Anomalous precipitation hardening in Al-(1 wt%)Cu thin films

\u3cp\u3eThis paper concentrates on the precipitation hardening of Al-(1 wt%)Cu thin films. It is shown that in contrast to bulk, the well-known approach of precipitation hardening in confined systems like thin layers and thin films does not operate in the conventional way. This work analyses and discusses the precipitate hardening and its relation to the precipitation mechanism of Al-(1 wt%)Cu thin films which are subjected to precipitation hardening, nano-indentation hardness measurements. Microfabricated Al-(1 wt%)Cu thin films were solution treated at 550 °C for 15 min, quenched and aged at 190 °C for various durations up to 48 h. Nano-indentation hardness measurements revealed for the first time an unexpected decrease in hardness at just ∼8 hours of aging, followed by a saturating increase. Microstructural analysis employing electron microscopy (high-resolution transmission, scanning, backscatter diffraction, energy dispersive spectroscopy) and x-ray diffraction revealed GP zones and θ precipitates but no θ’ in as-received films, and only θ precipitates for aging durations longer than 6 h in the precipitate hardened films. Through-thickness analyses of aged specimens highlighted that θ precipitates nucleate and grow essentially at grain boundary grooves and at the specimen surface as preferential nucleation sites, while depleting Cu from the grain interior. It is shown that the growing precipitation at the surface and grain boundary grooves depletes the Cu in the thin film interior explaining the weakening-hardening sequence observed in the hardness measurements. Hence, the work shows that precipitation kinetics, and not thermodynamics, determine the precipitation state in thin films.\u3c/p\u3

High resolution electron microscopy of misfit dislocations at metaloxide interfaces

Misfit dislocations at metal-oxide interfaces are studied using HRTEM. By observing misfit dislocations the bonding across the metal-oxide interface can be studied. This is done for two cases, a parallel Cu-MgO {111} interface and tilted interfaces in Pd-ZnO and Ag-ZnO. From the Cu-MgO micrographs the strain fields introduced by the misfit dislocation network could be observed. From the tilted ZnO interfaces it can be seen how different materials (Ag, Pd) adapt to this situation.</p

Advances in Laser Surface Engineering:Tackling the Cracking Problem in Laser-Deposited Ni-Cr-B-Si-C Alloys

<p>Laser-deposition technologies are being increasingly used for surface modification and three-dimensional manufacturing applications. The biggest technical obstacle to a wider usage of these technologies especially for deposition of hard alloys is cracking of the deposited samples. In this work, the idea of microstructural refinement as a toughening mechanism for Ni-Cr-B-Si-C alloys deposited by laser cladding is evaluated and a new idea for reducing the cracking tendency of these alloys is proposed. The results show that although a significant refinement of the Cr-rich precipitates in these alloys could be induced by a suitable addition of Nb, the cracking susceptibility of the deposits was unchanged. This was so because the continuous network of hard eutectics was still providing an easy route for crack growth. The outcome of this work shows that an effective toughening mechanism for these alloys should include not only a refinement of the hard precipitates but also modification of the eutectic structure.</p>

Tantalum-modified Stellite 6 thick coatings:microstructure and mechanical performance

<p>Thick Co-based coatings with different contents of tantalum were prepared by simultaneous powder feeding laser cladding technique on 304 stainless steel substrate, with the Ta wt% being 0, 2, 7 and 12. Laser processing was carried out with a continuous 3.3 kW Yt:YAG fiber laser. Microstructural observations were executed using scanning electron microscopy, energy dispersive X-ray spectroscopy analysis, and transmission electron microscopy. Observations indicated that, with an increase in the Ta contents, the Ta-rich MC-type carbides were formed in interdendritic regions. Also, hexagonal M7C3-type carbides were formed instead of orthorhombic M7C3-type carbides. The orientation relationships between different phases and the matrix were determined by electron diffraction. Mechanical properties were determined using microhardness measurement at room temperature and wear resistance measurement at room and elevated (500 A degrees C) temperatures. The research demonstrated that alloying any amount of tantalum, in spite of increasing the microhardness, could be detrimental for increasing the wear resistance of Stellite 6, both at room and elevated temperatures. The relationship between microstructure and mechanical properties is explained.</p>