Interaction of dislocations with grain boundaries in Ni3Al

The interaction between slip dislocations and grain boundaries in hypo-stoichiometric Ni₃Al, with and without boron, has been investigated by using the in situ TEM deformation technique. In both alloys, the slip dislocations were incorporated into the grain boundaries and remained at the point of entry. The difference between the alloys was in the dominant response mode of the grain boundary to the stress concentration associated with a dislocation pileup. In the boron-free material, the stress was relieved primarily by the nucleation and propagation of a crack along the grain boundary. In contrast, in the boron-doped material, relief occurred by the emission of dislocations from the grain boundary. These results are consistent with boron increasing the cohesive energy of the grain boundary. The slip system activated at grain boundaries in the ductile ordered alloy was shown to satisfy the same slip transfer criteria that operate in f.c.c. disordered alloys.     

Atom probe analysis of grain boundaries in rapidly-solidified Ni3Al

The distribution of boron in a rapidly solidified Ni-24 at.% Al alloy containing 0.24 at.% B was investigated by atom probe field-ion microscopy. Boron was found to segregate to both APBs and grain boundaries. Field-ion micrographs revealed that some of the APBs and most of the grain boundaries were decorated with bright spots identified by single atom analysis as boron atoms. A 0.4–1.2 nm thick boron-enriched phase was observed on most of the grain boundaries. The distribution of this phase was not uniform either along a boundary or from boundary to boundary. The degree of long-range order as measured by the atom probe was greater than 0.97. This state of order continued to the interface between the matrix and grain boundary phase.     

Sulfur segregation to grain boundaries in Ni3Al and Ni3(AI,Ti) alloys

The segregation of S to grain boundaries in Ni₃Al and Ni₃(Al, Ti) has been studied using Auger electron spectroscopy. The S concentration at the grain boundaries decreases more slowly with increasing temperature than would be predicted by segregation models based on a single solute binding energy to the grain boundaries. This behavior, which can be interpreted as an increase in the effective solute binding energy for a grain boundary as a function of temperature, is consistent with predictions of a model based on the existence of a spectrum of solute binding energies for grain boundaries. Formerly Professor and Head of the Department of Metallurgical Engineering, Michigan Technological University     

Roles of dislocations and grain boundaries in martensite nucleation

In order to elucidate roles of dislocations and grain boundaries in martensite nucleation, the transformation temperature (M_s) of specimens austenitized at various temperatures and subjected to prestrain has been measured, using Fe-Ni, Fe-Ni-C, and Fe-Cr-C alloys. It is concluded that the plastic accommodation, in austenite, of the shape strain of the transforming martensite is a vital step in the nucleation event. Any factors impeding such plastic accommodation, such as the lack of dislocations, work hardening, and grain refinement, suppress the transformation. Contrary to the general belief, dislocations themselves do not act as favorable nucleation sites. Grain boundaries provide nucleation site, but only certain types of grain boundaries are qualified to be potential nuclei. A quantitative analysis shows that the increasing difficulty for the plastic accommodation with decreasing grain size is the main factor to depress M_s in fine-grained specimens.     

Relation between ductility and grain boundary character distributions in Ni3Al

The grain boundary character distributions in cast, recrystallized and strain annealed Ni₃Al alloys with a composition of Ni23Al were examined to clarify the relation between ductility and the grain boundary character distributions in Ni₃Al. The percentages of CSL boundaries in cast, recrystallized and strain annealed Ni₃Al alloys are 26.8, 43.1 and 58.4%, respectively, of the total number of boundaries examined. The sum of the percentages for LAB (Low Angle Boundary) and Σ3 boundary in cast, recrystallized and strain annealed Ni₃Al alloys is 4.46, 10.8 and 29.2% respectively. This indicates that strain annealing is effective in enhancing the frequency of occurrence of CSL boundaries, especially LAB and Σ3 boundary, in Ni₃Al. The strain annealed Ni₃Al alloy having a much higher frequency of CSL boundaries is found to exhibit elongation to fracture of more than 45%. This significant increase in elongation of the strain annealed Ni₃Al alloys is attributed to the presence of relatively low energy LAB and Σ3 boundaries.     

Fabrication of Al2O3-reinforced Ni3Al composites by a novel in situ route

A novel in situ technique has been used to fabricate an Al₂O₃-reinforced Ni₃Al matrix composite. The composite was prepared by first incorporating a low volume fraction of continuous Al₂O₃ fibers in a Ni₃Al alloy containing 0.34 at. pct Zr. Pressure casting was used to embed the fibers. Casting resulted in partial reduction of the Al₂O₃ fiber by the Zr present in the matrix and the formation of a layer of ZrO₂ on the surface of the fibers. The final composite was then prepared by air annealing the precursor composite at 1100 °C for 10 days. Air annealing led to the formation of networks of Al₂O₃ around the fibers. The matrix in the immediate vicinity of the networks consisted of Ni₃Al particles in a matrix of disordered α-Ni(Al). The Al₂O₃ networks raised the yield and tensile strength of the material by 35 and 18 pct, respectively. The composite had a tensile ductility of 14 pct.     

Investigation of grain boundary diffusion in polycrystals by means of extrinsic grain boundary dislocations spreading rate

The spreading rate of extrinsic grain boundary dislocations (EGBDs) was used to study the diffusivity on grain boundaries in austenitic steel for a range of temperatures. The distribution function of the grain boundary diffusivity as well as the activation energy for grain boundary diffusion may be obtained by this method. This is an important characteristic of polycrystals. It is especially suited for investigating changes of distribution functions of grain boundary properties after various thermomechanical treatments or changes of grain boundary chemical composition as well as for studying the effect of grain boundaries on polycrystal properties. The optimal averaging methods of such distribution functions to a single diffusion coefficient were discussed. It was concluded, that EGBD spreading rate measurements provide a very sensitive method of grain boundary diffusion determination.     

Dissociation of grain boundaries induced by changes of composition,the ejection of dislocations from grain boundaries,and the nucleation of diffusion induced grain boundary migration

We present various observations of the dissociation of large and small angle grain boundaries in copper during the alloying of the specimens with zinc. In all large angle cases, this dissociation takes the form of the large angle boundary reacting to form another large angle boundary and a small angle boundary. This process may be of great importance in understanding the nucleation of diffusion induced grain boundary migration. Small angle boundaries can undergo complete disintegration upon the addition of zinc, and the magnitude of the stress involved in such a disintegration is estimated.     

The influence of grain boundary geometry on intergranular crack propagation in Ni3Al

The distribution of grain boundary types along intergranular cracks in Ni₃Al was measured, by Σ value, and compared to the distribution in the bulk, using statistically significant sample sizes. It was found that low angle (Σ1) and symmetrical Σ3 boundaries (twins) are particularly strong, and all high angle boundaries, independent of their Σ values are weak. In particular, low Σ, high angle boundaries, as a group, are also weak. These results are in qualitative agreement with predictions based on the structural unit model and imply that the fracture strength of an intergranularly brittle polycrystalline aggregate can be increased only by increasing the fraction of low angle and symmetrical Σ3 boundaries.     

Thermodynamic activities and phase boundaries for the alloys of the solid solution of Co in Ni3Al

The vaporization of the alloy samples of the compositions (Ni₃Al)_1-x Co _x (x = 0, 0.03, 0.06, 0.09, 0.12, or 0.15) was investigated at temperatures between 1326 to 1581 K by the use of Knudsen effusion mass spectrometry in order to obtain thermodynamic data for the solid solution γ′ phase of the type Ni₃Al. The partial pressures of Al, Co, and Ni were determined over the samples investigated. Excess chemical potentials at a temperature of 1473 K resulted for the components in the γ′ solubility range. New results on the phase boundaries of the γ′ solubility lobe were obtained by the analysis of quenched alloy samples and from the mass spectrometric studies. The results obtained are discussed with respect to the solubility behavior of Co in the γ′ phase.     

Effect of preoxidation and grain size on ductility of a boron-doped Ni3Al at elevated temperatures

The ductility of a preoxidized Ni₃Al (Ni-23Al-0.5Hf-0.2B. at.%) alloy with various grain sizes (17–193 μm) was evaluated by means of tensile tests at 600 and 760°C in vacuum. The preoxidation does not affect the ductility of the finest-grained material at either temperature, whereas it causes severe embrittiement in the largest-grained material, especially at 760°C. Auger studies revealed very little oxygen penetration along grain boundaries in the finest-grained material but substantial oxygen penetration in the largest-grained one. A continuous, thin Al-rich oxide layer which forms on the fine-grained samples protects the underlying alloy from oxygen penetration, preventing any loss of ductility, whereas the nickel-rich oxide which forms on the large-grained samples allows oxygen to penetrate along grain boundaries, causing severe embrittiement. The grain boundaries act as short-circuit paths for rapid diffusion of aluminum atoms from the bulk to the surfaces, and this is responsible for the change in oxidation product from Ni-rich to Al-rich oxide with decreasing grain size.     

Site occupation determination of Pd in Ni3Al by ALCHEMI

Atom location, by channeling enhanced microanalysis (ALCHEMI), was performed to examine the substitution behavior of Pd atoms added to a Ni₃Al intermetallic compound. From the quantitative analysis, it can be concluded that Pd has a strong tendency to locate in the Ni site of the compositions Ni₇₅Al₂₃Pd₂, Ni₇₄Al₂₄Pd₂, and Ni₇₃Al₂₅Pd₂, which means that the anti-structure defects are formed in the compositions of Ni_75−xAl_25−yPd_{x+ y}(x ≦ y, y ≠ 0). In addition, the above results are thermodynamically discussed by considering the formation of the anti-structure defects introduced by Pd addition. It is investigated as to whether the substitution behavior of ternary alloying elements, which are known to substitute for the Ni site in the composition of Ni_{75− x}Al₂₅C_x, depend on the composition of the host elements when they are added to the composition of Ni₇₅Al_25−xC_x.