Atomic structure of biodegradable Mg-based bulk metallic glass

Abstract

We have used highly accurate first-principles molecular dynamics simulations to elucidate the structure of Mg 60 Zn 35 Ca 5 and Mg 72 Zn 23 Ca 5 bulk metallic glasses, which are candidate materials for biomedical implants; these two compositions exhibit different behaviours when implanted. The environments of each species are different, and average coordination numbers are ∼ 13 for Mg, ∼ 11 for Zn and ∼ 18 − 19 for Ca. A wide range of local environments were found and icosahedral motifs, often seen in bulk metallic glasses, were among the most common for both Mg and Zn.Through the computation of a chemical short-range order parameter, a moderate avoidance of ZnZn bonding over Zn-Mg or Zn-Ca was observed. No statistically significant difference in structure was observed between the two compositions.

Atomic structure of Mg-based metallic glasses from molecular dynamics and neutron diffraction

“…The metallic, r m , and covalent, r c , bond distances, 25,26 bond distance values averaged for the five compositions, (r ij ) aver , and their deviations from r m and r c are also given across this range of composition. This is contrary to other studies 8,13,14,29 where, for example, the Zn-Zn interatomic bond distance changes significantly, by up to 0.1 Å. The average interatomic distances, (r ij ) aver , are generally slightly shorter for the Mg-rich compositions we consider in this system than for the Ca-rich compositions considered in other work, 29 which implies a more dense and efficient packing, as we discuss below in Section 3.3.…”

“…The cooling rate of these glasses is 0.7 K ps À1 , which is considerably slower than that used in ab initio MD simulations of similar BMG. 8,13,14 Although this is still orders of magnitude larger than cooling rates achieved experimentally, simulations using cooling rates of this order have been successful in generating glass structures in agreement with experiment. [20][21][22][23][24] Whilst the structures generated using this methodology are in agreement with experimental values, some properties of the glass, such as glass transition temperature, are more difficult to reproduce using simulation methods.…”

“…As Zn content increases, the first peak of G Zn-Zn (r) and G Zn-Ca (r) sharpens and becomes more intense; similar features were found in ab initio MD simulations of these and related Mg-Zn-Ca glass compositions. 13,14 The G Ca-Ca (r) partial PDF has considerable variations in peak shape at larger distances (6-12 Å). This effect has never been considered before as the amount of Ca atoms in models generated with ab initio MD is very low, so that the Ca-Ca partial PDF is liable to strong statistical noise.…”

“…Due to the potential large impact of these glasses, this has attracted substantial attention, but the structural studies of Mg-Zn-Ca metallic glasses are far from complete. Several recent works showed that the short-range order (SRO) in these glasses is formed to some extent with icosahedral and icosahedral-like structural units, 8,[11][12][13][14] but it is clear that distinguishing the subtle changes in SRO with glass composition is difficult. Based on ab initio molecular dynamics (MD) it was also proposed that a percolated network formed with Zn-centered structural units in Zn-rich Mg-Zn-Ca glasses (i.e.…”

“…13 Much of this previous work relies on ab initio MD. 8,11,13,14 Ab initio MD has the great advantage that is free from a priori established assumptions about interatomic interactions, but the associated very high computational expense means that it is restricted to small models, typically a few hundred atoms. This prohibits the possibility of analysing the structure at larger length scales.…”