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64CENTRIFUGAL CASTING-SHS PROCESS OF THE CAST CoCrFeNiMn-TYPE HIGH-ENTROPY ALLOY HARDENED BY THERMOMECHANICAL TREATMENT AND THE INTRODUCTION OF SILICON-BORIDE HARDENERS
V. N. Sanin*", D. M. Ikornikov", O. A. Golosova", D. E. Andreev", V. I. Yukhvid", and S. V. ZherebtsovA
aMerzhanov Institute of Structural Macrokinetics and Materials Science, Russian Academy of Sciences, Chernogolovka, Moscow, 142432 Russia bBelgorod State University, Belgorod, 308015 Russia *e-mail: [email protected]
DOI: 10.24411/9999-0014A-2019-10145
Conventional alloy design over the past centuries has been obliged by the concept of one or two prevalent base elements. As a breakthrough of this restriction, the concept of high-entropy alloys (HEAs) containing multiple principal elements has drawn great attention over the last 13 years due to the numerous opportunities for investigations in the huge unexplored compositional space of multicomponent alloys [1-6]. A large number of studies in this field have been motivated by the original HEA concept, which suggested that achieving maximized configurational entropy using equiatomic ratios of multiple principal elements could stabilize singlephase massive solid-solution phases [3]. However, an increasing number of studies have revealed that formation of single-phase solid solutions in HEAs shows weak dependence on maximization of the configurational entropy through equiatomic ratios of elements [2] and it was even found that maximum entropy is not the most essential parameter when designing multicomponent alloys with superior properties [4-8]. These findings encouraged efforts to relax the unnecessary restrictions on both the equiatomic ratio of multiple principal elements as well as the formation of single-phase solid solutions. In this context, non-equiatomic HEAs with single-, dual-, or multiphase structure have recently been proposed to explore the flexibility of HEA design and overcome the limitations of the original HEA design concept [9]. Also, deviation from the equimolar composition rule facilitates identification of compositions which allow the often-brittle intermetallic phases to be avoided. Thermodynamic investigations of non-equiatomic HEAs showed that the configurational entropy curve of these alloys is rather flat, indicating that a wide range of compositions alongside the equiatomic configuration assume similar entropy values. As compared with conventional alloys with one or two principal elements plus minor alloying components, as well as equiatomic HEAs with equimolar ratios of all alloy elements, non-equiatomic HEAs greatly expand the compositional space that can be probed. Indeed, recent studies have revealed that outstanding mechanical properties exceeding those of equiatomic HEAs can be achieved by non-equiatomic alloys [7, 9, 10]. As one of the possible pathways, a novel type of transformation-induced plasticity-assisted dualphase (TRIP-DP) HEA was developed [4]. The two constituent phases in the alloy, i.e., the face-centered cubic (FCC) matrix and the hexagonal close-packed (HCP) phase, are compositionally equivalent and thus can both be referred to as high-entropy phases [4]. This leads to a significantly improved strength- ductility combination compared with corresponding equiatomic HEAs, mainly due to the combination of massive solid-solution strengthening and the TRIP effect [11]. The above-mentioned findings clearly indicate that expanding the HEA design concept to nonequiatomic compositions has great potential for pursuing more compositional opportunities for design of novel materials with exceptional properties. For transition-metal HEAs, vacuum induction furnace is used to melt and cast various HEAs. In as-
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cast condition, the multiple principal elements are typically not homogeneously distributed in the bulk HEAs with their coarse dendritic microstructure owing to liquid segregation, although X-ray diffraction (XRD) analysis may suggest single- or dual-phase structures. Therefore, to obtain high-quality alloys with a controlled microstructure and the level of mechanical properties we have to use the post-processing thermo-mechanical treatment and controlled introduction of various modifying or strengthening additives during casting process.
Processing routes are not only decisive for the grain size and phase fraction corresponding to a targeted specific composition but are also critical with respect to the compositional homogeneity state, which also has significant effects on the mechanical behavior. To obtain from cast alloys the samples with different compositional homogeneity various processing routes are applied including hot-rolling, homogenization, cold rolling, and recrystallization annealing etc.
Today, the most studied alloys belonging to this HEAs group are the alloys in the Co-Cr-Fe-Ni-Mn system. The equiatomic Co-Cr-Fe-Ni-Mn alloy have a single-phase structure of a disordered substitution solid solution on the basis of a face-centered cubic fcc lattice. The multicomponent high-entropy fcc alloys Co-Cr-Fe-Ni-Mn exhibited high ductility at room and cryogenic temperatures [12] and record breaking fracture toughness at cryogenic temperatures; but the yield strength of the alloy was rather low (HEAs). There have been many efforts to improve the properties of the CoCrFeNiMn alloy. It was found than the highest strength can be attained in the alloys with a fcc matrix strengthened with the particles of some another compound. However, the design of dispersion strengthened HEAs with optimal properties requires additional efforts.
In frame of the work, we will provide some recent results of various strong and ductile Al, C-containing CoCrFeNiMn-type high-entropy alloys hardened by thermo-mechanical treatment and the introduction of silicon-boride hardeners. The compositional design, centrifugal casting-SHS processing routes, and microstructure/property relations will pointed out. With compere of the conventional multistage vacuum casting processes the relatedly new technique which is called the SHS-technology of high temperature melts or the metalothermic SHS [13], is promising route to produce cast HEAs. This is a low-energy consuming technique due to the use of internal energy released in high-caloric combustion reactions. Recently, cast high-entropy transition metal alloys were first prepared [14]. The use of highly exothermic SHS compounds of the thermite type makes it possible to produce melts of combustion products (at temperature above 2500°C) and, as a consequence, to obtain cast products (ingots). The synthesis was carried out using powdered SHS systems containing oxides of target elements (NiO, Cr2O3, Fe2O3, Co3O4, MnO2), reduced metal (Al) and alloy additives (C, silicon-boride hardeners (Ti(Cr)-B-Si)). The experiments were carried out using a centrifugal SHS machine [13] under the influence of an overload of 10 to 80 g, which in turn is a powerful tool for controlling the combustion processes and the formation of synthesis products [13, 14]. The effect of overload in the combustion stage makes it possible to suppress the spread of products during combustion, to realize intensive mixing of the melt over the combustion front and to obtain a high conversion of the initial mixture in the combustion front. At the stage of gravitational separation and cooling, the effect of overloading makes it possible to realize the high yield of the metallic phase in the ingot (close to the calculated one), to remove the gaseous products from it and to facilitate the equalization of the chemical composition by the HEA ingot volume, which was very important for the synthesis of polymetallic alloys. The general scheme of the process for the synthesis of cast alloys was described in [14]. Figure 1 illustrates XRD pattern and microstructure of SHS-produced CoCrFeNiMn alloy without additives.
The XRD pattern contains only fcc phase with lattice parameter a = 3.588 nm. XRD and EBSD data show strong crystallographic texture typical of the cast materials. The alloy has coarse structure with a grain size of 250-400 |im. The grain boundaries are often curved, and the shape of the grains is irregular. SEM and TEM studies revealed no second phases.
(a) (b) (c)
Fig. 1. (a) XRD pattern ( FCC (Im3m)), (b) SEM image, and (c) EBSD map of crystallography orientations of as-cast CoCrFeNiMn alloy.
The introduction of 0.2 wt % carbon into alloy does not have a noticeable effect on the phase composition formation but significantly increases the hardness of the alloy. The introduction of silicon-boride hardeners Ti(Cr)-B-Si affects the phase composition. The introduction of Al into composition of alloy markedly reduces the density of the alloy. The formation of intermetallic phase NiAl is observed at high concentrations of Al and it promotes to a sharp decrease in plasticity.
The synthesized Ab.4C0jCo22.3Cr19jFe22.9Ni22.4Mn8.6 alloy was subjected to thermomechanical treatment (cold rolling and following annealing). Mechanical properties of treated samples—yield strength (00.2), ultimate tensile strength (guts), uniform elongation (su), and elongation to fracture (sf)—are given in Table 1.
Table 1. Tensile properties of Al3.4Co.7Co22.35Cri9.7Fe22.9Ni22.4Mn8.6 alloy.
ct0.2, CTuts, £1^ £f, ct0.2, ctuts, £f,
MPa MPa % % MPa MPa % %
As-cast 210 455 74 80 Annealing at 700°C 870 1060 13 24
20% rolling 545 650 18 25 Annealing at 800°C 610 925 25 38
40% rolling 945 980 3.7 7 Annealing at 900°C 530 875 27 41
60% rolling 965 1140 2.3 5.4 80% Annealing at 1000°C 435 820 36 44
80% rolling 1310 1500 1.3 6.5 rolling Annealing at 1100°C 320 760 40 47
The stress-strain curves of the annealed Al3.4C0.7Co22.35Cr19.7Fe22.9Ni22.4Mn8.6 alloy demonstrate (Fig. 2) that mechanical properties of the alloy can be further tailored by annealing treatment. Apparently, the alloy becomes softer and more ductile with increasing annealing temperature. For example, after annealing at 700°C the alloy still retains high strength: the yield strength and ultimate tensile strength are 870 and 1060 MPa, respectively, at a reasonable ductility of 13 and 25%. An increase in the annealing temperature up to 900°C decreases the material strength, especially the yield strength (530 MPa).
0 -1-1-1-1-1--0 20 40 60 80
0 20 40 60 80 100 120
Engineering strain, %
Engineering strain,%
(a) (b)
Fig. 2. Tensile stress-strain curves of Ab.4C0jCo22.35Cr19jFe22.9Ni22.4Mn8.6 after: (a) cold rolling with different thickness reductions and (b) after subsequent annealing at 700-1100°C for 1 h.
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Thus, cast CoCrFeNiMn-type high-entropy alloy hardened with different additive components can be fabricated by centrifugal SHS metallurgy in optimized conditions. The analysis of the obtained data allows drawing a conclusion about the prospects of the materials under investigation and the method of their production for the formation of volumetric nanostructured materials. The production of metallic composite materials based on the new principle of formation of polymetallic alloys can significantly expand the basis for the creation of new materials and facilitate the creation of new technological models.
The research was supported by the Russian Foundation for Basic Research, (project no. 19-08-01108).
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