30079_06.pdf

CHAPTER 6

TITANIUM AND ITS ALLOYS

Donald Knittel

James B. C. Wu

Cabot Corporation

Kokomo, Indiana

6.1 INTRODUCTION

6.5.2 Drawing

100

6.5.3 Bending

104

6.2 ALLOYS

6.5.4 Cutting and Grinding

104

6.2.1 Aerospace Alloys

6.5.5 Welding

104

6.2.2 Nonaerospace Alloys

6.2.3 Other Alloys

6.6 SPECIFICATIONS, STANDARDS,

AND QUALITY CONTROL

105

6.3 PHYSICALPROPERTIES

6.7 HEALTH AND SAFETY

FACTORS

6.4 CORROSION RESISTANCE

107

6.5 FABRICATION

6.8 USES

107

6.5.1 Boiler Code

6.1 INTRODUCTION

Titanium was first identified as a constituent of the earth's crust in the late 170Os. In 1790, William

Gregor, an English clergyman and mineralogist, discovered a black magnetic sand (ilmenite), which

he called menaccanite after his local parish. In 1795, a German chemist found that a Hungarian

mineral, rutile, was the oxide of a new element he called titan, after the mythical Titans of ancient

Greece. In the early 190Os, a sulfate purification process was developed to commercially obtain high-

purity TiO 2 for the pigment industry, and titanium pigment became available in both the United States

and Europe. During this period, titanium was also used as an alloying element in irons and steels.

In 1910, 99.5% pure titanium metal was produced at General Electric from titanium tetrachloride

and sodium in an evacuated steel container. Since the metal did not have the desired properties,

further work was discouraged. However, this reaction formed the basis for the commercial sodium

reduction process. In the 1920s, ductile titanium was prepared with an iodide dissociation method

combined with Hunter's sodium reduction process.

In the early 1930s, a magnesium vacuum reduction process was developed for reduction of tita-

nium tetrachloride to metal. Based on this process, the U.S. Bureau of Mines (BOM) initiated a

program in 1940 to develop commercial production. Some years later, the BOM publicized its work

on titanium and made samples available to the industrial community. By 1948, the BOM produced

batch sizes of 104 kg. In the same year, E. I. du Pont de Nemours & Co., Inc., announced commercial

availability of titanium, and the modern titanium metals industry began. 1

By the mid-1950s, this new metals industry had become well established, with six producers, two

other companies with tentative production plans, and more than 25 institutions engaged in research

projects. Titanium, termed the wonder metal, was billed as the successor to aluminum and stainless

steels. When, in the 1950s, the DOD (titanium's most staunch supporter) shifted emphasis from

aircraft to missiles, the demand for titanium sharply declined. Only two of the original titanium metal

plants are still in use, the Titanium Metals Corporation of America's (TMCA) plant in Henderson,

Reprinted with additions from Kirk-Othmer Encyclopedia of Chemical Technology, 3rd ed., Wiley,

New York, 1983, Vol. 23, by permission of the publisher.

Mechanical Engineers' Handbook, 2nd ed., Edited by Myer Kutz.

Nevada, and National Distillers & Chemical Corporation's two-stage sodium reduction plant built in

the late 1950s at Ashtabula, Ohio, which now houses the sponge production facility for RMI Cor-

poration (formerly Reactor Metals, Inc.).

Overoptimism followed by disappointment has characterized the titanium-metals industry. In the

late 1960s, the future again appeared bright. Supersonic transports and desalination plants were

intended to use large amounts of titanium. Oregon Metallurgical Corporation, a titanium melter,

decided at that time to become a fully integrated producer (i.e., from raw material to mill products).

However, the supersonic transports and the desalination industry did not grow as expected. Never-

theless, in the late 1970s and early 1980s, the titanium-metal demand again exceeded capacity and

both the United States and Japan expanded capacities. This growth was stimulated by greater accep-

tance of titanium in the chemical-process industry, power-industry requirements for seawater cooling,

and commercial and military aircraft demands. However, with the economic recession of 1981-1983,

the demand dropped well below capacity and the industry was again faced with hard times.

6.2 ALLOYS

Titanium alloy systems have been studied extensively. A single company evaluated over 3000 com-

positions in 8 years. Alloy development has been aimed at elevated-temperature aerospace applica-

tions, strength for structural applications, and aqueous corosion resistance. The principal effort has

been in aerospace applications to replace nickel- and cobalt-base alloys in the 500-90O 0 C ranges. To

date, titanium alloys have replaced steel in the 200-50O 0 C range. The useful strength and corrosion-

resistance temperature limit is ~550°C.

The addition of alloying elements alters the a- /3 transformation temperature. Elements that raise

the transformation temperature are called a stabilizers; elements that depress the transformation tem-

perature are called /3 stabilizers; the latter are divided into /3-isomorphous and /3-eutectoid types. The

/3-isomorphous elements have limited a solubility, and increasing additions of these elements pro-

gressively depresses the transformation temperature. The /3-eutectoid elements have restricted beta

solubility and form intermetallic compounds by eutectoid decomposition of the /3 phase. The binary

phase diagram illustrating these three types of alloy systems is shown in Fig. 6.1

The important a-stabilizing alloying elements include aluminum, tin, zirconium, and the intersti-

tial alloying elements (i.e., elements that do not occupy lattice positions) oxygen, nitrogen, and

carbon. Small quantities of interstitial alloying elements, generally considered to be impurities, have

a very great effect on strength and ultimately embrittle the titanium at room temperature. 3 The effects

of oxygen, nitrogen and carbon on the ultimate tensile properties and elongation are shown in Table

6.1. These elements are always present and are difficult to control. Nitrogen has the greatest effect,

and commercial alloys specify its limit to be less than 0.05 wt %. It may also be present as nitride

(TiN) inclusions, which are detrimental to critical aerospace structural applications. Oxygen additions

increase strength and serve to identify several commercial grades. This strengthening effect diminishes

at elevated temperatures and under creep conditions at room temperature. For cryogenic service, low

oxygen content is specified (<1300 ppm) because high concentrations of interstitial impurities in-

crease sensitivity to cracking, cold brittleness, and fracture temperatures. Alloys with low interstitial

Fig. 6.1 The effect of alloying elements on the phase diagram of titanium: (a) ^-stabilized sys-

tem, (b) /3-isomorphous system, and (c) /3-eutectoid system. 2

Carbon"- 0

Table 6.1 Effects of O, N, and C on the Ultimate Tensile Strength 2 ' 3

Nitrogen* 3 ' 0

Oxygen 6 ' 0

Concentration of

Impurity, wt%

0.025

0.05

0.1

0.15

0.2

0.3

0.5

0.7

MPa d

330

365

440

490

545

640

790

930

MPa d

310

330

370

415

450

500

520

525

MPa d

Elong., %

380

460

550

630

700

embrittles

a Tests were conducted using titanium produced by the iodide process.

b UT = ultimate tensile stress.

c Elongation on 2.54 cm.

d To convert MPa to psi, multiply by 145.

content are identified as ELI (extra-low interstitials) after the alloy name. Carbon does not affect

strength at concentration above 0.25 wt % because carbides (TiC) are formed. Carbon content is

usually specified at 0.08 wt % max. 4

The most important alloying element is aluminum, an a stabilizer. It is not expensive, and its

atomic weight is less than that of titanium; hence, aluminum additions lower the density. The me-

chanical strength of titanium can be increased considerably by aluminum additions. Even though the

solubility range of aluminum extends to 27 wt %, above 7.5 wt % the alloy becomes too difficult to

fabricate and embrittles. The embrittlement is caused by a coherently ordered phase based on Ti 3 Al.

Other a-stabilizing elements also cause phase ordering. An empirical relationship below which or-

dering does not occur is 5

„ A ,

wt % Sn wt % Zr ^

^ ^

wt % Al +

+ 10 x wt % O < 9

The important /3-stabilizing alloying elements are the bcc elements vanadium, molybdenum, tan-

talum, and niobium of the /3-isomorphous type and manganese, iron, chromium, cobalt, nickel, cop-

per, and silicon of the j8-eutectoid type. The /3-eutectoid elements arranged in order of increasing

tendency to form compounds are shown in Table 6.2. The elements copper, silicon, nickel, and cobalt

are termed active eutectoid forms because of a rapid decomposition of /3 to a and a compound. The

other elements in Table 6.2 are sluggish in their eutectoid reactions.

Alloys of the (3 type respond to heat treatment, are characterized by higher density than pure

titanium, and are easily fabricated. The purpose of /3 alloying is to form an all-j8-phase alloy with

commercially useful qualities, form alloys with duplex a and /3 structure to enhance heat-treatment

Table 6.2 /3-Eutectoid Elements in Order of Increasing Tendency to

Form Compounds 2 ' 6

Composition for

j8 Retention

on Quenching,

Wt %

6.5

4.0

8.0

7.0

8.0

13.0

Eutectoid

Composition,

Wt %

0.9

Eutectoid

Temperature, 0 C

550

600

675

685

770

790

860

Element

manganese

iron

chromium

cobalt

nickel

copper

silicon

response (i.e., changing the a and /3 volume ratio), or use /3-eutectoid elements for intermetallic

hardnening. The most important commercial /3-alloying element is vanadium.

6.2.1 Aerospace Alloys

The alloys of titanium for aerospace use can be divided into three categories: an all-a structure, a

mixed a- /3 structure, and an all-/3 structure. The a- /3 structure alloys are further divided into near-a

alloys (<2% (3 stabilizers). Most of the approximately 100 commercially available alloys (approxi-

mately 30 in the United States, 40 in the USSR, and 10 in Europe and Japan) are of the a-/3 structure

type. 7 Some of these, produced in the United States, are given in Table 6.3 along with some wrought

properties. 8 " 1 0 The most important commercial alloy is Ti-6 Al-4 V, an a-(3 alloy with a good

combination of strength and ductility. It can be age-hardened and has moderate ductility, and an

excellent record of successful applications. It is mostly used for compressor blades and disks in

aircraft gas-turbine engines, and also in lower-temperature engine applications such as rotating disks

and fans. It is also used for rocket-motor cases, structural forgings, steam-turbine blades, and cryo-

genic parts for which ELI grades are usually specified.

Other commercially important a-j3 alloys are Ti-3 Al-2.5 V, Ti-6 Al-6 V-2 Sn, and Ti-IO V-2

Fe-3 Al (see Table 6.3). As a group, these alloys have good strength, moderate ductility, and can be

age-hardened. 10 ' 1 1 Weldability becomes more difficult with increasing (3 constituents, and fabrication

of strip, foil, sheet, and tubing may be difficult. Temperature tolerances are lower than those of the

a or near- a alloys. The alloy Ti-3 Al-2.5 V (called one-half Ti-6 Al-4 V) is easier to fabricate than

Ti-6 Al-4 V and is used primarily as seamless aircraft-hydraulic tubing. The alloy Ti-6 Al-6 V-2

Sn is used for some aircraft forgings because it has a higher strength than Ti-6 Al-4 V. The alloy

Ti-IO V-2 Fe-3 Al is easier to forge at lower temperatures than Ti-6 Al-4 V because it contains

more /3-alloying constituents and has good fracture toughness. This alloy can be hardened to high

strengths [1.24-1.38 GPa or (1.8-2) X 10 5 psi] and is expected to be used as forgings for airframe

structures to replace steel below temperatures of 30O 0 C 12 ,

Table 6.3 Properties, Specifications and Applications of Wrought Titanium Alloys 2 ' 9 ' 1 0

Average Physical Properties

CLTE 3 ,

^m/(m • K)

21-10O 0 C 21-538 0 C

Nominal

CAS

Modulus of

Elasticity b ,

GPa c

Modulus of

Rigidity",

GPa c

Poisson's 6

Ratio

Composition,

Registry

ASTM

B-265

Density,

g/cm 3

wt %

No.

Condition

commercially pure

99.5 Ti

99.2 Ti

99.1 Ti

99.0 Ti

99.2 Ti*

98.9 Ti*

Ti-5 Al-2.5 Sn' [11109-19-6]

Ti-8 Al-I Mo, [39303-55-4]

1 V

Ti-6 Al-2 Sn

grade 1

grade 2

grade 3

grade 4

grade 7

8.7

9.8

102

103

104

102

0.34

4.5

4.4

annealed

duplex

annealed

8.7

9.8

8.7

9.8

8.7

9.8

8.7

9.8

grade 6

9.4

9.6

110

124

8.5

10.1

0.32

[11109-15-2]

7.8

8.1

114

4.5

4 Zr-2 Mo'

Ti-3 Al-2.5 V [77709-23-2]

Ti-6 Al-4 V

9.6

9.9

107

114

110

annealed

4.5

4.4

4.5

[12743-70-3]

grade 5

8.7

9.6

0.342

Ti-6 Al-6 V,

[72606-77-5]

9.0

9.6

2Sn'

Ti-IO V-2 Fe, [51809-47-3]

3 Al'

112

4.6

solution

and age

a CLTE = coefficient of linear thermal expansion.

b Room temperature.

c To convert GPa to psi, multiply by 145,000.

d To convert MPa to psi, multiply by 145.

The only a alloy of commercial importance is Ti-5 Al-2.5 Sn. It is weldable, has good elevated-

temperature stability, and good oxidation resistance to about 60O 0 C. It is used for forgings and sheet-

metal parts such as aircraft-engine compressor cases because of weldability.

The commercially important near-a alloys are Ti-8 Al-I Mo-I V and Ti-6 Al-2 Sn-4 Zr-2

Mo. They exhibit good creep resistance and the excellent weldability and high strength of a alloys;

the temperature limit is ~500°C. Alloy Ti-8 Al-I Mo-I V is used for compressor blades because

of its high elastic modules and creep resistance; however, it may suffer from ordering embrittlement.

Alloy Ti-6 Al-2 Sn-4 Zr-2 Mo is also used for blades and disks in aircraft engines. The service

temperature limit of 470 0 C is ~70°C higher than that of Ti-8 Al-I Mo-I V. 5

Commercialization of /3 alloys has not been very successful. Even though alloys with high strength

[up to 1.5 GPa (217,500 psi)] were made, they suffered from intermetallic and o>-phase embrittlement.

These alloys are metallurgically unstable and have little practical use above 25O 0 C. They are fabricable

but welds are not ductile. This alloy type is used in the cold-drawn or cold-rolled condition and finds

application in spring manufacture (alloy Ti-13 V-Il Cr-3 Al). 1 3 There is one commercially available

alloy of the j3-eutectoid type (Ti-2.5 Cu) that uses a true precipitation-hardening mechanism to

increase strength. The precipitate is Ti 2 Cu. This alloy is only slightly heat treatable; it is used in

engine castings and flanges. 5

6.2.2 Nonaerospace Alloys

The nonaerospace alloys are used primarily in industrial applications. The four grades (ASTM grade

1 through grade 4) differ primarily in oxygen and iron content (see Table 6.4). ASTM grade 1 has

the highest purity and the lowest strength (strength is controlled by impurities). The two other alloys

of this group are ASTM grade 7, Ti-0.2 Pd, and ASTM grade 12, Ti-0.8 Ni-0.3 Mo. The alloys in

this group are distinguished by excellent weldability, formability, and corrosion resistance. The

strength, however, is not maintained at elevated temperatures (see Table 6.3). The primary use of

alloys in this group is in industrial-processing equipment (i.e., tanks, heat exchangers, pumps, elec-

Average Mechanical Properties

Extreme Temperatures

Tensile Yield Elonga-

Strength, Strength, tion,

MPa d

Room Temperature

Yield Elonga-

Strength, tion,

MPa*

Charpy

Impact

Strength,

J/m e

Reduction

in Area,

Test

Temperature,

0 C

Reduction

in Area,

Tensile

Strength,

MPa d

Hardness'

331

434

517

662

434

517

862

1000

241

315

540

152

HB 120

HB 200

HB 225

HB 265

HB 200

193

117

346

448

234

138

586

310

172

346

186

110

448

324

207

807

565

448

HRC 36

HRC 35

952

621

517

979

896

540

648

490

HRC 32

690

993

1069

586

315

540

315

483

345

924

531

427

HRC 36

HRC 38

931

807

1000

1276

1200

315

1103

979

e To convert J/m to ft-lb/in., divide by 53.38.

f HB = Brinnell, HRC - Rockwell (C-scale).

8 Also contains 0.2 Pd.

h Also contains 0.8 Ni and 0.3 Mo.

' Numerical designations = wt % of element.

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