Quickly Understand the Different Roles of Tungsten in Superalloys

What is Tungsten

Tungsten is a metal element with the element symbol W and atomic number 74. Tungsten is a non-ferrous metal and an important strategic metal. Tungsten is mainly used to make filaments, high-speed cutting alloy steels and superhard molds. It is also used in the manufacture of optical instruments, chemical instruments. China is the world's largest tungsten storage country.

	I																	O
1	1 H	II											III	IV	V	VI	VII	2 He
2	3 Li	4 Be											5 B	6 C	7 N	8 O	9 F	10 Ne
3	11 Na	12 Mg	III	IV	V	VI	VII	VIII			I	II	13 Al	14 Si	15 P	16 S	17 Cl	18 Ar
4	19 K	20 Ca	21 Sc	22 Ti	23 V	24 Cr	25 Mn	26 Fe	27 Co	28 Ni	29 Cu	30 Zn	31 Ga	32 Gc	33 As	34 Se	35 Br	36 Kr
5	37 Rb	38 Sr	39 Y	40 Zr	41 Nb	42 Mo	43 Tc	44 Ru	45 Rh	46 Pd	47 Ag	48 Cd	49 In	50 Sn	51 Sb	52 Te	53 I	54 Xe
6	55 Cs	56 Ba	57-71 La-Lu	72 Hf	73 Ta	74 W	75 Re	76 Os	77 Ir	78 Pt	79 Au	80 Hg	81 Tl	82 Pb	83 Bi	84 Po	85 At	86 Rn
7	87 Fr	88 Ra	89-103 Ac-Lr	104 Rf	105 Db	106 Sg	107 Bh	108 Hs	109 Mt	110 Ds	111 Rg	112 Uub

La-Lu		57 La	58 Ce	59 Pr	60 Nd	61 Pm	62 Sm	63 Eu	64 Gd	65 Tb	66 Dy	67 Ho	68 Er	69 Tm	70 Yb	71 Lu
Ac-Lr		89 Ac	90 Th	91 Pa	92 U	93 Np	94 Pu	95 Am	96 Cm	97 Bk	98 Cf	99 Es	100 Fm	101 Md	102 No	103 Lr

Item Name: Tungsten
Element Symbol: W
Atomic Number: 74
Atomic Weight: 183.84
Atomic Radius: 137pm
Density: 19.35g/cm3
Melting Point: 3410°C
Boiling Point: 5660°C
Electronic Layout: [Xe]4f145d46s2

Figure 1
The Atomic Structure of Tungsten

Physical Properties

Tungsten is a silver-white metal and is a rare metal. It is also a refractory metal. Tungsten can improve the high temperature hardness of steel. Tungsten has a high melting point, a low vapor pressure, and a low evaporation rate.

Tungsten has two crystal structures, α and β. At standard temperature and pressure, α-tungsten is a stable body-centered cubic structure. β-tungsten occurs only in the presence of oxygen. It is stable below 630 °C, and is converted into α-tungsten above 630 °C, and this process is irreversible.

Chemical Properties

Tungsten is chemically stable. It does not react with air and water at room temperature. Without heating, any concentration of hydrochloric acid, sulfuric acid, nitric acid, hydrofluoric acid, aqua regia and tungsten will not react. When the temperature rises to 80°C-100°C, other acids except hydrofluoric acid will only have a weak effect on tungsten.

Solid Solution Strengthening Effect of Tungsten

Tungsten plays a significant role in solid solution strengthening in superalloys. Tungsten has a larger atomic radius, 10%-13% larger than that of nickel, cobalt, and iron. Based on this property, tungsten causes significant expansion of alloy crystals in superalloys. This significantly increases the strength of the alloy.

The addition of tungsten to the superalloy also reduces the stacking fault energy of the alloy matrix. This also significantly improves the solid solution strengthening effect of the alloy, as we explained in our previous article.

The figure below shows the effect of tungsten content on the strength of superalloys.

Figure 2
The Effect of Tungsten Content on Alloy Strength

Among superalloys, many Hastelloy alloys use tungsten for solid solution strengthening. Such as: Hastelloy C-22, Hastelloy C-276, Hastelloy G-30. Also, the tungsten in Inconel 686 plays the same role.

C-22

C-276

G-30

686

Precipitation Strengthening Effect of Tungsten

For precipitation strengthened superalloys, tungsten has no direct precipitation strengthening effect. But in these alloys, 50% of the tungsten goes into the strengthening phase of the alloy. This affects the distribution of other alloying elements between the matrix and the strengthening phase. This will make the precipitation strengthening effect of the alloy more obvious.

In addition, the addition of tungsten will also promote the formation of other intercrystalline strengthening phases. This also affects the strength of the alloy.

Conclusion

Tungsten is a refractory metal with a very large atomic radius. Therefore, tungsten plays a significant role in solid solution strengthening in the alloy.

In addition, tungsten can also enter the precipitated phase and affect the partitioning of other elements. This gives it a precipitation strengthening effect at the same time.

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2022.06.27