Microstructural Evolution and Tensile Properties of Al0.3CoCrFeNi High-Entropy Alloy Associated with B2 Precipitates

Abstract

The room-temperature strength of Al_0.3CoCrFeNi high-entropy alloys (HEAs) is relatively low owing to its intrinsic fcc structure. In the present study, the as-cast HEAs were subjected to cold rolling and subsequent annealing treatment (800, 900, and 1000 °C) to adjust the microstructures and tensile properties. This treatment process resulted in the partial recrystallization, full recrystallization, and grain coarsening with increasing the annealing temperature. It was found that the large and spherical B2 precipitates were generated in the recrystallized grain boundaries of three annealing states, while the small and elongated B2 precipitates were aligned along the deformation twins in the non-recrystallized region of the 800 °C-annealing state. The former B2 precipitates assisted in refining the recrystallized grains to quasi ultra-fine grain and fine grain regimes (with the grain sizes of ~0.9, ~2.2, and ~7.2 μm). The tensile results indicated that the decreased annealing temperature induced the gradual strengthening of this alloy but also maintained the ductility at the high levels. The yield strength and ultimate tensile strength in 800 °C-annealed specimen were raised as high as ~870 and ~1060 MPa and the ductility was maintained at ~26%. The strengthening behavior derived from the heterogeneous microstructures consisting of quasi ultra-fine recrystallized grains, non-recrystallized grains, deformation twins, dislocations, and B2 precipitates. Current findings offer the guidance for designing the HEAs with good strength and ductility.

Keywords: B2 precipitates; high-entropy alloy; microstructures; strengthening; tensile properties.

Figure 1

XRD curves of as-cast HEA specimens and specimens after cold rolling and subsequent annealing.

Figure 2

Inverse pole figure (IPF) maps recorded from (a) 800 °C, (b) 900 °C, and (d) 1000 °C annealed HEA specimens. (c) is grain boundary map from rectangle region in (a).

Figure 3

Phase maps of HEA specimens annealed at (a) 800 °C, (b) 900 °C, and (c) 1000 °C. The precipitate phases are indicated by red colors. The lengths of scale bars in (a–c) are identical.

Figure 4

TEM-EDS elemental mapping for precipitate phase. (a) TEM image of precipitate and the inset is corresponding selected area electron diffraction (SAED) pattern; (b) Al, (c) Co, (d) Cr, (e) Fe and (f) Ni.

Figure 5

SEM micrographs of HEA specimens annealed at (a) 800 °C, (c) 900 °C, and (e) 1000 °C. (b,d,f) are magnified images in boxed regions of (a,c,e).

Figure 6

(a,d) TEM images of microstructures in non-recrystallized region of 800 °C-annealed specimen, showing the deformation twins, dislocations and precipitates; (b) magnified image of red rectangle region in (a); (c) corresponding selected area electron diffraction (SAED) pattern taken from the twin in (a); The inset is the dark field micrograph of (b).

Figure 7

The histograms of precipitate size distribution in HEAs annealed at (a) 800 °C, (b) 900 °C, and (c) 1000 °C; (d) precipitate size as a function of annealing temperature.

Figure 8

Volume fraction of precipitate in different HEA states.

Figure 9

The histograms of grain size distribution in HEAs annealed at (a) 800 °C, (b) 900 °C, and (c) 1000 °C; (d) grain size as a function of annealing temperature.

Figure 10

Relation between grain size and precipitate characteristics.

Figure 11

Vicker hardness variation in as-cast and different annealed HEA states.

Figure 12

(a) Engineering tensile stress-strain curves and (b) yield strength, ultimate tensile strength and elongation to fracture in as-cast and different annealed HEA states.

Figure 13

SEM images of surface damage and fracture surface features in (a–c) 800 °C-, (d–f) 900 °C-, and (g–i) 1000 °C-annealed HEA specimens.

Figure 14

Hall–Petch relationship plotted based on previous references [13,14,16] and the present work data.