The state-of-the-art metallic biomaterials are 316L, CoCrMo and Ti6Al4V but they all suffer from known issues relating to biocompatibility, wear resistance and corrosion resistance. Therefore, there ...is always the motivation to identify novel superior metallic biomaterials to 316L, CoCrMo and Ti6Al4V. The concept of refractory high-entropy alloys (RHEAs) provides an interesting research direction towards developing novel metallic biomaterials, initially because RHEAs consist of purely biocompatible elements, but a systematic study of the performance of RHEAs targeting biomedical applications, while comparing to that of the state-of-the-art 316L, CoCrMo and Ti6Al4V, was not existing before and constitutes the theme of the current work. Two exemplary RHEAs that are studied in detail in this work, TiZrTaHfNb and Ti1.5ZrTa0.5Hf0.5Nb0.5, show highly promising characteristics as novel superior metallic biomaterials in that they possess a desirable combination of wear resistance, wettability and pitting and general corrosion resistance, outperforming 316L, CoCrMo and Ti6Al4V almost in all these important aspects. In addition, it is also shown in this work that how appropriate alloying in RHEAs can be utilized to fine-tune their performance as better metallic biomaterials, such as the correlation between lattice strain and corrosion resistance.
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•As-cast TiZrTaHfNb and Ti1.5ZrTa0.5Hf0.5Nb0.5 were composed of a single BBC phase.•Ti1.5ZrTa0.5Hf0.5Nb0.5 showed lower lattice strain than TiZrTaHfNb.•RHEAs exhibited higher H and E ratios compared to 316L, CoCrMo and Ti6Al4V.•TiZrTaHfNb and Ti1.5ZrTa0.5Hf0.5Nb0.5 revealed no pitting effect.•Ti1.5ZrTa0.5Hf0.5Nb0.5 has more resistance of barrier oxide than TiZrTaHfNb.
The Hf0.5Nb0.5Ta0.5Ti1.5Zr refractory high-entropy alloy with excellent corrosion resistance in the 3.5 wt% NaCl solution is identified in this work. This refractory high-entropy alloy exhibits much ...better general corrosion resistance than that of the 316L stainless steel, due to its corrosion current density being about one fifth of that in the latter. Meanwhile, the pitting potential of Hf0.5Nb0.5Ta0.5Ti1.5Zr reaches an unusually high value of +8.36 V, much higher than that of reported high-entropy alloys. The superior passivity of Hf0.5Nb0.5Ta0.5Ti1.5Zr is accredited to the formation of a single-phase solid solution containing high amount of homogenously distributed passivity-promoting elements, and also the existence of metallic Ta and OH− species in the passive film, which contribute to the high immunity to passive film breakdown.
•Hf0.5Nb0.5Ta0.5Ti1.5Zr contains finely distributed passivity-promoting elements.•Hf0.5Nb0.5Ta0.5Ti1.5Zr exhibits better general corrosion resistance than 316L SS.•Epit of Hf0.5Nb0.5Ta0.5Ti1.5Zr reaches +8.36 V, beating all other reported RHEAs.
The effect of low temperature on tensile properties of a AlCoCrFeNi2.1 eutectic high entropy alloy (EHEA) was investigated in the present work. Transmission electron microscopy (TEM) showed that the ...initial as-cast microstructure consisted of B2 (ordered body centered cubic structure) and L12 (ordered face centered cubic structure) phases. Upon tensile testing at temperatures ranging from room temperature (RT) to −196 °C (77 K), the L12 phase became disordered, changing to a simple FCC (face centered cubic) crystal structure whereas the B2 phase maintained an ordered structure. An increase over 300 MPa was observed in the ultimate tensile strength (σUTS) of the −196 °C tensile tested sample compared to the room temperature tensile tested sample, while keeping almost similar amount of total elongation. TEM results indicated an increase in the dislocation activity in the FCC phase as well as B2 phase in the −196 °C tensile tested sample compared to the sample deformed at room temperature.
•Low temperature tensile behavior of AlCoCrFeNi2.1 high entropy alloy was studied.•The −196 °C tensile tested sample showed the highest ultimate tensile strength.•Disordering of L12 structure was observed upon tensile deformation.
Refractory high-entropy alloys (RHEAs) are promising candidates for new-generation high temperature materials, but they generally suffer from room temperature brittleness and unsatisfactory ...high-temperature oxidation resistance. There currently lack efforts to address to these two critical issues for RHEAs at the same time. In this work, the high temperature oxidation resistance of a previously identified ductile Hf0.5Nb0.5Ta0.5Ti1.5Zr RHEA is studied. An accelerated oxidation or more specifically, pesting, in the temperature range of 600–1000 °C is observed for the target RHEA, where the oxidation leads the material to catastrophically disintegrate into powders. The pesting mechanism is studied here, and is attributed to the failure in forming protective oxide scales accompanied by the accelerated internal oxidation. The simultaneous removal of zirconium and hafnium can eliminate the pesting phenomenon in the alloy. It is believed that pesting can also occur to other equiatomic and non-equiatomic quinary Hf-Nb-Ta-Ti-Zr or quaternary Hf-Nb-Ti-Zr and Hf-Ta-Ti-Zr RHEAs, where all currently available ductile RHEAs are identified. Therefore, the results from this work will provide crucial perspectives to the further development of RHEAs as novel high-temperature materials, with balanced room-temperature ductility and high-temperature oxidation resistance.
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•High temperature oxidation resistance of Hf0.5Nb0.5Ta0.5Ti1.5Zr RHEA is studied.•Pesting in the temperature range of 600–1000 °C is observed.•Pesting leads the material to catastrophically disintegrate into powders.•Pesting due to not forming protective oxide scales & accelerated internal oxidation.•Simultaneous removal of zirconium and hafnium can eliminate pesting.
Refractory high-entropy alloys (RHEAs) emerge as promising candidate materials for ultrahigh-temperature applications. One critical issue to solve for RHEAs is their balanced oxidation resistance and ...mechanical properties, mainly room-temperature ductility for the latter. Recently, it was found that existing ductile RHEAs are subject to catastrophic accelerated oxidation, also known as pesting. In this work, both alloying and surface coating, are applied to enhance the oxidation resistance of ductile RHEAs, with the focus on surface coating using the pack cementation method and more specifically, aluminizing. The oxidation resistance of two RHEAs, Hf0.5Nb0.5Ta0.5Ti1.5Zr, one recently identified ductile RHEA which pests in the temperature range of 600–1000 °C, and Al0.5Cr0.25Nb0.5Ta0.5Ti1.5, the newly designed ductile RHEA which does not pest but embrittles after oxidation, are studied after aluminizing at 900 °C using three different pack components. Aluminizing, if using the appropriate pack cementation parameters, can avoid pesting in Hf0.5Nb0.5Ta0.5Ti1.5Zr and alleviate the oxidation induced embrittlement in Al0.5Cr0.25Nb0.5Ta0.5Ti1.5, and holds the promise for further improving the RHEAs as potential ultrahigh-temperature materials.
•Pack cementation using different pack compositions were experimented on two RHEAs.•Proper aluminizing can avoid pesting in the Hf0.5Nb0.5Ta0.5Ti1.5Zr RHEA.•Proper aluminizing can improve the embrittlement in Al0.5Cr0.25Nb0.5Ta0.5Ti1.5.•Aluminizing is promising to further improve RHEAs for high-temperature applications.
Refractory high-entropy alloys (RHEAs), or multi-principal-element refractory alloys, have been intensively studied in recent years, due to their attractive potential for ultrahigh-temperature ...structural applications. One formidable challenge facing the materials development for RHEAs though, is to simultaneously achieve good oxidation resistance and good mechanical properties, which unfortunately often impose contradictory microstructural requirements. So far, there exist no ductile RHEAs that possess reasonable oxidation resistance, and the formation of protective oxide scales such as alumina in them is never seen. Here we report, for the first time, a strategy to form protective alumina scale in ductile RHEAs upon high temperature exposure, using a tailor-designed two-step pack aluminizing process. Very importantly, the oxidation resistance of ductile RHEAs improves tremendously, as evidenced from harsh cyclic oxidation tests, thus providing a huge impetus to push forward the further development of RHEAs, towards ultrahigh-temperature applications.
•a two-step pack aluminizing process forms protective alumina scale in ductile RHEAs.•a TiAl3/TiAl/Ti2AlNb and substrate multi-layered gradient structure is achieved.•Crack-free TiAl3 coating forms on the top surface of Al0.5Cr0.25Nb0.5Ta0.5Ti1.5•The oxidation resistance of ductile RHEAs improves tremendously after aluminizing.
The present work is focused on synthesis and heat treatment on non-equiatomic AlCoCrFeNiTi0.5 high entropy alloy (HEA) with a composite structure reinforced by TiC nanoparticles. The initial alloy ...was prepared by mechanical alloying (MA) in a planetary ball mill, compacted by spark plasma sintering (SPS) and heat treated at different temperatures. Mechano-chemical reactions during the MA process as well as the microstructure and hardness of the SPS-ed compacts prior to and after the heat treatment were investigated. During MA, Cr-based supersaturated solid solution with the BCC structure was formed. After SPS at 1100°C, the BCC solid solution decomposed into nano-grained microstructure consisting of FCC and ordered BCC solid solutions, σ phase, and in-situ formed TiC nanoparticles. The high hardness of the alloy (762HV) was retained after the subsequent heat treatment at 1100°C (603HV). It was shown that the fabrication of TiC reinforced nanocomposites from elemental powders without the use of expensive nanograined powders can be achieved.
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•High Entropy Alloy nanocomposite was produced by powder metallurgy.•Formation of TiC nanoparticles by in-situ reaction was achieved.•The structure exhibited high hardness of 762HV.•Prominent hardness of 603HV even after heat treatment at 1100°C was retained.•Detrimental σ phase was successfully eliminated by the heat treatment.
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