The effects of boron and hydrogen on the embrittlement of polycrystalline Ni3Al

The discrete-variational method within the framework of density functional theory is used to study the effects of both boron and hydrogen on the embrittlement of polycrystalline Ni₃Al. The calculated results show that there are strong repulsive interaction between the boron and the hydrogen atoms, if they occupy the nearest interstitial sites, respectively, in the Ni₃Al grain boundaries. It indicates that the boron atoms inhibit the diffusion of hydrogen atoms along the grain boundary. It may be the main reason why boron can suppress the moisture induced hydrogen embrittlement. Our results also show that the attractive interactions between boron and some substrate atoms are weakened, but the attractive interactions between boron and other substrate atoms are enhanced, when hydrogen atoms are forced into the grain boundary and occupy the nearest interstitial sites to boron atoms. As a result, the bonding states are polarized in the local region of the grain boundary. It may suppress the movement of slips across the grain boundary. Furthermore, the weakening effects of hydrogen to the grain boundary are hardly affected by the boron atoms, even though they are very near to each other. It can be concluded that hydrogen embrittlement takes place when the boron-doped polycrystalline Ni₃Al are charged with hydrogen.     

Calculation of electronic properties of boundaries in Ni3Al

The effects of boron doping on both the Ni-d DOS and the cohesive properties of Ni₃Al have been studied through the use of supercell models having the local composition of Ni₃Al with and without B at grain boundaries. The calculations of the density of states are compared with experimental results obtained from electron energy loss near edge structures. The results suggest an increase in the intensity of the Ni-L_2,3 white line when there is Ni enrichment; they also suggest that this increase disappears when B is added to the Ni-enriched model boundary, in agreement with experiment. The relationship between these results and cohesive enhancement as observed in B-doped Ni₃Al is discussed.     

Phase field modeling the growth of Ni3Al layer in the β/γ diffusion couple of Ni–Al binary system

The growth behavior of Ni₃Al phase layer in the β/γ diffusion couple of Ni–Al binary system, including the shape evolution and growth kinetics, have been simulated by using the KKS multiphase field model. Simulation results indicate that, when Ni₃Al layer growth is controlled completely by volume diffusion, it could be regarded as parabolic growth. However, if the fast grain boundary diffusion is taken into account, the growth rate of Ni₃Al phase is accelerated, and the growth kinetics deviates from parabolic growth, which is consistent with experiments and other simulation results. Simulation results also demonstrate some details of shape evolution of grains, such as the uneven Ni₃Al/γ and Ni₃Al/β interfaces and the grain boundary migration of Ni₃Al grains caused by fast grain boundary diffusion.     

Microstructure and mechanical properties of Ni3Al and Ni3Al–1B alloys fabricated by SHS/HE

The well-densified Ni₃Al alloys without and with boron addition were fabricated by self-propagation high-temperature synthesis and hot extrusion (SHS/HE) technology. Microstructure investigation showed that Ni₃Al and Ni₃Al-1B alloys contained fine grain structure. Analysis of X-ray spectra as well as transmission electron microscopy studies revealed that three phases present in all alloys: γ-Ni, Ni₃Al and dispersoids of α- Al₂O₃ and γ- Al₂O₃. However, β-NiAl, Ni₃B phase and twinned Ni₃Al crystal are observed in the Ni₃Al-1B alloy. In addition, dislocations with high density exist in all alloys. The mechanical test showed that the B addition leads to obvious improvement in yield strength and compressive ductility, and compared with the ones synthesized by combustion, SHS/HE synthesized Ni₃Al and Ni₃Al-1B alloys exhibit more excellent mechanical properties.     

Effect of coherency on interphase boundary sliding in NiAl(β) bicrystals with film-like Ni3Al(γ′) precipitate along boundary

Effect of the crystallography of Ni₃Al(γ^′) precipitates along grain boundaries of NiAl(β) on the interphase boundary sliding of (β/γ^′) interface was examined using β bicrystals. Interphase boundary sliding occurred preferentially at incoherent (β/γ^′) interface and the sliding displacement increased with increasing deviation angle from the Kurdjumov–Sachs orientation relationship.     

Microstructure and mechanical properties of Ni3Al fabricated by thermal explosion and hot extrusion

The dense Ni₃Al with different boron and chromium addition was fabricated by the thermal explosion and hot extrusion (TE/HE) technology. Microstructure examination showed that Ni₃Al contained fine grain structure (200 nm to 10 μm). Analysis of X-ray spectra as well as transmission electron microscopy studies revealed that three phases present in all alloys: γ-Ni, Ni₃Al and dispersoids of Al₂O₃. With the addition of B, β-NiAl phase and twinned Ni₃Al formed, but in the B and Cr doped alloy, Cr rich phase and a spot of Cr₇Ni₃ particles with stacking faults inside were observed. In addition, dislocations with high density existed in all alloys. The mechanical test showed that the B and Cr addition resulted in significant improvement in mechanical properties, and TE/HE synthesized Ni₃Al alloy owned more excellent mechanical properties, compared with the one synthesized by combustion.     

Predicting grain refinement by cold severe plastic deformation in alloys using volume averaged dislocation generation

Grain refinement during severe plastic deformation (SPD) is predicted using volume averaged number of dislocations generated. The model incorporates a new expansion of a model for hardening in the parabolic hardening regime, in which the work hardening depends on the effective dislocation-free path related to the presence of non-shearable particles and solute–solute nearest-neighbour interactions. These two mechanisms give rise to dislocation multiplication in the form of generation of geometrically necessary dislocations and dislocations induced by local bond energies. The model predicts the volume averaged number of dislocations generated and considers that they distribute to create cell walls and move to existing cell walls/grain boundaries, where they increase the grain boundary misorientation. The model predicts grain sizes of Al alloys subjected to SPD over two orders of magnitude. The model correctly predicts the considerable influence of Mg content and content of non-shearable particles on the grain refinement during SPD.     

Interphase boundary fracture and grain boundary precipitation of Ni3Al(γ′) phase in β-NiAl bicrystals

The effect of the crystallography of film-like Ni₃Al(γ′) precipitates along grain boundaries of NiAl(β) on the fracture stress at room temperature was examined using β bicrystals with controlled orientations. The selected variant of γ′-film satisfied the Kurdjumov–Sachs (K–S) relation with a neighbouring β crystal, and deviated from the relation with an adjacent β crystal. In the course of tensile deformation at ambient temperature, fracture occurred preferentially at the (β/γ′) interphase boundary deviating from the K–S relation, which showed no plastic flow, and the fracture stress decreased with increasing deviation angle. In contrast, slip transfer from γ′-film to β crystal across coherent (β/γ′) interface was observed, when the variant of γ′-film satisfied the K–S relation with both neighbouring β crystals. To clarify the relation between the interphase boundary fracture and the deviation angle from the K–S relation, the boundary structure of incoherent (β/γ′) interfaces was discussed using O-lattice theory and transmission electron microscopic observations.     

What can be learnt on the yield stress anomaly of Ni3Al using AFM observations

We examined by in situ scanning probe microscopy the slip traces at the free surface of Ni₃(Al,Ta) single crystals deformed in ultra high vacuum environment at various temperatures. It is evidenced that the mean free path of superdislocations in the primary octahedral planes strongly decreases with increasing temperatures. This is quantified by the decrease in the number of dislocations along slip traces corresponding to the primary (111) slip planes. Correlatively the number of cross-slip events increases with increasing temperature, together with a drastic increase of the distance travelled by the dislocations on the cube cross-slip plane. The experimental results are discussed in the frame of previous theoretical models that have been proposed to predict the yield stress anomaly of L1₂ intermetallic compounds.     

Dislocation slip and twinning in Ni-based L12 type alloys

We report theoretical results on dislocation slip and twinning in Ni₃ (Al, Ti, Ta, Hf) compositions with L1₂ crystal structures utilizing first-principles simulations. The lattice parameters of Ni₃Al, Ni₃Al_0.75Ta_0.25, Ni₃Al_0.5Ta_0.5, Ni₃Ta, Ni₃Ti and Ni₃Al_0.75Hf_0.25 are calculated, and the crystal structures with lower structural energies are determined. We established the Generalized Stacking Fault Energy (GSFE) and Generalized Planar Fault Energy (GPFE), and calculated stacking fault energies APB (anti-phase boundary) and CSF (complex stacking fault) matched other calculations and experiments. Based on the extended Peierls–Nabarro model for slip and the proposed twin nucleation model, we predict slip and twinning stress and the results show a general agreement with available experimental data. The results show that in the studied intermetallic alloys, twinning stress is lower than slip stress; Ta and Hf ternary addition are substantial to increase flow stress in Ni₃Al. The models proposed in the paper provide quantitative understanding and guidelines for selecting optimal precipitate chemistry and composition to obtain higher mechanical strength in Shape Memory Alloys.     

Influence of grain boundary composition on the moisture-induced embrittlement of Ni3(Si,Ti) alloys

Ni₃(Si,Ti) alloys containing different levels of boron were tensile tested in air at room temperature at two different strain rates. The grain boundary compositions and fracture modes of these alloys were determined by Auger spectroscopy and scanning electron microscopy, respectively. Tensile elongation and fracture mode depend upon the fabrication procedure, heat treatment, and strain rate. Widely different boron concentrations were observed at the grain boundaries, depending on the fabrication procedure and heat treatment. In addition, silicon and titanium were depleted while nickel was enriched at the grain boundaries in all specimens examined. Tensile elongations correlated well with the grain boundary concentration of boron and also with an embrittlement parameter defined as (Si+Ti−B)/Ti. A sharp brittle-ductile transition was found to occur with increasing grain boundary concentration of boron and with decreasing values of the embrittlement parameter (Si+Ti−B)/Ti. The critical grain boundary concentrations corresponding to this transition were found to be sensitive to the strain rate. All the results can be explained in terms of the effect of grain-boundary composition on moisture-induced environmental embrittlement.     

Effect of chromium on early stages of oxidation of Ni3Al alloys at 600°C

Initial oxide formation at 600°C in air on Ni₃Al alloys with and without chromium additions was studied by TEM. Significant lateral nickel diffusion (apparently stress-induced) occurred in both alloys producing bands of nickel and nickel oxide-enriched hillocks. Chromium additions clearly alleviate dynamic embrittlement in Ni₃Al; chromium additions were previously assumed to affect the oxidation process. Chromium additions significantly reduced the oxidation rate of the alloy. However, a continuous film of pure Cr₂O₃ had not yet formed after 45 sec oxidation. Grain boundaries preferentially oxidized to form Al₂O₃ or Cr₂O₃ and rejected nickel along both the surface and the grain boundary, deeper into the specimen. The dramatic effect of chromium on improving the ductility of Ni₃Al when tested at high temperature in air is apparently a result of a process that occurs at or near the tip of a propagating crack that is both faster and on a finer scale than that studied here.     

Hydrogen diffusivity in B-doped and B-free ordered Ni3Fe alloys

The hydrogen diffusion coefficient of the ordered Ni₃Fe–B alloys with and without boron additions was measured by a method of the cathodical precharging with hydrogen. The apparent hydrogen diffusion coefficient decreases with increasing the boron concentration doped in the ordered Ni₃Fe alloy. Comparing with the B-free ordered Ni₃Fe alloy, the activation energy of hydrogen diffusion for the ordered B-doped Ni₃Fe alloy increases by as high as 42% when the boron content is sufficient. The doping boron in the Ni₃Fe alloy is effective in reducing the hydrogen diffusion at the grain boundary.     

Discontinuous precipitation at the deformation band in copper alloy

The Cu-Ni-Si alloy is known as a precipitation hardening alloy, where the Ni₂Si intermetallic compound is precipitated in the matrix during aging. There are two types of precipitation of Ni₂Si: continuous and discontinuous cellular. The discontinuous cellular precipitation is generally initiated at interfaces especially grain boundaries in the matrix. To observe the grain boundary effect on the discontinuous precipitation, a large-grained Cu-Ni-Si-Ti alloy was intentionally fabricated by unidirectional solidification and plastically deformed by groove rolling. While discontinuous cellular precipitation has been generally known to occur only at the high angled grain boundaries in the alloys, we found that it was also generated inside the grains, at the deformation bands formed by plastic deformation.     

Dispersoid Formation and Stability in Alloys

Analysis of dispersoid formation and stability indicates that in-situ formation of an adequate volume fraction of fine particles can be thermodynamically incompatible with stability against high-temperature coarsening. The theory of particle coarsening is extended to include the effects of grain boundaries and dislocations. Theoretical analysis predicts that particle dragging by migrating grain boundaries combined with enhanced coarsening by grain boundary diffusion can give denuded regions near grain boundaries. These predictions of enhanced coarsening and particle dragging are in accord with experimental observations on α-Ti and Ti₃Al based alloys.     

Analytical and computational description of effect of grain size on yield stress of metals

Four principal factors contribute to grain-boundary strengthening: (a) the grain boundaries act as barriers to plastic flow; (b) the grain boundaries act as dislocation sources; (c) elastic anisotropy causes additional stresses in grain-boundary surroundings; (d) multislip is activated in the grain-boundary regions, whereas grain interiors are initially dominated by single slip, if properly oriented. As a result, the regions adjoining grain boundaries harden at a rate much higher than grain interiors. A phenomenological constitutive equation predicting the effect of grain size on the yield stress of metals is discussed and extended to the nanocrystalline regime. At large grain sizes, it has the Hall–Petch form, and in the nanocrystalline domain the slope gradually decreases until it asymptotically approaches the flow stress of the grain boundaries. The material is envisaged as a composite, comprised of the grain interior, with flow stress σ_fG, and grain boundary work-hardened layer, with flow stress σ_fGB. The predictions of this model are compared with experimental measurements over the mono, micro, and nanocrystalline domains. Computational predictions are made of plastic flow as a function of grain size incorporating differences of dislocation accumulation rate in grain-boundary regions and grain interiors. The material is modeled as a monocrystalline core surrounded by a mantle (grain-boundary region) with a high work hardening rate response. This is the first computational plasticity calculation that accounts for grain size effects in a physically-based manner. A discussion of statistically stored and geometrically necessary dislocations in the framework of strain-gradient plasticity is introduced to describe these effects. Grain-boundary sliding in the nanocrystalline regime is predicted from calculations using the Raj–Ashby model and incorporated into the computations; it is shown to predispose the material to shear localization.     

Effect of yttrium coating on the oxidation behavior of Ni3Al

Yttrium-coated Ni₃Al with post heat treatment has shown good high-temperature oxidation resistance. To understand the effect of the Y-coating and post heat treatment on the oxidation resistance of Ni₃Al, the specimens were coated with Y by an ion-plating method, and heat treatment was performed at low oxygen level before or after the Y-coating was applied. Performance of the Y-coated Ni₃Al was evaluated by isothermal and cyclic oxidation tests. A simple deposition of Y on Ni₃Al did not change the oxidation kinetics, but the post heat treatment after Y-ion plating significantly decreased the oxidation rate of Ni₃Al. The scale formed on Y-coated Ni₃Al with post heat treatment after Y-ion plating showed a fine and dense structure which was grain refined by the presence of a (Y, Al) O-type oxide in the scale. The coated Y layer becomes a Y-Al compound during heat treatment. The presence of the (Y, Al)O-type oxide in grain boundaries or the lattice of Al₂O₃ modify the diffusion rate of Al and oxygen, and the oxide microstructure during oxidation. Improvement of cyclic-oxidation resistance of Ni₃Al by the Y-coating occurs because the presence of (Y, Al)O-type oxide develops fine-grain oxides which can easily relieve the growth stress.     

Grain-boundary precipitation in Allvac 718Plus

Allvac 718Plus is a newly developed nickel-based superalloy designed to replace Inconel 718 in static and rotating applications in gas turbine engines. Fine γ′ precipitates act as its principal strengthening phase and a second plate-like phase is generally present at grain boundaries. This has been reported to be the δ phase (Ni₃Nb, D0_a, orthorhombic), and is used to pin grain boundaries and improve resistance to intergranular fracture. In Allvac 718Plus non-uniform precipitation of a δ-like phase was observed along the grain boundaries; however, no relation was found between grain-boundary misorientation and the occurrence of precipitation. The crystal structure and chemistry of this phase was found to be different from the orthorhombic Ni₃Nb δ phase reported previously in Inconel 718 and Allvac 718Plus. The true structure was found to be consistent with the hexagonal η-Ni₃Ti D0₂₄ structure, but its chemistry was close to Ni₆AlNb with partial ordering of Al and Nb over the prototype Ti sites. It was found that the serrated boundaries observed in the alloy were a result of discontinuous precipitation of η-Ni₆AlNb, which was a predominant precipitation mechanism throughout the microstructure.     

Beyond the Fisher model of grain boundary diffusion: effect of structural inhomogeneity in the bulk

An extension of the Fisher model of grain boundary diffusion is suggested, in which the diffusion along the short-circuit paths in the bulk of the crystalline grains (dislocations, subgrain boundaries, interphase boundaries in the lamellar structures) is taken into account. In the framework of the suggested model the bulk is treated as a stochastic mixture of defect-free crystalline regions and regions of bad material inside the short-circuit paths. The Harrison classification of the diffusion regimes is extended to the new D-regime, in which the kinetics of the penetration of the diffusing element along the grain boundaries is dominated by diffusion along these short-circuit paths. Three different kinetic modes during the grain boundary diffusion in the D-regime are uncovered: for the short annealing times the penetration kinetics follows the Whipple law, but with the bulk diffusion coefficient substituted by g²D_d, for the intermediate annealing times the penetration distance along the grain boundary is a weak function of time and for the long times the Whipple law is valid again, but with the bulk diffusion coefficient substituted by gD_d, where g and D_d are the volume fraction of the material inside the short-circuit paths and the diffusion coefficient along them, respectively. The applications of the suggested model to the analysis of experimental data are discussed.     

A dislocation pile-up model for the yield stress of a composite

A numerical model to simulate yielding in a composite is developed for the transmission of slip across a dissimilar interface through the formation of co-planar dislocation arrays in both phases. A pile-up of dislocations in the soft phase is assumed to nucleate dislocations in the hard phase in which movement is dictated by lattice friction stress. The polycrystalline composite yield stress is calculated by determining the equilibrium positions of the dislocation arrays as a function of the length scales, elastic constants and Burgers vectors in the two phases, with particular reference to melt oxidized Al–Al₂O₃, in which homophase boundaries are absent, and to the commercially important system Co–WC. The hardness values predicted from this model are in good agreement with experimentally measured values in the above systems. The implications of these results for the design of hard composite microstructures are elucidated.