View source for Identification and modeling of fatigue crack growth mechanisms in a die-cast AM50 magnesium alloy

== Introduction ==

[[Image:Figure_1_AM50_Acta_Mat.bmp|400px|thumb|left|Fig. 1. Backscatter FEG-SEM micrographs  of a polished AM50 sample cut from the grip section of a fractured  fatigue specimen showing (a) an overall view  of the microstructure depicting  shrinkage  pore  clusters  and  an Al-rich eutectic  layer  fully  occupying  the dendrite  cell boundaries, (b) a higher magniﬁcation  image revealing  the formation  of the beta phase and Mn-rich phase inside the Al-rich eutectic layer.]]
The high strength-to-weight ratio of contemporary magnesium alloys that are suitable for ultimate weight reduction purposes in automotive and aircraft components has stimulated in the last decade substantial interest in understanding their fatigue crack growth behavior with process rationalization. Experimental  observations on cast Mg alloys  have  accumulatively revealed  that the dendrite cell  size,  pores,  secondary phase  particles,  persistent  slip bands and twinning in the dendrite cells considerably aﬀect the fatigue durability  and crack growth mechanisms of dendritic magnesium alloys [3–7]<ref>

[3] Mayer H, Papakyriacou M, Zettl B, Stanzl-Tschegg SE. Int J Fatigue, 2003;25:245–56.
[4] Horstemeyer  MF, Yang N, Gall Ken, McDowell DL, Fan J, Gullett, PM. Acta Mater 2004;52:1327–36.
[5] Gall  Ken,  Biallas  Gerhard,  Maier  Hans  J,  Horstemeyer  Mark  F, McDowell David L. Mater Sci Eng A 2005;396:143–54.
[6] Horstemeyer  MF, Yang  N, Gall K, McDowell D, Fan J, Gullett P., Fatigue  Fract  Eng Mater Struct 2002;25:1045–56.
[7] Shih Teng-Shih, Liu Wen-Sun, Chen Yeong-Jern.  Mater Sci Eng A,00202002;325:152–62.
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Our interest  of studying  the  AM50 alloy was motivated by the universal  trend to reduce the  aluminum  content  of  Mg–Al  alloys   in  an  eﬀort  to increase ductility under impact and fatigue loading for automotive  components.  In this paper, relevant micromechanisms of fatigue crack growth in an AM50 alloy are carefully examined.  Fatigue tests were carried  out in a fully reversed strain control condition at strain amplitudes ranging  from  0.1%  to  0.7%.  The  microstructure before and after fatigue failure was analyzed by means of ﬁeld emission  gun-scanning  electron  microscopy  (FEG-SEM) and electron probe microanalysis (EPMA) using specimens extracted  from the grip regions and gage lengths of tested samples, respectively.

[[Image:Figure_2_AM50_Acta_Mat.bmp|400px|thumb|rigth|Fig. 2. Backscatter FEG-SEM micrographs of a polished AM50 sample cut from the grip section of a fractured  specimen showing (a) the Al-rich eutectic layer surrounding  continuously the dendrite cells and the b-phase is formed spreading in the center of the Al-rich eutectic layer, (b) a higher magniﬁcation image of an Mn-rich inclusion juxtaposed of two particles;  the two overlying  particles  are substantially richer in Mn than the underlying darker  particle corresponding to the Al8Mn5   phase, and (c) a secondary electron  (SE) image of the same inclusion  shown in (b), the underlying  phase presents  as an agglomeration of extremely  small particles  (5 nm in diameter).]]