Influence of alloying elements on mechanical and electronic properties of NbMoTaWX (X = Cr, Zr, V, Hf and Re) refractory high entropy alloys

Highlights

• Mechanical properties of alloyed NbMoTaW RHEA were investigated based on first-principle calculation and experiments.

• Cr, Zr, V, Hf and Re enhance the alloy’ strength, while only Zr alloying improved both ductility and strength.

• V, Cr, Hf and Re strengthening of NbMoTaW RHEA should be attributed to the stronger atomic interaction.

• Zr-Zr metallic bonds formed in the Zr-alloyed RHEA help to improve both the strength and ductility.

Abstract

NbMoTaW refractory high entropy alloy (RHEA) has shown great potential applications for high temperature components due to its high temperature mechanical strength. However, the brittleness at room temperature hinders its engineering application and further development. To overcome this deficiency, refractory alloying elements with melting temperatures over 1850 °C, i.e., Cr, Zr, V, Hf, and Re, were chosen to enhance the mechanical performance of NbMoTaW RHEA in the present work. The effects of refractory alloying elements on the strength and ductility of NbMoTaW RHEA were investigated via a combination of theory and experiment. To be specific, the first-principle calculations based on density functional theory were employed to predict the mechanical properties of the alloyed NbMoTaWX (X = Cr, Zr, V, Hf and Re) RHEAs and explain the alloying effect from the atomic and electronic level. Moreover, the phase structures of the alloyed RHEAs were determined based on the formation enthalpy and cohesive energy, as well as some empirical parameters, such as the average valence electron concentration VEC and atomic size difference δ. In combination with the experimental results, the calculated elastic constants and modulus indicated that most of these alloying elements enhanced the strength of NbMoTaW RHEA, while only Zr-alloying significantly improved the ductility. The strengthening mechanism of different elements was well analyzed based on the total and partial density of states, overlapping Mulliken population, charge density contour, and atomic distance. The improvement of the ductility for Zr-alloying was attributed to the formation of Zr–Zr metallic bonds in the alloyed NbMoTaWX RHEAs. The theoretic predictions were confirmed by the experimental investigations. The present work provides a good guidance for design and construction of NbMoTaW RHEA.

Introduction

High entropy alloys (HEAs), firstly reported by Yeh et al., are a kind of novel structure materials breaking through the conventional alloy design concept [[1], [2], [3]]. Composed of five to thirteen elements with equiatomic or near equiatomic content, HEAs are inclined to form single-phase solid solution structure, such as face-centered cubic (FCC), body-centered cubic (BCC) or close-packed hexagonal (HCP) [[4], [5], [6]]. Generally, HEAs are considered to have high entropy effects, lattice distortion effect, slow diffusion effect and cocktail effect [7,8]. They own excellent mechanical properties and have great application prospects. Particularly, refractory high entropy alloys (RHEAs), composed of refractory elements, such as Nb, Ta, Mo, W, have high temperature resistance and outstanding high temperature strength [[9], [10], [11], [12]]. They are considered to be new high temperature alloys for higher temperature applications that exceeding the serving temperature of Ni-based high-temperature alloys. For instance, the firstly reported RHEAs, Nb₂₅Mo₂₅Ta₂₅W₂₅ and V₂₀Nb₂₀Mo₂₀Ta₂₀W₂₀ [13,14], possess BCC single-phase solid solution structure, and have high temperature stability up to 1600 °C. Importantly, their yield strength maintains 405 MPa and 477 MPa up to 1600 °C. It should be noted that the mechanical strength of Nb₂₅Mo₂₅Ta₂₅W₂₅ alloy was significantly improved after adding alloying element V, demonstrating that alloying enhancement is an effective method to strengthen RHEAs as it works in the traditional alloys. Though Nb₂₅Mo₂₅Ta₂₅W₂₅ RHEA has excellent high temperature performance, its ductility is quite poor at room temperature, which hinders the alloy's engineering application. Some investigations have been done to improve the ductility and strength of Nb₂₅Mo₂₅Ta₂₅W₂₅ RHEAs by alloying with V, Ti, Re and Cu [[13], [14], [15], [16], [17], [18], [19]]. For example, the yield strength of NbMoTaW RHEA at room temperature was improved from 1058 MPa to 1246 MPa by V alloying while it still maintained the low fracture strain [14]. Ti alloying obviously enhanced both the strength (from 1058 MPa to 1343 MPa) and ductility (from 2.6% to 14.1%) of NbMoTaW RHEA [15]. It is confirmed that different alloying elements have different strengthening effect on the NbMoTaW RHEA. Some elements merely strengthen the alloy without any improvement of ductility while others enhance both the strength and ductility. However, the effect of alloying element on this alloy's mechanical properties, such as strength and ductility, is still unclear up to now, and needs systematic investigations. In present work, elements with high melting points over 1850 °C, such as Cr, Zr, V, Hf and Re, were employed to alloy the NbMoTaW RHEA and improve its mechanical properties. The alloying elements with high melting temperatures over 1850 °C were merely chosen in order to not weaken the alloy's high temperature resistance. The mechanical properties of the alloyed NbMoTaWX (X = Cr, Zr, V, Hf and Re) RHEAs were predicted by theoretical calculations combined with experimental verifications. The influence of alloying elements on the mechanical properties of NbMoTaWX (X = Cr, Zr, V, Hf and Re) RHEAs was investigated at the atomic scale based on the bonding strength and electronic structure.