Fatigue and Tensile Behavior of Cast,Hot-Rolled,and Severely Plastically Deformed AZ31 Magnesium Alloy

The work focuses on experimental examination of the fatigue behavior of magnesium alloy AZ31 produced by three different procedures: squeeze casting (SC), hot rolling (HR), and equal-channel angular pressing (ECAP). The microstructures produced were studied by light and transmission electron microscopy (TEM). Squeeze-cast AZ31 had low porosity and coarse grains, while hot-rolled material showed microstructure with grain size of 3 to 20 μm. The finest grain structure with the average grain size of about 1 to 2 μm was found in the material pressed 4 times at 200 °C using the ECAP technique, route B _c . It was shown that low- and high-cycle fatigue behavior under symmetric loading at room temperature and with loading frequency of 20 Hz is strongly dependent on the technique employed in producing the alloy. The ECAP was shown to improve the fatigue life of the material in the low-cycle region over that of the squeeze-cast material. However, the fatigue life of AZ31 after ECAP was slightly lower than that of the hot-rolled material. In the high-cycle region, the hot-rolled material and the material that underwent ECAP exhibit the same fatigue strength, which is superior to that of the squeeze-cast alloy. Fatigue crack initiation and the character of fracture were examined by means of scanning electron microscopy. This article is based on a presentation made in the symposium entitled “Ultrafine-Grained Materials: from Basics to Application,” which occurred September 25–27, 2006 in Kloster Irsee, Germany.

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Influence of Microstructure on the Spall Failure of Aluminum Materials

Laser-shock-induced spall failure is studied in thin aluminum targets at strain rates from 2 to 5 × 10⁶ s⁻¹. Targets were prepared from high-purity aluminum in the recrystallized condition and a low-impurity aluminum alloy containing 3 wt pct magnesium in both recrystallized and cold-rolled conditions. The effects of material and microstructure on spall fracture morphology are investigated. Recrystallized pure aluminum produced spall fracture surfaces characterized by transgranular ductile dimpling. Recrystallized aluminum-magnesium alloy with a 50-μm grain size produced less ductile spall surfaces, which were dominated by transgranular fracture, with some isolated transgranular ductile dimpling at fast strain rates. Transgranular ductile dimpling regions disappeared in recrystallized alloy specimens with a 23-μm grain size tested at faster rates. Cold-rolled alloy material produced spall failure surfaces consisting of brittle intergranular and transgranular fractures. Measured spall strength increases with increasing ductile fracture character. Spall failure preferentially follows grain boundaries, making grain size an important factor in spall fracture surface character. This article is based on a presentation made in the symposium entitled “Dynamic Behavior of Materials,” which occurred during the TMS Annual Meeting and Exhibition, February 25–March 1, 2007 in Orlando, Florida, under the auspices of The Minerals, Metals and Materials Society, TMS Structural Materials Division, and TMS/ASM Mechanical Behavior of Materials Committee.

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