Crystal NiAl Research Articles

Anisotropy energy for the ordered Ni3Al crystal

The approximate d bands for the ordered Ni3Al crystal are formulated by Deegan’s prescription and Slater and Koster’s formulas in the tight binding approximation. The electronic energies of this crystal with the spin direction parallel to [100] and 110] directions are calculated by using Gilat and Raubenheimer’s method. Then, the anisotropy constant K1 is estimated, which is in agreement with the experimental result. The temperature dependence of K1 is also discussed. It is found that the temperature variation of the Fermi level for these two states is appreciably different with each other by analyzing the experimental result, and this can be explained by the d-band structure of this crystal.

Magnetocrystalline Anisotropy in the Weak Itinerant Ferromagnet Ni3Al

The intermetallic compound Ni3Al orders in the simple cubic (Cu3Au) crystal structure and becomes ferromagnetic below about 40 K with a spontaneous magnetization extrapolated to absolute zero and zero external magnetic field corresponding to approximately 0.075 μB/Ni atom. Measurements of the first magnetocrystalline anisotropy constant K1(T, H) have been carried out in stoichiometric single crystals of Ni3Al in magnetic fields 0 ⩽ H ⩽ 16 kG and in the temperature range 4.2 ≲ T ≲= 42 K by means of a high sensitivity torque magnetometer. At 4.2 K and 0 kG (extrapolated from 6 ⩽ H ⩽ 16 kG) we find K1 = - (9.6 ± 0.7) × 103 erg cm-3, a value having the same sign but about a hundred times smaller magnitude than that for pure Ni at the same temperature and field. The above value of K1 has been reproduced within experimental error in three stoichiometric specimens having Curie temperatures (Tc) between 39 and 42 K and residual resistivity ratios [ρ(293 K)/ρ(4.2 K)] in the range 17 to 34.As in pure ferromagnetic Ni, the decrease of the magnitude of K1 with increasing temperature is, at least for T ≲ Tc/3, markedly more rapid than would be expected in terms of conventional Zener theory. The results are discussed qualitatively in relation to current models of magnetic anisotropy in itinerant electron ferromagnets and compared with recent Fermi surface studies carried out in this laboratory.

THE CREEP AND STRESS-RUPTURE PROPERTIES OF γ′ CRYSTALS

In this paper, the results of creep and stress-rupture tests on Ni3Al base γ′ monocrystals are presented. The results show that the steady state creep rate εs conforms to the relationship: εs=Cσmexp(-Qc′/RT). The exponent m depends only on temperature. m(850℃)=3.95, m(950℃) = 3.62. The apparent activation energy of creep Qc′ amounts to 88 k cal/g-at. for Ni3Al crystals. All of the alloying elements studied Ti, Nb and W raise Qc′, the effect of Nb being the greatest, while W proves to be most effective in improving the creep strength. The orientation of crystals has a marked influence on εs. oriented crystals have the lowest εs and the longest stress-rupture life tr. tr≥ 100 hr for T= 1050℃ and σ= 5 kg/mm2. The Monkman-Grant relation tr(εs)p=const. holds likewise for Ni3Al single crystals, with p = 0.82.The glide system of γ′ crystals in creep appears to be {111} .

The plastic deformation of NiAl single crystals between 300°K and 1050°K

Single crystals of stoichiometric NiAl have been compressed in the temperature range 300°K to 1050°K. Contrary to previous work it has been found that in this range of temperature the flow stress o...

Oxidation-induced defects in NiAl

Abstract The annealing of voids and vacancy dislocation loops previously found in slowly cooled crystals of NiAl (Fraser, Loretto, Smallman and Wasilewski, 1971) has been studied using transmission electron microscopy. It is observed that both types of defect grow during annealing even when it is carried out in a vacuum of about 10−7 torr and that the detailed behaviour is consistent with vacancy introduction by surface oxidation. The occurrence of voids in as-grown single crystals, and the formation of vacancy-type loops in quenched NiAl (Ball and Smallman 1968) may now be interpreted in terms of surface oxidation rather than a low energy of formation of vacancies in NiAl.

The orientation dependence of deformation mode and structure in stoichiometric NiAl single crystals deformed by high temperature steady-state creep

Single crystals of stoichiometric NiAl having predetermined orientations have been deformed by steady-state creep in the temperature range 750° to 1055°C, the stresses being varied from 1.76 to 7.02 kg mm-2. In crystals oriented 7.5 deg from [101], only (100) slip was observed; the primary slip plane was 100 although secondary slip occurred on 110. As predicted by elastic anisotropy considerations, dislocations on 100 with b = α(100) have a zig-zag shape with the segments aligned along (110). On 110 they tend to lie along (111) and (112). Cross-slip of short segments having screw orientations can occur between l00 and 110, giving rise to dipoles and prismatic loops. Crystals with tensile axes 14 deg from [1ll] slip in all three (100) directions on both cube and dodecahedral planes. Characteristic structures are dislocation entanglements, dipoles, loops, and networks containing nodes. Often two (100) type dislocations react and form a segment of (110) dislocation. Crystals approximately 19 deg from the cube orientation slip in both the (100) and (110) directions, and the contribution of (110) slip to the total glide strain increases at higher temperatures. The (110) dislocations are mostly pure screw as predicted by elastic anisotropy. Two sets of α(100) dislocations with mutually perpendicular Burgers vectors can form a network of twist or mixed character. If the contact plane is not one of the cube planes, the network is lozenge shaped, and small (110) segments form at the node points. Slip in the (111) direction was not observed with certainty; (100) slip occurred in all specimens, and (110) slip only in crystals reasonably close to the cube orientation.

A Study of Dislocation Contrast for g.b = 2 and g.b = 4

Shear of Ni3Al crystals usually takes place by the motion of paired a/2<110> dislocations. The pairs are strongly coupled by an antiphase boundary such that their separation is less than 40Å. These are imaged as single dislocations having an a<110> Burgers vector. Thus g.b equals 2 for “in contrast“ imaging with lower order reflections such as g = 111 or 200. The dislocation image contrast under such conditions is shown in Fig. 1 for varying deviations w = sξg from the Bragg orientation. This dislocation was shown by the usual method of identification to have a Burgers vector b = a [011] and, other than the screw segment A, it is mixed in character. (The accurate straightness of the screw segment is due to relaxation of the paired dislocations upon (100). For this reason its image is not representative of that for g.b = 2).

Slip systems in NiAl single crystals at 300°K and 77°K

Abstract Single crystals of NiAl have been examined by transmission electron microscopy after plastic deformation at 300°K and 77°K. In agreement with earlier work, it is found that single crystals generally slip along 〈100〉. Further, it is shown that crystals oriented so that there is zero applied stress on the dislocations of b = 〈100〉, deform by the movement of dislocations of b = 〈111〉 on {112} and {110}. A lower limit to the Peierls stress for dislocations of b = 〈111〉 is calculated from the observed spacing of opposite sign screw dislocations, and it is shown that these dislocations are far more difficult to move than mobility calculations would suggest.

The growth of single crystals of the intermediate phases NiAl and Ni3Al

Single crystals of stoichiometric NiAl, nickel-rich Ni3Al, and aluminium-rich Ni3(Al, Ti) have been grown by a modified Bridgman technique; a strain-anneal method has also been used to produce small, single crystals of non-stoichiometric NiAl.