Polymorphic transformation: The basis of thermocycling treatment of titanium alloys

Abstract

Thermocycling consists of numerous heating and cooling cycles that occur with phase and structural transformations that change the properties of the alloys. For example, the thermocycling treatment (TCT) of steels occurs with phase a ~ )' recrystallization that causes a considerable strain hardening. In its turn it causes primary recrystallization and grain disintegration. In titanium the volume effect of the polymorphic a ~- [3 transformation (h.c.p. ~-b.c.c.) is an order of magnitude lower than in steels [1]. Therefore, the phase hardening in pure titanium is lower and the heating is not accompanied by recrystallization.In titanium alloys numerous metastable and intermediate phases form in the form of intermetallic compounds, carbides, hydrides, etc. However, the main phases in commercial titanium alloys are a and [3 and therefore the polymorphic a ~ [3 transformation is a fundamental one. The effect of thermocycling is determined to a great degree by phase hardening in this transformation. It has been shown in [2,3] that the atomic volumes of the a-and [3-phases in commercial titanium alloys depend in the first place on the chemical composition of these phases and the treatment temperature. The maximum microstresses O'ma x that arise in continuous heating of quenched alloys VT23 and VT22 have been determined in [2] to be 320 and 430 MPa, respectively. The authors of [4,5] have shown the substantial role of vacancies and other defects of the atomic and crystalline structure on the process of polymorphic a ~ [3 transformation. In addition, the stresses due to thermal anisotropy on the boundaries of a-grains (h.c.p. lattice) have been calculated under heat cycles.Analyzing the results of [6][7][8] and the data of [9-11] we singled out the external and internal factors that determine the state and properties of titanium alloys after thermocycling.The external factors include the heating and cooling rates, the maximum temperature of the heating and minimum temperature of the cooling, the duration of the isothermal I Russian State Technological University (K. I~. Tsiolkovskii Aircraft Engineering Institute), Moscow, Russia. 128hold or its absence at these temperatures, the number of therreal cycles, and the final (after the TCT) heat treatment and its regimes. The internal factors include the difference in the specific volumes of the phases, the anisotropy of the coefficients of linear expansion, the difference in the diffusivities of the alloying elements and the admixtures, the anisotropy of the moduli of elasticity and other properties of the phases, the surface energy of the phases, the amount of vacancies, dislocations and other defects of the atomic and crystal structure, and the original state of the alloys (deformed, cast, annealed, etc.).It has been shown in [11,12] that the rates of heating and cooling in TCT of titanium alloys can be low in contrast to steels. For example, the reduction of the heating rate of alloy Ti -6% A1 -4% V from 3.3 to 0.3 K/sec in [12] and that of alloys Vt3-1 and VT6 from 2.2 t...

The Influence of Thermocyclic Treatment on the Structure and Mechanical Properties of Pseudo-Alpha Titanium Alloys for Steam Turbine Blades

“…It is known that a sufficiently effective way to increase the strength of titanium alloys while maintaining their high ductility and heat resistance can be thermocyclic threatment (TCT), which consists in repeated heating and cooling periods of the titanium alloy in the range of complete polymorphic transformation, i.e. α↔β transition [1][2][3][4][5][6][7][8][9][10][11][12][13][14].…”