Precipitation behavior of yttrium-rich nano-phases in AlCoCrFeNi2.1Yx high-entropy alloy

Abstract

In this paper, rare earth element yttrium (Y) was selected to be doped into AlCoCrFeNi _2.1 Y _x _ ( _x _ = 0, 0.1%, 0.3%, 0.5%, and 1.0%) high-entropy alloy in order to refine grain and increase yielding strength. The precipitation behavior of the Y-rich nano-phases in the face centered cubic (FCC (L1 _2 )) phase and the body centered cubic (BCC (B2)) phase was investigated by x-ray diffraction, scanning electron microscopy, high-resolution transmission electron microscopy and differential scanning calorimetry. Refined crystal grains were observed due to the high-density precipitation. The nano-phase within the BCC(B2) phase was (Al-Ni-Y)-rich phase and single cubic (SC) structure. The nano-phase formed within the FCC (L1 _2 ) phase was (Fe-Co-Cr-Y)-rich phase and FCC structure. Fine (Al-Ni-Y)-rich nano-particles were formed due to the addition of Y and the amount of the nano-phase increased with increasing Y content. The adoption of Y promoted dispersed precipitation of (Al-Ni-Y)-rich nano-phase under deformation. The more Y content, the more nano-precipitates. When Y =1.0 at.%, the lamellar structure was transformed into bamboo-like structure in the BCC phase due to the segregation of Y. Double yielding phenomenon occurred during the compression deformation of the AlCoCrFeNi _2.1 Y _x alloys (when x ≥ 0.5 at.%) and caused an increase of yielding strength by 40%. It was since the barrier effect of both the (Al-Ni-Y)-rich nano-precipitation and the (Fe-Co-Cr-Y)-rich nano-phases on the dislocations within grains and grain boundaries led to the second yielding. With the further increase of Y content, the lamellar BCC phase was separated from the bamboo-like BCC phase owing to the super-saturated precipitation and segregation of Y at the solidification front of B2 phase, leading to a degradation of mechanical properties.

Keywords

• yttrium
• double yielding
• microstructure
• nano-precipitate
• high-entropy alloy