloy fundamentally reflects the macro results of its internal microstructural deformation mechanism. A systematic study on the tensile deformation behavior of TC4 alloy within a broad temperature range for hot forming is currently lacking. Therefore, this paper conducted high-temperature tensile experiments at temperatures of 650, 700, 750, 800 and 850 ℃ with a strain rate of 0.02 s^-1, employing electron backscatter diffraction （EBSD） to analyze the tensile microstructure characteristics and deformation mechanisms of TC4 alloy post-deformation. The formation process of recrystallized grains was reflected by changes in orientation difference angles, and individual grains were selected to analyze the globularization process of lamellar structures. The phase diagrams and image quality maps were used to reveal α-β phase transformation of TC4 alloy. The results showed that：1） TC4 alloy exhibited different deformation characteristics during high-temperature tensile processes. With increasing deformation temperature, yield stress gradually decreased. At 650 ℃, yield stress increased with loading, peaks, and then rapidly decreased until fracture, exhibiting significant work hardening characteristics. At 700 and 750 ℃, yield stress reached a maximum early in deformation, followed by dynamic softening, indicating DRV characteristics. At 800 and 850 ℃, yield stress peaked early in deformation and then decreased gradually with increasing strain, where dynamic recrystallization played a dominant role, resulting in decreased strength and significantly increased elongation. 2） Fracture surface analysis revealed that at lower temperatures, dimples were smaller and deeper, with uneven distribution and presence of microcracks and numerous voids at fracture surfaces. As temperature increased, dimples became larger and shallower, indicating increased plasticity, consistent with macro trends in elongation. 3） During high-temperature deformation of TC4 alloy, significant differences were observed in microstructural morphology. At 650 ℃, primary α phase exhibited elongated distribution with relatively low equiaxed transformation. Local recrystallization occurred in grain boundaries of α-p phase, forming "necklace-like" recrystallized grains. As temperature rose to 700 and 750 ℃, dynamic softening did not fully dissipate energy from thermal deformation, leading to dynamic precipitation of lamellar α phase, with increased thickness of lamellar α phase with temperature rise. At 800 ℃, thinner α phase lamellae transformed into finer recrystallized grains, undergoing dynamic globularization process, with average DRX grain size approximately 1.04 μm. Increasing to 850 ℃, volume fraction of recrystallized grains increased, including those formed at α-p phase grain boundaries and globularization of lamellar α phase. Overall, with increasing tensile temperature, primary α phase tended towards equiaxial transformation, decreasing aspect ratio from 1.67 to 1.35, with grain size initially increasing and then decreasing. Low-angle grain boundaries（LAGBs）gradually transformed into high-angle grain boundaries（HAGBs）, with HAGBs content increasing from 11.7% to 36.1%, indicating enhanced DRX effects. High-temperature tensile process of TC4 alloy was controlled by multiple deformation mechanisms, primarily including DRV mechanism, DRX mechanism, α→β phase transformation, lamellar α phase globularization mechanism, and grain boundary sliding（GBS）mechanism. DRV and continuous dynamic recrystallization（CDRX）mainly controled flow softening process of TC4 alloy, with CDRX mechanism continuously transitioning from LAGBs to HAGBs. Discontinuous dynamic recrystallization（DDRX）phenomenon was observed even at lower temperatures（650 ℃）. α→β phase transformation was a critical factor in enhancing alloy plasticity, with decreasing volume fraction of α_p phase and increasing α→β phase transformation with temperature rise. Dynamic globularization of lamellar α phase initially occurred in thinner lamellar layers, eventually forming fine and uniform DRX grains.