Connectivity Of Random Boundaries Research Articles

Simultaneously enhanced strength-ductility synergy and corrosion resistance in liquid-solid bonded stainless steel cladding carbon steel plate by hot rolling and annealing treatment

Liquid-solid (L-S) bonded stainless steel/carbon steel (SS/CS) laminated composites have strong metallurgical bonding interface, while the solidification structure of SS deteriorates the mechanical properties and corrosion resistance. In this study, hot rolling and annealing were proposed to regulate the microstructures and balance the performance of L-S bonded SS/CS clad plates. SS cladding CS plates were prepared by horizontal continuous liquid-solid composite casting and hot rolled at 1200 °C with a reduction rate of 90%, and annealed at 1000 and 1050 °C for 30 min. The microstructural evolution was studied by scanning electron microscope and electron back-scatter diffraction, elemental distribution was investigated by electron probe microanalysis. Microhardness, tensile test and anodic-cathodic polarization test in a 3.5% NaCl solution were carried out on hot-rolled and annealed SS cladding CS plates, and the mechanism for the simultaneous enhancement of strength-ductility synergy and corrosion resistance by hot rolling and annealing was analyzed. Annealing at 1050 °C resulted in excellent properties with tensile strength of 550 MPa and elongation of 50%, corrosion potential of −0.21 V and corrosion current density of 0.13 μA cm−2. The strength and ductility of the SS cladding CS plate were synergistically improved by a strong metallurgical bonding cladding interface, high frequency of low-Σ CSLs and refinement of grains in 304 SS. The low-Σ CSLs in 304 SS restricted the formation of carbides in the carburized layer and broke down the connectivity of random boundaries, improving the corrosion resistance.

Effect of thermomechanical processing via rotary swaging on grain boundary character distribution and intergranular corrosion in 304 austenitic stainless steel

In the present work, the thermomechanical processing consisting of rotary swaging deformation and annealing treatment was performed to optimize the grain boundary character distribution (GBCD) of 304 austenitic stainless steel. To systematically investigate the effect of GBCD evolution on intergranular corrosion (IGC), double loop electrochemical potentiokinetic reactivation and electrolytic oxalic acid etch tests were employed. The experimental results show that the fraction of low-Σ coincident site lattice (CSL) boundaries increased from 58.1% to 74.0% for the specimen swaged to 0.06 true strain and then annealed at 1050 °C for 5 min duration. By characterizing the evolution of GBCD as a function of strain level in terms of low-Σ CSL boundaries fraction, average twin-related domain (TRD) size, average number of grain per TRD and fractal dimension of the maximum random boundary connectivity, the grain boundary engineering (GBE) microstructure was realized by the occurrence of prolific multiple twinning events during strain-induced boundary migration while static recrystallization has a detrimental effect on optimizing GBCD for the prolific of new strain-free grains with random boundaries. The IGC resistance of the GBE-treated 304 austenitic stainless steel is enhanced by inhibiting the nucleation and propagation of IGC cracks, resulting from the increase in the fraction of low-Σ CSL boundaries, especially Σ3 boundaries and the disruption of random boundary network connectivity.

EBSD study of the effect of electropulsing treatment on the microstructure evolution in a typical cold-deformed Ni-based superalloy

Evolution of grain structure and grain boundary character distribution in a cold-deformed Ni-based superalloy during the electropulsing treatment (EPT) was investigated by EBSD technique. Compared with the conventional heat treatments (CHT), the nucleation and growth of SRX grains were significantly accelerated by EPT, which was mainly attributed to the high mobility of dislocations and solute atoms, and the efficient energy transfer under the effect of EPT. Especially, with the introduction of electric current, the activation energy of dislocation motion could be reduced by generating additional energy near the dislocation line, which made the motion of dislocation easy to start. The dislocations would also be easy to move in the form of slipping and climbing, under the effect of electron wind force. On the other hand, most of Σ3 boundaries were newly formed through growth accidents in the form of coherent Σ3 boundaries during EPT, and the mutual interactions between different Σ3n (n = 1, 2, 3) boundaries were rather limited. In addition, EPT could indirectly promote the formation of Σ3 boundaries by accelerating the SRX process, but the formation of Σ9 and Σ27 boundaries was inhibited at the temperatures higher than 700 °C. Moreover, the connectivity of random boundaries in the cold-deformed alloy treated by EPT was also discussed.

Evaluation of Grain Boundary Network and Improvement of Intergranular Cracking Resistance in 316L Stainless Steel after Grain Boundary Engineering.

For understanding the improvement of intergranular stress corrosion cracking (IGSCC) propagation in grain boundary engineering (GBE)-processed metals exposed to a simulated pressurized water reactor (PWR) environment, characteristics of the grain boundary network of 316L stainless steel before and after GBE were investigated and compared, including proportions both in length and in number of ∑3n boundaries, sizes, and topology of grain clusters (or twin-related domains), and connectivity of random boundaries. The term through-view random boundary path (TRBP) was proposed to evaluate the random boundary connectivity. A TRBP is a chain of end-to-end connected crack-susceptible boundaries that passes through the entire mapped microstructure. The work provides the following key findings: (I) the length fraction of ∑3n boundaries was increased to approximately 75% after GBE, but the number fraction was only approximately 50%; (II) a connected non-twin boundary network still existed in the GBE sample due to the formation of grain clusters; (III) the GBE sample exhibited a higher resistance to IGSCC; and (IV) as the twin boundary fraction increased, the number of TRBPs decreased and the normalized length of the minimum TRBP increased monotonically, leading to a higher resistance to IGSCC.

Three-dimensional study of twin boundaries in conventional and grain boundary-engineered 316L stainless steels

Abstract

Three-dimensional geometrical and topological characteristics of grains in conventional and grain boundary engineered 316L stainless steel

The three-dimensional microstructures of a conventional 316L stainless steel and the same material after grain boundary (GB) engineering have been measured by serial sectioning coupled with electron backscatter diffraction mapping. While it is well known that GB engineered materials are differentiated from conventional materials because of the proportion of coincidence site lattice boundaries, the size of their twin-related domains, and their reduced random boundary connectivity, this work provides a quantitative comparison of the geometrical and topological characteristics of grains in 316L stainless steel before and after GB engineering. Specifically, the numbers of grain faces, triple lines, and quadruple unions per grain have been measured and compared. In addition, the distributions of grain sizes, surface areas, and grain boundary areas have been measured and compared. The results show that, in many ways, the three-dimensional geometrical and topological characteristics of the grains in the GB engineered and conventional materials are similar. In both materials, the distributions of the geometrical parameters are well represented by a log-normal distribution. Comparatively, the GB engineered microstructure has grains that, on average, have both fewer faces and higher (specific) surface areas that deviate more from an ideal equiaxed shape, but there are several eccentric or non-compact shaped grains that have a huge number of faces and extremely large surface area in the GB engineered material. All of these characteristics are likely to be a result of the increased number of twins in the GB engineered microstructure. These eccentric grains would have a positive influence on increasing the resistance to intergranular degradation.

A critical evaluation on efficacy of recrystallization vs. strain induced boundary migration in achieving grain boundary engineered microstructure in a Ni-base superalloy

A critical evaluation of recrystallization vs. strain induced boundary migration (SIBM) as the mechanism responsible behind the grain boundary engineering (GBE) in alloy 617 is made. Towards this hot deformation processing (in the temperature range of 1173–1473 K and strain rates of 0.001–10 s−1) as well as GBE-type iterative processing is performed on solution-annealed specimen. GBE-quantifying parameters such as Σ3n fraction, triple junction distribution, twin-related grain size ratio, twin-related domain (TRD) parameters and fractal dimension have been utilized to quantify the extent of GBE in the processed microstructures. Occurrence of dynamic recrystallization (DRX) during hot deformation processing does not induce notable multiple twinning leading to a microstructure analogous to that of solution-annealed condition even after complete DRX. Following GBE-type iterative processing, GBE microstructure has only been achieved when prolific multiple twinning occurred due to activation of SIBM leading to a high fraction of Σ3n boundaries (∼79%) with most of the twins being part of the grain boundary network. Consequently, an increasing proportion of J2 and J3 triple junctions with a concomitant increase in both the average number of grains per TRD and twin-related grain size ratio are achieved. However, occurrence of static recrystallization during GBE-type processing has led to smaller TRD size and copious random boundaries connectivity as confirmed by fractal analysis thus clearly suggesting that SIBM, not recrystallization catalyzes the profusion of Σ3n boundaries. As GBE relies on the multiple twinning mechanisms promoted via SIBM, recrystallization needs to be circumvented for successful attainment of GBE microstructure.

Control of grain boundary connectivity based on fractal analysis for improvement of intergranular corrosion resistance in SUS316L austenitic stainless steel

The connectivity of high energy random boundaries was investigated on the basis of the fractal analyses of grain boundary microstructures in SUS316L stainless steel, to prove the usefulness of a refined approach to grain boundary engineering (GBE) for more precise prediction and control of intergranular corrosion in polycrystalline materials. It was found that the maximum connectivity for random boundary network, termed the maximum random boundary connectivity (MRBC) had a fractal nature in the studied specimens of SUS316L stainless steel. The fractal dimension of MRBC tended to decrease with decreasing fraction of random boundaries, or in other words with increasing fraction of low-energy low-Σ coincidence site lattice (CSL) boundaries. The lower coefficient of variation of grain size distribution suggesting a more homogeneous grain structure, was found to result in the lower fractal dimension of MRBC for the specimens with a similar grain boundary character distribution (GBCD). The optimum grain boundary microstructure for enhanced intergranular corrosion resistance in the SUS316L stainless steel was discussed based on the results from the fractal analyses of MRBC for different grain boundary microstructures.

In situ observations of crack propagation and role of grain boundary microstructure in nickel embrittled by sulfur

In situ observations of crack propagation in sulfur-doped coarse-grained nickel were performed for the specimens with grain boundary microstructure pre-determined by SEM/EBSD analysis. The role of grain boundary microstructure was studied in the crack propagation in nickel embrittled by grain boundary segregation of sulfur. It was found that the main crack tends to predominantly propagate along random boundaries, and the crack propagation rate can be locally accelerated at the grain boundary network with a high connectivity of random boundaries. On the other hand, the cracks can propagated along fracture-resistant low-Σ coincidence site lattice (CSL) boundary only when the trace of the grain boundary is arranged being almost parallel to slip bands in the adjacent grains. The local crack propagation rate was found to become lower when a crack propagated along low-Σ CSL boundaries. Moreover, when the crack propagation is inhibited by low-Σ CSL boundaries, the branching of propagating crack occurs at partially cracked triple junctions. The crack propagation can locally slow down due to the occurrence of crack branching. The optimum grain boundary microstructure for the control of sulfur segregation-induced brittle fracture is discussed on the basis of new findings obtained from the in situ observations on crack propagation and fracture processes in polycrystalline nickel.

Grain boundary engineering based on fractal analysis for control of segregation-induced intergranular brittle fracture in polycrystalline nickel

The effect of grain boundary microstructure on the fracture resistance of sulfur-doped polycrystalline nickel was investigated using specimens with different grain boundary microstructures to reveal the usefulness of grain boundary engineering for control of segregation-induced intergranular brittle fracture of polycrystalline materials. The sulfur-doped polycrystalline nickel specimen with more homogeneous fine-grained structure and a higher fraction of low-Σ coincidence site lattice (CSL) boundaries shows higher fracture resistance than the specimen with coarse-grained structure and a lower fraction of low-Σ CSL boundaries. It was found that high-energy random boundaries play a key role as the preferential crack path in fracture processes. The resistance to sulfur segregation-induced intergranular brittle fracture was evaluated by analyzing the fractal dimension of random boundary connectivity in the polycrystalline nickel specimens studied. The fractal dimension of random boundary connectivity decreases with increasing fraction of low-Σ CSL boundaries, resulting in the generation of a higher fracture resistance by restricting more frequent branching and deflection of propagating crack path along random boundaries from the main crack.

1314 硫黄が偏析した多結晶ニッケルのき裂進展に及ぼす粒界微細組織の影響(G02-1 金属の機械的特性と樹脂成形不良,G02 機械材料・材料加工部門)

The effect of grain boundary microstructure such as grain boundary connectivity and spatial distribution of the grain boundaries on propagation path in sulfur-doped nickel was investigated to reveal the utility of grain boundary engineering for control of fracture resistance to impurity segregation-induced embrittlement in polycrystalline materials. Crack predominantly propagated along high energy random boundaries. Thus, the deflection of crack propagation path strongly depended on the connectivity of random boundaries associated with grain size and shape. Crack propagation rate was found to decrease and branching of cracks occurred when the front of crack reached to low-E coincidence site lattice boundaries.

One-step and iterative thermo-mechanical treatments to enhance Σ3 n boundaries in 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 Σ3 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 Σ3 n boundaries was observed after annealing at 1,173 K for 0.5 to 2 h. Prolific multiple twinning with minimum deviation of Σ3 and Σ9 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 Σ3-Σ3-Σ9/Σ3-Σ9-Σ27 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 Σ3 boundaries. The Σ3 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 Σ3 n boundaries (~73%) in alloy D9.

Correlation of grain boundary connectivity with grain boundary character distribution in austenitic stainless steel

The connectivity of grain boundaries can be an important microstructural parameter governing the bulk properties of polycrystalline materials as well as the grain boundary character distribution. On the basis of the percolation theory, the connectivity of random boundaries that are susceptible to percolative phenomena like corrosion in a polycrystal has been quantitatively evaluated and examined in connection with the grain boundary character distribution in austenitic stainless steel. The percolation threshold has been found to occur at a fraction of approximately 70 ± 5% coincident site lattice (CSL) boundaries (30 ± 5% random boundaries). The maximum random boundary cluster length drastically decreases and the frequency of resistant triple junctions increases with increasing frequency of the CSL boundaries near the percolation threshold. An increase in the frequency of resistant triple junctions can enhance corrosion resistance of polycrystalline austenitic stainless steel even if the grain boundary character distribution is the same.

A study on the effect of cold rolling and annealing on special grain boundary fractions in commercial-purity nickel

It has been known for decades that thermomechanical processing, through cold working and annealing can effectively increase the fraction of special low energy grain boundaries in many FCC metals. The focus of the reported work was to study the effects of cold rolling and annealing processing parameters on both the fraction of special boundaries and the random boundary connectivity in commercially pure nickel. It was found that by using relatively low strain levels combined with high temperature anneals, it was possible to increase the fraction of special boundaries to 65% and severely fragment the random boundary connectivity.