Effect of Thermomechanical Processing Induced Retained Strain on In Vitro, In Vivo Biodegradation Response and Cytocompatibility of Mg Alloy

The implications of retained strain on in vitro and in vivo degradation behavior as well as cytocompatibility of the Mg-4Zn-0.5Ca-0.8Mn alloy is comprehensively studied. The retained strain is induced in homogenized specimens by hard-plate hot forging (HPHF) at a temperature (523 K) lower than the recrystallization temperature of the alloy. The retained strain generated during deformation process deteriorated the corrosion response of the deformed alloy as compared to its homogenized counterpart. The strained area of deformed specimen with high dislocation density promoted defect generation (oxygen vacancies) in the film and facilitated preferential migration of ions, consequently leading to formation of a nonuniform product film with low protectiveness. In addition to strain magnitude, the distribution of retained strain also influenced the product film properties in the deformed specimens. An even distribution of retained strain improved the uniformity of the product film to certain extent by providing greater film coverage, resulting in higher film resistance. After 24 h of immersion, the protectiveness of the film was further improved in this specimen due to annihilation of defects through homogeneous ionic migration, which led to the development of a uniform and stable film that restricted further ionic diffusion. The dissolution of Zn(OH)₂ into Zn²⁺ ions was promoted at lower pH, resulting in enhanced antimicrobial activity in the specimen with the lowest degradation. Besides, the specimen with stable product film not only minimized the rate of further degradation but also served as an interface for new bone growth, as evident from in vivo studies.