Design and characterization of high and medium entropy alloys for biomedical applications

Abstract

High entropy alloys (HEAs) are recently attracting the metallurgy community because the vast unknown region of multicomponent alloys is yet to be explored. In this space, we can find alloys that can surpass any other conventional alloy regarding mechanical properties at high or at cryogenic temperatures. The field of fcc HEAs is explored extensively and these alloys have excellent strength and ductility at room temperature. However, HEAs having BCC structures are less studied due to their poor ductility at room temperature. This area is important for both academic as well as for commercial applications as we expect once we are fully able to understand the governing mechanism of these alloys, we can design alloys of commercial importance. There is a need for a novel solution to improve the ductility of BCC HEAs. On the other hand, BCC HEAs fabricated using biocompatible elements gives superior mechanical and electrochemical properties to commercial biomedical alloys. In this study we targeted the aforementioned issue of improving ductility while having a satisfactory strength in BCC HEAs, however, putting two constraints: first, only biocompatible elements (MoNbTaTiZr system) will be used

second, while improving the mechanical properties of the alloy elastic modulus should also be decreased. We have introduced three main ways to improve the mechanical properties of the MoNbTaTiZr system while considering the biomedical aspects of the alloy as well: i) not changing the equiatomic composition of the system but refining the harder phase of this two-phase HEA, ii) tailoring the composition so that we can find an intrinsically ductile alloy, iii) incorporating compositional heterogeneity in the system in the light of first two studies to delocalize the strain. In the first method, we refined the harder phase in the MoNbTaTiZr system as this system consists of two phases. This approach gave us higher strength as well as higher plasticity, but under compression only. Henceforth, we recommended that particles of this alloy can be used as reinforcements. Moving on to the second method, we systematically decreased the content of the element with highest shear moduli and which contributes the most to the solid solution hardening effect, in such a way the average valence electron concentration of the alloy is decreased. By decreasing the VEC, just similar to Pugh’s ratio (G/B) or Cauchy pressure criteria, we can find the intrinsically ductile composition. This way we were able to find a composition that has high tensile yield strength (1060 ± 18 MPa) and decent tensile ductility (~20%). We have described the increase in ductility of the system with help of generalized stacking fault energy along with the most common observable {112}<111> slip system. To address the lowering of elastic modulus of alloys, we used the concept of heterostructure: incorporating heterogeneities in the microstructure to delocalize the strain and hence improving mechanical properties, but not the elastic modulus as we used the medium entropy alloy which has a low modulus (Ti-33Nb-25Zr). To incorporate chemical heterogeneity, we mixed HEA powder from the first approach in the Ti-33Nb-25Zr matrix. The formation of a tailorable chemical gradient at the interface of HEA and Ti-33Nb-25Z allowed us to increase the strength as well ductility of the alloy while slightly increasing the elastic modulus of the alloy. This way we were able to increase the strength up to 400 MPa as well as increase the wear resistance of the alloy over commercial titanium alloy. This study gives a systematic investigation of the MoNbTaTiZr system only, however, the concept tried here can also be applied to other similar systems. We have suggested ways to improve ductility in BCC multicomponent alloys. Moreover, we have described the role of gradient interface in heterostructure alloy and how we can increase the strength of the system without affecting the elastic modulus considerably. In the end, we tested the biocompatibility of our alloy using both in vivo and in vitro approaches. The multicomponent alloys have shown non-toxic behavior in skeletal muscles of mice. The cell culture test has also shown a higher cell density on the multi-component composition.