Prediction on temperature dependent elastic constants of “soft” metal Al by AIMD and QHA

First-principles methods based on density functional theory(DFT)are nowadays routinely applied to calculate the elastic constants of materials at temperature of 0 K.Nevertheless,the first-principles calculations of elastic constants at finite temperature are not straightforward.In the present work,the feasibility of the ab initio molecular dynamic(AIMD)method in calculations of the temperature dependent elastic constants of relatively"soft"metals,taking face centered cubic(FCC)aluminum(Al)as example,is explored.The AIMD calculations are performed with carefully selected strain tensors and strain magnitude.In parallel with the AIMD calculations,first-principles calculations with the quasiharmonic approximation(QHA)are performed as well.We show that all three independent elastic constant components(C11,C12 and C44)of Al from both the AIMD and QHA calculations decrease with increasing temperature T,in good agreement with those from experimental measurements.Our work allows us to quantify the individual contributions of the volume expansion,lattice vibration(excluding those contributed to the volume expansion),and electronic temperature effects to the temperature induced variation of the elastic constants.For Al with stable FCC crystal structure,the volume expansion effect contributes the major part(about 75%~80%)in the temperature induced variation of the elastic constants.The contribution of the lattice vibration is minor(about 20%~25%)while the electronic temperature effect is negligible.Although the elastic constants soften with increasing temperature,FCC Al satisfies the Born elastic stability criteria with temperature up to the experimental melting point.

Thin films of binary amorphous Zn-Zr alloys developed by magnetron co-sputtering for the production of degradable coronary stents:A preliminary study

1.Introduction Vascular stents are endo-prostheses that allow the restoration of blood flow in case of obstruction of a vessel.A stent usually consists of a corrosion-resistant metal such as Nitinol,316L steel or a CoCr alloy[1].Many studies have been undertaken for the production of polymer degradable stents[2,3]or metallic stents[4,5]and,among these,devices made of pure zinc or crystallized alloy based on zinc obtained by molding and extrusion are promising in terms of biodegradability,biocompatibility and mechanical properties[6],but also concerning the degradation rate of the stent which should to be effective until arterial remodeling and tissue healing are complete[5].The objective of this preliminary study is the comparison of the degradation in simulated biological medium and the endothelial cytotoxicity of amorphous and crystallized thin ZnZr films according to their composition and structure.In this work,the thin layers were deposited on silicon substrates by magnetron co-sputtering.This technology is already used to make metal stents[7,8].