Transition from High-Entropy to Conventional Alloys: Which Are Better?

The study of the transition from high-entropy alloys (HEAs) to conventional alloys (CAs) composed of the same alloying components is apparently important, both for understanding the formation of HEAs and for proper evaluation of their potential with respect to that of the corresponding CAs. However, this transition has thus far been studied in only two types of alloy systems: crystalline alloys of iron group metals (such as the Cantor alloy and its derivatives) and both amorphous (a-) and crystalline alloys, TE-TL, of early (TE = Ti, Zr, Nb, Hf) and late (TL = Co, Ni, Cu) transition metals. Here, we briefly overview the main results for the transition from HEAs to CAs in these alloy systems and then present new results for the electronic structure (ES), studied with photoemission spectroscopy and specific heat, atomic structure, thermal, magnetic and mechanical properties of a-TE-TL and Cantor-type alloys. A change in the properties of the alloys studied on crossing from the HEA to the CA concentration range mirrors that in the ES. The compositions of the alloys having the best properties depend on the alloy system and the property selected. This emphasizes the importance of knowing the ES for the design of new compositional complex alloys with the desired properties.

Multimodal characterization of the bone-implant interface using Raman spectroscopy and nanoindentation.

Titanium implants are widely used in dental and orthopedic surgeries. Osseointegration phenomena lead to direct contact between bone tissue and the implant surface. The quality of the bone-implant interface (BII), resulting from the properties of newly formed bone, determines the implant stability. This study investigates the BII properties using a dedicated in vivo implant model consisting of a coin-shaped Ti-6Al-4V implant inserted in a rabbit femur for 10 weeks. A gap created below the implant was filled with newly formed bone tissue after healing. The properties of mature and newly formed bone tissues were compared using: i) Raman spectroscopy to assess the nanoscale compositional bone properties and ii) nanoindentation to quantify microscale elastic properties in site-matched regions. The mineral-to-matrix ratio, crystallinity (mineral size and lattice order), and the collagen cross-link ratio were significantly lower in newly formed bone tissue (e.g., a mineral-to-matrix ratio of 9.3 ± 0.5 for proline 853 cm-1) compared to mature bone (15.6 ± 1). Nanoindentation measurements gave Young's modulus of 12.8 ± 1.8 GPa for newly formed bone and 15.7 ± 2.3 GPa for mature bone. This multimodal and multiscale approach leads to a better understanding of osseointegration phenomena.

Influence of roughness and tilt on nanoindentation measurements: A quantitative model.

A model, which takes into account the indentation size effect, was proposed to relate hardness and its standard deviation to the quality of the sample surface, i.e., the roughness and the tilt. Stainless steel samples were mechanically polished so as to obtain various roughness and tilt. Then the quality of the surface and the hardness were measured respectively by scanning probe microscopy and by nanoindentation. The model was fitted according to those experimental data. It was shown that the standard deviation only depends on the ratio of the arithmetic roughness over the contact depth of the indent while the average hardness also depends on the contact depth and on the tilt. It was deduced that hardness measurements are performed with an uncertainty lower than 10% if: (i) the ratio of the arithmetic roughness over the contact depth of the indent is lower than 0.05, (ii) the tilt of the sample is lower than 2° and (iii) the difference of roughness between samples is lower than 50%.