Mechanical Properties of AZ91D Magnesium Alloy with Short Carbon Fibers Under Heat Treatment and Equal-Channel Angular Pressing

AZ91D is a lightweight, representative commercial magnesium alloy known for its excellent castability and specific strength. However, the mechanical properties of as-cast AZ91D remain limited by inherent brittleness, relatively low strength, and microstructural inhomogeneity caused by enrichment of secondary phases at grain boundaries. In this study, AZ91D/Csf (short carbon fiber at 0, 2.5, and 5 wt.%) composites were prepared by gravity casting with mechanical stirring, followed by post-casting T4 heat treatment and equal-channel angular pressing (ECAP). Material characterization included optical microscopy (OM), field-emission scanning electron microscopy (FESEM), energy-dispersive spectroscopy (EDS), X-ray diffraction (XRD), X-ray photoelectron spectroscopy (XPS), uniaxial tensile testing, and microhardness tests. The results demonstrate that T4 treatment reduced intermetallic β-Mg17Al12 segregation and homogenized the microstructure, whereas one-pass ECAP further refined the grains. Mechanically, these two processes enable the (AZ91D/5 wt.% Csf) composite to achieve higher ultimate tensile strength (280.7 MPa by T4/280.2 MPa by T4 + one-pass ECAP), larger maximum strain (12.1% by T4/5.4% by T4 + one-pass ECAP), and higher microhardness (63.9 HV by T4/78.1 HV by T4 + one-pass ECAP). Compared to as-cast AZ91D, these findings demonstrate that T4 treatment provides a better strength–ductility balance via solid solution, whereas one-pass ECAP preferentially enhances surface microhardness by plastic deformation. This study highlights the performance of AZ91D/Csf composites and their potential for lightweight, high-strength-demand applications.