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.

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The design of specific material properties of aluminum alloys demands for a detailed understanding of clustering and precipitation processes occurring during heat treatments. Positron lifetime spectroscopy in combination with high-precision dilatometry measurements were taken, allowing for a comprehensive analysis of the aging mechanisms occurring on different timescales and in different temperature regimes, during artificial aging. From the results, unambiguous experimental evidence for the following three main steps of the precipitation process is obtained. In the first seconds of artificial aging, a competitive process of dissolution and growth of different cluster types occurs. Subsequently, clusters start to transform into coherent precipitates, which are mainly responsible for the hardening effect. For prolonged artificial aging, the number density of the coherent precipitates increases, while positron lifetime spectroscopy already reveals the simultaneous formation of less coherent precipitates.

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