Abnormal Effect of Al on the Phase Stability and Deformation Mechanism of Ti-Zr-Hf-Al Medium-Entropy Alloys

Complex concentrated alloys, including high-entropy alloys (HEAs) and medium-entropy alloys (MEAs), offer another pathway for developing metals with excellent mechanical properties. However, HEAs/MEAs of different structures often suffer from various drawbacks. So, investigations on the effect of phase and microstructure on their properties become necessary. In the present work, we adjust the phase constitution and microstructure by Al addition in a series of (Ti2ZrHf)100−xAlx (x = 12, 14, 16, 18, 20, at.%, named Alx) MEAs. Different from traditional titanium, Al shows a β-stabilizing effect, and the phase follows the evolution of α′(α)→α″→β + ω + B2 with Al increasing from 12 to 20 at.%, which could not be predicted by the CALPHAD (Calculate Phase Diagrams) method or the Bo-Md diagram because of the complex interactions among composition elements. At a low Al content, the solid solution strengthening of the HCP phase contributes to the extremely high strength with a σ0.2 of 1528 MPa and σb of 1937 MPa for Al14. The appearance of α″ deteriorates the deformation capability with increasing Al content in the Al16 and Al18 MEAs. In the Al20 MEA, Al improves the formations of ordered B2 and metastable β. The phase transformation strengthening, including B2 to BCC and BCC to α″, together with the precipitation strengthening of ω, brings about a high work-hardening ratio (above 5 GPa) and improvements in ductility (6.8% elongation). This work provides guidelines for optimizing the properties of MEAs.