Elsevier

Materials & Design

Volume 119, 5 April 2017, Pages 141-150
Materials & Design

Microstructure and mechanical properties of Ni1,5Co1,5CrFeTi0,5 high entropy alloy fabricated by mechanical alloying and spark plasma sintering

https://doi.org/10.1016/j.matdes.2017.01.036Get rights and content

Highlights

  • One phase FCC ultra-fine grained HEA with no porosity was produced via powder metallurgy.

  • For the first time, tensile test specimens produced from SPS compacts were tested.

  • The alloy exhibited excellent tensile strength of 1384 MPa, superior to HEA produced by casting routes.

  • Ductile fracture mode with dimple morphology has been observed on fracture surfaces.

Abstract

The present work is focused on synthesis and mechanical properties evaluation of non-equiatomic Ni1,5Co1,5CrFeTi0,5, ductile single phase high entropy alloy (HEA) with excellent mechanical properties (bend strength Rmb = 2593 MPa, tensile strength Rm = 1384 MPa, tensile elongation to fracture of 4.01%, and elastic modulus of 216 GPa) surpassing those of traditional as-cast HEA. For the alloy production, a combination of mechanical alloying (MA) process in a planetary ball mill and spark plasma sintering (SPS) for powder densification was utilized. The tensile properties of a bulk material produced by a combination of MA + SPS are characterized for the first time. The feedstock powder and corresponding bulk material microstructure, elemental and phase composition, and mechanical properties were investigated by scanning (SEM) and transmission (TEM) electron microscopy, energy-dispersive X-ray spectroscopy (EDX), electron backscatter diffraction (EBSD), X-ray diffraction (XRD), as well as impulse excitation of vibration, Vickers microhardness and tensile and bend strength tests, respectively. The structure of the samples consisted of single-phase FCC high entropy solid solution of extremely fine-twinned grains and oxide inclusions inherited from the original powder feedstock. Dimple-like morphology corresponding to ductile fracture mode has been observed on the fracture surfaces, with crack initiation sites on the inclusions phases.

Introduction

In their pioneer work, Yeh et al. [1] designed an equiatomic elements alloy comprising of five elements with a simple disordered solid solution structure. Along with others like Cantor [2], [3], they provided a new direction in metal alloys development; instead of using the traditional element-doped-into-base-matrix design approach, they chose multi-element systems with near equimolar ratios. This new class of materials was denominated as high entropy alloys (HEA). Over the time, a number of various alloy systems have been examined [4]. However, most of such studied systems exhibited complicated structures containing e.g. intermediate phases, thereby deriving from the original idea of one solid solution [5], [6], [7]. Such multi-phase microstructure alloys of near equimolar element ratios are now referred to as multi-principal-element alloys (MPEA). As well as HEA, these alloys are, too, being developed considering their suitability for some targeted use such as high strength or high temperature applications (where their multi-phase structure is more desirable [8], [9]). In fact, Wu et al. [10] proposed that the idea of stabilization of simple solid solutions by high configurational entropy (i.e., the currently acknowledged basic HEA principle) may be wrong and highlighted more prominent role of the mixing enthalpy.

Regardless of the terminology, these new materials have attracted substantial scientific interest due to their excellent combination of properties like high strength and ductility [11], [12], wear resistance [13], or e.g. fracture resistance [14]. This behavior is a result of the inherent ductile nature of solid solutions with high strength, derived from severe lattice strains present in the microstructure [4].

Despite the variety of designed and studied HEA, their tensile strength test performance was rarely documented. Due to the relative simplicity of the tests, the reported mechanical behavior of HEA mainly come from compression loading, which usually yields higher apparent strength and ductility combination [15]. To boost the application of HEA, mechanical testing that better corresponds with the stress-strain conditions of real parts in service is needed (cf. the outcomes of compression testing only). Results indicate that in general, FCC lattice HEA usually possess good tensile ductility, but their strengths are low, while BCC lattice HEA can have a high strength but rather poor tensile ductility [4].

At the present, majority of HEA is produced by the melting and casting procedure. Due to chemical or segregation phenomena, additional time and energy consuming processes like homogenization and hot working [10], [16] are required to obtain the desired properties.

By utilizing powder metallurgy (PM) fabrication approach, in particular mechanical alloying (MA) of pure elemental powders and a subsequent powder densification by spark plasma sintering (SPS), it is possible to – relatively easily – obtain dense bulk materials, as proven in a number of previous studies [17], [18], [19], [20]. There are a number of benefits resulting from this manufacturing method, such as production of nano-grained materials [19], [21], ease of advanced composite preparation [22], [23], and no segregation problems [24]. Given these, it could be expected that the properties of PM fabricated HEA bulks have a potential of overcoming limitations encountered in the casting routes. At the moment, though, the high hardness and high strength HEA materials fabricated via PM routes usually possess very limited ductility values. This may present a problem for e.g. structural materials, where ductility is a critically important property for manufacturing of damage tolerant structures [25]. As such, there is a need to produce HEA with sufficient ductility values, too.

According to the authors knowledge, this paper present the first systematic study reporting bend and tensile strength test behavior and fracture mechanism of HEA materials produced by a combination of MA + SPS showing strengths superior to those of as-cast bulks, while maintaining reasonable ductility (and hence promising application potential).

Section snippets

Experimental

The nominal composition of the investigated alloy was Ni1,5Co1,5CrFeTi0,5 (expressed in molar ratio). The particular concentration was chosen from the previous studies due to its combination of superior corrosion properties, wear resistance and high temperature strength [13], [26], [27]. Importantly, the alloy also inherently represents a promising material of a good combination of strength-to-ductility. Elemental powders of Cr, Co, Ni, Fe, and Ti with high purity (all > 99.5 wt%, Sigma Aldrich)

Phase prediction

Several parameters have been proposed in the literature to predict phase composition of high entropy alloys without the need to use CALPHAD software or ab-initio calculations. Yang et al. [32] proposed enthalpy of mixing ΔHmix, configurational entropy ΔSconf, and element radii mismatch δ as the criteria for such predictions. The respective equations for the calculation of these parameters can be found in their work [33]. According to their findings, disordered solid solutions form when − 15  ΔHmix

Conclusions

In the presented study, Ni1,5Co1,5CrFeTi0,5 high entropy alloy with one phase face centered cubic solid solution structure was successfully synthesized by a combination of mechanical alloying and spark plasma sintering (cf. multi-phase cast alloys produced by traditional casting routes). Excellent combination of mechanical properties was achieved with bend strength Rmb = 2593 MPa, tensile strength Rm = 1384 MPa, tensile elongation to fracture of 4.01%, and elastic modulus of 216 GPa. The microhardness

Acknowledgments

Support of Czech Science Foundation project GACR 13-35890S and GACR GB 14-36566G is acknowledged. The research was co-funded by the Ministry of Education, Youth and Sports within the “National Sustainability Programme I” (NETME CENTRE PLUS - LO1202). The authors would like to thank Jan Cupera for his help with SEM analysis.

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