Sept_AMP_Digital

John H. Perepezko,

FASM*

University of

Wisconsin, Madison

Bruce A. Pint,

FASM*

Oak Ridge National

Laboratory, Tenn.

David R. Forrest,

FASM*

U.S. Dept. of Energy,

Washington

ADVANCED MATERIALS & PROCESSES •

SEPTEMBER 2014

aintaining our current standard of

living will require not only an abun-

dant energy supply, but also one that

is environmentally benign. This reality is moti-

vating the drive toward enhanced efficiency in

energy production. The majority of energy

sources that involve thermal production—in-

cluding fossil, nuclear, and even concentrated

solar power—require increased operating tem-

peratures and materials that can withstand an

aggressive, high-temperature environment for

long periods. Structural intermetallics offer a

number of attractive materials choices to ad-

dress these needs.

A myriad of intermetallic alloy phases are

available, with unique capabilities ranging from

electronic properties (such as permanent mag-

net and thermionic emission characteristics) to

specific chemical behavior (for use in catalysts

and sensors, and for corrosion and oxidation

resistance). However, the use of intermetallic

alloys in structural applications has lagged after

substantial progress in the 1980s and 90s. One

reason is that the fundamental characteristics

that impart intermetallics with exceptionally

high modulus and strength present challenges

to achieving useful ductility and fracture tough-

ness. However, the outlook for structural appli-

cations is changing. As the fundamental

understanding of deformation behavior and de-

fect structure has evolved, it has provided valu-

able guidance to enable advances in alloy design

and materials processing. These advances have

moved some intermetallics, such as Ti alu-

minides, from the laboratory into full scale

structural applications. Later this month, the

structural intermetallics symposium at Materi-

als Science & Technology 2014 in Pittsburgh

will highlight some of these advances as well as

progress in newer refractory metal silicide al-

loys for ultrahigh temperature applications. See

sidebar for details.

Commercial applications

Titanium aluminide alloys based on the γ-

TiAl phase have been widely studied

[1,2]

and cast

components are being introduced into gas tur-

bine engines and turbochargers (Fig. 1). These

commercial successes are the result of a multi-

faceted design approach to modify the natural

highly ordered structure to overcome pure

TiAl’s brittle nature. Lattice-distorting and par-

titioning alloying elements have been intro-

duced, phase transformations and secondary

phases are incorporated, and casting tech-

niques and heat treatments are being used to

homogenize and control grain structure. First

principle calculations are also helping to de-

velop new compositions. Resistance to disloca-

tion movement, which dominates the fracture

and fatigue crack growth behavior and limits

the extent of structural applications, is also in-

fluenced by deformation twinning that results

in cyclic hardening during fatigue.

In most structural applications, compo-

nents must be joined. Solid state joining by dif-

fusional annealing of a foil interlayer can be an

effective technique if there is control over the

intermetallic phases that can form within the

diffusion zone. A novel surface halogen treat-

ment significantly improves the high tempera-

ture oxidation resistance of TiAl-type alloys.

For Ti-Ni shape memory alloys, a refined and

uniform intermetallic precipitate structure is

favored by short annealing times.

Iron and nickel aluminides were researched

extensively in the 1980s and 90s in an attempt

to take advantage of their attractive high tem-

perature corrosion resistance

[3,4]

. Extensive ef-

forts were made to develop Fe

Al, FeAl, Ni

Al,

and NiAl for a variety of structural applications.

Great progress was made in increasing the high

temperature creep strength of NiAl

[5]

, but it did

not displace single crystal superalloys for high

temperature applications. Commercial applica-

tions include FeAl heating elements, Fe

porous hot gas filters, and cast Ni

Al rolls for

Structural Intermetallics:

Alloy Design, Processing, and Applications

Structural

intermetallics

offer a number

of attractive

choices to

address an

urgent need for

materials that

can endure

harsh

environments

for extended

periods.

*Member of ASM International

Fig. 1 —

Cast 50-mm diameter TiAl automotive

turbocharger.