Understanding the mechanical behaviour and the large strength/ductility differences between FCC and BCC AlxCoCrFeNi high entropy alloys

The deformation behaviour of three direct laser fabricated AlxCoCrFeNi high entropy alloys were examined. One alloy had a face-centered cubic (x = 0.3) structure, one had a two-phase structure comprising an ordered (B2) and disordered BCC phase (x = 0.85), and the third alloy contained a mixture of all three phases (x = 0.6). The deformation behaviour of all three alloys was examined by mechanical testing, scanning electron microscopy and transmission electron microscopy. The dislocation density of specimens was measured using x-ray diffraction. For the FCC alloy, it was found that the correlation between dislocation density, applied stress and compressive strain all benchmark closely with the behaviour of austenitic stainless steel. It is therefore found that this alloy does not show any unique deformation behaviours of note, and like austenitic steels, show a low yield point, moderately high work hardening rates and excellent plasticity. For the case of the alloy with a high aluminium concentration, which had a two-phase B2+BCC microstructure, an exceptionally high yield strength of 1400 MPa was observed. Nano-hardness examination of this alloy showed that the B2 phase had an exceptionally high hardness value. Since the alloy contained a large volume fraction of this hard intermetallic B2 phase, is was concluded that the B2 phase is responsible for the high strength of this alloy. It was shown that this alloy behaves like a composite, and its strength can be predicted closely using a simple rule of mixtures approach. The poor tensile ductility of BCC high entropy alloys has been suggested to be the result of the inability of the B2 phase to accommodate shape change, leading to interphase cracking that results in brittle fracture characteristics.