Dynamic recrystallization behavior of a newly developed Ni-Fe based superalloy during hot deformation

The dynamic recrystallization (DRX) behavior of a newly developed Ni-Fe based superalloy was investigated by isothermal compression tests performed at a wide temperature range of 950 to 1200 °C with a strain rate range from 0.01 to 10 s−1 under a true strain of 0.916. The electron backscatter diffraction (EBSD) technique was employed to investigate the effect of strain rate and temperature on the microstructure evolution. The results revealed that the size and volume fraction of DRX grains increased with increasing the temperature. The volume fraction of DRX grains first decreased and then increased with increasing the strain rate during the hot deformation. The preferred orientation of the overall grains changed from <001>//CD to <110>//CD with increasing the strain rate during the deformation at 950 °C, while the preferred orientation of overall grains consistently maintained as <001>//CD during the deformation at 1200 °C. DDRX was the dominant mechanism during the deformation, while CDRX was proved to be an assistant mechanism and was promoted with increasing the strain rate.


Introduction
The 700 °C-class advanced ultra-supercritical (A-USC) power units are currently the main research direction for the development and upgrading of thermal power units both domestically and internationally [1].As the core hot components of the 700 °C-class A-USC unit, large-diameter thickwalled pipes are subjected to significant stress because of its thick walls and large diameters.The service status of the large-diameter thick-walled pipes is closely related to the safe and stable operation of the unit.Besides, the processing and preparation process of the large-diameter thickwalled pipes are a core technology of the 700 °C-class A-USC unit.Therefore, the development process of 700 °C-class A-USC unit is determined by research and development as well as the industrialization of related components of 700 °C-class class large-diameter thick-walled pipes.
In recent years, a large amount of researches have been conducted on the materials used for 700 °C-class A-USC [2].Ni-Co based superalloys, represented by Inconel 740 and its modified alloy Inconel 740H, CCA 617 and Haynes 282, have excellent mechanical properties [3].However, high costs and difficulties in processability limit their application in large-diameter thick-walled pipes.Therefore, many researches have been focused on the low-cost and good processability Ni based or Fe based superalloys, which have been considered as the candidate materials for the 700 °C-class largediameter thick-walled pipes.Recently, a new low-cost and high-performance Ni-Fe based superalloy (named as HT700P alloy) with the superior rupture property and oxidation resistance property has been developed by Xi'an Thermal Power Research Institute Co. Ltd., which has been considered as a promising candidate material with broad application prospects for the 700 °C-class large-diameter thick-walled pipes [4].Over the past years, tensile properties and creep properties at elevated service temperatures of this alloy have been studied in detail [3,4], while there is still a lack of systematic research on the dynamic recrystallization (DRX) behavior of the new superalloy during hot deformation.
In this paper, the microstructure evolution of the HT700P alloy is mainly studied to reveal the DRX mechanism of the alloy, so as to provide a scientific theoretical basis for formulating the hot working process plan of the alloy and promote the commercial application of the alloy, as well as provide theoretical support for the selection of hot working process and microstructure control of the materials used for 700 °C-class large-diameter thick-walled pipes.

Material and Experimental
The nominal chemical composition (wt%) of the HT700P alloy is Ni-42 Fe-17 Cr-3.5 (Al+Ti)-3.0(Mo+Co+W)-0.2Nb-0.1 Mn-0.2 Si-0.05 C-0.003 B. Cylindrical compression specimens with 10 mm in diameter and 15 mm in height were machined from a forged bar.Then, the compression tests were carried out on the thermal simulation machine.The specimens were heated up to 950-1200 ℃ at a rate of 10 °C/s, respectively, and held for 5 min before hot compression.Subsequently, all the specimens were isothermally deformed up to a 0.916 true strain with the constant strain rate range from 0.01-10 s - 1 , followed by water quenching immediately to preserve the deformed microstructure.In addition, in order to study the initial microstructures just prior to hot compression as much as possible, some samples were heated up to 950-1200 ℃ and held for 5 min without compression followed by water quenching immediately.
All deformed specimens were sectioned along the compression axis through the center and the microstructures of the center area were observed.All samples for electron backscatter diffraction (EBSD) were mechanically polished and then electrochemically polished.EBSD maps were obtained with a step size of 0.1-2.0μm depending on the grain size and the acquired EBSD data was analyzed by Channel 5 software.Moreover, the grain boundaries with misorientation angles larger than 2° but less than 10° were defined as the low angle grain boundaries (LAGBs) and the grain boundaries with misorientation angles larger than 10° but less than 15° were defined as the medium angle grain boundaries (MAGBs) as well as the grain boundaries with misorientation angles larger than 15° were defined as the high angle grain boundaries (HAGBs).

Starting microstructures
Starting microstructures of the alloy heated at 950 °C and 1200 °C for 5 min without hot compression are shown in figure 1(a-b) and figure 1(c-d), respectively.Figure 1(a) and (c) present the inverse pole figure (IPF) plus color-coded grain boundary map, while figure 1(b) and (d) present the grain orientation spread (GOS) plus color-coded grain boundary map, respectively.As shown in figure 1(a) and (b), fine equiaxed grains with a size of 56 μm and a lot of annealing twins with a fraction of 43% can be observed in the starting microstructure of the alloy heated at 950 °C.Besides, the GOS values of the grains were almost lower than 2°.When soaked at 1200 °C for 5 min prior to hot compression, coarse equiaxed grains with a size of 234 μm and the fraction of annealing twins up to 63% can be observed in figure 1(c).In addition, the GOS values of all grains were lower than 2°, as shown in figure 1(d).

Microstructural evolution during hot deformation
GOS method has been extensively used to differentiate between recrystallized and deformed grains in polycrystalline materials [5].The GOS values of the recrystallized grains are lower than those of the deformed grains.According to the GOS distributions and their corresponding deformed microstructures, grains with GOS values below 2° were considered as recrystallized grains, while the rest were regarded as deformed grains.
In order to investigate the effect of strain rate on microstructural evolution during hot deformation, the GOS maps and IPF maps of the alloy deformed up to a 0.916 true strain with various strain rates of 0.01 s -1 , 0.1 s -1 , 10 s -1 at 950 °C are shown in figure 2(a-c) and (d-f), respectively.As can be seen in figure 2(a-c), the fraction of DRX grains (grains with GOS values less than 2°) decreased firstly and then increased slightly with increasing the strain rate.Compared with the initial microstructure (figure 1(a-b)), the deformed grains were elongated perpendicular to the compress direction and were surrounded by fine DRX grains, which presents the typical necklace structure and means the occurrence of discontinuous DRX (DDRX) [6], as shown in figure 2(a-f).The evolution of orientation gradient was investigated by carrying out misorientation analysis to understand the substructure development of the deformed grains.The orientation gradients along the grain boundaries (the lines marked as A1-C1, shown in figure 2(d-f)) and across the grain interior (the lines marked as A2-C2, shown in figure 2(d-f)) of the deformed grains were measured and the corresponding results are illustrated in figure 2(g-i) and figure 2(j-l), respectively.As can be seen in figure 2(g-l), the point to origin misorientations (cumulative misorientations) along the grain boundaries or across the grain interior both increased with increasing the strain rate.The cumulative misorientations along the grain boundaries were higher than those across the grain interior when deformed at the strain rate of 0.01 s -1 , while the opposite results can be observed when deformed at the strain rate of 0.1 s -1 and 10 s -1 .It is noteworthy that the cumulative misorientation within the grain at strain rate of 10 s -1 had reached beyond 30° and isolated DRX grains appeared in deformed grains, which means the occurrence of continuous DRX (CDRX) [7].The GOS maps and IPF maps of the alloy deformed up to a 0.916 true strain with various strain rates of 0.01 s -1 , 0.1 s -1 , 10 s -1 at 1200 °C are shown in figure 3(a-c) and (d-f), respectively.As can be seen in figure 3(a-c), the fraction of DRX grains decreased firstly and then increased with the increase of strain rate.Compared with the initial microstructure with coarse grains (figure 1(c-d)), the grains have been significantly refined after hot deformation at the strain rate higher than 0.01 s -1 .With increasing the strain rate, the degree of grain boundary bulging gradually decreased.As the strain rate increased to 10 s -1 , a relatively uniform equiaxed fine grains can be obtained.The orientation gradients along the grain boundaries (the lines marked as D1-F1, shown in figure 3(d-f)) and across the grain interior (the lines marked as D2-F2, shown in figure 3(d-f)) of the deformed grains are shown in figure 3(g-i) and figure 4(j-l), respectively.As can be seen in figure 3(g) and (j), the cumulative misorientations and local misorientations (point to point misorientations) along the grain boundaries or across the grain interior were both lower than 1° when deformed at the strain rate of 0.01 s -1 .
Although the misorientation increased with the increase of strain rate, its value does not exceed 10° at strain rate of 0.1 s -1 and 10 s -1 .Notably, isolated DRX grains in deformed grains can be still observed at strain rate of 0.1 s -1 .Figure 3. Microstructural evolution of the alloy deformed up to a 0.916 true strain with various strain rates at 1200 °C: GOS plus colorcoded grain boundary maps: (a) 0.01 s -1 , (b) 0.1 s -1 , (c) 10 s -1 , IPF plus color-coded grain boundary maps: (d) 0.01 s -1 , (e) 0.1 s -1 , (f) 10 s -1 , Misorientations measured along the lines marked as: (g) D1, (h) E1, (i) F1, (j) D2, (k) E2, (l) F2. respectively.It can be observed that the volume fraction of DRX grains first decreased and then increased with increasing the strain rate at 950-1200 °C.Besides, the higher deformation temperature resulted in the higher DRX volume fraction at the same strain rate.What's more, the grain sizes for DRX grains first decreased and then increased with increasing the strain rate at 950-1200 °C.The grain sizes for deformed grains first decreased and then increased with increasing the strain rate at 950-1050 °C, while the grain sizes for deformed grains decreased continuously with increasing the strain rate at 1200 °C.The variation of grain boundaries with various strain rates deformed at 950 °C and 1200 °C is shown in figure 5. Compared with the initial microstructure, the fraction of the HAGBs and twin boundaries significantly reduced, while the fraction of the LAGBs and MAGBs significantly increased.The grain orientation change of the alloy during hot deformation can be reflected through pole figure (PF) map.PF maps of all grains and DRX grains of the alloy deformed up to a 0.916 true strain with various strain rates at 950 °C and 1200 °C are presented in figure 6.When deformed at 950 °C, the preferred orientation of the overall grains changed from <001>//CD to <110>//CD with increasing the strain rate, while the preferred orientation of the DRX grains maintained a consistently <001>//CD.When deformed at 1200 °C, the preferred orientation of overall grains and DRX grains did not change with the strain rate and was consistently exhibited as <001>//CD.

Effect of strain rate and temperature on the volume fraction of DRX grains
The variation of DRX volume fraction with strain rate and temperature is mainly caused by the combined action of the following aspects [5,7,8].First, increasing the strain rate and decreasing the temperature would result in the increase of the critical dislocation density and thus raising the critical strain for the occurrence of DRX.Besides, adiabatic heating occurred during the hot compression process at the strain rate higher than 1 s -1 , leading to an increase in the actual deformation temperature.The higher strain rate resulted in the higher temperature rise value at the same strain and temperature, while the lower deformation temperature resulted in the higher temperature rise value at the same strain rate and strain.The significant adiabatic temperature rise led to the intensive DRX behavior of the alloy at high strain rates, which leads to an increase of the recrystallization grain size at high strain rates (strain rates higher than 1 s -1 ).What's more, the higher strain rate means the shorter deformation time for grain boundary migration, which can weaken the development of DRX process.To a certain extent, this will hinder the growth of recrystallized grains.At last, the starting grain size before hot deformation also significantly affects the DRX process.The smaller initial grain size, the more nucleation sites for DRX and the better deformation coordination, which is more conducive to the occurrence of DRX.High volume fraction of twins in the initial microstructure of the alloy can promote DRX process by accelerating the DRX kinetics via increasing the mobility of grain boundaries.In addition, the existence of twin boundaries can accelerate the bulging of grain boundaries and further promote the separation of bulging part from original grain.

Effect of strain rate and temperature on the orientation of grains
As the {100} oriented grains have the lowest strain energy in FCC (Face Center Cubic) structured materials [9], the preferred orientation of DRX grains exhibits <001>//RD.Besides, hot deformation causes the deformed grains to exhibit <110>//RD orientation in FCC structured materials, which is mainly related to the gradual rotation of the normal direction of slip surface of grains towards the compression axis during the uniaxial compression deformation of FCC materials [10].Similar phenomena have also been found in other Ni based superalloys [10].Due to the relatively high volume fraction of DRX grains during deformation at 950 °C/0.01 s -1 , few large-sized original deformed grains existed.Therefore, the preferred orientation of the overall grains was shown as <001>//RD for DRX grains.As the strain rate increased, the volume fraction and size of the deformed grains were relatively large, resulting in the preferred orientation of the overall grains exhibiting <110>//RD for deformed grains.When the deformation temperature raised to 1200 °C , the original coarse deformed grains almost disappeared, so the preferred orientation of the overall grains exhibited the preferred orientation of DRX grains constantly.

Effect of strain rate and temperature on the DRX mechanism
DDRX is frequently observed in cubic metals with low or medium stacking fault energy, where nucleation of new dislocation-free grains emerges and these grains subsequent grow at the expense of regions full of dislocations [7,11].The representative characteristics of DDRX are typical necklace structure or bulging grain boundaries.CDRX is typically featured by the formation of sub-grains, progressive subgrain rotation and eventual transformation into HAGBs, which will result in the formation of new isolated DRX grains in the original deformed grains.Besides, the significant increase in MAGBs indicates the progressive sub-grain rotation, which reveals the occurrence of CDRX [7].What's more, the high orientation gradients across the grain interior than that along the grain boundaries can also indicate the occurrence of the CDRX.The domination of DDRX or CDRX mechanism is mainly influenced by the imposed processing parameters.When deformed at 950 °C, the typical necklace structure and bulging grain boundaries at various strain rates demonstrate that DDRX is the dominant mechanism.The fraction of the MAGBs was about 5.3%-6.9%under various strain rates and isolated DRX grains can be observed in deformed grains, which means the occurrence of CDRX.However, with increasing the stain rate, the cumulative misorientations across the grain interior were gradually higher than those along the grain boundaries and the cumulative misorientation within the grain at strain rate of 10 s -1 had reached beyond 30°.This phenomenon demonstrates that CDRX was an assistant mechanism and was promoted with the increase of strain rate.When deformed at 1200 °C, grain boundary bulging and isolated DRX grains can also be observed in deformed grains, which indicates the occurrence of DDRX and CDRX.However, the fraction of the MAGBs was about 3.8%-6.0%under various strain rates, which was relatively low than those deformed at 950 °C.Besides, the extent of grain boundary bulging was weaker than that at 950 °C.Although the misorientation increased with the increase of strain rate, its value does not exceed 10° at all strain rates.These phenomena are all related to the significant acceleration of DRX process due to the increase in deformation temperature, which consumes deformation stored energy.Overall, the DRX mechanism deformed at 1200 °C was similar to that deformed at 950 °C.

Conclusions
1.With increasing the strain rate, the volume fraction of DRX grains first decreased and then increased during the hot deformation at 950-1200 °C.
2.The higher deformation temperature resulted in the higher DRX volume fraction at the same strain rate.
3.The preferred orientation of the overall grains changed from <001>//CD to <110>//CD with increasing the strain rate during the deformation at 950 °C, while the preferred orientation of overall grains was consistently exhibited as <001>//CD during the deformation at 1200 °C.
4.DDRX was the dominant mechanism during the deformation, while CDRX was proved to be an assistant mechanism and was promoted with the increase of strain rate.

Figure 1 .
Figure 1.Starting microstructures of the alloy heated at 950 °C and 1200 °C for 5 min without hot compression: (a) and (c) is the IPF plus color-coded grain boundary map (the inset image is color legend), (b) and (d) is the GOS plus color-coded grain boundary map, (a) and (b) of the alloy heated at 950 °C, (c) and (d) of the alloy heated at 1200 °C.

Figure 2 .
Figure 2. Microstructural evolution of the alloy deformed up to a 0.916 true strain with various strain rates at 950 °C: GOS plus colorcoded grain boundary maps: (a) 0.01 s -1 , (b) 0.1 s -1 , (c) 10 s -1 , IPF plus color-coded grain boundary maps: (d) 0.01 s -1 , (e) 0.1 s -1 , (f) 10 s -1 , Misorientations measured along the lines marked as: (g) A1, (h) B1, (i) C1, (j) A2, (k) B2, (l) C2.The GOS maps and IPF maps of the alloy deformed up to a 0.916 true strain with various strain rates of 0.01 s -1 , 0.1 s -1 , 10 s -1 at 1200 °C are shown in figure3(a-c) and (d-f), respectively.As can be seen in figure3(a-c), the fraction of DRX grains decreased firstly and then increased with the increase of strain rate.Compared with the initial microstructure with coarse grains (figure1(c-d)), the grains have been significantly refined after hot deformation at the strain rate higher than 0.01 s -1 .With increasing the strain rate, the degree of grain boundary bulging gradually decreased.As the strain rate increased to 10 s -1 , a relatively uniform equiaxed fine grains can be obtained.The orientation gradients along the grain boundaries (the lines marked as D1-F1, shown in figure3(d-f)) and across the grain interior (the lines marked as D2-F2, shown in figure3(d-f)) of the deformed grains are shown in figure3(g-i) and figure 4(j-l), respectively.As can be seen in figure3(g) and (j), the cumulative misorientations and local misorientations (point to point misorientations) along the grain boundaries or

Figure 4 .
Figure 4. (a) The variation of DRX volume fraction and (b) the variation of DRX grain size and deformed grain size with various strain rates and temperatures.The variation of DRX volume fraction as well as the variation of DRX grain size and deformed grain size with various strain rates and temperatures are shown in the figure 4(a) and figure 4(b), respectively.It can be observed that the volume fraction of DRX grains first decreased and then increased with increasing the strain rate at 950-1200 °C.Besides, the higher deformation temperature

Figure 5 .
Figure 5.The variation of grain boundaries with various strain rates deformed at (a) 950 °C and (b) 1200 °C (Starting grain boundaries represent the grain boundaries in the starting microstructure).The grain orientation change of the alloy during hot deformation can be reflected through pole figure (PF) map.PF maps of all grains and DRX grains of the alloy deformed up to a 0.916 true strain with various strain rates at 950 °C and 1200 °C are presented in figure6.When deformed at 950 °C, the preferred orientation of the overall grains changed from <001>//CD to <110>//CD with increasing the strain rate, while the preferred orientation of the DRX grains maintained a consistently <001>//CD.When deformed at 1200 °C, the preferred orientation of overall grains and DRX grains did not change with the strain rate and was consistently exhibited as <001>//CD.

Figure 6 .
Figure 6.Pole figure (PF) maps of the alloy deformed up to a 0.916 true strain with various strain rates at 950 °C and 1200 °C.(A1 indicate the compress direction, i.e.CD).