Effect of Li Addition on the Optimum Thermo-Mechanical Processing Parameters of the Mg Alloy AZ31
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MAGNESIUM and its alloys hold considerable promise for widespread use in aerospace and automobile industries due to their low density and moderate strength.[1,2] One of the main drawbacks, which is preventing their widespread use, is their poor workability, which is attributed to Mg’s hexagonal close packed (hcp) crystal structure. Consequently, significant research efforts are being made to improve this specific aspect of Mg alloys. Prior research has shown that the workability of Mg alloys can be improved through the control of texture,[3–5] microstructural refinement via thermo-mechanical processing,[6–9] and through alloying.[5,10,11] Amongst the alloying options, Li appears to be the most suitable candidate as alloying with it does
GOVIND BAJARGAN is with the Foundry Technology Division, Vikram Sarabhai Space Centre, Trivandrum 695022, India. GAURAV SINGH and U. RAMAMURTY are with the Department of Materials, Indian Institute of Science, Bangalore 560012, India. Contact e-mail: [email protected] Manuscript submitted March 30, 2017.
METALLURGICAL AND MATERIALS TRANSACTIONS A
not impose a weight penalty (Li—0.58 g/cm3 vs Mg—1.74 g/cm3). It also improves the room temperature ductility of Mg as well improving its workability, both through the promotion of cross-slip that occurs due to a decrease in the critical resolved shear stress (CRSS) for non-basal slip.[9–14] The reduction in CRSS due to 1.8 wt pct Li addition is reported to be higher than that occurs due to the addition of Al and Zn.[13,15] First principles calculations by Han et al.[16] show the stacking fault energy (SFE) of the basal planes in Mg is enhanced (from 33 to 46 mJ/m2) by Li addition, which makes the cross-slip easier, whereas Al was found to decrease the SFE from 33 to 23 mJ/m2, leading to slip planarity that is undesirable. The term workability refers to the ease with which an alloy can be thermo-mechanically processed into the desired form without any defects—both external as well internal—in the workpiece. It is broadly partitioned into two independent characteristics, viz., intrinsic and extrinsic workabilities.[17] Extrinsic workability is process dependent like geometry of deformation zone, externally imposed stress state, both of which depend on the type of metal deformation process. On the other hand, intrinsic workability is a material property and is sensitive to the microstructure of the alloy, as well as the temperature,
strain, and strain rate imposed. In trying to reconcile with such a multi-parameter space, Ashby[18] proposed the use of the deformation mechanism map that displays the dominance of different micromechanisms of deformation and failure in the stress and temperature space. Raj[19] developed this idea further on the basis of cavitation, dynamic recrystallization (DRX), and adiabatic heating led shear localization mechanisms. As these maps are based on atomistic mechanisms, they require many fundamental parameters for generation, which are not readily available for many commercial alloys. Both Ashby and Raj
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