Formation of Compositional Modulated Microstructure in Ti-Zr Binary Alloys

In this study, the effects of slow cooling treatment on the microstructure of Ti-xZr (x = 15–45 at%) binary alloys were investigated. The alloys fabricated by vacuum arc melting were annealed at 1000°C (β phase field) for 2 h and subsequently cooled to room temperature (α phase field) at 50°C/h. The crystal structure was investigated using X-ray diffraction, confirmed that the constituent phase of the slow-cooled Ti-xZr alloys was only α phase. However, optical microscope and scanning electron microscope observations for the alloys with x = 33–37 revealed that the layered microstructure was formed at various locations within α grains. In addition, the layer thickness decreased with increasing x. Compositional analyses for the layered microstructure were performed by energy dispersive X-ray spectroscopy, showing that Zr-rich and Zr-lean layers were arranged alternately. Therefore, it was found that a compositional modulation was occured in the layered microstructure. To investigate the crystallographic orientation of the layered microstructure, electron backscatter diffraction measurement was conducted. This result revealed that the c-axis direction of the hexagonal close-packed structure was oriented parallel to the layers. Moreover, the kernel average misorientation value in Zr-rich layer was higher than that in the Zr-lean layer. In fact, transmission electron microscope observation indicated that the unit cell orientation in the Zr-rich layer was slightly misaligned from that in the Zr-lean layer. The Vickers hardness of the layered microstructure was slightly lower than that of the random microstructure. From these results, it is expected that the slow cooling treatment for the Ti-Zr alloys is an effective approach to introduce periodicity into the microstructure.