Deformation mechanism diagram and deformation instability of a Ti-5Al-5Mo-5V-1Cr-1Fe titanium alloy during the hot compression

Abstract

Controlling the hot deformation mechanism of the titanium alloy and avoiding deformation instability are the key points in the hot forging of the titanium alloy. A high throughput experimental method was used to establish a deformation mechanism map efficiently. A continuously varying strain distribution from 0.3 to 1.4 was obtained by the deformation of the double-cone specimens. At the temperature of 910 °C and the strain rate of 1.37 s−1, Combining the analysis of metallography and strain-stress curves, the deformation mechanisms in different regions of the double-cone specimens were divided. Dynamic recovery (DRV) and dynamic recrystallization (DRX) work together in the low strain region (＜0.62), and DRX gradually becomes the dominant deformation mechanism in the medium strain region (0.62–1.02). In the large strain region (1.02–1.4), amounts of deformation bands (DBs) formed. Electron back scatter diffraction (EBSD) technique was used for the microstructure observation and the analysis of deformation mechanism. The results demonstrate that with an increase in strain, the recrystallization fraction increased from 83.8% to 96.3%. Combining the several results of the double-cone specimens, the hot deformation mechanism diagram with a temperature range of 850–930 °C, and a strain range of 0.3–1.4 was established. When the strain reaches a certain degree, it is difficult to further form new recrystallized grains at the grain boundaries. The formation of recrystallized grains within the deformation band has been characterized using transmission electron microscopy (TEM). Under the action of shear force, fine recrystallized grains form within the elongated grains. The concentrated fine recrystallized grains aggravate the microstructural nonuniformity of the Ti-55511 titanium alloy in this area, which finally leads to the deformation instability.