Ti-4Fe-7Al 合金の焼戻しに伴う特異な変態挙動と組織変化

抄録

  The transformation behavior and microstructural changes of Ti-4Fe-7Al alloy during tempering were investigated by performing a hardness test and using X-ray diffraction (XRD) and transmission electron microscopy (TEM) techniques. Within a brief time of 30 s during tempering at 450℃, the hardness rapidly increased to a maximum value of 600 Hv. The transformation behavior estimated from the increase in hardness indicated “a lower half of C-curve” in the TTT diagram, which suggests that the transformation occurs through certain thermal activation processes. However, it is difficult to assume that the transformation is controlled by the diffusion of substitutional atoms, since the mean diffusion distance of Fe atoms during tempering at 300℃ for 1×10³ s, which also maximized the hardness, is much less than the distance of the nearest neighbors in the β structure. The structure of the α″ phase formed by tempering depends on both the temperature and holding time; a high temperature and a long time bring the α″ phase close to an hcp structure. As determined by XRD measurement, the sample tempered at 150℃ for 60 min was composed of the β phase. However, the ω and α″ phases were also detected in selected area diffraction (SAD) patterns. The sample tempered at 450℃ for 60 min consisted of a single α″ phase and exhibited a tweed structure in the TEM observation. Although the tweed structure seemed to contain only one α″-variant in the SAD patterns, HR-TEM observation revealed that it was composed of multiple nanoscale variants. It was considered that the marked grain refinement due to the nanoscale variants led to the extreme hardening of the sample and the significant broadening of XRD peaks. When the tempering temperature was increased to 600℃, the α phase and finely dispersed TiFe precipitates were formed after 60 min by the diffusion of atoms, which resulted in the softening of the sample and the sharpening of the XRD peaks.