In-situ TEM Investigation of Rapid Solidification of Aluminum and Aluminum Copper Alloys

Measuring and observing rapidly evolving interfaces of irreversible transient states such as rapid solidification have been a long time standing problem in materials science as characterization techniques that deliver the necessary requirements, i.e. nanosecond temporal and nano-meter spatial resolution, have not been present. Dynamical TEM utilizes a process initiation photon laser pulse coupled with a timed electron pulse train to observe transient states of rapidly evolving phase transformations. Nanoscale spatio-temporal resolution in-situ TEM revealed growth mode changes and enabled quantitative measurements of locally resolved instantaneous and averaged interface velocities for pure Aluminum and hypo-eutectic Aluminum-Copper alloys. Post-mortem TEM was employed to gain insights on micro-structural evolution morphology regarding texture, morphology changes, grain size and grain size development, phase formation and orientations relationships during the laser processing. Post-mortem TEM studies revealed that resultant microstructures found in thin film solidification are equivalent with microstructures found in bulk sample experimentation utilizing CW-lasers. DTEM allows systematical studies of far-from equilibrium phase transformation and were employed to investigate PL initiated directional rapid solidification in Al and Al-Cu alloys

Resultado do Edital Nº 47 -CAPES - Programa de Doutorado Sanduíche no Exterior

Here we present measurements of surface tension and viscosity of the bulk glass-forming alloy Pd_(43)Cu_(27)Ni_(10)P_(20) performed during containerless processing under reduced gravity. We applied the oscillating drop method in an electromagnetic levitation facility on board of parabolic flights. The measured viscosity exhibits a pronounced temperature dependence following an Arrhenius law over a temperature range from 1100 K to 1450 K. Together with literature values of viscosity at lower temperatures, the viscosity of Pd_(43)Cu_(27)Ni_(10)P_(20) can be well described by a free volume model. X-ray diffraction analysis on the material retrieved after the parabolic flights confirm the glassy nature after vitrification of the bulk samples and thus the absence of crystallization during processing over a wide temperature range

In situ and ex situ characterization of the microstructure formation in Ni-Cr-Si alloys during rapid solidification-toward alloy design for laser additive manufacturing

Laser beam-based deposition methods such as laser cladding or additive manufacturing of metals promises improved properties, performance, and reliability of the materials and therefore rely heavily on understanding the relationship between chemical composition, rapid solidification processing conditions, and resulting microstructural features. In this work, the phase formation of four Ni-Cr-Si alloys was studied as a function of cooling rate and chemical composition using a liquid droplet rapid solidification technique. Post mortem x-ray diffraction, scanning electron microscopy, and in situ synchrotron microbeam X-ray diffraction shows the present and evolution of the rapidly solidified microstructures. Furthermore, the obtained results were compared to standard laser deposition tests. In situ microbeam diffraction revealed that due to rapid cooling and an increasing amount of Cr and Si, metastable high-temperature silicides remain in the final microstructure. Due to more sluggish interface kinetics of intermetallic compounds than that of disorder solid solution, an anomalous eutectic structure becomes dominant over the regular lamellar microstructure at high cooling rates. The rapid solidification experiments produced a microstructure similar to the one generated in laser coating thus confirming that this rapid solidification test allows a rapid pre-screening of alloys suitable for laser beam-based processing techniques.ISSN:1996-194

Surface tension and viscosity of liquid Pd43Cu27Ni10P20 measured in a levitation device under microgravity

Here we present measurements of surface tension and viscosity of the bulk glass-forming alloy Pd43Cu27Ni10P20 performed during containerless processing under reduced gravity. We applied the oscillating drop method in an electromagnetic levitation facility on board of parabolic flights. The measured viscosity exhibits a pronounced temperature dependence following an Arrhenius law over a temperature range from 1100 K to 1450 K. Together with literature values of viscosity at lower temperatures, the viscosity of Pd43Cu27Ni10P20 can be well described by a free volume model. X-ray diffraction analysis on the material retrieved after the parabolic flights confirm the glassy nature after vitrification of the bulk samples and thus the absence of crystallization during processing over a wide temperature range

Gradient Nanostructure and Residual Stresses Induced by Ultrasonic Nano-crystal Surface Modification in 304 Austenitic Stainless Steel for High Strength and High Ductility

In this study, the effects of Ultrasonic Nano-crystal Surface Modification (UNSM) on residual stresses, microstructure changes and mechanical properties of austenitic stainless steel 304 were investigated. The dynamic impacts induced by UNSM leads to surface nanocrystallization, martensite formation, and the generation of high magnitude of surface compressive residual stresses (−1400 MPa) and hardening. Highly dense deformation twins were generated in material subsurface to a depth of 100 µm. These deformation twins significantly improve material work-hardening capacity by acting both as dislocation blockers and dislocation emission sources. Furthermore, the gradually changing martensite volume fraction ensures strong interfacial strength between the ductile interior and the two nanocrystalline surface layers and thus prevents early necking. The microstructure with two strong surface layers and a compliant interior embedded with dense nanoscale deformation twins and dislocations leads to both high strength and high ductility. The work-hardened surface layers (3.5 times the original hardness) and high magnitude of compressive residual stresses lead to significant improvement in fatigue performance; the fatigue endurance limit was increased by 100 MPa. The results have demonstrated that UNSM is a powerful surface engineering technique that can improve component mechanical properties and performance