Si-addition contributes to overcoming the strength-ductility trade-off in high-entropy alloys

Highlights

• Si-addition overcomes the strength-ductility trade-off of CoCrFeNi HEA.

• Si-addition tunes the plastic deformation mechanism of the CoCrFeNi HEA.

• Si-addition enhances yield stress by multiple strengthening mechanisms.

• The plasticity mechanism is tuned partially due to the reduced SFE.

Abstract

Face-centered cubic single-phase high-entropy alloys (HEAs) containing multi-principal transition metals have attracted significant attention, exhibiting an unprecedented combination of strength and ductility owing to their low stacking fault energy (SFE) and large misfit parameter that creates severe local lattice distortion. Increasing both strength and ductility further is challenging. In the present study, we demonstrate via meticulous experiments that the CoCrFeNi HEA with the addition of the substitutional metalloid Si can retain a single-phase FCC structure while its yield strength (up to 65%), ultimate strength (up to 34%), and ductility (up to 15%) are simultaneously increased, owing to a synthetical effect of the enhanced solid solution strengthening and a reduced SFE. The dislocation behaviors and plastic deformation mechanisms were tuned by the addition of Si, which improves the strain hardening and tensile ductility. The present study provides new strategies for enhancing HEA performance by targeted metalloid additions.

Introduction

Metals and alloys are indispensable infrastructure materials for various load-bearing applications, where the superior combination of strength and ductility has attracted considerable attention from metallurgists. Conventional alloys such as steel, Ni alloys, and Co alloys are developed by adding alloying elements into a single host of metals (Fe, Ni, and Co, respectively). In 2004, a new class of metallic materials known as high-entropy alloys (HEAs) was proposed (Cantor et al., 2004; Yeh et al., 2004). HEAs consist of multi-principal metallic elements (more than four elements) with equiatomic or near-equiatomic proportions (Cantor et al., 2004; Miracle and Senkov, 2017; Tsai and Yeh, 2014; Yeh et al., 2004; Zhang et al., 2014). Subsequently, medium-entropy alloys (MEAs) comprising three principal elements have also emerged (Gludovatz et al., 2016). Certain HEAs and MEAs exhibit superior mechanical properties (Dai et al., 2022; Hua et al., 2021; Miracle and Senkov, 2017; Romero et al., 2022; Smith et al., 2020; Tsai and Yeh, 2014; Zhang et al., 2021, 2014), a good combination of mechanical property and soft magnetic property (Chen et al., 2020), exceptional corrosion resistance (Zhang et al., 2022), and excellent irradiation resistance (Lu et al., 2019), which have attracted extensive academic interest in the past decade. Among these, the face-centered cubic (FCC) single-phase equiatomic quinary CoCrFeMnNi, quaternary CoCrFeNi, and ternary CoCrNi and CrFeNi alloys are the most intensively investigated systems owing to their good combination of strength and ductility, superior fracture resistance and good fatigue resistance, particularly at cryogenic temperatures (Cantor et al., 2004; Gludovatz et al., 2016, 2014; Li et al., 2022a; 2022b; Lu et al., 2021; Otto et al., 2013; Rackwitz et al., 2020; Schneider et al., 2020; Schneider and Laplanche, 2021; Zhang et al., 2020b). The superior mechanical properties are attributed to the high-entropy stabilized single-phase structure, low stacking fault energy (SFE), a combination of large modulus and atomic-size misfit parameter, and possibly short-range order (SRO).

The SFE of these HEAs and MEAs ranges from 18 to 35 mJ/m², and the mechanical twinning that occurs during plastic deformation enhances the strain hardening and strength of the alloys (Gludovatz et al., 2016, 2014; Hua et al., 2021; Kaushik et al., 2021; Laplanche et al., 2016; Li et al., 2022a; Liu et al., 2018; Okamoto et al., 2016a; Otto et al., 2013; Schneider and Laplanche, 2021; Wagner et al., 2022; Zhang et al., 2020b). The SFE is the primary material property that determines the plasticity mechanism of the FCC metals, in which the plastic deformation switches from a dislocation glide (SFE: > 45 mJ/m²) to mechanical twinning (SFE: 15-45 mJ/m²) and further to the FCC → hexagonal close-packed (HCP) strain-induced phase transformation (SFE: <15 mJ/m²) (De Cooman et al., 2018; Wei et al., 2019c). A significant amount of effort has thus been made to further reduce the SFE of HEAs/MEAs by optimizing their compositions to concurrently improve the strength and ductility (Deng et al., 2015; Li et al., 2016; Wei et al., 2019a, 2019b, 2019c), benefiting from the well-known twinning-induced plasticity (TWIP) and/or transformation-induced plasticity (TRIP) mechanisms (Bahramyan et al., 2020; Connolly et al., 2022; De Cooman et al., 2018; Fang et al., 2019; Wang et al., 2022). Typically, precipitation-strengthening and grain-boundary strengthening are effective in increasing the strength but inevitably result in a loss of ductility of these alloys (He et al., 2021c, 2021b; Liu et al., 2022; Ye et al., 2022; Yi et al., 2021a, 2020; 2021b; Zhao et al., 2017). Microalloying of metallic elements such as Al, Ti, Hf, Mo, Nb, V, and W in HEAs marginally enhances the strength by solid-solution strengthening (SSS); however, the ductility is decreased significantly when incoherent precipitates are formed (He et al., 2021c; He et al., 2014; Liu et al., 2022, 2016, 2015; Ma and Shek, 2020; Ming et al., 2019; Wang et al., 2020; Ye et al., 2022; Lv et al., 2023). Recent studies reported that an increase in the local lattice distortion (LLD) correlates with an increase in the yield strength (Okamoto et al., 2016b; Romero et al., 2022; Varvenne et al., 2016; Zhao and Nieh, 2017), which has recently been understood through the correlation of the LLD with the solid-solution misfit parameter (Geslin and Rodney, 2020; Nöhring and Curtin, 2019). The SRO might arise due to sufficiently strong solute-solute interactions, which strengthen the alloy but should also increase the SFE and thus compromise ductility. Increases in both strength and ductility of HEAs have thus been difficult by usual compositional variations and processing.

The enhancement of mechanical performance of HEAs and MEAs by adding non-metal elements such as carbon and nitrogen was recently investigated. The improvement in strength was attributed to SSS by the effect of the interstitial atoms or the formation of ordered complexes and precipitates (He et al., 2021b, 2021a; Klimova et al., 2021, 2020). Metalloids such as B, Si, Ge, As, Sb, Te, and At possess properties that range between metals and nonmetals, but tend to form ordered compounds or intermetallics with transition metals (Co, Cr, Fe, Ni, and Mn) (Mann et al., 2000; Vernon, 2013). It is thus expected that metalloid elements will tend to create SRO with host transition metals when added to HEAs and MEAs. Metalloids usually have a notable size difference with transition metals, enabling solid-solution strengthening (and increased lattice distortions) in HEAs/MEAs. Indeed, Boron additions led to the formation of SRO and grain boundary segregation (Seol et al., 2020). Overall, neither the addition of interstitial non-metals (C, N) nor metalloids (B) into FCC-phase HEAs notably improves the strength, and at the expense of reduced ductility. On the other hand, among the metalloids, Si is widely utilized as a microalloying element in various alloys. Si is a strong SSS element in FCC austenitic stainless steels and Fe-Mn TWIP steels (De Cooman et al., 2018; Jeong et al., 2013; Ohkubo et al., 1994; Xiong et al., 2015). Its addition reduces the SFE of steels, which increases both the yield strength and ultimate tensile strength (UTS), owing to the SSS effect and high strain hardening caused by the low SFE by enhancing the activation of primary and secondary twinning (Jeong et al., 2013; Ohkubo et al., 1994; Xiong et al., 2015). Further, the addition of Si to Fe-Mn-based shape memory alloys lowers the magnetic transition temperature (Neel temperature) and promotes the FCC → HCP martensitic transformation (Gavriljuk et al., 2005; Klimova et al., 2021; Xiong et al., 2015). However, the influence of metalloid additions on multi-principal metallic HEAs and MEAs has not yet been totally revealed. The influence of Si additions on the mechanical behavior of the CoCrNi MEA was investigated quite recently but the results are contradictory (Chang et al., 2021; Liu et al., 2020). On the other hand, it was reported that the Si-addition tuned the phase equilibrium of CoFeMnNi and CoCrFeMnNi HEAs, and slightly increased the tensile strength of CoCrFeMnNi HEA (Yamanaka et al., 2021). The first-principle calculations and Monte-Carlo simulations revealed that Si-addition exerts a synergetic effect on the lattice features and elastic properties of HEAs (Lizárraga et al., 2021; Tsuru et al., 2022), whereas the yielding and hardening behavior has not been clarified. Thus, the effect of Si on plastic deformation mechanisms in HEAs deserves further investigation.

In the present study, we investigate the effect of metalloid addition on the mechanical behaviors of the CoCrFeNi HEA using electron microscopic characterizations and neutron diffraction measurements, assisted with the thermodynamic and first-principle calculations. The influences of Si on the microstructure, SFE, multidimensional mechanical responses, and plasticity of CoCrFeNi HEA are reported. We find that the single-phase FCC structure is retained up to (CoCrFeNi)₉₀Si₁₀ (at.%), and that Si reduces the SFE, increases the lattice distortions, and tunes the plasticity mechanism (influences the plastic deformation behaviors), which is associated with measured increases in both ductility and strength. The Si metalloid additions thus overcome the traditional strength/ductility tradeoff that is common to metal alloys, and open a new path for high-performance alloy development.