Plastic deformation and strengthening mechanisms in CoNiCrFe high entropy alloys: The role of lattice site occupancy

The work herein presents the designing of two γʹ strengthened high entropy alloys guided by density function theory (DFT) and thermodynamics calculations with compositions Co<inf>34</inf>Ni<inf>34</inf>Cr<inf>12</inf>Al<inf>8</inf>Nb<inf>3</inf>Ti<inf>4</inf>Fe<inf>5</inf> and Co<inf>31.5</inf>Ni<inf>31.5</inf>Cr<inf>12</inf>Al<inf>8</inf>Nb<inf>3</inf>Ti<inf>4</inf>Fe<inf>10</inf> (referred as 5Fe and 10Fe). These alloys in the peak aged condition (900 °C for 20 h) exhibit similar precipitates sizes, shapes, volume fractions and γ/γʹ lattice misfit (∼ 0.56). Intriguingly, despite their microstructural similarities, these alloys show different trends in yield strength (YS) evolution over a temperature range. The 5Fe alloy shows a better combination of strength and ductility at room temperature (RT), with YS and elongation of 970 ± 25 MPa, ∼ 18 (%), respectively, in comparison to 850 ± 20 MPa, and ∼ 15(%) in the 10Fe alloy. The precipitate chemistry analyses carried out by 3D atom probe tomography suggest that Fe atoms occupy B-sites in the 5Fe alloy, while it occupies both A and B-sites in the 10Fe alloy. The site occupancy behaviour rendered a higher stacking fault energy (SFE) of the 5Fe alloy, making the γʹ shearing more difficult compared to the 10Fe alloy. The synchrotron X-ray measurements further confirm higher stacking fault (SF) probability in the γ matrix compared to γʹ precipitates in the 5Fe alloy. The role of deformation substructure evolution is also carefully discussed to explain the differences in the high temperature behavior. These results on the effects of alloying chemistry in high entropy alloys enable tuning the mechanical properties of alloys and widening the alloy spectrum with improved high-temperature properties.