Diffusionless isothermal omega transformation in titanium alloys driven by quenched-in compositional fluctuations

Abstract

In titanium alloys, the ω(hexagonal)-phase transformation has been categorized as either a diffusion-mediated isothermal transformation or an athermal transformation that occurs spontaneously via a diffusionless mechanism. Here we report a diffusionless isothermal ω transformation that can occur even above the ω transformation temperature. In body-centered cubic β-titanium alloyed with β-stabilizing elements, there are locally unstable regions having fewer β-stabilizing elements owing to quenched-in compositional fluctuations that are inevitably present in thermal equilibrium. In these locally unstable regions, diffusionless isothermal ω transformation occurs even when the entire β region is stable on average so that athermal ω transformation cannot occur. This anomalous, localized transformation originates from the fluctuation-driven localized softening of 2/3[111] β longitudinal phonon, which cannot be suppressed by the stabilization of β phase on average. In the diffusionless isothermal and athermal ω transformations, the transformation rate is dominated by two activation processes: a dynamical collapse of {111} β pairs, caused by the phonon softening, and a nucleation process. In the diffusionless isothermal transformation, the ω-phase nucleation, resulting from the localized phonon softening, requires relatively high activation energy owing to the coherent β/ω interface. Thus, the transformation occurs at slower rates than the athermal transformation, which occurs by the widely spread phonon softening. Consequently, the nucleation probability reflecting the β/ω interface energy is the rate-determining process in the diffusionless ω transformations.

Evolution of microstructure and variations in mechanical properties accompanied with diffusionless isothermal ω transformation in β -titanium alloys

“…For the interested reader a detailed report can be found elsewhere [34]; where we elucidated that solute oxygen atoms retard the DI-ω transformation. Based on previous works [31,35], the athermal ω transformation temperatures T ω of the respective alloys are T ω (15V)…”

“…To clarify these unsolved problems and to construct a universal ω transformation theory, we recently explored the detailed mechanism of the diffusionless isothermal ω (DI-ω) transformation in the Ti-V alloy system [31]. Figures 1(b) and 1(c) summarize the fundamental mechanism of the ω transformation.…”

“…Note that this compositional fluctuation is not an inhomogeneity (failure) unexpectedly introduced during a sample fabrication process but the thermal equilibrium fluctuation that inevitably exists even in a single phase for a statistical reason. As described in the literature [31][32][33], the variance (σ 2 ) of the concentration distribution is inversely proportional to the volume L 3 of each subsystem in which we define the local concentration, that is, σ 2 ∼ kT /L 3 , where k and T are the Boltzmann constant and temperature, respectively. As shown in Fig.…”

“…In general, phase transitions are regarded as cooperative phe-nomena, and therefore many researchers tried to understand the ω transformation in this way. However, although the softening of the 2/3[111] β longitudinal phonon mode plays an important role, our previous study [31] showed that this softening cannot be completely prohibited only by adding β-stabilizing elements (V or Nb, etc.). The ω transformation is not a conventional cooperative phenomenon and the local lattice instability [ Fig.…”

Exploration of Nano-scale Structural Instabilities in Metastable β Titanium Alloys Using Advanced Electron Microscopy

“…With the development of the advanced characterization techniques, especially the atomic resolution aberration-corrected scanning transmission electron microscopy, a variety of nano-scale novel phases have been reported by different research groups in metastable β titanium alloys: A new B2 structured phase was characterized in the thin foil heat treated at 300°C via both in-situ and ex-situ TEM observations in Ti-15Mo [7]. Nanoscale diffusionless isothermal ω phase was reported for the first time in Ti-V alloys driven by the compositional fluctuation [8]. In the solution-and-quenched metastable β titanium alloys, Kim et.…”