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This study reports the interplay between grain boundary morphology and mechanical behaviour using micropillar compression. Quantitative findings indicate that serrated grain boundaries with multiple curvatures exhibit notably higher yield strengths compared to their straight counterparts. In a bi-crystal system, 18 pct increase in boundary length, achieved through multiple curvature boundaries, results in 21 pct increase in yield strength. The quasi-in-situ electron backscatter diffraction (EBSD) investigations show the concentration of plastic strain within preferentially oriented slip bands, with grain boundaries offering resistance, and slip band leads to changing directions as they traverse from one grain to another, with secondary slips emerging post-yielding during micropillar compression. As compression levels rise, a prominent uniform strain hardening rate emerges in grain boundaries characterized by multiple curvatures. Local resolved shear stress at grain boundaries experiences a pronounced reduction under the applied load, particularly when the serration wavelength exceeds 0.4, and the amplitude ranges from 0.3 to 0.5 times the total grain boundary length. An attempt is made here to shed light upon the underlying microscopic mechanisms that govern grain boundary micromechanics through a comprehensive three-dimensional analysis. It becomes evident that both boundary curvature and inclination of interface plane play critical roles in enhancing material strength, collectively contributing to a 21 pct increase in yield strength in current case of boundary with curvature. Additionally, these morphologies notably reduce the likelihood of heterogeneous plastic deformation compared to straight grain boundaries. [ABSTRACT FROM AUTHOR]

The formation of δ-ferrite in advanced structural materials is well known to impair mechanical and corrosion properties. In the present work, high-temperature phase stability and phase transformation characteristics of the γ → δ phase transformation of ferritic/martensitic steel were studied by differential scanning calorimetry (DSC) and electron backscatter diffraction (EBSD). The characteristics of martensitic transformation were studied by varying the temperature and time of austenitization. The results show that γ-austenite completely transforms to δ-ferrite during austenitization beyond 1523 K. The absence of martensite transformation was observed in the DSC thermogram for the sample annealed at 1523 K for 5 h. A detailed EBSD study of the time-dependent evolution of δ-ferrite revealed a significant reduction in low-angle and coincidence site lattice (CSL) grain boundaries of the martensite matrix. There was no reverse transformation (δ-ferrite → γ-austenite) observed during heating in DSC. Further, the Kolmogorov–Johnson–Mehl–Avrami (KJMA) model was employed to study the kinetics of the γ-austenite → δ-ferrite transformation. The activation energy and growth exponent obtained for this transformation were 335 kJ mol−1 and 2.1, respectively. Tis result has significant technological implications as it revealed an important fact that δ-ferrite, once formed in the material, does not dissolve by heat treatment. The characteristics of martensitic transformation were studied by varying the temperature and time of austenitization. It is observed that the γ-austenite completely transforms to δ-ferrite during austenitization beyond 5 h at 1523 K. A detailed EBSD study of the time-dependent evolution of δ-ferrite revealed a significant reduction in low-angle and coincidence site lattice (CSL) grain boundaries of the martensite matrix. For δ-ferrite, there was no reverse transformation (δ-ferrite → γ-austenite) observed during heating in DSC which signifies the difficulty associated with its dissolution by heat treatment. Further, Kolmogorov–Johnson–Mehl–Avrami (KJMA) model was employed to study the kinetics of the γ-austenite → δ-ferrite transformation. The activation energy and growth exponent obtained for this transformation were 335 kJ mol−1 and 2.1, respectively. [ABSTRACT FROM AUTHOR]

This paper presents the details on predicted and experimental microstructural evolution and high-temperature tensile behavior of a hot isostatically processed (HIPed) and heat-treated Ni-based superalloy EP741NP for aerospace applications. The phase prediction carried out using JMatPro® software from the composition of the alloy as a function of heat treatment has been found to agree with the results of characterization by scanning and transmission electron microscopy. Prediction has revealed the evolution of secondary and tertiary gamma prime (γ′) with phase fraction of ~46% during solutionization at 1483 K (1210 °C) for 8 h and ~12% during the course of aging treatment at 1144 K (871 °C) for 32 h, respectively. A uniform distribution of secondary γ′ with typical octocube morphology within a size range of 0.5–0.8 µm and a sparse distribution of tertiary γ′ of fine spherical particles with average size < 0.1 µm have been observed during study of microstructure. Electron backscattered diffraction (EBSD) studies showed epitaxial growth for γ′ phase within a grain with similar crystallographic orientation. Tensile tests conducted at room temperature and in the temperature range from 823 K to 1123 K (550 °C to 850 °C) have shown that yield strength (YS) and ultimate tensile strength (UTS) remain almost unchanged up to temperature 923 K (650 °C) due to microstructural stability of strengthening γ′ precipitate. Thereafter, it starts decreasing because tertiary precipitates cannot hinder grain boundary motion at high temperature, first because of their size and more importantly because they are usually dissolved back into the matrix at high temperature, which assist the annihilation and rearrangement of dislocations leading to reduction of dislocation density. Fractographic studies have revealed that the fracture mode is found to be mostly mixed in nature (both ductile and brittle fracture). [ABSTRACT FROM AUTHOR]

This article presents a systematic study to highlight micromechanisms associated with the grain interface character distribution of SS304HCu. The low-energy coincidence-site-lattice (CSL) boundary fraction is found to be increased (33 to 62 pct) with an increase in annealing temperature from 1073 K to 1323 K after 50 pct cold rolling. Furthermore, an increase in annealing temperature has shown reduction/saturation for CSL (~ 58 pct) due to diffusion-assisted grain boundary migration. Associated effective grain boundary energy has been calculated to rationalize the energy minimization process during annealing. [ABSTRACT FROM AUTHOR]

This paper focuses on studying the influence of texture on the deformation mechanism of oxide dispersion strengthened (ODS) 18Cr ferritic steel subjected to high-temperature tensile testing after consolidation by hot extrusion. The initial microstructure of the hot extruded steel showed preferential (1 1 0) planes alignment in perpendicular and parallel direction to extruded (ED) and transverse (TD), respectively. Two types of anisotropy have been highlighted: (1) A strong anisotropy of elongation properties with a transverse ductility is lower as compared to the axial ductility; (2) yield stress anisotropy (mechanical strength anisotropy) which is found to be temperature dependent. The material flow in 〈1 1 1〉 direction is observed during the application of load along extruded direction during the tensile test; however, (1 1 0) slip plane normal is parallel to the direction of loading for transverse direction and brings the Schmid factor to a lower level which makes slip activation unlikely. Deformation band is found to be aligned with the loading direction in the ED specimen and covering across a number of neighboring ferrite grains could be noticed irrespective of material flow. However, the areas near the grain boundaries in the microstructure are found to be the strain gradient regions. Further, it shows a preferential accommodation of strain in proximity to the boundary network for TD specimen. High temperature (973 K) tensile test in ED is found to promote grain-subdivision of elongated bands by dynamic recrystallization, and inverse pole figure distribution reveals a preferential increase in the intensity of clustering toward (1 1 1) plane parallel to the loading direction. [ABSTRACT FROM AUTHOR]

The creep deformation and rupture behavior of P92 steel weld joint fabricated by narrow-gap TIG (NG-TIG) welding process have been investigated at 923 K over a stress range of 80-140 MPa. The prior-austenite grain size, M23C6 precipitate size and hardness have been found to vary significantly in the weld joint. The reduction in hardness from weld metal to base metal with a trough at the outer edge of heat-affected zone (HAZ) has been observed. Coarsening of M23C6 precipitate, recovery of martensite lath dislocation structure and formation of subgrain structure led to lower hardness in the intercritical HAZ. The creep rupture life of NG-TIG weld joint was lower than the base metal, the difference in creep rupture life between base metal and weld joint has been increased significantly with decrease in applied stress. The fracture location in the joint changed from base metal at high-stress regime to the fine grain (FG) HAZ (Type IV cracking) under lower stress level. Fracture in the FGHAZ evidenced the significant reduction in ductility, localized deformation and extensive localized cavitation. Extensive Laves phase formation with significant loss of solution strengthening contribution from tungsten and coarsening of M23C6 precipitate with prolonged creep exposure led to reduction in hardness and more extensive cavitation in the FGHAZ resulting in premature Type IV failure of the weld joint. Weld strength reduction factor about 0.59 has been evaluated for 105 h at 923 K. [ABSTRACT FROM AUTHOR]

This paper presents the results of an experimental study aimed to identify hot working domains in oxide dispersion strengthened (ODS) 18Cr ferritic steel over a wide range of temperatures (1323-1473 K) and strain rates (0.01-10 s−1). The experimental data were obtained by uniaxial compression test using the Gleeble-1500D simulator in this range of temperature and strain rate. An inverse relationship with temperature and positive strain rate sensitivity associated with dynamic recovery and recrystallization, which is influenced by temperature and strain rate, was derived from the flow stress. Based on the processing map generated at 0.5 true strain, using rate dynamic material model (DMM) approach and the calculated instability parameter ξϵ´>0, the optimum processing domain has been determined for this steel. The most favorable processing parameters are found in the temperature ranges of 1350-1450 K with a strain rate of 0.01 s−1 and 1473 K with a strain rate 0.1 s−1 with peak efficiency of 30 and 35%, respectively. The material flow behavior was studied using scanning electron microscopy (SEM)-based EBSD microstructural characterization. The steel subjected to 1323 K at high strain rate 10 s−1 in the low-efficiency workability region showed low aspect ratio as compared to the elongated bamboo-like initial microstructure; however, minimum strain rate (0.01 s−1) showing that localized slip/shearing is the key mechanism and fiber texture studied from the intensity distribution of inverse pole figure showed the presence of significant amount of θ-fibers. In contrast, dynamic recrystallization dominated at higher efficiency region in the safe domain of processing map and γ-fiber texture was evident in the specimen deformed at 1373 and 1473 K with strain rate of 0.01 and 0.1 s−1, respectively, which is responsible for the change in the initial 〈1 1 0〉//ED α-fiber texture. [ABSTRACT FROM AUTHOR]

This paper presents the results of a study on grain boundary characteristics in cold deformed and annealed 304HCu grade austenitic stainless steel (SS 304HCu) using electron backscatter diffraction. The microstructure exhibited an increasing fraction of Σ1 to 29 coincidence site lattice boundaries with annealing temperature, resulting up to ~60 pct at 1573 K with 92 pct contribution from Σ3-type twin boundary. However, the twin boundary interaction at the triple points with a network of Σ3-Σ3-Σ9 was found to decrease from 4 to 0.5 pct with annealing temperature. To understand the resultant boundary advancement of the Σ3n (n = 1, 2, 3) boundaries, their migration was traced in the annealed specimen. However, in the specimen with extended annealing Σ3 boundary fraction was found to be higher with a concomitant decrease in the boundary fraction generated by the Σ3 interactions. In this study, a procedure to analyze the coherency of Σ3 boundaries and its interfaces that form due to Σ3 interactions has been evolved based on single-section analysis using the pole concentration across the grains. Further, a crystallographic description of the two planes meeting at the interface of Σ3-type boundary has been provided by adopting serial sectioning methods, which help to understand the morphological changes. The quantitative deviation from exact coherent Σ3 has been estimated to be within ~6 deg in this study. [ABSTRACT FROM AUTHOR]