In-situ observations of high cycle fatigue mechanisms in cast AM60B magnesium in vacuum and water vapor environments
Gall, Ken and Biallas, Gerhard and Maier, Hans Jürgen and Gullett, Phil and Horstemeyer, Mark F. and McDowell, David L. and Fan, Jinghong (2004) In-situ observations of high cycle fatigue mechanisms in cast AM60B magnesium in vacuum and water vapor environments. International Journal of Fatigue, 26 (1), pp. 59-70.
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We present in situ scanning electron microscopy (SEM) observations regarding the formation and propagation of small fatigue cracks in cast AM60B magnesium. Using an environmental SEM, observations were made in vacuum and in the presence of water vapor at 20 Torr. In the vacuum environment, fatigue cracks in the magnesium formed preferentially at pores, sometimes precluded by observable cyclic slip accumulation. At higher cycle numbers in the vacuum environment, additional cracks were discovered to initiate at persistent slip bands within relatively large magnesium dendrite cells. The propagation behavior of small fatigue cracks (a ‹ 6–10 dendrite cells) was found to depend strongly on both environment and microstructure. Small fatigue cracks in the magnesium cycled under vacuum were discovered to propagate along interdendritic regions, along crystallographic planes, and through the dendrite cells. The preference to choose a given path is driven by the presence of microporosity, persistent slip bands, and slip incompatibilities between adjacent dendrite cells. Fatigue cracks formed more rapidly at certain locations in the water vapor environment compared to the vacuum environment, leading to a smaller total number of cracks in the water vapor environment. The majority of small cracks in magnesium cycled in the water vapor environment propagated straight through the dendrite cells, at a faster rate than the cracks in the vacuum. In the water vapor environment, cracks were observed to grow less frequently through interdendritic regions, even in the presence of microporosity, and cracks did not grow via persistent slip bands. The propagation behavior of slightly larger fatigue cracks (a › 6–10 dendrite cells) was found to be Mode I-dominated in both environments.
|Title:||In-situ observations of high cycle fatigue mechanisms in cast AM60B magnesium in vacuum and water vapor environments|
|Journal or Publication Title:||International Journal of Fatigue|
|In ISI Web of Science:||Yes|
|Page Range:||pp. 59-70|
|Keywords:||cast magnesium, in-situ fatigue, small cracks, environmental degradation, surface layer, hydrogen embrittlement, damage mechanisms|
|HGF - Research field:||Aeronautics, Space and Transport|
|HGF - Program:||Aeronautics|
|HGF - Program Themes:||L AR - Aircraft Research|
|DLR - Research area:||Aeronautics|
|DLR - Program:||L AR - Aircraft Research|
|DLR - Research theme (Project):||L - Structures & Materials|
|Institutes and Institutions:||Institute of Materials Research > Mechanical Testing of Materials|
|Deposited By:||Dr.-Ing. Gerhard Biallas|
|Deposited On:||27 Jan 2006|
|Last Modified:||27 Apr 2009 04:55|
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