Effect of Ti content and ball-milling time on the microstructural, thermal, and microhardness properties of AlCoCuNiTi high entropy alloys

Abstract

In this study, AlCoCuNiTi high entropy alloy was produced by high-energy ball-milling (mechanical alloying) and sintering methods. The microstructural, thermal, and microhardness properties of the milled powders and sintered bulk alloy samples were examined by X-ray diffraction (XRD), field emission scanning electron microscopy with energy dispersive X-ray (FESEM-EDX), differential thermal analyser (DTA), optical microscope (OM), and Vickers hardness tester. XRD results revealed that the intermetallic and solid solution phases which consist of body-centered cubic (BCC) and face-centered cubic (FCC) structured phases were formed after 80–120 h of milling. It was found that the crystallite size decreased to ⁓5.67 nm, the lattice strain increased and the dislocation density increased to ⁓31.14 × 1015/m2 after the increase in Ti ratio and milling up to 120 h in the produced samples. FESEM/EDX analyses also revealed that as the mechanical milling time increased, fracture-fragments, welds, re-fractures, and agglomerations occurred, the particle size decreased to a minimum of 8 μm, and a more homogeneous structure containing spherical-shaped particles was formed. DTA curves showed an exothermic peak indicating the crystallization of the Al–Cu-based FCC solid solution phase between 550 °C and 600 °C temperatures in the samples milled up to 30 h, while it did not show any reaction peak in the samples milled up to 120 h. Additionally, it was observed that the mechanically milling time and sintering, and adding Ti led to an increase in the microhardness of the bulk alloy samples due to the deformation hardening, and the appearance of new intermetallic phases and FCC + BCC solid solution phases. Finally, after 120 h of milling and sintering at 815 °C, the maximum microhardness of the 8 % Ti-doped AlCoCuNiTi alloy was measured as 882 ± 20 HV1, which indicates that its mechanical properties are improved.