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Effect of Li Addition on the Plasticity of AZ31 Mg-Alloy

Electronic Theses of Indian Institute of Science

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Title Effect of Li Addition on the Plasticity of AZ31 Mg-Alloy
 
Creator Govind, *
 
Subject Magnesium Alloys
AZ31 Magnesium Alloys
Plasticity
Magnesium-Lithium Alloys
AZ31 Lithium Alloys
AZ31 Magnesium Alloys - Deformation
Dynamic Recrystallization
Processing Maps
AZ31 Mg-Alloy
Twinning
Dynamic Recrystallization (DRX)
Materials Science
 
Description Mg-alloys, despite being the lightest structural metallic materials, find limited applications due to their poor workability, which is due to the hcp structure that does not provide sufficient number of independent slip systems for compatible deformation. Workability improves with the increase in the deformation temperature, when non-basal slip starts playing a larger role in deformation. Efforts were made to improve the workability through control of texture, grain refinement and alloying. Alloying activates non-basal slip by decreasing the critical resolved shear stress (CRSS) on non-basal planes or by promoting cross slip through an increase in the stacking fault energy (SFE) on basal planes. In this thesis, the effect of Li addition to the most widely used wrought Mg-alloy AZ31 on its workability is examined. Plastic deformation behaviour of a series of AZ31-Li alloys with temperature, T, and strain rates, ε , as variables was studied, so as to identify the optimum Li content that results in highly workable alloy. The T and ε combinations that are best suited for hot deformation of these alloys were also identified through processing maps and microstructural analysis.
First, deformation behaviour of the base AZ31 is examined in detail. Compression tests were carried out, with T ranging between 150 and 400 °C and at ranging from 10-3 to 102 s-1, covering entire hot working range of the alloy. The results suggest that the deformation behaviour of AZ31 could be partitioned into three temperature regimes. In low T regime, twinning played an important role. It changes the orientation and increases hardening rate, θ (given by dσ/dε where σ and ε are true stress and strain respectively); material exhibits macroscopic flow localization and cracking along twin boundaries. The onset of twinning was examined in detail by examining the local maxima before ϵpeak strain in plot between d2σ/dε2 vs. ε. Twinning was found to occur at all the deformation conditions. Dynamic recrystallization (DRX) was observed at temperatures above 250 °C whereas deformation at low T (< 250 °C) led to extensive twinning at all . ε . At intermediate T of 250-300 °C, plastic strains tend to localize near grain/twin boundaries, confining DRX only to these regions. Increase in T promotes non-basal slip, which, in turn, leads to uniform deformation; DRX too becomes uniform. The dependence of critical stress (σc) for the onset of DRX and peak flow stress (σp) on Zener-Hollomon parameter (Z) indicates that these stresses increase with Z. Activation energy (Q) for the deformation of AZ31 was estimated at peak stress and steady state conditions. High values of Q (150-200 kJ/mol) indicate cross slip as the rate controlling mechanism, at the peak, in the stress-strain responses. For steady state, Q corresponds to lattice/grain boundary diffusion (90-150 kJ/mol).
Next, the effect of Li on deformation behaviour of AZ31 was examined. In addition to AZ31 without any Li (0Li), three alloys 1 (1Li), 3 (3Li) and 5 (5Li) wt% Li were prepared with the aid of a specially designed set-up for melting and casting of Li containing alloys. Experimental results on homogenized alloys show that 1Li alloy’s overall response is similar to that of 0Li alloy, but 3Li and 5Li alloys exhibit distinctly different deformation behaviour. Li addition facilitates cross slip by increasing SFE on basal planes, thus leading to change in the deformation mechanism of the alloy. Increased softening due to cross slip decreases θ and also the twin density at low ϵ (
 
Contributor Ramamurty, U
Sivakumar, D
 
Date 2017-11-14T09:46:27Z
2017-11-14T09:46:27Z
2017-11-14
2014
 
Type Thesis
 
Identifier http://etd.iisc.ernet.in/handle/2005/2755
http://etd.ncsi.iisc.ernet.in/abstracts/3620/G25988-Abs.pdf
 
Language en_US
 
Relation G25988