Improvements in elongation and tradeoffs in strength and ductility of several Mg sheet alloys through cyclic bending under tension and annealing

Abstract

This paper presents results acquired from experimental investigations into determining the influence of cyclic-bending-under-tension (CBT) and annealing on elongation-to-fracture (ETF) and tradeoffs in strength and ductility of three Mg sheet alloys: ZEK100, BioMg250, and Mg4Li. The CBT process imparts uniform deformation greater than achievable in simple tension (ST) incrementally by subjecting a sheet specimen to simultaneous tension with a crosshead motion and bending with a set of rollers reciprocating along the specimen. The space of process parameters including crosshead velocity and bending depth is explored initially to achieve the greatest ETF of ZEK100 alloy. Improvements in ETF of about 40% are attained using CBT relative to ST. Given the uniform deformation imparted by CBT to large plastic strains, tradeoffs in strength and ductility of the alloy are investigated next by subjecting the alloy sheets to a certain number of CBT cycles under the optimized parameters and subsequent annealing. Strength of the alloy is found to increase by a factor of 1.4 along the sheet strongest direction, the rolling direction, and a factor of 2 along the sheet softest direction, the transverse direction. Since the strength improved more along the soft direction than along the hard direction, the alloy anisotropy reduces. Significantly, the strength can increase for about 40% along the soft direction, while reducing the anisotropy and preserving at least 10% of the alloy ductility in every direction. Characterization of microstructural evolution using electron-backscattered diffraction and texture evolution using neutron diffraction revealed slip dominated deformation of the alloy. Similar processing and testing of BioMg250 and Mg4Li sheet alloys produced even better results in terms of enhancing elongation and improving the contrasting strength and ductility properties. Comprehensive data for the three alloys and insights from the investigations are presented and discussed.