Extraordinary combination of strength and ductility in an additively manufactured Fe-based medium entropy alloy through in situ formed η-nanoprecipitate and heterogeneous microstructure

Abstract

The current study investigated the mechanical properties of Fe₆₅Ni₁₅Co₈Mn₈Ti₃Si (at%) medium entropy alloy (MEA), printed by laser-based direct energy deposition (DED) in the room and liquid nitrogen environments. The as-printed microstructure contains various levels of heterogeneities, including different grain sizes, dual-phase microstructure, in situ formed η nanoprecipitate, cellular structure, and elemental segregation generated during the DED process. The impact of each observed heterogeneity on the mechanical properties and the predominant deformation mechanisms were examined. The corresponding MEA revealed a high ultimate tensile strength (UTS) of ∼1.02 GPa, total elongation (TE) of ∼46%, and exceptional cryogenic mechanical properties with a UTS and TE of 1.83 GPa and 40%, respectively. According to microstructural observations, the deformation-induced phase transformation from face-centered cubic (FCC) to body-centered cubic (BCC) is the predominant strengthening mechanism at both room and liquid nitrogen temperatures in addition to solid-solution strengthening and dislocation-mediated plasticity. The non-shearable η nanoprecipitates also enhanced the mechanical properties through precipitation/stacking fault strengthening. Hetero-deformation-induced (HDI) strengthening also occurred in the presence of dual-phase microstructure, cellular microstructure, and elemental segregation in the FCC phase. This upgraded synergy of tensile strength and ductility at liquid nitrogen temperature can be explained through the phase transformation and additional activation of twinned martensite formation, which results in two-step strain-hardening behavior. The presented results expand possibilities for developing DED-processed ferrous HEAs/MEAs to overcome the strength and ductility trade-off at room and liquid nitrogen temperatures.

Introduction

Multi-principle-element alloys (MPEAs), also called high/medium entropy alloys (HEAs/MEAs), have been developed as a new class of alloys to break through the traditional single-principal-based alloy design strategy for potential applications [1], [2], [3]. Taking advantage of multiple components (five principal elements or more) in an equiatomic or near-equiatomic composition, an excellent combination of mechanical properties at cryogenic-to-high temperatures can be achieved by proper alloying design [4]. Due to the high mixing entropy of the principal alloying elements, HEAs mainly consist of simple solid-solution structures with face-centered cubic (FCC) or body-centered cubic (BCC) phases [5]. Since single BCC-structured HEAs/MEAs possess superior strength but very poor ductility [6], promoting the solid-solution hardening effect of the FCC matrix is an excellent strategy to improve the mechanical properties of the corresponding components [7], [8], [9]. The high strain-hardening capability of FCC-structured HEAs/MEAs results from the intrinsically activated multiple dislocation slip system during straining, which promotes tensile ductility. Simultaneous improvement in strength and ductility of single-phase HEAs is the primary concern of recent research because of the limited inherent solid-solution hardening of HEAs/MEAs [10].

Among several production methods, e.g., casting and sintering, metal additive manufacturing (MAM) or three-dimensional (3D) printing is a promising method for layer-by-layer production of lower-defect HEA/MEA components [11], [12]. As a near-net-shape manufacturing method with high production design freedom and less production time, material wastage, and processing cost, MAM is considered a great production option for the HEAs/MEAs industry [12], [13], [14]. The corresponding samples possess exclusive anisotropic and heterogeneous microstructures consisting of fine subgrain cellular dislocation structure, unidirectional epitaxial columnar grain structure, and fine second phases, which can be engineered by deposition parameters based on performance conditions [15], [16]. Given the advantages of the compositional diversity, several HEAs/MEAs with uniform microstructures and properties comparable to conventionally produced HEAs/MEAs are currently fabricated using MAM [12], [14], [16]. However, the mechanical performance of MAM-processed HEAs/MEAs is still unsatisfactory despite numerous attempts at improvement.

Several strategies have been used to overcome the strength–ductility trade-off by proper alloying design to coactivate other strengthening mechanisms such as solid-solution strengthening [16], [17], [18], [19], precipitation strengthening [16], composite effects [17], transformation/twinning-induced plasticity (TRIP/TWIP) [18], [19], and hetero-deformation-induced (HDI) strengthening [20], [21]. As a novel strengthening approach, HDI strengthening can be promoted by the heterogeneous grain structure in as-printed samples, leading to exceptional strength–ductility synergy [20]. This heterogeneity, identified through various domains with significantly different characteristics, causes strain partitioning between different domains and, subsequently, the formation and development of geometrically necessary dislocations (GNDs) during straining. The resulting HDI stresses to enhance the strain-hardening ability of the corresponding MAM-processed HEA/MEA to prevent early necking [5], [20], [21], [22].

In the present work, a novel dual-phase Fe₆₅Ni₁₅Co₈Mn₈Ti₃Si MEA has been fabricated using laser-based direct energy deposition (DED) to achieve an improved combination of mechanical strength and ductility at both room and liquid nitrogen temperatures. The FeNiCoMnTiSi MEAs also have a high potential for the formation of Fe₂SiTi and Ni₃Ti nanoprecipitate upon an aging treatment, which results in enhanced mechanical properties and acceptable plasticity due to the activation of the TWIP and TRIP effects [23]. During the printing of samples, some alloying elements, including Mn, Ti, and Si, segregated in the middle of FCC grains due to different diffusivity and solubility in the matrix, causing another heterogeneity within the DED-processed microstructure. This resulted in a nonequilibrium condition that led to in situ formation of newly developed η nanoprecipitate. The design concept of the current study takes advantage of both the microstructural and metastability engineering strategies within one step by DED instead of common and complex thermomechanical treatments. This process enables the achievement of an extraordinary combination of strength and ductility at room and liquid nitrogen temperatures; both strength and ductility are superior to most MAM-processed HEAs/MEAs reported thus far.