Influence of Cooling Rate from the β Region on Phase Transformations in the Zr–2.5Nb Alloy

The morphological and crystallographic features of two-phase α + β states in the Zr–2.5Nb alloy formed during cooling from the β region at different cooling rates (water, air, and furnace) were studied using X-ray diffraction analysis and scanning electron microscopy, including orientation analysis using electron backscatter diffraction. In addition to orientation maps (EBSD), the crystallographic analysis used spectra of misorientations of intercrystallite and interphase boundaries, as well as the restoration of high-temperature β grains using Burgers orientation relationships. It is shown that with significant differences in the morphological features of the structural states caused by the cooling rates, almost the same picture is observed crystallographically: complete coincidence of the spectra of misorientations of intercrystallite (α'/α' and α/α) and interphase (α'/β and α/β) boundaries. X-ray analysis did not record the presence of the β phase in the alloy structure upon quenching in water, but showed its noticeable amount at lower cooling rates. In this case, the chemical composition of the β phase is close to the point of invariant transformation (~20 at % Nb). An assumption is made that at all cooling rates, phase transformations are realized by the same mechanism: shear rearrangement of the bcc ↔ hcp lattice, accompanied by directed jumps of individual atoms. It has been confirmed that the observed β phase is not retained from the high-temperature region, but is released during phase transformations by the shift-shuffling mechanism at previously formed α'/α' or α/α boundaries.