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To reduce fuel consumption and greenhouse gas emissions, magnesium alloys are being considered for automotive and aerospace applications due to their low density, high specific strength and stiffness, and other attractive traits. Structural applications of magnesium components require low-cycle fatigue (LCF) behavior, since cyclic loading or thermal stresses are often encountered. The aim of this article was to study the cyclic deformation characteristics and evaluate LCF behavior of a recently developed AM30 extruded magnesium alloy. This alloy exhibited a strong cyclic hardening characteristic, with a cyclic strain-hardening exponent of 0.33 compared to the monotonic strain-hardening exponent of 0.15. With increasing total strain amplitude, both plastic strain amplitude and mean stress increased and fatigue life decreased. A significant difference between the tensile and compressive yield stresses occurred, leading to asymmetric hysteresis loops at high strain amplitudes due to twinning in compression and subsequent detwinning in tension. A noticeable change in the modulus was observed due to the pseudoelastic behavior of this alloy. The Coffin-Manson law and Basquin equation could be used to describe the fatigue life. At low strain ratios the alloy showed strong cyclic hardening, which became less significant as the strain ratio increased. The lower the strain ratio, the lower the stress amplitude and mean stress but the higher the plastic strain amplitude, corresponding to a longer fatigue life. Fatigue life also increased with increasing strain rate. Fatigue crack initiation occurred from the specimen surface and crack propagation was mainly characterized by striation-like features. Multiple initiation sites at the specimen surface were observed at higher strain amplitudes.

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samples were produced at CANMET-MTL, and the casting parameters were as follows: 725°C casting temperature, 350°C to 400°C mold temperature, 2-to 3-second filling time, and 0.125 MPa filling pressure. The cooling rate for the LPDC samples was about 0.8°C/s. The HPDC samples were cast at the Gibbs Die Casting Corporation (Henderson, KY), and the casting parameters were as follows: 7.6 m/s injection speed, 690°C metal temperature, and 230°C mold temperature. The cooling rate of the HPDC sample was estimated to be at least two orders of magnitude faster than that of the LPDC sample.Metallographic samples were examined first with an optical microscope and then in a PHILIPS* XL30 scanning *PHILIPS is a trademark of

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Two types of specimen for crack tip opening angle (CTOA) measurement have been investigated for pipeline applications, i.e., the modified double cantilever beam (MDCB) (at NIST) and the drop-weight tear test (DWTT) specimen (at CANMET). Results of effects of specimen types, thicknesses and loading rates on CTOA are summarized and discussed. The main observations include: (i) For both MDCB and DWTT specimens tested at quasi-static loading rate, crack front tunnelling (i.e., with a deep triangular crack-tip shape) was present in high-strength steels; (ii) For DWTT specimens, CTOA values measured optically at the surface were significantly higher than those from the simplified single-specimen method (S-SSM) and those measured at mid-thickness [on sections cut using electric discharge machining (EDM)]; and (iii) CTOA values from surface measurement of MDCB specimens were comparable to those derived from S-SSM of DWTT specimens, but the surface values of DWTT were higher than those of MDCB specimens.

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The results of an experimental study on the grain coarsening behavior, M 23 C 6 carbide precipitation, and secondary MC carbide precipitation kinetics in UDIMET 520 are presented. Primary MC carbides and M(C, N) carbonitrides strongly influence the grain growth, with their dissolution near 1190 ЊC and 1250 ЊC, respectively, resulting in two distinct grain coarsening temperatures (GCTs). M 23 C 6 carbides precipitate in the alloy over a wide range of temperatures varying between 600 ЊC and 1050 ЊC. A discrete M 23 C 6 grain boundary carbide morphology is observed at aging temperatures below 850 ЊC. Secondary MC carbides formed at temperatures ranging between 1100 ЊC and 1177 ЊC, in specimens in which primary MC dissolution had been obtained at solution treatment temperatures of 1190 ЊC to 1250 ЊC. A schematic time-temperature-transformation (TTT) diagram for understanding the microstructure and precipitation inter-relationships in UDIMET 520 alloy is also presented.