טכניון מכון טכנולוגי לישראל
הטכניון מכון טכנולוגי לישראל - בית הספר ללימודי מוסמכים  
Ph.D Thesis
Ph.D StudentTokar Alexander
SubjectSolid State Transformations in an Iron-Alloyed Titanium
Aluminide
DepartmentDepartment of Materials Science and Engineering
Supervisor Professor Emeritus Lev Levin


Abstract

Phase equilibria and solid state transformations under non-equilibrium conditions were investigated in the iron-alloyed titanium aluminide TiAl-2Fe. During water quenching from 1400oC, a massive b-Ti® t2 phase transformation was observed. The t2 formed  has characteristic serrated morphology and a tetragonally distorted lattice. FeTi found after water quenching from 1300oC differs from the known equilibrium modifications, being over-saturated in Al.  The FeTi phase, formed at 1300oC, being transferred to the intermediate temperature range 700-1070oC, decomposes via two subsequent reactions: aluminum depletion of the parent phase, and its decomposition to the stable a2 and t2 phases. The kinetics of the depletion reaction was analyzed using the Larche-Cahn formalism for diffusion in crystalline media and the quasi-chemical model for Gibbs energy of intermetallic compounds. It was found that the diffusion that enables Al depletion is governed by concentration gradients of the   MFe-Al and MTi-Al diffusion potentials, being sufficiently fast to permit the depletion reaction at all intermediate temperatures observed. The rate of a2 + t2 nucleation has a maximum in the vicinity of the 800oC. This nucleation rate leads to the formation of the very fine a2 + t2 mixture after sample holding at this temperature. A new phase, t'2, was detected after water quenching from 1200oC. The t'2 phase is a product of the FeTi long-range ordering.  An orientation relationship between the t2 and the parent FeTi was suggested. Furnace cooling from 1400 and 1300oC results in transformation of the high-temperature b-Ti (or FeTi) into an a2+t2 mixture of non-uniform grain size  and- chemical composition. The observed non-uniformity stems from kinetic constraints due to continuous cooling conditions.