Compression-compression fatigue of Pd_(43)Ni_(10)Cu_(27)P_(20) metallic glass foam

Compression-compression fatigue testing of metallic-glass foam is performed. A stress-life curve is constructed, which reveals an endurance limit at a fatigue ratio of about 0.1. The origin of fatigue resistance of this foam is identified to be the tendency of intracellular struts to undergo elastic and reversible buckling, while the fatigue process is understood to advance by anelastic strut buckling leading to localized plasticity (shear banding) and ultimate strut fracture. Curves of peak and valley strain versus number of cycles coupled with plots of hysteresis loops and estimates of energy dissipation at various loading cycles confirm the four stages of foam-fatigue

Nindestructive Evaluation of Metal Matrix Composite Products with Implanted Defects

The Westinghouse Science and Technology Center has undertaken a program to develop nondestructive evaluation (NDE) techniques for characterizing the internal structure of SiC particle-reinforced aluminum matrix composites at critical stages during fabrication [1–5]. Because of the large number of processing variables in the manufacture of metal matrix composites (MMC), the likelihood of having detrimental discontinuities is high. The detection of potential defects early in the processing cycle would increase the overall system yield, lower costs, and enhance final product quality [4]. The aim of this investigation was to develop and conduct NDE at various stages of MMC fabrication, correlate the results with microstructural characterization, and establish qualified product quality assurance processes. A large-scale billet was fabricated specially using powder metallurgy techniques to facilitate this objective. The billet contained implanted silicon-carbide particle and aluminum powder clusters as inspection targets. The billet was subsequently extruded into a primary cylindrical extrusion, and finally into a flat plate. The NDE objectives included evaluating the detectability and mapping the implanted defects through each of the processing steps. Comprehensive evaluation of MMC structures requires the use of multiple NDE techniques, including ultrasonic, eddy current, and radiographic testing. This paper concentrates on the results of the ultrasonic investigations. Our experimental approach was: (1) fabricate a MMC billet with intentionally placed inhomogeneities; (2) develop and implement NDE techniques to characterize the MMC internal structure; (3) extend the NDE techniques to intermediate processing and final product forms; and (4) correlate the NDE data with microstructural characterization and mechanical testing results

Physics-Based Machine-Learning Approach for Modeling the Temperature-Dependent Yield Strengths of Medium- or High-Entropy Alloys

Machine learning is becoming a powerful tool to predict temperature-dependent yield strengths (YS) of structural materials, particularly for multi-principal-element systems. However, successful machine-learning predictions depend on the use of reasonable machine-learning models. Here, we present a comprehensive and up-to-date overview of a bilinear log model for predicting temperature-dependent YS of medium-entropy or high-entropy alloys (MEAs or HEAs). In this model, a break temperature, Tbreak, is introduced, which can guide the design of MEAs or HEAs with attractive high-temperature properties. Unlike assuming black-box structures, our model is based on the underlying physics, incorporated in form of a priori information. A technique of global optimization is employed to enable the concurrent optimization of model parameters over low- and high-temperature regimes, showing that the break temperature is consistent across YS and ultimate strength for a variety of HEA compositions. A high-level comparison between YS of MEAs/HEAs and those of nickel-based superalloys reveal superior strength properties of selected refractory HEAs. For reliable operations, the temperature of a structural component, such as a turbine blade, made from refractory alloys may need to stay below Tbreak. Once above Tbreak, phase transformations may start taking place, and the alloy may begin losing structural integrity

Elastic properties of Zr-based bulk metallic glasses studied\ud by resonant ultrasound spectroscopy

We report measurements of the elastic properties of Zr-based bulk metallic glasses,\ud Zr52.5Cu17.9Ni14.6Al10Ti5, Zr50Cu30Ni10Al10, and Zr50Cu40Al10 between 5 K and\ud 300 K. Both the shear and longitudinal modulus have been measured as a function of\ud temperature, allowing accurate determination of the Poisson’s ratio and the related\ud ratio of bulk modulus to shear modulus, K/G. These data make it possible to assess the\ud influence of the alloy’s composition on the mechanical properties and enable an\ud evaluation of the correlation between the elastic moduli and the ductility of the alloys

Elastic properties of Zr-based bulk metallic glasses studied by resonant ultrasound spectroscopy

We report measurements of the elastic properties of Zr-based bulk metallic glasses, Zr52.5Cu17.9Ni14.6Al10Ti5, Zr50Cu30Ni10Al10, and Zr50Cu40Al10 between 5 K and 300 K. Both the shear and longitudinal modulus have been measured as a function of temperature, allowing accurate determination of the Poisson’s ratio and the related ratio of bulk modulus to shear modulus, K/G. These data make it possible to assess the influence of the alloy’s composition on the mechanical properties and enable an evaluation of the correlation between the elastic moduli and the ductility of the alloys

Scaling and complexity of stress fluctuations associated with smooth and jerky flow in a FeCoNiTiAl high-entropy alloy

Recent observations of jerky flow in high-entropy alloys (HEA) revealed a high role of self-organization of dislocations in their plasticity. The present work reports first results of investigation of stress fluctuations during plastic deformation of a FeCoNiTiAl alloy, examined in a wide temperature range covering both smooth and jerky flow. These fluctuations, which accompany the overall deformation behavior representing an essentially slower stress evolution controlled by the work hardening, were processed using complementary approaches comprising the Fourier spectral analysis, the refined composite multiscale entropy, and multifractal formalisms. The joint analysis at distinct scales testified that even a macroscopically smooth plastic flow is accompanied with nonrandom fluctuations, disclosing self-organized dynamics of dislocations. Qualitative changes in such a fine-scale "noise" were found with varying temperature. The observed diversity is significant for understanding the relationships between different scales of plasticity of HEAs and crystal materials in general.Comment: 13 pages main body, 6 figures, 2 appendices, 65 citations (22 pages overall