The origin and role of ∑3 boundaries during dynamic recrystallization (DRX) and grain boundary engineering (GBE) of a Ti-modified austenitic stainless steel (alloy D9) is studied. Hot deformation tests were carried out on solution-annealed (SA) specimens to study the DRX behavior whereas a series of cold deformation and annealing were performed on SA specimens to realize GBE microstructure. A linear relationship between the area fraction of DRX and the number fraction of ∑3 boundaries was observed during hot deformation. This high fraction of ∑3 boundaries could account for the formation of coherent annealing twins by "growth accidents" during DRX. For certain combinations of cold deformation and annealing, a significant increase in ∑3 boundaries was observed. In contrast to hot deformation, majority of these new ∑3 boundaries during cold deformation and annealing were formed by geometrical interactions between the pre-existing ∑3 boundaries. The role of the ∑3 boundaries during DRX and on tailoring microstructure through grain boundary engineering approach is discussed. © (2012) Trans Tech Publications, Switzerland.

V. Subramanya Sarma

Department of Metallurgical and Materials Engineering

Alloy D9

Annealing twins

Area fraction

AUSTENITIC

Cold deformation

GRAIN BOUNDARY ENGINEERING

HOT DEFORMATION TESTS

Linear relationships

Thermo-mechanical processing

Annealing

Austenitic stainless steel

Behavioral research

Dynamic recrystallization

Dynamics

Grain boundaries

Simulated annealing

Textures

Deformation

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IIT Madras

Materials Science Forum

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

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 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

Acta Materialia

The role of annealing twins during recrystallization of Cu

Scripta Materialia

On the role of iterative processing in grain boundary engineering

Mechanisms of grain boundary engineering

Origin and role of ∑3 boundaries during thermo-mechanical processing of a Ti-modified austenitic stainless steel

Journal	Materials Science Forum
Publisher	Trans Tech Publications Ltd
ISSN	02555476
Open Access	No