Phase transformations during continuous heating and cooling in a Ti-Al-Ru alloy
Open Access
- Author:
- Rajsiri, Supphachan
- Graduate Program:
- Materials Science and Engineering
- Degree:
- Doctor of Philosophy
- Document Type:
- Dissertation
- Date of Defense:
- December 14, 2011
- Committee Members:
- Earle Richard Ryba, Dissertation Advisor/Co-Advisor
Earle Richard Ryba, Committee Chair/Co-Chair
Pual R Howell, Committee Member
Todd Palmer, Committee Member
Michael T Lanagan, Committee Member - Keywords:
- Phase transformation
Ti-Al-Ru alloy
Synchrotron X-ray diffraction - Abstract:
- The phase transformations in a Ti-Al-Ru alloy were studied by performing high temperature X-ray diffractions using synchrotron radiation. A Ti-32.4 at% Al-4.1 at% Ru alloy was used; it undergoes various phase transformations. Specimens were prepared by arc-melting. The alloy was beta-homogenized at 1300 oC and ice-water quenched. Powdered specimens with particle size equivalent or smaller than 45 micron were prepared, and packed in a quartz capillary. The X-ray measurements were performed during the step-wise heating and cooling between room temperature and 1220 oC. A single beta phase with relatively small amounts of alpha2, G and alpha phases were initially present in the sample. Decomposition of the as-quenched beta phase to the equilibrium phases was investigated as well as the phase relationships among the major phases (beta, alpha2 and G). At an average heating rate of 5.3 oC/min, most of the beta phase decomposed to form the alpha2 and G phases, possibly through eutectoid reaction taking place between 687 oC-709 oC. The gamma phase formed at approximately 917 oC. A reverse eutectoid reaction for the formation of beta phase was observed between 1018 oC and 1220 oC, but was incomplete due to two possible reasons: suspected low diffusion rates in the alpha2 and G phases, and the presence of a relatively large amount of dissolved oxygen, which increases the stability of the alpha2 phase. beta-ordering could not be detected due to the overlap of the beta’ (001) reflection, the most intense superlattice reflection for the beta’ phase, and the Ti5Si3 (020) diffraction peaks. During cooling from 1220 oC to room temperature at an average heating rate of 4.8 oC/min, the beta phase partially decomposed to alpha2 and G phases,but no significant amount of transformation was observed. The (Ti,Al)Ru phase was observed on the X-ray patterns recorded at 1068 oC through 36 oC. The overall reaction rates of the as-quenched Ti-32.4 at% Al-4.1 at% Ru alloy were sluggish. Mathematical expressions for the relationships between lattice parameters and alloy compositions were proposed for the G, beta, and alpha2 phases. It was suspected that the presence of Ru affects the stability of the beta phase. A slight change in Ru content was also detected in the alpha2 and G phases during the phase reactions. Simulated step-wise heating and cooling microstructure contained plate-like alpha2 phase and the G-phase matrix as the product of the beta decomposition between 683 oC and 709 oC. Small alpha2 particles were observed nearby the interface between the plate-like alpha2 phase and the G phase matrix as a result of the beta decomposition during cooling from 1220 oC. Fundamental information for the processing of an alloy similar to this Ti-Al-Ru alloy is proposed: a deformation processing of single beta phase alloys should be done below 680 oC, and any thermal processing above 680 oC should be avoided due to the presence of the brittle G phase.