development of spd techniques for producing ufg mg-zn-ca alloys with superior properties for medical purposes

In the last decade, numerous works have appeared on the use of severe plastic deformation (spd) methods for the formation of ultrafine-grained (ufg) structures in magnesium alloys [1-6]. for magnesium alloys, studies have shown that the use of spd methods can be beneficial in terms of both mechanical properties and corrosion behavior, which is necessary for their medical applications. however, due to the limited workability of magnesium alloys, their processing by spd is a difficult task since higher temperatures and lower strain rates are required. such processing conditions can retard or even suppress the formation of ufg structures. in this regard, most of the studies on spd processing of magnesium alloys were exploratory in nature and were performed on laboratory facilities using small samples. in this work, we will demonstrate the prospects for using spd processing to obtain magnesium alloys with superior properties for medical implants. along with a brief review of the effect of previously studied spd techniques on the strength, ductility and corrosion rate of mg and its alloys, this report will present new approaches to modify these properties. in this manner, first the effect severe straining on the microstructure of the mg alloys and therefore its properties will be discussed. on the other hand, as mentioned above, two of the most prominent challenges in the path of industrialized processing of mg alloys by spd techniques for medical applications include (a) lowering the processing temperature, and (b) mass production. hence, in this report, using the mg-zn-ca alloy as an example, the possibility of achieving a strength of more than 250-280 mpa with a ductility of more than 15% and a corrosion rate in ringer's solution of less than 1 mm/year by less-considered spd methods will be demonstrated. such superior properties could be achieved by recently developed spd techniques not only in small laboratory specimens of the alloy, but also in pilot semi-finished items – rods and strips, intended for the manufacture of various implants. while the effect of some spd techniques, such as ecap and hpt, on the microstructure and properties of mg and mg alloys are well discussed in the literature, in this report we focus on the use of ecap-conform and circular simple shear extrusion (csse) [7] techniques. the ecap-conform method has the ability to be used industrially and for mass production. also, by using this method, it is possible to have different combinations processing conditions including ecap at different temperatures and inter pass annealing time and temperatures in one production line. on the other hand, the investigations showed that simple shear extrusion (sse) technique, which is the origin of csse, results in the higher workability of mg and more homogenous ufg structure compared to ecap [8]. csse itself, has the privilege of having more capability of becoming industrialized. in this manner, these two techniques (ecap-conform and csse) seem to be promising for the mass production of mg alloys for biomedical applications.