Precipitate Redistribution During Creep of Alloy 617

Nickel-based superalloys are being considered for applications within advanced nuclear power generation systems due to their high temperature strength and corrosion resistance. Alloy 617, a candidate for use in heat exchangers, derives its strength from both solid solution strengthening and the precipitation of carbide particles. However, during creep, carbides that are supposed to retard grain boundary motion are found to dissolve and re-precipitate on boundaries in tension. To quantify the redistribution, we have used electron backscatter diffraction and energy dispersive spectroscopy to analyze the microstructure of 617 after creep testing at 900 and 1000°C. The data were analyzed with respect to location of the carbides (e.g., intergranular vs. intragranular), grain boundary character, and precipitate type (i.e., Cr-rich or Mo-rich). We find that grain boundary character is the most important factor in carbide distribution; some evidence of preferential distribution to boundaries in tension is also observed at higher applied stresses. Finally, the results suggest that the observed redistribution is due to the migration of carbides to the boundaries and not the migration of boundaries to the precipitates

Influence of Grain Boundary Character on Creep Void Formation in Alloy 617

Alloy 617, a high temperature creep-resistant, nickel-based alloy, is being considered for the primary heat exchanger for the Next Generation Nuclear Plant (NGNP) which will operate at temperatures exceeding 760oC. Orientation imaging microscopy (OIM) is used to characterize the grain boundaries in the vicinity of creep voids that develop during high temperature creep tests (800-1000oC at creep stresses ranging from 20-85 MPa) terminated at creep strains ranging from 5-40%. Observations using optical microscopy indicate creep rate does not significantly influence the creep void fraction at a given creep strain. Preliminary analysis of the OIM data indicates voids tend to form on grain boundaries parallel, perpendicular or 45o to the tensile axis, while few voids are found at intermediate inclinations to the tensile axis. Random grain boundaries intersect most voids while CSL-related grain boundaries did not appear to be consistently associated with void development

Modification of Sensitization Resistance of AISI 304L Stainless Steel through Changes in Grain Size and Grain Boundary Character Distributions

Sensitization behavior of thermomechanically processed AISI 304L stainless steel has been investigated. The mechanical processing was carried out at deformations of 30 to 90 pct (reduction in thickness), and annealed subsequently at temperatures ranging from 800 °C to 950 °C for 15 to 60 minutes. Stainless steel was then sensitized at 675 °C both for short (2 hours) and long (53 hours) durations. Thus treated specimens were characterized for grain boundary character distribution, grain size, and degree of sensitization (DOS). The increase in annealing temperature and time following mechanical processing showed an increase in grain size (up to 37 μm) and in the DOS. The fraction of coincident site lattice (CSL) boundaries (Σ3 to Σ29) was also noticed to increase with the annealing temperature, which implied that an increasing fraction of low energy boundaries did not cause a decrease in the DOS. The grain size through its effect on grain boundary surface area and the effective grain boundary energy correlated well with the extent of sensitization. Grain growth reduces the grain boundary surface area and the effective grain boundary energy as well, which, in turn, enhanced the DOS. A critical grain size (∼0.05), above which, sensitization reduced to insignificant levels was observed

The role of Σ9 boundaries in grain boundary engineering

In a grain boundary engineered (GBE) microstructure Σ9 boundaries are the second most abundant boundary type. This paper presents data showing that in GBE copper most Σ9s were special. Σ9 boundaries are also a geometrically necessary component of a GBE microstructure. It is suggested that there are competing requirements for Σ9s: during iterative GBE processing they are required to be mobile, whereas afterwards it is desirable that they are special boundaries