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 . . . [...]... resistance and susceptibility to stress-corrosion cracking, and have much higher ductility On the other hand, the copper-base alloys are much less expensive, can be melted and extruded in air with ease, and have a wider range of potential transformation temperatures The two alloy systems thus have advantages and disadvantages that must be considered in a particular application Nickel-Titanium Alloys The... shape memory alloys becomes difficult to define between the Ms and the As transformation temperatures At these temperatures, the alloys exhibit nonlinear elasticity, and the modulus is both temperature- and strain-dependent Fig 4 Ms temperatures and compositions of Cu-Zn-Al shape memory alloys The melting of Cu-base shape memory alloys in similar to that of aluminum bronzes Most commercial alloys are... 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 Condition Tensile... SME Alloys The only two alloy systems that have achieved any level of commercial exploitation are the Ni-Ti alloys and the copperbase alloys Properties of the two systems are quite different The Ni-Ti alloys have greater shape memory strain (up to 8% versus 4 to 5% for the copper-base alloys) , tend to be much more thermally stable, have excellent corrosion resistance compared to the copper-base alloys' ... annealing Alloys with higher aluminum content are not as easily cold workable Cu-Al-Ni alloys, on the other hand, are quite brittle at low temperatures and can only be hot finished Manganese depresses transformation temperatures of both Cu-Zn-Al and Cu-Al-Ni alloys and shifts the eutectoid to higher aluminum content (Ref 10) It often replaces aluminum for better ductility Because copper-base shape memory alloys. .. but the effort has opened up new classes of alloys for exploration as shape memory alloys These new classes included β-Ti alloys and ironbase alloys References cited in this section 19 Product brochure, Dynalloy Inc., Irvine, CA 20 M Foos, C Frantz, and M Gantois, Shape Memory Effects in Alloys, J Perkins, Ed., Plenum Press, 1975, p 407 21 T Sohmura, R Oshima, and F.E Fujita, Scr Metall., Vol 14, 1980,... 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... oxygen and carbon can also shift the transformation temperature and degrade the mechanical properties, it is also desirable to minimize the amount of these elements The major physical properties of the basic binary Ni-Ti system and some of the mechanical properties of the alloy in the annealed condition are shown in Table 2 Note that this is for the equiatomic alloy with an Af value of about 110 °C... forging, bar rolling, and extrusion can be used for initial breakdown The alloys react slowly with air, so hot working in air is quite successful Most cold-working processes can also be applied to these alloys, but they work harden extremely rapidly, and frequent annealing is required Wire drawing is probably the most widely used of the techniques, and excellent surface properties and sizes as small as... copper-base shape memory alloys are available in ternary Cu-Zn-Al and Cu-Al-Ni alloys, or in their quaternary modifications containing manganese Elements such as boron, cerium, cobalt, iron, titanium, vanadium, and zirconium are also added for grain refinement The major alloy properties are listed in Table 3 The martensite-start (Ms) temperatures and the compositions of Cu-Zn-Al alloys are plotted in . 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. 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. 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