B2 Crystal Structure Research Articles

Hardening behavior of nickel beryllides

Future aerospace systems depend strongly on the availability of high temperature structural materials which are both strong and lightweight. Unfortunately, the mechanical strengths of almost all structural materials decrease as the temperature increases; an exception to this general rule is found in certain groups of long-range-order (LRO) alloys (or intermetallic compounds). A disadvantage of LRO alloys is that they are either intrinsically brittle because of their complex crystallographic structures, or they are too dense for aerospace applications. The brittleness problem has been studied heavily in recent years and some breakthroughs have been made. For example, polycrystalline Ni/sub 3/Al exhibits almost no ductility, but Ni/sub 3/Al containing a small amount (0.2 to 0.5 wt.%) of B can have a room temperature ductility of up to 40%. The high density of Ni/sub 3/Al remains an intrinsic problem. Amongst all the LRO alloys, those in the beryllide group probably have the lowest densities. In addition, it has been demonstrated that some transition beryllides exhibit attractive specific strengths at elevated temperatures, and these are of considerable interest for structural applications. The beryllide NiBe is of particular interest because it has a relatively simple ordered B2 crystal structure and exhibits a wide range of stoichiometry.more » These properties are similar to those of NiAl and CoAl. In the case of NiAl and CoAl, the alloys exhibit a hardness minimum at the equiatomic composition. The hardness increases observed in off-stoichiometric compositions have been attributed to defect structures developed in the alloys. In this paper, new data are presented on the hardness of NiBe for hypo- and hyper-stoichiometric compositions and comparisons are made with CoAl and NiAl.« less

The influence of grain size and composition on 1000 to 1400K slow plastic flow properties of NiAl

The compressive slow plastic flow behaviour of several B2 crystal structure NiAl intermetallics has been studied in air between 1000 and 1400 K (∼ 0.53 to 0.74TM). Small grain-sized Ni-48.25 at % Al (≈ 5 and 10 µm) was found to be stronger than the previously studied 17 µm diameter material. While grain refinement improved the strength at all test temperatures, the exact mechanism is not clear. Experiments at lower temperature revealed that composition as well as grain size can be an important factor, since Ni-49.2Al was weaker than Ni-48.25 Al. Pronounced yield points were found during slow strain-rate testing at 1000 K; however, continued deformation appears to take place by the same mechanism(s) as found at high temperatures. Small changes in thermomechanical processing (TMP) schedules to fabricate Ni-49.2 Al indicated that basic deformation characteristics (stress exponent and activation energy) are not affected; however, the pre-exponential term could be modified if TMP alters the grain structure.

High T c superconducting B1 phase MoN films prepared by low-energy ion-assisted deposition

Metastable B1 phase MoN was synthesized by low-energy ion-assisted thin-film growth techniques at deposition temperatures TD =400, 500, and 700 °C. The films were condensed under the action of mixed Ar+ and N2+ ion species at an ion to condensing atom arrival rate ratio of 1.0. For each deposition temperature, films were prepared at various ion energies in the range 2–200 eV. The presence of chemically reactive N2+ ions and the use of low ion energies allow both structural and chemical selectivity, facilitate increased incorporation of nitrogen, and promote ordering of the defect structure of cubic MoN. The films were characterized by x-ray diffraction, by measurements of superconducting temperature Tc and by temperature-dependent resistivity ρ(T). Collectively the film properties strongly depend on ion energy and deposition temperature. The composition, lattice parameter, crystalline preferred orientation, and grain size can be controlled by varying ion energy. The resistivity shows an anomalous temperature dependence that is typical of disordered metals, i.e., electron localization governs electrical conduction with the temperature coefficient of resistivity changing sign according to the Mooij correlation. At optimum deposition conditions the resistivity is metallike (phonon contribution ρthermal >0), Tc=6.2 K, the resistivity ratio RR=1.1, and ρ300 K =120 μΩ cm. Annealing of films at 700 °C causes an improvement in properties with Tc≂12 K, RR=1.7, and ρthermal =60–80 μΩ cm. In films with large thermal resistivities we observe a ρ∼T2 behavior at low temperatures and saturation of resistivity at high temperatures (ρsat =200–280 μΩ cm) which suggest that MoN with the B1 crystal structure is a strongly coupled and disordered high Tc superconductor.

Effect of composition and grain size on slow plastic flow properties of NiAl between 1200 and 1400 K

A series of ∼ 15 μm diameter, polycrystalline B2 crystal structure NiAl alloys ranging in composition from 43.9 to 52.7 Al (at%) have been compression tested at constant velocities in air between 1200 and 1400 K. All materials were fabricated via powder metallurgy techniques with hot extrustion as the densification process. Seven intermediate compositions were produced by blending various amounts of two master heats of prealloyed powder; in addition a tenth alloy of identical composition, 48.25 AI, as one of the blended materials was produced from a third master heat. Comparison of the flow stress-strain rate behaviour for the two 48.25 AI alloys revealed that their properties were identical. The creep strength of materials for AI/Ni ⩽ 1.03 was essentially equal, and deformation could be described by a single stress exponent and activation energy. Creep at low temperatures and faster strain rates is independent of grain sizes and appears to be controlled by a subgrain mechanism. However, at higher temperatures and slower strain rates, diffusional creep seems to contribute to the overall deformation rate.

Slow plastic flow properties of B2 CoFeAl and CoFeAl between 1100 and 1400 K

The compressive slow plastic flow behavior of iron- and nickel-modified B2 crystal structure CoAl-base intermetallics has been studied in air between 1100 and 1400 K. In comparison with a binary CoAl material, replacement of cobalt by either iron or nickel improves the properties; however, iron is a more effective solid solution strengthener than nickel. The grain size was also found to play an important role where at slow strain rates the creep strength of the aluminides increased as the grain size increased.

Superconducting NbNxCy thin films fabricated with a dual ion-beam sputtering method

We have fabricated refractory superconducting NbNxCy thin films on unheated Si substrates with a low-energy dual ion-beam sputtering technique. Films fabricated with this technique have predominantly B1 crystal structure with maximum Tc ∼13.2 K, resistivity of 80–150 μΩ cm and residual resistance ratio ∼1.0. The use of low ion-beam energies and the absence of substrate heating make this method well suited for producing NbNxCy films for superconducting microelectronic applications.

Room temperature deposition of superconducting NbN for superconductor–insulator–superconductor junctions

Reactive dc magnetron sputtering is used to deposit hard, refractory, superconducting thin films of niobium nitride on amorphous (glass) as well as crystalline (sapphire) substrates held at room temperature. Stoichiometric NbN films so obtained possess the desired B1 crystal structure with a distinct (111) texture, and exhibit high (∼16 K) superconducting transition temperature. Room temperature deposition of the high Tc films is achieved primarily by optimizing the reactant fluxes of niobium and nitrogen reaching the substrate, through a systematic study of the nitrogen consumption versus injection characteristics which are found to be an absolute indicator of the quality of NbN formed. Superconductor–insulator–superconductor (SIS) junctions are fabricated by using high Tc NbN as the base electrode, a thin layer of native oxide grown in oxygen plasma or in room air as the insulator, and vacuum deposited Pb as the counterelectrode. The current versus voltage (I–V) characteristics of the junctions exhibit high (∼110) nonlinearity parameter and the superconducting gap parameter for NbN, Δ∼2.8 meV. The transition temperature, superconducting gap parameter, and nonlinearity parameter of the junctions are identical for the films deposited at room temperature with predominant (111) texture as well as the films deposited at elevated temperatures possessing (200) texture.

Total-energy calculations for intermetallic compounds with a first-principles linear combination of atomic orbitals method.

For the compounds LiMg and LiAl the difference in the total energy in the B2 (CsCl) and B32 (NaTl) crystal structures has been calculated with use of a first-principles linear-combination-of-atomic-orbitals method. The calculations have been performed as a function of volume---this allows determination of the volume and the heat of formation for each of the compounds. At zero pressure the correct stable structure and a reasonable value for the heat of formation is found for each of the compounds. The bonding characteristics are studied by analyzing the electron-density distributions and the difference electron densities. For LiMg (B2) the bonding is found to be predominantly metallic, while for LiAl (B32) the formation of an Al---Al ``bond charge'' points to an at least partially covalent character of the chemical bond.

The influence of grain size and composition on slow plastic flow in FeAl between 1100 and 1400 K

The slow plastic flow properties of a series of B2 crystal structure FeAl materials ranging in aluminum content from 41.7 to 48.7 at.% have been determined. All intermetallics were fabricated by hot extrusion of blended pre-alloyed powders, and flow behavior was measured via constant-velocity compression tests at strain rates from about 2 × 10 −3 to about 2 × 10 −7 s −1 between 1100 and 1400 K. Two types of true compressive stress-strain diagram were noted: (1) flow at an approximately constant stress and (2) continuous work hardening. Analysis of the stress-strain rate data revealed that two independent deformation mechanisms with the same activation energy exist. Both mechanisms are dependent on grain size; however, for the high stress exponent mode the strength increases with a decreasing grain size while for the low stress exponent regime the opposite is true. The grain size effect in the low stress exponent mechanism is probably due to the action of diffusional creep, and for the high stress exponent mode the grain size strengthening seems to be the result of Hall-Petch behavior. Even though grain growth occurred, creep in the high stress exponent regime can be described in terms of the initial grain size. This is apparently possible because the large-angle grain structure is replaced by a small-angle grain microstructure of similar grain diameter during deformation.

High T c superconducting NbN films deposited at room temperature

Thin films of niobium nitride with superconducting transition temperature (Tc ) of 15.7 K have been deposited on a variety of amorphous as well as crystalline substrates including glass, glazed ceramic, fused quartz, and sapphire, maintained at room temperature, by dc reactive magnetron sputtering in a mixture of Ar and N2 gases. The effects of the deposition conditions, particularly the carrier gas pressure and composition, on the film crystal structure, orientation, and resistivity have been studied in an effort to maximize the superconducting transition temperature. A study of the variation of nitrogen consumption with nitrogen injection pressures for constant background argon pressures is conducted and found to be an absolute indicator of the NbN formation systematics. Initially, the consumption increases linearly with the injection pressure but beyond a certain threshold, it shows a distinct drop. The desired high Tc NbN with B1 crystal structure is formed in the vicinity of this turning point of the reactive gas consumption-injection characteristic. High Tc films possess B1 (fcc, NaCl-type) crystal structure as revealed by their x-ray diffraction patterns. An initial increase in the injection pressure of the reactive gas (N2) results in a remarkable increase in the (111) diffraction line intensity along with an increase in the film Tc. This trend continues up to the turning point of consumption-injection characteristic, beyond which the crystal structure distorts into the substoichiometric tetragonal phase with a consequent reduction in the transition temperature. A general protocol for studying the formation systematics of transition metal nitrides has thus emerged.

Structural phase stability of B2 and B32 intermetallic compounds.

For each of the compounds CsAu, LiAu, LiB, LiAl, LiGa, LiIn, LiTl, LiZn, LiCd, LiHg, NaIn, and NaTl the difference is calculated between the total energy of the compounds in the B2 (CsCl) and B32 (NaTl) crystal structures. The calculations are carried out over a pressure (P) range large enough to predict pressure-induced phase transitions. The structural differences in free energy at P=0 agree in sign with the observation of stable structures. The bonding of CsAu and LiAu appear to be quite different: CsAu is ionic and LiAu metallic. In the present theory where only these two structures are compared, CsAu has, at P=0, the B2 structure, but is predicted to undergo a transition to B32 at P\ensuremath{\approxeq}45 kbar. LiAu is, according to the calculations, not stable at P=0 in the CsCl structure; it requires an external pressure g145 kbar to exist in that structure. Trends of structural energy differences for the I-II and I-III compounds are analyzed by the examination of charge distributions, partial pressures, and their relation to constituent-atom ``sizes.'' The I-II and I-III compounds which at P=0 have the NaTl structure are predicted to undergo a structural phase transition at moderately high pressure (\ensuremath{\sim}200 kbar for LiAl). The bonding characteristics are examined by calculation of (nonspherical) charge distributions. Further analysis is supported by ``frozen-potential'' energy-difference calculations. The (hypothetical) LiB compound is the only one among the Zintl phases studied here which is semiconducting. It is also the case in which the covalent bonding is most pronounced. A few results of calculations for other compounds with structures related to those of the main topic of the work are also presented. These are for the pnictides, ${\mathrm{Li}}_{3}$Sb and ${\mathrm{Cs}}_{3}$Sb, which, in spite of expected similarities, show substantial differences as far as charge distributions are concerned.

Ternary metal hydrides for energy storage

Many metal hydride systems have been proposed for energy storage purposes, but the one which seems to offer the best practical advantages is that based on the intermetallic compound FeTi. It is important that any FeTi alloy consists solely of the FeTi phase with the B2 (CsCl) crystal structure. Excess iron causes the formation of the Fe/sub 2/Ti Laves phase, which does not readily form a hydride. Excess titanium results in the presence of some Ti solid solution, which forms a highly stable hydride, locking up hydrogen in a nonrecoverable form. The presence of either of these phases reduces the efficiency of the hydride.

Ternary metal hydrides for energy storage

Many metal hydride systems have been proposed for energy storage purposes, but the one which seems to offer the best practical advantages is that based on the intermetallic compound FeTi. It is important that any FeTi alloy consists solely of the FeTi phase with the B2 (CsCl) crystal structure. Excess iron causes the formation of the Fe/sub 2/Ti Laves phase, which does not readily form a hydride. Excess titanium results in the presence of some Ti solid solution, which forms a highly stable hydride, locking up hydrogen in a nonrecoverable form. The presence of either of these phases reduces the efficiency of the hydride.

A simple application of the Bailey-Orowan creep model to Fe-39.8 at % AI and γ/γ′-α

A method which allows calculation of recovery rates and work hardening coefficients for creep from simple constant velocity compression experiments is described. Stressing rates and straining rates are computed from the measured load-time curves, and these quantities are then fitted to the universal form of the Bailey-Orowan equation for creep. The techniques were applied to B2 crystal structure Fe-39.8 at % Al intermetallics and the directionally solidified eutectic alloyγ/γ′-α compression tested between 1200 and 1400 K. The recovery rates were found to be functions of nominal strain rate, stress, and temperature; the hardening coefficients were dependent only on temperature. Hardening coefficients for the aluminides are small (<0.002 of the elastic modulus) and are believed to be due to the lack of dislocation barriers. A more typical value for the hardening coefficient (∼ 0.05 of the elastic modulus) was observed inγ/γ′-α. Recovery rates and hardening coefficients can be described in terms of stress and temperature, and the equations used to estimate creep rates.

J-B-T-ε interaction and strain limits in A15, B1, and C15 crystal structure superconductors

Experimental evidence is presented which indicates, that the elastic strain effect on the critical current of high-field compound superconductors correlates strongly with the type of superconductor crystal structure. Large strain effects are observed in all practical A15 superconductors examined to date, including Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn, Nb-Hf/Sn-Ga, Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ge, Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Al, and V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga. Strain is observed to have no measurable effect, however, on either the critical current or the critical field of superconductors having the B1 crystal structure [NbN] or the C15 crystal structure [V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">2</inf> (Hf, Zr)]. Strain limits placed on the mechanical design of superconducting devices are evaluated as a function of magnetic field for several A15 superconductors (Nb <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Sn, Nb-Hf/Cu-Sn-Ga, and V <inf xmlns:mml="http://www.w3.org/1998/Math/MathML" xmlns:xlink="http://www.w3.org/1999/xlink">3</inf> Ga) and compared with the strain limits imposed by B1 and C15 superconductors. It is shown that the latter materials have mechanical design advantage in applications where the superconductor is subjected to strain in excess of 0.2% (either compressive or tensile).

Structure and formation of the metastable phase m-TiBe

The metastable ordered phasem-TiBe was discovered in the course of studying liquidquenched Ti-Be and Ti-Be-Zr alloys. m-TiBe forms, together with the α-Ti solid solution, on heating binary (and ternary) metallic glasses to 700°C and is also observed directly in as-quenched alloys lying outside the composition limits for glass formation. This phase has a B2 (CsCl-type) crystal structure. Its existence can be justified in terms of crystal chemical factors leading to the conclusion that TiBe most likely does not appear as an equilibrium phase in normally cooled alloys due to the greater stability of the Laves phase TiBe2.

Phase transformations occurring in Ni-rich NiAl during quenching

Quenching of Ni-rich NiAl is accompanied by the formation of fine planar precipitates on {110} matrix planes and martensite is produced. Martensite is shown to have a body centred tetragonal crystal structure which can form from the high temperature NiAl B2 crystal structure by a process involving an inverse Bain correspondence. Since further ordering of martensite occurs during quenching a superordered martensite crystal structure is proposed. This structure is characterized by alternate stacking of Ni atoms and Ni or Al atoms on (011) planes. The martensite plates are internally twinned in {101}T [101]T.