Structure and mechanical properties of low-density AlCrFeTiX (X = Co, Ni, Cu) high-entropy alloys produced by spark plasma sintering

Abstract

In this study, low-density (< 6 g/cm3) AlCrFeTiX (x = Co, Ni, Cu) high-entropy alloys were produced by mechanical alloying and spark plasma sintering (SPS), and their structures and mechanical properties after SPS and annealing at 1000 °C for 24 h were reported. Both after SPS and annealing, the AlCrFeTiX (x = Co, Ni, Cu) alloys consisted of an L21 matrix phase with embedded bcc and C14 Laves phase particles. All the alloys had a nano-sized structure after SPS that retained in the AlCrFeTiCo and AlCrFeTiNi alloys after annealing. In the AlCrFeTiCu alloy, the annealing led to a coarsening, with an increase in the size of structural constituents above 1 μm. Compression tests showed that the AlCrFeTiX (x = Co, Ni, Cu) alloys after SPS were brittle at 25 °C, but the AlCrFeTiCo alloy exhibited the highest peak strength of 3792 MPa. At 600 °C, the AlCrFeTiCo alloy after SPS also demonstrated the best performance, with the yield strength, peak strength, and plastic strain of 1960 MPa, 2121 MPa, and 1.8 %, respectively. The annealing did not eliminate the room-temperature brittleness, but improved the mechanical properties at 600 °C. The AlCrFeTiCo alloy showed a 15 %-increase in yield strength (2264 MPa) and more than 2.5 times higher compressive plasticity (4.7 %). The AlCrFeTiNi alloy became more ductile (1 %) and stronger (2200 MPa). For the AlCrFeTiCu alloy, the annealing had a negative effect that resulted in a halved plasticity at 600 °C. The AlCrFeTiCo and AlCrFeTiNi alloys after annealing showed record-high specific yield strength values at 600 °C, which were 383 and 371 MPa*cm3/g, respectively. With these values, the AlCrFeTiCo and AlCrFeTiNi alloys outperformed all the low-density medium-/high-entropy alloys available in literature to date obtained both by SPS or conventional casting. The structure formation and mechanical properties, as well as the response of these features to annealing, were extensively discussed.