**1. Introduction**

#### *Biodegradable Mg Alloys*

Magnesium alloys, as the lightest structural materials, are becoming increasingly popular for numerous applications, especially for biodegradable implants. When the clinical function of permanent implants is served, they typically must be removed because of allergy problems and/or mechanical instabilities, e.g., the stress shielding effect [1], which arises from different Young's moduli of bone and implant materials. Such a removal can be avoided when using biodegradable materials for implants provided the degradation time can be adjusted to the healing time. The implants dissolve in the human body after fulfilling their purposes by a corrosion process initiated by the body fluids [2].

MgZn-based alloys have been proposed as very suitable biodegradable materials for load-bearing applications due to their low density, comparably high strength and low Young's modulus—coming close to that of bones and thus avoiding stress shielding.

In general, improvements in mechanical properties can be achieved through increased alloy content and/or precipitation formation. In the case of Mg, the first strategy is limited since the solubility of most alloying elements is limited [3,4]. Concerning the formation of precipitates in Mg-Zn systems, Mima and Tanaka [5] identified three important low-temperature ranges for Mg-Zn systems: (i) below 60 ◦C, the formation of stable Guinier–Preston (GP1) zones; (ii) 60–110 ◦C, the formation of stable rod-type and basal platelet-type precipitates along with unstable GP1 zones followed by growth of the former at the expense of dissolution of the latter; and (iii) above 110 ◦C, the formation of stable rod-type and basal platelet-type precipitates, the most stable ones being the rod type [6,7]. In a commercial Mg5.5Zn0.6Zr (wt%) alloy (ZK60), Orlov et al. [8] found intermetallic precipitates similar to the GP1 zones, as a result of special ageing conditions after plastic deformation.

Enhancing solid solution and/or precipitation for the sake of mechanical properties, however, increases the chemical reactivity and finally causes unacceptably large rates of corrosion in most environments [9]. Moreover, in the case of precipitate formation, it may exhibit a markedly enhanced Young's modulus, thereby increasing the shielding effect. For all these reasons, using plastic deformation for the generation of lattice defects acting as barriers to dislocation movement, is an interesting alternative aiming at higher mechanical properties [10]. Importantly, the elastic moduli in texture-free polycrystalline aggregates do not change during plastic deformation.

In comparison to fcc and bcc metals, the critical resolved shear stresses in slip systems of hcp metals have large variations. Therefore, in Mg and its alloys, plastic deformation occurs by slip and/or twinning on a much lower number of systems, which significantly limits the ductility, especially at low temperatures [11]. This problem can be overcome by deformation at elevated temperatures, but then the production of lattice defects becomes increasingly balanced by their thermally activated annihilation, resulting in a decreasing total number of lattice defects. A better way is to process the materials by methods of severe plastic deformation (SPD) [12–16]. Those provide an enhanced hydrostatic pressure that is prevalent during deformation which suppresses the formation of cracks and extends deformability. In many works, equal channel angular pressing (ECAP, [17–23]) was used to deform Mg and Mg alloys for the sake of hardening through refining the microstructure, but still, cracks were formed during deformation at room temperature (RT), and continuous deformation was possible only above 200 ◦C allowing for grain sizes beyond 0.5 μm. As HPT yields much higher hydrostatic pressures (up to 10 GPa) than ECAP (1.5 GPa), processing can be extended to 10–100 times larger strains, thereby providing grain sizes down to 100 nm at room temperature (RT) processing [24–28]. Therefore, in the present investigation of low-concentration biodegradable Mg alloys, HPT was applied; we were expecting substantial grain refinement through massive dislocation production, by redistribution of solutes and also by a high concentration of vacancies [29–31]. Zehetbauer [32,33] and especially Horky et al. [33] for two selected Mg-alloys reported that these deformation-induced vacancies can form agglomerates that inhibit the dislocation movement and therefore increase the macroscopic strength; however, an extensive study of this effect has not been performed yet.

It was therefore the aim of this work to investigate thoroughly the strengthening capabilities of five biodegradable Mg alloys with low alloy content through the formation of both deformation-induced defects, including vacancy agglomerates, and precipitates via severe plastic deformation and heat treatments. The biodegradable MgZnCa-systems chosen here only included mineral nutrients that are not harmful to the human body. With the alloy constituents Zn and Ca, precipitates were formed, which not only impeded dislocation movement and thereby increased both strength and work hardening, but also stopped grain growth during solidification and thermal treatments [29]. Due to the fact that Ca is less noble than Mg, low MgZnCa systems also provide—according to Hofstetter et al. [9]—desirably slow and homogeneous degradation behavior with the low Mg alloys selected here.
