S. M. Bruemmer

Study of grain boundary character along intergranular stress corrosion crack paths in austenitic alloys

Abstract Samples of austenitic stainless alloys were examined by means of scanning and transmission electron microscopy. Misorientations were measured by electron backscattered diffraction. Grain boundary distributions were analyzed with special emphasis on the grain boundary character along intergranular stress corrosion cracks and at crack arrest points. It was established that only coherent twin Σ3 boundaries could be considered as “special” ones with regard to crack resistance. However, it is possible that twin interactions with random grain boundaries may inhibit crack propagation. The results suggest that other factors besides geometrical ones play an important role in the intergranular stress corrosion cracking of commercial alloys.

Evolution of fine-scale defects in stainless steels neutron-irradiated at 275 C

Six austenitic stainless steel heats (three heats each of 304SS and 316SS) neutron-irradiated at 275 °C from 0.6 to 13.3 dpa have been carefully characterized by TEM and their hardness measured as a function of dose. The characterization revealed that the microstructure is dominated by a very high density of small Frank loops present in sizes as small as 1 nm and perhaps lower, which could be of both vacancy and interstitial-type. Frank loop density saturated at the lowest doses characterized, whereas the Frank loop size distributions changed with increasing dose from an initially narrow, symmetric shape to a broader, asymmetric shape. Although substantial hardening is caused by the small defects, a simple correlation between hardness changes and density and size of defects does not exist. These results indicate that radiation-induced segregation to the Frank loops could play a role in both defect evolution and hardening response.

Influence of the particle size on recrystallization and grain growth in Al-Mg-X alloys

Abstract Recrystallization and grain growth studies of Al-Mg-X (where X is Mn, Zr or Sc) alloys have been carried out using a combination of optical and transmission electron microscopy of interrupted heating tests, changes in hardness, and in-situ observations in the transmission electron microscope. Nucleation sites have been identified as typically large (> 1 μm diameter) eutectic constituent particles, but also occasionally clusters of sub-micron dispersoids. By altering the density and size of the Mn-, Sc- and Zr-based particles with selected thermomechanical treatments, recrystallization could be suppressed up to temperatures near the melting point. These results can be explained by examination of the stored energy from cold work and the particle-substructure interactions due to the precipitate morphology. Implications of these results for grain size control in Al-Mg based alloys will be discussed.

Lattice defect/grain boundary interactions related to IASCC

Abstract Radiation-induced segregation (RIS) of major alloying elements to grain boundaries in austenitic stainless steels has emerged as a critical aspect of irradiation-assisted stress corrosion cracking (IASCC). Discriminating interactions between individual solute species and vacancy and interstitial defects as they migrate to grain boundaries result in redistribution of solute. Measurements of grain boundary Ni enrichment and Cr depletion indicate that RIS of major alloying elements is in reasonable agreement with the inverse-Kirkendall mechanism. The discriminating interactions for inverse-Kirkendal segregation are the relative rates of solute diffusion by vacancy exchange. Mechanistically, the ternary composition path, defined by change in Cr relative to Ni, depends on relative diffusivities. The absolute change in the composition, that is, extent along the composition path, depends on the kinetics of vacancy formation and migration. The composition path approach is used to quantify diffusional characteristics at low temperatures. Lastly, model predictions suggest a significant influence of grain boundary defect characteristics in addition to matrix defect characteristics. These grain-boundary sensitive characteristics may influence IASCC.

Formation of partial dislocations during intersection of glide dislocations with Frank loops in f.c.c. lattices

Interactions of moving dislocations with Frank loops in quenched or irradiated materials can result in either coalescence or intersection reactions. The latter lead to the formation of partial dislocations associated with the resulting half loops. Alloy stacking fault energy (SFE) and specific deformation conditions are shown to control the reaction of the partial dislocations with the half loops. Based on this analysis, a partial dislocation mechanism is proposed that allows explanation of partial unfaulting, stacking fault extension, and twin formation, previously observed during plastic deformation of irradiated austenitic stainless steel.

Evidence for excess vacancies at sliding grain boundaries during superplastic deformation

Rapid quenching of Al-Mg alloys during superplastic deformation has revealed the presence of nano-scale cavities along many grain boundaries. They were observed only under deformation conditions where grain boundary sliding was the dominant mechanism. Fine-probe compositional measurements revealed that the cavity surface is enriched in Mg, and in situ heating in the transmission electron microscope demonstrated that they are not stable above 175 C. Kinetic analysis of cavity formation during a quench concludes that the cavities did not exist during deformation but were formed as the sample cooled. It is proposed that these cavities are evidence for a localized excess of vacancies during grain boundary sliding.

Defect Microstructures and Deformation Mechanisms in Irradiated Austenitic Stainless Steels

Microstructural evolution and deformation behavior of austenitic stainless steels are evaluated for neutron, heavy-ion and proton irradiated materials. Radiation hardening in austenitic stainless steels is shown to result from the evolution of small interstitial dislocation loops during lightwater-reactor (LWR) irradiation. Available data on stainless steels irradiated under LWR conditions have been analyzed and microstructural characteristics assessed for the critical fluence range (0.5 to 10 dpa) where irradiation-assisted stress corrosion cracking susceptibility is observed. Heavy-ion and proton irradiations are used to produce similar defect microstructures enabling the investigation of hardening and deformation mechanisms. Scanning electron, atomic force and transmission electron microscopies are employed to examine tensile test strain rate and temperature effects on deformation characteristics. Dislocation loop microstructures are found to promote inhomogeneous planar deformation within the matrix and regularly spaced steps at the surface during plastic deformation. Twinning is the dominant deformation mechanism at rapid strain rates and at low temperatures, while dislocation channeling is favored at slower strain rates and at higher temperatures. Both mechanisms produce highly localized deformation and large surface slip steps. Channeling, in particular, is capable of creating extensive dislocation pileups and high stresses at internal grain boundaries which may promote intergranular cracking.

Grain Boundary Composition and Effects on Environmental Degradation

Environmental degradation of metallic alloys is shown to depend on its local microstructure and microchemistry in many instances. Equilibrium and non-equilibrium processes can produce significant changes to the grain boundary composition and promote intergranular stress corrosion cracking (IGSCC). Austenitic stainless steels are used as an example to illustrate equilibrium effects of Gibbsian segregation and second-phase precipitation as well as non-equilibrium effects of quench-induced and radiation-induced segregation. Interfacial Cr concentration is shown to be the dominant material variable promoting IGSCC of austenitic stainless steels in oxidizing environments. Cracking susceptibility is a direct function of the boundary Cr content regardless of depletion width. However, grain boundary Cr depletion does not adequately explain IGSCC in stainless steels strengthened by cold work or neutron irradiation. Significant interfacial enrichment of Cr, Mo and B are often present in annealed stainless steels which may play a role in the IGSCC of cold-worked materials and delay IASCC to higher radiation doses. Impurity segregants (e.g., P) can promote IG hydrogen-induced cracking but do not have a strong detrimental effect on cracking in high-temperature water environments.

Pinning effect of solute atoms on grain boundary dislocation dissociation

The dissociation of extrinsic grain boundary dislocations (EGBDs) in Al, Al-Mg and Al-Mg-Sn alloys was studied at various temperatures. While EGBDs were commonly observed in low- and high-angle boundaries in the alloy materials deformed to small strains, they could only be seen in low-angle grain boundaries in Al. Annealing of the lightly deformed alloy samples at 100 or 150°C led to the disappearance of the EGBDs with critical disappearance kinetics depending on grain boundary composition. A modified EGBD dissociation mechanism incorporating a solute drag effect has been developed in this work. Predictions based on the model agree reasonably well with the observed thermal behavior of the EGBDs in the three Al alloys.

Deformation microstructures in ion-irradiated stainless steel

Although radiation damage in stainless steel has been studied extensively, the primary focus has been on microstructural development and welling in alloys irradiated to high dose and elevated temperatures. Considerably less effort has been directed to stainless steel irradiated to low doses at low temperatures. In particular, deformation of stainless steel, irradiated to low dose levels (<10 dpa) at lower temperatures (<300 C), has received relatively little attention. Earlier work has shown that the microstructure of stainless steel, neutron irradiated at temperatures <300 C, is comprised of a high density of small dislocation loops. Deformation bands were observed in material deformed at ambient temperature but no analysis of the bands was made. As part of a larger program to understand the underlying causes for irradiation assisted, stress-corrosion cracking (IASCC), the deformation of irradiated stainless steel is being evaluated. In this study, ion irradiation is used to simulate the microstructure expected after neutron irradiation to dose levels [<=] 10 dpa at temperatures near 300 C.