Date of Award

January 2019

Document Type

Thesis

Degree Name

Master of Science (MS)

Department

Mechanical Engineering

First Advisor

Meysam Haghshenas

Abstract

Globally, magnesium (Mg), as the lightest metallic material, imparts a significant long term impact on the stipulation of lightweight structures in aerospace and automotive industries. However, the deformation behavior of magnesium at ambient (room) temperature is not acceptable for most of the structural applications because of its hexagonal closed pack (hcp) structure and limited active slip systems which result in an unacceptable level of brittleness (literally no formability at ambient temperature). Having said this, alloying Mg with an element with more active slip system in its crystalline structure (i.e. lithium with body center cubic crystalline structure) might be a solution to improve strength and ductility of the Mg. Addition of the lithium (Li) as the lightest element (density 0.54 g/cm3) in Mg (density ~1.74 g/cm3) results in enhanced plasticity producing ultra-light metallic alloys of Mg-Li with density of 1.35-1.65 g/cm3. The Mg-Li alloys are considered as the lightest metallic alloys which make them unique for many weight-saving applications.

Aluminum reinforcement in Mg-Li matrix develops the strength and corrosion resistance without introducing any second phase in the alloy. Because of its high solubility in Mg, aluminum can improve the strength without deterioration of density of Mg-Li alloy by means of grain size refining, solid solution hardening, and compound reinforcements. Although the alloy (Mg-Li-Al) possesses acceptable elastic modulus to density ratio, specific strength, damping capabilities, and electromagnetic shielding capability, extensive applications of Mg-Li alloys are still limited due to relatively low strength, poor corrosion resistance, and limited thermal stability.

In the present project a single phase Mg-3.5Li-Al (wt%) and a dual phase Mg-14Li-Al (wt%) are first thermomechanically processed through hot compression tests. Then microstructure, post thermomechanical properties, and hot compression stress-strain curves are studied. The main objective is to correlate the microstructure with the small scale properties and processing parameters (i.e. thermomechanical parameters like temperature and strain rate). The thermomechanical cycle consists of uniaxial compression at temperatures of 250°C, 350°C, and 450°C and strain rates of 1, 0.1, 0.01, and 0.001 /s using a Gleeble® 3500 thermal-mechanical simulation testing system. True stress-true strain curves plotted from the thermomechanical tests were used to assess the working behavior of the materials, to analyze and to understand the microstructure evolution which reflect intrinsic mechanical properties. Microstructural changes, hot compression stress-strain curves, and nanoindentation load/displacement response, to study post processing mechanical properties, for both alloys were discussed in detail in the present research.

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