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Metall. Res. Technol.
Volume 117, Number 4, 2020
Article Number 406
Number of page(s) 6
DOI https://doi.org/10.1051/metal/2020038
Published online 07 July 2020

© EDP Sciences, 2020

1 Introduction

Generally, the elastic modulus of metals and alloys decreases with increasing temperature. However, some ferromagnetic alloys based on Fe–Ni–Cr exhibit unusual elastic behavior and poses constant temperature coefficient of the elastic modulus [1]. The deformation of ferromagnetic alloy is divided into two different parts including: elastomechanical and mechanostriction components. By heating the alloy, the elastomechanical component of the elastic modulus decreases as a result of weakening of the forces of interatomic interactions. Also, the mechanostriction component decreases simultaneously due to lowering of the magnetization [2]. Therefore, the elastic modulus remains invariable by heating to the Curie point. Based on this effect, the constant modulus alloys nominated Elinvar has been developed [3].

One of Fe–Ni–Cr–Ti based alloy is Ni-Span-C 902 superalloy. This superalloy is developed to overcome the tendency of metals to lose stiffness and increase in length as they are heated. The most important application of this alloy is in helical and flat springs, resonant vibrating systems, diaphragms, electro-mechanical filters and Bourden tubes [4,5]. Thermoelastic coefficient (TEC) describing the elastic modulus changes with temperature of alloys can be controlled by chemical composition and the stress present in them. Ni-Span-C 902 composition creates an alloy in which the TEC can be controlled by balancing cold working and heat treatment. Cold working produces internal strains making the coefficient more negative while thermal treatments in the lower temperature ranges tend to relieve the strain. Therefore, by controlling cold working and heat treatment optimal TEC can be achieved [6]. This alloy contains precipitation hardening elements such as Ti and Al for improving its elastic properties and increasing room temperature and high temperature strength. Therefore, by controlling the size, distribution and volume fraction of γ׳ (Ni3(Ti,Al)) precipitates it is possible to adjust the thermoelastic coefficient within the certain range by suitable cold working and aging cycles [7]. The ordered γ׳ phase becomes unstable by increasing temperature and time during aging and transforms to a plate-like precipitate called ɛ which has a HCP structure and nominal composition of Ni3Ti [8,9]. It is well known [10] that excessive precipitation of ɛ phase in the matrix makes Fe–Ni superalloy brittle and remarkably deteriorates its mechanical properties.

In a research work done by Khomenko [11] it has been reported that, aging of Fe–Ni–Co alloy changes the Curie temperature, elastic modulus and TEC, also increasing aging temperature is accompanied with high TEC. In another research work done by Sazykina [12], it has been revealed that plastic deformation of Fe–Ni alloy before aging can significantly change elastic modulus and TEC which is related to the changes in matrix texture after cold wok.

Despite various studies [8,9,13,14] conducted on the effect of aging and variation of chromium on hardness and Curie tempearture of Ni-Span-C 902 superalloy, few comprehensive researches have been published on the influence of cold work and subsequent age treatment on mechanical properties and TEC. Therefore, the main purpose of this study was to examine the effect of different amount of cold rolling and aging at different temperatures on the tensile properties and TEC of Ni-Span-C 902 superalloy.

2 Material and methods

In this study, Ni-Span-C 902 superalloy was melted in vacuum induction melting furnace (VIM) and then refined by electro slag remelting (ESR) process. The chemical composition of the alloy used in this study is presented in Table 1. After homogenization and hot rolling, ingot was annealed for one hour at temperatures of 1000 °C then it was quenched immediately. Four samples were taken from as-received strip. Three samples were cold rolled to a reduction in thickness of 30, 40 and 50%. Then the samples, aging in the temperature range of 850–550 °C with 100 °C interval for 5 h. Tensile specimens were prepared according to the ASTM E8 standard with the axis along the rolling direction of the strips. The shape of tensile specimen was flat with the gage length of 25 mm. Also, the tensile tests were carried out at constant strain rate (i.e. ɛ = 0.001). The hot tension tests were performed using Instron 8502 machine equipped with resistance furnace at the temperatures of 30, 40, 60, 80 and 100 °C. After test, the specimens were cut along their longitudinal axes and prepared by the standard metallographic techniques for microstructural observations by optical microscope (OM) and electron microscopy (SEM). All of the samples after sanding and polishing were etched by Glyceregia solution with a combination of 45% nitric acid, 10% Glycerin and 45% hydrochloric acid.

Table 1

Chemical composition of material used in this study (wt.%).

3 Results and discussion

3.1 Microstructure and tensile properties

Figure 1 shows the effect of aging at different temperatures for 5 h on the tensile properties of Ni-Span-C 902 superalloy at ambient temperature. As shown in this figure, variation of yield strength (YS) and ultimate tensile strength (UTS) are entirely similar. By increasing aging temperature, the UTS is gradually increased and at 650°C shows sharp increase. On the other hands, failure strain of the alloy decreases with increasing aging temperature and most of the decline occurs at 650°C.

The results of studies on nickel–iron base superalloy showed [15] that, in the earlier stages of aging, ɣ’-phase formed by spinodal decomposition of saturation austenite and formation of titanium and aluminum-rich areas. According the same lattice parameter and coherent interface, ɣ’-phase precipitates homogeneous in the matrix. So the ɣ’-phases hinder movement of the dislocations on slip plane and the strength of Ni-Span-C 902 alloy increases.

Figure 2 shows variation of the tensile properties of cold-rolled and aged Ni-Span-C 902 alloy versus different temperatures for 5 h. As shown in this figure, the UTS significantly increase due to the cold rolling. Cold rolling with 30 and 50% reduction leads to an increase of 37 and 57% of the UTS. Cold work leads to increasing density of dislocations. It has been reported [16] that dislocations are energetic location in the matrix that results in reduction of the total strain energy of heterogeneous nucleation. Also dislocations with providing rapid diffusion routes for soluble elements, including titanium, increase its diffusion rate. Therefore, in these situations, precipitation of ɣ’-phase increases and strength incremented. Variation in the tensile properties of the cold-rolled specimens is similar to as-rolling ones. The rate of UTS and YS increasing and reduction in ductility for 30% cold rolled alloys in the temperature ranging of 550–750 °C and for 50% cold-rolled alloys in the temperature ranging between 550–650 °C is maximal. For 30% a cold rolled sample, the value of maximum UTS is 1480 MPa which is achieved via aging at 750 °C. On the other hand, for 50% cold rolled sample, the value of maximum UTS is 1504 MPa which is achieved during aging at 650 °C. Optimum tensile properties are obtained with aging 50% cold rolled samples at 650 °C for 5 h. Due to overaging, with increasing aging temperature the UTS is decreased and failure strain increased. It was clearly observed that aging temperature corresponding to maximum UTS decreases from 750 to 650 °C by increasing cold rolling reduction from 30 to 50%, respectively.

Figure 3 shows SEM microstructure of 50 pct cold rolled that were aged at 650 and 750 °C for 5 h. As shown in Figure 3a, fine and spherical particles of ɣ’-phase with 80 ± 20 nm diameter is precipitated. Thus, precipitation of this phase can be the reason of maximum UTS. As can be seen, with increasing aging temperature to 750 °C, size of particles significantly increased. In this case, particles size is about 100 ± 20 nm. Hence one reason of decline of UTS and increment of ductility at 750 °C can be the growth of ɣ’-phase.

In order to analyze how much the precipitation of ɣ’-phase affects the tensile properties variation, the contributions of both cutting and Orowan (bowing) mechanism responsible for precipitation strengthening were estimated in terms of the critical resolved shear stress (CRSS). It has been reported [17] that in the initial stages, as precipitation or aging continues, the precipitate particles increase in size and volume. As the size and amount of particles increase, more work needs to be done by the dislocation in shearing the particles. It turns out [15] that the shear strength (τ) of the alloy depends on the particle radius (r) and the particle volume fraction (f) according to the equation (1): (1)

As shown in Figure 3a, square and fine ɣ’-phase with high volume fraction is precipitated in the matrix. So, strength increases and ductility decreases due to precipitation hardening. Therefore, the maximum UTS with aging at 650 °C, as mentioned before, can be related to the precipitation of fine and spherical particles of strengthen phase and high volume fraction of ɣ’-phase. With increasing aging temperature, both r and f increase. Soon, however, a stage is reached in which the precipitate volume fraction does not increase any more. The precipitate size, however, continues to increase on further aging, because larger particles tend to grow as a result of precipitate coarsening. This growth is called precipitate coarsening. It is reported that [16] in nickel based superalloys, the term Ostwald ripening is also used for this phenomenon. The thermodynamic driving force for precipitate coarsening is the decrease in surface area, and thus in surface energy, of the precipitate with increasing size. In the initial stages of aging, both r and f increase, and the strength of the alloy increases. This, however, does not go on indefinitely, because, as precipitate coarsening occurs, the inter particle distance (x) increases. In fact, x becomes so large, that an alternative deformation process begins viz., dislocation bowing or looping around the particles via the Orowan mechanism. This happens because the shear stress required to bow the dislocation between the particles is less than that required to shear them. As shown in Figure 3b, the UTS of specimens that were aged at 750 °C for 5 h is reduced due to the growth of ɣ’-phase.

The UTS peak value of specimens which were cold rolled by different percentages has been changed due to the competition between the precipitation and growth of ɤ’-phase and formation of the plate-like ɛ phase. The UTS of 30 and 50 pct cold rolled specimens is reduced with increasing temperature over the 750 and 650 °C, respectively. This reduction in UTS can be related to growth of ɤ’-phase and formation of ɛ phase. As shown in Figure 2, increasing the aging temperature over the 850 °C has led to a substantial reduction in UTS. This phenomenon is caused by the overaging. Figure 4 shows optical microstructure of specimens which were cold rolled and aged at 850 °C for 5 h. As can been see in this figure, the plate-like ɛ phase is formed in the grain boundary with aging at 850 °C. Volume fraction of the ɛ phases are increased with increasing degree of cold work from 30 to 50%. It is reported [9,10] that ɛ phases precipitate formed in expense of ɤ’-phases. As a result, formation of ɛ phase decreases the UTS and increases the ductility of the alloy.

thumbnail Fig. 1

Effect of aging temperature on the tensile properties of Ni-Span-C 902 superalloy at ambient temperature.

thumbnail Fig. 2

Effect of aging temperature on the tensile properties of cold rolled Ni-Span-C 902 superalloy: (a) 30 pct cold rolled and; (b) 50 pct cold rolled.

thumbnail Fig. 3

Back scattered electrons image of the 50% cold rolled Ni-Span-C 902 superalloy aged at (a) 650 °C and (b) 750 °C for 5 h.

thumbnail Fig. 4

Optical microstructure of Ni-Span-C 902 aged at 850 °C for 5 h (a) 30% cold rolled and (b) 50% cold rolled

3.2 Elastic modulus (E)

Figure 5 shows the variation of E with cold working and aging at different temperature for 5 h. As shown in Figure 5 due to the precipitation of ɤ’ phases and subsequent changes in the chemical composition of the austenite with aging, E is increased. It has been reported [18] that the effect of aging on E is low, but in ferromagnetic alloys changes in E is greater due to the ΔE effect. However, studies on ferromagnetic alloys [19] showed that magnetoelastic strain created due to the reorientation of domain vectors by the applied stress. So this extra strain can decrease E. As shown in Figure 3, in cold rolled specimens, precipitation of ɤ’ phases occur with aging. These particles hindered the movement of the magnetic moment and decreased magnostriction so, elastic modulus is increased.

It has been also reported [9] that in Ni-Span-C alloy during formation and growth of ɤ’ E rises as the effect of nickel withdrawal overrides any changes due to the associated change in titanium content of the matrix. Formation of ɛ phase at the expense of ɤ’ however, reverses this process owing to its higher iron content. As can be seen E decreases for 30 and 50% cold rolled specimens with aging at 750 and 650 °C respectively. It was shown in Figures 3 and 4, E decreased due to coarsening of ɤ’ phases and formation of ɛ phase with HCP structure.

As you can see, variation of E is similar with changes in the UTS and YS of the alloy (Fig. 2) in the same condition. E value of the specimens, 30 and 50% cold rolled, increase 4 and 6% respectively. Studies on the effect of cold work before aging on E and UTS of similar alloys [16] showed that increasing in E and UTS with cold working and aging can be caused by the accumulation of vacancies around movable dislocation. As shown in Figure 5, the variation of E, with increasing aging temperature, for cold rolled specimens is more than other ones. It has been also reported that [20] Elinvar alloys after cold work have a dual axial texture <111> and <100>. It is known that the E of Fcc metals is the highest in direction <111> and the lowest in direction <100>. The increasing of E observed for Ni-Span-C 902 superalloy is evidently can due to an increase in the effect of orientation <111> in the deformation texture.

thumbnail Fig. 5

Effect of cold work and aging temperature on elastic modulus of Ni-Span-C 902 superalloy.

3.3 Termoelastic coefficient (TEC)

Figure 6 shows the variation of TEC with cold work and aging at different temperature for 5 h. As can be seen, all specimens have a positive temperature coefficient of the E up to Curie temperature, that is to say E increases with increasing aging temperature. Different temperature coefficient of the E in this figure can related to variation of Curie temperature with cold working and aging [5]. It has been reported [11] that total deformation of a ferromagnetic alloys under an external force consists of an elastomechanical and a mechanostriction (caused by the internal magnetic field) components. Deformation of ferromagnetic nature lowers somewhat the E. In Elinvar alloys this effect is quite important and causes an anomalous change in the E. During heating, the 650 °C/30% cold work elastomechanical component of the E decreases due to weakening of the forces of interatomic interaction; the mechanostriction component decreases simultaneously due to lowering of the magnetization. As a result, the E can remain invariable or even grow in heating to the Curie point. Moreover, the model proposed by other researchers [21,22] showed that there are two possible states for face-centered cubic (Fcc) ɤ-Fe: the ferromagnetic high-volume state (ɤ2) and the antiferromagnetic low-volume state (ɤ1). When iron is alloyed with sufficient nickel, palladium, or platinum the order of the levels can be reversed with the ferromagnetic ɤ2 (high volume) level being stabilized. In this cases where the ɤ2 level is stabilized, thermal excitation of the ɤ1 level decreases the atomic volume in opposition to the normal anharmonic source of expansion and depending on the energy difference of the two levels. So, TEC can have positive value or even zero up to Curie temperature. It has been reported [21] that for Ni-Span-C 902 superalloy the Curie temperature is 140 °C. Therefore, achieving positive TEC for Ni-Span-C 902 below the Curie temperature due to the ferromagnetic nature of these alloys is justified. As shown in Figure 6, TEC of specimens that have been 50 pct cold rolled and aged at 550 °C for 5 h are decreased from 3.95 × 10−6/°C to 3.00 × 10−6/°C. This could be caused by changes in the texture deformation of samples. It is reported [16] that in Fe–Ni alloys TEC is affected by plastic deformation. The TEC decreases after plastic deformation. Moreover, It was shown [16] that TEC is anisotropic in Fe–Ni alloys (TEC(111} < (TEC(100}), while the intensification of aging as the result of preliminary plastic deformation leads to an increase of TEC. Study on the Ni-Span-C alloy [14] showed that the ɤ’-precipitates produce magnetic inhomogeneity and hinder the movement of the Bloch walls. As a result, the saturation magnetostriction of the material is reduced. So as mentioned before, precipitation of ɤ’ phases occur with aging. As a result, TEC is increased. As shown in Figure 6, the lowest value of TEC is obtained with aging at 550 °C for 50 pct cold rolled samples. Minimum value of TEC is equal to 3.00 × 10−6/°C. As mentioned in the previous section, this temperature is related to the initial ɤ’ precipitation stage. TEC value increased during aging at 550 °C because of ɤ’ precipitation phenomenon. With increasing volume fraction of ɤ’ phases due to increasing aging temperature, the saturation magnetostriction of the material is reduced. So TEC is increased. But cold work due to changes in the texture of the materials reduces TEC. So the low value of TEC with aging at 550 °C can return to the interaction effect of aging and cold working. With increasing aging temperature to 650 °C due to increasing the volume fraction of ɤ’ phases, TEC value for 30 pct cold rolled specimens increases from 1.63 × 10−6/°C to 2.75 × 10−6/°C and for 50 pct cold rolled samples increase from 2.24 × 10−6/°C to 3.11 × 10−6/°C. Therefore, the value of TEC is increased by increasing the volume fraction of ɤ’ phases.

thumbnail Fig. 6

Effect of cold work and aging at different temperature for 5 h on the TEC of Ni-Span-C 902 superalloy: (a) aging temperature = 550 °C, cold working = 0, 30%; (b) aging temperature = 550 °C, cold working = 40, 50%; (c) aging temperature = 650 °C, cold working = 30, 50%.

4 Conclusion

In Ni-Span-C 902 superalloy, optimum tensile properties obtained with aging 50% cold rolled samples at 650 °C for 5 h with respect to the precipitation of fine ɤ’ particles.

Aging of the alloy in the temperature range of 750–850 °C decreased UTS and E and increased ductility due to the coarsening of ɤ’-phase and formation of plate like ɛ phase as a result of dissolving ɤ’ phases.

Cold rolling leads to reduced overaging temperature which for 50 pct and 30 pct cold rolled specimens, overaging temperature occurs above the 750 and 850 °C respectively.

The E in Ni-Span-C 902 is increased due to the precipitation of ɤ’ phases and subsequent changes in the chemical composition of the austenite with aging. Also cold working increases the E due to an increase in the effect of orientation <111> in the deformation texture.

Results showed that TEC increased with increasing aging temperature and decreased with cold work which lowest value of TEC is obtained with aging at 550 °C for 50% clod rolled specimens.

References

  1. Z. Kaczkowski, Physica B + C 149, 232–239 (1988) [CrossRef] [Google Scholar]
  2. A. Shatsov, Met. Sci. Heat Treat. 50, (2008) [CrossRef] [Google Scholar]
  3. K. Fukamichi, T. Masumoto, M. Kikuchi, IEEE Trans. Magnet. 15, 1404–1409 (1979) [Google Scholar]
  4. S.J. Patel, JOM 58, 18–20 (2006) [Google Scholar]
  5. M.D. De Sihues, C. Durante-Rincón, J. Fermin, J. Magnet. Magnet. Mater. 316, e462–e465 (2007) [Google Scholar]
  6. M. Lozovan, H. Chiriac, Bull. Inst. Polit. IASI 7, 471–476 (1994) [Google Scholar]
  7. G.R. de Souza, S.B. Gabriel, J. Dille, D.S. dos Santos, L.H. de Almeida, Mater. Sci. Eng.: A 564, 102–106 (2013) [Google Scholar]
  8. A. Tavassoli, Scr. Metall. 7, 345–350 (1973) [Google Scholar]
  9. A. Tavassoli, A. Miodownik, Met. Sci. 9, 493–495 (1975) [Google Scholar]
  10. K. Kusabiraki, E. Amada, T. Ooka, S. Saji, ISIJ Int. 37, 80–86 (1997) [Google Scholar]
  11. O. Khomenko, A. Sazykina, G. Tarnovskii, Met. Sci. Heat Treat. 22, 50–53 (1980) [Google Scholar]
  12. A. Sazykina, O. Khomenko, Met. Sci. Heat Treat. 18, 1040–1043 (1976) [Google Scholar]
  13. Y. Chen, Z. Liu, Y. Cao, Z. Zhu, Scr. Metall. 22, 1075–1078 (1988) [Google Scholar]
  14. Z. Liu, T. Al-Kassab, P. Haasen, Scr. Metall. Mater. 24, 655–660 (1990) [Google Scholar]
  15. K. Kusabiraki, E. Amada, T. Ooka, ISIJ Int. 36, 208–214 (1996) [Google Scholar]
  16. T. Otomo, H. Matsumoto, N. Nomura, A. Chiba, Mater. Trans. 51, 434–441 (2010) [Google Scholar]
  17. J. Singh, C. Wayman, Mater. Sci. Eng. 94, 233–242 (1987) [Google Scholar]
  18. J.-H. Oh, I.-C. Choi, Y.-J. Kim, B.-G. Yoo, J.-I. Jang, Mater. Sci. Eng.: A 528, 6121–6127 (2011) [Google Scholar]
  19. D. Sander, Rep. Progr. Phys. 62, 809 (1999) [Google Scholar]
  20. E. Vlasova, N. D’yakonova, V. Matorin, Fizika Metallov i Metallovedenie 77, 122–129 (1994) [Google Scholar]
  21. M. Gallas, J. da Jornada, J. Phys.: Condensed Matter 3, 155 (1991) [Google Scholar]
  22. F. Qin, F. Lu, X. Zhao, Mater. Charact. 148, 81–87 (2019) [Google Scholar]

Cite this article as: Maryam Morakabati, Peyman Ahmadian, Mohammad Rasoul Moazami Goodarzi, Effect of cold rolling and subsequent aging on tensile properties and thermoelastic coefficient of Ni-Span-C 902 superalloy, Metall. Res. Technol. 117, 406 (2020)

All Tables

Table 1

Chemical composition of material used in this study (wt.%).

All Figures

thumbnail Fig. 1

Effect of aging temperature on the tensile properties of Ni-Span-C 902 superalloy at ambient temperature.

In the text
thumbnail Fig. 2

Effect of aging temperature on the tensile properties of cold rolled Ni-Span-C 902 superalloy: (a) 30 pct cold rolled and; (b) 50 pct cold rolled.

In the text
thumbnail Fig. 3

Back scattered electrons image of the 50% cold rolled Ni-Span-C 902 superalloy aged at (a) 650 °C and (b) 750 °C for 5 h.

In the text
thumbnail Fig. 4

Optical microstructure of Ni-Span-C 902 aged at 850 °C for 5 h (a) 30% cold rolled and (b) 50% cold rolled

In the text
thumbnail Fig. 5

Effect of cold work and aging temperature on elastic modulus of Ni-Span-C 902 superalloy.

In the text
thumbnail Fig. 6

Effect of cold work and aging at different temperature for 5 h on the TEC of Ni-Span-C 902 superalloy: (a) aging temperature = 550 °C, cold working = 0, 30%; (b) aging temperature = 550 °C, cold working = 40, 50%; (c) aging temperature = 650 °C, cold working = 30, 50%.

In the text

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