Boundary Engineering Processing Research Articles

Novel role of thermomechanical grain boundary engineering in the microstructure evolution of austenitic stainless steel

The effect of special grain boundary development via the grain boundary engineering process (GBE) was explored in a bimodal microstructured 316L austenitic stainless steel with an average grain size of 1.92 μm. The GBE process was carried out by the rolling-annealing method via two routes of low and medium applied strain, followed by a short annealing period in a single-step and iterative manner. The grain boundary character distribution was obtained by Electron Backscatter Diffraction (EBSD) analysis, and the intergranular corrosion resistance and the tensile behavior of the samples were examined. Microstructural characterization showed that applying low strain repetitively increased the coincidence site lattice (CSL) and Σ3 boundaries percentage and created large twin-related domains (TRD). The reduced sensitization was attributed to a high proportion of CSLs and large-sized TRDs by applying a low-strain route. In the early stages of the low-strain route, the grain boundary character distribution showed a high number of primary twin nuclei in coarse grains and a continuous network of single twin nuclei in smaller grains, creating a high percentage of Σ3-grain boundaries. The low applied strain and the absence of the necessary driving force for the recrystallization and GBs migration led to the formation of twin nuclei, which would assist with the reduction of microstructure energy. A high percentage of Σ3 boundaries and an increase in the percentage of triple points consisting of low energy boundaries were found to be influential factors in increasing elongation.

Effects of various grain boundary engineering processing on microstructure and corrosion behaviors of 304 stainless steel analyzed with a fractal model

Grain boundary engineering (GBE) is designed for metals to optimize the grain boundary characteristics and improve their resistance to intergranular corrosion (IGC). In this study, the effect of various GBE processing on the fraction of low ΣCSL grain boundaries and random large-angle grain boundary connectivity in 304 stainless steel was investigated quantitatively with a fractal model. The corrosion weight loss rate, corrosion current density and sensitization degree of samples with various grain boundary characteristics were measured, and these corrosion behaviors were studied in term of the fractal dimension. The results showing the specimen after alternating directions cold rolling plus annealed at 1100 °C obtained the highest ΣCSL score that was 67.4%. And the value of fractal dimension for specimen after solid solution annealing, cold rolled 5% or cold rolled 20% reduction plus annealed at 1100 °C for 5 min in GBE was 1.75, 1.53 and 1.46, respectively. An indicator defined by fractal dimension for grain boundary characteristic on corrosion behaviors of 304 stainless steel was verified and clarified, and the mechanisms of GBE to improve corrosion resistance were confirmed with the observation of low-energy grain boundary fragments introduced by annealing twins and triple junctions in grain boundaries.

Grain boundary engineering process for nano reinforced aluminum matrix composites

It is challenging to obtain a balance of strength and ductility for alloys and their composites. They are expected to exhibit good comprehensive properties by adjusting the grain boundary character and/or introducing twin structures. However, for coarse grained aluminum with high stacking fault energies, twin structures are rarely found. In this study, a grain boundary engineering process was carried out on coarse grained aluminum containing nano-reinforcements via an alternated compression deformation, followed by an annealing treatment. The effects of the thermo-mechanical process and alloy elements on the microstructures of the composites were analyzed. The mechanical properties and inter-granular corrosion properties of the composites were evaluated. Twin structures could be generated in these alloys. After thermo-mechanical process, the composites exhibited good plasticity as well as higher strength. A Σ3 twin boundary hindered inter-granular corrosion crack propagation. The formation mechanism of the twin structures and the strengthening mechanism are discussed.

“Strain-annealed” grain boundary engineering process investigated in Hastelloy-X

The mechanisms responsible for microstructure evolution during “strain-annealed” grain boundary engineering in Hastelloy-X have been investigated. GBE-quantifying parameters such as Σ3n density and fraction, triple junction density and fraction have been utilized to quantify the extent of GBE in the processed microstructures. “Strain-annealed” GBE process favors the formation of Σ3 boundaries through dissociation mechanism. When the amount of strain/cycle is high, the change in microstructure is limited and most of the Σ3 boundaries are remnant annealing twins from the initial microstructure. When the amount of strain/cycle decreases, more grain boundary migration takes place and majority of Σ3 boundaries start integrating into the GB network through dissociation mechanism. AGG is found to take place under special conditions: when (i) the dislocation density stored in the microstructure is low and (ii) the fraction of Σ3 boundaries that are integrated into the general grain boundary network is high. It is the preferential growth advantage of CSL boundaries that leads to abnormal grain growth.

Effects of initial grain size and strain on grain boundary engineering of high-nitrogen CrMn austenitic stainless steel

18Mn18Cr0.5N steel with an initial grain size of 28–177 μm was processed by 2.5%–20% cold rolling and annealing at 1000°C for 24 h, and the grain boundary character distribution was examined via electron backscatter diffraction. Low strain (2.5%) favored the formation of low-Σ boundaries. At this strain, the fraction of low-Σ boundaries was insensitive to the initial grain size. However, specimens with fine initial grains showed decreasing grain size after grain boundary engineering processing. The fraction of low-Σ boundaries and the (Σ9 + Σ27)/Σ3 value decreased with increasing strain; furthermore, the specimens with fine initial grain size were sensitive to the strain. Finally, the effects of the initial grain size and strain on the grain boundary engineering were discussed in detail.

Unraveling the origin of twin related domains and grain boundary evolution during grain boundary engineering

Grain boundary engineering of Fe-based austenitic stainless steels and other materials has been successful in producing a large increase in twin and twin related grain boundaries from a wide range of thermomechanical treatments. However, the exact mechanisms and effective grain boundary network descriptors to create the heavily twinned microstructures are yet to be fully understood. In this study, we provide insight into the grain boundary engineering process by examining sequential progression of the same spatial location of a twin related microstructure through thermomechanical processing. The results show that clusters of twin related grain boundaries called twin related domains form during primary recrystallization. The size of the twin related domains increases as the level of strain falls toward the critical strain for recrystallization. Growth of twin related domains during recrystallization results in the formation of twin boundaries behind the migrating grain boundary front. Formation of higher order twin boundaries occurs when two separate grain boundary fronts of the same twin related domain impinge upon each other. We also present relevant microstructural descriptors with emphasis on twin related domain statistics to recrystallization phenomena in grain boundary engineering materials.

Grain boundary plane distributions and single-step versus multiple-step grain boundary engineering

The ‘five-parameter’ (i.e. both misorientation and grain boundary plane) distribution in type 304 austenitic stainless steel has been measured and evaluated for an ‘as-received’ (AR) specimen and specimens undergoing both single-step grain boundary engineering processing (SSGBE) and multiple-step grain boundary engineering processing (MSGBE) comprising three iterations. The results showed that the fundamental requirement for twinning-related GBE is to maximise concomitantly the proportion of both Σ3 and Σ9 boundaries, which in turn supports the development of special planes in the grain boundary network. 〈1 1 0〉 and 〈1 1 1〉 tilt and twist boundaries play a key role in the formation of ‘special’ grain boundary planes. MSGBE added increased proportions of Σ3 boundaries and resulted in development of different characteristics in the planes distribution compared to SSGBE. These modifications are likely to result in improved grain boundary properties after MSGBE compared to SSGBE.

Five‐parameter grain boundary analysis of a grain boundary–engineered austenitic stainless steel

Two different grain boundary engineering processing routes for type 304 austenitic stainless steel have been compared. The processing routes involve the application of a small level of strain (5%) through either cold rolling or uni-axial tensile straining followed by high-temperature annealing. Electron backscatter diffraction and orientation mapping have been used to measure the proportions of Sigma3(n) boundary types (in coincidence site lattice notation) and degree of random boundary break-up, in order to gain a measure of the success of the two types of grain boundary engineering treatments. The distribution of grain boundary plane crystallography has also been measured and analyzed in detail using the five-parameter stereological method. There were significant differences between the grain boundary population profiles depending on the type of deformation applied.

Changes in Interface Parameters and Tensile Properties in Copper as a Consequence of Iterative Processing

Both one-step and subsequent multistep grain boundary engineering processing of copper have been investigated. Specimens were tensile tested to failure after each stage of processing, and the microhardness was measured. These properties were linked to measurements of both the grain boundary misorientation and grain boundary plane distribution in order to provide insight into the mechanisms of microstructure evolution. Analysis of the stress-strain behavior revealed the balance between deformation and microstructure restoration occurring during processing. An increase in work hardening and a reduction in ductility were observed with successive processing iterations, indicating an accumulation of retained strain. This brought about stagnation in the microstructure, whereby no new Σ3s were generated and grain boundary migration was stabilized. It was shown that Σ3 boundaries are effective agents for dislocation pileup, and this strain-retention ability plays an important role in the early stages of iterative processing. Analysis of the data in terms of “incorporated” and “nonincorporated” Σ3s indicated that iterative treatments are mechanistically different from one-step treatments. It was also shown that the Σ9 and Σ27 boundaries added to the microstructure as a consequence of iterative processing were not “special,” because they were on irrational boundary planes.

Orientation Distribution of Σ3 Grain Boundary Planes in Ni before and after Grain Boundary Engineering

The distribution of grain boundary plane orientations in polycrystalline Ni has been measured before and after grain boundary engineering. The grain boundary engineered microstructure has a relatively higher concentration of Σ3 grain boundaries and, when compared to the initial structure, more of these boundaries have orientations that are inclined by more than 10° from the (111) orientation of the ideal coherent twin. Although the conventionally measured grain size is not affected by the grain boundary engineering process, the average size of the regions containing only Σ3n grain boundaries increases by nearly a factor of two. The observations indicate that the increase in the relative population of Σ3 grain boundaries results both from the preferential elimination of random grain boundaries and the generation of new Σ3 grain boundaries which do not have (111) grain boundary plane orientations.

Corrosion Behaviors of Random and Special Grain Boundaries in a Sensitized 304 Stainless Steel

Abstract The corrosion behaviors of random and special grain boundaries were compared in a sensitized type 304 stainless steel sheet previously treated by particular grain boundary engineering process. In order to evaluate the corrosion susceptibility of various grain boundaries, the samples were corroded (etched) in a particular solution and then the characters of grain boundaries were determined by electron backscatter diffraction (EBSD), associating with the microstructure acquired by optical microscopy. The results show that random boundaries are susceptible to corrosion whereas most of CSL grain boundaries (mainly Σ3 and Σ9) exhibit better resistance to corrosion. The incoherent Σ3 boundaries, located in the random grain boundary network, play a more significant role in arresting the development of corrosion along the random grain boundary path than coherent ones. The symmetrical incoherent Σ3 boundaries are more resistant to corrosion than asymmetrical ones. In addition, the more the boundary plane deviates from the exact CSL position, the more susceptible to corrosion it is.