High-Strength Low-Alloy (HSLA) Mg–Zn–Ca Alloys with Excellent Biodegradation Performance

Abstract

This article deals with the development of fine-grained high-strength low-alloy (HSLA) magnesium alloys intended for use as biodegradable implant material. The alloys contain solely low amounts of Zn and Ca as alloying elements. We illustrate the development path starting from the high-Zn-containing ZX50 (MgZn5Ca0.25) alloy with conventional purity, to an ultrahigh-purity ZX50 modification, and further to the ultrahigh-purity Zn-lean alloy ZX10 (MgZn1Ca0.3). It is shown that alloys with high Zn-content are prone to biocorrosion in various environments, most probably because of the presence of the intermetallic phase Mg 6 Zn 3 Ca 2. A reduction of the Zn content results in (Mg,Zn) 2 Ca phase formation. This phase is less noble than the Mg-matrix and therefore, in contrast to Mg 6 Zn 3 Ca 2 , does not act as cathodic site. A finegrained microstructure is achieved by the controlled formation of fine and homogeneously distributed (Mg,Zn) 2 Ca precipitates, which influence dynamic recrystallization and grain growth during hot forming. Such design scheme is comparable to that of HSLA steels, where low amounts of alloying elements are intended to produce a very fine dispersion of particles to increase the material's strength by refining the grain size. Consequently our new, ultrapure ZX10 alloy exhibits high strength (yield strength R p = 240 MPa, ultimate tensile strength R m = 255 MPa) and simultaneously high ductility (elongation to fracture A = 27%), as well as low mechanical anisotropy. Because of the anodic nature of the (Mg,Zn) 2 Ca particles used in the HSLA concept, the in vivo degradation in a rat femur implantation study is very slow and homogeneous without clinically observable hydrogen evolution, making the ZX10 alloy a promising material for biodegradable implants.

Producing Bulk Ultrafine-Grained Materials by Severe Plastic Deformation: Ten Years Later

“…This helps to avoid the detrimental effects of stress shielding on the bone tissue healing. 67 SPD processing ensures that the relatively low strength of as-cast Ti-Nb is significantly improved through grain refinement, 68,69 suggesting that this material still qualifies for bone replacement applications. However, the SPD parameters need to be chosen carefully to avoid the occurrence of deformationinduced new phases that may raise the Young's modulus of the alloy to undesirably high levels (see, for example, Panigrahi et al 68 ).…”

“…Mg-Zn-Ca alloys have proven to exhibit both these advantages at the same time. 69 With respect to the time necessary for tissue healing, however, these alloys may degrade too quickly and the concomitant hydrogen evolution is faster than the human body can absorb. The degradation was found to take longer with less alloyed Mg systems, 70 albeit at the cost of their strength.…”

Evolution of Mechanical Twinning during Cyclic Deformation of Mg-Zn-Ca Alloys

“…However, for biomedical applications, they have to meet many challenging mechanical, electrochemical and biological requirements. The Mg-Zn-Ca alloy system has been proven, offering a very attractive combination of functional properties including a reasonably high strength and ductility [4][5][6][7]. The strength and corrosion resistance can improve with small additions of Ca in Mg-Zn systems.…”

Building towards a standardised approach to biocorrosion studies: a review of factors influencing Mg corrosion in vitro pertinent to in vivo corrosion

“…Too rapid corrosion of the implant can cause issues stemming from (i) rapid ion release, (ii) excessive H 2 gas production forming gas pockets, (iii) local pH increases, and (iv) premature loss of structural integrity of the implant. The creation of hydrogen gas pockets has been of particular concern for biodegradable Mg implants [23,24], and the size of the gas pockets is linked to the samples' corrosion rate [25]. Ideally, the corrosion rate of the implant would match the rate at which the body tissue heals, as depicted in Fig.…”