Correction for ‘Progressive changes in crystallographic textures of biominerals generate functionally graded ceramics’ by David Wallis et al. , Mater. Adv. , 2022, DOI: 10.1039/d1ma01031j.

Biominerals with gradually-changing crystallographic textures are an unrecognized class of elastically-graded materials that are intrinsically toughened by stress delocalisation.

Colloidal supraparticles are finite, spherical assemblies of many primary particles. To take advantage of their emergent functionalities, such supraparticles must retain their structural integrity. Here, we investigate their size-dependent mechanical properties via nanoindentation. We find that the deformation resistance inversely scales with the primary particle diameter, while the work of deformation is dependent on the supraparticle diameter. We adopt the Griffith theory to such particulate systems to provide a predictive scaling to relate the fracture stress to the geometry of supraparticles. The interplay between primary particle material and cohesive interparticle forces dictates the mechanical properties of supraparticles. We find that enhanced stability, associated with ductile fracture, can be achieved if supraparticles are engineered to dissipate more energy via deformation of primary particles than breaking of interparticle bonds. Our work provides a coherent framework to analyze, predict, and design the mechanical properties of colloidal supraparticles.

Ultrafine-grained and nanocrystalline materials frequently show an enhanced rate sensitivity of their mechanical behavior, which is generally assumed to be the consequence of interface sliding or thermally activated dislocation processes at the boundaries. Although this has been well documented on many different materials, the underlying mechanisms and their effect on the ductility of the material are still not well understood. Therefore, here, the deformation behavior of the ultrafine-grained, heterogeneous superplastic alloy Zn-22% Al was analyzed by small-scale nanoindentation and micropillar testing. The results show a breakdown of the superplastic deformation behavior in terms of a reduced strain rate sensitivity at small scales, which has not been reported before, although grain boundary sliding is still prevalent at the nanoscale. These results suggest that grain boundary sliding does not necessarily result in a high strain rate sensitivity and high ductility. Instead, a pronounced strain rate dependent flow behavior requires grain boundary sliding to be controlled by dislocation creep. IMPACT STATEMENT Superplastic flow is shown to persist down to a small material volume corresponding to a few grains. This has far-reaching consequences for the production of small-scale devices with complex geometries by superplastic forming.

Biomineralizing organisms are widely noted and extensively studied due to their ability to generate structures exhibiting exceptional crystallographic control. Primarily, it is the organisms, such as sea-urchins or bivalves, that generate nearly single-crystalline biocrystals that have attracted attention. In contrast, biomineralizing organisms with seemingly disordered polycrystalline bio-armor have been left relatively unstudied. However, the crystalline ordering in the black-lipped pearl oyster, Pinctada margaritifera, reveals that biominerals with varying crystal textures are an unrecognized class of functionally graded materials. Changing crystal textures inevitably cause a variation in Young modulus due to the orientation-dependent mechanical properties of crystals. The case of Pinctada margaritifera demonstrates that bioceramics with such crystallographical gradients are toughened by stress delocalization and reduced stress intensity factors, outperforming non-graded counterparts. These findings suggest that a multitude of biominerals, which are perceived as poorly ordered because of their polycrystallinity and changing crystal textures, may be considered as graded materials with hitherto unidentified emergent mechanical properties. The underlying design principle is remarkably simple and applicable to a wide range of crystalline material classes and may thus serve as a blueprint for future bioinspired functional materials.

Here we report the synthesis of metallic, ultraincompressible (bulk modulus K_0 = 428(10) GPa) and very hard (nanoindentation hardness 36.7(8) GPa) rhenium (V) nitride pernitride Re_2(N_2)N_2. While the empirical chemical formula of the compound, ReN_2, is the same as for other known transition metals pernitrides, e.g. IrN_2, PtN_2, PdN_2 and OsN_2, its crystal chemistry is unique. The known pernitrides of transition metals consist of a metal in the oxidation state +IV and pernitride anions N_2^4-. ReN_2 contains both pernitride N_2^4- and discrete N^3- anions, which explains its exceptional properties. Moreover, in the original experimental synthesis of Re_2(N_2)N_2 performed in a laser-heated diamond anvil cell via a direct reaction between rhenium and nitrogen at pressures from 40 to 90 GPa we observed that the material was recoverable at ambient conditions. Consequently, we developed a route to scale up its synthesis through a reaction between rhenium and ammonium azide, NH_4N_3, in a large-volume press at 33 GPa. Our work resulted not only in a discovery of a novel material with unusual crystal chemistry and a set of properties attractive for potential applications, but also demonstrated a feasibility of surmounting conceptions common in material sciences.

The remarkable mechanical performance of biominerals often relies on distinct crystallographic textures, which complicate the determination of the nanohardness from indentations with the standard non-rotational-symmetrical Berkovich punch. Due to the anisotropy of the biomineral to be probed, an azimuthal dependence of the hardness arises. This typically increases the standard deviation of the reported hardness values of biominerals and impedes comparison of hardness values across the literature and, as a result, across species. In this paper, we demonstrate that an azimuthally independent nanohardness determination can be achieved by using a conical indenter. It is also found that conical and Berkovich indentations yield slightly different hardness values because they result in different pile-up behaviors and because of technical limitations on the fabrication of perfectly equivalent geometries. For biogenic crystals, this deviation of hardness values between indenters is much lower than the azimuthal variation in non-rotational-symmetrical Berkovich indentations.

Due to their high ductility, superplastic metallic alloys are promising candidates for microforming processes. However, it is not yet clear if the macroscopically observed superplastic material behavior also persists at micrometric length scales. For this reason, the superplastic deformation behavior of the eutectoid alloy Zn-22% Al was investigated at the microscale as a function of temperature, strain-rate and grain-size. Nanoindentation strain-rate jump tests at room temperature and elevated temperatures were correlated with the deformed microstructure. A sigmoidal relationship between the hardness and the strain-rate is evidenced, which is typical for superplastic alloys at the macroscale. In the central part of the sigmoidal curve strong indications for superplastic flow at the microscale are found. Microstructural investigations of the residual indents reveal noticeable intercrystalline deformation processes in combination with some intracrystalline dislocation activity regardless of the applied testing conditions. These experimental findings were further coupled with finite element analysis to determine the size of the plastic zone beneath the indents as a function of the penetration depth. Based on this, a critical cubic material volume of 8 grains over the edge length is found to be sufficient for superplastic flow at local length scales.

The strain-rate sensitivity of the flow stress represents a crucial parameter for characterizing the deformation kinetics of a material. In this work a new method was developed and validated for determining the local strain-rate sensitivity of the flow stress at different plastic strains. The approach is based on spherical nanoindentation strain-rate jump tests during one deformation experiment. In the case of ultrafine-grained Al and ultrafine-grained Cu good agreement between this technique and macroscopic compression tests has been achieved. In contrast to this, individual spherical nanoindentation experiments at constant strain-rates resulted in unrealistically high strain-rate sensitivities for both materials because of drift influences. Microstructural investigations of the residual spherical imprints on ultrafine-grained Al and ultrafine-grained Cu revealed significant differences regarding the deformation structure. For ultrafine-grained Cu considerably less activity of grain boundary sliding has been observed compared to ultrafinegrained Al.

Lead halide perovskite semiconductors are in general known to have an inherently high X-ray absorption cross-section and a significantly higher carrier mobility than any other low-temperature solution-processed semiconductor. So far, the processing of several-hundred-micrometres-thick high-quality crystalline perovskite films over a large area has been unresolved for efficient X-ray detection. In this Article, we present a mechanical sintering process to fabricate polycrystalline methyl ammonium lead triiodide perovskite (MAPbI 3 ) wafers with millimetre thickness and well-defined crystallinity. Benchmarking of the MAPbI 3 wafers against state-of-the-art CdTe detectors reveals competitive conversion efficiencies of 2,527 µC Gy air −1 cm −2 under 70 kV p X-ray exposure. The high ambipolar mobility–lifetime product of 2 × 10 −4 cm 2 V −1 is suggested to be responsible for this exceptionally high sensitivity. Our findings inform a new generation of highly efficient and low-cost X-ray detectors based on perovskite wafers.

Solid solution strengthening of the γ matrix is one key factor for improving the creep strength of single crystal nickel-base superalloys at high temperatures. Therefore a strong partitioning of solid solution hardening elements to the matrix is beneficial for high temperature creep strength. Different Rhenium-free alloys which are derived from CMSX-4 are investigated. The alloys have been characterized regarding microstructure, phase compositions as well as creep strength. It is found that increasing the Titanium (Ti) as well as the Tungsten (W) content causes a stronger partitioning of the solid solution strengtheners, in particular W, to the γ phase. As a result the creep resistance is significantly improved. Based on these ideas, a Rhenium-free alloy with an optimized chemistry regarding the partitioning behavior of W is developed and validated in the present study. It shows comparable creep strength to the Rhenium containing second generation alloy CMSX-4 in the high temperature / low stress creep regime and is less prone to the formation of deleterious topologically close packed (TCP) phases. This more effective usage of solid solution strengtheners can enhance the creep properties of nickel-base superalloys while reducing the content of strategic elements like Rhenium.