Mechanical properties, microstructure and thermal stability of a nanocrystalline CoCrFeMnNi high-entropy alloy after severe plastic deformation

Abstract

Severe plastic deformation Compositionally complex alloys 3 dimensional atom probe tomography Microstructure a b s t r a c t An equiatomic CoCrFeMnNi high-entropy alloy (HEA), produced by arc melting and drop casting, was subjected to severe plastic deformation (SPD) using high-pressure torsion. This process induced substantial grain refinement in the coarse-grained casting leading to a grain size of approximately 50 nm. As a result, strength increased significantly to 1950 MPa, and hardness to 520 HV. Analyses using transmission electron microscopy (TEM) and 3-dimensional atom probe tomography (3D-APT) showed that, after SPD, the alloy remained a true single-phase solid solution down to the atomic scale. Subsequent investigations characterized the evolution of mechanical properties and microstructure of this nanocrystalline HEA upon annealing. Isochronal (for 1 h) and isothermal heat treatments were performed followed by microhardness and tensile tests. The isochronal anneals led to a marked hardness increase with a maximum hardness of 630 HV at about 450°C before softening set in at higher temperatures. The isothermal anneals, performed at this peak hardness temperature, revealed an additional hardness rise to a maximum of about 910 HV after 100 h. To clarify this unexpected annealing response, comprehensive microstructural analyses were performed using TEM and 3D-APT. New nano-scale phases were observed to form in the originally single-phase HEA. After times as short as 5 min at 450°C, a NiMn phase and Cr-rich phase formed. With increasing annealing time, their volume fractions increased and a third phase, FeCo, also formed. It appears that the surfeit of grain boundaries in the nanocrystalline HEA offer many fast diffusion pathways and nucleation sites to facilitate this phase decomposition. The hardness increase, especially for the longer annealing times, can be attributed to these nano-scaled phases embedded in the HEA matrix. The present results give new valuable insights into the phase stability of single-phase high-entropy alloys as well as the mechanisms controlling the mechanical properties of nanostructured multiphase composites.

The BCC/B2 Morphologies in AlxNiCoFeCr High-Entropy Alloys

“…This new type of alloys has attracted much attention due to the formation of simple crystalline structures, such as face-centered cubic (FCC)-, body-centered cubic (BCC)-, and close-packed hexagonal (HCP)-solid solutions, as well as their derivatives, instead of complex intermetallic compounds [6][7][8]. Until now, many HEAs have been reported in diverse systems, e.g., CoCrFeNi-based HEAs [9][10][11][12][13][14], Al-TMs HEAs (TMs: transition metals) [15][16][17][18][19][20][21][22][23], and refractory HEAs [24][25][26][27]. The special structures render HEAs with the unique performance of excellent mechanical properties and superior corrosion-/oxidation-resistances, as examplified by the fact that the single FCC CoCrFeMnNi HEA shows a remarkably high fracture toughness that outperforms all conventional alloys at cryogenic temperatures [3,6,11].…”

Evidence for superplasticity in a CoCrFeNiMn high-entropy alloy processed by high-pressure torsion

“…There are only a few reports describing the influence of HPT processing on HEAs [16][17][18][19][20] but it was shown for the CoCrFeNiMn alloy that processing by HPT leads to exceptional grain refinement to ~10 nm with a significant strength of ~1.75 GPa, a hardness of ~4. 41 GPa and with a very low ductility of ~4% at room temperature.…”

Estimation of Resistance to Destruction of Multi-Element Coatings

“…В последние годы получила развитие концепция высокоэнтропийных или многоэлементных сплавов и по-крытий на их основе [7][8][9][10][11].…”