Development of an Al–Ti–Hf Composite Alloy Strengthened with High Volume Fraction of In-situ formed Al₃(Ti, Hf)-Type Trialuminide Intermetallic Phase

We have used CALPHAD-guided design methodology to develop a novel lightweight Al–Ti–Hf alloy with nearly 50 vol% in-situ formed trialuminide reinforcement. Through compositional optimization, Al_87.5Ti_6.25Hf_6.25 (at%) was chosen as the experimental alloy composition. Using the Differential scanning calorimetry (DSC) and CALPHAD-derived melting point data, the as-cast alloy was subjected to 24 h homogenization heat treatment at 475 °C to achieve equilibrium. X-ray diffraction (XRD), Scanning electron microscopy (SEM), and DSC analyses revealed that the developed alloy has a dual-phase microstructure, composed of approximately 50 vol% of an FCC Al-rich matrix and 50 vol% of an Al₃(Ti, Hf)-type (D0₂₂) phase, which matches closely with the thermodynamic calculations. The experimental onset melting point of the Al-rich matrix was determined to be 638 °C which is significantly higher than Al–Si-based high-temperature (HT) alloys, indicating the potential of the developed alloy as a HT structural material. Nanoindentation (NI) tests demonstrated the remarkable phase-specific nanomechanical properties of the alloy. The developed alloy possessed a microhardness of 3075 MPa, which not only surpasses 7075-Al, A390-Al alloys and CP-Ti, but also rivals the microhardness of Ti–6Al–4V alloy at nearly 18% lower density. The study highlights the potential of this novel alloy in applications that demand for materials with low density, high hardness, and superior wear resistance.