Modification of Structure and Surface Properties of Hypoeutectic Silumin by Intense Pulse Electron Beams

Abstract

Methods of contemporary physical materials science are applied for the analysis of structural and phase states, tribological and mechanical properties of hypoeutectic silumin treated by electron beams with parameters as follow: energy density — 10–35 J/cm2, pulse duration — 10 µs, number of pulses — 3, pulse-repetition frequency — 0.3 Hz. The initial structure of silumin comprises grains of aluminium-based solid solution, eutectic grains, inclusions of silicon and intermetallic compounds with different shapes and sizes. Electron beam treatment (EBT) with energy density of 20–35 J/cm2 causes melting of the surface layer, dissolution of silicon inclusions and intermetallic compounds. A structure of high-speed cellular crystallization is formed, and submicro- and nanosize particles of the second phase are reprecipitated. An average size of crystallization cells are of 0.3–0.5 µm at the irradiated surface and of 0.4–0.8 µm on the lower edge of the layer with the cellular structure. The graded structure and phase states are analysed at a depth of up to 120 µm. The submicron grains of lamellar eutectic are detected at a depth of 15 µm. The lateral sizes of eutectic lamellae are within the range of 25–50 nm. The study indicates that nanohardness of irradiated silumin changes nonmonotonously and reaches its maximum at a depth of about 30 µm, which is approximately four times higher than hardness in the initial state. Hardness of the layer close to the irradiated surface (that is at a depth of about 5 µm) is higher by a factor of about 1.6 than that of as-cast silumin. The paper provides physical interpretation of the changes occurring in structure and properties during irradiation.

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