Exceptional phase-transformation strengthening of ferrous medium-entropy alloys at cryogenic temperatures

Abstract

High-entropy alloys (HEAs) are a newly emerging class of materials that show attractive mechanical properties for structural applications. Particularly, face-centered cubic (fcc) structured HEAs and medium-entropy alloys (MEAs) such as FeMnCoNiCr and CoNiCr alloys, respectively, which exhibit superior fracture toughness and tensile properties at liquid nitrogen temperature, are the potential HEA materials available for cryogenic applications. Here, we report a ferrous Fe₆₀Co₁₅Ni₁₅Cr₁₀ (at%) MEA exhibiting combination of cryogenic tensile strength of ∼1.5 GPa and ductility of ∼87% due to the multiple-stage strain hardening. Astonishingly, detailed microstructural observations at each stage reveal the sequential operation of deformation-induced phase transformation from parent fcc to newly formed bcc (body-centered cubic) phases. No compositional heterogeneity is observed at phase boundaries, indicating diffusionless phase transformation, as confirmed by atom probe tomography. The transformation to bcc phase occurs predominantly along grain boundaries (GBs) at the early stage of plastic deformation. Simultaneously, numerous deformation-induced shear bands (SBs) having stacking faults associated to the Shockley partial dislocations and thin hcp plates, form within fcc grains. Further deformation leads to the intense nucleation and growth of the bcc phase at the intersections of SBs within fcc grains. These micro-processes consecutively enhance the strain hardening rate, which play a key role in the multiple strain hardening behavior. The in-situ neutron diffraction studies make it clear that the martensite formation and the concurrent load partitioning between the fcc and bcc phases play an important role in the increase in strength. Furthermore, replacing high-cost alloying elements cobalt and nickel with iron, as well as introduction of metastability-engineering at liquid nitrogen temperature, distinguishes the new ferrous MEAs from previously reported equiatomic HEAs. This result underlines insights to provide expanded opportunities for the future development of HEAs for cryogenic applications.

Introduction

With recent advances in the aerospace, marine shipbuilding, and natural gas industries, the demand for metallic alloys having desirable strength and ductility in cryogenic environments has been increased. The challenge in designing alloys is that many engineering alloys become embrittled at low temperatures, hence, their use at cryogenic temperatures is often limited [[1], [2], [3]]. Multicomponent alloys, referred to as high-entropy alloys (HEAs) or medium-entropy alloys (MEAs) depending on their configurational entropy of mixing, have recently drawn much interest due to their outstanding properties at elevated temperatures [[4], [5], [6]]. In particular, equiatomic FeMnCoNiCr and CoNiCr alloys compromising a single face-centered cubic (fcc) structure have been potential materials for cryogenic engineering uses owing to their superior synergy of fracture toughness and tensile properties at liquid nitrogen temperature [[7], [8], [9], [10], [11]]. The underlying strengthening mechanism upon cryogenic-temperature plastic deformation of these materials is the substantial interaction between lattice dislocations and nano-twins. Additionally, Miao et al. [12] suggested that the formation of nanotwin and hexagonal close-packed (hcp) lamellar structures at larger strain level is responsible for the superior tensile properties of CoNiCr alloy.

Recently, classical methods for strengthening materials, including nanostructuring, micro-band formation, mechanical twinning, or precipitation strengthening, have been successfully applied for improving the room-temperature mechanical properties of HEAs due to the demand for further enhancing mechanical properties [[13], [14], [15], [16], [17]]. Among these, metastability-engineering strategy [18], as a breakthrough of HEA design concept, enables the development of transformation-induced plasticity-assisted dual-phase Fe₅₀Mn₃₀Co₁₀Cr₁₀ (at%) HEA. Destabilizing high-temperature fcc phase through reducing the Mn content in the HEA system, promotes deformation-induced phase transformation from fcc to hexagonal close-packed (hcp) phase at room temperature, improving both strength and ductility. Also, the similar approach that was based on destabilizing body-centered cubic (bcc) phase by changing Ta content offers phase-transformation ductilization to brittle bcc structure in Ta_xHfZrTi HEA system [4]. These efforts suggest that the transformation-induced plasticity (TRIP) effect manipulates the internal defect landscape of high-performance HEA or MEA for improving their strength without loss of ductility. However, despite such profound progress, the utilization of metastability-engineering strategy has so far been realized only for room-temperature conditions, and even few alloying systems. For exploiting the metastability measures having potentially great impacts on material's cryogenic-temperature mechanical properties, thus, research into a new HEA design approach is required.

Herein, we successfully exploit the metastability-engineering strategy under cryogenic conditions. Working toward this goal, we introduce fcc-structured Fe_x(CoNi)_90-xCr₁₀ (at%) MEA system, and the rationale for this selection is that: (i) The Cr-rich σ phase has been reported in four-element FeCoNiCr system [19,20], thus reducing the Cr content down to 10 at% can suppress the formation of intermetallic compound. (ii) Ni stabilizes the fcc phase, given that substitution of Ni for Fe broadens fcc single-phase region in Fe-Ni binary system [21]. (iii) Co acts as an fcc stabilizer at high temperatures, while raises the martensite starting temperature at low temperatures in Fe-Co binary system [[22], [23], [24]].

To most likely achieve the TRIP effects at cryogenic temperatures, we identify effects of alloying elements on phase constitution and on phase stability to introduce metastable fcc-structured solid solution, and newly design a ferrous Fe_x(CoNi)_90-xCr₁₀ (x = 55, 57.5, and 60 at%) MEA system. The objective of the present work is to study the evolution of phase stability and transformation-induced microstructures during tensile loading at temperature ranging 77–298 K of the present MEA materials. Based on the results of microstructural analysis, combined with in-situ neutron diffraction analysis during tensile deformation at room- and low-temperatures (298 K and 136 K, respectively), we found that the underlying micro-processes of the present alloys under cryogenic loading involve deformation-induced phase transformation from metastable fcc to bcc phases, thereby leading to the TRIP effects; we were able to achieve more than triple its tensile strength than that of room-temperature one, while further gaining its ductility rather than sacrificing it. Our results are expected to provide a potent means for the future development of HEAs available for cryogenic applications.