# Different-Shaped Ultrafine MoNbTaW HEA Powders Prepared via Mechanical Alloying

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## Abstract

**:**

## 1. Introduction

## 2. Materials and Methods

## 3. Results and Discussion

_{mix}= H

_{mix}− TS

_{mix}, free energy of the system will be smaller with higher mixing entropy, and thus, the solid solution is more favorably formed [21]. Furthermore, several parameters, such as mixture enthalpy (ΔH) [20], atomic size mismatch (δ) [22], valence electron concentration (VEC) [23], and thermodynamic parameter (Ω) [20] are employed to predict the phase formation and stability of single-phase solid solution in HEAs. They are defined and calculated based on the following equations:

_{i}and c

_{j}are the atomic percentage of the ith and jth components, respectively, r

_{i}is the atomic radius of the ith component, (VEC)

_{i}is the VEC of the ith component, and (T

_{m})

_{i}is the melting temperature of the ith element. Normally, the formation of simple BCC solid solutions in HEAs is concluded: −22 ≤ ΔH

_{mix}≤ 7 kJ·mol

^{−1}, 0 ≤ δ ≤ 6.6, VEC < 6.87, and Ω > 1.1 [20,22,23]. The above parameters ΔH, δ, VEC, and Ω for the MoNbTaW alloy in the present investigation were calculated to be −6.5 KJ/mol, 2.78, 5.5, and 5591, respectively, well meeting the criterion for the formation of a single BCC solid solution structure. Therefore, the MoNbTaW alloy tends to form a BCC single-phase structure rather than a complex structure alloy during mechanical alloying.

## 4. Conclusions

## Author Contributions

## Funding

## Conflicts of Interest

## References

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**Figure 1.**XRD patterns of the initial mixed pure powders and the MoNbTaW high-entropy alloys (HEA) powders prepared by mechanical alloying with different process control agents.

**Figure 2.**XRD patterns of the HEA powders prepared without process control agents before and after annealing at 1200 °C.

**Figure 3.**Micro-morphologies of the starting powder and the MoNbTaW HEA powders prepared by mechanical alloying with different process control agents, (

**a**,

**b**) the starting powder, (

**c**,

**d**) without process control agent, (

**e**,

**f**) stearic acid, and (

**g**,

**h**) ethanol.

**Figure 4.**(

**a**) EDS analysis of the MoNbTaW HEA powders milled without process control agent, (

**b**,

**c**) TEM image of the MoNbTaW HEA powders with the selective electron diffraction patterns.

**Table 1.**Crystallite size, lattice constant, and strain of the MoNbTaW powders prepared with different process control agents.

Powders Prepared with Different Process Control Agents | Crystallite Size (nm) | Lattice Constant (Å) | Strain (%) |
---|---|---|---|

None | 11.8 | 3.1636 | 0.688 |

Ethanol | 24.2 | 3.1549 | 0.483 |

Stearic acid | 14.7 | 3.1648 | 0.590 |

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**MDPI and ACS Style**

Tong, Y.; Qi, P.; Liang, X.; Chen, Y.; Hu, Y.; Hu, Z.
Different-Shaped Ultrafine MoNbTaW HEA Powders Prepared via Mechanical Alloying. *Materials* **2018**, *11*, 1250.
https://doi.org/10.3390/ma11071250

**AMA Style**

Tong Y, Qi P, Liang X, Chen Y, Hu Y, Hu Z.
Different-Shaped Ultrafine MoNbTaW HEA Powders Prepared via Mechanical Alloying. *Materials*. 2018; 11(7):1250.
https://doi.org/10.3390/ma11071250

**Chicago/Turabian Style**

Tong, Yonggang, Peibu Qi, Xiubing Liang, Yongxiong Chen, Yongle Hu, and Zhenfeng Hu.
2018. "Different-Shaped Ultrafine MoNbTaW HEA Powders Prepared via Mechanical Alloying" *Materials* 11, no. 7: 1250.
https://doi.org/10.3390/ma11071250