Microstructure and Properties of Hypoeutectic Silumin Treated by High-Current Pulsed Electron Beams

Abstract

The structural-phase states, microhardness, and tribological properties of hypoeutectic silumin after electron-beam treatment are studied by the methods of contemporary physical materials science. The object of the study is hypoeutectic АК10М2Н-type silumin containing 87.88 wt.% of Al and 11.1 wt.% of Si as the base components. The silumin surface is subjected to electron-beam treatment in six various regimes distinct in the density of electron-beam energy. The microhardness measurements of the modified silumin-surface layers enabled to determine three optimal impact regimes (with electron-beam energy densities of 25, 30, and 35 J/cm2), when the modified-layer microhardness exceeds that for the cast silumin. The obtained parameters are as follow: 0.86 ± 0.41 GPa for the cast state; 0.93 ± 0.52 GPa for 25 J/cm2; 0.97 ± 0.071 GPa for 30 J/cm2; 0.96 ± 0.103 GPa for 35 J/cm2. As found, the electron-beam treatment with the optimal parameters results in the formation of the surface whose mechanical and tribological characteristics sufficiently exceed corresponding values for the cast state of silumin. The atomic-force microscopy data correlate with the results on microhardness. The samples treated in the presented regimes are characterised with the fine-grained cellular structure and have the least roughness of the treated layer (of 17–33 nm) and substrate (of 45–57 nm) as compared to other regimes. As revealed, in the treated layer, the fine-grained, graded, and cellular structure is formed, and it transforms into the mixed-type structure when deepening away from the surface of treatment. Depending on the parameters of electron-beam treatment, the thickness of homogenized layer varies and reaches the maximum values of 100 m at the energy density of 35 J/cm2. As detected, the modified layer is free from intermetallides and consists of the nanocrystalline structure of cellular crystallization. As assumed, these two factors are responsible for the increased mechanical and tribological characteristics of the modified layer. The formation mechanism for structure of cellular and columnar crystallization consisting in the initiation of thermocapillary instability over the ‘evaporated substance/liquid phase’ interface is offered. The mathematical model of the thermal effect of electron beam on the silumin-surface layers is developed.

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