The second phase behavior in magnesium alloy MA14 under multistep isothermal forging and further isothermal rolling

D.R. Nugmanov, O.S. Sitdikov, M.V. Markushev

Abstract

Methods of transmission and scanning electron microscopy were used to investigate an influence of the processing, involving severe isothermal deformation with stage-by-stage decreasing temperature in the range of 200-400 °C, realized by multidirectional forging and subsequent rolling, on distribution and size of the second phases in a commercial rod of MA14 (Mg-6Zn-0,6Zr) alloy. It is established that forging at 400 and 3000C with a total true strain of е ~10 practically eliminates the string type structure of coarse excess phases peculiar to the pressed semi-finished products. Besides it considerably reduces the size of these particles with no changes in their volume fraction, preserving at a level of 3-4%. Decreasing the temperature of forging up to 2000C, below the solvus point of the main strengthening -phase (MgZn2), intensifies decomposition of the supersaturated by Zn magnesium solid solution. The processes of excess phase refinement and solid solution decomposition with precipitation of near 8% of phases were continued at rolling under the same temperature conditions. They resulted in anisotropic character of phase distribution in the form of secondary string structure of the coarsest excess phases elongated in the rolling direction. Besides, the strong anisotropy in size and densities of -phase plate-shape precipitates, as in interparticle spacing, determined by direct measurements in long and transverse sheet sections, was formed. It was concluded that the last factor is the main reason for the yield strength anisotropy that observed under ambient temperature tension in the ultrafine-grained alloy sheet processed via mentioned complex treatment.

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