Precipitation and decomposition in CoCrFeMnNi high entropy alloy at intermediate temperatures under creep conditions

The precipitation and decomposition behaviors of CoCrFeMnNi high entropy alloy (HEA) were studied at intermediate temperatures under creep conditions. One of the most interesting observation is the nanoscale precipitation of bcc Cr particles at dislocations during creep at 515 degrees C-580 degrees C. At 580 degrees C, the formation of intragranular Cr-rich sigma phase particles was also observed in addition to nanoscale bcc Cr particles. Typical Kurdjumov-Sachs orientation relationship was exhibited between bcc Cr precipitates and the matrix. Bcc Cr precipitates can be easily nucleated from segregated Cr atoms along < 111 >(fcc) directions because of the excellent fit between closed packed directions, <111 >(fcc) and <101 >(bcc). Strain field contrast at the interface of bcc Cr precipitates suggests that Cr particles is semi-coherent with the matrix. The precipitation of intragranular sigma phase precipitates only at a higher temperature (580 degrees C) is attributed to higher nucleation energy of intragranular sigma phase due to the lack of coherency at the interface. The decomposition and precipitation of various phases at grain boundaries were also observed. Interganular sigma, NiMn (L1o), and FeCo (B2) phases were observed at 515 degrees C. As the creep temperature increased, the formation of intergranular phase particles became less significant. The successive and agglomerated formation of three phases could be attributed to the enrichment of expelled atoms (from the transformed phase) and sluggish diffusion in HEA matrix. To improve the predictability of thermodynamic prediction of various phases in high entropy alloys, the local enrichment factor should be taken into consideration in the region of the significant heterogeneous transformation.