Opposing and Driving Forces Associated with the Dynamic Transformation of Ti-6Al-4V
- PDF / 520,079 Bytes
- 5 Pages / 593.972 x 792 pts Page_size
- 84 Downloads / 159 Views
The dynamic transformation of austenite to ferrite was first reported to occur in steel by Yada and co-workers in the 1980’s.[1] A thermodynamic basis for the occurrence of this phenomenon was later proposed [2–4] and it was shown to take place in titanium alloys as well. For example, Koike et al. found that there was an increase in the beta phase fraction during the tensile testing of Ti-5.5Al-1.5Fe in the two-phase region.[5] They took the view that the energy increase of the alpha phase accounted for the transformation, although the source of the energy increase was unknown at that time. Yang et al.[6] and Prada et al.[7] also observed that the beta phase fraction in Ti-6Al-4V increased with strain. More recently, Matsumoto et al.,[8] Zhang et al. [9] and Lu et al.[10] showed that the transformation of alpha to beta takes place during superplastic deformation and is promoted by straining.
BAOQI GUO, BINHAN SUN, and JOHN J. JONAS are with the Materials Engineering, McGill University, Montreal H3A 0C5, Canada. Contact e-mail: [email protected] S.L. SEMIATIN is with the Air Force Research Laboratory, Materials and Manufacturing Directorate, Wright-Patterson Air Force Base, OH 45433-7817. JIANGTAO LIANG is with the Collaborative Innovation Center of Steel Technology, University of Science and Technology Beijing, Beijing 100083, China Manuscript submitted November 25, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
Here the opposing forces inhibiting dynamic transformation were derived from torsion tests (Table I). In these experiments, isothermal torsion tests were performed on a Ti-6Al-4V at a strain rate of 0.01 s1 to a strain of 2.0 followed by water quenching. The microstructures were examined using backscattered electron imaging (BSEI) in a scanning electron microscope (SEM). These volume fractions were determined using the ImageJ software. Results showed that the beta phase fraction increased by about 10 pct during isothermal deformation.[11] In order to illustrate the source of driving force, the stored energy and mechanical driving force are compared in the present work with the opposing forces. Knowledge of the mechanical driving force is employed to derive a metastable phase diagram for Ti-6Al-4V undergoing deformation. The opposing forces for dynamic transformation comprise those which are chemical in nature, the shear work, and the dilation work associated with the phase transformation. Deformation increases the beta phase fraction, resulting in an increase in the Gibbs energy. This increase can be quantified by the following expression based on solution thermodynamics[12]: Cb Ca RT ln 1 Cb ð1 Cm Þ DGb!a ¼ ½1 Cb 1 þ @ ln v=@ ln Cb Here Cb and Ca denote the equilibrium vanadium concentrations in the beta and alpha phases (vanadium is the rate-limiting solute in the beta matrix), respectively, Cm represents the vanadium concentration of the matrix beta (determined from mass conservation), v is the activity coefficient of the vanadium in the beta, and the term (1 + ¶lnv/ ¶lnCb
Data Loading...
