1. Introduction
Oxide dispersion strengthened (ODS) ferritic materials are candidates for long-life core components of fast breeder reactors and elevated temperature heat exchangers due to their creep strength, resistance to prolonged high temperature exposure up to 1300°C,1,2,3) and irradiation resistance.4,5,6) The excellent creep strength at elevated temperatures is given by nano-scale and uniformly dispersed oxide particles in a ferritic matrix, which inhibit dislocation movement and therefore delay recovery and recrystallization.7) INCOLOY MA956 (INCOLOY is a trademark of the Special Metals Corporation group of companies, UNS S67956) is one of the commercialized iron-based mechanically alloyed (MA) ODS materials with Y2O3 dispersoids. There are a number of appropriate aspects to the MA956 alloy, including: (1) a low thermal coefficient of expansion, which provides better thermal fatigue life than nickel and cobalt superalloys, (2) improved oxidation, sulfidation, and carburization resistance by the formation of a stable and adherent alumina and aluminum-chromium oxide spinel on the surface at elevated temperatures,8) and (3) improved creep resistance from a coarse-grained microstructure with a high grain aspect ratio (GAR), or high ratio of grain length to width, oriented in the loading direction.8,9)
Commercial MA956 product (sheet, bar, and tube) microstructures are optimized for high-temperature performance via consolidation of mechanically alloyed powder, hot-working, cold-working, and annealing. During hot-working, the initial fine-grained microstructure undergoes recovery, dynamic recrystallization, and secondary recrystallization. Since controlled processing is the key to performance, microstructural development as a function of thermomechanical processing has been discussed by a number of authors.10,11,12,13,14,15,16,17) Research has focused on texture development and recrystallization behavior during primary recrystallization of the deformed structure (via extrusion, hot isostatic pressing, or hot-rolling) and secondary recrystallization to obtain coarse columnar grains.
While a coarse columnar grain structure is desirable for high-temperature applications, it results in anisotropic properties and is problematic for processing and applications where the loading direction may vary. The coarse, anisotropic grain microstructure reduces ductility and also promotes cleavage fracture along (100) crystal planes at lower temperature.16,18) The anisotropic properties have thus made room temperature forming of MA956 difficult. In order to supply MA956 products with more isotropic properties, a process designed to develop equiaxed microstracture is required. Thus, it is important to systematically assess the effect of thermomechanical processing parameters on the microstructure and texture development in MA956.
The objective of this study is to investigate microstructural development in a hot-rolled MA956 plate with an inhomogeneous starting microstructure to help inform thermomechanical processing strategies intended to produce more isotropic microstructures and optimize final properties in service. The investigation was performed by utilizing various cold-rolling and annealing processes followed by hardness measurements and microstructural characterization to clarify mechanisms of microstructural development as a function of starting grain structure.
3. Results and Discussion
3.1. As-Received Plate Microstructure
LOM images of the as-received plate are shown in Fig. 2. The microstructure varies significantly through-thickness, and can be divided into distinct 7 layers, as described in Table 2. The microstructures are described following the nomenclature in Table 1, and are described in more detail below.
Table 2. Description of layers in the as-hot-rolled plate.
| Layer | Distance from Top | Hardness, Hv Range (Average) | Microstructure | Texture |
|---|
| (mm) | (%) |
|---|
| 1 | 0.0–0.4 | 0–4 | 260–280 (265) | TEG | Random |
| 2 | 0.4–1.8 | 4–18 | 260–280 (270) | CEG | {318} <311> |
| 3 | 1.8–4.8 | 18–49 | 260–280 (264) | CEG, FEG | {318} <311>, {100} <110> |
| 4 | 4.8–6.3 | 49–65 | 260–280 (264) | CEG, FEG | {318} <311>, {100} <110> |
| 5 | 6.3–8.1 | 65–83 | 260–280 (267) | CEG, FEG | {318} <311>, {100} <110> |
| 6 | 8.1–9.3 | 83–96 | (297) | FEG band | {100} <110> |
| 7 | 9.3–9.7 | 96–100 | (260) | CEG | {318} <311> |
Layer 1 ranges from the top surface and includes about 4% of the thickness and consists of thin elongated grains (TEG) that are less than 100 μm in thickness with a random texture. Layer 2 ranges from about 4 to 18% of the depth and consists of thicker (>100 μm), coarse elongated grains (CEG) than Layer 1, and has a preferential texture that will be discussed in more detail later. Layers 3–5 range from about 18 to 83% of the depth and have CEG and regions of fine elongated grains (FEG) illustrated with the overlaid arrows. The thickness of the FEG regions is typically less than 50 μm. The morphology of the FEGs is similar to Layer 1; the FEGs have a plane normal close to the <110> direction parallel to the plate transverse direction. While the microstructures in layers 3–5 are comparable in the as-hot-rolled plate, their annealing responses differ, which will be described later. Layer 6 ranges from about 83 to 96% of the depth, but the visual contrast is different from the other CEG layers. The dominant texture in Layer 6 is the same as the FEGs observed in Layers 3–5, and is described as FEG bands in Table 2. Layer 7 ranges from about 96% of the depth to the other surface and consists of CEG. The CEGs observed in Layers 2, 3, 4, 5, and 7 primarily have a {318} <311> texture.
The hardness profile from the top to bottom surfaces was measured, with only Layer 6 (300 vs 260–280 HV) having a noticeably different hardness. Higher magnification IPF and boundary maps of Layer 6 are shown in Fig. 3. These results show that Layer 6 contains elongated grains less than 5 μm in thickness, with high angle boundaries (HABs, blue lines) along with low angle boundaries (LABs, red lines) and intermediate angle boundaries (IABs, green lines); these grains have a strong {100} <011> texture component. The high hardness in this layer may be attributed to the fine grain size. Similarly oriented fine grain regions are distributed within the {318} <311> matrix in Layers 3–5. The {100} <011> component has been observed in various ferritic alloys and MA956 in the as-deformed condition,10,16,17) but the {318} <311> texture has not been previously reported.
3.2. Annealing Response of the As-Received Hot-Rolled Plate
Since the as-hot-rolled plate has a layered and inhomogeneous microstructure as discussed above, the annealing response of the different layers is also inhomogeneous. After annealing at 1000 and 1200°C, there was little change in hardness as shown in Fig. 4, which plots the hardness profile through the thickness of the annealed plates. Softening occurred at 1380°C.
The higher hardness in Layer 4 of 1000°C and 1200°C annealed plates is due to microstructure variation within the hot-rolled plate rather than the annealing response. After annealing at 1380°C, the hardness decreased to approximately 250 HV and several regions, around 1 mm (Layer 2) and 7 mm (Layer 5) in depth and near the bottom surface (Layer 7), had hardness values as low as 220 HV. After annealing at 1380°C, the relatively high hardness in Layer 6 was reduced to 250 HV.
Figure 5 shows IPF maps of Layer 4 and Layers 6 and 7 in the as-received condition and after annealing at 1000, 1200, and 1380°C. While the high-temperature anneal did not change the grain structure substantially in Layers 1 and 2, the FEG regions in Layers 4, 6, and 7 diminished in favor of CEG. The changes in annealed grain structure of Layers 3 and 5 were comparable to Layer 4. Figure 5 shows that Layer 6 and Layer 7 no longer exhibit a clear difference in microstructure after annealing at 1380°C. Also, the {100}<110> texture (green-colored grains) is greatly reduced in Layers 4–7.
There are two possible processes for the disappearance of the FEG regions: (1) recrystallization, and/or (2) consumption by grain growth of adjacent CEG. The FEG regions may be nucleation sites for recrystallization since the regions have a large amount of HABs and substructure within the grains as shown in Fig. 3. However, the texture in these regions after annealing at 1380°C is the same as that of the neighboring CEG, {318} <311>, and therefore it is suggested here that FEG regions are likely consumed by grain growth.
3.3. Cold-Rolled Microstructures
After cold-rolling, all plates have layered microstructures elongated parallel to the rolling direction, and the microstructure still exhibits 7 layers similar to the as-hot-rolled plate. The hardness values of the 40, 60, and 80% cold-rolled conditions are distributed around 340–360 HV, 350–370 HV, and 360–380 HV, respectively, as shown in Fig. 6. The hardness increases with the amount of reduction in thickness, and the high hardness layer (Layer 6) of the as-hot-rolled plate is no longer apparent. This result suggests possible strain partitioning to the lower hardness layers; i.e. higher strain energy was introduced in the CEG than the FEG regions.
The 40% cold-rolled plate displays deformation bands (DB) in some regions of the plate as shown in the LOM image in Fig. 7(a). The DBs are inclined at approximately 40 degrees with respect to the rolling direction. The DBs in Layer 2 are sharply-defined in the prior CEGs with a {318}<311> texture. In the 60 and 80% cold-rolled conditions, DBs and a significantly flattened grain structure are observed in Layer 2 as shown in Figs. 7(b) and 7(c); these regions are hereafter referred to as heavily deformed regions (HDR).
KAM maps obtained from EBSD are presented to show changes in local misorientation due to cold-rolling. Greater deformation generally produces higher KAM angles,19) and high KAM angles indicate large stored energies from cold-rolling. Figure 8 shows a KAM map in each layer of the 40% cold-rolled condition. In Layer 1, high KAM values exist near grain boundaries, and high values inclined at about 40° may also indicate the formation of DBs, which are not obvious in LOM. In Layer 2, high KAM values, over 10°, are associated with deformation bands that are thicker than in Layer 1. In Layers 3 to 6, the higher KAM values are found within regions that are elongated in the rolling direction, with some local regions of KAM near 20–25°. Some faint deformation bands are also apparent in Layer 5. The regions with higher KAM are denser in parts of Layers 3 and 6 than in Layers 4 and 5.
Figure 9 shows IPF and KAM maps from all seven layers in the 80% cold-rolled condition. The elongated regions in Layers 1–5 and 7 are subdivided into finer regions of various orientations, with the elongated grain structure still present in some regions. The FEG structure and orientation is still largely present in Layer 6, and regions of high KAM are less prominent in this region, possibly due to lower strain relative to other layers resulting from the higher initial hardness. The regions of high KAM values are much more dense in Layers 1–5 and 7 than in the 40% cold-rolled condition.
3.4. Annealing Response of Cold-Rolled Microstructures
As with the as-hot rolled condition, the annealing response of the cold-rolled conditions varies in each layer. Hardness profiles for the annealed 40% cold-rolled plates are shown in Fig. 10.
After annealing at 1000°C and 1200°C, the hardness in all layers decreases significantly, with further decreases in hardness of all layers after annealing at 1380°C. The hardness values are generally greater than the as-hot rolled plate due to the refinement in grain size in some locations and the substructure remaining from cold-rolling. However, the cold-rolled and annealed plates do not have increased hardness in Layer 6 compared to the same region in the as-hot-rolled plate.
Figure 11 shows IQ maps with boundaries superimposed for the 40% cold-rolled condition before and after annealing treatments. As indicated by the amount of IABs and LABs in the micrographs from the annealed conditions (red and green), a large amount of substructure remains even after annealing at 1380°C. Recrystallization (see arrows) is apparent in the regions of the microstructure with very low populations of IABs and LABs. These regions are obvious in Layer 2 where DBs existed in the cold-rolled condition. The recrystallized grains nucleate and grow parallel to the DBs. Recrystallization with growth along RD is present to a limited extent in the other layers at the lower annealing temperatures and in larger volume fractions after annealing at 1380°C.
Figure 12 shows IPF and KAM maps from Layer 2 for the 40% cold-rolled condition before and after annealing treatments. The grains that form in the DBs are more randomly oriented than grains in the as-hot rolled condition and exhibit a reduction in KAM values with annealing compared to the cold-rolled condition. The reduced area of high KAM values suggests reduced local strain energy after annealing, as expected for recovery and recrystallization processes.
Hardness profiles of the annealed 80% cold-rolled and annealed plates are shown in Fig. 13. After annealing, the hardness in all layers decreased substantially, with more variability in hardness apparent after annealing than in the as-hot rolled or 40% cold-rolled conditions. Hardness reduced continuously with increasing annealing temperature, with some regions of relatively high hardness remaining in Layer 5 even after annealing at 1380°C.
Figures 14 and 15 show IQ maps with superimposed boundaries for Layers 1–7 of the 80% cold-rolled and annealed plate.
Recrystallized grains tend to be promoted in HDR in Layer 2 and in FEG regions in the 80% cold-rolled and annealed conditions. And also, the size and volume fraction of recrystallized grains increases with increasing annealing temperature. While Layer 2 consists entirely of recrystallized grains after annealing at 1000°C, the other layers consist of a mixture of recrystallized grains and non-recrystallized regions.
In Layers 3–5, the 80% cold-rolled microstructure consists of CEGs and FEG regions, while some of the FEG regions disappear and several coarser grains elongated parallel to the RD appear after annealing at 1000 and 1200°C. In addition, the IQ and boundary map of Layer 3 after annealing at 1000°C shows that FEG regions change to coarser elongated grains. After annealing at 1380°C, almost all of the FEG regions disappear and Layers 3–4 are exclusively grains elongated parallel to the RD. The grain sizes of the elongated grains in Layer 3 are larger than in Layers 4 and 5 at all annealing temperatures.
Layer 6 consists of a few grains elongated parallel to the RD and the FEGs disappear after annealing at 1200 and 1380°C. The grains observed in Layer 6 after annealing at 1200 and 1380°C are larger than the elongated grains in the other layers.
Layer 7 consists of TEGs containing HDR in the 80% cold-rolled condition. This layer evolves to contain TEG and several grains elongated parallel to RD after annealing at 1380°C.
These microstructures can be correlated to the results of the hardness measurements; hardness values less than 280 HV are found in the recrystallized regions and hardness values over 280 HV appear in the partially recrystallized or non-recrystallized regions.
After annealing at 1200°C, the recrystallized grains in Layers 3 and 6 become elongated parallel to the RD and occupy the whole layer, while the grains remain much finer and less elongated in Layer 2. After annealing at 1380°C, recrystallized grains appear in all layers and largely consume the entire thickness except Layers 1 and 5. LABs and IABs are still retained in both Layers 1 and 5 and correlate to non-recrystallized regions.
Figure 16 shows KAM maps for all layers of the 80% cold-rolled plate before and after annealing. High KAM values are prominent in the regions where HABs and/or IABs are concentrated in the IQ and boundary maps. High KAM values are dense in Layers 2, 3, 4, and 7. The fraction of regions with KAM values over 10° is higher in the 80% condition than in the 40% cold-rolled conditions. KAM values tend to decrease with increasing annealing temperature due to recovery and recrystallization during annealing for 40 or 80% cold-reduction. As shown by the IQ and boundary maps for each cold-rolled condition, the frequency of occurrence of recrystallization increases and the recrystallization temperature decreases with increasing KAM in the cold-rolled state.
After cold-rolling, DBs and/or HDRs possessing large amounts of HABs and high KAM angles appear, and the length, thickness, and KAM values of those deformation structures vary according to the thickness and distribution of the FEG regions. Recrystallization in DBs and HDRs can occur after annealing at 1000°C in all cold rolling conditions, while FEG regions seem to need more thermal energy for recrystallization, since recrystallization in FEG occurs at annealing temperatures over 1200°C in the 40% cold-rolled plates.
Additionally, recrystallized grains can be divided into three morphologies. The recrystallized grains from DBs form grains elongated along DBs, the recrystallized grains from FEG regions result in grains elongated parallel to the RD, and recrystallized grains from HDRs result in fine grains with low GAR. The density of high KAM values is high in Layer 2 which consists of CEGs in the as-hot-rolled condition, while it is lower in Layers 1 and 5 which respectively consist of TEG and CEGs with FEG regions in the as-hot-rolled condition. Correspondingly, non-recrystallized regions appear in Layers 1 and 5 even in the most preferable condition for recrystallization, 80% cold-rolled and annealed at 1380°C.
3.5. Discussion of Recrystallization Behavior
The heterogeneity of the as-hot-rolled and cold-rolled plates enabled observation of different microstructural, recrystallization, and texture development behaviors during annealing. In this section, microstructure evolution before and after recrystallization are discussed. The onset of recrystallization associated with three microstructural regions is discussed: FEG regions, DBs, and HDRs.
The FEG regions were already present in the as-hot-rolled condition. However, it is considered that these regions do not have enough stored energy for recrystallization to occur in the as-hot-rolled condition because the strain energy is low, and the driving force of recrystallization is also low due to the stored energy depending on the orientation of the deformed matrix.20) Instead, the FEG regions are consumed by growth of the surrounding matrix grains in both the as-hot rolled and cold-rolled + annealed at 1380°C condition. Furthermore, the strong {100} <110> texture disappears while the CEG texture {318} <311> remains in the as-hot-rolled + annealed at 1380°C conditions. It is perhaps not surprising that the FEG regions are consumed by grain growth of the surrounding matrix in the as-hot rolled condition since the regions have large amounts of HABs and IABs as shown in Fig. 3. Figure 17(a) shows low magnification IPF and KAM maps of the FEG region in Layer 6 of the as-hot rolled condition. This region in the as-hot rolled plate seems to have a large amount of high KAM angles as shown by KAM mapping at a low magnification, though the interior regions of the fine grains have small KAM values as shown in the higher magnification images in Fig. 17(b). On the other hand, the FEGs in the cold-rolled plate have a larger amount of high KAM values within the grain interior, as depicted in Fig. 17(c). Introducing strain energy by cold-rolling is necessary to activate these regions as recrystallization sites. Once recrystallization occurs, it rapidly develops, consuming adjacent grains and/or sub-grains. Recrystallized grains nucleated in the FEG regions form elongated grains parallel to the rolling direction. After recrystallization, although the morphology of recrystallized grains is similar to the initial condition, the texture of the recrystallized grains is random in contrast to the as-hot-rolled condition.
A second important site for recrystallization is the DB regions, where grains elongated along DBs are observed after annealing even at 1000°C for 1 hr. The most prominent example of DBs appears in Layer 2 of the 40% cold-rolled condition. DBs have also been reported in cold-swaged bar of MA956 and were also nucleation sites for recrystallization after annealing.9) The misorientation between the DBs and the matrix CEG is less than 30°; this misorientation is lower than the misorientation associated with twinning deformation (> 60°).21) Hence, the DBs are likely formed by slip deformation. The length and KAM angles of DBs are dependent on the thickness of the matrix layer (TEG or CEG) and the distribution of FEG regions in the matrix. The longer DBs and higher KAM angles tend to be associated with thicker matrix layers. In addition, FEG regions, which contain a large amount of HABs, interrupt the propagation of deformation bands. Coarser grains, such as those shown in Layer 2, contain long DBs with high KAM angles, while Layer 1 consisting of thin elongated grains and perhaps Layer 5 containing FEG regions, display short DBs and low KAM angles in the 40% cold-rolled condition as shown in Fig. 18. Since high KAM angles, and thus high strains, are associated with the deformation bands, recrystallized grains develop along the DBs with high aspect ratio.
The third important site for recrystallization is HDRs, shown as multi-colored gradations in IPF maps that appear after 60 and 80% cold-rolling. Since HDRs contain high amounts of strain energy (shown by high KAM values), nucleation of recrystallized grains is promoted, and the growth of recrystallized grains is inhibited by the surrounding grains with less stored strain energy. Therefore, the size and shape of recrystallized grains tends to be finer and more equiaxed (lower in GAR) compared with the grains recrystallized on DBs in the annealed 40% cold-rolled plate.
Based on these three types of observed recrystallization behavior, the evolution of recrystallized grains depends upon the initial microstructure, the deformation volume, and annealing temperature. As mentioned above, the inhomogeneity of the as-hot-rolled plate influenced all the aspects of microstructural development: generation of DBs and HDR, texture, recovery, generation of nuclei for recrystallization, and growth of recrystallized grains. Microstructural development of MA956 during cold-rolling and annealing processes is often considered to result in the formation of coarse elongated grains with preferential texture due to secondary recrystallization. However, the microstructural development of the MA956 plate in the current study is more complicated due to the initial inhomogeneous and layered microstructure in the as-hot-rolled condition. The misorientation introduced by cold-rolling varies with the thickness of each matrix layer and the distribution of FEG regions. The occurrence of recrystallization during annealing is dependent on whether a local region has high enough stored energy for grain nucleation. Layers which do not have sufficiently high stored energy indicated by low KAM values in EBSD images are unable to generate nuclei and only recover. Specific examples of non-recrystallized regions are distinguished in Layers 1 and 5 of the 80% cold-rolled condition. Layer 1 consists of TEGs, and KAM angles in this layer are relatively low even after 80% cold-rolling. The amount of accumulated stored energy in each TEG is apparently less than in the CEG regions. Layer 5 consists of CEGs and FEG regions and is adjacent to layer 6, which is entirely composed of FEGs. In Layer 5, several recrystallized grains are observed, though most of the region is non-recrystallized. It is assumed that plastic deformation by cold-rolling in this layer is restrained by Layer 6. As a result, non-recrystallized regions remained in Layer 5 even after annealing at 1380°C.
