In-situ investigation of phase transformations during heating of AlCoCrCuNi high entropy melt-spun ribbons

The decomposition of rapidly cooled, equiatomic high-entropy AlCoCrCuNi alloy ribbons was studied using in situ transmission electron microscopy (TEM) and X-ray diffraction (XRD) analysis. The high cooling rate achieved during melt spinning led to the formation of a two-phase microstructure consisting of large amounts of bcc based partially B2-ordered grains in a Cu-enriched fcc (A1) solid solution matrix. The microstructure appears to be similar to that obtained earlier by classical casting into steel molds, but detailed studies of the A2 grains (bcc) revealed the presence of spinodal modulations with 5-8 nm spacings with disordered and B2 ordered regions. TEM investigations of the grains after annealing at 400 degrees C for 10 min showed Moire fringes suggesting the presence of fine precipitates, the size of which has been estimated as 20 nm. In-situ TEM heating experiments revealed that heating the foil up to 500 degrees C and holding it for 30 min led to the formation of disc-like precipitates of a omega-type phase with a hexagonal structure (P6/mmm), while at 600 degrees C rods of a sigma-type phase appeared. A significant increase in the ribbon hardness from 659 HV (as-cast state) to 806 HV was observed after annealing at 500 degrees C. Heating further up to 700 and 800 degrees C led to the formation of lamellar-shaped precipitates (probably sigma phase). The absence of (001) B2 superlattice spots at 800 degrees C indicated the occurrence of a B2 -> A2 transformation. Therefore, the sequence of processes active during melt spinning of the AlCoCrCuFe alloy could be expressed as follows: melt -> solidification to A1 + A2 -> A1 + spinodal modulations in A2 with partial ordering of A2 to B2. Heating the ribbons activates the following transformation sequence in the grains: full ordering of A2 to B2 -> B2 + omega -> B2 (Cr2Ni3) + sigma CrCo -> A2.