Hardness–Deformation Energy Relationship in Metals and Alloys: A Comparative Evaluation Based on Nanoindentation Testing and Thermodynamic Consideration

Nanoindentation testing using a Berkovich indenter was conducted to explore the relationships among indentation hardness (H), elastic work energy (We), plastic work energy (Wp), and total energy (Wt = We + Wp) for deformation among a wide range of pure metal and alloy samples with different hardness, including iron, steel, austenitic stainless steel (H ≈ 2600–9000 MPa), high purity copper, single-crystal tungsten, and 55Ni–45Ti (mass%) alloy. Similar to previous studies, We/Wt and Wp/Wt showed positive and negative linear relationships with elastic strain resistance (H/Er), respectively, where Er is the reduced Young’s modulus obtained by using the nanoindentation. It is typically considered that Wp has no relationship with We; however, we found that Wp/We correlated well with H/Er for all the studied materials. With increasing H/Er, the curve converged toward Wp/We = 1, because the Gibbs free energy should not become negative when indents remain after the indentation. Moreover, H/Er must be less than or equal to 0.08. Thermodynamic analyses emphasized the physical meaning of hardness obtained by nanoindentation; that is, when Er is identical, harder materials show smaller values of Wp/We than those of softer ones during nanoindentation under the same applied load. This fundamental knowledge will be useful for identifying and developing metallic materials with an adequate balance of elastic and plastic energies depending on the application (such as construction or medical equipment).

Characterization and Application of Bioactive Glass and Chitosan Nanoparticles for Tooth Enamel Remineralization

We investigated the remineralization of demineralized enamel surfaces through the application of bioactive glass (BG) and chitosan nanoparticles (CNPs). BG and CNPs were immersed in artificial saliva for 7 days. The resulting crystals were analyzed by X-ray diffraction (XRD) and scanning electron microscopy (SEM). Embedded enamel blocks were immersed in demineralization solution and then classified into five treatment groups: (1) No material applied; (2) toothpaste containing NaF (F); (3) CNP hydrogel; (4) distilled water slurry containing BG; and (5) CNP hydrogel containing BG (BG-CNP). The enamel blocks were immersed in an artificial saliva solution for 1 month; each material was applied twice per day. Nanoindentation testing of enamel surfaces was performed during the immersion period. We found that unidirectional rod-shaped crystals formed in the artificial saliva solutions treated with BG and BG-CNP, showing multiple XRD peaks for hydroxyapatite. The mechanical properties of enamel surfaces decreased markedly following immersion in the demineralization solution, and significantly recovered after 1 month of BG-CNP and BG treatment compared to other the specimens. Porous demineralized enamel surfaces were filled with a remineralization layer after immersion in saliva and application of NaF, BG, and CNP-BG. Thus, daily application of CNP-BG or BG facilitates enamel remineralization.

Nanoindentation Test Conducted on Single Scan Tracks for the Development of New Multi-Principal Element Alloys

Urgent environmental challenges and emerging additive manufacturing (AM) technologies push research towards more performant and new materials. In the field of metallurgy, high entropy alloys (HEAs) have recently represented a topic of intense research because of their promising properties, such as high temperature strength and stability. Moreover, this class of multi-principal element alloys (MPEAs) have opened up researcher community to unexplored compositional spaces, making prosper literature of high-throughput methodologies and tools for rapidly screening large number of alloys. However, none of the methods has been aimed to design new MPEAs for AM process known as selective laser melting (SLM) so far. Here we conducted nanoindentation testing on single scan tracks of elemental powder blends and pre-alloyed powders after ball milling of AlTiCuNb and AlTiVNb. Results show that nanoindentation can represent an effective technique to gain information about phase evolution during laser scanning, contributing to accelerate the development of new MPEAs.

Experimental Correlation of Mechanical Properties of the Ti-6Al-4V Alloy at Different Length Scales

This article focuses on a systematic study of a Ti-6Al-4V alloy in order to extensively characterize the main mechanical properties at the macro-, micro- and submicrometric length scale under different stress fields. Hardness, elastic modulus, true stress–strain curves and strain-hardening exponent are correlated with the intrinsic properties of the α- and β-phases that constitute this alloy. A systematic characterization process followed, considering the anisotropic effect on both orthogonal crystallographic directions, as well as determining the intrinsic properties for the α-phase. An analytical relationship was established between the flow stress determined under different stress fields, testing geometries and length scales, highlighting that it is possible to estimate flow stress under compression and/or tensile loading from the composite hardness value obtained by instrumented nanoindentation testing.

Morphology and Mechanical Properties of 3Y-TZP Nanofiber Mats

The mats of yttria-stabilized tetragonal zirconia nanofibers were prepared using electrospinning. The effect of calcination temperature in the range of 600–1200 °C on their microstructure, phase composition and mechanical properties was investigated. Phase composition of the nanofibers did not change in all ranges of the calcination temperatures, while the average grain size increased from 8 to 39 nm. Nanoindentation testing of the mats showed a decrease in the hysteresis loop energy in samples with higher calcination temperature. Hardness and the elastic modulus measured with the indentation technique were the highest for the mats calcined at 900 °C.

Nanomechanical Characterization for Cold Spray: From Feedstock to Consolidated Material Properties

Cold gas-dynamic spray is a solid-state materials consolidation technology that has experienced successful adoption within the coatings, remanufacturing and repair sectors of the advanced manufacturing community. As of late, cold spray has also emerged as a high deposition rate metal additive manufacturing method for structural and nonstructural applications. As cold spray enjoys wider recognition and adoption, the demand for versatile, high-throughput and significant methods of particulate feedstock as well consolidated materials characterization has also become more notable. In order to address the interest for such an instrument, nanoindentation is presented herein as a viable means of achieving the desired mechanical characterization abilities. In this work, conventionally static nanoindentation testing using both Berkovich and spherical indenter tips, as well as nanoindentation using the continuous stiffness measurement mode of testing, will be applied to a range of powder-based feedstocks and cold sprayed materials.