Materials Science and Engineering Doctor (2019)
Magnesium has attracted attention worldwide because it is the lightest structural metal. However, a high strength-to-weight ratio remains its only attribute, since an intrinsic lack of strength, ductility and low melting temperature severely restricts practical applications of Mg. In this work, interface strain engineering of Mg with Nb was exploited to pseudo-morphically transform hexagonal closed packed (hcp) Mg into a stable body centered cubic (bcc) Mg at ambient pressures and the adjacent Mg/Nb interfaces were spaced within a few nanometers forming multilayered Mg/Nb nanocomposite. Structure and properties of pseudomorphic bcc Mg were studied and compared with bulk bcc Mg, nano-laminated bcc Mg and nano-laminated hcp Mg. These research activities obtained a fundamental understanding of pseudomorphic bcc Mg, as well as the effect of individual layer thicknesses on the mechanical response of Mg/Nb nanocomposites.By using physical vapor deposition (PVD) techniques, Mg-Nb multilayered nanocomposites were synthesized, over a range of layer thicknesses ranging from 2 nm to 50 nm. We encountered a mix phase region in Mg/Nb multilayers, and we found out that below 7-8 nm Mg was completely bcc. Transmission electron microscopy (TEM) and X-Ray diffraction (XRD) studies revealed that Mg in the 50nm/50nm and 20nm/20nm Mg/Nb nanocomposites was in its traditional hcp structure, while for the lower layer thicknesses of 5nm/5nm and 2nm/2nm Mg had undergone an interface strain induced phase transition from hcp to bcc structure. Structure and stability of the hitherto-unknown bcc Mg phase in the Mg/Nb multilayer nanocomposite were investigated under high pressures in a diamond anvil cell experiment using synchrotron radiation x-ray diffraction (XRD). The hcp Mg present in the larger layer thicknesses exhibited an hcp-to-bcc phase transformation at pressures greater than 44 GPa, and this pressure value was found to vary between the equal (1:1 Mg:Nb) and unequal (10:1 Mg:Nb) Mg/Nb nanocomposite thickness ratios. Additionally, the compressibility of the pseudomorphic bcc Mg structure under pressure was shown to be fundamentally different from the bulk (non-laminated) bcc Mg structure formed under high pressures. These results indicated that interface strain engineering, and an appropriate choice of the adjacent layer material, might be a viable pathway for tuning the structure and properties of the pseudomorphic bcc Mg structure. The mechanical response and deformation behavior of pseudomorphic bcc Mg in these multilayered nano-composites were also investigated using a combination of nanoindentation, in-situ SEM compression testing of micro-pillars, and in-situ SEM micro-tensile testing. Micropillar testing for three different orientations, with the interfaces oriented normal, parallel and oblique (45o) to the compression axis, enabled us to explore the anisotropy in the mechanical response of the multilayer system. We demonstrated that when introduced into a nanocomposite bcc Mg is far more ductile, 50% stronger, and retains its strength after extended exposure to 200 C, which is 0.5 times its homologous temperature. Both bcc Mg and hcp Mg nanolaminate displayed plastic anisotropy, where normal orientation (iso-stress condition) showed higher strength and hardening compared to parallel and inclined 450 orientations. In addition to ‘smaller is stronger’ effect, by introducing pseudomorphic bcc Mg phase in Mg/Nb nanocomposites, ~55% higher strain at instability was achieved for bcc Mg nanolaminates as compared to hcp Mg nanolaminates. Micro-pillar compression tests on two different pillar shapes (circular vs. square) exhibited no significant effect of pillar shape until the instability on the mechanical response, however post instability effects were observed. Comparison of tension-compression mechanical response in parallel orientation in Mg/Nb nanocomposites illustrated that the mechanical properties were more severely affected due to porosity in case of tension than in compression. Tension-compression asymmetry was not observed in both bcc and hcp nanolaminates.