Air embrittlement of a cobalt-base superalloy

The stress-rupture and tensile properties of a cast cobalt base superalloy, MM509, are compared after high temperature exposure in air and vacuum. A loss in tensile ductility after air exposure is observed over the entire temperature range from room temperature to 1000 °C with the effect being most severe at 400 ° to 600 °C. This appears to be related to grain boundary oxygen penetration and is compared and contrasted with similar observations in nickel base alloys. The cobalt base alloy has high intrinsic ductility at high temperatures and, even after air exposure, there is no loss in creep life associated with the embrittlement. It is argued that in this alloy the critical strain for fracture of embrittled grain boundaries is much higher than that for high strength nickel base superalloys.     

Creep–Environment Interactions in Dwell-Fatigue Crack Growth of Nickel Based Superalloys

A multi-scale, mechanistic model is developed to describe and predict the dwell-fatigue crack growth rate in the P/M disk superalloy, ME3, as a function of creep–environment interactions. In this model, the time-dependent cracking mechanisms involve grain boundary sliding and dynamic embrittlement, which are identified by the grain boundary activation energy, as well as, the slip/grain boundary interactions in both air and vacuum. Modeling of the damage events is achieved by adapting a cohesive zone (CZ) approach which considers the deformation behavior of the grain boundary element at the crack tip. The deformation response of this element is controlled by the surrounding continuum in both far field (internal state variable model) and near field (crystal plasticity model) regions and the intrinsic grain boundary viscosity which defines the mobility of the element by scaling up the motion of dislocations into a mesoscopic scale. This intergranular cracking process is characterized by the rate at which the grain boundary sliding reaches a critical displacement. A damage criterion is introduced by considering the grain boundary mobility limit in the tangential direction leading to strain incompatibility and failure. Results of simulated intergranular crack growth rate using the CZ model are generated for temperatures ranging from 923 K to 1073 K (650 °C to 800 °C), in both air and vacuum. These results are compared with those experimentally obtained and analysis of the model sensitivity to loading conditions, particularly temperature and oxygen partial pressure, are presented.     

Grain boundary pest of boron- doped Niin3Al at 1200 °C

A ductile Ni_77.4Al₂₂Zr_0.6B_0.2 alloy suffered severe intergranular embrittlement after air exposure at 1200 °C for 100 hours. However, the material survived after 1038 °C air exposure for 100 hours. Auger analysis showed enormous oxygen segregation on the grain boundaries in the 1200 °C, air exposed, boron-doped Ni₃Al, while the boron segregation remained unchanged. To elucidate this type of grain boundary damage, a modified fracture mechanism was proposed. Finally, anomalous grain growth was found in this alloy after 1200 °C air exposure, and an explanation for this phenomenon was suggested.     

Grain boundary embrittlement of the iron-base superalloy IN903A

It is shown that a low coefficient of expansion, iron-base superalloy, IN903A, suffers severe tensile embrittlement following high temperature air exposure at 1000 °C. This embrittlement involves a transition to intergranular failure at low strains, with no reduction in yield strength, and is manifested in the room temperature to 800 °C range. In parallel with earlier observations on nickel-base superalloys, ductility is regained at 1000 °C. However, in contrast to these earlier results, air exposure enhances rather than hinders grain growth in the near surface regions, and, in addition, suppresses the occurrence of the jerky flow seen in vacuum-exposed material. Oxygen is demonstrated to be the damaging species, and it is show’n that boundaries are embrittled far ahead of any matrix internal oxidation. Small additions of boron are successful in eliminating the embrittlement, as they were in nickel-base alloys. The results of stress rupture tests are then reviewed, and it is concluded that the rapid failures which occur on air testing are a consequence of embrittled grain boundaries failing in tension, rather than the stress accelerated grain boundary oxidation mechanism previously proposed.     

Study on rare earth electrolyte of SDC

The grain boundaries of polycrystalline oxygen ion conductors presented a blocking effect on the oxygen ionic transport across them.It was found that the apparent specific grain boundary conductivity was 2-3 orders of magnitude lower than the bulk conductivity in the temperature range of 200-500 ℃ for normal purity Ce0.85Sm0.15O1.925(SDC)with an average grain size of 320-580 nm.The apparent specific grain boundary conductivity increased with decreasing average grain size.It was found that the space charge potential was nearly independent of grain size,and the reason was analyzed.The increase of the conduction path width was resportsible for the increase in the apparent specific grain boundary conductivity.     

Delamination Fracture Related to Tempering in a High-Strength Low-Alloy Steel

The delamination or splitting of mechanical test specimens of rolled steel plate is a phenomenon that has been studied for many years. In the present study, splitting during fracture of tensile and Charpy V-notch (CVN) test specimens is examined in a high-strength low-alloy plate steel. It is shown that delamination did not occur in test specimens from plate in the as-rolled condition, but was severe in material tempered in the temperature range 500 °C to 650 °C. Minor splitting was seen after heating to 200 °C, 400 °C, and 700 °C. Samples that had been triple quenched and tempered to produce a fine equiaxed grain size also did not exhibit splitting. Microstructural and preferred orientation studies are presented and are discussed as they relate to the splitting phenomenon. It is concluded that the elongated as-rolled grains and grain boundary embrittlement resulting from precipitates (carbides and nitrides) formed during reheating were responsible for the delamination.     

On the mechanisms of high-temperature intergranular embrittlements of Ni3Al-Zr Alloys

Recent studies on the room-temperature fracture behavior of Ni₃Al-Zr alloys after preexposure at elevated temperatures show various types of intergranular failure. In the presently studied Ni₇₈Al₂₁Zr₁B_0.2 alloy, a strong intergranular fracture tendency at room temperature has been found after preexposure at 750 °C, which is caused by the grain boundary precipitation in this alloy. After short-term exposure above 1200 °C and bending fracture at room temperature, the alloy also suffers intergranular embrittlement due to grain boundary melting. The intergranular fracture appearance is quite different from that observed in a previous study for a Ni_77.4Al₂₂Zr_0.6B_0.2 alloy after air exposure for 100 hours at 1200 °C. In that case, the intergranular fracture was accompanied by grain boundary diffusion (invasion) and segregation of oxygen. The mechanisms of these types of grain boundary failure are discussed. Formerly Doctoral Candidate, Institute of Materials Science and Engineering, National Taiwan University.     

Temperature and strain rate dependence of cyclic deformation response and damage accumulation in ofhc copper and 304 stainless steel

The effects of temperature and strain rate on deformation behavior and dislocation structure were investigated for OFHC copper and type 304 stainless steel. It is shown that the cyclic stress response is inversely related to the cell size for copper cycled at different temperatures ranging from -75 to 650°C. Type 304 stainless steel underwent a change from a planar to a wavy slip character as the temperature was changed from room temperature to 760°C. At elevated temperatures, cells were observed and the size of the cells tended to increase with increase in temperature. The effects of temperature on the cyclic stress-strain parameters were investigated for copper, type 304 stainless steel and Ferrovac “E” iron. On studying the effects of temperature and strain rate on the fracture mechanisms it was found that a time dependent fracture mode was dominant at high temperature levels and low strain rates. However, at high strain rates the life was insensitive to temperature. The role of grain boundary migration on the fracture process was investigated. Grain boundary migration was found to be dependent on strain rate for copper. However, for type 304 stainless steel, the grain boundary migration was inhibited at high temperature (760°C) due to the presence of precipitates at the grain boundaries. In strain cycling of OFHC copper and type 304 stainless steel, it was found that the addition of creep-type damage to fatigue damage resulted in a total damage which was not equal to unity for failure when these different modes were imposed sequentially. The sense of the damage accumulation appeared to have no effect on this summation.     

The wetting transition in high and low energy grain boundaries in the Cu(In) system

Wetting of two symmetrical tilt grain boundaries, 77° 〈110〉 and 141° 〈110〉, in synthetic copper bicrystals with a Cu(In) melt was studied in the temperature range 690°990°C. The contact angle at the site of GB intersection with the solid-melt interface was measured. A wetting transition occurred at T_w = 960 ± 6°C for the 77° 〈110〉 grain boundary and at T_w = 930 ± 5°C for the 141° 〈110〉 grain boundary. The contact angle approached zero for this transition. The relative surface energies of the two boundaries were measured using the thermal grooving technique. The energy of the 77° 〈110〉 grain boundary is about 40% lower than that of the 141° 〈110〉 grain boundary. Therefore, it has been shown experimentally that the lower the grain boundary energy, the higher the wetting transition temperature. This is in agreement with the thermodynamic model of wetting transitions on 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.     

Brief review of oxidation kinetics of copper at 350 °C to 1050 °C

Copper’s oxication mechanism and purity effects were elucidated by oxidizing 99.99 pct (4N), 99.9999 pct (6N), and floating zone refined (>99.9999 pct) specimens in 0.1 MPa oxygen at 350 °C to 1050 °C. Throughout the temperature range, the oxidation kinetics for all specimens obeys the parabolic oxidation rate law. The Cu₂O scale grows predominantly, and the rate-determining step is concluded to be outward diffusion of copper atoms in Cu₂O. The activation energy at high temperatures, where the lattice diffusion predominates, is 173 kJ/mol, but it becomes lower at intermediate temperatures and even lower at low temperatures because of the contribution of the grain boundary diffusion. At high temperatures, oxidation kinetics is almost uninfluenced by purity, but the lattice-diffusion temperature range is wider for higher-purity copper. At intermediate temperatures, copper oxidation is enhanced because trace impurities can impede growth of Cu₂O grains to facilitate grain boundary diffusion. At low temperatures, grain boundary diffusion is possibly hindered by impurities segregated at grain boundaries.     

Microstructural Features Leading to Enhanced Resistance to Grain Boundary Creep Cracking in ALLVAC 718Plus

This study focuses on the microstructural features that enhance the resistance of ALLVAC 718Plus to grain boundary creep cracking during testing of samples at 704 °C in both dry and moist air. Fully recrystallized structures were found to be susceptible to brittle grain boundary cracking in both environments. Detailed transmission electron microscopy (TEM) microstructural characterization reveals features that are believed to lead to resistance to grain boundary cracking in the resistant microstructures. It is suggested that dislocation substructures found within the grains of resistant structures compete with the high-angle grain boundaries for oxygen, thereby reducing the concentration of oxygen on the grain boundaries and subsequent embrittlement. In addition, electron backscatter diffraction (EBSD) misorientation maps reveal that special boundaries (i.e., Σ3 boundaries) resist cracking. This is in agreement with previous findings on the superalloy INCONEL 718. Furthermore, it is observed that cracks propagate along high-angle boundaries. This study also shows that in this case, the presence of delta phase at the grain boundaries does not by itself produce materials that are resistant to grain boundary cracking.     

Creep and intergranular cracking of Ni-Cr-Fe-C in 360 °C argon

The influence of carbon and chromium on the creep and intergranular (IG) cracking behavior of controlled-purity Ni-xCr-9Fe-yC alloys in 360 °C argon was investigated using constant extension rate tension (CERT) and constant load tension (CLT) testing. The CERT test results at 360 °C show that the degree of IG cracking increases with decreasing bulk chromium or carbon content. The CLT test results at 360 °C and 430 °C reveal that, as the amounts of chromium and carbon in solution decrease, the steady-state creep rate increases. The occurrence of severe IG cracking correlates with a high steady-state creep rate, suggesting that creep plays a role in the IG cracking behavior in argon at 360 °C. The failure mode of IG cracking and the deformation mode of creep are coupled through the formation of grain boundary voids that interlink to form grain boundary cavities, resulting in eventual failure by IG cavitation and ductile overload of the remaining ligaments. Grain boundary sliding may be enhancing grain boundary cavitation by redistributing the stress from inclined to more perpendicular boundaries and concentrating stress at discontinuities for the boundaries oriented 45 deg with respect to the tensile axis. Additions of carbon or chromium, which reduce the creep rate over all stress levels, also reduce the amount of IG fracture in CERT experiments. A damage accumulation model was formulated and applied to CERT tests to determine whether creep damage during a CERT test controls failure. Results show that, while creep plays a significant role in CERT experiments, failure is likely controlled by ductile overload caused by reduction in area resulting from grain boundary void formation and interlinkage. Thomas M. Angeliu, formerly Graduate Student Research Assistant, Department of Materials Science and Engineering, the University of Michigan, Ann Arbor, MI,.     

Low temperature sensitization of type 304 stainless steel pipe weld heat affected zone

Large-diameter Type 304 stainless steel pipe weld heat-affected zone (HAZ) was investigated to determine the rate at which low temperature sensitization (LTS) can occur in weld HAZ at nuclear reactor operating temperatures and to determine the effects of LTS on the initiation and propagation of intergranular stress corrosion cracks (IGSCC). The level of sensitization was determined with the electrochemical potentiokinetic reactivation (EPR) test, and IGSCC susceptibility was determined with constant extension rate tests (CERT) and actively loaded compact tension (CT) tests. Substructural changes and carbide compositions were analyzed by electron microscopy. Weld HAZ was found to be susceptible to IGSCC in the as-welded condition for tests conducted in 8-ppm-oxygen, high-purity water at 288 °C. For low oxygen environments (i.e., 288 °C/0.2 ppm O₂ or 180 °C/1.0 ppm O₂), IGSCC susceptibility was detected only in weld HAZ that had been sensitized at temperatures from 385 °C to 500 °C. Lower temperature heat treatments did not produce IGSCC. The microscopy studies indicate that the lack of IGSCC susceptibility from LTS heat treatments below 385 °C is a result of the low chromium-to-iron ratio in the carbide particles formed at grain boundaries. Without chromium enrichment of carbides, no chromium depleted zone is produced to enhance IGSCC susceptibility.     

Weldability Evaluation of a Cu-Bearing High-Strength Blast-Resistant Steel

BlastAlloy160 (BA-160) steel, with a nominal composition of Fe-0.05C-3.65Cu-6.5Ni-1.84Cr-0.6Mo-0.1V (wt pct), is strengthened by Cu-rich precipitates and M₂C carbides. This alloy was subjected to several weldability tests to assess its susceptibility to certain weld cracking mechanisms. Hot ductility testing revealed a liquation cracking temperature range (LCTR) of 148 K (–125 °C), which suggested moderate susceptibility to heat-affected zone (HAZ) liquation cracking. The enrichment of Ni and Cu was measured along the prior austenite grain boundaries in the simulated partially melted zone (PMZ) and was consistent with similar enrichment at interdendritic boundaries of the simulated fusion zone (FZ). Good wetting and penetration of liquid films along the austenite grain boundaries of the PMZ was also observed. Associated with that finding were thermodynamic calculations indicating a completely austenitic (face-centered cubic) microstructure at elevated temperatures. In testing to determine reheat cracking susceptibility, ductility values of 41 to 78 pct RA were established for the 723 K to 973 K (450 °C to 700 °C) temperature range. The good ductility values precluded susceptibility to reheat cracking according to the test criterion. Dilatometric measurements and thermodynamic calculations revealed the formation of austenite in the reheat cracking temperature range, which was attributed to the high Ni content of the BA-160 alloy.     

Some observations of grain boundary ledges and ledges as dislocation sources in metals and alloys

Direct observations of grain boundary ledges have been made in annealed Ni, Al, Cu, Mo, Ta, Ir, 304 Stainless Steel and Inconel 600 by transmission electron microscopy. Grain boundary ledges have been observed to be sources of dislocations during and after plastic deformation, and to resemble the appearance of dislocation pileups in the transmission electron microscope. Ledge density (number per unit length of grain boundary) has been observed to increase with an increase in grain boundary misorientation in Ni and 304 Stainless Steel, and the distribution of misorientations was observed to be continuous over the range 0 deg <θ< 90 deg at an annealing temperature of 1060°C. The mean grain boundary misorientation in 304 Stainless Steel was also observed to decrease with a decrease in the recovery temperature following cold reduction and to vary from 10 to 45 deg in the temperature range of 660 to 1060°C. An essential point of this investigation is that Li’s theoretical treatment of the flow-stress, grain-size relation based on the existence of grain boundary ledges and their action as sources of dislocations under stress is shown to be correct.     

The influence of temperature and strain rate on the stress-strain behaviour of “in-situ solidified” steel during tensile test

The stress-strain behaviour of a low-carbon low-alloyed steel after solidification in-situ was studied at four temperatures (1000°C, 1150°C, 1300°C and 1400°C) and at four deformation rates (6 mm s^?1, 0,6 mm s^?1, 0,06 mm s^?1 and 0,006 mm s^?1) by tensile tests. The resulting microstructures after solidification and deformation were examined by scanning electron microscope and optical microscope. The results showed that the crystals developed after solidification in the melted part of the tensile specimen are cellular dendritic, similar to the crystallization mode of steel in continuous casting. It was also shown that slip in the dendritic crystals is the main mode of deformation mechanism in the tensile tests within the temperature range mentioned above, but at higher levels within this temperature range (i.e. 1400°C) and at a low deformation rate (i.e. 0,006 mm s^?1) grain boundary migration takes part in the deformation process. The data from the tensile test were analysed.     

Effect of strain wave shape on high temperature fatigue life of a type 316 steel and application of the strain range partitioning method

Effects of strain rate and strain wave shape on low-cycle fatigue life of a Type 316 stainless steel were investigated at 600 and 700 °C. A great reduction in the fatigue life corresponded with a variation in the fracture mode. Especially extensive grain boundary microcracks such as wedge- and cavity-type cracks were observed in slow-fast sawtooth wave shape tests and in tension hold time tests, respectively. The test results were analyzed by the strain range partitioning method proposed by Manson, Halford and Hirschberg. In applying the method, a new technique of partitioning the inelastic strain range was proposed and used. Four component strain range vs life relationships were dependent on the testing temperature between 600 and 700 °C. However, the difference between 600 and 700 °C was eliminated by using normalized component strain range by dividing by ductility of creep rupture test and tensile test.     

The role of sulfur in the air embrittlement of nickel and its alloys

A mechanism leading to the embrittlement of nickel and its alloys following high temperature air exposure is proposed. This mechanism involves the internal oxidation of sulfides to oxides, accompanied by a release of embrittling sulfur onto the grain boundaries. The mechanism is shown to work in a model system of nickel containing MnS precipitates, in which a ring of internal oxidation 250 μm in depth forms during 200 hours air exposure at 1000 °C. Auger analysis shows very high sulfur levels on grain boundaries within this region, but also reveals considerable sulfur concentrations beyond it. This massive release of free sulfur had the effect of rendering the alloy brittle over the entire temperature range investigated (25 to 1000 °C). The contribution of this mechanism to the known air embrittlement of pure nickel (Ni270) and a nickel base superalloy (IN738) is investigated. Although enhanced O/Ni peak height ratios were observed in the air exposed samples of both materials, the only significant sulfur concentrations were observed on the surfaces of grain boundary cavities formed in Ni270. However, the starting sulfur levels were extremely low in both cases, and the mechanism may contribute to high temperature air embrittlement in other systems. R.A. MULFORD, formerly with the General Electric Corporate Research and Development Laboratory, Schenectady     

Some observations of grain boundary ledges and ledges as dislocation sources in metals and alloys

Direct observations of grain boundary ledges have been made in annealed Ni, Al, Cu, Mo, Ta, Ir, 304 Stainless Steel and Inconel 600 by transmission electron microscopy. Grain boundary ledges have been observed to be sources of dislocations during and after plastic deformation, and to resemble the appearance of dislocation pileups in the transmission electron microscope. Ledge density (number per unit length of grain boundary) has been observed to increase with an increase in grain boundary misorientation in Ni and 304 Stainless Steel, and the distribution of misorientations was observed to be continuous over the range 0 deg <θ< 90 deg at an annealing temperature of 1060°C. The mean grain boundary misorientation in 304 Stainless Steel was also observed to decrease with a decrease in the recovery temperature following cold reduction and to vary from 10 to 45 deg in the temperature range of 660 to 1060°C. An essential point of this investigation is that Li’s theoretical treatment of the flow-stress, grain-size relation based on the existence of grain boundary ledges and their action as sources of dislocations under stress is shown to be correct.