Influence of the grain size and phase content on the mechanical behaviour of magnesium alloys for biodegradable implants. Gravity casting and SPS manufactured

Recent biomedical engineering development has made temporary implants not only possible but a certain widespread reality in the short term. These implants can be used in the healing process of bones or as vascular stents. Inside the patient, they are progressively degraded and absorbed, completely disappearing after fulfilling their purpose. Notable amongst their many advantages is the avoidance of an extraction surgery. Magnesium, an essential biological element with low density but good mechanical properties, similar to those of human bones, is a perfect candidate for implant applications. However, Mg has to be alloyed to tailor its corrosion and mechanical properties to match specific requirements and prevent premature mechanical failure. Unfortunately, a usual alloying element of Mg, aluminium, is suspected to be related to cancer and Alzheimer’s occurrence. Therefore Al-free Mg-based alloys are the scope of this work. In this sense, biocompatible alloying elements were added to Mg in small quantities intending to improve corrosion and mechanical behaviour thanks to the solid solution phenomenon, aiming to avoid detrimental excessive formation of secondary phases. Ca, Mn and Sr were the alloying elements used in this work, all three essential nutrients. More specifically, two ternary (Mg-Ca-Mn, Mg-Mn-Sr) and a quaternary (Mg-Ca-Mn-Sr) alloy systems were studied. Favourable alloy compositions were predicted and determined through thermodynamic calculations. Microstructural and mechanical behaviour analyses were performed on as-cast produced samples. After characterization, a correlation between mechanical properties, composition phase content and grain size was obtained. Based on this relation, recommended values for alloying elements were obtained. These are 0.5wt.% Ca, 0.5wt.% Mn and 0.75wt.% Sr. At these percentages solid solution strengthening occurs. A higher percentage of alloying elements leads to appearance of secondary phases that can be deleterious for corrosion or even mechanical resistance. Furthermore, the possibility of mechanical alloying followed by Spark Plasma Sintering (SPS) as a manufacturing method for these alloys was studied. The MgCa-Mn system was chosen for this purpose owing to the beneficial properties of Ca in the process. As a result some process parameters were optimized and near full density samples were produced.