Damage Susceptibility of Grain Boundaries in HT9 Steel Subjected to High-Temperature Creep

HT9 steel is an attractive ferritic/martensitic steel that is used in components of nuclear and fossil power plants because of its high strength and good swelling resistance. Specific phenomena (such as segregation, voiding, cracking, etc.) are prevalent along grain boundaries since these interfaces act as efficient sources for vacancies. The accumulation of vacancies in grain boundaries may result in intergranular fracture. In this study, HT9 steel was subjected to creep tests at elevated temperature (about 0.5 T _m) and two different creep conditions (where creep lifetimes were about 100 and about 1000?hours, respectively). The grain boundaries in HT9 steel after creep tests were studied by the use of scanning electron microscopy in order to establish the relationship between the grain boundary structure and creep damage. Images and data obtained using electron backscatter diffraction reveal a high susceptibility of high-angle boundaries to creep cavitation, as expected. In addition, the ??3 boundaries are also susceptible to damage under these conditions at a similar or even higher rate as compared with random high-angle boundaries.     

Characterization of Creep-Damaged Grain Boundaries of Alloy 617

Intergranular cracking and void nucleation occur over extended periods of time in alloy 617 when subjected to stress at high temperatures. Damage occurs inhomogeneously with some boundaries suffering failure, while others are seemingly immune to creep. Crack propagation associated with grain size, and grain boundary character was investigated to determine which types of grain boundaries are susceptible to damage and which are more resistant. Electron backscatter diffraction and a stereological approach to obtain the five-parameter grain boundary distribution were used to measure the proportions of each type of boundary in the initial and damaged structures. The samples were crept at 1273.15 K (1000 °C) at 25 MPa until fracture. It was found that in addition to low-angle and coherent twin boundaries, other low index boundary plane grain boundaries with twist character are relatively resistant to creep.     

The Role of grain boundary misorientation in intergranular cracking of Ni-16Cr-9Fe in 360 °C argon and high-Purity water

The effect of grain boundary misorientation on the intergranular cracking behavior of pure Ni-16Cr-9Fe was assessed by determining if low-angle boundaries (LABs) or coincident site lattice boundaries (CSLBs) are more crack resistant than general high-angle boundaries (GHABs) in argon and high-purity water. Cracking susceptibility of boundary types was determined using constant extension rate tensile tests (CERTs) in 360 °C argon and in deaerated, high-purity water. Annealed samples contained 12 to 20 pct CSLBs, while CSLB-enhanced samples contained 27 to 44 pct CSLBs; GHAB proportions varied accordingly. Cracked boundary fractions for CSLB-enhanced samples tested in either environment ranged from 0.01 to 0.08, while those for annealed samples ranged from 0.07 to 0.10, indicating that samples with increased proportions of CSLBs are more crack resistant. No LABs cracked in either environment. In annealed samples, the proportion of CSLBs that cracked in water was 6.7 pct compared to 1.5 pct in argon; the proportion of GHABs that cracked in water was 9.3 pct compared to 6.6 pct for argon. Thus, CSLBs are more crack resistant than GHABs in either environment, and both are more crack resistant in argon than in water. The higher amounts of cracking and the higher CSLB cracking susceptibility in high-purity water indicate the presence of an environmental effect on cracking behavior. The beneficial effect of LABs and CSLBs is likely due to the ability of these boundaries to induce slip in neighboring grains by either transmitting or absorbing and re-emitting lattice dislocations, thereby reducing grain boundary stresses and the propensity for crack initiation. The results indicate that control of grain boundary proportions can improve the intergranular stress corrosion cracking susceptibility of pure Ni-16Cr-9Fe. Formerly Graduate Research Assistant, The University of Michigan.     

On the Role of Grain Boundary Serration in Simulated Weld Heat-Affected Zone Liquation of a Wrought Nickel-Based Superalloy

The effects of grain boundary serration on boron segregation and liquation cracking behavior in a simulated weld heat-affected zone (HAZ) of a wrought nickel-based superalloy 263 have been investigated. The serrated grain boundaries formed by the developed heat treatment were highly resistant to boron segregation; the serrated sample contained 41.6 pct grain boundaries resistant to boron enrichment as compared with 14.6 pct in the unserrated sample. During weld thermal cycle simulation, liquated grain boundaries enriched with boron were observed at the peak temperature higher than 1333 K (1060 °C) in both unserrated and serrated samples; however, serrated grain boundaries exhibited a higher resistance to liquation. The primary cause of liquation in this alloy was associated with the segregation of the melting point depressing element boron at grain boundaries. The hot ductility testing result indicated that the serrated grain boundaries showed a lower susceptibility to liquation cracking; the grain boundary serration led to an approximate 15 K decrease in the brittle temperature range. These results reflect closely a significant decrease in interfacial energy as well as a grain boundary configuration change by the serration.     

Influence of cold work and recovery on the strength and toughness of iron

In this work some of the structures typical of those found in thermomechanically processed steels were reproduced by cold work, cold work and recovery, and recrystallization treatments of vacuum-melted iron single-crystals and polycrystals. The mechanical properties of the microstructural features such as subgrain formation, texture development, and grain elongation were recorded. It was shown that although the dislocation subboundaries produced on recovery add an increment of strength to that produced by grain boundaries, they are less effective strengtheners than high-angle grain boundaries. Further, the data suggests that yield strength is related not only to subgrain size but also to the angle of misfit of the subgrain boundaries. Although strength increased with subgrain boundary formation, toughness remained constant. Consequently, the introduction of subgrain boundaries offers a means of improving strength while maintaining toughness. The ductile-to-brittle transition temperature of cold-worked as well as cold-worked and recovered polycrystalline iron varied with specimen orientation relative to the direction of deformation. These variations were primarily a function of the anisotropy of grain dimension that is produced by cold deformation. Toughness was not influenced by the preferred orientations produced by the various processing techniques.     

Effect of initial microstructure on microstructural instability and creep resistance of XD TiAl alloys

A number of lamellar structures were produced in XD TiAl alloys (Ti-45 at. pct and 47 at. pct Al-2 at. pct Nb-2 at. pct Mn+0.8 vol pct TiB₂) by selected heat treatments. During creep deformation, microstructural degradation of the lamellar structure was characterized by coarsening and spheroidization, resulting in the formation of fine globular structures at the grain boundaries. Grain boundary sliding (GBS) was thought to occur in local grains with a fine grain size, further accelerating the microstructural degradation and increasing the creep rate. The initial microstructural features had a great effect on microstructural instability and creep resistance. Large amounts of equiaxed γ grains hastened dynamic recrystallization, and the presence of fine lamellae increased the susceptibility to deformation-induced spheroidization. However, the coarsening and spheroidization were suppressed by stabilization treatments, resulting in better creep resistance than the microstructures without these treatments. Furthermore, well-interlocked grain boundaries with lamellar incursions were effective in restraining the onset of GBS and microstructural degradation. In the microstructures with smooth grain boundaries, a fine lamellar spacing significantly lowered the minimum creep rate but rapidly increased the tertiary creep rate for the 45 XD alloy. For the 47 XD alloy, well-interlocked grain boundaries dramatically improved the creep resistance of nearly and fully lamellar (FL) structures, in spite of the presence of coarse lamellar spacing or equiaxed γ grains. However, it may not be feasible to produce a microstructure with both a fine lamellar spacing and well-interlocked grain boundaries. If that is the case, it is suggested that the latter feature is more beneficial for creep resistance in XD TiAl alloys with relatively fine grains.     

Microstructure development during hot deformation of aluminum to large strains

During deformation, the original grains change their shape and the surface area per unit volume increases with strain until a certain critical strain has been reached. The structure of high-angle boundaries has been monitored at increasing strains with the aim of finding the effect of grain breakup and strain-induced boundary migration. It has been found that the distance between the high-angle boundaries does not depend only on geometrical considerations. At high Zener-Hollomon parameters, the distance between the high-angle boundaries was found to be smaller than predicted from geometry, indicating that high-angle boundaries are formed during deformation. In the case of deformation at very low Zener-Hollomon parameters, the distance between the high-angle boundaries was found to be larger than predicted from geometry, which indicates migration of the original grain boundaries in a direction opposite to the one imposed by the deformation. The evolution in grain-boundary structure during deformation has been successfully modeled on the basis of expressions for the grain breakup and restoration reactions.     

Ultrafine-Grained Al-Mg-Sc Alloy via Friction-Stir Processing

Friction-stir processing (FSP) of twin-roll cast (TRC) Al-Mg-Sc alloy resulted into ultrafine-grained microstructure. The alloy was processed in as-received and aged (563 K [290 °C], 22 hours) conditions and at three different tool rotation rates: 800, 400, and 325 rpm. The microstructural features were characterized using electron backscattered diffraction (EBSD), scanning electron microscopy (SEM), and transmission electron microscopy (TEM). The grain size varied from 0.89 μm to 0.39 μm depending on the processing and initial thermo-mechanical conditions of the alloy. The TRC alloy processed at 325 rpm in aged condition had all the grains less than 1 μm, and 95 pct of grains had high-angle grain boundaries (HAGBs). In all the cases, the fraction of HAGBs were more than 80 pct. The variation of misorientation angle distribution was similar to the theoretical MacKenzie distribution for cubic crystal materials. Grain size analysis at different sections and locations on the transverse section of the dynamically recrystallized zone showed a homogeneous and equiaxed microstructure. The average dispersoid (Al₃(Sc,Zr)) size was ~8.0 nm in diameter obtained using high-resolution TEM. Grain size reduction was observed with increase in Zener–Hollomon parameter. It was shown that under the current microstructural and deformation conditions, dynamic recrystallization via particle-stimulated nucleation might not be possible during FSP.     

Frequent Occurrence of Discontinuous Dynamic Recrystallization in Ti-6Al-4V Alloy with α′ Martensite Starting Microstructure

The microstructural conversion mechanism in an α′ martensite starting microstructure during hot deformation (at 973 K (700 °C)-10 s^?1) of the Ti-6Al-4V alloy is studied through detailed microstructural observations, kinetic analysis of deformation in the microstructure, and various theoretical models. After compressing the α′ starting microstructure at 973 K (700 °C)-10 s^?1 and at a height strain of 0.8, it is observed that the α′ starting microstructure with acicular morphology evolved into an ultrafine-grained microstructure with an average grain size of 0.2 μm and a high fraction of high-angle grain boundaries. At the initial stage of deformation, subgrain formation in martensite variants and the formation of new grains with high-angle boundaries at interfaces of martensite variants, and $ \{ 10\bar{1}1\} $ twins are dominant. On increasing the height strain to 0.8, discontinuous dynamic recrystallization (DDRX) along with heterogeneous nucleation and fragmentation of grains with high-angle boundaries becomes dominant. In contrast, in the case of an (α + β) starting microstructure, continuous dynamic recrystallization (CDRX) is dominant throughout the deformation process. Thus, we found that DDRX becomes dominant by changing the starting microstructure from the conventional (α + β) to the acicular α′ martensite one. This behavior of the α′ martensite microstructure is attributed to the considerable number of nucleation sites such as dislocations, interfaces of martensite variants and $ \{ 10\bar{1}1\} $ twins, and the high-speed grain fragmentation along with subgrain formation in the α′ starting microstructure during the initial stage of deformation.     

Heterogeneous Creep Deformations and Correlation to Microstructures in Fe-30Cr-3Al Alloys Strengthened by an Fe<Subscript>2</Subscript>Nb Laves Phase

A new Fe-Cr-Al (FCA) alloy system has been developed with good oxidation resistance and creep strength at high temperature. The alloy system is a candidate for use in future fossil-fueled power plants. The creep strength of these alloys at 973 K (700 °C) was found to be comparable with traditional 9 pct Cr ferritic–martensitic steels. A few FCA alloys with general composition of Fe-30Cr-3Al-.2Si-xNb (x = 0, 1, or 2) with a ferrite matrix and Fe₂Nb-type Laves precipitates were prepared. The detailed microstructural characterization of samples, before and after creep rupture testing, indicated precipitation of the Laves phase within the matrix, Laves phase at the grain boundaries, and a 0.5 to 1.5 μm wide precipitate-free zone (PFZ) parallel to all the grain boundaries. In these alloys, the areal fraction of grain boundary Laves phase and the width of the PFZ controlled the cavitation nucleation and eventual grain boundary ductile failure. A phenomenological model was used to compare the creep strain rates controlled by the effects of the particles on the dislocations within the grain and at grain boundaries. (The research sponsored by US-DOE, Office of Fossil Energy, the Crosscutting Research Program).     

Effect of boron segregation at grain boundaries on heat-affected zone cracking in wrought INCONEL 718

Susceptibility to heat-affected zone (HAZ) cracking during electron-beam welding was studied in two INCONEL 718-based alloys doped with different levels of boron. By lowering the carbon, sulfur, and phosphorous concentrations to be “as low as possible,” the occurrence of HAZ cracking was related directly to the level of segregation of boron at grain boundaries, which occurred by nonequilibrium segregation during a preweld heat treatment. The study has demonstrated a direct correlation between the amount of boron segregated at grain boundaries and their susceptibility to HAZ cracking, in terms of the total crack length and number of cracks observed in the HAZ. The analysis of results suggests that both the melting and resolidification temperatures of the boron-segregated grain boundaries can be about 100 °C to 200 °C lower than those of the grain boundaries that were susceptible to constitutional liquation of Nb carbides on them, making boron more deleterious in causing HAZ cracking.     

Evolution of Grain Boundary Precipitates in Al 7075 Upon Aging and Correlation with Stress Corrosion Cracking Behavior

Transmission electron microscopy (TEM) was employed to investigate the microchemistry and microstructure of grain boundary precipitates in Al 7075 aged at room temperature for several hours, at 393 K (120 °C) for 12 hours (under aged), at peak aged (T651) and over aged (T73) conditions. High resolution TEM analysis of precipitates at grain boundaries and fine probe energy dispersive spectrometry showed that the grain boundary precipitates at peak and over aged conditions are hexagonal η phase with stoichiometry Mg(Cu _x Zn_1?x )₂. Considerable increase in Cu content in the grain boundary η in the over aged condition compared to the peak aged condition was observed. The average Cu content in the over aged condition was found to be 20 at. pct. The higher Cu content of the precipitate is associated with a lower stress corrosion cracking plateau velocity.     

Effect of homogenization heat treatment on the microstructure and heat- affected zone microfissuring in welded cast alloy 718

The effect of homogenization temperature on microfissuring in the heat-affected zones of electronwelded cast INCONEL 718 has been studied. The material was homogenized at various temperatures in the range of 1037 ° to 1163 ° and air-cooled. The homogenized material was then electron-beam welded by the bead-on-plate welding technique. The microstructures and microfissuring in the heat-affected zone (HAZ) were evaluated by analytical scanning electron microscopy (SEM). The grain boundary segregation of various elements was evaluated by secondary ion mass spectroscopy (SIMS). It was observed that the total crack length (TCL) of microfissures first decreases with homogenization temperature and then increases, with a minimum occurring in the specimen heat treated at 1163 °. This trend coincides with the variation in segregation of B at grain boundaries with homogenization temperature and has been explained by equilibrium and nonequilibrium segregation of B to grain boundaries during the homogenization heat treatment. No other element was observed to segregate at the grain boundaries. The variation in volume fraction of phases like δ-Ni₃Nb, MC carbide, and Laves phases does not follow the same trend as that observed for TCL and B segregation at the grain boundaries. Therefore, microfissuring in HAZ of welded cast INCONEL 718 is attributed to the segregation of B at the grain boundaries.     

Embrittlement of copper due to segregation of oxygen to grain boundaries

The tensile and creep properties of oxygen free OF- and oxygen saturated OS-polycrystalline copper have been investigated in the temperature range 25 to 500 °C. Oxygen increases the yield strength by a factor of 2 or 3, by solid solution hardening, and for coarse-grained copper, causes a severe embrittlement, particularly under creep conditions. Both direct and indirect evidence indicate that the embrittlement is caused by the segregation of oxygen to the grain boundaries in copper, thus promoting grain boundary decohesion and intergranular fracture. Auger electron spectroscopy is used to indicate the presence of oxygen at the grain boundaries in OS-Cu. The embrittling effects of oxygen are reversible in the sense that both tensile and creep ductility are restored when oxygen is removed.     

Cooperative phenomena at grain boundaries during superplastic flow

In situ observations in a scanning electron microscope (SEM) performed on different microstructural scales in Pb—62%Sn specimens, superplastically deformed in single shear and in simple tension, showed sliding of grain groups [cooperative grain boundary sliding (CGBS)], rotation of grain groups (cooperative grain rotation) and cooperative grain boundary migration (correlated migration of sliding grain boundaries). The observed macroscopic pattern of the CGBS surfaces is consistent with predictions of slip-lines field theory. The progress of the sliding of grain blocks at the scale of grain groups can be modeled in terms of cellular dislocations. The micromechanism for such sliding and the migration of sliding grain boundaries at the scale of the individual interface might be interpreted from the viewpoint of grain boundary dislocations.     

The effects of microstructure,strength level,and crack propagation mode on stress corrosion cracking behavior of 4135 steel

The stress corrosion cracking (SCC) susceptibility of 4135 steel in a simulated sea water solution has been analyzed in an attempt to understand the effect that microstructural changes associated with the corresponding changes in strength level have on both intergranular (IG) and transgranular (TG) crack propagation modes. After a selection of heat treatments, the following different microstructural variables were studied: the effect of grain size on IG fracture processes; the influence of the grade of tempering on the SCC resistance and crack propagation mode; and the effect of type and content of bainite and the effect of ferrite in mixed microstructures. A global analysis shows that the typical SCC resistance-strength level inverse relationship can only be applied when the microstructure re-mains invariable. An important microstructural control of SCC behavior was found for TG processes at moderate and low strength levels. The data analysis showed the following: a beneficial effect of increasing the grain size when crack propagates at grain boundaries without precipitates; the existence of a critical tempering temperature so that a sudden IG-TG change happens without any apparent relation to microstructural changes; the beneficial effect of bainite presence as a substitute for mar-tensite and high SCC resistance of structures containing over 50 pct ferrite, associated with their low strength levels.     

The influence of sulfur on stress-rupture fracture in inconel 718 superalloys

The effects of sulfur, with content variations of 15 to 175 ppm, on the stress-rupture and tensile properties in nickel-base alloy 718 are reported. The stress-rupture life dramatically decreased with increasing sulfur content. This was especially noticeable in the ductility loss at 650 °C. Auger electron spectroscopy of stress-rupture tested specimens provided direct evidence of sulfur and phosphorus segregation to grain boundaries and carbide/matrix interfaces. The stress-rupture life and fracture morphology were both found to correlate with the segregation of sulfur at grain boundaries in alloy 718. Sulfur was also preferentially segregated at the carbide matrix surfaces, and phosphorus was found to be distributed on grain boundaries. However, the phosphorus segregation did not correlate with stress-rupture behavior. Sulfur contents in the range of 15 to 50 ppm had little effect on the stress-rupture life. However, the stress-rupture life decreased dramatically with increasing sulfur content above 50 ppm.