Formation and evolution of platelet-like Ti-rich precipitates in the V–4Cr–4Ti alloy

• Precipitates in the V–4Cr–4Ti alloy are Ti-(CN) with NaCl structure.
• The Ti-rich precipitates are platelet-like in three dimensional space.
• Twins occur in both ends of the precipitates and the midst of these is twin-free.
• Twinning and its elimination is explained by displacive followed by diffusion.

The goal of the present investigation is to explain the obviously different appearances of Ti-rich precipitates in vanadium alloy and in steels. To achieve the goal, the formation and evolution of the precipitates in the as-cast and the heat treated V–4Cr–4Ti samples were investigated using optical and electron microscopies. The precipitates were found to be rare in the as-cast samples, and a high density of the precipitates occur in the samples subjected to isothermal holding at 600–1300 °C. The precipitates preferentially distribute within the grains rather than at the grain boundaries. All of the precipitates are platelet-like, with NaCl structure, in three-dimensional space. The further observation using high-resolution electron microscopy (HREM) reveals that a high density of twins occurs in the growth front of the precipitates, whereas the middle of the precipitates is twin-free. Meanwhile, enrichment of titanium atoms was observed in the middle of the precipitates. These results indicate that the precipitates form by a displacive transformation, followed by a diffusional process to enrich titanium further and eliminate the twined structure.