Deformation behavior in lightweight alloys: effects of rare-earth microalloying and carbon nanotube reinforcement

Fatma Mokdad

doi:10.32920/ryerson.14652621

Abstract

One of the most important strategies for improving fuel efficiency and reducing anthropogenic emissions is vehicle lightweighting by the use of lightweight materials such as Mg and Al alloys in the automotive industry. The structural application of these alloys inevitably requires the mechanical properties and their continuous performance improvement to meet the increasingly stringent safety and durability requirements. An effective method to enhance the deformation resistance is to alloy with rare-earth (RE) elements for Mg alloys or develop composites with the addition of reinforcement for Al alloys. The objective of this dissertation was to identify the effects of RE element and carbon nanotube (CNT) reinforcement on the deformation behavior, focusing mainly on the deformation mechanisms. The deformation behavior of a RE-free extruded AZ31 Mg alloy was first studied. It was observed that the propagation of distinct twin variants led to the confinement of the spaces constrained by the fine twin lamellas. Various double twinning structures acknowledged through atomistic simulations were experimentally observed via progressive electron backscatter diffraction (EBSD) analyses during stepwise compression. The vanishing of primary {1121} embryonic twins via the nucleation and growth of either single or multiple {1012} secondary extension twins was detected, and two new ladder-like and branching-like twin-twin interaction phenomena were observed. Then a low-RE containing Mg alloy was exploited via texture and cyclic deformation studies. The addition of 0.2 wt.% Nd in ZEK100-O Mg alloy led to a weaker basal texture in comparison with AZ31 Mg alloy. Fatigue life of ZEK100 alloy was longer than that of AZ31 alloy, due to a good combination of strength with ductility. Asymmetry of hysteresis loops was improved because of texture weakening and grain refinement, however anelastic behavior largely remained arising from the presence of twinning and detwinning. The last investigation involved deformation behavior of CNT reinforced Al composites where the addition of 2.0 wt.% CNT in a 2024Al alloy led to considerable grain refinement. Deformation resistance of the composite was effectively enhanced due to CNT load transfer, Hall-Petch strengthening, thermal mismatch and Orowan looping. In a nutshell, this work constitutes a valuable benchmark for understanding the factors affecting the performance of two lightweight alloys in the automotive and aerospace applications.

Highlights

Background and motivationBecause of the huge environmental concerns and mounting global energy demands in the transportation industry, vehicle lightweighting has been recently recognized as one of the most important methods to improve fuel efficiency and reduce anthropogenic climate-changing and human death-causing 1 emissions [1-5]
Finding alternatives to reduce their fuel consumption has been of major interest, attracting a great deal of research curiosity, especially since fuel efficiency was found to be ameliorated by ~6-8% for each 10% weight reduction [8]
Lightweight alloys such as Mg and Al have been of tremendous interest as well as factors mainly affecting the enhancement of their mechanical properties, such as rare earth (RE) microalloying and carbon nanotube (CNT) reinforcement

Summary

Introduction

Background and motivationBecause of the huge environmental concerns and mounting global energy demands in the transportation industry, vehicle lightweighting has been recently recognized as one of the most important methods to improve fuel efficiency and reduce anthropogenic climate-changing and human death-causing 1 emissions [1-5]. RE elements were reported to alternate the bonding energy between Mg and RE atoms, increasing the possibility of non-basal slip and inhibiting deformation twinning [15] Detailed information of such RE-Mg alloys under dynamic cyclic loading condition is still lacking and it is unclear how RE-elements impact the tensile-compressive yield asymmetry generally occurring in RE-free extruded Mg alloys, and to what extent the RE addition would affect the fatigue life. Several typical areas on the sample surface were observed after each compression, aiming to capture distinctive, typical and special twinning features from the polished sample surface It should be noted that, to avoid the loss of generality, locations L1 to L7 were indicated on the micrographs of Fig.4.1 after carefully examining the whole microstructure so as to identify the most typical and prevailing deformation features

Objectives

Discussion

Conclusion