Abstract
This study investigates the crystal structural changes and pressure-induced tetragonal distortion in rapidly solidified niobium powder subjected to pressures up to 9.9 ± 0.2 GPa. Despite extensive research on niobium at higher pressures and temperatures, a significant gap exists in understanding its behavior under moderate pressures, particularly in ultrafine microstructures formed during rapid solidification. Synchrotron X-ray diffraction revealed a systematic reduction in lattice parameters with increasing pressure and a notable 2.5-fold rise in defect density, underscoring significant microstructural changes. At 9.9 ± 0.2 GPa, a tetragonal distortion with a c/a ratio of 1.0019 ± 0.0002 was detected, marking a unique transition rarely observed in transition metals at such moderate pressures. The relationship between unit-cell volume and pressure was accurately modeled using the 3rd order Birch-Murnaghan equation, yielding a bulk modulus of K0= 140 ± 2 GPa. This distinct behavior contrasts with bulk niobium, highlighting the influence of ultrafine microstructures. These findings provide valuable insights into niobium's deformation mechanisms under moderate pressures, advancing the understanding of high-pressure material science and offering implications for its application in extreme environments.