Stoichiometric NiAl Research Articles

Point defects in NiAl: The effect of lattice vibrations

We investigate the effect of atomic vibrations on point defect free energies and equilibrium concentrations in the B2 NiAl compound using the quasiharmonic approximation in combination with a recently developed embedded-atom potential. The entropy term appears to be the dominant contribution to the Gibbs free energies of point defects. Vibrational entropies of the main defect complexes: triple-Ni defect, exchange defect, divacancy, and even of the interbranch-Al defect turn out to be positive in the whole range of temperatures studied here (0--1700 K). This leads to an increase in the concentrations of all four types of point defects in Ni-rich and stoichiometric NiAl. On the Al-rich side, the effect of lattice vibrations is to shift the minimum on the vacancy concentration versus temperature curve towards lower temperatures. The effect of zero-point vibrations is shown to be too small to affect the type of constitutional defects in NiAl. The constitutional defects remain nickel antisite atoms on the Ni-rich side and nickel vacancies on the Al-rich side.

Defects and hyperfine interactions in Ni–Y intermetallics(Y = Al, Ga, In,Ti) via 27Al, 47Ti, 61Ni, 69,71Ga and 115In nuclear resonance

Defect structures and nuclear electric hyperfine interactions have been investigated in a series ofbinary Ni–Al, Ni–Ga, Ni–In intermetallic compounds, and the shape memory compound NiTi using27Al,47,49Ti,61Ni,71Ga and115In nuclear resonance.Components of the 61Ni NMRspectrum for a series of cubic Ni1−xAlx and Ni1−xGax specimenson either side of x = 0.5 have been identified as due to Ni substitutions and Al (or Ga) vacancies. For stoichiometric NiAl,Ni2Al3 and NiAl3 the 61Ni lines are narrow and distinguished by well-separated Knight shifts, although the61Ni lineshape for the ordered vacancy compoundNi3Al4 indicates a substantial nuclear quadrupole interaction at the Ni site.The substitution for Al of the group III elements Ga and In,which have isoelectronic outer shells, acts to increase the61Ni Knight shiftsuch that 61K(Al)<61K(Ga)<61K(In). This trend is observed not only for cubic NiAl and NiGa, but also for trigonalNi2Al3,Ni2Ga3 and Ni2In3.In NiTi the sharp first-order transition between the (high-temperature) cubic phase andthe (low-temperature) monoclinic phase has been observed while monitoring the61Ni and 49,47Ti NMR lineshapes as a function of temperature.In the non-cubic phases of these materials (including hexagonalε-NiIn) the electric field gradient tensor componentsVzz and η at the atomic sites have been determined from the nuclear quadrupole perturbed27Al,47Ti,61Ni,71Ga NMR lineshapestogether with 69Ga and 115In NQR transition frequencies, and compared with values derived from ab initio calculation.

The hot extrusion reaction synthesis of nickel aluminide alloys

Elemental powder compacts of Ni–13.3 wt.% Al, the composition that corresponds to stoichiometric Ni3Al, have been processed by hot extrusion reaction synthesis (HERS) to form a dense rod. A narrow processing window has been determined for the experimental scale rig used. The success of the processing route has been shown to be highly dependent on the initial billet temperature, the heating time of the billet, and the extrusion speed. HERS has been successful at relatively low processing temperatures (less than 600 °C) and offers a highly economical method of producing aluminide extrusions. The extruded microstructure consisted of a mixture of ordered gamma prime precipitates in a Ni-rich matrix and grains of twinned martensite. Both a {200} texture and a weaker {111} texture were observed along the extrusion direction. The Vickers hardness of the extruded material was 340 kgf /mm2. The 0.2% compressive yield stress at room temperature at a constant strain rate of 10−3 s−1 was 600 MPa.

Orientation dependence of texture evolution in cold-rolled Ni3Al single crystals

Texture evolutions of cold-rolled thin foils of binary stoichiometric Ni3Al single crystals were examined as a function of the initial crystal orientation. In the cases of the initial rolling direction (RD) close to ⟨001⟩, a double {110} {112} texture associated with a banded structure tends to develop irrespective of the initial foil normal direction (ND). The macroscopic shape of these foils with the banded structure is straight and simply elongated along RD, keeping their rectangular shape. In contrast, when the initial RD is close to {112}, the texture and the microstructure are rather uniform without the texture splitting and the banded structure formation. The foils of group B are macroscopically curved, twisted and eventually cracked from the side edge of the samples. Tendencies of the lattice rotations vary with the initial ND in the latter case. According to the plastic strain analysis, the formation of the banded structure is found to be essential for fabricating the thin straight foils in the case of the cold rolling of Ni3Al single crystals.

The phosphorus effect on the superplastic deformation of NiAl

The P effect on the superplastic deformation of NiAl has been investigated. It is shown that P segregates to grain boundaries and the segregation has profound influence on the high temperature tensile properties of the NiAl alloy. The segregation retards dynamic recovery and recrystallization that leads to a decrease in elongation and enhancement in cavitation compared with the stoichiometric NiAl. Therefore, the P has harmful effect on the superplastic deformation of NiAl.

Transmission electron microscopy of diffusion-soldered Ni/Al/Ni interconnections

Transmission electron microscopy (TEM) was used to study cross-sections of diffusion-soldered Ni/Al/Ni interconnections. Precipitation of coherent Ni-rich AlNi particles (3–10nm) was observed within the stoichiometric NiAl compound. A new phase was detected in the peninsulas of Ni-rich AlNi embedded in AlNi3. This phase is described as enriched with Ni and with the structure of NiAl L1o (CuAu I type). It is suggested that the Ni-rich NiAl phase with cubic B2 (CsCl type) structure transforms martensitically into a close-packed tetragonal L1o (CuAu I type) structure upon cooling. It is an unexpected result because previous electron probe microanalysis and X-ray diffraction analysis of this region revealed it as a NiAl one-phase layer enriched with Ni.

Mechanical behaviour of Σ3 boundaries in Ni 3Al

The mechanical properties of Σ3 boundaries in binary stoichiometric Ni 3Al were studied by performing tensile tests on miniature bicrystal specimens. It was found that Σ3 boundaries do not fracture in air. They are intrinsically strong and show no environmental embrittlement, independent of the grain boundary plane. An upper angular deviation limit, Δθ c = 15 Σ -1 ~ 15 Σ -2/3, is suggested for the Σ3 boundary depending on the fracture behaviour. The boundaries lying within Δθ c do not fracture, whereas the boundary with an angular deviation larger than Δθ c exhibits brittle fracture. The macroscopic slip behaviour at the grain boundary was found to be related to specimen geometry. when the common slip system in both halves of the bicrystal is the primary slip system, slip can transfer readily across the grain boundary, leading to uniform deformation. Otherwise, slip is impeded at the grain boundary.

Microstructural evolution in a PH13-8 stainless steel after ageing

The precipitation process in a wrought PH13-8 steel during ageing was investigated using a position sensitive atom probe. The precipitates formed are enriched in Ni and Al, and depleted of Fe and Cr, yet the composition is far from the stoichiometric NiAl phase. They may take on different shapes at different temperatures. The hardening effects observed during early stages of ageing should be due to the redistribution of atoms such as Fe, Cr, Ni, and/or Al. Particle coarsening takes place simultaneously with the development of the composition of the NiAl-enriched precipitates. Mo segregation at the precipitate/matrix interface was not found.

Determination of the constitution of the quasi–binary eutectic NiAl–Re system by DTA and microstructural investigations

The constitution of the quasi–binary eutectic NiAl–Re section of the ternary Al–Ni–Re phase diagram was reinvestigated by using differential thermal analysis, metallography and scanning electron microscopy. The alloys studied were stoichiometric NiAl and various NiAl–Re x (0.75< x<2.5 at.%) alloys. The microstructure of the as-cast NiAl–Re eutectic is of irregular fibrous morphology. Ingots with Re concentrations ⩾1.25 at.% reveal non-equilibrium microstructures which consist of primary Re crystals, NiAl crystals, and eutectic. An ideal eutectic microstructure was achieved by unidirectional solidification of a NiAl–Re 1.25 alloy using Bridgman technique at a low growth velocity of 30 mm h −1. The melting temperature of the stoichiometric NiAl compound is 1674 °C. The eutectic point of the quasi–binary section of the NiAl–Re system is determined to be at 1.25 at.% Re and a temperature of 1668 °C. The solidus temperatures of the NiAl–Re ingots exhibit a slight dependence on the Re concentration. The solid solubility of Re in NiAl is 0.6 at.% at the eutectic temperature and 0.2 at.% at 1300 °C.

Microstructure of ion-irradiated stoichiometric NiAl intermetallic: a comparative image simulation and transmissionelectron microscopy study

A comparative image simulation and transmission electronmicroscopy (TEM) study has been addressed in order to assess whetherNiAl intermetallic undergoes a crystalline-to-amorphoustransformation directly induced by the cascade. For this purpose,an NiAl single-crystal specimen has been irradiated with6 MeV nickel ions to 0.1 dpa (maximum) and observed byconventional TEM in weak-beam imagingmode. This work shows that a crystalline-to-amorphoustransformation directly induced by the cascade does not occur(absence of amorphous halo). Image simulation reported a defectcluster size of 2.8 nm as compared to a mean value of 3.0 nmmeasured on the experimental micrograph. The same image simulationstudy reported a defect cluster density of 1.7×1024 m-3 whereas a direct measurement from theexperimental micrograph indicates 1.4×1023 m-3, avalue about 10× lower, most probably due to the resolutionlimit of the technique and also due to the contrast transferfunction oscillation along the specimen thickness, which mightrender a fraction of defect clusters invisible.

Room-temperature mechanical properties of cold-rolled thin foils of binary, stoichiometric Ni3Al

In our previous works, thin foils of boron-free stoichiometric Ni3Al, with thicknesses ranging from 57 to 315 µm, were fabricated by cold rolling of single-crystalline sheets which were sectioned from directionally solidified ingots. In this article, the room-temperature mechanical properties of the 83 and 95 pct cold-rolled foils were examined. Depending on the initial rolling direction, the foils exhibited two types of deformation microstructures: a banded structure with dual {110} textures and a band-free structure with a single {110} texture. The 83 pct cold-rolled foils showed very high Vickers hardness numbers: 649 and 604 for the banded and band-free structures, respectively. The foils possessed very high tensile fracture stress (1.7 to 2.0 GPa), with no appreciable plastic elongation along the rolling direction. The fracture stress of the 95 pct cold-rolled foils was slightly higher than that of the 83 pct cold-rolled foils. The banded-structure foils showed slightly higher fracture stress than the band-free-structure foils at the 83 pct reduction, but there was no difference between both the structures at the 95 pct reduction. Although there was no appreciable tensile elongation, slip traces were clearly observed on the surfaces of the foil specimens after the tensile test, indicating traces of some plastic deformation. The 95 pct cold-rolled foils possessed bending ductility, which was estimated as about 12 pct elongation on the tension-side surface of the bent specimen.

Grain-boundary fracture strength in Ni3Al bicrystals

Abstract The fracture properties of grain boundaries in binary stoichiometric Ni3Al were studied by performing tensile tests at room temperature in air on miniature bicrystal specimens with various grain-boundary types, namely. ∑ = 1, ∑ = 3, ∑ = 5, ∑ = 1, ∑ = 9, ∑ = 13a and random boundaries (RBs). All the specimens exhibited some tensile ductility prior to fracture. A fairly good relationship was found between fracture mode, fracture strength and ∑ value. The ∑ = 1, ∑ = 3 and ∑ = 9 boundaries were strongly crack resistant and the adjacent bulk fractured instead. In contrast, the ∑ = 5, ∑ = 7 and ∑ = 13a boundaries and RBs were less crack resistant, showing a typical intergranular fracture. The fracture strength of these boundaries was low and widely varied with the orientation of the component grains and the grain-boundary plane. The cohesive strengths of ∑ = 1, ∑ = 3 and ∑ = 9boundaries were estimated to be high in comparison with that in bulk Ni3Al, that of RBs much smaller and those of ∑ = 5, ∑ = 7 and ∑ = 13a boundaries slightly larger than that of RBs.

Void formation and morphology in NiAl

Void formation in stoichiometric NiAl was studied through controlled heat treatments and microstructural characterization through transmission electron microscopy. Voids were observed to form at temperatures as low as 400°C, but were noted to dissolve during annealing at 900°C. Two distinct void shapes, cuboidal and rhombic dodecahedral, were observed, often at the same annealing temperature. At higher temperatures (≥800°C) extensive dislocation climb, rather than void formation, was noted. The relative incidence of void formation and dislocation climb can be related to the mobility of vacancies at each annealing temperature. The shape of void type is described in terms of their relative surface energy and mode of nucleation.

High Temperature Resistant Intermetallic Nial-Based Alloys with Refractory Metals Cr, Mo, Re - Structures - Properties - Applications -

ABSTRACTThe stoichiometric intermetallic compound NiAl with B2 superlattice structure exhibits superior physical and high-temperature mechanical properties, and excellent oxidation resistance. The main disadvantages of polycrystalline NiAl are the lack in plasticity and fracture toughness below the brittle-to-ductile-transition temperature of about 550°C. Insufficient high-temperature strength and creep resistance occur at temperatures above 800°C. Despite these facts NiAl-based alloys are still considered as promising structural materials for high-temperature applications. The refractory metals Cr, Mo, and Re with b.c.c. and h.c.p. lattice structures form with NiAl quasi-binary eutectic systems, showing high melting temperatures and thermally stable microstructures. Elasticity, solid solution hardening, fibre reinforcement, and creep properties were investigated in view of the constitutional defect structure and microstructural features. Especially the fibre reinforced NiAl matrix composites possess optimum high-temperature strength up to 1200 °C, and improved creep resistance as well.

Orientation Dependence of the Texture Evolution in Cold-Rolled Thin Foils of Ni3Al Single Crystals

ABSTRACTTexture evolutions of cold-rolled thin foils of binary stoichiometric Ni3Al single crystals were examined as a function of the initial crystal orientation. In the cases of the initial rolling direction (RD) close to &lt;001&gt;, the rolling texture above 90% reduction is composed of two {110}&lt;112&gt; textures, which resulted from the formation of the banded structure with two types of differently oriented matrix bands. The macroscopic shape of these cold-rolled foils is straight and simply elongated along RD keeping their rectangular shape. In contrast, when the initial RD is close to &lt;112&gt;, the texture and the microstructure are rather uniform but the foils are curved, twisted, and eventually cracked from the side edge of the samples. Based on the analysis of the texture evolution and the microstructure observation, the operative slip systems were determined.

The effect of long range order on the activation energy for atomic migration in NiAl alloys; resistivity study

The influence of the degree of long range order (S) on the migration energy in NiAl was investigated by electrical resistivity and X-ray diffraction. Iron (12at.%) was over-alloyed to the stoichiometric NiAl in order to decrease the order-disorder transition temperature.(NiAl12Fe) The degree of LRO in NiAl12Fe decreased gradually as temperature increased. The activation energies for migration of iron in NiAl12Fe (QAFe* that were measured from the temperature dependence of jumping frequency (1/τFe*) were 1.42 ± 0.19 eV, 1.35 ± 0.18 eV, 1.16 ± 0.06 eV for the alloys quenched from 1073 K, 1273 K, and 1473 K respectively. The activation energies for migration of excess vacancy (QAv* in NiAl single crystal, were 1.55 ± 0.02 eV for the specimen quenched from 1473 K and 1.40 ± 0.11 eV for one quenched from 1673 K. The degree of LRO (S) in single crystalline NiAl calculated from the result of Hughes Lautensclager, Cohen and Brittain, J. Appl. Phys.42 (1971) 3705 seems to be consistent with this decreasing tendency in the activation energy. The reduction of the degree of LRO (S) can explain these variations in the migration energy as the Girifalco's model.

Fabrication of thin foils of binary Ni–Al γ/γ′ two-phase alloys by cold rolling

Recently we have successfully fabricated thin foils (57 μm in thickness with 94% reduction) of boron-free binary stoichiometric Ni 3Al by using directional solidification and cold rolling [1]. In this study the same process was applied to binary Ni–Al γ/γ′ two-phase alloys. Single crystals were grown for Ni–16 and Ni–18 at.% Al by a floating zone method, while columnar-grained polycrystals for Ni–20 and Ni–22 at.% Al. It turned out to be possible to cold-roll to 200 μm-thick foils with 80% reduction without any cracks for Ni–16 and Ni–18 at.% Al similar to the case of the stoichiometric Ni 3Al, while some cracking occurred up to the same reduction for Ni–20 and Ni–22 at.% Al. Strong {110} rolling texture evolved on all the foils. The tensile properties of the rolled foils were measured at room temperature.

Texture induced plastic anisotropy of NiAl polycrystals

Textured stoichiometric NiAl polycrystals have been deformed in compression at room temperature under 0.4 GPa confining pressure at a strain rate of about 10 −4 s −1. The strength of the samples depends on the preferred orientation with respect to the compression axis, with 〈100〉 being much harder than those with 〈110〉 and 〈111〉. Similarly, the frequency of intercrystalline microcracking shows the same tendency. The orientation distribution of the microcracks indicates that they mainly result from internal stresses released after unloading. The plastic anisotropy at elevated strains correlates with the Taylor energy calculated. The deviation from axially symmetric deformation is due to the tendency of minimizing the Taylor energy.

Ordering kinetics in ternary Ni 3Al–X alloys

Measurements of electrical resistivity were used to study the kinetics of ordering in stoichiometric Ni 3Al alloyed with a third element (Nb, Ti, V, W, Cr, Co, Fe). The activation energies of ordering and disordering were found.

Hydrogen diffusivity in single crystal stoichiometric NiAl

The hydrogen diffusion behavior of single crystal stoichiometric NiAl was investigated. The results show that the hydrogen diffusivity and permeabilty of single crystal stoichiometric NiAl obey Arrhenius relationship in the experimental temperature range. The activation energy of hydrogen diffusion in single crystal stoichiometric NiAl is about 45 kJ/mol.