The application potential of zirconium and titanium alloys can be seen in automobile, aircraft industry and in medicine as well. Thus, they are intensively studied mainly for their mechanical properties such as corrosion resistance, biocompatibility and specific strength. The mechanical properties are strongly influenced by the microstructure and phase composition of these alloys. Both pure zirconium and titanium can exist in two distinct crystal structures - alpha and beta phase. The beta phase is stable only at high temperatures above 700 °C (so-called beta-transus temperature), however the stability and coexistence of phases are strongly influenced by adding beta-stabilizing elements (stabilizes the beta phase) to the alloy, such as Mo or Nb. A higher content of the beta-stabilizers results in stable beta phase even at room temperature. The beta phase has a cubic body-centred unit cell. The alpha phase has a slightly distorted hexagonal close packed (hcp) structure where the most densely packed planes {110} of beta become the basal planes {0001} of alpha. In alloys with higher amount of the beta-stabilizing elements (~ 10-15 wt.%) a so-called omega phase may form. The omega phase has a hexagonal structure which is a result of a collapse of two-thirds of the {111} beta layers into one layer. The omega phase causes a brittleness of the material – its presence is undesirable. However, it forms in the alloys via martensitic transformation upon quenching from temperatures above beta-transus temperature and disappears upon heating to higher temperatures ~ 500 °C. Therefore, a complete understanding of phase transformations occurring upon heating in the alloys is necessary in order to choose an appropriate thermomechanical treatment to achieve the desired mechanical properties of the materials.
The presented study is focused on the investigation of phase content and stability in Zr12Nb, Zr15Nb and Ti15Mo alloys in temperature interval 30 – 800 °C. The high energy X-ray diffraction measurements were performed on polycrystalline bulk samples. In all three alloys the formation of the omega phase is observable which completely disappears above 500 °C and is followed by the formation of alpha phase. Although the sequence of the formation of phases is the same in both type of alloys, the mechanism and way of the formation of the omega phase is different.