Outstanding tensile properties of a precipitation-strengthened FeCoNiCrTi_0.2 high-entropy alloy at room and cryogenic temperatures

Abstract

A FeCoNiCrTi_0.2 high-entropy alloy strengthened by two types of coherent nano-precipitates but with the same composition was fabricated, and its tensile properties at room (293 K) and cryogenic temperatures (77 K) and the corresponding defect-structure evolution were investigated. Compared with the single-phase FeCoNiCr parent alloy, the precipitation-strengthened FeCoNiCrTi_0.2 high-entropy alloy exhibits a significant increase in yield strength and ultimate tensile strength but with little sacrifice in ductility. Similar to the single-phase FeCoNiCr high-entropy alloy, the deformation behavior of this precipitation-strengthened FeCoNiCrTi_0.2 high-entropy alloy shows strong temperature dependence. When the temperature decreases from 293 K to 77 K, its yield strength and ultimate tensile strength are increased from 700 MPa to 860 MPa and from 1.24 GPa to 1.58 GPa, respectively, associated with a ductility improvement from 36% to 46%. However, different from the single-phase FeCoNiCr high-entropy alloy with a twinning-dominant deformation mode at 77 K, multiple-layered stacking faults with a hierarchical substructure prevail in the precipitation-strengthened FeCoNiCrTi_0.2 high-entropy alloy when deformed at 77 K. The mechanism of twinning inhibition in this precipitation-strengthened high-entropy alloy is the high energy barrier for twin nucleation in the ordered γ′ nano-particles. Our results may provide a guide for the design of tough high-entropy alloys for applications at cryogenic temperatures through combining precipitation strengthening and twinning/stacking faults.

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 Al_0.3CoCrFeNi 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.