Influence of heat treatment and severe plastic deformation (SPD) ON Zn-3Mg alloy properties for bio-implant application

Abstract

Unlike permanent implant materials, biodegradable metallic implants can avoid a revision surgery for implant removal. Recently, Zn and its alloys have received a lot of attention as an alternative to Mg-based alloys, especially for temporary implant applications such as fractured bone fixation devices. Advantage of zinc, apart from its significance for many biological functions, it also supports wound healing and exhibits superior degradation performance in physiological environment than Mg-based alloys. Earlier investigations have shown that, Zn-based alloys have limitations on its strength and ductility. In addition, the biocompatibility status of this alloy is also unknown. This work attempts to improve these properties, particularly on Zn-3Mg alloy via heat treatment followed by severe plastic deformation technique, i.e equal channels angular pressing (ECAP). Eutectic Zn-3Mg alloy samples were prepared using the casting process. During casting, solidification behaviours were analysed to determine the feasible range of heat treatment temperature. Heat treatment was conducted using a vacuum tube furnace at 370?C for 5, 10, 15 and 25 hours dwelling time followed by quenching in three different media: water bath, inside the furnace and open air to room temperature. Corrosion behaviours of the untreated and treated alloy were evaluated using electrochemical polarisation and immersion methods. The experiments were conducted in Kokubo simulated body fluid (SBF). Apart from mechanical properties (hardness, tensile and compressive strengths), the samples were subjected to cytotoxicity test. As-cast microstructure consisted of star-like dendrites of Zn-rich and rectangular structures of Zn2Mg11 phases dispersed in segregated pattern. These phases were partially dissolved after heat treatment and became more homogenised. It was noticed that the ductility of the alloy improved by 64 % while the strength reduced by 45 %. A remarkable decrease in grains size up to 96.34 % was observed after the cast alloy was subjected to heat treatment followed by 2-ECAP passes. In addition, other properties such as ultimate tensile strength, yield strength and elongation were substantially increased by 2.63 fold, 3.15 fold and 4.98 fold respectively. Improvements on strength and ductility were attributed to the combined influence of microstructural changes, elimination of dendrite structure, as well as the existence of high-volume density of dislocations that occurred on the refined microstructure during 2-ECAP pressing. Assessment of corrosion showed that the corrosion rate decreased from 0.269 to 0.188 mm/year after the cast alloy was severely deformed. This was attributed to improved microstructure homogeneity and reduction in casting defects. The study also reveals that extract of Zn-3Mg alloy exhibits good biocompatibility towards normal human osteoblast cells (NHOst) in low concentration (<0.5 mg/ml). The proposed hybrid processing method seems able to enhance the properties of developed Zn-3Mg alloy after 2-ECAP passes. These encouraging findings would improve the prospects of Zn-3Mg alloy as a new alternative metallic biodegradable implants material.