A First Principles Investigation of W1 - x Ir x Alloys: Structural, Electronic, Mechanical, and Thermal Properties
HUANGFU Hao1, WANG Zilong1, LIU Yongli1(), MENG Fanshun2, SONG Jiupeng3, QI Yang1
1.College of Materials Science and Engineering, Northeastern University, Shenyang 110819, China 2.School of Science, Liaoning University of Technology, Jinzhou 121001, China 3.China National R&D Center for Tungsten Technology, Xiamen Tungsten Co. Ltd. , Xiamen 361021, China
Cite this article:
HUANGFU Hao, WANG Zilong, LIU Yongli, MENG Fanshun, SONG Jiupeng, QI Yang. A First Principles Investigation of W1 - x Ir x Alloys: Structural, Electronic, Mechanical, and Thermal Properties. Acta Metall Sin, 2022, 58(2): 231-240.
Tungsten (W) possess comprehensive physical and chemical properties that are suitable for aerospace and space nuclear power applications, including the highest melting temperature (3410oC) among metals, high elastic modulus, thermal shock resistance, and high temperature strength. However, its poor ductility at room temperatures significantly hinders its fabricability and potential use in the above-mentioned fields. Accordingly, to improve the ductility of W, solid solution strengthening is the primary method considered besides grain refining and deformation strengthening. Experimental studies have shown that Ir is a brittle metal with an fcc structure, but it can greatly improve the ductility of W; however, the corresponding mechanism is still unclear. Thus, using the first principles method based on density functional theory together with phonon spectrum calculations, the effect of the addition of different contents of Ir on the structure, phase stability, mechanical properties, and thermodynamic properties of W were studied. The relation between the addition of different contents of Ir and above-mentioned properties of W-Ir alloys were theoretically investigated. It was found that Ir can induce instability in the W-Ir alloy in the ground state due to the occupation of its antibonding electrons below the Fermi level. When content of Ir added is less than 7.4%, the formation of the W-Ir alloy becomes stable in the ground state. With an increase in temperature and the content of Ir, the thermodynamic stability is improved, implying that Ir is suitable for incorporation with W for application at high temperature. The addition of Ir helps to improve the toughness of the W alloy, which is consistent with the experimental observation. Besides, Ir can simultaneously improve the planar shear resistance. Furthermore, the pCOHP analysis revealed that the inherent mechanism of the ductile effect of brittle Ir in W is attributed to their different modes of electron transition and overlapping. For Ir, electrons transfer from its higher energy orbital of to the lower energy d xz and d yz orbitals. In contrast, for W, the electrons transfer from its low energy orbital of to the d xzand d yz orbitals. The d xzand d yzorbitals of Ir and W form a metallic bond, which is further enhanced with an increase in the content of Ir added. Therefore, Ir acts as a toughness-enhancing element in W-Ir alloys.
Fig.1 The illustrations of W-Ir solid solution models
Element
Source
a / nm
B / GPa
G / GPa
E / GPa
G / B
Cp / GPa
W
Present
0.3171
305
158
404
0.52
41.71
Exp.
0.3165[33], 0.3166[34]
314[35], 315[36]
163[35], 164[36]
418[35], 419[36]
0.52[35], 0.52[36]
41.82[35], 41.79[36]
Calc.
0.3171[37]
323[37], 310[38],
176[37], 145[38],
447[37], 377[38],
0.55[37], 0.47[38],
29[37], 66.70[38],
304[39], 301[40]
147[39], 148[40]
379[39], 382[40]
0.49[40], 0.48[41]
52.7[40], 59[41]
Ir
Present
0.3877
342
226
555
0.66
-39.07
Exp.
0.3839[42]
363[43], 353[44]
221[43], 217[44]
550[43], 540[44]
0.61[43], 0.61[44]
-13[43], -14[44]
Calc.
0.3871[45]
351[45], 364[46],
232[45], 223[46],
570[45], 555[46],
0.66[45], 0.61[46],
-43[45], -15[46],
405[47], 347[48]
288[47], 222[48]
698[47], 549[48]
0.71[47], 0.64[48]
-88[47], -29[48]
Table 1 Comparisons between the calculated lattice constant (a), elastic moduli (bulk modulus(B), shear modulus (G) and Young's modulus(E)), Pugh value (G / B) and Cauchy pressure (Cp) of W and Ir according to Equations (6)-(8) and those from the literatures [33-48]
Fig.2 Variation of formation energy (ΔE) of W-Ir binary alloys changes with Ir addition
Fig.3 Relationship between a and Ir content for W-Ir binary alloys
Fig.4 Variations of elastic constants (C11, C12, C44) (a), elastic moduli (B, G, E) (b), G / B (c), and Cp (d) with Ir additions for W-Ir binary alloys (The corresponding data of bcc-W from literatures [35-41]are also presented for comparison)
Fig.5 Relationships between energy and projected crystal orbital Hamiltonian population (pCOHP) for different W-Ir alloys (Ir—W1 (Ir—W2) refers to the bonding between Ir and its 1st (2nd) nearest neighbor W atoms)
Fig.6 Differential charge densities of different W-Ir binary alloys
Fig.7 Average bonding lengths between the first neighbor atorns (L) of W—W1, Ir—W1, and Ir—Ir1 for different W-Ir binary alloys, and Ir—Ir1 and W—W1 for pure metal
Fig.8 Temperature dependences of free energy (F) (a), entropy (S) (b), enthalpy(H) (c), and heat capacity (cV ) (d) for different W-Ir binary alloys (Insets show the locally enlarged plots)
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