A Study of Slight Addition of Silicon on Mg-Li Alloy
Date Issued
2010
Date
2010
Author(s)
Chou, Sheng-Yao
Abstract
Magnesium and magnesium alloys are used in a wide variety of structural and nonstructural applications. Because of their high specific strength and specific stiffness, great damping properties, they are applied to 3C products. It is commonly recognized that magnesium possesses poor formability because of its hexagonal close-packed structure. To make up for this shortcoming and further reduce weight, alloying magnesium with lithium of extremely low density, 0.534 g/cm3, can achieve both goals.
Mg–Li alloys exhibit two phase structures between 5.7 and11 mass% Li contents consisting of the Li-rich BCC-structured phase and the Mg-rich HCP-structured phase at room temperature. The single phase structure could exist for Li contents greater than 11 mass%. As the amount of Li added to the Mg–Li alloy increases, the phase still possesses HCP structure, but the crystal lattice axes ratio, c/a decreases such that slip between crystal planes become less difficult, the co-existence of the phase makes the Mg–Li alloy possible to be cold-worked.
Because of poor mechanical properties and work hardening of Mg–Li alloy, the structural applications of Mg–Li alloy are confined. Therefore, the thesis aims to investigate how alloying 1.2Si% into Mg-9 wt% Li(LZ91), influences the properties of the alloys, including the microstructure, the mechanical properties and the corrosion property. And using different process of work hardening such as extrusion, rolling, ECAP to investigate the relationship between the microstructure and mechanical properties, and determine the optimistic working process. Furthermore, using the TEM instrument to observe and analyze the structure of the precipitate.
Alloying the silicon elements to LZ91 alloy, because of the low solution ratio, the most silicon elements would precipitate as compound Mg2Si in the matrix. By 70％ cold-rolling, LZ91 alloy can improve their tensile strength as 40MPa and keep its ductility. In addition, the Mg2Si in the 3.5％NaCl solution would dissolve into SiO2 covering the MgO/ Mg(OH)2 thin film and reduces the corrosion rate of LZ91 alloy.
After annealing and rolling process of the LZS911extrusion, the α phase would precipitate thinner particles from the matrix and improve the ultimate tensile strength about 40 MPa. After different temperature, process and extrude angle of ECAE process, the grain refining would become more obvious as temperature decreasing, and the optimistic temperature of the extrusion is about 100℃.By increasing the angle and process of ECAE, α phase would separate to the discontinuous structure and the precipitate α phase would be denser. 90˚-100℃-8 would have the maximum of the UTS, about 200MPa.
Subjects
Mg-Li alloys
Mg2Si
Microstructure
Mechanical properties
Corrosion resistance
Annealing treatment
Equal channel angular extrusion
Type
thesis
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