Development of cost-effective high-entropy alloys with superior mechanical properties

Abstract

High-entropy alloys (HEAs) have attracted increasing attention since the concept of HEAs was firstly proposed in 2004 because of their extraordinary properties, such as exceptional high strength, low-temperature fracture toughness, good corrosion resistance and high-temperature properties. The original HEA was defined as an alloy that consists of single-phase solid-solution (SS) with five or more equiatomic principal elements.  But, the definition has been recently extended to non-equiatomic multicomponent alloys with multi-phases, which exhibit superior mechanical properties resulting from their integrated strengthening mechanisms. Due to the multi-phase structure and high content of Cr or Al, most HEAs also show good corrosion resistance and high-temperature properties. Hence, some HEAs have the potential to partially replace Ni-base superalloys for aerospace applications.However, one of the most critical issues that restrict their industrial applications is the cost of this type of alloys. The majority of the current HEAs contain a high content of expensive metals, such as the most frequently used Co. This inevitably increases the cost of raw materials. In addition, the mechanical properties of most HEAs are not better than some traditional alloys with much lower cost, such as the ultra-high strength maraging steels and quenching & partitioning steels. To improve the mechanical properties of HEAs, substantial research efforts have been devoted to microstructure control. This includes introducing one or more strengthening mechanisms into the alloys, such as short-range order clusters, ultra-fine grains, uniformly distributed nanoprecipitates, high-density dislocations, high-density nano-twins and metastable phase. However, in most cases, the increase in strength is with the sacrifice of ductility.To address the above issues, a novel alloy design strategy is proposed in the present work. The design started with an equiatomic HEA system. Then, the composition can be experimentally specified by adjusting the atomic content ratio (RI/F) of the intermetallic forming elements (I) to the face-centred cubic (FCC) forming elements (F). Using this new approach, a series of Fe-rich HEAs were designed and produced by decreasing the ratio of Cr/Mo to Fe. The mechanical properties of Fe-rich HEAs were sensitive to the RI/F value that can be optimized to manipulate the strength and ductility of the HEAs. Since the new alloys are Fe-rich without containing expensive metals, such as Co, the cost of the raw materials is significantly reduced.On the basis of the above results, two alloys were chosen for further investigation. One is the hard yet brittle dual-phase (DP) eutectic HEA (Fe35Ni25Cr25Mo15) and another is the soft yet ductile FCC-dominated HEA (Fe45Ni25Cr25Mo5). It is aimed to improve the mechanical properties from two different approaches. One is to enhance the ductility of the brittle DP eutectic high entropy alloy (EHEA) while another is to strengthen the ductile FCC-dominated HEA. The feasibility of malleableizing the brittle Fe35Ni25Cr25Mo15 EHEA was investigated using two conventional methods, heat treatment and microalloying. It was found that the as-melted Fe35Ni25Cr25Mo15 EHEA was a pseudo eutectic alloy comprised of the alternant σ phase and FCC phase. Unlike softening of traditional eutectic alloys, spheroidization treatment was considered invalid to improve the ductility of pseudo-eutectic HEA with a high fraction of intermetallic phase. By contrast, eutectic modification through minor addition of boron was more effective in the improvement of malleability of brittle EHEAs. However, it is still a great challenge to remarkably increase the ductility of the brittle Fe35Ni25Cr25Mo15 EHEA alloys without sacrificing strength.To develop a cost-effective HEA with superior mechanical properties, the rest work focuses on the strengthening of a ductile FCC-dominated HEA with a composition of Fe45Ni25Cr25Mo5. Similarly, heat treatment was firstly performed to further improve the mechanical properties of the ductile Fe45Ni25Cr25Mo5 HEA. The high-temperature aging behaviour was investigated to evaluate the efficiency of precipitation strengthening on the mechanical properties of FCC-dominated HEAs. It was found that the Fe-rich alloy showed a high age-hardening response at temperatures up to 900 °C due to the precipitation of intermetallic phases. Unfortunately, although the as-cast Fe-enriched HEA possessed balanced high tensile strength and ductility compared with previously reported as-cast FCC-dominated HEAs, the peak-aging resulted in brittleness at room temperature, which was attributed to the precipitation of large needle-shaped intermetallics.To simultaneously increase the strength and ductility, the recently proposed concept of heterogeneous microstructure was adopted to achieve hetero-deformation induced (HDI) strengthening and strain hardening. Thus, heterogeneous lamella (HL) design strategy was proposed to improve the mechanical properties of the cost-effective FCC-dominated Fe35Ni35Cr25Mo5 HEA through thermomechanical processing. An HL structure with nanoprecipitates was produced by cold rolling and single-step heat treatment (800 °C for 1h), resulting in a superior tensile property, with the yield strength over ~1.0 GPa and ductility of ~13%. The formation of the nanoprecipitates-decorated HL structure was a result of the preferential and concurrent nucleation of partially recrystallized grains and precipitates at the shear bands as well as the “pinning-effect” of precipitates. The superior yield strength was mainly attributed to the HDI strengthening and precipitation strengthening caused by the heterogeneous lamella structures with nanoprecipitates. The good ductility was originated from the coexistence of multiple deformation mechanisms, which began with dislocation slip and formation of stacking faults at the initial stage, followed by an additional deformation mechanism by nano-twinning at the higher strain level.In addition, it is noteworthy that the cost-effective FCC-dominated Fe45Ni25Cr25Mo5 HEAs has demonstrated a high-temperature age-hardening response at temperatures up to 900 °C and comparable high oxidation resistance with the Inconel 625 superalloy at 1200 °C, indicating its high potency to achieve superior properties at high temperatures. Thus, to further exploit its potency to be used at elevated temperatures, an in-depth investigation of the high-temperature oxidation behaviour of this newly developed HEA was carried out. Results proved that the superior oxidation and spallation resistance were attributed to the formation of an exclusive and compact chromia scale with high hardness and elastic modulus, protecting the substrate from further oxidation and providing high resistance against cracking. The superior mechanical properties of the chromia scale originated from its high compactness, fine-grains and solid-solution strengthening.