Characterization of lattice distortions in refractory metal complex concentrated alloys using positron annihilation spectroscopy

Severe distortions of crystalline lattice represent an inherent feature of complex concentrated alloys (CCAs). Lattice distortions are caused by atomic size differences of alloy constituent ions which are distributed randomly over the lattice sites. It is believed that lattice distortions play the key role in extraordinary properties of CCAs. However, characterization of lattice distortions is difficult since they occur on the atomic scale and have a stochastic nature. In the present work we employ positron annihilation spectroscopy for characterization of lattice distortions in series of 18 refractory metal CCAs with the bcc structure. Our study revealed that the bulk positron lifetime increases with increasing magnitude of lattice distortions. It indicates that positron delocalized in the lattice can be used as a probe of lattice distortions in CCAs since it is located with higher probability in interstitial regions with extended open volume. This experimental observation is supported by theoretical calculations of positron lifetimes in CCAs. Moreover, it was confirmed that lattice distortions cause strengthening of CCAs.