Full length articleOutstanding tensile properties of a precipitation-strengthened FeCoNiCrTi0.2 high-entropy alloy at room and cryogenic temperatures
Graphical abstract
Introduction
As emerging structural materials, single-phase high-entropy alloys (HEAs) with equiatomic or near-equiatomic concentrations have rapidly garnered much attention for transforming the conventional alloy design concept by shifting the search space of useful alloys from the corners of phase diagrams to their center regions [[1], [2], [3], [4], [5]]. Some single-phase HEAs with face-centered cubic (fcc) structure exhibit excellent resistance under irradiation [[6], [7], [8], [9]] and outstanding mechanical performance especially at cryogenic temperatures [[10], [11], [12], [13]], making them strong candidates for engineering applications in extreme environments. For example, when the temperature decreases from room to cryogenic temperatures, the single-phase fcc HEAs made of 3d transition metals (e.g., FeCoNiCr and FeCoNiCrMn) show both enhanced strength and ductility due to the onset of twinning at cryogenic temperatures [11,[13], [14], [15], [16]], making them the toughest alloys. However, their applications are limited by their low yield strength (YS). Though these HEAs have a large fluctuation of local lattice distortion [[17], [18], [19]], experiments revealed that it only provides Labusch-type weak pinning on dislocation motion [19,20]. An effective solution to the low YS is to incorporate other strengthening mechanisms, including grain refinement [[21], [22], [23]] and precipitation strengthening [[24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]].
Strength and ductility are mutually exclusive properties in metals and alloys, and grain refinement and precipitation strengthening inevitably lead to a strength-ductility trade-off. These single-phase fcc HEAs, however, are extremely ductile, making balancing YS and ductility relatively easy through proper grain refinement or precipitation strengthening. Several works have been conducted to improve the YS of the FeCoNiCr-based HEA by adding precipitate-forming elements, such as Mo, Nb, Cu, Al and Ti [[24], [25], [26], [27], [28], [29], [30], [31], [32], [33], [34], [35]]. For the case of adding Mo [24,25], incoherent precipitates of the Mo-rich σ and μ phases were formed. At room temperature the YS of the Mo-rich precipitation-strengthened FeCoNiCr HEA reached 800 MPa and meanwhile 20% ductility was obtained. For the FeCoNiCr HEA strengthened by coherent L12, Ni3(Al, Ti)-type γ′ precipitates under the tensile test at room temperature, a higher YS (∼1 GPa) was realized with remaining ∼20% ductility [32]. The CoCrNi medium-entropy alloy (MEA) also strengthened by coherent L12, Ni3(Al, Ti)-type γ′ precipitates shows a YS of ∼800 MPa and a good ductility of ∼40% at room temperature [36]. These works demonstrated that a combination of high YS and good ductility can be achieved at room temperature especially in the HEAs or MEAs strengthened by coherent precipitates.
However, the mechanical performance and deformation mechanism of the precipitation-strengthened HEAs at cryogenic temperatures are rarely explored. It is known that twinning can be enhanced in the single-phase FeCoNiCrMn and FeCoNiCr HEAs and NiCoCr MEA as a result of the reduced stacking fault energy (SFE) with decreasing temperature [16,[37], [38], [39]]. It is worth mentioning that twinning occurs at 77 K in the Al0.3CoCrFeNi HEA strengthened with incoherent B2 particles [13] and the fcc CuNiSi alloy with incoherent Ni2Si precipitates [40]. But a recent study shows that at cryogenic temperatures, rather than twins, stacking faults (SFs) prevail in the deformed FeCoNi-based MEA strengthened by coherent L12, Ni3(Al, Ti)-type γ′ precipitates [41], suggesting that the presence of coherent γ′ precipitates hinders the formation of twins. Many works have been conducted to examine the effect of coherent precipitates on twinning behavior in conventional alloys, such as Mg alloys and Ni-based superalloys [[42], [43], [44], [45], [46]]. Chun et al. [42] reported that after an aging treatment, twinning deformation in the MgZn alloy at 4.2 K was postponed to double the stress of the quenched state without precipitates. But it remains under debate about whether twin nucleation or twin growth is inhibited [43]. SFs and twinning in the γ′-strengthened Ni-based superalloys are often observed during the creep test at intermediate temperatures, and the formation of microtwins in Ni-based superalloys at intermediate temperatures is controlled by a diffusion-mediated reordering within the γ′ precipitates [44], which is different from the case of γ′-strengthened HEAs or MEAs under deformation at cryogenic temperatures because of negligible diffusion.
To better understand the effect of precipitate on the twinning behavior of low SFE HEAs at cryogenic temperatures, the FeCoNiCr HEA strengthened by coherent precipitates was prepared, and its tensile property and deformation mechanism at cryogenic temperatures were examined. Based on the evolution of deformation defects in the deformed samples, the effect of precipitates on the deformation mechanisms was discussed. The present research provides a guide for the design of tough HEAs for applications at cryogenic temperatures through combining precipitation strengthening and twinning/SF deformation.
Section snippets
Experimental procedure
An ingot with a composition FeCoNiCrTi0.2 was produced by arc melting Fe, Co, Ni, Cr and Ti metals with high purity (>99.9 wt%) in an argon atmosphere. After being repeatedly melted five times, the ingot was then drop-casted into a copper mold to make a slab with dimensions of 5 mm × 10 mm × 50 mm. The slab was homogenized at 1373 K for 5 h, followed by water quenching. Then, the homogenized slab was cold rolled with a total thickness reduction of 80%, subsequently recrystallized at 1373 K for
Microstructure
The XRD pattern obtained from the aged FeCoNiCrTi0.2 HEA (denoted by Ti-HEA hereafter) was shown in Fig. 1a. It appears that the aged Ti-HEA has only one phase with the fcc structure. The lattice constant of this fcc phase is 3.585 Å determined by the Rietveld refinement method [49]. Compared with the FeCoNiCr HEA [50], the lattice constant of the aged Ti-HEA slightly increases by 0.5%. Its microstructure was characterized by EBSD, as shown in Fig. 1b. We can see that the aged Ti-HEA has
Strengthening mechanisms
The contribution from precipitation strengthening to the YS can be approximately estimated as follows. The YS is a combination of a lattice friction stress and other strengthening mechanisms, including the solid-solution strengthening, precipitation strengthening and grain-boundary strengthening. After the subtraction of the YS of the single-phase FeCoNiCr parent HEA, grain-boundary strengthening and internal friction can be roughly removed. Compared with the single-phase FeCoNiCr HEA [12], the
Conclusions
A precipitation-strengthened FeCoNiCrTi0.2 HEA was fabricated. Microstructure was characterized by combining XRD, SEM, APT and TEM techniques. Its tensile properties at room temperature (293 K) and liquid nitrogen temperature (77 K) and the corresponding defect-structure evolution were investigated by TEM. Based on our observations, the following conclusions can be drawn.
- (a)
After aging at 1073 K for 1 h, precipitates in lamellar and spherical shapes were formed near the grain boundaries and in the
Acknowledgements
Y. T. and J. J. K. gratefully acknowledge for the financial support from the Hong Kong Research Grant Council (RGC) with grant No. 9380075 and 9042204, and T. Y. and C. T. L. for the RGC grant No. 9042048. P. K. L. would like to acknowledge the Department of Energy (DOE), Office of Fossil Energy, National Energy Technology Laboratory (DE-FE-0008855, DE-FE-0024054, and DE-FE-0011194), with Mr. V. Cedro and Mr. R. Dunst as program managers. P. K. L. and R. F. appreciate the support of the U.S.
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Y. T., D. C, B. H. and J. W. contribute equally to the present work.