Martienssen W., Warlimont H. (Eds.). Handbook of Condensed Matter and Materials Data

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302 Part 3 Classes of Materials

1400

1300

1200

1100

1000

900

800

700

600

500

10 20 30 40 50 60 70 80 85 90 95

Al Cu10 20 30 40 50 60 70 80 90

Al–Cu

(wt %)Cu

(at. %)Cu

T(K)

933.5 K

821 K

836 K

864 K

897 K

(Al)

840 K

636 K

(Cu)

1320 K

1356 K

Fig. 3.1-135 Cu

−

Al phase diagram [1.106]

1800

1700

1600

1500

1400

1300

10 20 30 40 50 60 70 80 90

10 20 30 40 50 60 70 80 90Cu

(at.%)

(wt %)

T(K)

Cu–Ni

1357.87 K

1728 K

(Cu, Ni)

Fig. 3.1-136 Cu

−

Ni phase diagram [1.106]

10 20 30 40

Specific electrical conductivity (MS/m)

Zn, Ni (wt %)

Cu-Zn 20 °C

Cu-Zn 200 °C

Cu-Ni 20 °C

Cu-Ni 200 °C

Fig. 3.1-137 Electrical conductivity of Cu

−

Ni and Cu

−

alloys [1.103]

Part 3 1.8

Metals 1.9 Refractory Metals and Alloys 303

Table 3.1-107 Composition and properties of characteristic copper–nickel and copper–nickel–zinc alloys

Material UNS No. Purity; other Yield Ultimate Fracture Thermal Electrical

elements (wt%) stress R

po.2

tensile strain conductivity κ resistivity ρ

(MPa) strength R

(%) (Wm

−1

) (µ cm)

(MPa)

Copper nickel C70400 92.4 Cu–5.5Ni 276–524 262–531 2–46 67 13.79

–1.5 Fe–0.6Mn

Copper nickel C70600 87.7 Cu–10 Ni 110–393 303–414 10–42 42 21.55

–1.5 Fe–0.6Mn

Copper nickel C71500 67 Cu–31 Ni 138–483 372–517 15–45 21 38.31

–0.7 Fe–0.5Be

Nickel silver 10% C74500 65 Cu–25 Zn–10 Ni 124–524 338–896 1–50 37 20.75

Nickel silver 12% C74500 65 Cu–23 Zn–12 Ni 124–545 359–641 2–48 30 22.36

Nickel silver 15% C75200 65 Cu–20 Zn–15 Ni 124–545 365–634 2–43 27 24.59

Nickel silver 18% C74500 65 Cu–17 Zn–18 Ni 172–621 386–710 3–45 28 27.37

400

350

300

250

200

150

100

20 30 40

Zn, Ni (wt %)

Thermal conductivity (Wm

–1

)

Cu-Zn 200 °C

Cu-Zn 20 °C

Cu-Ni 200 °C

Cu-Ni 20 °C

Fig. 3.1-138 Thermal conductivity of Cu

−

Ni and Cu

−

alloys [1.103]

3.1.9 Refractory Metals and Alloys

Several books and reviews have been published on the

technology and properties of refractory metals and their

alloys [1.107–116]. According to the most common deﬁ-

nition, refractory metals comprise elements of the Group

Va and VIa with a melting point higher than 2000

◦

these are niobium, tantalum, molybdenum, and tungsten.

In some publications the VIIa metal rhenium is also in-

cluded, as it does not ﬁt in any other classiﬁcation. Less

common deﬁnitions describe a refractory metal as metal

with a melting point equal to or greater than that of

chromium, thus additionally including vanadium, tech-

netium, the reactive metal hafnium, and the noble metals

ruthenium, osmium, ruthenium, and iridium. This chap-

ter will give data on molybdenum, tungsten, tantalum,

niobium, and their alloys. Powder metallurgy (P/M) is

the only production route for commercial W and W al-

loys and for 97% of Mo, the remainder is processed

by electron-beam melting (EB) and vacuum-arc casting

(VAC) [1.117]. The ﬁner grain structure of P/M-material

is advantageous for both the further processing and

the mechanical properties of the ﬁnished product. For

some alloys such as those doped with potassium silicate,

, and Y

, P/M is the only possible production

technique.

Part 3 1.9

304 Part 3 Classes of Materials

Die-pressing

Mo, W, Ta

Powder

Mo, W, Ta

Cold-isostatic pressing

Mo, W, Ta

Powder-injection

molding Mo, W

Extrusion of plasti-

fied powder Mo, W

Spray-forming

W, Ta

Indirect heating

(resistance heating elements

or induction radiant heating

Mo, W, Ta

Direct heating

(self-resistance heating)

Mo, W, Ta

Deformation

Mo, W, Ta

Hot isostatic pressing

or uniaxial hot pressing

Mo, W, Ta

Infiltration

Mo, W

Shaping

and

compaction

Sintering

Full density

processing

Fig. 3.1-139 Large-scale P/M production routes for Mo, W and Ta [1.118]

The mechanical properties and homogenous mi-

crostructure required are again the reasons to apply P/M

in producing Ta wire as employed extensively in the

manufacture of capacitors. The larger fraction of the Ta

sheet production is based on the use of EB sheet bars, as

this is more economical. The techniques of VAC and EB

dominate in the production of Nb and Nb alloys. The in-

dustrial P/M production routes of Mo, W, Ta, and their

alloys are given in Fig. 3.1-139.

Net shape techniques such as powder injection

molding and spraying on a lost core (vacuum-plasma-

spraying, chemical vapor deposition) are limited to some

speciﬁc applications in the ﬁeld of electronic devices

and aerospace products. Less than 10% of the produc-

tion quantity is delivered in the as-sintered state. The

most common fully-dense-processing techniques are

deformation by rolling, forging, swaging, and drawing.

Initially refractory metals were applied mainly in

the pure state. Extensive developments mainly driven by

US aerospace programs led to a wide variety of alloys

now commercially available. The compositions of solid-

solution, precipitation- and dispersion-strengthened

alloys are given in Table 3.1-108. Tungsten heavy met-

als and refractory-metal-based composite materials are

not included here.

Carbide precipitation hardening (in TZM, MHC)

is effective up to 1400

◦

C. The addition of the de-

formable oxides La

(in ML) and xK

O· y SiO

(in

−

Mo), respectively, results in oxide reﬁnement by

deformation and in the possibility of tailoring the mech-

anical properties. This is the main alloying mechanism

for Mo applied in lamps [1.119].

The tungsten alloy which is commercially most

important is Al

−

K-silicate doped W (AKS–W). It is

a dispersion-strengthened, micro-alloyed metal with

a directionally recrystallized microstructure. Spherical

bubbles stabilized by containing potassium gas are inter-

acting with dislocations, sub-boundaries and high-angle

boundaries [1.120,121]. The high stability of these bub-

bles can be explained by the low solubility of K in W,

even at operating temperatures up to 3200

◦

A current task is to reduce the low-temperature brit-

tleness of Mo and W, which is essentially due to a rigid

covalent component of the interatomic bond along the

edges of the bcc unit cell. This causes a low solubility for

interstitial elements which occupy the octahedral sites

of the lattice and give rise to its tetragonal distortion and

a strong interaction of dislocations with the elastic strain

ﬁeld surrounding the interstitial solutes, thus impeding

the dislocation movement [1.122]. One possibility to

increase the low-temperature ductility is alloying with

rhenium which lowers the brittle-to-ductile-transition

temperature of both W and Mo [1.123–125]. But the

insufﬁcient supply and the high price of rhenium limit

the application of these alloys.

The addition of oxides such as ThO

,BaO,SrO,

,andSc

lowers the electron work function

of W, which is important for its application in elec-

trodes. The production quantities of W

−

and

−

as electron emitting materials are increas-

ing at the expense of the slightly radioactive W

−

ThO

material.

The production of capacitors, the dominating ap-

plication of Ta, requires material in its purest state.

Part 3 1.9

Metals 1.9 Refractory Metals and Alloys 305

Table 3.1-108 Typical compositions of commercial refractory metal alloys (in weight percent, analyses of base-metal correspond to metallic purity) [1.118]

Alloy

designation Mo W Ta Nb Re C O Si K Y La Ce Th Ti Zr Hf

Molybdenum alloys

Pure Mo 99.97

UHP-Mo 99.9995

TZM 99.3 0.025 0.02 0.5 0.08

MHC 98.6 0.08 0.035 1.2

−

99.2–99.6 0.048–0.1 0.27–0.6

(ML)

−

99.42 0.10–0.12 0.37–0.43 0–0.06

(MY)

−

Mo 99.8–99.9 0.01–0.07 0.013–0.07 0.005–0.03

Mo50Re 52.4 47.5

Mo30W 69.7 30

Tungsten alloys

Pure W 99.99

UHP-W 99.9995

AKS-W 99.98 0.004–0.01

−

97.9–98.9 0.015–0.03 0.85–1.7

(WL)

−

98.0 0.28 1.62

(WC)

−

ThO

98.0 0.24 1.71

(WT)

AKS- 98.0–99.0 0.12–0.24 0.001–0.005 0.86–1.71

−

ThO

W5Re 94.9 5.0

W26Re 73.9 26

AKS-W3Re 96.9 3.0 0.004

Tantalum alloys

Pure Ta 99.95

Ta2.5W 2.5 97.4

Ta10W 10.0 89.9

Niobium alloys

Pure Nb 99.9

Nb1Zr 98.9 1.0

C-103 88.8 1 10

FS-85 10 28 60.8 1

WC-3009 9 60.8 0.1 30

Nb−46.5Ti 46.5

Part 3 1.9

306 Part 3 Classes of Materials

Solid-solution-strengthened Ta2.5W is used for compo-

nents in chemical apparatus. Superconducting Nb46.5Ti

accounts for more than half of all Nb alloys produced.

Hafnium is the main addition for niobium-based alloys

used by the aerospace industry.

Refractory metals and their alloys are used in a wide

variety of ﬁelds of application and products such as

electrical and electronic devices; light sources; medical

equipment; automotive, aerospace, and defense indus-

try; chemical and pharmaceutical industry; or premium

and sporting goods.

The producers of electrical and electronic devices,

including the lighting industry, are the largest consumers

of refractory metal products. In 1998, 1850 t of W prod-

ucts was used for ﬁlaments and electrodes in lamps

only. Signiﬁcant quantities of Mo are used for semi-

conductor base-plates for power rectiﬁers and in various

products for lamps, such as dipped beam shields or sup-

port wires. Rapid growth in multimedia and wireless

communication networks systems has boosted the need

for W

−

Cu and Mo

−

Cu heat sink materials. These ma-

terials possess a high thermal conductivity combined

with a low thermal expansion, close to those of Si

and GaAs semiconductors or certain packaging mater-

ials. Also, the amount of Mo sputtering targets applied

in the production of wiring for large format thin-ﬁlm

transistor LCDs and PDPs has risen signiﬁcantly ow-

ing to the unique combination of low resistivity and

high resistance against hillock formation. The electronic

industry is the largest market (around 70%) for Ta prod-

ucts, employing the metal mainly in the manufacture of

capacitors.

Refractory metals are also widely used by the ma-

terials processing industry. Molybdenum glass melting

electrodes, TZM and MHC isothermal forging tools,

weighing several tons per part, TZM piercing plugs

for the production of stainless steel tubes, Mo and Ta

crucibles for synthesizing artiﬁcial diamond, or TIG-

welding electrodes are examples of products in this ﬁeld.

In order to improve the tribological properties of trans-

mission and engine components for automobiles they

are coated with Mo.

Recent products in the ﬁeld of aerospace and defense

industry are shaped charge liners made of Mo and Ta

penetrators formed explosively. The X-ray anode, a com-

posite product made of W5Re or W10Re, TZM and

optionally graphite, is the essential item of computer-

Fig. 3.1-141 Thermal conductivity versus temperature of

molybdenum [1.126], tungsten [1.129], niobium [1.127],

and tantalum [1.127]

tomography equipment, a very demanding application

that critically depends on the users’ expertise.

3.1.9.1 Physical Properties

The atomic and structural properties of the pure refrac-

tory metals are listed in Chapt. 2.1, Tables 2.1-12 and

2.1-14. Special features of refractory metals are their

low vapor pressure, low coefﬁcient of thermal expan-

sion, and the high thermal and electrical conductivity of

Mo and W. This combination of physical properties has

opened up a wide range of new applications during the

last decade, especially in the ﬁeld of electronics.

The coefﬁcient of linear thermal expansion, the

thermal conductivity, the speciﬁc heat, and the elec-

trical resistivity as function of temperature are shown

in Figs. 3.1-140 – 3.1-143. The vapor pressure and rate

of evaporation are shown in Fig. 3.1-144. In the case of

precipitation- and dispersion-strengthened molybdenum

alloys, such as TZM, MHC, ML, MY, and K

−

Mo,

0 200 400 600 800 1000 1200 1400 1600

T(°C)

(× 10

–6

)

Coefficient of linear thermal

expansion (m/m

)

Mo Nb

Ta W

Fig. 3.1-140 Coefﬁcient of linear thermal expansion ver-

sus temperature of molybdenum [1.126], tungsten [1.126],

niobium [1.127], and tantalum [1.128]

180

160

140

120

100

0 200 400 600 800 1000 1200 1400

T(°C)

Thermal conductivity (W/mK)

Mo Nb

Ta W

Part 3 1.9

Metals 1.9 Refractory Metals and Alloys 307

0.4

0.3

0.2

0.1

Specific heat (J/g

0 200 400 600 800 1000 1200 1400 1600

T(°C)

Mo Nb

Ta W

Fig. 3.1-142 Speciﬁc heat versus temperature of molybde-

num, tungsten, niobium, and tantalum [1.126]

0.6

0.5

0.4

0.3

0.2

0.1

0.0

200

400

600

800

1000

1200

1400

1600

T(°C)

Mo Nb

Specific electrical resistivity (Ω mm

/m)

Ta W

Fig. 3.1-143 Speciﬁc electrical resistivity versus tem-

perature of molybdenum [1.126], tungsten [1.126],

niobium [1.127], and tantalum [1.127]

as well as the tungsten based alloys AKS–W, WL, WC,

and WT, the physical properties do not differ signif-

icantly from those of the pure metals. Values for the

Young’s modulus and its temperature dependence are

plotted in Fig. 3.1-145. TheYoung’s moduli of the Group

Va metals are considerably lower than those of the

Group VIa metals due to the differences in electronic

structure.

The strong inﬂuence of the surface conditions on

the emissivity and lack of information in the literature

concerning pre-treatment make it difﬁcult to interpret

emissivity data. An overview of emissivity measure-

ments for W, Nb, and Ta at 684.5 nm from 1500

◦

up to the liquid phase using laser polarimetry is given

in [1.130].

–10

2900

2700

2500

2300

2100

1900

1700

1500

Vapor pressure (Pa)

–9

–8

–7

–6

–5

–4

–3

–2

–1

–7

2900

2700

2500

2300

2100

1900

1700

1500

1300

Evaporation rate (mg/cm

–6

–5

–4

–3

–2

–1

T(°C)

Mo Nb

Ta W

T(°C)

Mo Nb

Ta W

Fig. 3.1-144a,b Vapor pressure (a) and evaporation rate

(b) versus temperature of molybdenum, tungsten, niobium,

and tantalum [1.129]

450

400

350

300

250

200

150

100

0 200 400 600 800 1000120014001600180020002200

T(°C)

Young’s modulus (GPa)

Fig. 3.1-145 Young’s moduli versus temperature of molyb-

denum, tungsten, niobium, and tantalum [1.128]

Part 3 1.9

308 Part 3 Classes of Materials

3.1.9.2 Chemical Properties

Refractory metals are highly resistant to many chem-

ical agents. Tantalum is outstanding in its performance

as it is inert to all concentrations of hydrochloric and

nitric acid, 98% sulphuric acid, 85% phosphoric acid,

and aqua regia below 150

◦

C. Tantalum can be attacked

by hydroﬂuoric acid and strong alkalis, however. The

excellent corrosion resistance of Ta is attributed to

dense natural oxide layers which prevent the chem-

ical attack of the metal. Niobium is less resistant

than Ta and embrittles more easily. Nevertheless the

more economical Nb has replaced Ta in some applica-

tions. Molybdenum and W are highly resistant to many

molten glasses and metals as long as free oxygen is

absent.

The interaction with H, N, and O is widely differ-

ent for Mo and W on the one hand, and Nb and Ta

on the other. Molybdenum and W have almost no sol-

ubility, whereas Nb and Ta can dissolve a considerable

amount of these elements. Hydrogen can be removed

from Nb at 300

◦

C to 1600

◦

C and from Ta at 800

◦

to 1800

◦

C without metal loss by degassing in high vac-

uum. For the removal of N, temperatures higher than

1600

◦

C are recommended. The evaporation of volatile

oxides at temperatures above 1600

◦

C in high vacuum

leads to a reduction of the oxygen content in Nb and

Ta. But during such heat treatments, metal is evaporated

simultaneously.

An overview on the resistance of pure Mo, W, Nb,

and Ta against different media is given in Tables 3.1-109

Table 3.1-109 Metal loss of Mo, W, Nb and Ta in millimeter per year (mm/yr) in acids, alkalis and salt solutions

Corroding agent Temperature Metal loss (mm/yr)

(aqueous solution)

Mo W Nb Ta

10% HCl 20

◦

C < 0.003 0.002 0 < 0.001

10% H

◦

C < 0.005 < 0.1 < 0.1 < 0.1

10% HNO

◦

C 18.6 < 0.25 < 0.013 < 0.013

3% HF 20

◦

C < 0.001 < 0.1 > 3.0 > 3.0

10% CH

COOH 20

◦

C 0.07 < 0.05 < 0.1 < 0.013

10% KOH 20

◦

C < 0.1 < 0.1 < 0.2 < 0.1

3% NaCl 20

◦

C < 0.1 < 0.1 < 0.1 < 0.1

10% HCl 100

◦

C < 0.025 0.005 0.005 (embrittlement) < 0.025

10% H

100

◦

C 0.17 < 0.25 < 0.001 0

10% HNO

100

◦

C 150 < 0.25 < 0.076 < 0.025

3% HF 100

◦

C 0.18 0.15 > 3.0 > 3.0

10% CH

COOH 100

◦

C 0.033 < 0.05 < 0.1 < 0.013

10% KOH 100

◦

C 0.054 0.01 1.2 (embrittlement) < 0.003 (embrittlement)

3% NaCl 100

◦

C < 0.1 < 0.1 < 0.1 < 0.1

to 3.1-111. In accordance with the deﬁnition given

in [1.131], a material is considered “stable” against acor-

rosive medium if the metal loss is < 0.1 mm per year. If

the loss of material is between 0.1 and 1.0 mm per year,

the material is “considerably stable;” and it is “fairly un-

stable” if the loss of material is between 1.0 and 3.0 mm.

The material is “unsuitable” in an environment if the

metal loss is > 3.0 mm per year.

Oxidation Behavior

Refractory metals require protection from oxidizing en-

vironment as they do not form protective oxide layers.

Oxidation of Mo and W leads to a loss of material by the

formation of volatile oxides above 600

◦

C, but without

any signiﬁcant impact on the mechanical properties.

The low temperature oxidation of Mo and W of-

ten causes problems in practical use as thin corrosion

ﬁlms are formed during storage in moist air. The surface

topography of Mo has a strong impact on the reac-

tion rate. The corrosion ﬁlm contains oxygen but also

chemically-bonded nitrogen, which is incorporated in

the ﬁlm during ﬁlm growth. C

residues are the nu-

clei for the oxidative attack in the early stage of ﬁlm

growth [1.132].

There is extensive evidence that between 300 and

500

◦

C, oxide-dispersion-strengthened (ODS) refrac-

tory metal alloys (e.g., Mo

−

and Mo

−

grades) possess markedly reduced oxidation rates

compared to the pure metals [1.133]. In-situ oxida-

tion and evaluation of the binding state reveal that

MoO

dominates over MoO

both for Mo and Mo–

Part 3 1.9

Metals 1.9 Refractory Metals and Alloys 309

Table 3.1-110 Maximum temperatures for the resistance of Mo, W, Nb, and Ta against metal melts (up to the stated

temperatures the solubility of the refractory metal in the metal melts and vice versa is negligible)

Metal melt Maximum resistance temperature (

◦

Mo W Nb Ta

Al Not resistant 680 Not resistant Not resistant

Pb 1100 1100 850 1000

Fe Not resistant Not resistant Not resistant Not resistant

Ga 300 800 400 450

K 1200 900 1000 1000

Cu 1300 Resistant Resistant Resistant

Mg 1000 600 950 1150

Na 1030 900 1000 1000

Hg 600 600 600 600

Zn 500 750 Not resistant 500

Sn 550 980 Not resistant 260

Ag Resistant Resistant Resistant 1200

Table 3.1-111 Maximum temperatures for the resistance of Mo, W, Nb, and Ta against gaseous media at atmospheric

pressure

Gaseous media Maximum resistance temperature

Mo W Nb Ta

Air and oxygen see Sect. 3.1.9.2

Hydrogen Resistant Resistant 250

◦

(T > 250

◦

embrittlement owing to

dissolved hydrogen)

300

◦

(T > 300

◦

embrittlement owing to

dissolved hydrogen)

Nitrogen Resistant Resistant 300

◦

(T > 300

◦

embrittlement owing to

dissolved nitrogen)

400

◦

(T > 400

◦

embrittlement owing to

dissolved nitrogen)

Ammonia Resistant

(except 1000–1100

◦

nitration)

Resistant

(except 700–1150

◦

nitration)

300

◦

(T > 300

◦

embrittlement owing to

dissolved nitrogen)

700

◦

(T > 700

◦

embrittlement owing to

dissolved nitrogen)

Water vapor 700

◦

C 700

◦

C 200

◦

C 200

◦

Carbon monoxide 800

◦

(T > 800

◦

C carburization)

1000

◦

(T > 800

◦

C carburization)

800

◦

C 1100

◦

Carbon dioxide 1200

◦

(T > 1200

◦

C oxidation)

1200

◦

(T > 1200

◦

C oxidation)

400

◦

C 500

◦

0.47 wt%Y

–0.08 wt%Ce

(Fig. 3.1-146). These

results are conﬁrmed when comparing the thickness of

the oxide layer formed at 500

◦

C in air (Fig. 3.1-147).

The enhanced oxidation resistance of doped sam-

ples cannot be attributed to a chemical but rather to

a morphological-mechanical effect. Investigations of the

surface of pure Mo samples subjected to long-term ox-

idation reveal a network of cracks in the MoO

layer

which drastically diminishes the passivating effect of

this layer by facilitating a further attack by O. Such

cracks cannot be found on the surface of oxidized

−

and Mo

−

samples [1.133].

Above 700

◦

C for Mo and approximately 900

◦

for W, evaporation of the volatile oxides MoO

and WO

is the rate-controlling process and the oxidation follows

a linear time dependence. Above 2000

◦

C for Mo and

2400

◦

C for W, the metal loss increases because of the

increasing vapor pressure of the pure metals. The time

Part 3 1.9

310 Part 3 Classes of Materials

100

(%)

Langmuir (L)

0 5000 10000 15000

20000

Mo/percentage

MY/percentage

MoO

MY/percentage

MoO

2400 2100 1800

1600 1400 1300

4×10

–1

–2

6×10

–3

3.6 4.4 5.2 6.0 6.8

/T (K

–1

)

3000 2400 2100

1800 1400 1200

–1

–2

4×10

–3

34567

/T (K

–1

)

1600

2×

–1

T(°C)

∆

(µm/h)

∆

(mg/

min)

0.67 Pa

0.27 Pa

0.13 Pa

0.07 Pa

0.03 Pa

T(°C)

∆

(µm/h)

∆

(mg/

min)

0.03 Pa

0.07 Pa

0.13 Pa

0.27 Pa

0.53 Pa

Fig. 3.1-148a,b Metal loss in air for Mo (a) and W (b) at T ≥

1200

◦

C [1.134]

Fig. 3.1-146 ESCA measurements and evaluation of

Mo/MoO

/MoO

fraction versus in-situ oxidation con-

ditions in Langmuir (1 L = 10

−6

Torr s). Mo: 4N5 Mo

ribbon, electro-polished surface; MY: Mo–0.47 wt%Y

–

0.08 wt%Ce

ribbon, electro-polished surface, test

conditions: 500

◦

C/air [1.133]

Crack length/

area (mm/mm

)

Time (h)

0 100 200 300 400 500 600

Oxide layer

thickness (µm)

Mo (oxide layer thickness)

MY (oxide layer thickness)

Mo (crack length)

MY (crack length)

Fig. 3.1-147 Thickness of oxide layer and crack length

per area versus testing time. Mo: 4N5 Mo rib-

bon, electropolished surface; MY: Mo–0.47 wt%Y

–

0.08 wt%Ce

ribbon, electropolished surface, test

conditions: 500

◦

C/air [1.133]

dependence of metal loss for Mo and W at T ≥ 1200

◦

is shown in Fig. 3.1-148 [1.134].

The oxidation rates of Nb and Ta strongly depend

on temperature, pressure, and time. Different mecha-

nisms causing different oxidation rates can be observed

and metastable sub-oxides are formed during use in

O-containing atmospheres [1.131, 134]. Oxygen is dis-

solved in the metal matrix which leads to signiﬁcant

changes of the mechanical properties. A poorly adher-

ent, porous pentoxide is formed on the metal surface

which does not protect the metal from further attack.

Oxygen which has penetrated the porous oxide layer

diffuses along the grain boundaries of the metal, leading

to drastic embrittlement.

The metal loss at 1100

◦

C due to oxidation in air is

shown for Mo, W, Nb, and Ta in Fig. 3.1-149 [1.135].

The embrittled zone caused by oxygen diffusion into the

substrate is not considered in this diagram.

Only a few coating systems have been found to pre-

vent the refractory metals from oxidation. In the case

of Mo, Pt-cladded components with a diffusion barrier

interlayer on the basis of alumina, which prevents the

formation of brittleintermetallic molybdenum–platinum

Part 3 1.9

Metals 1.9 Refractory Metals and Alloys 311

Time (h)

024681012

Metal loss (mm)

Fig. 3.1-149 Oxidation behavior of Mo, W, Nb, and Ta at

1100

◦

C [1.135]

phases, are used; e.g., for stirrers used to homogenize

special glasses. Mo components for glass tanks and glass

melting electrodes are protected from oxidation by coat-

ings based on silicides, such as silicon–boron [1.136].

Silicon–boron and other silicide based coatings, such as

silicon–chromium–iron or silicon–chromium–titanium,

are also used to protect Mo and Nb based alloys from

oxidation in aerospace applications [1.135,137–139]

3.1.9.3 Recrystallization Behavior

The mechanisms and kinetics of recovery and recrys-

tallization processes signiﬁcantly affect the processing

and application of refractory metals. The homologous

temperatures for obtaining 50% recrystallized struc-

ture during annealing for one hour, range from 0.39

for Mo, 0.41 for W, 0.42 for Nb, to 0.43 for P/M Ta.

The fraction of recrystallized structure as a function of

the annealing temperature of Mo, W, P/M Ta, TZM,

and Mo–0.7wt%La

sheets of 1-mm thickness is

shown in Fig. 3.1-150. Experimental data on the evo-

lution of grain size with the annealing temperature for

pure Mo and Ta sheets of 1-mm thickness are shown

in Fig. 3.1-151 [1.126,140]. A recrystallization diagram

for Mo is published in [1.141], and for P/M Ta in [1.142].

Compared to pure Mo, the recrystallization tempera-

ture of the carbide-precipitation-hardened alloy TZM is

increased by 450

◦

C and that of Mo–0.7wt%La

550

◦

C. The data listed in Table 3.1-112 show that all

those alloys containing particles, which deform together

with the matrix metal (ML, K

−

Mo, WL10, WL15,

WT20, and AKS

−

ThO

), reveal a signiﬁcantly

increased recrystallization temperature in the highly de-

100

Annealing temperature (°C)

600 800 1000 1200 1400 1600 1800

TZM

Mo–0.7 wt%

Percentage of recrystallized structure

Fig. 3.1-150 Fraction of recrystallized structure versus an-

nealing temperature (annealing time t

= 1h) for Mo,

P/M Ta, W, TZM, and Mo–0.7 wt% La

sheets

with a thickness of 1 mm, degree of deformation,

ϕ =

−th

100[%],whereth

= thickness of sintered

plate, and th

= thickness of rolled sheet. Mo: ϕ = 94%,

P/M Ta: ϕ = 98%, W: ϕ =94%, TZM: ϕ = 98%, Mo–

0.7 wt% La

: ϕ = 99% [1.126]

10000

1000

100

Annealing temperature (°C)

600 800 1000 1200 1400 1600 1800 2000 2200

Average grain size (µm)

EB Ta

P/M Mo

P/M Ta

Fig. 3.1-151 Grain size versus annealing temperature

(annealing time t

=1 h) for Mo, P/M Ta, and EB (electron-

beam melted) Ta sheets with a thickness of 1 mm. Mo:

ϕ = 94%, P/M Ta: ϕ = 98%, EB Ta: ϕ = 98% [1.126,140]

formed state. Increasing the degree of deformation from

ϕ = 90% to ϕ =99.99% leads to a further increase of

the recrystallization temperature (100% recrystallized

structure, 1 h annealing time) ranging from 600

◦

C for

−

Mo, 700

◦

C for ML, 950

◦

C for WT20, 1000

◦

for WL10, to 1050

◦

C for WL15 and WC20. This can

be explained by particle reﬁnement. During the de-

formation process the particles are elongated. During

annealing they break up and rows of smaller particles

Part 3 1.9