Defect Study in CoCrFeMnNi High Entropy Alloy by Positron Annihilation Lifetime Spectroscopy

Positron annihilation lifetime spectroscopy is applied to study the crystal defects in CoCrFeMnNi high entropy alloy. The material is measured in the as-received (electro eroded; non-etched) state; it shows a defect-related positron lifetime of 220ps, corresponding most likely to divacancies, with a relatively high concentration. Furthermore, this sample is annealed in the temperature range up to 1000K and positron lifetime is measured after each annealing step. The average positron lifetime is found to decrease with annealing and reaches 121ps after annealing at 1000K, indicating the existence of some remaining defects in the material. To show whether the observed defects are present in the bulk sample or only in the surface area, another sample is deeply etched and shows only one lifetime component of 112ps, which is assigned to the defect-free bulk lifetime in this alloy. This study indicated that the investigates high entropy alloy (in etched state) does not contain open-volume defects within the sensitivity range of positron annihilation. Here, the defects observed in the non-etched sample are present in the subsurface region, which are assumed to be generated most probably during wire-erosion of the samples. In addition, X-ray diffraction results indicate that the crystal structure is FCC with a lattice constant of (3.597 +/- 0.002)angstrom, which is not affected by annealing.