The refractory metals are conveniently described as those which, first
of all, melt at temperatures well above the melting points of the common
alloying bases, iron, cobalt, and nickel. Second, it seems appropriate to
consider the refractory metals as those which have higher melting points
than do titanium (melting point 1660°C) and zirconium (1850°C), which
are used chiefly at intermediate temperatures. Therefore chromium (melting
point 1875°C) is usually classed as a refractory metal.
When the refractory metals are considered to be those metals melting at
temperatures above 1850°C, twelve metals are in this group:
W (melting point 3410°C), Re,
Os, Ta, Mo, Ir,
Nb, Ru, Hf, Rh,
V, Cr. Metalloids are elements of small atomic
size, which form interstitial solid solutions or interstitial compounds with
metals. They include hydrogen, oxygen, nitrogen, and carbon. In certain cases,
small metallic atoms, like boron and beryllium, may enter into restricted
interstitial solid solutions. However, the atomic sizes of these metals
are such as to preclude extensive interstitial solution, and they will
not be considered.
Alloying of tungsten (W) has been relatively
less studied than of some of the other refractory metals. Most of the tungsten
used thus far in aerospace applications has been in the unalloyed form, which
is much easier and less expensive to produce and fabricate. Also, it has been
found that, particularly at temperatures above 2200°C (4000°F), the
strengthening effects of many alloying agents decrease disproportionately.
Tungsten is consumed in four forms:
- Tungsten carbide
- Alloying additions
- Pure tungsten
- Tungsten-based chemicals
Tungsten carbide accounts for about 65% of tungsten consumption. It is combined
with cobalt as a binder to form the so-called cemented carbides, which are
used in cutting and wear applications. Characteristically, most of these
carbides have high hardness, good electrical and thermal conductivity, and
high stability. These properties account for the principal applications:
structures resistant to chemical reaction, uses in which wear resistance
is of major importance, and high-temperature radiant-energy sources. The
brittleness of carbides, however, has prevented their use as single-phase
materials in highly stressed structural applications and has led to the
development of metal-bonded composites (cemented carbides or cermets).
Metallic tungsten and tungsten alloy mill products account for about 16% of
consumption. Tungsten and tungsten alloys dominate the market in applications
for which a high-density material (19.3 g/cm3) is required, such
as kinetic energy penetrators, counterweights, flywheels, and governors.
Other applications include radiation shields and x-ray targets. In wire form,
tungsten is used extensively for lighting, electronic devices, and
Tungsten chemicals make up approximately 3% of the total consumption and are
used for organic dyes, pigment phosphors, catalysts, cathode-ray tubes, and
The high melting point of tungsten makes it an obvious choice for structural
applications exposed to very high temperatures. Tungsten is used at lower
temperatures for applications that can use its high elastic modulus, density,
or shielding characteristics to advantage.
Tungsten and tungsten alloys can be pressed and sintered into bars and
subsequently fabricated into wrought bar, sheet, or wire. Many tungsten
products are intricate and require machining or molding and sintering to
near-net shape and cannot be fabricated from standard mill products.
Tungsten mill products can be divided into three distinct groups on
the basis of recrystallization behavior.
The first group consists of EB-melted, zone-refined, or arc-melted unalloyed
tungsten; other very pure forms of unalloyed tungsten; or tungsten alloyed with
rhenium or molybdenum. These materials exhibit equiaxed grain structures upon
primary recrystallization. The recrystallization temperature and grain size
both decrease with increasing deformation.
The second group, consisting of commercial grade or undoped P/M tungsten,
demonstrates the sensitivity of tungsten to purity. Like the first group, these
materials exhibit equiaxed grain structures, but their recrystallization temperatures
are higher than those of the first-group materials. Also, these materials do not
necessarily exhibit decreases in recrystallization temperature and grain size
with increasing deformation. In EB-melted tungsten wire, the recrystallization
temperature can be 900°C (1650°F) or lower, whereas in commercially
pure (undoped) tungsten it can be as high as 1205 to 1400°C (2200 to 2550°F).
The third group of materials consists of AKS-doped tungsten (that is, tungsten
doped with aluminum-potassium-silicon), doped tungsten alloyed with rhenium, and
undoped tungsten alloyed with more than 1% ThO2. These materials are
characterized by higher recrystallization temperatures (>1800°C, or 3270°F)
and unique recrystallized grain structures. The structure of heavily drawn wire
or rolled sheet consists of very long interlocking grains.
This structure is most readily found in AKS-doped tungsten or in doped tungsten
alloyed with 1 to 5% Re. The potassium dopant is spread out in the
direction of rolling or drawing; when heated, it volatilizes into a linear array of
submicron-size bubbles. These bubbles pin grain boundaries in the manner of a
dispersion of second-phase particles. As the rows of bubbles become finer and
longer with increasing deformation, the recrystailization temperature rises, and the
interlocking structure becomes more pronounced.
Tungsten Alloys. Three tungsten alloys are produced commercially:
tungsten-ThO2, tungsten-molybdenum, and tungsten-rhenium. The
W-ThO2, alloy contains a dispersed second phase of 1 to 2% thorium.
The thorium dispersion enhances thermionic electron emission, which in turn improves
the starting characteristics of gas tungsten arc welding electrodes. It also increases
the efficiency of electron discharge tubes and imparts creep strength to wire at
temperatures above one-half the absolute melting point of tungsten.
Tungsten mill products, sheet, bar, and wire are all produced via powder
metallurgy. These products are available in either commercially pure (undoped)
tungsten or commercially doped (AKS-doped) tungsten. These additives improve the
recrystallization and creep properties of tungsten, which are especially important
when tungsten is used for incandescent lamp filaments. Wrought P/M stock can be
zone refined by EB melting to produce single crystals that are higher in purity
than the commercially pure product. Electron beam zone-melted tungsten single
crystals are of commercial interest for applications requiring single crystals
with very high electrical resistance ratios.
Tungsten Heavy-Metal Alloys (WHAs). These are a category of
tungsten-base materials that typically contain 90 to 98 wt% W.
Most commercial WHAs are two-phase structures, the principal phase being nearly
pure tungsten in association with a binder phase containing the transition metals
plus dissolved tungsten. As a consequence, WHAs derive their fundamental properties
from those of the principal tungsten phase, which provides for both high density
and high elastic stiffness. It is these two properties that give rise to must
applications for this family of materials.
The current uses of WHAs are spanning a wide range of consumer, industrial,
and government applications that include:
- Damping weights for computer disk drive heads
- Balancing weights for ailerons in commercial aircraft, helicopter rotors,
and for guided missiles
- Kinetic energy penetrators for defeating heavy armor
- Fragmentation warheads
- Radiation shielding, radio isotope containers, and collimalion apertures
for cancer therapy devices
- High performance lead-free shot for waterfowl hunting
- Gyroscope components
- Weight distribution adjustment in sailboats and race cars.
Many applications that require high gravimetric density for balance weights,
inertial masses, or kinetic energy penetrators or high radiographic density for
radiation shielding and collimaiion necessitate rather large bulk shapes. Such a
requirement eliminates all but a few candidates on the basis of prohibitive cost,
typically reducing the choice of very dense alloys down to either tungsten- or
Uranium alloys, like lead, are eliminated from an increasing number of potential
applications based on toxicity considerations, with uranium-base materials requiring
a license except for very small quantities. While the precious metals listed possess
attractive densities and offer essentially no toxicity, their cost is prohibitive
for all but a few density applications.
WHAs typically consist of 90 to 98 wt% W in combination with some
mix of nickel, iron, copper, and/or cobalt. The bulk of WHA production falls into
the 90 to 95% W range.
The choice of alloy composition is driven by several considerations. The primary
factor is the density required by the given application. Further considerations
include corrosion resistance, magnetic character, mechanical properties, and
postsinter heat treatment options.
The first WHA developed was a W-Ni-Cu alloy. Alloys of this ternary system are
still occasionally used today, primarily for applications in which ferromagnetic
character and electrical properties must be minimized. W-Ni-Cu alloys otherwise
offer inferior corrosion resistance and lower mechanical properties than the present
industry standard W-Ni-Fe alloys.
The majority of current uses for WHAs are best satisfied with the W-Ni-Fe system.
Alloys such as 93W-4.9Ni-2.lFe and 95W-4Ni-lFe represent common compositions.
The addition of cobalt to a W-Ni-Fe alloy is a common approach for slight
enhancement of both strength and ductility. The presence of cobalt within
the alloy provides solid-solution strengthening of the binder and slightly
enhanced tungsten-matrix interfacial strength. Cobalt additions of 5 to 15%
of the nominal binder weight fraction arc most common.
For extremely demanding applications, even higher mechanical properties are
obtainable from the W-Ni-Co system with nickel-to-cobalt ratios ranging from 2
to 9. Such alloys require resolution/quench, however, due to extensive
intermetallic (Co3W and others) formation on cool down from sintering.
A number of special WHAs are known as well. An example is the W-Mo-Ni-Fe quaternary
alloy, which utilizes molybdenum to restrict tungsten dissolution and spheroid
growth, resulting in higher strengths (but reduced ductility) in the as-sintered
There are also a number of alloy systems in various stages of development for
kinetic energy penetrators that are intended to provide a WHA that will undergo
high deformation rate failure by shear localization in a manner similar to
quenched and aged U-0.75Ti for more efficient armor defeat. These alloys to
date have not exhibited a property set of interest for industrial applications,
Mechanical and Physical Properties. Tungsten has high tensile
strength and good creep resistance. However, its high density, poor low-temperature
ductility, and strong reactivity in air limit its usefulness. Maximum service
temperatures for tungsten range from 1925 to 2500°C (3500 to 4500°F), but
surface protection is required for use in air at these temperatures.
Wrought tungsten (as-cold worked) has high strength, strongly directional mechanical
properties, and some room-temperature toughness. However, recrystallization occurs
rapidly above 1370°C (2500°F) and produces a grain structure that is crack
sensitive at all temperatures.