Development of ultralight and ultrafine grained Mg-Li-Ca alloy by compositional optimisation and severe plastic deformation

This PhD thesis aimed at developing a new superlight Mg-Li-Ca alloy with superior properties. Binary Mg-30Ca and Mg-14Li (wt %) master alloys were melted successively in the induction furnace to obtain a Mg-Li-Ca ternary alloy containing 3.99 % Li and 1 % Ca. The alloy is designated Mg-4Li-1Ca (LX41). The as-cast material was homogenised at 350° C for 2 hr followed by furnace cooling. The homogenised alloy was then independently processed via normal hot rolling (60 % reduction in thickness), newly designed two step rolling (TSR, 60 % reduction in thickness) and multi temperature equal channel angular pressing (ECAP, 4 passes). Samples of the alloy in each processing condition were further annealed at 350 °C for 15-90 min followed by air cooling. The microstructures of as-cast, homogenised, conventionally rolled (AR), two step rolled (TSR), Equal channel angular pressed (ECAP) and subsequently annealed conditions were examined by optical, scanning and transmission electron microscopy. It clearly showed the presence of α-Mg and eutectic (α-Mg + Mg2Ca) phases in as-cast and homogenised samples. Fine recrystallised grains along with twins were seen in AR structure which subsequently transformed to equiaxed grains structure upon annealing. TSR and ECAP microstructures showed extensive grain refinement leading to average grain size of 200 and 300 nm, respectively, along with substantial fragmentation of the Mg2Ca phase during deformation. LX41 alloy also showed significant change in the quality of the texture upon different processing. AR alloy showed significant pole splitting towards TD whereas TSR alloy showed substantial spread in the texture along the transverse direction (TD) of rolling. Annealing subsequent to rolling and TSR resulted in substantial weakening of basal texture (texture intensity of 3.7 multiples of random distribution (MRD)) and especially after 30 min of annealing subsequent to TSR (TA30 material), no pronounced texture persisted and a fine equiaxed grain structure with a variety of orientations away from basal pole occurred. ECAP led to development of an even weaker texture, the cross-section and extrusion direction having texture intensities of 3 and 4.5 MRD, respectively. Annealing subsequent to ECAP was found to stabilise the texture. In addition, an extra texture component developed in the pole figure suggesting activation of non-basal slip components to accommodate deformation in the polycrystal. Tensile samples corresponding to all processing histories were deformed to failure at room temperature at a strain rate of 10-4 s-1. ECAP processed alloy showed the greatest strength of 270 MPa, whereas TSR led to a strength of 250 MPa, while that after normal rolling was 208 MPa. Further improvement of the properties was achieved by annealing. Thus, TA30 material showed a remarkable strain hardening ability leading to a true UTS of 250 MPa (engineering UTS of 210 MPa), which is on par with true UTS after TSR (270 MPa). The main gain was a significant improvement in the ductility of the alloy upon 30 min annealing after TSR. The strain to failure achieved was as high as 18.1 % - larger than that after ECAP (11 %). Thus, this microstructural engineering in terms of a well designed TSR and annealing step opened up a new way of obtaining a favourable combination of strength and ductility for LX41 alloy. While investigating the bio-response of the material it was found that pH variation during bio-degradation was significantly changing with starting microstructure for a given time of immersion thereby showing the highest pH value for as-cast alloy and the lowest one for TSR alloy in the early stage of degradation (just after 3 days of immersion). Hence, the important role of microstructure in determining the biodegradation (corrosion) mechanism (uniform or localised) was demonstrated. Also, the release of H2 gas and Li+ and Mg2+ ions after 7 days of immersion was found to be controlled by varying the microstructure of the LX41 alloy. The measured levels of hydrogen and Li+ and Mg2+ ions after immersion in a simulated body fluid (EBSS) were found to be below tolerable levels in the human body. SEM-EDAX and XRD analysis of the surfaces of immersed specimens confirmed formation of hydroxyapatite (HA) containing Mg2+ and Li+ ions and it was also noted that the driving force for the formation of HA was governed by OH- ions released during degradation. Once HA with entrapped Mg2+ and Li1+ ions was formed, these ions, as well as Ca2+, acted as mediators between the cells and the alloy. Hence, as understood from the above discussion, the formation of a HA layer with entrapped Mg2+ and Li1+ ions plays a key role in enhancing cell response and thus, indirectly, promoting cytocompatibility. As the formation of HA is a function of the amount of OH- ions released, which is controlled by microstructure, there exist a strong microstructure-biodegradation-cytocompatibility relationship for LX41 alloy. Finally, development of the new ultralight Mg-4Li-1Ca alloy having an excellent combination of density, strength, ductility and cytocompatibility has resulted as a very promising candidate for use in biodegradable medical implants as well as for lightweight applications. The alloy is considered as a strong competitor to conventional structural Mg alloys of the AZ and ZK series processed in a similar way. Thesis submitted in partial fulfillment of the requirements for the degree of Doctor of Philosophy of the Indian Institute of Technology Bombay, India and Monash University, Australia