On the Degree of Plastic Strain during Laser Shock Peening of Ti-6Al-4V.

Laser shock peening (LSP) is an innovative technique that is used to enhance the fatigue strength of structural materials via the generation of significant residual stress. The present work was undertaken to evaluate the degree of plastic strain introduced during LSP and thus improve the fundamental understanding of the LSP process. To this end, electron backscatter diffraction (EBSD) and nano-hardness measurements were performed to examine the microstructural response of laser-shock-peened Ti-6Al-4V alloy. Only minor changes in both the shape of α grains/particles and hardness were found. Accordingly, it was concluded that the laser-shock-peened material only experienced a small plastic strain. This surprising result was attributed to a relatively high rate of strain hardening of Ti-6Al-4V during LSP.

Measuring Dental Enamel Thickness: Morphological and Functional Relevance of Topographic Mapping.

The interest in the development of dental enamel thickness measurement techniques is connected to the importance of metric data in taxonomic assessments and evolutionary research as well as in other directions of dental studies. At the same time, advances in non-destructive imaging techniques and the application of scanning methods, such as micro-focus-computed X-ray tomography, has enabled researchers to study the internal morpho-histological layers of teeth with a greater degree of accuracy and detail. These tendencies have contributed to changes in established views in different areas of dental research, ranging from the interpretation of morphology to metric assessments. In fact, a significant amount of data have been obtained using traditional metric techniques, which now should be critically reassessed using current technologies and methodologies. Hence, we propose new approaches for measuring dental enamel thickness using palaeontological material from the territories of northern Vietnam by means of automated and manually operated techniques. We also discuss method improvements, taking into account their relevance for dental morphology and occlusion. As we have shown, our approaches demonstrate the potential to form closer links between the metric data and dental morphology and provide the possibility for objective and replicable studies on dental enamel thickness through the application of automated techniques. These features are likely to be effective in more profound taxonomic research and for the development of metric and analytical systems. Our technique provides scope for its targeted application in clinical methods, which could help to reveal functional changes in the masticatory system. However, this will likely require improvements in clinically applicable imaging techniques.

Mechanical Behavior of a Medium-Entropy Fe65(CoNi)25Cr9.5C0.5 Alloy Produced by Selective Laser Melting.

Specimens of a medium-entropy Fe65(CoNi)25Cr9.5C0.5 (in at.%) alloy were produced using additive manufacturing (selective laser melting, SLM). The selected parameters of SLM resulted in a very high density in the specimens with a residual porosity of less than 0.5%. The structure and mechanical behavior of the alloy were studied under tension at room and cryogenic temperatures. The microstructure of the alloy produced by SLM comprised an elongated substructure, inside which cells with a size of ~300 nm were observed. The as-produced alloy demonstrated high yield strength and ultimate tensile strength (YS = 680 MPa; UTS = 1800 MPa) along with good ductility (tensile elongation = 26%) at a cryogenic temperature (77 K) that was associated with the development of transformation-induced plasticity (TRIP) effect. At room temperature, the TRIP effect was less pronounced. Consequently, the alloy demonstrated lower strain hardening and a YS/UTS of 560/640 MPa. The deformation mechanisms of the alloy are discussed.

Combustion Synthesis and Reactive Spark Plasma Sintering of Non-Equiatomic CoAl-Based High Entropy Intermetallics.

The present work reports the direct production of a high-entropy (HE) intermetallic CoNi0.3Fe0.3Cr0.15Al material with a B2 structure from mechanically activated elemental powder mixtures. Fast and efficient combustion synthesis (CS), spark plasma sintering (SPS), and reactive SPS (RSPS) methods were used to synthesize the HE powders and bulks. The formation of the main B2 phase along with some amounts of secondary BCC and FCC phases are reported, and L12 intermetallic (CS scheme) and BCC based on Cr (CS + SPS and RSPS schemes at 1000 °C) were observed in all samples. The interaction between the components during heating to 1600 °C of the mechanically activated mixtures and CS powders has been studied. It has been shown that the formation of the CoNi0.3Fe0.3Cr0.15Al phase occurs at 1370 °C through the formation of intermediate intermetallic phases (Al9Me2, AlCo, AlNi3) and their solid solutions, which coincidences well with thermodynamic calculations and solubility diagrams. Compression tests at room and elevated temperatures showed that the alloy obtained by the RSPS method has enhanced mechanical properties (σp = 2.79 GPa, σ0.2 = 1.82 GPa, ε = 11.5% at 400 °C) that surpass many known alloys in this system. High mechanical properties at elevated temperatures are provided by the B2 ordered phase due to the presence of impurity atoms and defects in the lattice.

Gradient Microstructure and Texture Formation in a Metastable Austenitic Stainless Steel during Cold Rotary Swaging.

The paper aimed to study the evolution of the microstructure and texture gradient of a 321-type metastable austenitic stainless steel during cold rotary swaging. Cold rotary swaging was carried out with a reduction of up to 90% at ambient temperature. Pronounced gradients of the α'-martensite volume fraction, the axial texture of austenite (⟨111⟩ and ⟨001⟩) and α'-martensite (⟨101⟩), and non-uniform microhardness distribution along the rod diameter were obtained after a reduction of 80-90%. According to the finite element analysis, moderate tensile stresses were attained in the center, whereas high compressive stresses operated at the edge. Due to water cooling of the rod surface and heating of the rod center during processing, a temperature gradient was also derived. Features of strain-induced martensitic transformation, microstructure and texture evolution, and non-uniform hardening during cold rotary swaging were discussed.

Performance-oriented multistage design for multi-principal element alloys with low cost yet high efficiency.

Multi-principal element alloys (MPEAs) with remarkable performances possess great potential as structural, functional, and smart materials. However, their efficient performance-orientated design in a wide range of compositions and types is an extremely challenging issue, because of properties strongly dependent upon the composition and composition-dominated microstructure. Here, we propose a multistage-design approach integrating machine learning, physical laws and a mathematical model for developing the desired-property MPEAs in a very time-efficient way. Compared to the existing physical model- or machine-learning-assisted material development, the forward-and-inverse problems, including identifying the target property and unearthing the optimal composition, can be tackled with better efficiency and higher accuracy using our proposed avenue, which defeats the one-step component-performance design strategy by multistage-design coupling constraints. Furthermore, we developed a new multi-phase MPEA at the minimal time and cost, whose high strength-ductility synergy exceeded those of its system and subsystem reported so far by searching for the optimal combination of phase fraction and composition. The present work suggests that the property-guided composition and microstructure are precisely tailored through the newly built approach with significant reductions of the development period and cost, which is readily extendable to other multi-principal element materials.

Machine Learning-Based Strength Prediction for Refractory High-Entropy Alloys of the Al-Cr-Nb-Ti-V-Zr System.

The aim of this work was to provide a guidance to the prediction and design of high-entropy alloys with good performance. New promising compositions of refractory high-entropy alloys with the desired phase composition and mechanical properties (yield strength) have been predicted using a combination of machine learning, phenomenological rules and CALPHAD modeling. The yield strength prediction in a wide range of temperatures (20-800 °C) was made using a surrogate model based on a support-vector machine algorithm. The yield strength at 20 °C and 600 °C was predicted quite precisely (the average prediction error was 11% and 13.5%, respectively) with a decrease in the precision to slightly higher than 20% at 800 °C. An Al13Cr12Nb20Ti20V35 alloy with an excellent combination of ductility and yield strength at 20 °C (16.6% and 1295 MPa, respectively) and at 800 °C (more 50% and 898 MPa, respectively) was produced based on the prediction.

Exceptionally high strain-hardening and ductility due to transformation induced plasticity effect in Ti-rich high-entropy alloys.

Ti-rich body-centered cubic (BCC, ß) high-entropy alloys having compositions Ti35Zr27.5Hf27.5Nb5Ta5, Ti38Zr25Hf25Ta10Sn2, and Ti38Zr25Hf25Ta7Sn5 (in at%) were designed using bond order (Bo)-mean d-orbital energy level (Md) approach. Deformation mechanisms of these alloys were studied using tensile deformation. The alloys showed exceptionally high strain-hardening and ductility. For instance, the alloys showed at least twofold increment of tensile strength compared to the yield strength, due to strain-hardening. Post-deformation microstructural observations confirmed the transformation of ß to hexagonal close packed (HCP, α') martensite. Based on microstructural investigation, stress-strain behaviors were explained using transformation induced plasticity effect. Crystallographic analysis indicated transformation of ß to α' showed strong variant selection (1 1 0)ß//(0 0 0 1)α', and [1 - 1 1]ß//[1 1 - 2 0]α'.

From Metallic Lead Films to Perovskite Solar Cells through Lead Conversion with Polyhalide Solutions.

Solutions of methylammonium and formamidinium polyhalides (AX1+n, A = MA, FA, X = I, Br) in isopropanol are introduced as novel versatile precursors for the fabrication of APbX3 hybrid perovskite thin films via oxidation of metallic Pb. The polyhalide solution with adjustable reactivity is distributed over a metallic Pb layer followed by iodine vapor postprocessing to tune the morphology and composition of the film using only the elements inherently present in the perovskite. This method is easily reproducible in any materials science laboratory with equipment commonly used for perovskite solar cell fabrication and resulted in power conversion efficiencies of 16.2 and 17.2% for planar solar cells using MAPbI3 and MA0.25FA0.75PbI2.75Br0.25 perovskites, respectively, as a proof of concept. Implementation of metallic lead thin films as the single Pb-containing precursors reduces a number of in-lab handling hazards compared to classical PbI2 powder and solutions and provides a variety of scalable deposition options.

Effect of High-Pressure Torsion on Structure and Properties of Ti-15Mo/TiB Metal-Matrix Composite.

The microstructure and microhardness evolution of a Ti-15(wt.%)Mo/TiB metal-matrix composite (MMC) during high-pressure torsion (HPT) at 400 °C was studied. The composite was fabricated by spark plasma sintering of a Ti, Mo and TiB2 powders mixture at 1200 °C. In the initial condition, the structure of the composite consisted mainly of body-centered cubic (bcc) Ti solid solution and TiB whiskers. An increase in dislocation density, a considerable decrease in a grain size in the bcc Ti matrix, and breaking/rearrangement of the TiB whiskers were observed during HPT. The (sub)grain size in the bcc Ti matrix attained after 1 revolution was ~75 nm and then gradually decreased to ~55 nm after 5 revolutions. The TiB particle sizes after 5 revolutions was found to be 130⁻210 nm. The microhardness increased with strain from 575 HV in the initial state to 730 HV after 5 revolutions. Various hardening mechanisms' contributions in the Ti-15Mo/TiB were evaluated.

Oxidation Behavior of Refractory AlNbTiVZr0.25 High-Entropy Alloy.

Oxidation behavior of a refractory AlNbTiVZr0.25 high-entropy alloy at 600⁻900 °C was investigated. At 600⁻700 °C, two-stage oxidation kinetics was found: Nearly parabolic oxidation (n = 0.46⁻0.48) at the first stage, transitioned to breakaway oxidation (n = 0.75⁻0.72) at the second stage. At 800 °C, the oxidation kinetics was nearly linear (n = 0.92) throughout the entire duration of testing. At 900 °C, the specimen disintegrated after 50 h of testing. The specific mass gains were estimated to be 7.2, 38.1, and 107.5, and 225.5 mg/cm² at 600, 700, and 800 °C for 100 h, and 900 °C for 50 h, respectively. Phase compositions and morphology of the oxide scales were analyzed using X-ray diffraction (XRD) and scanning electron microscopy (SEM). It was shown that the surface layer at 600 °C consisted of the V2O5, VO2, TiO2, Nb2O5, and TiNb2O7 oxides. Meanwhile, the scale at 900 °C comprised of complex TiNb2O7, AlNbO4, and Nb2Zr6O17 oxides. The oxidation mechanisms operating at different temperatures were discussed and a comparison of oxidation characteristics with the other alloys was conducted.

Microstructure and Mechanical Properties Evolution of the Al, C-Containing CoCrFeNiMn-Type High-Entropy Alloy during Cold Rolling.

The effect of cold rolling on the microstructure and mechanical properties of an Al- and C-containing CoCrFeNiMn-type high-entropy alloy was reported. The alloy with a chemical composition (at %) of (20-23) Co, Cr, Fe, and Ni; 8.82 Mn; 3.37 Al; and 0.69 C was produced by self-propagating high-temperature synthesis with subsequent induction. In the initial as-cast condition the alloy had an face centered cubic single-phase coarse-grained structure. Microstructure evolution was mostly associated with either planar dislocation glide at relatively low deformation during rolling (up to 20%) or deformation twinning and shear banding at higher strain. After 80% reduction, a heavily deformed twinned/subgrained structure was observed. A comparison with the equiatomic CoCrFeNiMn alloy revealed higher dislocation density at all stages of cold rolling and later onset of deformation twinning that was attributed to a stacking fault energy increase in the program alloy; this assumption was confirmed by calculations. In the initial as-cast condition the alloy had low yield strength of 210 MPa with yet very high uniform elongation of 74%. After 80% rolling, yield strength approached 1310 MPa while uniform elongation decreased to 1.3%. Substructure strengthening was found to be dominated at low rolling reductions (<40%), while grain (twin) boundary strengthening prevailed at higher strains.

New way to substitute tetracyanocyclopropanes: one-pot cascade assembling of carbonyls and malononitrile by the only bromine direct action.

THE NEW TYPE OF THE CHEMICAL CASCADE REACTION WAS FOUND: formation of cyclopropanes from carbonyl compounds and CH acid by the only bromine direct action. The action of aqueous bromine on the carbonyl compounds and malononitrile in EtOH-H2O solutions in the presence of NaOAc results in the formation of 3-substituted 1,1,2,2-tetracyanocyclopropanes in 48-93% yields. The latter are well-known precursors for the different bicyclic heterosystems, among them those containing cyclopropane ring and those possessing different types of pharmacological activity.