Area of Research

Hip and Knee osteoarthritis bones and Total hip and knee joint replacement implants [2].

Components used in joint replacement are femoral stem, femoral head, and acetabular cup in these femoral head is one of the major component. Important properties required for a material to be used as bearing surfaces in joint replacement are biocompatibility, resistance to corrosion in bodily fluids, high wear resistance, and high hardness. Currently used materials for bearing surfaces are alumina/zirconia based ceramics and CoCrMo alloys. However, these materials are failing after 10-15 years of use. Due to the brittleness of ceramic, components lead to sudden failure and in case of CoCrMo alloys reports indicate that prolonged articulation results in nanometer sized wear debris. The large surface area of these nanoparticles enhances release of metal ion (Co ions) which triggers osteolysis leading to further degradation of THA. Further studies have shown that elevated Co ions concentration is associated with neurological, cardiac and endocrine symptoms. Due to the increasing demand on the joint replacements, the implants are now expected to serve for a longer without complications and failure leading to revision surgery. Hence there is a global need to improve the existing technology available and develop implants with better performance.

Recently, a new class of materials have been introduced in alloy world with compositional features of multi-component (five or more elements) at equi-molar ratio and are commonly known as “high entropy alloys (HEAs)”, due to their potentially interesting properties. Among different types of HEAs, the alloys consisting of non-toxic and non-allergenic refractory elements such as Ti, Ta, Hf, Nb, Zr, and Mo, known as refractory HEAs or RHEAs, could be potential candidates for biomedical applications. Currently a number of equiatomic and non equiatomic bio-RHEAs such as TiNbTaZrMo, TiTaNbZrHf Ti_2.6NbTaZrMo, and Ti_1.7ZrNbTaMo_0.3 etc., are developed. These alloys have high hardness, more compressive yield strength, excellent corrosion resistance and biocompatibility compared to ceramics and CoCrMo alloys. Although hardness and compressive yield strength of these alloys are well studied, a comprehensive understanding of tensile, wear, fatigue, and corrosion properties are missing. Moreover, a complete biocompatibility evaluation and correlating with corrosion data is not reported in literature. Therefore, within the scope of this work, novel bio-RHEAs will be developed, and their mechanical, corrosion, and biocompatibility characterizations will be carried out in detail.

Work done so far

To make an implant economical, it is necessary to decrease its melting point and density. To achieves these properties refractory high entropy alloys; such as Ti_xTa_2-xNbZrMo (X = 0.5, 0.75, 1, 1.25, 1.75) and Cr_xTiNbMoZr (X = 0.2, 0.4, 0.6) were designed by tailoring the alloy composition. After that, the solid solution formation ability of RHEAs was checked by the empirical parameters. Then phase stability and elastic properties of the developed RHEAs were studied through the DFT. For DFT studies special quasi-random structures (SQSs) were generated via the mcsqs program in the Alloy-Theoretic Automated Toolkit (ATAT). Then the accuracy of the present theoretical model was predicted by the constituent elements Wigner -Seitz radius and bulk modulus.

Further, the accuracy of RHEAs was calculated by the equilibrium WS radius which was calculated by using Vegard’s rule of mixtures. Then the phase stability of Ti_xTa_2-xNbZrMo (X = 0.5, 0.75, 1, 1.25, 1.75) RHAEs was calculated by using the energies of the fcc and hcp phases relative to that of the bcc phase, equilibrium Winger-Seitz radii, the equilibrium bulk moduli, and enthalpy of formation. Moreover, lattice constants and densities of the alloys were calculated theoretically and compared with the literature results.

Along with the above properties, mechanical properties such as single elastic constants are computed. With these single crystal elastic constants polycrystalline elastic moduli, Poisson's ratio, Pugh's ratio, Cauchy pressure, and Hardness were calculated by using Voigt-Reuss-Hill (VRH) averaging approximations. Further, the elastic anisotropy of present RHEAs was studied by using the Zener anisotropy ratio and elastic anisotropy ratio, and they were visualized by the 3D distribution plots.

Along with the DFT calculations, to study the deformation mechanism and tensile properties of developed alloy systems through the molecular dynamics simulations the embedded-atom method (EAM) interatomic potentials for RHEAs were developed. Then these interatomic potentials were parameterized with DFT results.

Presently, experiments on Ti_xTa_2-xNbZrMo (X = 0.5, 0.75, 1, 1.25, 1.75) refractory high entropy alloys are going on to validate first principles results. Moreover, DFT calculations on Cr_xTiNbMoZr (X = 0.2, 0.4, 0.6) alloys are also going on to study the elastic properties.

Future work

Future work is, the developed alloys mechanical properties will be studied through the molecular dynamics simulations and compared with experiments. Furthermore, the developed alloys will be fabricated via mechanical alloying and sintered with presser associated techniques (hot press (HP) and spark plasma sintering (SPS)) and studied their mechanical properties. In vitro corrosion properties and in vitro biocompatibility of RHEAs will be evaluated in simulated body fluid (SBF) and cell-materials interactions.

References

[1] M. Geetha, A.K. Singh, R. Asokamani, A.K. Gogia, Ti based biomaterials, the ultimate choice for orthopaedic implants - A review, Prog. Mater. Sci. 54 (2009) 397–425. https://doi.org/10.1016/j.pmatsci.2008.06.004.

[2] https://www.mayoclinic.org/diseases-conditions/osteoarthritis/symptoms-causes/syc-20351925

https://orthoinfo.aaos.org/en/diseases--conditions/arthritis-of-the-knee/

https://orthoinfo.aaos.org/en/treatment/total-hip-replacement

http://www.drrkmathur.com/knee-replacement.html