Although face-centered cubic (fcc) H/MEAs (high and medium entropy alloys) are promising future structural materials at room and low temperatures, their relatively low yield strength limits their potential in many engineering applications. Traditional methods of strengthening metal materials, such as reducing grain size or employing precipitation hardening, may be effective, but they can adversely affect fracture toughness due to the inherent conflict between strength and toughness. Additive manufacturing (AM) processes, such as laser powder bed fusion (LPBF), provide an alternative microstructure design pathway to overcome strength-toughness trade-offs by introducing micro- and mesoscale structures. This method has been shown to enhance the strength of the material while maintaining (or even strengthening) its fracture toughness and damage tolerance. Punit Kumar et al. from the Department of Materials Science and Engineering at the University of California, Berkeley, USA, used a nonlinear elastic fracture mechanics method to evaluate the resistance of the microscopic and mesoscopic layered structures of LPBF CrCoNi to crack propagation by measuring the R-curve behavior in the form of J-integral for crack propagation. The specific characteristics of the hierarchical structure at different length scales provide a basis for the enhanced toughness performance of entropy alloys in additive manufacturing. The correlation between this deformation and the layered structure of different length scales provides guidance for improving the fracture toughness of medium-entropy alloys in the future.
CrCoNi blocks with a size of 25×25×25 mm3 were prepared by LPBF process using Cr, Co and Ni prealloy powders. The dimensions of the compact tensile specimen C(T) and the dogbone tensile specimen are shown in Fig. 1C and D, respectively. Uniaxial tensile and fracture toughness measurements were performed at room temperature (298 K) and liquid nitrogen (77 K). Prior to testing, the tensile specimen is ground on all sides with SiC paper to remove the oxide layer formed during EDM. According to the uniaxial tensile engineering stress-strain curve, the yield strength, ultimate tensile strength and elongation at break of the specimen were determined.
Figure 1. (A) Scanning strategy for the LPBF process in CrCoNi samples. (B) Compact tensile specimen C(T) and the relative orientation of the tensile specimen relative to the build plate, where the z-axis is parallel to the build direction (BD), the y-axis is parallel to the scan direction (SD), and the x-axis is parallel to the transverse (TD). (C) (T) SPECIMEN SIZE, (D) TENSILE SPECIMEN SIZE.
Figures 2B and C show grain growth within the laser scan trajectory and across the melt pool boundary, respectively. On a plane parallel to the build direction (BD), the average grain size d is ~13±15 μm. The IPF plot in Figure 2B shows that the size of the grains is limited by the width of the laser scan trace on the BD plane, resulting in a synthetic microstructure. The microstructure of LPBF CrCoNi is similar to that of other FCC alloys produced by additive manufacturing processes.
Figure 2. (A) Three-dimensional microstructure showing the laser scan trajectory (white dotted line) on a plane parallel to the build direction and the melt pool on a plane perpendicular to the build direction. (B) IPF plot obtained by electron backscatter diffraction (EBSD) showing multiple grains within the laser scan trajectory and columnar grains in the SD and TD planes. (C) Image showing anomalous grains growing on multiple melt pool boundaries. (D) Transmission electron microscopy (TEM) image showing the dislocation complex structure inside the grain.
Figure 3. SEM fracture morphology of C(T) specimen after fracture toughness measurement at (A) 298 K and (B) 77 K.
In the process of plastic deformation of LPBF CrCoNi, the lack of nanotwins and extensive lamination formation, especially in the presence of dislocation cell structure, the movement of dislocations is limited. In addition, the critical twin stress of additively manufactured CrCoNi is 800-900 MPa, which is much lower than the stress experienced by the tensile specimen in this study. This points to the role of the chemical composition of the material in influencing the deformation mechanism. For example, the activation of the deformation mechanism is directly related to the lamination energy of the FCC alloy, which in turn depends on the alloy composition.
In order to study the influence of crack paths and microstructure during fracture, compression specimens used for fracture toughness testing were sectioned in medium thickness to expose the main fracture paths under planar strain conditions. The IPF plots in Figures 4A and B show the areas of crack initiation and subsequent propagation in the 298 K and 77 K fracture tests, respectively. These images show that the cracks propagate mainly along the boundaries of the laser scan trajectory (indicated by the white dotted line in Figure 4) as they coincide with the boundaries of the grains within the scan trajectory, which results in the deflection of the crack path.
Figure 4.Electron Backscatter (EBSD) IPF plot showing the crack path during overload fracture and the interaction of the crack with the microstructure. IPF diagram of crack initiation location: (A) 298 K, (B) 77 K; (C) IPF diagram of crack stop location during 298 K fracture toughness test.
While the dislocation cell structure strengthens the LPBF CrCoNi alloy, the material is intrinsically toughened by plastic deformation before the crack tip and extrinsically toughened by the distortion of the crack path distortion due to the distorted grains of the laser scanning trajectory/><110 texture. It has long been thought that natural materials are processed "from the bottom up" into a hierarchical structure, which often gives them a good combination of strength and toughness. With traditional manufacturing using "top-down" machining, it is difficult to manipulate the structure at multiple length scales to introduce this layering feature, such as increasing the strength of the material without compromising its toughness. This study shows that the layered microstructure features introduced by additive manufacturing of medium-entropy CrCoNi alloys using the laser powder bed fusion process can easily provide reinforcement without affecting fracture toughness. The strength of the CrCoNi samples prepared by the LPBF process was higher at 298 K and 77 K than that of the conventionally prepared CrCoNi. In addition, despite the significant increase in yield strength, the ideal fracture toughness is maintained at 298 K and 77 K. This study suggests that the additive manufacturing (AM) process may be a viable approach to address the trade-off between strength and toughness of materials.
相关研究成果以“On the strength and fracture toughness of an additive manufactured CrCoNi medium-entropy alloy”为题发表在Acta Materialia（Volume: 258，2023，119249）上，论文第一作者为Punit Kumar，通讯作者为Robert O. Ritchie。
Paper Links:
https://doi.org/10.1016/j.actamat.2023.119249
Source: Multiscale Mechanics
If readers want to understand the current situation and development trend of laser application in new energy vehicles,
Welcome to join us in July 2024 to participate in our Yangtze River Delta G60 Laser Alliance
Conference on the application of laser in new energy vehicles