A Study of the Kinetics of Phase Transformations in Nb-Ti-Al Alloys
Abstract
The work reported herein was performed by two Ph.D. students Keith J. Leonard and Joseph C. Mishurda, under the supervision of the PI. The phase equilibria and solid state transformations within fifteen Nb-rich Nb-Ti-Al alloys were investigated. The alloys ranged in composition between 15 and 40 at.% Al with Nb:Ti ratios of 1:1.5 to 4:1. Examination of the as-cast microstructures revealed that all alloys solidified from the beta phase field, with subsequent solid-state transformations occurring within four of the alloys during cooling. The range of primary beta phase solidification was determined to extend beyond the limits of previous liquidus projections. The high temperature beta phase field was verified in each alloy through quenching experiments. The beta phase exhibited B2 ordering at room temperature with the order-disorder transition temperatures evaluated for select alloys. The site occupancy preferences within the beta phase were evaluated through the ALCHEMI technique, which determined that Ti substitution occurred for Nb on Nb sublattice sites with the degree of sublattice partitioning found to depend upon alloy composition. The phase equilibria and transformations that occurred within the alloys were explored over a complete range of temperatures, with experimental isotherms produced at 1400, 1100, 900 and 700 degrees C, along with a plethal section between the beta and sigma phase fields. Formation of metastable O-Ti2AlNb plates occurred within several of the Al lean alloys at 700 deg C, instead of either the delta- Nb3Al or sigma-Nb2Al phases due to the slow diffusion growth kinetics of these phases below 900 deg C. The beta to O transformation was investigated through DTA analysis, with volume diffusion ahead of the advancing precipitate found to be the rate controlling mechanism. This feature was supported by the values of n equal to values between 1.3 and 1.4, along with values of Q applicable to the diffusion limited region of the TTT diagram.c
Document Details
- Document Type
- Technical Report
- Publication Date
- Jun 19, 2000
- Accession Number
- ADA382596
Entities
People
- Joseph C. Mishurda
- Keith J. Leonard
- Vijay K. Vasudevan
Organizations
- University of Cincinnati