Retardation of the σ phase formation in the AlCoCrFeNi multi-component alloy

Highlights: • In the dendrite area BCC phase does not decompose at 850 °C. • In the iterdendrite area at 850 °C (B2 + BCC) transforms into (σ + FCC). • Transformation to σ can be retarded by strain. • Long range order of the Anti-Phase Boundaries (APBs) relaxes the matrix. • Following the heat treatment, APBs attained long range order across the sample. -- Abstract: In the last decades, multi-component or high entropy alloys (HEA) are in the center of researchers' attention due to their exceptional properties. AlCoCrFeNi alloy is one of the most studied HEAs. However, this alloy is not commercial due to several drawbacks, one of which is brittleness at high temperatures caused by transformation to σ phase. Our study of the heat treated AlCoCrFeNi alloy shows that this transformation occurs only in the inter-dendrite (ID) area and not in the dendrite (DR). Estimation of strain in the vicinity of the particles in the ID and DR areas allows concluding that in the DR area, transformation to σ is retarded by strain. Furthermore, several relaxation mechanisms were identified – long range order of the Anti-Phase Boundaries (APBs) in the matrix and high density of structural defects in the FCC particles, which appeared adjacent to the σ in the ID area. Phase transformation occurring in this area was identified as B2 + BCC → σ + FCC + B2.