View source for Structure-property relations of cyclic damage in a wrought magnesium alloy

== Abstract ==

The fatigue properties of an extruded  Mg–3Al–0Mn magnesium  alloy  component  were evaluated  experimentally. Fully reversed, strain control fatigue tests were conducted on specimens extracted from regions with a varying grain size and texture. Scanning electron microscopy was employed to establish structure–property relations between microstructure and cyclic damage. Relations were drawn between microstructural features such as particle size, grain size, initial Taylor factor and the number of cycles to failure.

== Introduction ==

[[Image:Figure_1_S-P_AM30_GM.bmp|400px|thumb|left|Figure. 1.  The initial average  grain size and initial Taylor factor in the extruded  magnesium  AM30 alloy.  The fatigue  specimens  were machined  from  sections  A,  B,  C, E, G,  and  I  as  indicated  in  the cross-sectional  view of the extruded  alloy.]]
General Motors Research and Development Center (GM R&D) recently created a new extruded magnesium alloy, designated  AM30 [17]<ref> [17] A.A. Luo, A.K. Sachdev, Met. Mater. Trans. A 38 (2007) 1184.</ref>. In terms of microstructural inﬂuences  on fatigue  damage,  research has  previously been  conducted  on  similar  magnesium alloys with regards to microstructural eﬀects on the strain rate sensitivity  [21] <ref> [21] H. Kim, Y. Lee, C. Chung, Scr. Mater. 52 (2005) 473. </ref> and grain  size reﬁnement eﬀects on fatigue properties  [22,23] <ref> [22] F. Yang, S.M. Yin, S.X. Li, Z.F. Zhang, Mater. Sci. Eng. A 491 (2008) 131.
[23] Z.B. Sajuri,  Y. Miyashita,  Y. Hosokai,  Y. Mutoh, Int. J. Mech. Sci. 48 (2006) 198.
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Magnesium alloys with a ﬁner grain size exhibited a reduction in crack growth rate when compared to the same alloy with larger grains. Material orientation with respect to the extrusion direction and manganese content on fatigue life were shown to have a profound impact on both short and  long  crack development  [24] <ref> [24] H. Somekawa,  T. Mukai,  Scr. Mater. 53 (2005) 541. </ref>. Furthermore, the authors suggested that the crack closure behavior was largely responsible for variations in the fatigue life of diﬀerent specimen orientations. Further studies into the texture eﬀects were conducted on a magnesium alloy, AZ31 [25] <ref> [25] D. Raabe,  Acta Mater. 43 (1995) 1531.</ref>. These ﬁndings indicated that the fracture toughness of the material was signiﬁcantly  aﬀected by the orientation  of the specimen with respect to the extrusion  direction.
The objective of this research is to further  quantify  the cyclic  behavior   of  AM30 and  to  obtain  relations  be- tween the microstructure of the material and cyclic damage. As such, structure–property relationships that correlate material texture, grain size and critical ﬂaw size to fatigue life were determined.

== Results ==

[[Image:Figure_4_S-P_AM30_GM.bmp|300px|thumb|left|Figure.  4.  Number  of  cycles  to  failure  vs.  the  square   root  of  the particle  area  (a), average  grain size (b), and average  Taylor  factor  (c) for  the magnesium  AM30 alloy.  The locations  from  the rail  cross- section of the specimens are also shown.]]
[[Image:Figure_2_S-P_AM30_GM.bmp|300px|thumb|right|Figure.  2.  EBSD  mapping   showing   the  texture   characteristics of section A in the extruded  direction  (ED).]]
The specimens  were  machined  from  an  extruded  AM30 magnesium alloy crash rail parallel to the extrusion direction.  These cylindrical  dog-bone-shaped  specimens had  a gage length of 30 mm and  a diameter  of 6 mm. Since diﬀerent regions of the rail exhibited varying average grain sizes and texture, multiple specimens were machined from six predetermined  sections shown in Figure 1.

The average  initial  grain  size and  initial  Taylor  factors based on electron backscatter diﬀraction (EBSD) results of the rail cross-section are shown in Figure. 1. Figure. 2 shows the speciﬁc EBSD results from section A. The initial Taylor factors were only used to provide a comparative analysis of the distribution of the material strength in the rail component.


The objective of this study was to correlate structure–property relationships  in order  to predict the scatter  that is typically observed  in fatigue studies. The square  root of the particle area vs. the number of cycles to failure is shown in Figure.  4a. The area  of each particle  was calculated using  an  image  analyzing  software  program. The trend shows that specimens with smaller particles had greater fatigue resistance, and therefore better fatigue life, when compared  to specimens with larger particles.

To   further characterize   the  microstructure  of  the AM30 alloy in regards  to the fatigue behavior,  the average  grain  size  and  initial  Taylor   factors   are  plotted against  the number of cycles to failure. Again, the spec- imens were machined from sections that contained  various  grain  sizes  and  calculated   initial  Taylor  factors. Figure.  4b displays  the  number  of  cycles  vs. average grain  size.  While this  plot  displays  signiﬁcant  scatter, the trend suggests that the alloy  exhibits greater fatigue resistance   for  smaller  grains  compared   to  specimens with larger  grains. This observation is somewhat  unexpected considering larger grains typically provide better fatigue  resistance  in steel and aluminum  alloys  by providing grain  boundary blocking  that slows the rate of crack propagation. However, in this present study, smaller grains provided better fatigue resistance, which is consistent  with a similar extruded  Mg alloy  [22].

== References ==
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Citation:
Structure–property relations of cyclic damage in a wrought magnesium alloy, Bernard, J.D., Jordon, J.B., Horstemeyer, M.F., El Kadiri, H., Baird, J., Lamb, David, and Luo, Alan A., Scripta Materialia, v 63, n 7, p 751-756, 2010.



[[Category:Research Paper]]
[[Category:Fatigue]]
[[Category:Magnesium Alloys]]
[[Category:Structure–property]]
[[Category:Magnesium]]
[[Category:Aluminum]]