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.
Highlights
IntroductionSince the introduction of high entropy alloys (HEAs) [1,2], researchers have continuously explored the factors that affect the mechanical properties of HEAs [3,4,5,6,7,8,9,10]
The structure and crystal type of the CoCrFeNiX0.4 (Al, Ta, Nb) high entropy alloys (HEAs) produced by powder plasma arc additive manufacturing (PPA-AM) [23] are shown in the TEM
Laves in Co-Ta and the 1216 Laves phase rich in Fe-Nb. This result is consistent with previous XRD [23] and is similar to HEAs produced by vacuum arc melting and direct laser deposition AM [5,14,17,27], except for the alumina with an HCP structure in CoCrFeNiAl0.4
Summary
Since the introduction of high entropy alloys (HEAs) [1,2], researchers have continuously explored the factors that affect the mechanical properties of HEAs [3,4,5,6,7,8,9,10]. The solid solution phase is the main component of the high entropy alloy, and different phases (such as FCC, BCC, and HCP) have a great influence on the mechanical properties of HEAs. In recent research [11,12,13], solid solution strengthening has been more used in CoCrFeNi-based HEAs added with a large atomic radius to explain the improvement of alloy properties. The researchers did not give specific proof of solid solution strengthening. The strengthening effect of the precipitated phase is significant [14,15,16,17], and
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