Localized dislocation interactions within slip bands and crack initiation in Mg-10Gd-3Y-0.3Zr alloy
Introduction
Magnesium (Mg) and its alloys, as the lightest structural metallic materials, are actively being developed for their potential use in automotive, aircraft, and aerospace industries, in which the weight savings translate to lower energy consumption [1], [2], [3]. The structural application of Mg alloys in these industries inevitably suffers cyclic deformation at very low-stress levels [4], which may result in fatigue failure after a high number of load cycles. Therefore, the high cycle fatigue behavior of Mg alloy has become one of the significant concerns that require a deep understanding of its failure mechanisms.
Fatigue failure of materials must experience a process of crack initiation, which is dominated by the deformation mechanism at the crystallographic scale. Under quasi-static loading conditions, it is well known that Mg alloys exhibit high directional anisotropy and are hard to deform at room temperature owing to their insufficient independent slip modes [5], [6], [7]. 〈c + a〉 dislocations are thought to be intrinsically unstable and cannot contribute to the c-axis plastic strain [5], [6]. In this regard, deformation twinning plays an essential role in the accommodation of c-axis strain to reduce the incompatibility of plastic deformation [8], [7]. Consequently, only basal slip and twinning can operate during plastic deformation at room temperature. Addition of rare-earth (RE) elements into Mg is a promising approach to improve its room-temperature ductility. For example, Gd and Y reduce the difference in critical resolved shear stress (CRSS) between the basal slip and 〈c + a〉 slip [9], [10], bringing in the improvement of monotonous plasticity. However, the cyclic deformation mechanism of RE-Mg alloy is quite different from its monotonic staining. It was reported the deformation twinning was absent when the cyclic stress/strain was applied at relatively low levels [11], [12], [13], [14]. Recently, further investigations found that basal slip is the solo deformation mode and slip bands (SBs) are only visible at isolated grains with favorable orientations [13], [15], [16]. Therefore, the improvement of RE elements on the deformation compatibility is questionable when the applied cyclic stresses are much lower than its yield strength.
The cyclic plasticity of metallic materials in fatigue is generally localized within slip bands (SBs), characterized by the formation of surface relief [17], [18]. Extensive and long-range research of the persistent SBs leads a progressive acknowledge of the sub-dislocation structure [17], [19], [20], [21], [22]. For Mg alloys, the characterization of dislocation activities strictly at the SB was limitedly studied. Kwadjo et al. found no sub-dislocation structures s in the interior of SBs but just dislocation arrays in the form of planner slip [23], while further correlations between the slip and crack are unknown. Due to the limited available slip systems at low stresses, the fatigue crack is found to initiate along the basal SBs. More importantly, the crack initiation process from the SBs within the size of the initial grain consumes a vast majority of the total fatigue life in a high cycle fatigue regime [15], [16]. Despite severe incompatible deformation at low-stress levels, the RE-Mg alloys exhibit enhanced resistances to slip-induced crack initiation [24]. However, its underlying mechanism is still unknown, which requires further insight into the characteristics of dislocation motion and interaction during the development of SBs.
The deformation of Mg alloys at low-stress amplitudes are associated with the dislocation slip. The movement and interaction of dislocations at a pathway form the SBs and play a decisive role in the initiation of fatigue cracks. Since the process of SB-cracking in primary grain consumes an extremely high proportion of total cyclic loadings in high cycle fatigue, it is significant to correlate the and dislocation activities and microstructural evolution in SBs with the fatigue crack initiation to improve the fatigue resistance of Mg alloys. In our present work, the cyclic deformation mechanisms and the development process of the SBs in RE-containing Mg alloys have been investigated to figure out slip-induced crack initiation mechanism in high cycle fatigue. We found the development of basal SBs is assisted by the motion of 〈c + a〉 dislocations, resulting in the formation and growth of dislocation intensity zones (DIZs) distributed along the SBs. The evolution process of DIZs is believed to retards the crack initiation from the SBs. These findings reveal a new formation mechanism of the SBs that can enhance the fatigue resistance of RE-Mg alloys.
Section snippets
Material and methods
The material used in this investigation is a RE-Mg alloy with the following nominal chemical composition: Mg-10Gd-3Y–0.5Zr (wt%, GW103K). High purity Mg (99.95%) ingots, Mg-25Gd (wt%), Mg–25Y (wt%) and Mg–30Zr (wt%) master alloys were melted under the protection of mix atmosphere of SF6 and CO2, and then poured into a permanent mold. The as-cast GW103K alloys were extruded at 400 °C with an extrusion ratio of 16:1 to achieve a 10-mm-diameter bar. Fig. 1a presents the optical microstructure of
Facet morphology at fatigue crack initiation site
Fig. 2 shows a typical fracture surface of a specimen tested at 120 MPa and finally failed at 8.9106 cycles. The fatigue crack initiation site is located at the specimen surface (Fig. 2a). The enlarged view of framed area “b” is presented as Fig. 2b, in which numerous facets indicated with hollow arrows are separately distributed around the crack initiation site. These facets are characterized by flat and smooth fracture morphologies with their sizes in a specific range (Fig. 2c), generally,
Cyclic deformation at low cyclic stress levels
The cyclic deformation and fatigue behaviors of Mg-Gd-Y alloy have been previously investigated with the strain amplitudes varying from 0.0275% to 5.0% [32]. The results showed that a kink point corresponding to a strain amplitude of 0.75% was detected according to the changes of predominant deformation mechanisms from deformation twinning to slipping. The Mg alloys prefer to deform in slip mode when the applied strain is lower than 0.75%. However, the activated slip system was still unclear in
Conclusions
In the present work, cyclic deformation at low-stress levels in an Mg-Gd Y-Zr alloy has been experimentally investigated to reveal the fatigue crack initiation mechanism from the SBs. The following conclusions can be drawn:
- (1)
Fatigue cracks initiate from the basal slip bands, causing a facet-like fracture surface around the initiation site.
- (2)
Basal slip is the solo deformation mode at the grain scale. Non-basal slip bands are invisible, but gliding of 〈c + a〉 dislocation also occurs at the localized
Declaration of Competing Interest
The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper.
Acknowledgements
This work was financially supported by the National Natural Science Foundation of China (Nos. 12072212 and 11832007), National Key Research and Development Program of China (No. 2018YFE0307104), and Sichuan University & ZiGong government Support Program (No. 2019-CDZG-4). Special thanks for the microstructural characterization at the ultramicroscopy research center of Kyushu University in Japan.
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