Elsevier

Scripta Materialia

Volume 137, August 2017, Pages 88-93
Scripta Materialia

Regular article
Precipitation strengthening of ductile Cr15Fe20Co35Ni20Mo10 alloys

https://doi.org/10.1016/j.scriptamat.2017.05.019Get rights and content

Abstract

Non-equiatomic Cr15Fe20Co35Ni20Mo10 (Mo10) high-entropy alloys (HEAs) are produced by arc-melting and subsequent thermal-mechanical processing. The single-phase Mo10 HEAs with face centered cubic (FCC) structure exhibit an exceptional ductility and strain hardening capability associated with twinning. Tailoring thermal-mechanical processing condition enables the evolvement of single-phase Mo10 HEAs to multiphase microstructure that comprises simple FCC solid solution and nanoscale precipitates of Mo-rich μ phase. These precipitates primarily grow at grain boundaries and twin boundaries, strengthening Mo10 HEAs. This work sheds light on realizing a superb strength-ductility combination of HEAs by coupling solid solution hardening, nanotwinning and precipitation hardening mechanisms.

Introduction

High-entropy alloys (HEAs) that are composed of five or more elements in an equiatomic or near-equiatomic composition have attracted increasing attentions because of their unique compositions, microstructures, and adjustable properties [1], [2], [3], [4], [5], [6]. According to the structure (simple/complex) and ordering (ordered/disordered) of phases, HEAs are classified into three types: simple disordered phase, simple ordered phase and complex ordered phase [3]. Simple structures are the most frequently seen in as-cast HEAs, and generally refer to solid solutions of simple face-centered cubic (FCC), body-centered cubic (BCC), or hexagonal close-packed (HCP) structures [7], [8], [9], [10]. Besides simple phases, different kinds of complex ordered phases, such as σ, μ, Laves, etc., are also observed in HEAs [11], [12], [13], [14]. These complex ordered phases strengthen HEAs while often decrease their ductility. However, it has been demonstrated experimentally that manipulating the shape, dimension and distribution of strengthening units, e.g. hard and brittle particles, fibers, and precipitates, can optimize the strength and ductility of structural materials [15], [16], [17], [18]. For example, nanoscale precipitate or lamina can plastically co-deform with matrix associated with dislocations transmission across the strengthening unit [17], [18].

HEAs with single phase FCC structure generally exhibit low yield strength but superb ductility and strain hardening capability [1], [2]. Plastic deformation of FCC HEAs (for example of Co-Cr-Fe-Ni system plus one or two other elements) is accommodated by slip of 1/2 〈110〉 type dislocations on {111} planes and/or twinning associated with successive gliding of 1/6 〈112〉 type dislocations on {111} planes [19], [20]. Deformation twinning generates twin boundaries that block dislocation motion and further strengthen materials (referred to as twin boundary strengthening mechanism) [21], [22]. Besides boundary strengthening associated with grain refinement and twinning, FCC HEAs can be strengthened through adjusting elements and their composition ratio corresponding to solid solution hardening mechanism [23], [24] and precipitation hardening mechanism [25]. In this work, we aim to optimize strengthen and ductility of Cr-Fe-Co-Ni-Mo HEAs by coupling the three strengthening mechanisms.

We fabricate a series of non-equiatomic Cr20  xFe20Co20 + 3xNi20Mo20  2x HEAs. The Cr-Fe-Co-Ni-Mo system is chosen for this study because Cr-Fe-Co-Ni HEAs are FCC phase solid solutions [26], [27]. Although the role of each element in developing microstructure and changing mechanical properties of HEAs is not understood comprehensively, previous study of Cr-Fe-Co-Ni system provides valuable guidance of designing the composition ratio of Cr-Fe-Co-Ni HEAs [28], [29], [30]. For example, equiatomic CoCrNi alloys [28], [29] that have low stacking fault energy exhibit superior mechanical properties (good ductility and toughness) compared to equimolar CoCrFeMnNi due to the earlier onset of twinning [31], [32]. Therefore, adjusting Cr concentration may achieve low stacking fault energy of {111} planes and promote twin boundary strengthening effect. To promote solid solution hardening and precipitation hardening effects, we simultaneously adjust the concentration of Mo and Co elements. Lattice distortion due to size misfit of atoms impedes dislocation movement and leads to the pronounced solid solution strengthening. For example, increasing concentration of Al (larger radius than others) in AlxCrFeCoNi HEAs significantly increases yield and flow strength of the HEAs [33]. Among the five elements in Cr-Fe-Co-Ni-Mo system, Mo has the largest metallic radius (Ni (0.124 nm), Co (0.125 nm), Fe (0.126 nm), Cr (0.128 nm), Mo (0.139 nm)), and thus potentially maximizes solid solution strengthening. In addition, Mo has been demonstrated to facilitate formation of hard and brittle intermetallic phases in AlCoCrFeNiMo and AlCrFeNiMo alloys [34], [35]. Interestingly, increasing the concentration of Co may inhibit the formation of intermetallic phases in (FeNiCrMn)(100  x)Cox HEAs [36]. Therefore, we hypothesize that optimizing the composition ratio of Cr, Co and Mo simultaneously may develop Cr20  xFe20Co20 + 3xNi20Mo20  2x HEAs with superb mechanical properties.

Section snippets

Experimental methods

Alloy ingots with nominal compositions of Cr20  xFe20Co20 + 3xNi20Mo20  2x (x = 0, 2.5, 5, 7.5 at.%) are prepared by arc-melting under an argon atmosphere, subsequently homogenized in vacuum at 1200 °C for 48 h and then cooled in the furnace. Purity of the raw materials is greater than 99.9%. The synthesized HEAs are referred to as Homogenized-HEAs. The homogenized HEA ingots are further hot-rolled at 1100 °C to the sheet with a thickness of 8 mm, then cold-rolled to 2.4 mm with a thickness reduction of

Mechanical properties

Tension testing shows that homogenized Cr20  xFe20Co20 + 3xNi20Mo20  2x alloys as x = 0 at.% and 2.5 at.% (referred to as Mo20 and Mo15 HEAs, respectively) are extremely brittle without apparent plasticity. The XRD patterns and optical micrographs reveal that Mo20 and Mo15 HEAs contain σ phase networks in FCC matrix (Fig. S1 in Supplementary information), which is consistent with previous work [37], [38]. With decreasing the concentration of Mo, homogenized Mo10 and Mo5 HEAs (corresponding to x = 5 at.%

Conclusions

In summary, we fabricate a series of Cr-Fe-Co-Ni-Mo high-entropy alloys (HEA) with simultaneous adjustment of Mo, Co and Cr contents by arc-melting technique. These HEAs are further recrystallized through thermal-mechanical processing and annealing at different temperatures. We obtain single FCC-phase, non-equiatomic Cr15Fe20Co35Ni20Mo10 (Mo10) and Cr12.5Fe20Co42.5Ni20Mo5 (Mo5) HEAs. Both exhibit an exceptional strength-ductility combination due to the significant solution strengthening effect

Acknowledgments

This work is supported by the Science Fund for Creative Research Groups (61271043). J.W. acknowledges financial support provided by the Research Council at the University of Nebraska-Lincoln (URC-26-1123-9001-006).

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