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Frontiers of Mechanical Engineering

Front. Mech. Eng.    2019, Vol. 14 Issue (1) : 113-127
Fatigue crack initiation of magnesium alloys under elastic stress amplitudes: A review
B. J. WANG1, D. K. XU2(), S. D. WANG2, E. H. HAN2
1. School of Environmental and Chemical Engineering, Shenyang Ligong University, Shenyang 110159, China
2. CAS Key Laboratory of Nuclear Materials and Safety Assessment, Institute of Metal Research, Chinese Academy of Sciences, Shenyang 110016, China
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The most advantageous property of magnesium (Mg) alloys is their density, which is lower compared with traditional metallic materials. Mg alloys, considered the lightest metallic structural material among others, have great potential for applications as secondary load components in the transportation and aerospace industries. The fatigue evaluation of Mg alloys under elastic stress amplitudes is very important in ensuring their service safety and reliability. Given their hexagonal close packed structure, the fatigue crack initiation of Mg and its alloys is closely related to the deformation mechanisms of twinning and basal slips. However, for Mg alloys with shrinkage porosities and inclusions, fatigue cracks will preferentially initiate at these defects, remarkably reducing the fatigue lifetime. In this paper, some fundamental aspects about the fatigue crack initiation mechanisms of Mg alloys are reviewed, including the 3 followings: 1) Fatigue crack initiation of as-cast Mg alloys, 2) influence of microstructure on fatigue crack initiation of wrought Mg alloys, and 3) the effect of heat treatment on fatigue initiation mechanisms. Moreover, some unresolved issues and future target on the fatigue crack initiation mechanism of Mg alloys are also described.

Keywords Mg alloys      fatigue behavior      microstructure      crack initiation      deformation mechanism     
Corresponding Authors: D. K. XU   
Just Accepted Date: 12 December 2017   Online First Date: 29 December 2017    Issue Date: 30 November 2018
 Cite this article:   
B. J. WANG,D. K. XU,S. D. WANG, et al. Fatigue crack initiation of magnesium alloys under elastic stress amplitudes: A review[J]. Front. Mech. Eng., 2019, 14(1): 113-127.
Fig.1  Microstructure image of the as-cast Mg-12%Zn-1.2%Y-0.4%Zr alloy. Image (a) shows the location of Image (b) [5]
Fig.2  Nonpropagating fatigue crack initiation at (a) slip bands in the grain interior, (b) slip bands near the I-phase/α-Mg matrix interfaces, and (c) magnified image of the squared area in Image (b) [5]
Fig.3  Retarding effect of the I-phase on fatigue crack propagation. The square in Image (a) shows the location of Image (b) [5]
Fig.4  S-N curve of the as-cast Mg-12%Zn-1.2%Y-0.4%Zr alloy [5]
Fig.5  Overall fracture surfaces of Samples (a) S1, (b) S2, (c) S3, and (d) S4, showing the fatigue crack initiation sites at the interior porosities (indicated by red cycles) [5]
Fig.6  Overall fracture surfaces of Samples (a) S5, (b) S6, (c) S7, and (d) S8, showing the fatigue crack initiation sites at the external porosities (indicated by red cycles) [5]
Fig.7  High-magnification images of the porosity located at the crack initiation site of Sample S1 (cycled in Fig. 6(a)): (a) Secondary SEM image, (b) backscatter SEM image, and (c) fatigue striations. The arrowhead in (a) indicates the location of Image (c) [5]
Fig.8  SEM images showing porosities originating fatigue crack in the GPM-NZ30K-T6 alloy (105 MPa cycled for 2.4×105 cycles). Fatigue crack formation sites are indicated by a white arrow. The high-magnification image of the area indicated by the square in Image (a) is shown in Image (b) [24]
Fig.9  SEM images showing the free surface-originating fatigue cracks in the LPS-NZ30K alloy (105 MPa cycled for 2.35×106 cycles). The arrows indicate the crack initiation sites. the high-magnification image of the area indicated by squares in Image (a) is shown Image (b) [24]
Fig.10  Optical images of the specimen sections parallel to the gauge length beneath the fracture surface of the LPS-NZ30K alloys: (a) T4 and (b) T6 [24]
Fig.11  Twinning accumulation of the sample fatigue tested at a stress amplitude of 30 MPa for (a) 103, (b) 104, (c) 105 and (d) 106 cycles [13]
Fig.12  Microstructural analyses: (a) EBSD analysis for orientation mapping (inserted) and the misorientation angle distribution between the twinned area and the Mg matrix for the sample fatigue tested at a stress amplitude of 30 MPa for 105 cycles; (b,c) surface images and (d,f) fractography of the fatigue sample, which failed after 5×105 cycles at a stress amplitude of 35 MPa; (c) high-magnification images of the squared area in (b); (e,f) high-magnification images of the squared areas labeled “e” and “f” in (d); the EDS result at the crack initiation is inset in (e); “SB” and “TB” in (e) denote slip band and twin boundary, respectively [13]
Fig.13  Optical microscopy images of the as-forged and T4 treated samples subjected to fatigue testing at stress amplitudes of 100 and 70 MPa: (a,b) 0, (c,d) 103, (e,f) 105, and (g,h) 5 × 106 cycles. Images of (a, c, e, and g) were taken on an as-forged sample surface; Images (b, d, f, and h) were taken on a T4 treated sample surface [11]
Fig.14  SEM images of the surface of the surviving fatigue samples after 5 × 106 cycles: (a) As-forged, 100 MPa, showing microcracks initiate along slip bands; (b) T4, 70 MPa, showing microcracks initiated at twin boundaries “SB” and “TB” in Image (b) denote slip band and twin boundary, respectively [11]
Fig.15  High-magnification images of the fatigue fracture surface morphology of Samples S1, S2, N1, and N2. Images (a,b) are secondary electron images of the oxide film located at the fatigue crack initiation regions of Samples S1 and S2, respectively; Images (c,d) are secondary electron images of the crack initiation sites of Samples N1 and N2, respectively. Notably, the inserts in Images (a,b) are EDS results of the corresponding marked areas in the images; the inserts in Images (c,d) are high-magnification images of the corresponding marked areas [11]
Fig.16  Fatigue lifetime behaviors of the three conditions of WE43 with the data points from step tests. The similarities between the microstructures of the under-aged and T6 conditions result in little difference in the fatigue lifetime behavior, which becomes more evident with the inclusion of step tests. Subsurface initiation was observed in the coarse-grained conditions at high lifetimes [44]
Fig.17  Crack initiation sites in T5 WE43 showing (a) single facets, (b) groupings of multiple facets, and fatigue damage sites at (c) intergranular and (d) transgranular locations [44]
Fig.18  Crack initiation sites in coarse-grained WE43 showing (a,b) large single facets, and (c) nonfatal crack initiation site at a grain boundary [44]
Fig.19  Subsurface crack initiation sites in large-grained WE43 showing (a) ‘‘supergrain” groupings and (b) chevron groupings of facets [44]
Fig.20  Diameters and orientations (angle of plane normal with respect to load) of facets present at (a) the T5 surface initiation sites, (b) the under-aged and T6 surface initiation sites, and (c) under-aged and T6 subsurface initiation sites [44]
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