Raman Jayaram

Characterization of phosphorus segregation in neutron-irradiated Russian pressure vessel steel weld

An atom probe field ion microscopy characterization of three Russian pressure vessel steels has been performed. Field ion micrographs of several lath boundaries have indicated that they are decorated with a semicontinuous film of discrete brightly-imaging precipitates that were identified as molybdenum carbonitrides. In addition, extremely high phosphorus levels were measured at the lath boundaries. The phosphorus was found to be confined to an extremely narrow region indicative of monolayer type segregation. The phosphorus coverage determined from the atom probe results of the unirradiated materials agree with predictions based on McLeans equilibrium model of grain boundary segregation. The boundary phosphorus coverage of a neutron-irradiated weld material was significantly higher than in the unirradiated material. Ultrafine darkly-imaging copper- and phosphorus-enriched precipitates were also observed in the matrix of the neutron-irradiated material.

An APFIM analysis of grain boundaries and precipitation in boron-doped NiAl

An atom probe characterization of a stoichiometric NiAl alloy doped with 100 wt. ppm of boron has been performed. Field ion micrographs revealed boron segragation to the grain boundaries. Atom probe analysis of the grain boundaries measured a boron coverage of up to 30% of a monolayer. A low number density of (Ti, V, Cr)B2 precipitates with a size distribution ranging from 10 to < 1 nm radius was also observed in the matrix. Atom probe analysis of the matrix revealed that approximately 23 of the boron remained in solid solution. The increase in the yield stress observed over the undoped alloy is due to a combination of boron solid solution strengthening and precipitation hardening from the boride precipitates.

APFIM characterization of single-crystal PWA 1480 nickel-base superalloy

Abstract An atom-probe field-ion microscopy and analytical electron microscopy characterization of a PWA 1480 nickel-based single-crystal alloy has revealed a high number density of 5 to 10 nm diameter γ precipitates in the interior of coarse cuboidal γ′ precipitates in material that was heat-treated for 4 h at 1288°C, 4 h at 1079°C and 32 h at 871°C. Atom-probe analysis revealed that the chromium, cobalt and tungsten partitioned preferentially to the γ precipitates and γ matrix and the aluminum, titanium and tantalum partitioned to the γ′ precipitates. No evidence of tungsten clustering in the γ matrix was found.

APFIM characterization of 15Kh2MFA Cr-Mo-V and 15Kh2NMFA Ni-Cr-Mo-V type steels

Abstract A microstructural characterization of 15Kh2MFA Cr-Mo-V and 15Kh2NMFA Ni-Cr-Mo-V type steels that are used in the pressure vessels of Russian VVER 440 and VVER 1000 nuclear reactors, respectively, has been performed with the use of the techniques of atom-probe field-ion microscopy (APFIM) and transmission electron microscopy. The microstructure of these materials was found to be tempered martensite and bainite. A high number density of coarse (≈ 50 to ≈ 500 nm) blocky M7C3 carbides and some inclusions were observed. In addition to these coarse carbides, some finer (≈ 10 nm diameter) approximately spherical MC carbides were also observed in the VVER 440 steel. Field-ion microscopy has revealed that the lath boundaries in both unirradiated VVER 440 and VVER 1000 reactor steels are decorated with an ultrathin semicontinuous film of molybdenum-carbonitride precipitates. Atom-probe analysis has revealed a high enrichment of phosphorus at the lath boundaries.

Some factors affecting analysis in the atom probe

Abstract Preferential evaporation and retention of elements during pulsed field evaporation of a specimen is known to affect the results of a chemical analysis in the atom probe field ion microscope. Field evaporation produced by increasing the standing voltage on the specimen alters the surface chemistry because the more easily field evaporated elements will be removed preferentially. This change in surface chemistry will result in incorrect compositions being measured for small precipitates, boundaries or surfaces. The use of pulsed field evaporation should be adopted in these cases to avoid this problem. The shape of the specimen was found to change between DC and pulsed field evaporated conditions.

An atom probe characterization of carbon-doped NiAl

Abstract The atom probe field ion microscope has been used to characterize stoichiometric NiAl doped with 300 ppm (0.1 at%) carbon. Field ion images of grain boundaries showed no decoration indicative of segregation. This observation was consistent with atom probe analyses which did not detect carbon at the grain boundaries. Matrix analyses revealed that only a small percentage (3–6%) of the carbon was in solid solution the remainder being in the form of ultrafine MC precipitates ranging from less than 1 nm to ∼ 20 nm in diameter. Atom probe composition measurements revealed that the metallic component of the precipitates consisted of Ti, V, W and Cr which are present as trace impurities in the alloy. The enormous increase in the yield stress of carbon-doped over the undoped alloy is predominantly due to precipitation hardening from the carbide precipitates.

An atom probe study of the substitutional behavior of beryllium in NiAl

Abstract Stoichiometric NiAl doped with 0.24 at% beryllium has been studied using atom probe field ion microscopy (APFIM). Composition measurements revealed that almost all the beryllium remained in solid solution in the matrix. Atom probe analysis of the (100) alternating pure nickel and pure aluminum planes determined the occupancy of beryllium to be almost exclusively at aluminum sites. A low number density of coarse MC precipitates, originating from trace impurities in the alloy, were found along the grain boundaries and within the grains. These coarse MC precipitates occurred in too low a number density to make any significant contribution to the observed increase in yield strength. The small increase in the yield strength that accompanied the beryllium addition to NiAl was from a weak solid solution strengthening mechanism from the substitutional beryllium atoms in the matrix.

Characterization of phosphorus segregation in neutron-irradiated pressure vessel steels by atom probe field ion microscopy

An atom probe field ion microscopy characterization of A533B and Russian VVER 440 and 1000 pressure vessel steels has been performed to determine the phosphorus coverage of grain and lath boundaries. Field ion micrographs of grain and lath boundaries have revealed that they are decorated with a semi-continuous film of discrete brightly-imaging precipitates that were identified as molybdenum carbonitride precipitates. In addition, extremely high phosphorus levels were measured at the boundaries. The phosphorus segregation was found to be confined to an extremely narrow region indicative of monolayer-type segregation. The phosphorus coverages determined from the atom probe results of the unirradiated materials were in excellent agreement with predictions based on McLean`s equilibrium model of grain boundary segregation. The boundary phosphorus coverage of a neutron-irradiated weld material was significantly higher than that observed in the unirradiated material.

An APFIM/TEM study of crept model NiMoTaAl superalloys

The influence of initial microstructure and Mo composition on the high-temperature creep behaviour of two crept model Ni-x at.%Mo-2.0 at.%Ta-13.7 at.%Al(x = 8.7 and 9.2 at.%) superalloys has been investigated by the techniques of transmission electron microscopy (TEM) and atom probe field ion microscopy (APFIM). Acicular δ phase precipitates of approximate stoichiometry NiMo have been observed by TEM and APFIM in both alloys. However, a marked difference in γ′ morphology is observed between the alloys due to different initial microstructures. Small variations in the initial morphologies of the γ′ microstructure rather than Mo composition play a significant role in affecting creep strength in this alloy system.

A field-ion microscopy study of nanocrystalline Ni and Ni3Al

Abstract Electrodeposition with Ni has been shown to be a feasible technique to fabricate atom probe field-ion microscope (APFIM) specimens from ball-milled nanocrystalline Ni3Al powder particles. Field-ion images of the electrodeposited nanocrystalline Ni matrix revealed atomically sharp grain boundaries in the majority of the cases and occasionally some dark regions at the boundaries. Atom-probe analysis of the dark regions did not reveal any compositional differences. In some instances, an unidentified dark region was observed at the boundaries. Three distinct phases were identified from the field-ion images obtained from the ball-milled Ni3Al specimens. Brightly-imaging, well-defined atomic terraces were characteristic of the L12-ordered phase, poorly-defined atomic terraces that were separated by darkly-imaging regions indicated the intermediate disordered phase and randomly-imaging spots gave some indication of the amorphous phase of Ni3Al.