Strengthening Mechanisms in CoCrFeNiX0.4 (Al, Nb, Ta) High Entropy Alloys Fabricated by Powder Plasma Arc Additive Manufacturing

Abstract

In high entropy alloys (HEAs), the addition of large-size atoms results in lattice distortion and further leads to solid solution strengthening or precipitation strengthening. However, the relationship between atomic radius, solid solution strengthening and precipitation strengthening has not been discerned yet. In this work, CoCrFeNiX0.4 (X = Al, Nb, Ta, with an equi-atomic radius) HEAs were prepared by powder plasma arc additive manufacturing (PPA-AM) and evaluated for their mechanical properties. Compression and nano-indentation hardness tests showed that the HEA with Ta showed the best properties. The influence of atomic radius and solid solubility on solid solution strengthening was investigated and the main strengthening mechanism that determines the mechanical properties of the developed HEAs was analyzed. The results showed that (i) the CoCrFeNiAl0.4 alloy did not show any solid solution strengthening effect and that a clear relation between solid solution strengthening and atomic size was not observed; (ii) in both CoCrFeNiTa0.4 and CoCrFeNiNb0.4 HEAs, precipitation strengthening and grain boundary strengthening effects are observed, wherein the difference in mechanical properties between both the alloys can be mainly attributed to the formation of fine eutectic structure in CoCrFeNiTa0.4; and (iii) from the microstructural analyses, it was identified that, in the CoCrFeNiTa0.4 HEA, the location containing a fine eutectic structure is accompanied by the formation of low-angle grain boundaries (LAGBs), which is also the region where deformed grains gather, giving rise to improved mechanical strengthening.

The Effects of Annealing at Different Temperatures on Microstructure and Mechanical Properties of Cold-Rolled Al0.3CoCrFeNi High-Entropy Alloy

“…Work hardening and refining hardening can be employed to increase the yield strength of HEA alloys [10,11]. Moreover, alloying is also an effective method to increase the yield stress by solid solution hardening or precipitation hardening [12][13][14][15][16]. The FCC Al 0.3 CoCrFeNi is the most widely studied precipitation hardening HEA alloy [16][17][18][19][20].…”

Investigation of the Effect of the Thermo‐Mechanical Processing of Additively Manufactured CoCrFeNiAl0.4 High‐Entropy Alloy

“…The random orientation of X0 (Figure 4a) is different from the <001> direction along the additive manufacturing direction as reported in the literature. [ 33 ] The reason is that the direction of EBSD samples was taken perpendicular to the direction of the building direction (i.e., the direction was along the sample rolling direction (RD) instead of the building direction (ND)). As the amount of rolling deformation increases, the FCC phase grows along the <001> direction.…”