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.     

X-ray diffraction study of the oxidation of Ni3(Al,Ti) alloys

Conclusions A study was made of the phase composition of the scale forming on Ni₃(Al + 15 at.% Ti) and Ni₃(Al + 5 at.%Ti) alloys during atmospheric oxidation at temperatures ranging from 600 to 1000°C. The scale was found to contain the oxides NiO, TiO₂ (rutile),-Al₂O₃, NiO · TiO₂, NiO · Al₂O₃, Ti₃O₅, and Ti₂O₃ together with Ni and a nickel base solid solution (Ni_ss). The character of the distribution of these phase constituents in successive scale layers after oxidation under various conditions is described. A correlation was found between the phase composition of the scale and certain kinetic oxidation characteristics of the alloys.Translated from Poroshkovaya Metallurgiya, No. 7 (151), pp. 70–74, July, 1975.     

Stacking faults in gamma prime Ni3(Al,Ti) precipitation hardened nickel-base alloys

Most high temperature nickel-base alloys are strengthened by a coherent, orderedγ′ precipitate of Ll₂ structure. Examination of thin foils of these materials after various thermal and/or mechanical treatments has shown the presence of stacking faults in a variety of forms. Stacking faults are developed in conjunction with certain phase reactions, and during plastic deformation. The experimental evidence indicates that stacking fault energy is an important parameter determing the slip mode and strength at low and intermediate temperatures.     

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.     

A model describing neutron irradiation-induced segregation to grain boundaries in dilute alloys

A model describing neutron irradiation-induced grain boundary segregation at a given temperature is established for dilute alloys based on a complex diffusion mechanism and combined with Mc-Lean’s equilibrium segregation model. In the model, irradiation-enhanced solute diffusion is taken into consideration. The diffusion equations are more rigorously solved than in earlier models, so that an accurate definition of the grain boundary solute concentration is given as a function of time. The effect of the temperature dependence of dislocation density is accommodated and the estimation method for complex diffusion is reappraised. Theoretical predictions are made for segregation of phosphorus in neutron-irradiatedα-Fe. There exists a transition temperature below which combined irradiation-induced nonequilibrium and irradiation-enhanced equilibrium segregation is dominant and above which thermal equilibrium segregation is dominant. The peaks in the temperature dependence of segregation shift to lower temperatures with decreasing neutron dose rate and/or increasing neutron dose. The combined radiation-induced nonequilibrium and radiation-enhanced equilibrium peak segregation temperature is about 150 °C forP grain boundary segregation in neutron-irradiatedα-Fe at dose rate=10⁻⁶ dpa/s and dose=1 dpa. The thermal equilibrium segregation peak is around 550 °C for the same conditions. Comparison of some experimental and predicted results shows that the predictions are generally consistent with the observations.     

Strength and fracture of single-crystalline Ni3(Al,Ti) and Ni3(Al,Ta) intermetallic compounds at 290 K

Tensile tests at room temperature were performed on single crystals of Ni₇₅Al₂₀Ti₅, Ni₇₅Al₁₅Ti₁₀ and Ni₇₅Al₂₀Ta₅ to determine the effects of ternary addition on the deformation and fracture behavior of the Ni₃Al-base intermetallic compounds. The yield stress and work-hardening rate increase steadily in the sequence of Ni₇₅Al₂₀Ti₅→ Ni₇₅Al₁₅Ti₁₀→ Ni₇₅Al₂₀Ta₅. This fact is well expressed by the effects of solid-solution hardening and Kear-Wilsdorf mechanism. While {001} cleavage fracture is predominant for Ni₇₅Al₂₀Ta₅ and Ni₇₅Al₁₅Ti₁₀, the mixed modes of cleavage and {111} cracking are found for Ni₇₅Al₂₀Ti₅. Moreover, the fracture stress for Ni₇₅Al₁₅Ti₁₀ is governed by the hydrostatic component of the stress system, whereas the shear stress criterion exists in the case of Ni₇₅Al₂₀Ta₅ and Ni₇₅Al₂₀Ti₅ crystals. These facts are reasonably explained by considering the plastic deformation behavior and the cohesive strength of a cleavage plane. All these properties are directly related to the ductility of crystals; the elongation increases with both increasing the fracture stress and decreasing the yield stress and the work-hardening rate, depending on ternary addition and orientation.     

Investigation of the segregation behavior of boron in Ni3Al alloys by PTA technique

The particle-tracking autoradiograph (PTA) technique has been used, combined with quantitative statistical analysis using the image processing system, to study the effect of such factors as aluminium content of base alloy, the bulk boron concentration and different heat treatments on the grain boundary segregation behaviors of boron in Ni₃Al alloys. Moreover, the mechanical properties of Ni₃Al alloys subjected to different heat treatments and therefore with various quantities of segregated boron have been tested. Their fracture surfaces have also been observed in SEM. The relation between the level of boron segregation and ductility has been inspected.     

The intergranular segregation of boron in Ni3Al: Equilibrium segregation and segregation kinetics

The grain boundary B content of high-purity Ni-24 at.% Al alloys containing 0.048, 0.144, 0.240 and 0.480 at.% B (100, 300, 500, 1000 ppm mass) has been determined for samples aged from 1323 to 873 K for sufficient times to attain equilibrium. The B content was derived from Auger electron spectra of the intergranular fracture facets. Many facets were exposed during fracture at ≈ 300 K, and additional facets were formed upon fracturing following hydrogen charging after heat treatment. For each alloy sample, about 25 facets were analyzed. The grain boundary B contents were in the range of 0.5–2.5 at.%. The grain boundary B content increased with decreasing temperature and with increasing bulk B content in the alloys. The energy of binding of a B atom to the grain boundary was calculated using McLean's segregation theory and assuming a unique binding energy for each alloy. The values were in the range of 0.15–0.45 eV/atom, and increased with increasing temperature and with decreasing bulk B content. These results have been rationalized in terms of a spectrum of binding energies for a given alloy. However, when the entropy of adsorption was taken into account, an enthalpy of adsorption of B to the grain boundary of 0.13 eV/atom was obtained, independent of temperatire and bulk B content. This is interpreted to mean that the spectrum of binding energies is quite restricted. The grain boundary B content of these alloys has also been measured as a function of annealing time at 773, 873, 973 and 1173 K. The diffusion coefficient of B in Ni₃Al at 773 K is about 5 × 10⁻²¹ m²/s, and the equilibrium grain boundary B content is attained at about 3000 s. The diffusion coefficient at 973 K is between 10⁻¹⁶ and 10⁻¹⁷ m²/s. The activation energy for diffusion of B in Ni₃Al is between 200,000 and 300,000 J/mol.     

Statistical and structural effects during solute-atom segregation to grain boundaries

Mean field theory of a rigid grain boundary segregation in binary mixture is developed on the basis of local equilibrium and geometrical considerations. The application of the Guggenheim adsorption isotherm equation to the problem of internal interfaces is proved to be valid for the case of regular substitutional solid solution. The segregation coefficient is found to be a function of a new interaction constant which yields the relative stability of an interface between a crystal made of pure solute and a crystal of pure solvent, with respect to the stability of grain boundaries in solvent and solute materials. Three typical structures are obtained when a local miscibility gap is formed and two other structures when the bulk tends to order. Segregation can stabilize new boundary structures which may involve changes in the intergranular translation, faceting, etc.     

Hydrogen embrittlement of L12-type Ni3 (Al,Ti) single crystals

The deformation and fracture behavior of the L1₂-type Ni₃ (Al,Ti) single crystals were investigated on the effect of testing atmosphere and strain rate. The elongation and the ultimate tensile stress decreased in the sequence, vacuum > air > hydrogen and also these values decreased as strain rate decreased. The cleavage fracturing with the river-like patterns was dominant in the single crystals which were tensile-tested in air and hydrogen. Whereas, the ductile fracturing with the ridge patterns was dominant in the single crystals which were tensile-tested in vacuum and at a high strain rate. Macroscopic fracture planes changed from non-crystallographic plane, through {001} plane, to {111} and {011} planes as used atmosphere changed from vacuum through air to hydrogen gas. As the micro-mechanism responsible for the observed embrittlement, the “decohesion” mechanism was likely. Hydrogen accumulates at the microdefect created by dislocation reaction (or at a resultant micro crack tip) and thereby lowers the cohesive strength of the lattice at which the bond rupture occurs.     

Thermodynamically stable β[Ni(Al,Ti)]-β′[Ni2AITi]-γ[Ni3(Al,Ti)] metal-metal composites

Single-phase intermetallic compounds are often quite brittle. Experiments are reported in which a metallic composite consisting of three different intermetallic compounds (β, β′, and γ′ ) in the Ni-Ti-Al system was fabricated by extruding a mechanical mixture of the compounds. In this way, the microstructure could be engineered to disperse the most ductile intermetallic in between the more brittle components. An additional feature was to choose the β, β′, and γ′ powders with compositions which allowed them to coexist in equilibrium when coextruded to form the composite. It is found that this approach confers elevated temperature microstructural stability, a crack arresting ability, reasonable ductility, and considerable strength even at high temperatures.     

Mechanical twinning in Ti50Ni47Fe3 and Ti49Ni51 alloys

Pseudo-twinning and mechanical twinning have been observed in a transmission electron microscopy study of Ti₅₀Ni₄₇Fe₃, and Ti₄₉Ni₅₁ alloys which have the B2(CsCl) structure. Observation of twinning in ordered alloys is rare and this is the first observation of twinning reported in a B2 structure. The twin planes are the {112} and {114} planes. For {112} pseudo-twins, the composition plane is not the twin plane and the pseudo-twin does not have the B2 structure. For {114} mechanical twins, the composition plane is the twin plane and the twin does have the B2 structure. It is shown that a shear on the {114} plane plus a shuffle of the atoms results in the ordered B2 structure in the twinned region.