Handbook Properties and Selection Nonferrous Alloys and Spl Purpose Mtls (1992) WW Part 11 docx

total of less than 1%) has a coefficient of expansion so low that its length is almost invariable for ordinary changes in
temperature. This alloy is known as Invar, which is a trade name (of Imphy, S.A.) meaning invariable.

Fig. 1 Coefficient of linear expansion at 20 °C versus Ni content for Fe-Ni alloys containing 0.4% Mn and 0.1% C

After the discovery of Invar, an intensive study was made of the thermal and elastic properties of several similar alloys.
Iron-nickel alloys that have nickel contents higher than that of Invar retain to some extent the expansion characteristics of
Invar. Alloys that contain less than 36% nickel have much higher coefficients of expansion than alloys containing 36% or
more nickel. Further information on iron-nickel alloys besides Invar is given in the section "Iron-Nickel Alloys Other
Than Invar" in this article.
Invar
Invar (UNS number K93601) and related alloys have low coefficients of expansion over only a rather narrow range of
temperature (see Fig. 2). At low temperatures in the region from A to B, the coefficient of expansion is high. In the
interval between B and C, the coefficient decreases, reaching a minimum in the region from C to D. With increasing
temperature, the coefficient begins again to increase from D to E, and thereafter (from E to F), the expansion curve
follows a trend similar to that of the nickel or iron of which the alloy is composed. The minimum expansivity prevails
only in the range from C to D.

Fig. 2 Change in length of a typical Invar over different ranges of temperature

In the region between D and E in Fig. 2, the coefficient is changing rapidly to a higher value. The temperature limits for a
well-annealed 36% Ni iron are 162 and 271 °C (324 and 520 °F). These temperatures correspond to the initial and final
losses of magnetism in the material (that is, the Curie temperature). The slope of the curve between C and D is then a
measure of the coefficient of expansion over a limited range of temperature.
Table 1 gives coefficients of linear expansion of iron-nickel alloys between 0 and 38 °C (32 and 100 °F). The expansion
behavior of several iron-nickel alloys over wider ranges of temperature is represented by curves 1 to 5 in Fig. 3. For
comparison, Fig. 3 also includes the similar expansion obtained for ordinary steel.
Table 1 Thermal expansion of Fe-Ni alloys between 0 and 38 °C

Ni, %

Mean coefficient, μm/m · K

31.4 3.395 + 0.00885 t
34.6 1.373 + 0.00237 t
35.6 0.877 + 0.00127 t
37.3 3.457 - 0.00647 t
39.4 5.357 - 0.00448 t
43.6 7.992 - 0.00273 t
44.4 8.508 - 0.00251 t
48.7 9.901 - 0.00067 t
50.7 9.984 + 0.00243 t
53.2 10.045 + 0.00031 t


Fig. 3 Thermal expansion of Fe-Ni alloys. Curve 1, 64Fe-31Ni-5Co; curve 2, 64Fe-36Ni (Invar); curve 3, 58Fe-42Ni; curve 4, 53Fe-
47Ni;
curve 5, 48Fe-52Ni; curve 6, carbon steel (0.25% C)
Effects of Composition on Expansion Coefficient. The effect of variation in nickel content on linear expansivity is shown in
Fig. 1. Minimum expansivity occurs at about 36% Ni, and small additions of other metals have considerable influences on
the position of this minimum. Because further additions of nickel raise the temperature at which the inherent magnetism
of the alloy disappears, the inflection temperature in the expansion curve (Fig. 2) also rises with increasing nickel content.
The addition of third and fourth elements to Fe-Ni provides useful changes of desired properties (mechanical and
physical), but significantly changes thermal expansion characteristics. Minimum expansivity shifts toward higher nickel
contents when manganese or chromium is added, and toward lower nickel contents when copper, cobalt, or carbon is
added. Except for the ternary alloys with nickel-iron-cobalt compositions (Super-Invars), the value of the minimum
expansivity for any of these ternary alloys is, in general, greater than that of a typical Invar alloy.
The effects of additions of manganese, chromium, copper, and carbon are shown in Fig. 4. Additions of silicon, tungsten,
and molybdenum produce effects similar to those caused by additions of manganese and chromium; the composition of
minimum expansivity shifts towards higher contents of nickel. Addition of carbon is said to produce instability in Invar,
which is attributed to the changing solubility of carbon in the austenitic matrix during heat treatment.

Fig. 4 Effect of alloying elements on expansion characteristics of Fe-
Ni alloys. (a) Displacement of nickel content caused by additions of
manganese, chromium, copper, and carbon to alloy of minimum expansivity. (b) Change in value of minimum coefficient of expansion
caused by additions of manganese, chromium, copper, and carbon
Effects of Processing. Heat treatment and cold work change the expansivity of Invar alloys considerably. The effect of heat
treatment for a 36% Ni Invar alloy is shown in Table 2. The expansivity is greatest in well-annealed material and least in
quenched material.
Table 2 Effect of heat treatment on coefficient of thermal expansion of Invar

Condition Mean coefficient, μm/m · K

As forged
At 17-100 °C (63-212 °F)
1.66
At 17-250 °C (63-480 °F)
3.11
Quenched from 830 °C (1530 °F)
At 18-100 °C (65-212 °F) 0.64
At 18-250 °C (65-480 °F) 2.53
Quenched from 830 °C and tempered
At 16-100 °C (60-212 °F) 1.02
At 16-250 °C (60-480 °F) 2.43
Cooled from 830 °C to room temperature in 19 h


At 16-100 °C (60-212 °F) 2.01
At 16-250 °C (60-480 °F) 2.89

Hot workability is enhanced by very close control of deoxidation and degassing during the melt process. Considerable

care must be used in hot working of iron-nickel alloys because at hot-working temperature they have a tendency to check
and break up when carelessly handled. Invar and related alloys should be annealed in a reducing atmosphere. Because
they are susceptible to intercrystalline oxidation during annealing, they should be processed in an atmosphere that
contains a large percentage of a neutral gas (such as nitrogen) and a small percentage of a reducing gas. Cold rolling and
drawing of iron-nickel alloys are quite similar to corresponding processing procedures for nickel.
Heat Treatment. The iron-nickel binary alloys are not hardenable by heat treatment. Annealing practice should be adjusted
to be consistent with requirements of the intended application. Exposure to temperatures and times that promote excessive
grain growth will limit further fabricating steps that require extreme bending, forming, deep drawing, chemical etching,
and so forth.
Annealing is done at 750 to 850 °C (1380 to 1560 °F). When the alloy is quenched in water from these temperatures,
expansivity is decreased, but instability is induced both in actual length and in coefficient of expansion. To overcome
these deficiencies and to stabilize the material, it is common practice to stress relieve approximately at 315 to 425 °C (600
to 800 °F) and to age at a low temperature 90 °C (200 °F) for 24 to 48 hours.
Cold drawing also decreases the thermal expansion coefficient of Invar alloys. The values for the coefficients in the
following table are from experiments on two heats of Invar:

Material condition Expansivity, ppm/°C

Direct from hot mill 1.4 (heat 1)

1.4 (heat 2)
0.5 (heat 1) Annealed and quenched
0.8 (heat 2)
0.14 (heat 1) Quenched and cold drawn (>70% reduction with a diameter of 3.2 to 6.4 mm, or 0.125 to 0.250 in.)

0.3 (heat 2)

By cold working after quenching, it is possible to produce material with a zero, or even a negative, coefficient of
expansion. A negative coefficient may be increased to zero by careful annealing at a low temperature. However, these
artificial methods of securing an exceptionally low coefficient may produce instability in the material. With lapse of time

and variation in temperature, exceptionally low coefficients usually revert to normal values. For special applications
(geodetic tapes, for example), it is essential to stabilize the material by cooling it slowly from 100 to 20 °C (212 to 68 °F)
over a period of many months, followed by prolonged aging at room temperature. However, unless the material is to be
used within the limits of normal atmospheric variation in temperature, such stabilization is of no value. Although these
variations in heat-treating practice are important in special applications, they are of little significance for ordinary uses.
Magnetic Properties. Invar and all similar iron-nickel alloys are ferromagnetic at room temperature and become
paramagnetic at higher temperatures. Because additions in nickel contents raise the temperature at which the inherent
magnetism of the alloy disappears, the inflection temperature in the expansion curve rises with increasing nickel content.
The loss of magnetism in a well-annealed sample of a true Invar begins at 162 °C (324 °F) and ends at 271 °C (520 °F).
In a quenched sample, the loss begins at 205 °C (400 °F) and ends at 271 °C (520 °F). Figure 5 shows how the Curie
temperature changes with nickel content in iron.

Fig. 5 Effect of nickel content on the Curie temperature of iron-nickel alloys

The thermoelastic coefficient, which describes the changes in the modulus of elasticity as a function of temperature, varies
according to the nickel content of iron-nickel low-expansion alloys. Invar has the highest thermoelastic coefficient of all
low-expansion iron-nickel alloys, while two alloys with 29 and 45% nickel have a zero thermoelastic coefficient (that is,
the modulus of elasticity does not change with temperature). However, because small variations in nickel content produce
large variations in the thermoelastic coefficient, commercial application of these two iron-nickel alloys with a zero
thermoelastic coefficient is not practical. Instead, the iron-nickel-chromium Elinvar alloy provides a practical way of
achieving a zero thermoelastic coefficient.
Electrical Properties. The electrical resistivity of 36Ni-Fe Invar is between 750 and 850 nΩ · m at ordinary temperatures.
The temperature coefficient of electrical resistivity is about 1.2 mΩ/Ω · K over the range of low expansivity. As nickel
content increases above 36%, the electrical resistivity decreases to ~165 nΩ · M at ~80% NiFe. This is illustrated in Fig.
6.

Fig. 6 Effect of nickel content on electrical resistivityof nickel-iron alloys

Other Physical and Mechanical Properties. Table 3 presents data on miscellaneous properties of Invar in the hot-rolled and
forged conditions. The effects of temperature on mechanical properties of forged 66Fe-34Ni are illustrated in Fig. 7.

Table 3 Physical and mechanical properties of Invar
Solidus temperature, °C (°F) 1425 (2600)
Density, g/cm
3
8.1
Tensile strength, MPa (ksi) 450-585 (65-85)

Yield strength, MPa (ksi) 275-415 (40-60)

Elastic limit, MPa (ksi) 140-205 (20-30)

Elongation, % 30-45
Reduction in area, % 55-70
Seleroscope hardness 19
Brinell hardness 160
Modulus of elasticity, GPa (10
6
psi) 150 (21.4)
Thermoelastic coefficient, μm/m · K 500
Specific heat, at 25-100 °C (78-212 °F), J/kg · °C (Btu/lb · °F) 515 (0.123)
Thermal conductivity, at 20-100 °C (68-212 °F), W/m · K (Btu/ft · h · °F)

11 (6.4)
Thermoelectric potential (against copper), at -96 °C (-140 °F), μV/K 9.8


Fig. 7 Mechanical properties of a forged 34% Ni alloy. Alloy composition: 0.25 C, 0.55 Mn, 0.27 Si, 33.9 Ni, balance Fe.
Heat treatment:
annealed at 800 °C (1475 °F) and furnace cooled
The binary iron-nickel alloys are not hardenable by heat treatment. Significant increases in strength can be obtained by

cold working some product forms such as wire, strip, and small-diameter bar. Table 4 shows tensile and hardness data for
both 36% and 50% nickel-iron alloys after cold working various percent cross-section reduction.
Table 4 Mechanical properties of Invar and a 52% Ni-48% Fe glass-sealing alloy
UNS number (alloy name) 0.2%
yield strength

Ultimate
tensile strength

Elongation,

%
Approximate
equivalent
hardness, HRB


MPa ksi MPa ksi

hardness, HRB

K93601 (Invar 36% Ni)
As annealed
260 38 470 68 37 75
10% cold worked
370 54 565 82 23 86
30% cold worked 550 80 675 98 10 95
50% cold worked 640 93 725 105 5 96
70% cold worked 703 102 730 106 3 97
K14052 (glass-sealing alloy 52% Ni)

As annealed 235 34 538 78 32 83
10% cold worked 525 76 640 93 19 92
30% cold worked 715 104 750 109 6 99
50% cold worked 770 112 814 118 3 100
70% cold worked 800 116 834 121 2 26 HRC

Mechanical properties such as tensile strength and hardness decrease rapidly with increasing service temperatures.
Selected elevated-temperature data for iron-nickel alloys are shown in Table 5.
Table 5 Typical tensile properties at elevated temperatures for some low-expansion nickel-iron alloys

Test
temperature

0.2% yield

strength
Ultimate
tensile
strength
UNS number (alloy name)
°C °F MPa

ksi MPa

ksi
Elongation,

%

Reduction

of area,

%
24 75 265 38.5

483 70 44 81.5
150 300 139 20.2

405 59 44.5 77.5
Invar 36% Ni (K9360l)
315 600 95 14 420 61 50 73

480 900 90 13 275 40 63 73
24 75 295 43 550 80 43.7 73.5
150 300 225 32.5

510 74 45.6 67.1
315 600 188 27.3

495 71.8

52.8 67.1
42% Ni low-expansion alloy (K94100)

480 900 157 22.8

370 54 43 58.4
24 75 300 43.3

538 78 46.2 79.3
150 300 243 35.3

483 70 43.2 75.6
315 600 223 32.3

462 67 42 73.5
49% Ni low expansion alloy
480 900 217 31.5

385 55.8

35.5 51.9

Corrosion Resistance. The iron-nickel low-expansion alloys are not corrosion resistant, and applications in even relatively
mild corrosive environments must consider their propensity to corrode. A comparison to corrosion of iron, in both high
humidity and salt spray environments, is shown in Fig. 8 and Table 6. Rust initiation occurs in approximately 24 hours for
nickel contents less than ~40% in high-humidity tests. Severe corrosion occurs after 200 hours exposure to a neutral salt
spray at 35 °C (95 °F).
Table 6 Effects of relative humidity on selected nickel-iron low-expansion alloys
Specimens exposed to 95% relative humidity for 200 h at 35 °C (95 °F)
Alloy type (UNS number) Condition Portion of surface

rusted (average), %
(a)


First rust

(three specimens), h

Annealed 70 1,1,1 Electrical iron
Cold rolled

50 1,1,2
Annealed 5 24, 24, 24 30% Ni temperature-compensator alloy
(b)


Cold rolled

>5 24, 24, 24
Annealed Few rust spots 48, 48, 96 Invar 36% Ni (K93601)
Cold rolled

Few rust spots 96, 96, 96
42% Ni low-expansion alloy (K94100) Annealed 0 . . .

Cold rolled

0 . . .
Annealed 0 . . . 49% Ni low-expansion alloy
Cold rolled

0 . . .
Annealed 0 . . . 52% Ni glass-sealing alloy (K14052)
Cold rolled

0 . . .

Annealed 0 . . . 80% Ni-4.5% Mo
(c)

Cold rolled

0 . . .

(a)
Visual estimate of the percentage of surface rusted.
(b)

Provided under the tradename of "Temperature Compensation 30" by Carpenter Technology Corporation.
(c)
Provided under the tradenames of "MolyPermalloy" by Allegheny Ludlum and "HyMu80" by Carpenter Technology Corporation


Fig. 8 Rust versus nickel content from 200 h neutral salt spray at 35 °C (95 °F)

Machinability. The iron-nickel alloys can be machined using speeds or feeds that are modified to accommodate their
gummy and stringy characteristics. As a general comparison to other austenitic alloys, they are similar to 316 stainless
steel. Using single point turning as a measure of machinability, the iron-nickel low-expansion alloys exhibit a 25%
machinability rating compared to resulfurized carbon steel (such as B 1112). Some general machining parameters for
iron-nickel alloys are shown in Table 7. There are "free-cut" varieties of Invar-type alloys available. These require minor
additions of other elements (such as selenium) which when combined with moderate adjustments to other residual
elements (such as manganese) will produce a twofold improvement in the machinability characteristic of these alloys.
Some increase in thermal expansion characteristics results from the modified compositions.
Table 7 Examples of various machining parameters for iron-nickel low-expansion alloys

Turning (single-point and box tools)
Roughing

Depth of cut, mm (in.) 2.5 (0.1)
Speed, m/min (ft/min) 9 (30)
Feed, mm/rev (in./rev) 0.25 (0.010)
Finishing
Depth of cut, mm (in.) 0.5 (0.020)
Speed, m/min (ft/min) 6 (20)
Feed, mm/rev (in./rev) 0.05 (0.002)
Turning (cutoff and form tools)
Speed, m/min (ft/min) 6 (20)
Feed, mm/rev (in./rev) with a tool width of:
3.2 mm (0.125 in.) 0.025 (0.001)
6.4 mm (0.250 in.) 0.025 (0.001)
13 mm (0.50 in.) 0.038 (0.0015)

25 mm (1.0 in.) 0.025 (0.001)
50 mm (2.0 in.) 0.018 (0.0007)

Drilling
Speed, m/min (ft/min) 10 (35)
Feed, mm/rev (in./rev) for a drill diameter of:
1.6 mm (
1
16
in.)
0.025 (0.001)
3.2 mm (
1
8
in.)
0.075 (0.003)

6.4 mm (
1
4
in.)
0.10 (0.004)
13 mm (
1
2
in.)
0.20 (0.008)
20 mm (
3
4
in.)
0.25 (0.010)
25 mm (1 in.) 0.30 (0.012)
38 mm (1
1
2
in.)
0.38 (0.015)
50 mm (2 in.) 0.45 (0.018)
Tapping speed, m/min (ft/min)
≤
7 threads per 25 mm (1 in.)
1.8 (6)
8-15 threads per 25 mm (1 in.) 2.4 (8)
6-24 threads per 25 mm (1 in.) 3.65 (12)
>24 threads per 25 mm (1 in.) 4.5 (15)
End milling parameters

With 0.5 mm (0.020 in.) radial depth of cut:
Speed, m/min (ft/min) 20 (65)
Feed, mm/tooth (in./tooth), with 13 mm (
1
2
in.) cutter diam
0.05 (0.002)
Feed, mm/tooth (in./tooth), with 25-50 mm (1-2 in.) cutter diam

0.10 (0.004)
With 1.5 mm (0.06 in.) radial depth of cut:
Speed m/min (ft/min) 15 (50)
Feed, mm/tooth (in./tooth), with 13 mm (
1
2
in.) cutter diam
0.075 (0.003)
Feed, mm/tooth (in./tooth), with 25-50 mm (1-2 in.) cutter diam

0.125 (0.005)

Welding. Invar can be successfully welded using most standard arc-welding processes. In general, preparation for welding
should be similar to stainless steels and should include proper cleaning and handling. Joint designs should allow easy
access to the weld because of poor weld pool fluidity but also should limit total weld volume to reduce shrinkage
problems. Preheating and postheating are not required and should be avoided. A low interpass temperature (150 °C, or
300 °F max) should be maintained.
Welding is most commonly performed using the gas-tungsten-arc or gas-metal-arc processes. Gas-tungsten-arc welding
can be accomplished with argon and/or helium shielding gases. Welding is best performed with a freshly ground thoriated
tungsten electrode. Gas-metal-arc welding can be successfully performed in all metal transfer modes, depending primarily
on base metal thickness. Shielding gases should be argon or argon-helium mixtures. Other nonarc welding processes

(such as resistance welding) may also be used.
When a filler metal is needed, a matching composition will provide the best match in thermal expansion properties.
Invarod weld filler metal (a 36Ni-Fe alloy containing ~1% Ti and 2.5% Mn) has been successfully used for matching
expansion characteristics. If a matching composition is not available, a high-nickel filler metal conforming to AWS A5.14
ERNi-1 or ER-NiCrFe-5 can be used. These materials will result in a weld with different thermal expansion properties.
Iron-Nickel Alloys Other Than Invar
Although iron-nickel alloys other than Invar have higher coefficients of thermal expansion, there are applications where it
is advantageous to have nickel contents above or below the 36% level of Invar. The alloy containing 39% Ni, for
example, has a coefficient of expansion corresponding to that of low-expansion glasses.
Alloys that contain less than 36% Ni have much higher coefficients of expansion than alloys with a higher percentage.
Alloys containing less than 36% Ni include temperature-compensator alloys (30 to 34% Ni). These exhibit linear changes
in magnetic characteristics with temperature change. They are used as compensating shunts in metering devices and
speedometers.
Iron-nickel alloys that have nickel contents higher than that of Invar retain to some extent the expansion characteristics of
Invar. Because further additions of nickel raise the temperature at which the inherent magnetism of the alloy disappears,
the inflection temperature in the expansion curve (Fig. 2) rises with increasing nickel content. Although this increase in
range is an advantage in some circumstances, it is accompanied by an increase in coefficient of expansion. Table 8 and
Fig. 9 present additional information on the coefficients of expansion of nickel-iron alloys at temperatures up to the
inflection temperature. They also give data on alloys with up to 68% Ni.
Table 8 Expansion characteristics of Fe-Ni alloys
Composition, % Inflection
temperature

Mn Si Ni °C °F
Mean coefficient
of expansion, from 20 °C to
inflection temperature, μm/m · K

0.11

0.02

30.14

155 310 9.2
0.15

0.33

35.65

215 420 1.54
0.12

0.07

38.70

340 645 2.50
0.24

0.03

41.88

375 710 4.85
. . . . . . 42.31

380 715 5.07
. . . . . . 43.01

410 770 5.71
. . . . . . 45.16

425 800 7.25
0.35

. . . 45.22

425 800 6.75
0.24

0.11

46.00

465 870 7.61
. . . . . . 47.37

465 870 8.04
0.09

0.03

48.10

497 925 8.79
0.75

0.00

49.90

500 930 8.84
. . . . . . 50.00

515 960 9.18
0.25

0.20

50.05

527 980 9.46
0.01

0.18

51.70

545 1015 9.61
0.03

0.16

52.10

550 1020 10.28
0.35

0.04

52.25

550 1020 10.09
0.05

0.03

53.40

580 1075 10.63
0.12

0.07

55.20

590 1095 11.36
0.25

0.05

57.81

None 12.24
0.22

0.07

60.60

None 12.78
0.18

0.04

64.87

None 13.62
0.00

0.05

67.98

None 14.37


Fig. 9 Effect of nickel content on expansion of Fe-
Ni alloys. (a) Variation of inflection temperature. (b) Variation of average coefficient of
expansion between room temperature and inflection temperature
Of significant commercial interest are those alloys containing approximately 40% to 50% nickel-iron alloys. Typical
compositions and thermal expansions for some of these alloys are given in Table 9.
Table 9 Composition and typical thermal expansion coefficients for common iron-nickel low-expansion alloys

Composition(a), % Alloy ASTM
specification

C(max)

Mn(max)

Si(max)

Ni(nom)

42 Ni-Iron F 30 0.02 0.5 0.25 41
46 Ni-Iron F 30 0.02 0.5 0.25 46
48 Ni-Iron F 30 0.02 0.5 0.25 48
52 Ni-Iron F 30 0.02 0.5 0.25 51
42 Ni-Iron (Dumet) F 29 0.05 1.0 0.25 42
42 Ni-Iron (Thermostat)

B 753 0.10 0.4 0.25 42
Typical thermal expansion coefficients from room temperature to:

300 °C (570 °F) 400 °C (750 °F) 500 °C (930 °F)
Alloy
ppm/°C ppm/°F mmp/°C ppm/°F ppm/°C ppm/°F
42 Ni-Iron 4.4 2.4 6.0 3.3 7.9 4.4
46 Ni-Iron 7.5 4.2 7.5 4.2 8.5 4.7
48 Ni-Iron 8.8 4.9 8.7 4.8 9.4 5.2
52 Ni-Iron 10.1 5.6 9.9 5.5 9.9 5.5
42 Ni-Iron (Dumet) . . . . . . 6.6 3.7 . . . . . .
(a)

Balance of iron with residual impurity limits of 0.25% max Si, 0.015% max P, 0.01% max S, 0.25% max Cr, and 0.5% max Co.
(b)

From room temperature to 90 °C (200 °F).
(c)

From room temperature to 150 °C (300 °F).
(d)

From room temperature to 370 °C (700 °F).

The 42% Ni-irons are widely used in applications for their low-expansion characteristics. These include semiconductor
packaging components, thermostat bimetals, incandescent light bulb glass seal leads (copper clad), and seal beam lamps.
Dumet wire is an alloy containing 42% Ni. It is clad with copper to provide improved electrical conductivity and to prevent
gassing at the seal. It can replace platinum as the seal-in wire in incandescent lamps and vacuum tubes.
The 43 to 47% Ni-iron alloys are commonly used for glass seal leads, grommets, and filament supports. This group of
alloys includes Platinate (36% Ni to 64% Fe), which has a coefficient of thermal expansion equivalent to that of platinum
(9.0 ppm/°C).
Iron-Nickel-Chromium Alloys
Elinvar is a low-expansion iron-nickel-chromium alloy with a thermoelastic coefficient of zero over a wide temperature
range. It is more practical than the straight iron-nickel alloys with a zero thermoelastic coefficient, because its
thermoelastics coefficient is less susceptible to variations in nickel content expected in commercial melting.
Elinvar is used for such articles as hair-springs and balance wheels for clocks and watches and for tuning forks used in
radio synchronization. Particularly beneficial where an invariable modulus of elasticity is required, it has the further
advantage of being comparatively rustproof.
The composition of Elinvar has been modified somewhat from its original specification of 36% Ni and 12% Cr. The
limits now used are 33 to 35 Ni, 61 to 53 Fe, 4 to 5 Cr, 1 to 3 W, 0.5 to 2 Mn, 0.5 to 2 Si, and 0.5 to 2 C. Elinvar, as
created by Guillaume and Chevenard, contains 32% Ni, 10% Cr, 3.5% W, and 0.7% C.
Other iron-nickel-chromium alloys with 40 to 48% Ni and 2 to 8% Cr are useful as glass-sealing alloys because the
chromium promotes improved glass-to-metal bonding as a result of its oxide-forming characteristics. The most common
of these contain approximately 42 to 48% nickel with chromium of 4 to 6%. Although chromium additions increase the
minimum thermal expansion and lower inflection points (Curie temperature), they have a beneficial effect on the glass-
sealing behavior of these alloys. The chromium promotes formation of a surface chromium oxide that improves wetting at

the metal/glass interface. Some of this metal oxide is absorbed by the glass during the actual glass seal and promotes a
higher-strength metal/glass bond (graded seals). Compositions and thermal expansions for some Fe-Ni-Cr alloys are
shown in Table 10.
Table 10 Type, composition, and typical thermal expansion for some iron-nickel-chromium glass-seal alloys

Composition
(a)
, % Alloy type

ASTM
specifications

Mn(max)

Si(max)

Cr(nom)

Ni(nom)

42-6 F 31 0.25 0.25 5.75 42.5
45-5 . . . 0.25 0.30 6.00 45.0
48-5 . . . 0.30 0.20 6.00 47.5
Alloy type

Average thermal expansion coefficients from room temperature to:
200 °C (390 °F) 300 °C (570 °F) 400 °C (750 °F) 500 °C (930 °F)

ppm/°C

ppm/°F

ppm/°C

ppm/°F

ppm/°C

ppm/°F

ppm/°C

ppm/°F

42-6 7.1 3.9 8.3 4.6 10.0 5.55 11.5 6.4
45-5 8.2 4.55 8.7 4.8 10.0 5.55 11.2 6.2
48-5 . . . . . . 9.4 5.2 10.3 5.7 . . . . . .

(a)

Balance of iron with 0.05% max C, 0.015% max P, 0.015% max S, and 0.50% max Co


Iron-Nickel-Cobalt Alloys
Replacement of some of the nickel by cobalt in an alloy of the Invar composition lowers the thermal expansion coefficient
and makes the alloy's expansion characteristics less susceptible to variations in heat treatment. These iron-nickel-cobalt
alloys (known as Super-Invars), however, have a more restrictive temperature range of useful application. In its restricted
temperature range, the expansion coefficient of a Super-Invar alloy is lower than that of Invar (unless the Invar is in the
cold-worked condition).
Super-Invar. Substitution of ~5% Co for some of the nickel content in the 36% Ni (Invar) alloy provides an alloy with an

expansion coefficient even lower than that Invar. A Super-Invar alloy with a nominal 32% Ni and 4 to 5% Co will exhibit
a thermal expansion coefficient close to zero, over a relatively narrow temperature range. Figure 10 compares thermal
expansion for 32% Ni-5% Co Super-Invar with that of an Invar alloy.

Fig. 10 Comparison of thermal expansion for Super-Invar (63% Fe, 32% Ni, 5% Co) and Invar (64% Fe, 36% Ni) alloys

Cobalt has been added to other Fe-Ni alloys in amounts as high as 40%. Such additions increase the coefficient of
expansion at room temperature. However, because they also raise the inflection temperature, they produce an alloy with a
moderately low coefficient of expansion over a wider range of temperature. If
Θ
is inflection temperature in °C, X is
nickel content, Y is cobalt content, and Z is manganese content. The inflection temperature of any low-expansion Fe-Ni-
Co alloy is approximated by θ= 19.5 (X + Y) - 22Z - 465. Carbon content does not significantly affect the inflection
temperature.
For practical applications, these Fe-Ni-Co alloys require that Ni+Co content be sufficient to lower the martensite start
temperature (M
s
) to well below room temperature. Nickel-cobalt contents for M
s
temperatures of about -100 °C (-150 °F)
can be approximated by:
Y = 0.0795 θ+ 4.82 + 19W - 18.1


X = 41.9 - 0.0282 θ- 37Z - 19W


where W is carbon content.
Kovar is a nominal 29%Ni-17%Co-54%Fe alloy that is a well-known glass-sealing alloy suitable for sealing to hard
(borosilicate) glasses. Kovar has a nominal expansion coefficient of approximately 5 ppm/°C and inflection temperature

of ~450 °C (840 °F) with an M
s
temperature less than -80 °C (-110 °F). The Dilver-P alloy produced by Imphy, S.A., is a
competitive grade with the Kovar alloy of Carpenter Steel.
Special Alloys
Iron-Cobalt-Chromium Low-Expansion Alloys. An alloy containing 36.5 to 37%Fe, 53 to 54.5% Co, and 9 to 10% Cr has an
exceedingly low, and at times, negative (over the range from 0 to 100 °C, or 32 to 212 °F) coefficient of expansion. This
alloy has good corrosion resistance compared to low-expansion alloys without chromium. Consequently, it has been
referred to as "Stainless Invar." Fernichrome, a similar alloy containing 37% Fe, 30% Ni, 25% Co, and 8% Cr, has been
used for seal-in wires for electronic components sealed in special glasses.
Hardenable Low-Expansion Alloys. Alloys that have low coefficients of expansion, and alloys with constant modulus of
elasticity, can be made age hardenable by adding titanium. In low-expansion alloys, nickel content must be increased
when titanium is added. The higher nickel content is required because any titanium that has not combined with the carbon
in the alloy will neutralize more than twice its own weight in nickel by forming an intermetallic compound during the
hardening operation.
As shown in Table 11, addition of titanium raises the lowest attainable rate of expansion and raises the nickel content at
which the minimum expansion occurs. Titanium also lowers the inflection temperature. Mechanical properties of alloys
containing 2.4% titanium and 0.06% carbon are given in Table 12.
Table 11 Minimum coefficient of expansion in low-expansion Fe-Ni alloys containing titanium

Ti, %

Optimum Ni, %

Minimum coefficient of

expansion, μm/m · K
0 36.5 1.4
2 40.0 2.9
3 42.5 3.6

Table 12 Mechanical properties of low-expansion Fe-Ni alloys containing 2.4 Ti and 0.06 C
Tensile strength

Yield strength

Condition
MPa ksi MPa ksi
Elongation
(a)
,

%
Hardness,

HB
42Ni-55.5Fe-2.4Ti-0.06C
(b)

Solution treated 620 90 275 40 32 140
Solution treated and age hardened 1140 165 825 120 14 330
Solution treated, cold rolled 50% and age hardened

1345 195 1140 165 5 385
52Ni-45.5Fe-2.4Ti-0.06C
(c)

Solution treated 585 85 240 35 27 125
Solution treated and age hardened 825 120 655 95 17 305

(a)
In 50 mm (2 in.).
(b)
Inflection temperature, 220 °C (430 °F); minimum coefficient of expansion, 3.2 μm/m · K.
(c)
Inflection temperature, 440 °C (824 °F); minimum coefficient of expansion, 9.5 μm/m · K

In alloys of the constant-modulus type containing chromium, addition of titanium allows the thermoelastic coefficients to
be varied by adjustment of heat-training schedules. The alloys in Table 13 are the three most widely used compositions.
The recommended solution treatment for the alloys that contain 2.4% Ti is 950 to 1000 °C (1740 to 1830 °F) for 20 to 90
min., depending on section size. Recommended duration of aging varies from 48 h at 600 °C (1110 °F) to 3 h at 730 °C
(1345 °F) for solution-treated material.
Table 13 Thermoelastic coefficients of constant modulus Fe-Ni-Cr-Ti alloys

Composition, %
Ni

Cr

C Ti
Thermoelastic coefficient,

annealed condition,

μm/m · K
Range of possible

coefficients
(a)
,

μm/m · K
42

5.4

0.06

2.4

0 18 to -23
42

6.0

0.06

2.4

36 54 to 13
(a)

Any value in this range can be obtained by varying the heat treatment.

For material that has been solution treated and subsequently cold worked 50% aging time varies from 4 h at 600 °C (1100
°F) to 1 h at 730 °C (1350 °F). Table 14 gives mechanical properties of a constant-modulus alloy containing 42% Ni,
5.4% Cr, and 2.4% Ti. Heat treatment and cold work markedly affect these properties.
Table 14 Mechanical properties of constant-modulus alloy 50Fe-42Ni-5.4Cr-2.4Ti
Tensile
strength
Yield

strength
Modulus of
elasticity
Condition
MPa

ksi MPa ksi
Elongation
(a)
,

%
Hardness,

HB
GPa

10
6

psi
Solution treated 620 90 240 35 40 145 165 24
Solution treated and aged 3 h at 730 °C (1345 °F) 1240

180

795 115 18 345 185 26.5
Solution treated and cold worked 50% 930 135

895 130 6 275 175 25.5

Solution treated, cold worked 50% and aged 1 h at 730 °C
(1345 °F)
1380

200

1240 180 7 395 185 27

(a)

In 50 mm (2 in.)

High-Strength, Controlled-Expansion Alloys. There is a family of Fe-Ni-Co alloys strengthened by the addition of niobium
and titanium that show the strength of precipitation-hardened superalloys while maintaining low coefficients of thermal
expansion typical of certain alloys from the Fe-Ni-Co system. Compositions of the alloys are shown in Table 15; typical
mechanical properties are presented in Table 16. The combination of exceptional strength and low coefficient of
expansion makes this family useful for applications requiring close operating tolerances over a range of temperatures.
Several components for gas turbine engines are produced from these alloys. Further information on low-expansion
superalloys is contained in the article "Nickel and Nickel Alloys" in this Volume.
Table 15 Composition and thermal expansion coefficients of high-strength controlled-expansion alloys
Coefficient of thermal expansion,
from room temperature to:
260 °C (500 °F) 370 °C (700 °F) 415 °C (780 °F)
Inflection
temperature

Alloy
designation
Composition, %
ppm/°C

ppm/°F

ppm/°C

ppm/°F

ppm/°C

ppm/°F

°C °F
Incoloy 903 and
Pyromet CTX-1
0.03 C, 0.20 Si, 37.7 Ni, 16.0 Co,
1.75 Ti, 3.0 (Nb + Ta), 1.0 Al,
0.0075 B, bal Fe
7.51 4.17 7.47 4.15 7.45 4.14 440 820
Incoloy 907 and
Pyromet CTX-3
0.06 C max, 0.5 Si, 38.0 Ni, 13.0
Co, 1.5 Ti, 4.8 (Nb + Ta), 0.35 Al
max, 0.012 B max, bal Fe
7.65 4.25 7.50 4.15 7.55 4.20 415 780
Incoloy 909 and
Pyromet CTX-
909
0.06 C max, 0.40 Si, 38.0 Ni, 14.0
Co, 1.6 Ti, 4.9 (Nb + Ta), 0.15 Al
max, 0.012 B max, bal Fe

7.75 4.30 7.55 4.20 7.75 4.30 415 780

Table 16 Typical tensile properties of high-strength, controlled-expansion alloys
Test Temperature

Ultimate
tensile
strength
0.2% yield

strength
Alloy designation
°C °F MPa

ksi MPa

ksi
Elongation,

%
Reduction in area,

%
Room temperature 1480

215

1310

190

15 45 Incoloy 903 and Pyromet CTX-1
540 1000 1310

190

1035

150

15 45
Room temperature 1170

170

825 120

15 25 Incoloy 907 and Pyromet CTX-3
540 1000 1035

150

690 100

15 40
Incoloy 909 and Pyromet CTX-909

Room temperature 1310

190

1070

155

10 20
Engineering Applications
Use of alloys with low coefficients of expansion has been confined mainly to such applications as geodetic tape, bimetal
strip, glass-to-metal seals, and electronic and radio components. Almost all variable condensers are made of Invar. Struts
on jet engines are made of Invar to ensure rigidity with temperature changes. Close control of residuals (such as sulfur,
phosphorus, aluminum, and nitrogen) has resulted in a readily weldable alloy (Invar M63 in Table 17) which has been
extensively used for making tanks of liquid natural gas ships.
Table 17 Tradenames of various low-expansion alloys
Nominal composition,
%
UNS
number
Tradename and producing company
Iron-nickel alloys
36% Ni, bal Fe K93601 Invar (INCO and Imphy, S.A.) Invar M63 (Imphy, S.A.) AL-36 (Allegheny Ludlum) Invar
"36" (Carpenter Steel)
39% Ni, bal Fe . . . Low expansion "39" (Carpenter Steel)
42% Ni, bal Fe K94100 Low expansion "42" (Carpenter Steel) AL-42 (Allegheny Ludlum) N42 (Imphy, S.A.)
46% Ni, bal Fe . . . Platinate (same expansion coefficient as platinum) Glass Sealing "46" (Carpenter Steel)
47-48% Ni, bal Fe . . . N47, N48 (Imphy, S.A.)
49% Ni, bal Fe . . . AL-4750 (Allegheny Ludlum) Low expansion "49" (Carpenter Steel)
52% Ni, bal Fe K14042 Glass Sealing "52" (Carpenter Steel) AL-52 (Allegheny Ludlum) N52 (Imphy, S.A.)
Iron-nickel-cobalt alloys
. . . Super Invar (INCO) 32% Ni, 5% Co, bal Fe
Super Invar 32-5 (Carpenter Steel)

Iron-nickel-chromium alloys
42% Ni, 6% Cr, bal Fe K94760 Sealmet 4 (Allegheny Ludlum) Glass Sealing "42-6" (Carpenter Steel) N426 (Imphy, S.A.)
SNC-K (Toshiba)

More recent applications of the iron-nickel "low" expansion alloys include structural components for optical and laser
measurement systems, and lay-up tooling for graphite/epoxy composite components. Significant quantities of these alloys
find application in substrates and housings for hermetic packaging of semiconductors where ceramic components require
some matching of thermal expansion.
There is increasing use of Invar-type alloys for shadow masks in color television picture tubes. The low thermal
expansion of Invar prevents excessive distortion of this shadow mask as internal temperatures increase during operation
of the picture tube.
Shape Memory Alloys
Darel E. Hodgson, Shape Memory Applications, Inc.; Ming H. Wu, Memry Corporation; and Robert J. Biermann, Harrison Alloys, Inc.

Introduction
THE TERM SHAPE MEMORY ALLOYS (SMA) is applied to that group of metallic materials that demonstrate the
ability to return to some previously defined shape or size when subjected to the appropriate thermal procedure. Generally,
these materials can be plastically deformed at some relatively low temperature, and upon exposure to some higher
temperature will return to their shape prior to the deformation. Materials that exhibit shape memory only upon heating are
referred to as having a one-way shape memory. Some materials also undergo a change in shape upon recooling. These
materials have a two-way shape memory.
Although a relatively wide variety of alloys are known to exhibit the shape memory effect, only those that can recover
substantial amounts of strain or that generate significant force upon changing shape are of commercial interest. To date,
this has been the nickel-titanium alloys and copper-base alloys such as Cu-Zn-Al and Cu-Al-Ni.
A shape memory alloy may be further defined as one that yields a thermoelastic martensite. In this case, the alloy
undergoes a martensitic transformation of a type that allows the alloy to be deformed by a twinning mechanism below the
transformation temperature. The deformation is then reversed when the twinned structure reverts upon heating to the
parent phase.
History
The first recorded observation of the shape memory transformation was by Chang and Read in 1932 (Ref 1). They noted

the reversibility of the transformation in AuCd by metallographic observations and resistivity changes, and in 1951 the
shape memory effect (SME) was observed in a bent bar of AuCd. In 1938, the transformation was seen in brass (copper-
zinc). However, it was not until 1962, when Buehler and co-workers (Ref 2) discovered the effect in equiatomic nickel-
titanium (Ni-Ti), that research into both the metallurgy and potential practical uses began in earnest. Within 10 years, a
number of commercial products were on the market, and understanding of the effect was much advanced. Study of shape
memory alloys has continued at an increasing pace since then, and more products using these materials are coming to the
market each year (Ref 3, 4).
As the shape memory effect became better understood, a number of other alloy systems that exhibited shape memory
were investigated. Table 1 lists a number of these systems (Ref 5) with some details of each system. Of all these systems,
the Ni-Ti alloys and a few of the copper-base alloys have received the most development effort and commercial
exploitation. These will be the focus of the balance of this article.
Table 1 Alloys having a shape memory effect
Transformation-temperature range

Transformation

hysteresis
Alloys Composition
°C °F ∆°C ∆°F
Ag-Cd 44/49 at.% Cd -190 to -50 -310 to -60 ≈15 ≈25
Au-Cd 46.5/50 at.%Cd 30 to 100 85 to 212 ≈15 ≈25
12/14.5 wt% Al

3/4.5 wt% Ni
Cu-Al-Ni
3/4.5 wt% Ni
-140 to 100 -220 to 212 ≈35 ≈65
Cu-Sn ≈15 at.% Sn -120 to 30 -185 to 85
Cu-Zn 38.5/41.5 wt% Zn -180 to -10 -290 to 15 ≈10 ≈20
Cu-Zn-X (X = Si, Sn, Al)

a few wt% of X -180 to 200 -290 to 390 ≈10 ≈20
In-Ti 18/23 at.% Ti 60 to 100 140 to 212 ≈4 ≈7
Ni-Al 36/38 at.% Al -180 to 100 -290 to 212 ≈10 ≈20
Ni-Ti 49/51 at.% Ni -50 to 110 -60 to 230 ≈30 ≈55
Fe-Pt ≈25 at.% Pt ≈-130 ≈-200 ≈4 ≈7