Influence of Ti addition on microstructure and mechanical behavior of a FCC-based Fe30Ni30Co30Mn10 alloy
Introduction
Alloys possessing five or more principal elements with equiatomic or near-equiatomic elements which were proposed and defined as high entropy alloys (HEAs) by Yeh [1], have been intensively studied since 2004. According to physical metallurgy intuition, alloys with a variety of elements may solidify as multi-phase microstructures composed of several different intermetallic compounds or other complex compounds. However, multi-component HEAs usually consist of simple solid solutions with face-centered cubic (FCC) or/and body-centered cubic (BCC) and/or hexagonal close-packed (HCP) structures in the previous studies [2], [3], [4], [5], [6], [7]. Most of the researchers attribute the “abnormal” phase composition of HEAs to high configurational entropy originated from multi-principal elements which could overwhelm the enthalpy of compound generation [1], [5], [8]. Furthermore, due to the sluggish diffusion and severe lattice distortion effects, nanosized precipitations and amorphous phases can even be observed in HEAs fabricated by casting [3], [9], [10], [11]. Owing to the about-mentioned intriguing microstructural features, HEAs can achieve many unique properties, such as high strength and hardness [12], [13], good resistance to wear and corrosion [14], [15], high thermal stability and outstanding cryogenic performance [4], [16], etc., and accordingly HEAs is a promising material for widely engineering applications.
In order to explore advanced alloys possessing excellent properties, tailoring alloying elements and developing advanced processing techniques are commonly taken into consideration. Like conventional alloys, alloying elements also have remarkable influence on the phase composition, microstructure and properties in the novel HEAs. Inspection of the published literature reveals that 3d TM-based (TM means transition metals) HEAs with different aluminum contents have been widely studied up to date [1], [3], [17], [18], [19], [20], [21], [22], [23]. Published results indicate that in AlxCoCrCuFeNi and AlxCoCrFeNi alloy systems, phase compositions of these HEAs change from single FCC phase to FCC + BCC phases, and then single BCC structured phase on account of increasing Al proportion (molar ratio) [3], [17], [18]. In addition, similar phenomenon also have been observed in both AlxCoCrFeNiTi and AlxFeCoNiCrMn system HEAs [19], [22]. Although having FCC crystal structure, Al has been considered as a strong BCC phase stabilizer in HEAs. The formation of BCC structured phase which plays a key role in hardening and strengthening HEAs leads to the strength increasing significantly from 1 GPa to 2.28 GPa when x value increases in CoCrFeNiTiAlx HEA system [19]. When it comes to intrinsic BCC structured HfNbTaTiZr HEA, the addition of Al does not have evident influence on the microstructure, but can significantly improve the strength and reduce the density of the alloy [23]. Additionally, as the main constituent in HEAs, for examples, Ni, Cr, W and Ti transition elements, also have distinct effects on these novel HEAs [12], [24], [25], [26], [27], [28], [29], [30], [31]. In detail, Cr and Nb have been classified as BCC stabilizers as well as Al, while Ni, Co and Cu are regarded as FCC phase formation drivers [5], [24]. Among TM, the effects of Ti which has relative large atomic size and active nature on HEAs have also been intensively investigated. Note that Ti can strengthen HEAs via promoting the formation of BCC or/and Laves phases [12], [27], [31], however, it can also contribute to ductility through increasing the volume fraction of FCC phase [30].
Inspection of the HEA literature reveals that casting is the most common method used to fabricate HEAs, including induction melting [2], arc-melting and arc-melting followed by copper mold casting [3], [4], [10], etc. Additionally, processing routes such as reactive DC sputtering [32], laser cladding [33] and mechanical alloying (MA) [34] have been explored to manufacture coatings or alloy powders of HEAs. Rolling with subsequent annealing which is adopted to optimize properties tend to decrease casting defects and to obtain fine grains and precipitation-hardened HEAs bulks [9], [35]. What's more, the combination of MA and spark plasma sintering (SPS) is a appropriate processing route to synthesize bulk HEAs with ultra-fine grains or even nanosized grains [25], [30], [31], [36]. As a field-assisted sintering technique, SPS can produce dense HEAs bulks with unique properties via adjusting experimental parameters, such as sintering current and pressure [37]. On one hand, previous studies show that the bulk HEAs prepared by MA and SPS possessing FCC+BCC phases usually have high strength but low ductility [30], [31]. On the other hand, single FCC structured HEAs or medium-entropy alloys produced by casting, such as as-cast FeNiCoCrMn, FeNiCoMn and FeNiCoCr, usually exhibit outstanding ductility but low strength [38], [39], [40], [41]. Accordingly, using the combination of MA and SPS to produce FCC structured HEAs or medium-entropy alloys, may achieve a balance of ductility and strength.
In view of the above discussion, the present work was motivated by the following three factors. First, there are very few reports on FCC structured HEAs having good ductility and high strength. Second, fundamental information on single FCC structured (especially FCC structured single-phase) HEAs and medium-entropy alloys processed by MA and SPS remains very limited. Third, influence of Ti addition on the alloying behavior, microstructure and mechanical properties of FeNiCoMn medium-entropy alloy have not been studied in detail. Additionally, Mn content was lowered down based on the equiatomic FeNiCoMn alloy which has single FCC phase aiming at achieving FCC structured single-phase, and therefore a novel Fe30Ni30Co30Mn10 alloy was designed. In addition to that a Fe27Ni27Co26Mn10Ti10 HEA was also designed to investigate the influence of Ti addition on microstructure and mechanical behavior of the Fe30Ni30Co30Mn10 medium-entropy alloy.
Section snippets
Experimental details
To prepare the two alloys with nominal compositions of Fe30Ni30Co30Mn10 and Fe27Ni27Co26Mn10Ti10 (expressed in molar ratio), elemental powders of Fe, Ni, Co, Mn and Ti with purity higher than 99.7 wt% and particle size less than 45 µm (325 mesh) were used as raw materials. Blended powders and tungsten carbide balls (8 mm in diameter) were placed into high performance stainless steel vials with the ball to powder ratio of 10:1. Then the MA process was carried out in a high energy planetary ball
Influence of Ti addition during the MA process
The XRD patterns of Fe30Ni30Co30Mn10 and Fe27Ni27Co26Mn10Ti10 powders as a function of milling time are shown in Fig. 1. In full view of the MA process, it is obvious that the diffraction intensity of peaks decreases with the increased milling duration, meanwhile, some peaks disappear gradually. The pattern of initial mixture (0 h) in Fig. 1(a) displays peaks for all component elements of the Fe30Ni30Co30Mn10 alloy. After milling for 5 h, the relative intensity of peaks corresponding to Fe
Conclusions
Two alloys with the nominal compositions of Fe30Ni30Co30Mn10 and Fe27Ni27Co26Mn10Ti10 were successfully synthesized via MA and SPS. After 40 h ball milling, single FCC structured phase with nanosized crystalline size and severe lattice distortion was formed in powders of the two studied alloys. Following SPS, microstructure analyses of the bulk specimens indicate that the bulk Fe30Ni30Co30Mn10 alloy exhibited a FCC single solid-solution phase, but two FCC structured phases (FCC1 and FCC2) with
Acknowledgment
The authors wish to acknowledge the financial support by the National Natural Science Foundation of China (Grant no. 51271080).
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