TY - JOUR

T1 - In-situ observations of high cycle fatigue mechanisms in cast AM60B magnesium in vacuum and water vapor environments

AU - Gall, Ken

AU - Biallas, Gerhard

AU - Maier, Hans J.

AU - Gullett, Phil

AU - Horstemeyer, Mark F.

AU - McDowell, David L.

AU - Fan, Jinghong

N1 - Funding Information: The authors thank Anja Puda for her careful surface preparation of the cast Mg alloy specimens for in situ studies. This work was sponsored by the US Department of Energy under contract DE-AC04-94Al85000, and was performed with the support of Dick Osborne and Don Penrod for the USCAR Lightweight Metals Group. Funding for K. Gall was provided by a DOE PECASE award from Sandia National Laboratories.

PY - 2004/1

Y1 - 2004/1

N2 - 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.

AB - 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.

UR - http://www.scopus.com/inward/record.url?scp=0242490145&partnerID=8YFLogxK

U2 - 10.1016/S0142-1123(03)00079-3

DO - 10.1016/S0142-1123(03)00079-3

M3 - Article

AN - SCOPUS:0242490145

VL - 26

SP - 59

EP - 70

JO - International journal of fatigue

JF - International journal of fatigue

SN - 0142-1123

IS - 1

ER -