Grain Boundaries In Ni3Al Research Articles

Computer simulation on surfaces and [001] symmetric tilt grain boundaries in Ni, Al, and Ni3Al

We have used “local volume” (embedded atom) type potentials to study the surfaces and grain boundaries of Ni, Al, and Ni3Al. The simulations show that with appropriately fit potentials, the surface and grain boundary structure can be realistically calculated. The surface rippling and relaxation show good agreement with experiments. The energies of most surfaces and grain boundaries also agree with existing data. The structural unit model for grain boundaries in Ni3Al shows the same generic units as in pure metals, but with large variations due to distortions and multiplicity. The utility of the structural unit model is thus more limited for alloys. The grain boundary energies were found to be the highest for Al-rich Ni3Al grain boundaries, and depend significantly on the local composition of the grain boundary. The cusps in the grain boundary energy as a function of misorientation angle are different for different grain boundary stoichiometries. The Ni3Al grain boundaries have approximately the same grain boundary energy and cohesive energy as that of Ni.

Characterization and Statistical Distribution of Grain Boundaries in Ni3Al

Grain Boundaries in two specimens of Ni 4 Al of different compositions and mechanical behavior were studied by electron microscopy. SEM was used to determine the distribution of Σ values in both samples. TEM results were combined with CSL theory to completely characterize the nature of the boundaries, including Σ values and the grain boundary plane orientation. The boundaries most commonly observed in the experiments were also studied using a computer simulation technique and the results correlated with the experimental observations.

Theoretical Studies of Ni3Al and Nial with Impurities

ABSTRACTIntermetallic compounds have been extensively studied because of their superior strength, low creep rate, and high melting point [1,2]. However, room temperature ductility for the L12 and B2 phases are a continuing problem. Both L12 Ni3Al [3,4] and B2 NiAl [5,6] exhibit an intergranular fracture mode. Understanding grain boundaries in these materials is of particular importance since intergranular fracture limits the applicability of these otherwise promising materials. In an effort to understand the fracture mechanism, we have used embedded atom potentials [7] to study the properties of Ni 3Al [8,9,10] and NiAl [11]. We also consider the effect of boron, sulfur, and nickel segregation on the strength of grain boundaries in Ni3Al and NiAl. Many of the results presented here appear in literature elsewhere [8,9,10,11].

On the cleavage strength of grain boundaries in Ni 3Al: Effect of impurity segregation

The cleavage strength of grain boundaries in Ll 2 type of intermetallic compounds (Ni 3Al) is investigated using the tight-binding (TB) electronic theory of s, p and d basis orbitals. It is shown that cleavage strength of the grain boundaries depends strongly on the segregation of impurity atoms at the grain boundaries, in agreement with experimental results. The stoichiometry effect of the Ll 2 compound is also discussed.

Effect of boron on the mechanism of strain transfer across grain boundaries in Ni3Al

The effect of boron on the mechanism of strain transfer across grain boundaries in Ni3Al has been investigated by dynamic recording of events occurring during in-situ straining in the transmission electron microscope. Boundaries in both doped and undoped material can act as effective barriers to dislocation motion, large numbers of dislocations being incorporated into the boundary without any plastic strain occurring in the adjacent grain. In the undoped material, the grain-boundary strain is relieved by the sudden failure of the grain boundary. In the doped material the strain is relieved by the sudden generation and emission of large numbers of dislocations from the grain boundary. This effect may be understood by boron either increasing the grain-boundary cohesion or reducing the stress required to operate grain-boundary dislocation sources, rather than easing the passage of slip dislocations through the grain boundary.

Computer simulation of grain boundaries in Ni 3Al: The effect of grain boundary composition

The authors have performed atomistic simulations on three (001) symmetric tilt grain boundaries: 5(210), (310), and 13(320). Depending on which sub-lattice in each of the two grains is occupied by Al, the grain boundary may have different stoichiometries. All of the simulations show that the Al-rich grain boundaries have the highest grain boundary energies. Thus Al-rich grain boundaries are more likely to fail than those which have the bulk stoichiometry or are Ni-rich. This conclusion is consistent with the observed stoichiometry dependence of the beneficial boron effect. The similarity between the grain boundary energies (cohesive energies) of Ni/sub 3/Al and Ni and the much higher yield stress of Ni/sub 3/Al provides a justification for the ''inherent'' brittleness of Ni/sub 3/Al grain boundaries.

Effect of boron on grain-boundaries in Ni 3Al†

The effects of boron additions (up to 0.4 wt% B) on grain-boundary chemistry and tensile properties of Ni 3Al containing 24–26 at.% Al were studied. Room-temperature ductility and fracture behavior of B-doped Ni 3Al depended critically on deviation from alloy stoichiometry. As the aluminum content of B-doped Ni 3Al is decreased below 25 at.%, the ductility increases dramatically and the fracture mode changes from intergranular to transgranular. Auger studies indicate that the intensity of boron segregation to grain boundaries increases and the concentration of grain-boundary aluminium decreases significantly with decreasing bulk aluminum concentration. These results suggest that alloy stoichiometry strongly influences grain-boundary chemistry which in turn, affects the grain-boundary cohesion. Boron exhibits an unusual segregation behavior in Ni 3Al, i.e. it has a strong tendency to segregate to the grain boundaries but not to cavity (free) surfaces. On the other hand, sulfur, an embrittling impurity, tends to segregate more strongly to free surfaces than to grain boundaries. The beneficial effect of boron is in agreement with existing theories of solute segregation effects on grain-boundary cohesion. The yield stress of B-doped Ni 3Al decreases with increasing grain size produced by long-term annealing at 1000°C The yield stress obeys the Hall-Petch relation: σ y= α o,y+ k y d 1 2 with σ y = 163 MPa and k y = 8.2 MPa cm 1 2 . The tensile elongation was initially independent of grain size, and showed only a moderate decrease from about 50–40% with grain diameters larger than 110μm.

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.