Microstructure Evolution of a Nickel-Base Alloy Resistant to High Temperature during Aging

In the present study, the microstructural evolution during aging at 1023, 1073, 1123 and 1173 K of a 35Cr-45Ni heat resistant alloy, produced in the form of centrifugally cast tubes, was characterized by means of light optical microscopy, scanning electron microscopy (SEM) with secondary and backscattered electron imaging, energy-dispersive X-ray spectroscopy (EDS) and Vickers hardness tests. The evolution of the Vickers hardness at 1023 K for aged samples shows that the microstructure is stable during the analyzed aging period. At 1073 K, the rate of increase in hardness is lower than 1023 K and this behavior would be associated with morphological changes observed in primary interdendritic carbides and secondary carbides in the matrix. At 1123 K and 1173 K, an atypical behavior in Vickers hardness curve is presented; where it can be seen that at certain aging times, the hardness decreases significantly. A microstructural analysis of these samples indicates that they have a region free of precipitates (near interdendritic edges) where the hardness is lower. Probably, these regions are areas poor in chromium.


Experimental Procedure
The alloy was produced as centrifugally cast pipes by a conventional procedure, with a 110 mm diameter and a 11 mm wall thickness. Specimens of cast material were obtained from a ring extracted of the tube and cut transversely with a 12 mm width.
The chemical composition of the Ni-Cr-Fe alloy studied is indicated in Table 1.
The aging temperatures were 1023, 1073, 1123 and 1173 K and aging times of up to 4000, 2000, 3000 and 1000 h, respectively. Aging heat treatment was made using resistive furnaces in air atmosphere, and then each sample was cooled in air. The samples were ground with silicon carbide papers from 80 to 2000 grit. Polishing was done with 1 μm alumina paste. Polished specimens were electrolytically etched with a 10% aqueous solution of KOH. Metallographic observations were made with a Leica light microscopy equipped with a DFC 295CCD camera. The identification of primary and secondary particles was done using scanning electron microscopy (SEM) in secondary electrons mode (SE) and back-scattered electron imaging mode (BSE) and electron probe micro-analysis with energy-dispersive X-ray spectroscopy (EDS). It was determined Vickers hardness (macro) and Vickers microhardness using an OSHMA hardness tester with a load of 1kgf and a Future Tech FM300 series microhardness tester using a load of 10 g, respectively. The Vickers hardness determined for a given temperature and time, corresponds to a single sample. Figure 1 shows low magnification micrographs of centrifugally cast microstructures of the studied alloy, where the austenite matrix with intergranular eutectic like primary carbides can be seen.  Using SEM (Q4) in the SE and BSE, it is easy to observe differences between chromium-rich carbides (M 23 C 6 and M 7 C 3 ) and niobium-rich carbides (MC). Indeed, on back-scattered electron images, the bright niobiumcontaining phases can be distinguished easily from the dark grey chromium-rich phases. Nevertheless, this technique cannot differentiate M 23 C 6 from M 7 C 3 , because they have similar electronic densities (1.96 electrons/Å 3 for Cr 23 C 6 and 1.92 electrons/Å 3 for Cr 7 C 3 ) and therefore similar chemical contrasts in BSE. Figure 2 shows SEM observation in the SE and BSE mode of the microstructure as-cast.

Microstructural Characterization of As-Cast Material
X-ray analyses of these carbides were also obtained (EDS) (Figure 3). X-ray diffraction analyses were carried out on a sample in their as-cast condition (Figure 4). The microstructure of the as-cast alloy is composed by austenitic matrix and a network of primary carbides of two types: MC carbides (M = Nb, Ti) and M7C3 carbides (M = Cr, Ni, Fe). In addition, it is observed that both types of carbides present different morphologies. The M 7 C 3 types show morphological characteristics of a eutectic composite; whilst the MC types have a more regular morphology. Several authors [6]- [10], have noted that these carbides cannot be distinguished by optical microscopy, it is necessary to resort to using a backscattered electrons SEM micrograph.

Effect of Aging Treatment on the Microstructure at T = 1023 K
In Figure 5, the respective microstructures are presented corresponding to aging treatments performed at T = 1023 K and different aging times.
Regarding the as-cast microstructure, the aged samples show a microstructure in which the following can be observed: a) initiation and growth of secondary precipitation in the matrix (increase in area fraction) as a function of the aging time, b) secondary precipitation initiated in the interdendritic regions near edges and grain boundaries and ingrowth of dendrites, iii) morphology in the secondary precipitates of needle type with a welldefined orientation.
A detailed analysis of the secondary particles by X-ray scattering indicated that these particles are rich in Cr. Figure 6 shows an X-ray spectrum obtained for these particles.
Some authors [6], studied the aging 1023 K in an alloy very similar to ours and found that the precipitated secondary particles were needle-shaped carbides of M 23 C 6 (M = Cr) type and distributed homogeneously with orientation on (110) planes. Similarly, [11] determined that during ageing 1023 K conducted in high-temperature-resistant alloy of the 35Cr-45Ni type, carbides M 23 C 6 type (M = Cr) precipitated in the matrix and in a similar arrangement to that observed in our case. Thus, we assume that the carbides in the matrix found in our case were M 23 C 6 type carbides (with M = Cr, Ni, Fe). On the other hand, in certain austenitic stainless steels, the carbides of the type M 23 C 6 are found to lie in strings along <110> matrix directions. Studies of the early stages of ageing have shown that discrete particles are nucleated on the existing dislocations, and they grow partially coherently with the matrix. In order to accommodate the strains resulting from the difference in atomic volume between the two lattices, prismatic loops of dislocation are punched out by the growing particle into the matrix.   These punched loops (which have <110> glide axes) then act as sites for further precipitation of carbide, resulting in the formation of the stringers observed after long ageing times [12].

Evolution of the Hardness during the Aging at 1023 K
In Figure 7, the evolution of the Vickers macrohardness is presented as a function of aging time for T = 1023 K. It is noted that Vickers macrohardness increases with aging time from its as-cast condition (Hv = 173) to a value of about 250 and then, remains, approximately, constant. Clearly, growth hardness is associated with the increase of the volume fraction of secondary precipitation. Subsequently, hardness values determined for time near 4000 h aging processes could indicate that the Vickers hardness is maintained constant.

Effect of Aging Treatment on the Microstructure at T = 1073 K
In Figure 8, the corresponding microstructures are presented at different aging times for samples heat treated at T = 1073K.
In this case, a similar microstructure to the previous case can be observed, with a small difference that is characteristic in some samples. It was found that some samples show a zone free of precipitates near the grain boundary; as indicated in the previous figure. Also, regions may be observed within the dendrites where the volume fraction of secondary precipitates is very low. It is likely that this phenomenon is associated with the low carbon concentration in that region. Figure 9 shows the evolution of the Vickers macrohardness as a function of aging time for samples heat treated at T = 1073 K.

Evolution of the Hardness during the Aging at 1073 K
It is noted that the value of the plateau is reached quickly; indicating that at this temperature the precipitation of secondary carbides is faster than for the previous temperature. Subsequently, it is observed that the hardness increases and decreases again. To understand this behavior, samples that have maximum and minimum hardness were compared in their microstructures; for example, samples aged during 480, 687, 1167 and 2007 h [points (a), (b), (c) y (d) in Figure 9]. It was found that microstructures having a minimum in hardness (points (a) and (c)) shown a region free of precipitates near the grain boundaries and interdendritic edges; while in those samples having higher hardness values, this region is almost negligible (points (b) and (d)). Vickers microhardness was determined in these regions free of precipitates (see Figure 10). A value of Hv = 184 ± 6 was obtained by using a load of 10 g. It is indicating that the region free of precipitates is softer than the rest. Probably, we are in the  presence of the phenomenon of sensitization. In these cases, chromium confers to steels, special alloys in general and to nickel alloys in particular a good resistance to intergranular attack and to intergranular stress corrosion cracking. The high chromium concentration destabilizes the solution by generating complex chromium carbides of type Cr 3 C 2 , Cr 7 C 3 , Cr 23 C 6 and intermetallic phases which are rich in chromium. These compounds precipitate more or less quickly between 673 and 1173 K according to the content of carbon, chromium and molybdenum of ternary alloys Ni-Cr-Fe [13]- [18]. They often respect a sequence of heterogeneous nucleation, beginning with intergranular precipitation, then precipitation on incoherent twin boundaries and on the dislocations. Their growth, which is controlled by an intragranular thermally activated diffusional process, depends on the chromium content in the adjacent precipitates and on the matrix carbon content [19]- [26]. The consequence of the intergranular precipitation of these phases in general, and of the carbides in particular, is to create chromiumdepleted zones near the grain boundaries. These depleted zones get wider as the temperature and/or the aging duration is higher and reduces the local resistance to intergranular corrosion [27]. Sourmail [28], mention that M 23 C 6 precipitation is generally related with intergranular corrosion, since these carbides causes a local depletion in chromium near the grain boundaries. This process is known as sensitization.

Effect of Aging Treatment on the Microstructure at T = 1123 K
In this case, one can see how the morphology of the primary carbides is modified in function of the aging time and also how these begin to differentiate in their tonality through microstructures obtained by optical microscopy. In Figure 11, these differences can be observed.

Evolution of the Hardness during the Aging at 1123 K
In Figure 12, Vickers macrohardness determinations for samples aged at 1123K and different times are presented. As it can be seen, Vickers hardness gradually increases to a maximum value and then remains constant until a 3000 h aging. This behavior in the evolution of the hardness is somewhat different to that for the previous two temperatures of 1023 and 1073 K. First, in this case we find that the maximum value is reached after a rather longer time than the two previous cases. Moreover, the Vickers hardness values are lower (maximum value Hv = about 230). This behavior can be explained by the change in the morphology observed at T = 1123 K in the primary carbides, which shows that the eutectic type structure is disappearing and thus contributing to a softening hardness. Thus, the Vickers hardness values are lower at higher aging temperatures. Another additional contribution to softening of the microstructure is the size of the secondary precipitates and separation. In this regard, these no longer represent impenetrable barriers to dislocations and then the latter can cut the precipitates and move on sliding planes.

Effect of Aging Treatment on the Microstructure at T = 1173 K
In this case, there are no significant differences in the evolution observed in the microstructure at first, compared to the previous cases; however, for times of aging processes above about 500 h heterogeneities were observed in the microstructure, where the density of precipitates is too low and the free zones near grain boundaries precipitates is greater than the aged samples by time less than 500 h. In Figure 13, such regions are indicated.

Evolution of the Hardness during the Aging at 1173 K
In this case, Vickers macrohardness gradually increases from its value as cast to a maximum value of about 245 Hv; and then it decreases slightly above the 500 h of aging, the hardness drops sharply to values of about 195 Hv and increases again in the last sample aged for up to 1000 h (see Figure 14). Note that, as temperature increase, aging times decrease because nucleation of precipitates take place in shorter times and the kinetic evolution of precipitation are faster. This somewhat strange behavior, led us to repeat the heat treatment in these samples. Finally, this trend could be confirmed (that is, the sharp decrease in hardness). To understand this behavior effect, the samples microstructures were analyzed and compared, among those, the hardest and those which showed a sharp drop. It was found that the low hardness having these two samples is associated with the heterogeneous microstructure. Such heterogeneity comprises significant fractions of precipitated free area, mainly near the grain boundaries and regions of low density of precipitates inside dendrites regions. Vickers microhardness measurements were per-  formed with a load of 10 g in these regions and an average value of Hv = 184 ± 6 was obtained; this indicates that these areas are soft in relation to the rest of the microstructure regions. Therefore, we can say that the fluctuations that occur in the hardness curves for temperatures of 1123 and 1173 K can be explained from the heterogeneities observed in the microstructures of samples that had lower hardness.

Conclusions
The conclusions of this study are summarized as follows:  The microstructure as cast is dendritic type and is constituted by an austenitic matrix with carbides of two types; a) chromium carbides M 23 C 6 type (eutectic) (M = Cr, Ni, Fe) and niobium carbides of MC type (M = Nb); which are located in interdendritic edges and grain boundaries.  In the aged samples at 1023K, secondary precipitation of fine carbides of M 23 C 6 type, Vickers hardness increases to a saturation value and then remains approximately constant up to 3000 h of aging. In line with this, the microstructure does not change after reaching the saturation time or maximum hardness. At 1073 K, the hardness gradually increases at a slower rate than 1023 K. The microstructure in the aged samples shows changes in the morphology of the eutectic interdendritic primary precipitates and in secondary precipitates in the matrix. This could justify the behavior in the evolution of hardness.  In the case of aged samples at 1123 and 1173 K, it is observed that the hardness shows fluctuations with different aging times. It is found that they are associated with heterogeneity observed in the microstructure due to the presence of free regions near the grain boundaries and interdendritic edges. The microhardness values determined in these areas were lower than those found by macrohardness determinations; which it would indicate that these regions had lower mechanical strength.