Effect of second phase particles on mechanical properties and grain growth in a CoCrFeMnNi high entropy alloy

Abstract

Effect of annealing of a cold-worked CoCrFeMnNi alloy at temperatures of 500-900°C for 1-50 h on the structure and mechanical properties was studied in the present work. Annealing for an hour resulted in: i) recrystallization of the face-centered cubic (fcc) matrix at 600-900°C; ii) precipitation of a Cr-rich body-centered cubic (bcc) phase at 500-700°C or a sigma phase particles at 600-800°C. Moreover, an increase in the annealing time to 50 h at 600°C resulted in a continuous growth of both the fcc grans and bcc/sigma particles and in an increase in the fraction of the sigma phase at the expense of the bcc phase particles. The fcc grains growth was found to be controlled by the pinning effect of the second phase particles. Soaking for an hour at 500-600°C resulted in a substantial increase in strength of the alloy due to the second phases precipitation. Meanwhile annealing at the higher temperatures as well as an increase in the annealing time at 600°C resulted in softening; however, even after 50 h annealing, the alloy demonstrated reasonably high strength. In the latter case fine fcc grains, preserved due to the pinning effect by the second phases particles, contributed to strength mainly.

Microstructure and Mechanical Property Evolution during Annealing of a Cold-Rolled Metastable Powder Metallurgy High Entropy Alloy

“…The nano-precipitates can effectively improve the mechanical properties of the alloy. It has been reported that precipitation of second phase particles can pin grain boundaries, suppress grain growth, and result in the formation of fine-grain microstructures in the FCC type HEAs [ 37 , 38 , 39 ]. According to Figure 1 c, the average grain size of the extruded alloy is about 20 μm.…”

“…In their work, the tensile strength was as high as 1.2 GPa and the ductility was about 19%. In fact, the size, content, and distribution of precipitates have been seen to have great effects on the mechanical performance of CoCrFeNi-based HEAs [ 20 , 21 , 31 , 32 , 33 , 34 , 35 , 36 , 37 , 38 , 39 , 40 , 41 ]. It is known that precipitation behavior is closely related to the heat treatment process.…”

Fatigue Behaviour and Crack Initiation in CoCrFeNiMn High-Entropy Alloy Processed by Powder Metallurgy

“…On the contrary, when the powder metallurgy approach for the alloy preparation was applied, the ultra-fine-grained structure was reached. The precipitation of secondary phases (usually Cr rich phase) during the alloy preparation and processing was also described by some authors [1,6].…”

“…A large variety of HEAs exists, but single-phase equiatomic five-element HEA Co-Cr-Fe-Mn-Ni, the so-called Cantor alloy, is one of the most studied [5]. The single-phase solid solution high-entropy alloys have a potential for solid solution hardening and if the single-phase fcc lattice structure is reached, a large number of slip systems ensures good ductility of the material [6][7][8][9]. Nevertheless, the experimental data about mechanical behaviour of this alloy type are still not fully understood and in some cases are controversial.…”

“…2 GPa [18,19] with the corresponding ductility in tenths of a percent, respectively, depending mostly on the grain size and processing route. The major active deformation mechanism at room temperature is the dislocation slip, while deformation-induced nanoscale twinning (nano twinning) is active at cryogenic temperatures, leading to a significant strain hardening [6,11,16,20]. In some cases, the (nano) twinning is connected with significant ductility of the HEA alloy at low temperatures.…”