Solution-annealed alloy D9 (a 15Cr-15Ni-2.2Mo-Ti-modified austenitic stainless steel) samples were subjected to annealing in the temperature range 1173-1273 K with and without 5% prior deformation. Anomalous grain growth (AGG) was observed in these processing conditions. Electron back scattered diffraction (EBSD) analysis revealed that the proportion of Σ3 boundaries increased during AGG. This was attributed to the formation of new annealing twins during grain growth. Fraction of Σ27 boundaries remained constant in all processing conditions, whereas the fraction of Σ9 boundaries increased marginally at higher annealing temperatures. This indicates that only a few interactions between Σ3 boundaries happened during grain growth. The AGG in alloy D9 during annealing is attributed to inhomogeneous distribution of local strain in material coupled with intrinsic misorientation dependant boundary mobility. The inhomogeneous grain size distribution in the solution-annealed specimen would have provided the 'size-advantage' so that bigger grains could grow abnormally during annealing. © 2009 Elsevier B.V. All rights reserved.

V. Subramanya Sarma

Department of Metallurgical and Materials Engineering

Alloy D9

ANNEALED ALLOYS

Annealing temperatures

Annealing twins

ANOMALOUS GRAIN GROWTH

BOUNDARY MOBILITIES

ELECTRON BACK-SCATTERED DIFFRACTION ANALYSIS

Grain boundary character distribution

Grain size distributions

INHOMOGENEOUS DISTRIBUTIONS

Local strains

MIS ORIENTATIONS

Processing conditions

Temperature ranges

Annealing

Austenite

Austenitic stainless steel

Chromium

Grain boundaries

Grain size and shape

Metal analysis

Molybdenum

Steel

Steel metallurgy

Grain growth

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It currently offers undergraduate, postgraduate and research degrees across 16 disciplines in Engineering, Sciences, Humanities and Management. About 596 faculty belonging to science and engineering departments and centres of the Institute are engaged in teaching, research and industrial consultancy.

IIT Madras

Materials Science and Engineering A

In this work, the role of grain boundary engineering (GBE) on the weld heat-affected-zone (HAZ) liquation cracking resistance of austenitic stainless steel AISI 316Ti was investigated. Standard wrought-processed alloy 316Ti was cold rolled to 5% strain and subsequently annealed at 1373 K for 30 min to achieve the optimum grain boundary character distribution (GBCD). The GBE samples were found to consist of a significantly higher fraction (72%) of low Ʃ coincident site lattice (CSL) boundaries as compared to the as received (AR) samples (45%). To study the liquation cracking behavior, single-track longitudinal varestraint tests was performed on AR and GBE samples. Microstructural examination revealed that the grain coarsening and the liquation events were less prominent in the GBE samples as compared to the AR samples. It is believed that the reduced segregation of impurities to the special boundaries (coherent Ʃ3 twins, in particular) present in the GBE samples is responsible for the observed improvement in the HAZ liquation cracking resistance. © 2018 Elsevier Inc.

Materials Characterization

Effect of grain boundary character distribution on weld heat-affected zone liquation cracking behavior of AISI 316Ti austenitic stainless steel

Dynamic recrystallization (DRX) kinetics and its association with annealing twins in alloy 617 has been studied in a range of strain rates (0.001 s−1 to 10 s−1) and temperatures (1173–1473 K). In both low (&lt; 0.1 s−1) and high (≥ 1 s−1) strain rate domains, DRX fraction is high as compared to intermediate strain rate (~ 0.1 s−1). The lower critical strain for DRX initiation with higher growth of DRX grains is responsible for large DRX fraction with coarse grains microstructure in the low strain rate domain. In contrast, DRX at higher strain rates is mainly controlled by the large stored energy along with adiabatic temperature rise which leads to higher DRX nucleation. It is also noticed that the evolution of twins is greatly influenced by deformation temperature at all imposed strain rates. Here, the evolution of twins is quantified by measuring twin boundary length fraction and length density. The twin-length density increases with increase in DRX fraction at low temperatures (&lt; 1423 K) but decreases with increasing DRX fraction at high temperatures (≥ 1423 K). An exponential rise in recrystallized grain size with DRX fraction is observed at a temperature ≥ 1423 K. This suggests that rapid migration of high-angle boundaries happened at ≥ 1423 K which may not be favorable for nucleation of twins during DRX. © 2017 Elsevier B.V.

Influence of processing parameters on dynamic recrystallization and the associated annealing twin boundary evolution in a nickel base superalloy

Thermo-mechanical processing (one-step and iterative) comprising strain (5, 10, and 15 pct cold rolling) and annealing [at 1273 K, 1323 K, and 1373 K (1000 °C, 1050 °C, and 1100 °C) for different times of 30 minutes, 1 and 2 hours] were employed to realize a grain boundary engineered (GBE) microstructure in alloy 617. Among the single-step routes, the process employing 15 pct cold reduction and annealing at 1373 K (1100 °C) for 1 hour was found to be effective in increasing the fraction of Σ3 boundaries; however, it also induced partial recrystallization. The iterative processing employing lower reductions and higher annealing temperatures failed to realize GBE microstructure. The second-phase carbides in this material effectively pin the boundaries thus requiring higher pre-strain to initiate the boundary migration and subsequent multiple twinning events. The iterative processing designed based on the outcomes of the single step route resulted in GBE microstructure by significantly increasing the Σ3 fraction and substantially disrupting the random high-angle grain boundaries connectivity. The newly added Σ3 boundaries in the GBE microstructure predominantly terminated on (111) plane indicating that they have low-energy configuration. The GBE specimen has shown remarkable resistance to intergranular corrosion as compared to the as-received condition. © 2015, The Minerals, Metals & Materials Society and ASM International.

Metallurgical and Materials Transactions A: Physical Metallurgy and Materials Science

Role of Carbide Precipitates and Process Parameters on Achieving Grain Boundary Engineered Microstructure in a Ni-Based Superalloy

Dynamic recrystallization (DRX) behavior in hot deformed (by uniaxial compression in a thermomechanical simulator in the temperatures range 1173 K to 1373 K [900 °C to 1100 °C]) Ti-modified austenitic stainless steel was studied using electron back scatter diffraction. Grain orientation spread with a "cut off" of 1 deg was a suitable criterion to partition dynamically recrystallized grains from the deformed matrix. The extent of DRX increased with strain and temperature, and a completely DRX microstructure with a fine grain size ∼4 μm (considering twins as grain boundaries) was obtained in the sample deformed to a strain of 0.8 at 1373 K (1100 °C). The nucleation of new DRX grains occurred by the bulging of the parent grain boundary. The DRX grains were twinned, and a linear relationship was observed between the area fraction of DRX grains and the number fraction of ∑3 boundaries. The deviation from the ideal misorientation of ∑3 boundaries decreased with an increase in the fraction of ∑3 boundaries (as well as the area fraction of DRX) signifying that most ∑3 boundaries are newly nucleated during DRX. The generation of these ∑3 boundaries could account for the formation of annealing twins during DRX. The role of ∑3 twin boundaries on DRX is discussed. © 2010 The Minerals, Metals & Materials Society and ASM International.

A study on microstructural evolution and dynamic recrystallization during isothermal deformation of a Ti-modified austenitic stainless steel

The article discusses the results of a study on low-strain thermo-mechanical (one-step and iterative) processing to enhance ∑3 n boundaries in a Ti-modified austenitic stainless steel (alloy D9). Solution annealed (SA) specimens were subjected to 10% thickness reduction by rolling followed by annealing at 1173, 1223, and 1273 K for 0.5, 1, and 2 h. Anomalous grain growth with moderate increase in ∑3 n boundaries was observed after annealing at 1,173 K for 0.5 to 2 h. Prolific multiple twinning with minimum deviation of ∑3 and ∑9 boundaries from ideal orientation was achieved after annealing at 1,273 K for 0.5 to 2 h. A significant disruption in random boundary connectivity was obtained in these conditions due to the presence of large number of ∑3-∑3-∑9/∑3- ∑9-∑27 triple junctions. Iterative processing (up to 4 cycles) employing 10% thickness reduction followed by annealing at 1,273 K for 0.5 h revealed fluctuations in the evolution of ∑3 boundaries. The ∑3 fraction increased after 2nd and 4th iteration and there is a drop after 3rd iteration. This was attributed to the increased driving force for grain boundary migration due to dislocation pile-up at twin boundaries during earlier iterations. A two step iterative processing comprising of 10% deformation followed by annealing at 1,273 K for 0.5 h is the recommended thermo-mechanical processing to achieve enhanced fraction of ∑3 n boundaries (∼73%) in alloy D9. © 2010 Springer Science+Business Media, LLC.

Journal of Materials Science

One-step and iterative thermo-mechanical treatments to enhance ∑3 n boundaries in a Ti-modified austenitic stainless steel

The present study discusses the grain boundary microstructural control in a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel (commonly known as alloy D9) through a one-step thermomechanical treatment. The experimental methodology adopted in this investigation was based on the strain annealing approach in which a small amount of strain (5 to 15 pct) was imparted on the solution-annealed (SA) sample. The cold-deformed samples were subsequently annealed at various temperatures (1173 to 1273 K) for different time periods (0.5 to 2 hours). It was observed that annealing after 5 pct deformation induces anomalous grain growth with a moderate increase in number fraction of coincidence site lattice (CSL) boundaries. However, a prestrain of 10 to 15 pct followed by annealing at 1273 K for 0.5 to 2 hours was found to be a suitable thermomechanical processing schedule to increase the number fraction of CSL boundaries (particularly Σ3 and its variants) significantly. Further, the well-connected network of random grain boundaries present in the SA specimen was substantially disrupted in these processing conditions due to the incorporation of Σ3 and its variants. The preceding results were discussed with reference to strain-induced grain growth vis-à-vis strain-induced boundary migration (SIBM) following deformation and annealing. © The Minerals, Metals & Materials Society and ASM International 2008.

Grain boundary microstructural control through thermomechanical processing in a titanium-modified austenitic stainless steel

The Measurement of Grain Boundary Geometry

A C-scan ultrasonic imaging system was used to investigate the microstructural evolution during dynamic recrystallization (DRX) of a 15Cr-15Ni-2.2Mo-Ti modified austenitic stainless steel (alloy D9). Four specimens were forged at 1273 K to different strains in the range 0.1-0.5. Specimens with true strains of 0.2 or lower did not show any variation in the amplitude of the first back-wall echo. However, a visible variation in the C-scan image was observed at and above the 0.3 strain level. This variation was attributed to the evolution of fine grains. The formation of fine grains was related to DRX, as indicated by electron backscattered diffraction. This study also revealed the characteristics of the DRX or 'necklace grains', as opposed to the so-called parent grains or rest of the microstructure.

Philosophical Magazine

Evolution and characterization of dynamically recrystallized microstructure in a titanium-modified austenitic stainless steel using ultrasonic and EBSD techniques

Scripta Materialia

Grain growth and twinning in nickel

Journal of Microscopy

Evaluation of intragranular misorientation parameters measured by EBSD in a hot worked austenitic stainless steel

Studies on twinning and grain boundary character distribution during anomalous grain growth in a Ti-modified austenitic stainless steel

Journal	Materials Science and Engineering A
ISSN	09215093
Open Access	No