Subramanya Sarma Vadlamani

In the present work, the hot deformation behaviour of as-cast medium Mn steels with nominal Mn content of 6, 8, and 10 wt% were studied using the thermomechanical simulator in the temperature and strain rate range of 1173 to 1373 K and 10−3 - 10 s−1 respectively. The flow curves of the three steels revealed significant work hardening followed by a plateau at strain rates ≥1s−1 in the entire temperature range. At lower strain rates (< 10−2 s−1) noticeable softening is observed after a strain of 0.2 in all three steels. The extent of softening is lower in 10 wt% Mn steel in comparison to the 6 and 8 wt% Mn steels. Analysis of processing maps reveals that in steels with 6 and 8 wt% Mn, regions of ‘high’ strain rate sensitivity (m) (0.2 - 0.25) are found to occur in similar temperature and strain rate regimes of 1273–1323 K and 10−2 s−1 respectively. These steels also showed similar apparent activation energies for hot deformation (380–390 kJmol−1), stress exponents (4.6) and dynamic recovery/recrystallization evolution with strain. The steel with 10 wt% Mn, has higher apparent activation energy for hot deformation (450 kJmol−1), higher stress exponent of 5.1, higher dynamic recovery and lower dynamic recrystallization. The observed activation energies correlate well with the activation energy for self-diffusion (QSD) of Fe in austenite containing Mn as an alloying element. In the ‘low’ m regimes, higher dynamic recovery (evaluated using kinetic analysis) was observed in the 10 % Mn steel as compared to the 6 and 8% Mn steels. In the ‘high’ m regimes, the dynamic recrystallization fraction (estimated from kinetic analysis and from parent grain reconstruction analysis) was lower in the 10 % Mn steel when compared to the 6 and 8 wt % Mn steels. This behaviour can be attributed to the higher dynamic recovery observed in the 10 % Mn steel. From the analysis of processing maps, kinetic analysis and microstructure, the optimum strain, strain rate and temperature for the thermomechanical processing of as-cast medium Mn steels with Mn in the range of 6 to 10 wt% are ≤ 0.6, ≤ 10−2 s−1 and 1273 to 1373 K respectively.

In this work, we report the results of a systematic study on the evolution of the precipitates during aging in a Ni-based superalloy (Alloy 617) and their effect on the high-temperature hot corrosion (HTHC). It was observed that the extent of Cr-rich precipitation increased with the aging time while the other microstructural parameters such as grain size, retained strain, and Σ3n (n ≤ 3) boundaries fraction did not change in comparison to the as-received (AR) condition. To evaluate the role of precipitation on hot corrosion, the AR and the aged specimens were subjected to the HTHC test involving complete immersion of the specimens in a 75 wt% Na2SO4 + 20 wt% NaCl + 5 wt% V2O5 salt mixture at 1273 K for 24 h. The characterization of surface and cross-section morphologies of the corroded layers through scanning electron microscopy revealed that the oxide film formed on the AR specimen is highly porous and loose. In contrast, the layer formed on the specimen aged for 100 h (AR-100 h) is significantly thinner and dense. Post-HTHC analysis aided with the thermodynamic and diffusion calculations revealed that the improved HTHC performance of the AR-100 h specimen is due to the rapid formation of an oxide layer owing to the enhanced supply of Cr as a consequence of carbide dissolution. Also, the undissolved carbides at the grain boundaries acted as a potential obstruction to the inward transport of corrosive species, resulting in a significantly lower percolation depth in the AR-100 h specimen (~66 ± 6 µm) than the AR condition (~305 ± 15 µm).

There is considerable research interest in developing medium Mn steels as part of the 3rd generation of advanced high strength steels, mainly due to the possibility to achieve high tensile strength-high ductility combination at an affordable cost. In the present work, we have designed a steel chemistry and its thermomechanical processing route based on a computational approach based on CALPHAD with an objective to achieve tensile strength and uniform elongation in excess of 1000 MPa and 20%, respectively. The influence of alloying elements on factors such as weldability, coatability and formability are also taken into account while designing the steel chemistries. The alloy chemistry was optimised to achieve at least 50% of retained austenite and a stacking fault energy in the range 12–20 mJ m−2 to activate transformation induced plasticity (TRIP) and twinning induced plasticity (TWIP) effects. The designed steel was cast and thermomechanically processed to produce ultrafine-grained ferrite and austenite microstructure with ~50% of retained austenite. Analysis of local composition by atom probe tomography revealed preferential partitioning of Mn and C to austenite during intercritical annealing, thereby enhancing its stability. The optimised microstructure resulted in tensile strength and uniform elongation in excess of 1300 MPa and 26%, respectively. The stress-strain curves revealed serrations and a staircase type of strain hardening. A detailed study of the strain hardening behaviour showed that this can be attributed to the occurrence of discontinuous TRIP effect and deformation twinning in the austenite. This was further corroborated by transmission electron microscopy of the deformed samples which showed the presence of nano-twins in the austenite phase while the XRD and EBSD of the deformed samples showed a significant drop in the austenite fraction post deformation.

The classical JMAK model of recrystallization kinetics has been widely used to describe the growth of randomly distributed nuclei under constant driving force. These conditions are not satisfied in many systems. The driving force for growth generally varies with time, and the nuclei are usually not uniformly distributed. In this paper, we present a physically motivated alternative model of recrystallization kinetics, which accounts for the variation of driving force due to recovery and the effect of clustering of the nuclei. The model is based on the growth kinetics of initially circular, strain free nuclei in a deformed matrix. The effect of recovery on the recrystallization kinetics is studied in terms of the parameters governing initial stored energy and the decay rate. The model predicts cessation of recrystallization when the stored energy decreases below a certain critical limit. This cessation depends on both the initial stored energy value as well as the recovery time constant. The effect of clustering of nuclei on recrystallization kinetics is analyzed by considering representative volume elements with different nuclei distributions. It is shown that recrystallization kinetics becomes slower with increased clustering. The results of this mean field model are compared to phase field simulations. Graphical abstract: [Figure not available: see fulltext.].

In this work, a novel idea of combining two established microstructural engineering approaches viz., grain boundary engineering (GBE) and grain boundary serration (GBS) through optimization of thermomechanical and thermal processing in Alloy 617 is investigated and the superior resistance of GBE+GBS microstructure to the high-temperature hot corrosion is demonstrated. To achieve the GBE+GBS microstructure, the GBS treatment was introduced as a part of the GBE processing schedule (i.e., incomplete GBE+GBS) and following the GBE treatment (i.e., complete GBE+GBS). The extent of GBS is found to be similar in all the processed specimens. However, the extent of GBE is observed to be lower in the specimens undergoing incomplete GBE+GBS. This is due to the occurrence of recrystallization and consequent infrequent multiple twinning. On the other hand, a higher extent of GBE is achieved in the specimens subjected to complete GBE+GBS owing to the retention of the optimized GBE microstructure following the GBS treatment. The synergistic influence of GBE and GBS on the hot corrosion behavior is assessed by exposing the as-received (AR) as well as optimized grain boundary engineered and serrated (GBES) specimens to a salt mixture at 1273 K. The percolation depth after 24h and 48h exposure is significantly lower in the GBES specimen (∼55 µm and ∼115 µm, respectively) than the AR condition (∼305 µm and ∼630 µm, respectively). This is ascribed to the incorporation of Σ3n (n ≤3) and serrated boundaries in the GBES microstructure which obstructed the infiltration of harmful species into the alloy.

In this work, we studied the effect of initial grain size on the hot compression characteristics of super-304H austenitic stainless steel in a range of strain rates (0.001–1 s−1) at a fixed temperature of 1223 K. Analysis of the flow curves reveals that the flow stress is inversely proportional to the average sub-grain diameter in both coarse and fine-grained specimens at higher strain (≥0.5) levels. Further, the fine-grained specimen following deformation at a low strain rate (0.001 s−1) reveals the occurrence of both continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) mechanisms. In this condition, the CDRX is characterized by a progressive increase of boundary misorientation, while the DDRX is characterized by bulging and strain-induced boundary migration. In contrast, the CDRX characterized by the formation of microbands is majorly responsible for the grain refinement in the fine-grained specimen at higher strain rates (∼1 s−1) and in the coarse-grained specimen at all strain rates (0.001–1 s−1). The superposition of different DRX mechanisms leads to significant variations in grain refinement kinetics with strain rates and initial grain sizes. The findings of this investigation provide a foundation for the accurate control of the microstructures in the studied alloy with different initial grain sizes during the hot working at various strain rates.

Dynamic microstructural evolution and recrystallization mechanism during hot deformation of intermetallic-hardened duplex Fe-9Al-10.8Mn-4.5Ni-0.7C (wt.%) lightweight steel have been comprehensively examined at various deformation temperatures at a fixed strain rate of 0.001 s−1. The flow curves are predicted employing Avrami exponent obtained from the strain dependent Johnson-Mehl-Avrami-Kolmogorov relation, which is further corroborated with the dynamic microstructural response. A detailed analysis of the intermetallic precipitates and elemental partitioning in both the ferrite and austenite phases are performed. The ferrite matrix having uniformly distributed nano-sized B2 (NiAl) precipitates has a higher micro-hardness as compared to the austenite matrix, which corroborates the strain partitioning in the austenite phase during hot deformation. Two distinct restoration mechanisms are observed in this alloy viz. continuous dynamic recrystallization (CDRX) and discontinuous dynamic recrystallization (DDRX) following hot deformation. The CDRX mechanism in the ferrite and austenite phase is characterised by the progressive misorientation development of subgrains into high-angle boundary during straining. The ferrite phase is associated with CDRX mechanism at all the deformation temperatures (1223–1423 K) albeit DDRX-like mechanism, facilitated by austenite/ferrite interphase is found to be an assisting mechanism towards the higher temperatures (1323–1423 K). The austenite phase, on the other hand, exhibits DDRX mechanism during the initial stage and dominant CDRX at the later stage of deformation at lower temperature (1223–1323 K). With the increasing deformation temperature to 1423 K, the dissolution of boundary-B2 precipitates in austenite facilitates the boundary migration, thus promoting the DDRX accompanied by the twinning in this phase.

In the present study, strain-annealing based thermo-mechanical processing was employed to achieve a grain boundary engineered (GBE) microstructure in an equiatomic CoCrFeMnNi high entropy alloy (Cantor alloy) with a single phase, fcc structure. Cast, homogenized and recrystallized strips were cold rolled to 5%, 10% and 15% thickness reductions and annealed at temperatures from 1173 K to 1373 K for 1–6 h duration. Deformation twins were observed following cold rolling to 15%. From the deformed and annealed specimens, GBE microstructure was identified based on coincident site lattice (CSL) (Σ ≤ 29) boundary length fraction, number of twins per grain, triple junctions (TJs) character distribution and grain boundary plane orientations. Specimens rolled to 5% and annealed at 1223 K for 1 h exhibited GBE microstructure. The Σ3 fraction was enhanced from ~44% to ~62% in the GBE specimen with concurrent increments in TJs containing at least two CSL boundaries from ~20% to ~40% compared to as-recrystallized (AR) specimen. Potentiodynamic polarization studies revealed that the GBE specimen exhibited lower corrosion rate in both 0.1 M and 0.6 M NaCl solutions as compared to AR counterpart. The GBE specimen also displayed better passive film resistance due to higher polarization and charge transfer resistance, as evaluated from electrochemical impedance spectroscopy studies.

Grain boundary engineering involves manipulation of grain boundary character and network to improve the properties of materials. Twin boundaries which form during annealing of low and medium stacking fault energy face centered cubic materials play an important role in grain boundary engineering. In this work, we study the formation, growth, interactions and annihilation of annealing twins using a phase field model. The model incorporates twin formation through growth accidents on grain boundaries and triple junctions. The results indicate that higher stored energy gradients in the microstructure enhance the twin formation and their rate of growth. Also, recovery with a slower rate of stored energy decay increases twin formation events and delays the annihilation of the twins. The influence of the variant selection (based on the axes of misorientation of the twin boundaries with the parent grains) on the nature of their interactions with other twins and coincident site lattice boundaries is examined. The variant selection is found to affect the interactions of twins with other coincident site lattice boundaries thereby influencing the grain boundary character distribution.

Uniaxial hot deformation was carried out in a wide range of temperatures (1173 K to 1423 K) and strain rates (0.001 to 10 s−1) in a super austenitic stainless steel employing a Gleeble thermomechanical simulator. The evolution of fine grains due to dynamic recrystallization (DRX) with near-random texture was evident following deformation at strain rates 0.001 to 10 s−1 and temperatures 1173 K to 1273 K. The deformed grains in this domain exhibited the typical formation of 〈110〉//ND or α fiber, and its volume fraction gradually increased when the resistance to deformation increases. Weak deformation texture was persisted in the fine DRX grains (≤ 3 µm) which gradually converted to a strong 〈001〉//ND fiber following grain growth. The deformation texture was simulated by employing a crystal plasticity finite element method (CPFEM). The texture inhomogeneity was observed in the typical compressed specimen which was associated with variation in stress/strain paths at different locations of the hot compressed specimen. Further, the ∑3 twin boundary evolution was analyzed employing Pande’s relationship and twin-related domains (TRDs) analysis. The growth accident was identified as the major mechanism of ∑3 twin boundary evolution during DRX in this alloy. Moreover, the ∑3 regeneration was also evident in a small domain of hot working (1323 K/0.001 s−1) which was substantiated by texture randomization and formation of relatively larger TRDs in the microstructure.