Microstructural Constituents, Local Hardness Contrast, and Tensile Variability of As-Cast AM50 Magnesium Alloy

Commercially produced as-cast AM50 magnesium alloy was investigated to assess the relationship between cast microstructural heterogeneity, local hardness contrast, tensile variability, and fracture behavior. Optical microscopy (OM), scanning electron microscopy coupled with energy-dispersive X-ray spectroscopy (SEM/EDS), X-ray diffraction (XRD), semiquantitative image analysis, hardness testing, tensile testing, and fractography were employed. The microstructure was dominated by an α-Mg matrix containing interdendritic Al-enriched constituents consistent with β-Mg17Al12 and Al–Mn-rich particles. XRD confirmed α-Mg and β-Mg17Al12, whereas the Al–Mn-rich constituents were supported by SEM/EDS but could not be reliably indexed by XRD. Image analysis of eight optical fields yielded an apparent area fraction of 27.6 ± 1.1% for dark interdendritic/secondary regions; this value represents combined etched microstructural constituents rather than an individual phase fraction. Vickers microhardness showed a pronounced local contrast between the matrix (39.1 ± 5.3 HV) and constituent-rich regions (119 ± 12 HV). Tensile testing of six specimens extracted from comparable upper regions of different commercial ingots yielded an average ultimate tensile strength of 129.42 ± 17.63 MPa, while recorded fracture strain showed substantially greater relative scatter (coefficient of variation ≈122%) than strength. Fractography revealed mixed ductile–brittle features, including dimples, tear ridges, and localized brittle or rough regions. The combined results show that intermetallic-rich regions contribute to local mechanical heterogeneity, but their presence alone is insufficient to explain the tensile variability. Casting-related discontinuities, sampling history, and between-ingot variability must also be considered.