Grain refinement of Al-Mg-Sc alloy by ultrasonic treatment

In foundry practice, ultrasonic treatment has been used as an efficient technique to achieve grain refinement in aluminium and magnesium alloys. This article shows the strong effect of pouring temperature and ultrasonic treatment at various temperatures on the grain refinement of Al-1 wt% Mg-0.3 wt% Sc alloy. Without ultrasonic treatment, a fine grain structure was obtained at the pouring temperature of 700 °C. The average grain size sharply decreases from 487 ± 20 to 103 ± 2 μm when the pouring temperature decreases from 800 to 700 °C. Ultrasonic vibration proved to be a potential grain refinement technique with a wide range of pouring tem- perature. A microstructure with very fine and homogeneous grains was obtained by applying ultrasonic treatment to the melt at the temperature range between 700 and 740 °C, before pouring. Cavitation-enhanced hetero- geneous nucleation is the mechanism proposed to explain grain refinement by ultrasound in this alloy. Moreover, ultrasonic treatment of the melt was found to lead to cast samples with hardness values similar to those obtained in samples submitted to precipitation hardening, suggesting that ultrasonic treatment can avoid carrying out heat treatment of cast parts.This research was supported by The Project Bridging The Gap, funded by the Erasmus Mundus External Cooperation Window Programme. Acknowledgements also to the University of Minho, for the provision of research facilities

Cyclic fatigue effect in particulate ceramic composites

A new model is presented that provides an improved understanding of the time dependent fatigue behavior of two phase brittle particulate ceramic composites under static and cyclic loading conditions. The proposed model takes into consideration cyclic fatigue effects, which are responsible for the accelerated fatigue crack propagation in the cyclic loading as compared to the static loading. It also takes into account the effect of both thermal residual stresses and bridging stresses acting in the composite during time dependent crack propagation. Experimental results for the fatigue behavior of ZrB2–45 vol%SiC ceramic composite were used as a case study to valid the proposed model. The model gives insight both into the time dependent mechanical behavior of ceramic composites and, at the same time, allows determination of important structural parameter, such as, size of the bridging zone in the material under cycling loading

Derivation of a general three-dimensional crack-propagation law: A generalization of the principle of local symmetry

We derive a general crack propagation law for slow brittle cracking, in two and three dimensions, using symmetry, gauge invariance, and gradient expansions. Our derivation provides explicit justification for the ``principle of local symmetry,'' which has been used extensively to describe two dimensional crack growth, but goes beyond that principle to describe three dimensional crack phenomena as well. We also find that there are new materials properties needed to describe the growth of general cracks in three dimensions, besides the fracture toughness and elastic constants previously used to describe cracking.Comment: 31 pages, including several figure