Compared with traditional CrMnFeCoNi high-entropy alloy, 3D printed carbon-containing FeCoCrNiMn high-entropy alloy exhibited excellent high-temperature creep resistance (i.e. creep rate and threshold stress minimized). Inha University and Korea Institute of Materials Science studied the high-temperature creep behavior of laser powder bed fusion (LPBF) carbon-containing high-entropy alloys for the first time, and explained the influence of nano-scale carbides on creep resistance.
The carbon-containing CrMnFeCoNi HEA (hereinafter referred to as C-HEA) powder contained 1.5 at% C and an average particle size of 23.7 μm. The scanning speed of laser powder bed fusion (LPBF) is 600 mm/s, the power is 90W, the scanning distance is 0.08mm, and the layer thickness is 0.025mm. To stabilize the subgrains and form additional nanoscale carbide particles, the samples were heat-treated at 650 °C for one hour.
The high-temperature creep test of LPBF C-HEA was carried out under a constant stress of 175–325 MPa at a temperature of 873 K (the temperature stability of 0.2 K was maintained during the creep test, as shown in Figure 1), and the creep test of the specimen The interval is 86.4 K. In order to stabilize the creep strain, a creep test of 259.2 ks was performed at 150 MPa, followed by a multi-step creep test.
Figure 2 shows the SEM–EDS spectrum and EBSD analysis results of LPBF C–HEA. The constituent elements in LPBF C–HEA were found to be uniformly distributed even after heat treatment, suggesting that LPBF and subsequent heat treatment do not affect the compositional uniformity of micron-scale HEA.Figure 2b shows the EBSD inverse pole figure (IPF) map at low magnification and reveals that the alloy has a layered and non-uniform grain structure.After heat treatment, the average grain size (AGS) did not change significantly and was similar to that of the as-built C–HEA.Remark that the EBSD results and XRD patterns in Figure 2b confirm that the present alloy has a single phase of FCC,The high-magnification IPF map clearly shows highly jagged grain boundaries (GBs), which significantly improves high-temperature creep by inhibiting GB sliding (Fig. 2C 1). Geometrically necessary dislocations (GNDs) form low-angle grain boundaries (LAGBs) within the grains (Fig. 2C), and the alloy still exhibits an extremely high GND density after heat treatment at 650 °C.
The formation of jagged grain boundaries is mainly seen in the second phase particles contained in metallic materials, such as nickel-based superalloys and magnesium alloys.The formation of jagged GBs due to the pinning effect of second phase particles during grain growth has been well documented.In other words, heat treatment leads to grain growth, and the second-phase particles inhibit the grain growth in localized areas, resulting in the zigzag appearance of GB.However, the aging treatment used in this study did not induce any grain growth, suggesting that the highly jagged grain boundaries in this alloy are caused by the melting and solidification steps of LPBF.In a recent report, 3D printed metallic materials with in situ precipitation also exhibited jagged GB.Notice that the highly jagged GB has been seen in the as-built C-HEA.This suggests that the pinning effect is caused by the high density of in-situ carbides at the grain boundaries during the cyclic heat treatment, which results in highly jagged grain boundaries.
Figure 3a is the ECC image of LPBF C–HEA, showing the existence of substructures induced by the dislocation network.The measured average width of these substructures is 534.2 ± 16.3 nm.Previous studies have shown that the substructure is stabilized by additionally formed nanoscale carbide precipitates with partially rearranged dislocations.Figure 3b shows that there are a large number of irregularly shaped nano-sized carbides (white arrows) at the substructure boundaries.HAADF STEM images and corresponding EELS maps were acquired to further understand the chemical heterogeneity inside carbides, as shown in Fig. 3c. The nanocarbides are mainly composed of Cr and C, indicating that these carbides are rich in Cr.
As shown in Fig. 4, in support of these findings, thermodynamically calculated equilibrium phase diagrams for the chemical composition of LPBF C–HEA using the Thermo–Calc software and an upgraded version of the TCFE2000 database.The phase diagram shows that M23C6 type carbides are mainly formed in the temperature range of 500–1000 °C, indicating that the Cr23C6 phase is the main component of LPBF C–HEA.On the other hand, in the literature, the Cr23C6 carbides of CoCrFeMnNi HEA exist in the scale of several microns, and the carbon content is 1.3-1.8 at%.In contrast, the alloy contains nanosized carbides even after heat treatment, suggesting that a metastable substructure with a high density of dislocations controls the formation of nanosized carbides with a uniform distribution.Meanwhile, manganese-rich oxides were also observed on the EELS maps, and they were reported to consist of MnO in LPBF C–HEA.However, the strengthening effect of the MnO phase is low relative to Cr23C6; therefore, carbides are considered as the main contributors to the strength in this study.
Figure 5a shows the multilevel creep curves of LPBF O–HEA, LPBF C–HEA, and LPBF CrMnFeCoNi reinforced with nano-oxides. In all creep stress ranges, LPBF C–HEA exhibited lower creep strain (i.e., higher creep resistance) than the reference materials (LPBF CrMnFeCoNi and LPBF O–HEA).Furthermore, compared with the creep results of LPBF CoCrFeMnNi, LPBF C–HEA exhibited the lowest minimum creep rate in all creep stress ranges.In particular, at an applied stress of 225 MPa, the minimum creep rate of LPBF C–HEA is about two orders of magnitude lower than that of conventionally processed alloys.This means that heat treatment not only greatly improves the room-temperature mechanical properties, but also improves the high-temperature creep resistance in the additively manufactured HEA, which contains supersaturated carbon induced by rapid solidification. The black dots for the single-step creep in Fig. 5b indicate the good reliability and reproducibility of the multi-stage creep test.
As shown in Figure 6, the high-temperature creep deformation behavior of LPBF C-HEA was explored by examining the large-scale microstructure using the GND distribution map and the IPF map.An earlier study of the creep behavior of equiatomic CrMnFeCoNi HEAs found a significant increase in strain during creep at 873 K, especially when a large amount of stress was applied, suggesting microstructural evolution.However, the IPF plot in Figure 6 shows that no microstructural evolution occurred in the creep sample with a creep strain of 7%, even at an applied stress of 325 MPa.Furthermore, as shown in Fig. 7a, substructures not observed in the EBSD map of the initial sample were found to appear in the creep microstructure. This indicates that the unique initial microstructure suppresses dislocation motion and microstructure evolution, and leads to the excellent creep resistance of LPBF C-HEA.As indicated by the black arrows in Figure 6, ultrafine grains with a size of ~2 μm were observed in some regions, which will be discussed later.
As shown in Figure 7a,High resolution IPF map of a creep sample.The severely jagged GBs observed in the creep microstructure suggest that nanoscale carbides cause severe GB jaggedness during creep deformation.In many cases of FCC-based metallic materials, jagged GBs hinder grain boundary sliding, thereby improving creep resistance at high temperatures.Reported that the enhanced creep resistance was associated with lower cavitation rates and crack propagation through GB serrations.For austenitic stainless steels, the formation mechanism of jagged grain boundaries is usually related to the interaction between grain boundaries and carbide precipitates: 1) grain boundary migration between pinned grains and 2) influence of carbide growth. LPBF C–HEA did not show any growth of carbides after creep deformation (Fig. 7c–d).Therefore, it can be inferred that the formation of jagged grain boundaries may be attributed to the grain boundary migration between pinned particles.
The GND profile in Fig. 7b shows the subgrains in the creep sample. Although the ECC image (Fig. 3a) shows that the initial sample has substructures decorated with dislocation networks, by EBSD observation, the substructures are indistinguishable.In contrast, the creep samples clearly contained subgrains with high GND density, indicating that dislocations accumulated at the substructure boundaries as well as grain boundaries during high temperature creep.This demonstrates that the substructure boundaries can successfully block dislocation motion even under high-temperature creep deformation. High-magnification ECC images support highly accumulated dislocations at subgrain boundaries (Fig. 7c).Here, the mechanism of lattice pinning and dislocation junctions of HEA is explained by the joint effect of forest dislocations and concentrated solution hardening.However, the present alloy exhibits subgrains with high GND density after creep deformation, suggesting that the creep mechanism of LPBF HEA nanocomposites is somewhat different from that of deformed HEA.Next, ECCI was used to examine the recrystallized ultrafine grains in the creep samples (Fig. 7d), which have a low internal dislocation density and are confined by carbides.For metallic materials, the driving force for recrystallization gradually increases with increasing temperature. However, considering that LPBF C–HEA generates a large amount of precipitation, which leads to Zenner pinning pressure, recrystallization is suppressed even at high temperature.Therefore, LPBF C–HEA did not undergo any microstructural evolution, such as recovery and recrystallization, under high-temperature creep deformation after applying a stress of 325 MPa.Although recrystallized ultrafine grains were observed in some regions, they were confined by nanometer-sized carbides, which prevented further grain growth.Careful examination of the creep deformation structure by ECCI and EBSD led to the conclusion that stable subgrains with a dislocation network and nano-sized carbides retard recovery and recrystallization during creep deformation while further strengthening the dislocation network induced substructure.
Summary:
The additive manufacturing process and subsequent heat treatment of carbon-containing CrMnFeCoNi HEA lead to the formation of not only heterostructure grains with substructures decorated with dislocation networks, but also uniformly distributed carbides at grain and subgrain boundaries.
The high-temperature creep resistance of LPBF C-HEA is better than that of reported CrMnFeCoNi high-entropy alloys. The creep rate of C-HEA is two orders of magnitude lower than conventionally processed HEA.
Microstructural observation confirms that stable subgrains induce the formation of extremely jagged grain boundaries, which further strengthen the subgrains and inhibit recrystallization during high-temperature creep, resulting in excellent creep resistance sex.
Key words:Additive Research, Metal additive manufacturing,Mana Materials, Metal 3D printing
