## 1 Introduction  
Hypoeutic Al-Si alloys have garnered significant attention as high-performance materials capable of maintaining lightweight characteristics, high thermal conductivity, and an excellent wear and corrosion resistance even under elevated temperature conditions. Owing to these properties, they are extensively applied in components exposed to extreme service environments, such as those in the automotive, aerospace, and marine sectors [1, 2]. Especially, their ability to retain mechanical stability and long-termdurability under cyclic thermo-mechanical stress makes them promising candidates for next-generation lightweight structural applications, including engine blocks, pistons, and heat exchangers [3, 4, 5]. To fully optimize their performance in high-temperature damage environments, it is essential to go beyond mere compositional modifications. A systematic approach to microstructural engineering--encompassing precise control over the distribution of strengthening phases, stabilization of solute atoms in the Al matrix, and maintenance of interfacial coherence between phases--is required [6, 7, 8, 9].  
Among the various alloying elements, Copper (Cu) and Magnesium (Mg) are critical in determining the high-temperature strength, wear resistance, and corrosion behavior of Al-Si-based alloys, primarily by inducing solid solution strengthening and precipitation hardening, respectively [4, 8, 9]. Cu enhances the tensile strength and frictional resistance at elevated temperatures by forming substitutional solid solutions within the Al lattice, thereby introducing lattice distortion and impeding dislocation motion [10, 11, 12, 13]. In contrast, Mg contributes to high-temperature strength through the formation of thermally stable Mg\({}_{2}\)Si precipitates. However, the excessive precipitation or agglomeration of Mg\({}_{2}\)Si may result in microstructural inhomogeneity, localized embrittlement, and an increased susceptibility to corrosion [10, 13].  
Thus, the synergistic interaction between Cu and Mg significantly influences the thermodynamic and mechanical stability of Al-Si alloys--extending far beyond the effects of their individual concentrations [14, 15]. Nonetheless, most existing studies have focused primarily on the independent effects of either Cu or Mg or have been limited to first-order mechanical properties such as hardness and tensile strength [10, 11, 13]. Structural-property relationship studies examining the influence of the Cu:Mg ratio on the phase distribution, interfacial stability, and high-temperature wear resistance remain scarce. In particular, quantitative microstructural investigations capable of explaining the complex damage mechanisms under extreme thermal and mechanical environments are largely lacking.  
This study aims to quantitatively elucidate the relationship between microstructural stability and the high-temperature mechanical and tribological performance in heat-treated hypoeutectic Al-Si alloys by varying the Cu:Mg ratio. Two alloys were designed with Cu:Mg ratios of 3:1 (Alloy A) and 1:3 (Alloy B), and their phase distributions, solute behavior, and thermodynamic stability were systematically evaluated using X-ray diffraction (XRD), scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), and electron probe microanalysis (EPMA). Subsequent tensile, hardness, and wear tests were conducted at 25 \({}^{\circ}\)C, 150 \({}^{\circ}\)C, and 250 \({}^{\circ}\)C to identify the relationship between thermodynamic stability and the resistance to mechanical degradation. This study proposes a microstructural design roadmap for the performance optimization of Al-Si-based alloys and demonstrates that the precise control of the Cu:Mg ratio enables the simultaneous enhancement of the phase stability and damage resistance.