Temperature-dependent fatigue response of a Fe44Mn36Co10Cr10 high entropy alloy: A coupled in-situ electron microscopy study and crystal plasticity simulation
Graphical abstract
Introduction
The coming of high entropy alloys (HEAs) with concentrated compositional constituents has given rise to a profuse platform in expediting the search for optimal mechanical and functional performances [1], [2], [3], [4], [5], [6]. For structural applications, numerous reports have revealed the more promising strength-ductility synergy [7], [8], [9], fatigue [10], [11], [12], [13], wear [14] and creep resistance [15] that HEAs can exhibit compared with their dilute counterparts. Amongst them, temperature-dependent fatigue response has drawn particular attention, since practical structural components are inevitably subjected to cyclic loading ranging from room to elevated temperatures. Hemphill and Tang et al. [10], [16] studied fatigue property of Al0.5CoCrCuFeNi HEA at room temperature (RT), which exhibited long fatigue lives even at relatively high stresses. The fatigue endurance limit was obtained and surpassed conventional alloys. Mechanical twinning was identified as the major deformation module, giving rise to micro-crack nucleationand propagation. Lu et al. [17] studied low cycle fatigue (LCF) behavior of equiatomic CoCrFeMnNi HEA at 550 °C. Compared to conventional FCC steels, the HEA exhibited a better LCF life. Microscopic observations after the LCF experiments showed that the dislocation density increased significantly, and the dislocation slip, accumulation and entanglement were observed. Cracks were found to initiate at the local defects induced by the casting process and propagated transgranularly.
The fatigue behavior associated with microstructural evolutions is important for clarifying the deformation mechanism of HEA. Tu-Ngoc [18] and Kai [19] et al. studied the fatigue crack behavior of CoCrFeMnNi HEA at RT, and found the dislocation slip in the plastic zone at the crack tip is the main factor causing fatigue crack propagation. Kaiju et al. [20] conducted LCF experiments of CoCrNi and CoCrFeMnNi at RT, and transmission electron microscopy (TEM) observation was conducted to study the deformed microstructures after failure. The results showed CoCrFeMnNi exhibited wavy dislocation configuration, while for CoCrNi planar dislocations were observed, which reduced the localization of plastic deformation and improved the fatigue property. S. Picak et al. [21] studied the CoCrFeMnNi HEA at RT, and suggested there were significant differences in dislocation structure between monotonic and cyclic loading. Takeshi et al. [22] investigated the fatigue crack behavior of a dual-phase HEA at RT. Electron back-scatter diffraction (EBSD) and electron channeling contrast imaging (ECCI) analysis was employed on the specimens after fracture. Evident misorientation change and high dislocation density were revealed, indicating the desirable strain hardenability of this HEA. However, although considerable effort has been made to study the fatigue performance of various HEAs, most research is limited to fracture analysis and microscopic observation after fracture, there has been little work done on employing in-situ SEM to reveal microstructure evolution during fatigue process and the temperature effect on deformation mechanism.
With the advancement of EBSD technique, several quantitative parameters have been proposed to characterize misorientation and plastic deformation, such as Kernel Averaged Misorientation (KAM) and Grain Reference Orientation Deviation (GROD) [23], [24], [25], [26]. Further, the calculation method of geometrically necessary dislocation (GND) density based on EBSD was proposed and developed [27], [28], [29], [30], and has been applied to the analysis of dislocation density and deformation mechanism of various metal materials [31], [32], [33], [34], [35], [36], [37], [38], [39], [40], [41].
EBSD can also be combined with CPFE simulation for polycrystalline metals to improve the simulation capability by providing the microstructural information especially crystallographic orientation. CPFE simulation for polycrystals involves not only the description and integration of crystal plasticity law, but also the assignment of material microstructural features such as grain morphology and crystal orientation of each grain. The current methods largely exploit the EBSD statistical parameters such as average grain size, variance and Orientation Distribution Function (ODF), and employ algorithms such as Voronoi Tessellation Method (VTM) to generate random polycrystalline grain geometries and representative volume element (RVE). There are already softwares such as DAMASK [42], Dream3D [43] and Neper [44], [45], [46] that can implement these pre-processing function and connect with various finite element solvers. There have been some effective applications such as compression and tension [47], [48], [49], indentation [50] and fatigue [51], [52], [53], [54], [55], [56]. However, CPFE simulation of polycrystals based on real EBSD-measured grain morphology is still in its fancy, especially the polycrystalline HEA fatigue simulation is very lacking. Moreover, dominant slip system map can be obtained from CPFE simulation and has been proved to be closely related to slip line and crack behaviors [57], [58], [59], [60], but there has been little work done on the investigation of dominant slip system of HEA fatigue.
The major objective of this paper is to obtain a comprehensive understanding of the fatigue behavior of Fe44Mn36Co10Cr10 HEA at RT and elevated temperature using in-situ SEM observation, EBSD and CPFE simulation. Firstly, in-situ SEM fatigue experiments of HEA at RT and 300 °C will be conducted, and the microstructure evolution including slip line and short crack will be observed. The difference of microstructure evolution between RT and 300 °C will be revealed and associated with fatigue behaviors. Secondly, EBSD analysis of non-deformed and specimens containing short cracks will be carried out, and the misorientation and dislocation density will be presented. Finally, a method to establish polycrystal CPFE model based on the EBSD-measured microstructures will be proposed, and cumulative plastic strain and dominant slip system map will be obtained. The in-situ SEM, EBSD and CPFE simulation results will be analyzed together to provide a comprehensive clarification of fatigue deformation mechanism of Fe44Mn36Co10Cr10 HEA.
Section snippets
Material fabrication
The HEA with a nominal composition of Fe44Mn36Co10Cr10 (at.%) was fabricated by vacuum arc melting. The metallic components had purities higher than 99.9 wt% and were repeatedly melted 5 times to ensure homogeneous microstructure. Then the as-cast ingot was processed with hot-rolling, followed by 1200 °C homogenization for 6 h in vacuum and furnace cooling. Energy Dispersive Spectroscopy (EDS) analysis was conducted and the element distributions of non-deformed Fe44Mn36Co10Cr10 HEA were shown
In-situ fatigue experimental results at RT
Fig. 6 shows the fatigue crack initiation and propagation process of Fe44Mn36Co10Cr10 HEA at RT. When fatigue loading reaches 2 × 104 cycles, a clusters of parallel slip lines appeared in the left side of the notch area (see Fig. 6(a)), which indicates plastic deformation occurred in the notch area under fatigue loading. As can be seen from the local enlarged image, the slip lines were almost perpendicular to the arc of the notch and slip line spacing was nearly uniform. As the slip lines
Conclusion
In this paper we investigated the fatigue deformation behavior of Fe44Mn36Co10Cr10 HEA at RT and 300 °C using in-situ SEM, EBSD and crystal plasticity simulation. The main conclusions are as follows:
- (1)
The damage behaviors of HEA including slip line and short crack evolution were observed by in-situ SEM fatigue experiments. At RT and 300 °C, a large number of slip lines were observed in the pre-notched area. At RT, the slip lines were mainly along the direction perpendicular to the edge of the
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.
Acknowledgment
This work is financially supported by the National Natural Science Foundation of China (Nos. 12002155, 91860101 and 11632010), National Major Science and Technology Projects of China (No. 2017-VI-0003-0073). The authors gratefully acknowledge Shaolou Wei for the valuable discussion on high entropy alloys and the critical feedback about dislocation plasticity. Manqiong Xu and Qianjin Wang are also acknowledged for their support with the experiments.
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