Characterization of hot workability of Ti-6Cr-5Mo-5V-4Al alloy based on hot processing map and microstructure evolution

Ti-6Cr-5Mo-5V-4Al (Ti-6554) alloys with excellent comprehensive properties are expected to become the preferred material for large-scale parts in the aviation field. However, the processing parameters of large-scale parts have a significant impact on the microstructure, so it is necessary to optimize the hot working process to improve the comprehensive mechanical properties of the alloy. In this paper, hot compression experiments of Ti-6554 alloy were carried out in the temperature range from 680 to 830 & DEG;C and strain rate range from 0.001 to 10 s-1. The hot workability of the Ti-6554 alloy was studied based on the hot processing map and microstructure evolution. The Arrhenius constitutive model of the two phase region and single phase region was established. It was found that the thermal activation energy was higher at high value in the adopted range of strain rate, and the average thermal activation energy gradually decreased with raising the strain. The peak efficiency in the hot processing map occurred at 680 ?/0.001 s(-1) and 770 ?/0.001 s(-1 )with efficiency values of 0.47 and 0.48, respectively. The instability regions were mainly concentrated at a high value in the adopted range of strain rate, and the typical instability phenomenon was flow localization. With raising the strain rate and temperature, the volume fraction and average size of the alpha phase decreased due to the dynamic phase transformation. Dynamic recovery (DRV) was the major deformation mechanism at low temperatures and low strain rates. The deformation mechanism gradually changed to DRX with the increase of temperature and strain rate. However, the increase of temperature at a higher strain rate could not improve the level of dynamic recrystallization (DRX). Based on electron backscatter diffraction (EBSD) characterization, the DRX types of the beta phase were determined to be discontinuous dynamic re-crystallization (DDRX) and continuous dynamic recrystallization (CDRX). DDRX mainly occurred at high temperatures and low strain rates. Furthermore, the spheroidizing mechanism of the equiaxed alpha phase was also analyzed. First, under the action of compressive stress, the aspect ratio of the equiaxed alpha phase gradually increased and became the lamellar alpha phase. Subsequently, the low angle grain boundaries (LAGBs) gradually changed to high angle grain boundaries (HAGBs), accompanied by wedging of the beta phase. Finally, the spheroidization process was completed. (C) 2022 Elsevier B.V. All rights reserved.

Automated summary

This paper investigates the hot working properties of Ti-6554 alloy. Hot compression experiments were conducted in a temperature range of 680 to 830°C and a strain rate range of 0.001 to 10 s-1. The hot workability of the alloy was analyzed based on the hot processing map and microstructure evolution. The results revealed that the thermal activation energy was higher at higher strain rates, and the average thermal activation energy decreased with increasing strain. The peak efficiency in the hot processing map was observed at specific temperature and strain rate conditions. The instability regions were concentrated at higher strain rates, and flow localization was observed as a typical instability phenomenon. The volume fraction and average size of the alpha phase decreased due to the dynamic phase transformation with increasing strain rate and temperature. The deformation mechanism transitioned from dynamic recovery to dynamic recrystallization. The types of dynamic recrystallization for the beta phase were identified as discontinuous and continuous, depending on temperature and strain rate. Additionally, the spheroidization mechanism of the equiaxed alpha phase was analyzed. The aspect ratio of the equiaxed alpha phase increased under compressive stress, transforming into lamellar alpha phase. The low angle grain boundaries gradually transformed into high angle grain boundaries with wedging of the beta phase, resulting in spheroidization.