A set of embedded atom method model interatomic potentials is presented to represent a high-entropy alloy with five components. The set is developed to resemble but not model precisely face-centered cubic (fcc) near-equiatomic mixtures of Fe–Ni–Cr–Co–Cu. The individual components have atomic sizes deviating up to 3%. With the heats of mixing of all binary equiatomic random fcc mixtures being less than 0.7 kJ/mol and the corresponding value for the quinary being −0.0002 kJ/mol, the potentials predict the random equiatomic fcc quinary mixture to be stable with respect to phase separation or ordering and with respect to bcc and hcp random mixtures. The details of lattice distortion, strain, and stress states in this phase are reported. The standard deviation in the individual nearest neighbor bond lengths was found to be in the range of 2%. Most importantly, individual atoms in the alloy were found to be under atomic strains up to 0.5%, corresponding to individual atomic stresses up to several GPa.

In this work, corrosion-resistant fluoridated Ca–Mg–P composite coatings were prepared on magnesium alloys via a hydrothermal assisted sol–gel process. All these coatings derived from Coating Sols with different F− concentrations are composed of fluoridated hydroxyapatite, magnesium hydroxide, and dittmarite. When F− concentration of Coating Sol is 0.03 M, the coating exhibited uniform and dense surface, and its thickness reached 32 μm, thus possessing a high charge transfer resistance of 312 ± 12.69 kΩ cm2 in simulated body fluid (SBF). Immersion test in SBF showed that this coating could quickly induce the formation of the mineralized layer, implying relatively high bioactivity. After 49 days of immersion, the original composite coating and newly formed mineralized layer reached 60 μm in thickness, providing effective long-term protection for magnesium alloys. These attractive results indicate that this fluoridated Ca–Mg–P composite coating is a promising protective coating on biodegradable magnesium and magnesium alloy implants for orthopaedic applications.

BaTiO3-based lead-free piezoelectric materials have long been known as “a mediocre class of piezoelectric materials.” However, they have seen significant renewed interest in recent years ever since the discovery of high piezoelectricity in Ba(Zr, Ti)O3-(Ba, Ca)TiO3 as well as the related Ba(Sn, Ti)O3-(Ba, Ca)TiO3 and Ba(Hf, Ti)O3-(Ba, Ca)TiO3 systems. The unexpectedly high piezoelectricity in this class of BaTiO3 (BT)-based materials is still not well understood and has stimulated significant research activity. We present a concise discussion of the notions leading to high piezoelectricity in BaTiO3-based systems. In particular, the possible role of a multiphase-coexisting point is highlighted.

High-entropy alloys (HEAs) consisting of multiprincipal elements have demonstrated many interesting structural, physical, and chemical properties for a wide range of applications. This article is a review of the current theoretical research on the elastic parameters of HEAs. The performance of various ab initio-based computational models (effective medium and supercell approaches) is carefully analyzed. Representative theoretical elastic parameters of different HEAs, including single-crystal elastic constants, polycrystalline elastic moduli, elastic anisotropy, and Debye temperature, are presented and discussed. For comparison, simple mixtures of the elastic moduli of pure elements are calculated and contrasted with the ab initio results. The present work provides a reference for future theoretical investigation of the micromechanical properties of systems based on HEAs.

Piezoelectric ceramics generate strain through the intrinsic piezoelectric effect, the motion of ferroelectric domain walls, or through field-induced phase transitions. The enhanced piezoelectric properties observed in morphotropic solid solutions arise from several distinct, but interrelated, mechanisms associated with the near degeneration of the energy surface from cubic to spherical symmetry. The phenomenological theory of ferroelectricity is used to explain the thermodynamic origins of strain generation mechanisms in these solid solutions. The displacement of ferroelectric domain walls is an extrinsic contribution to the piezoelectric response that can be controlled by modifying the host material with small concentrations of dopants. The concept of “hardening” is introduced; hardening can be useful in applications where piezoelectric energy conversion and low energy loss are more important than large strain. The operative mechanisms of strain generation and energy conversion in technologically important lead-based and lead-free piezoelectric materials are summarized.

The piezoelectric properties of lead-free ferroelectric materials have been dramatically improved over the past two decades. For some limited applications, their properties have reached the same levels or have even surpassed the properties of the benchmark lead-based material Pb(Zr,Ti)O3 (PZT). Initial commercial lead-free products, including powders, ceramic components, films, and devices (e.g., ultrasonic cleaner, knocking sensor), are now available on the market. Several prototype devices, such as inkjet printheads, ultrasonic motors, angular sensors, and energy harvesters, have been developed. Their overall performance is still inferior to that of PZT-based devices; however, these prototypes and products point the way for future applications. Here, we provide an overview of recent industrial developments in the field and discuss the main advantages and disadvantages of lead-free piezoceramics for individual applications.

The focus on piezoelectric ceramics based on the potassium sodium niobate system began in 2004. After years of dedicated research, these materials can be considered one of the most promising lead-free piezoceramics with comprehensive performance. While their structure–property relationships are still not completely understood, the thermal stability issue is partly resolved, which leaves further room for phase-boundary engineering. Technological advancement has recently focused on using base metals as inner electrodes for multilayer actuators, which provides cost benefits as compared to lead zirconate titanate devices. The remaining challenges, however, such as poor sinterability and weak reproducibility of functional properties, still hinder extensive applications of these materials.