Dislocation structures in disordered and ordered Ni3Fe

Transmission electron microscopy (TEM) methods have been used to examine the dislocation structures in thin foils of Ni₃Fe in four different states, corresponding to disordered and deformed; fully ordered and deformed; deformed when disordered and afterwards fully ordered; and deformed when disordered, afterwards fully ordered and additionally deformed. The study has been carried out on single crystals deformed at room temperature. In the disordered alloy slip is coarse and group motion of dislocations is prevailing, as is confirmed by the abundance of planar dislocation arrays. The dislocation structure of such a disordered deformed crystal remains unchanged after additional ordering by annealing 1000 h at 460°C. No rearrangement of the unit dislocations into superlattice dislocations is observed. The dislocations are preserved and since they are unit dislocations they are sessile. The additional deformation of this disordered deformed and afterwards fully ordered crystal proceeds by the glide of superlattice dislocations. They originate in the high internal stresses of the preserved unit dislocations. Cross slip from (111) onto (11̄1) planes is very frequent, whereas cross slip from (111) onto (010) planes is rather rare. The structure of the superlattice dislocations in fully ordered and deformed specimens consists of dipoles and bundles of dipoles of near edge orientation. Superlattice dislocations of near screw orientation are rarely observed, since they cross slip from (111) onto (11̄1) planes and annihilate in most cases. The experimental results on ordered Ni₃Fe samples differ characteristically from those reported in the literature on other alloys having the L1₂ long range ordered structure (e.g. Ni₃Al, Cu₃Au, Ni₃Ga).     

Formation of coatings with nanostructures and amorphous structures

A technique for spraying coatings as thick as 10 mm (one-point spraying technique) is designed to analyze the changes in the structure of rapidly quenched coatings of nickel-and iron-based eutectic alloys at high heating and cooling rates. Zones with amorphous, nano-, and microcrystalline structures form across the coatings. The microhardness of the coating of a grade 12496 nickel-based alloy (Ni-0.8% C-15% Cr-4.1% Si-3.1% B) is maximal in the zone with nanophases, and the microhardness increases from 11351.3 ± 461 to 16372 ± 2066 MPa as the load decreases from 0.2 to 0.01 N. This scale dependence is explained by the effect of microporosity at the boundaries of the sprayed particles. The microhardness of the initial grade 12496 alloy powder with strengthening microphases at a load of 0.01 N is 11572.9 ± 1996 MPa, which is lower than the maximum microhardness of the coating by a factor of 1.41. Thermoplastic treatment of the grade 12496 alloy coating increases its microhardness at a load of 0.05 N from 12372.8 ± 1226.8 to 13287 ± 612.5 MPa due to the strong cohesion of the particles in the coating and the formation of strengthening nanophases in it.     

Formation of pearlitic banded structures in ferritic-pearlitic steels

The conditions for ferrite and pearlite banding in strip and plate made of structural steels were investigated. Factors found to influence the formation of banded structures were the cooling rate during the γ/α-transformation, the former austenite grain size, and the work-hardened condition of the former austenite. Analyses with the aid of an electron beam microprobe made it possible to demonstrate that the carbon-rich bands correspond locally with banded manganese enrichments, yet that they do not form before the course of the γ/α-transformation as a result of secondary segregation. It was possible to explain the mechanism of action of the influencing factors on the basis of this model.     

Phase transformations of disordered structures of graphite-like boron nitride under high-temperature shock compression

The pattern of phase transformations in boron nitride under high-temperature shock compression has been studied using a previously proposed method for high-temperature shock-wave synthesis of high-pressure phases followed by rapid quenching. Fine powders of turbostratic and partially ordered graphite-like BN were used as initial structures. Shock compression was carried out in ring devices at a pressure of 30 GPa and a temperature above 2500 K. A mixture of dense phases (wurtzitic and sphaleritic) was found to form from the graphite-like structures under those conditions; the total yield of those phases and the relative amount of the sphaleritic modification are considerably higher when turbostratic BN is the starting material. Both of the dense phases formed have a nanocrystalline grainstructure. The wurtzitic phase does not transform into the sphaleritic phase under those conditions, which points to cubic BN forming directly from the graphite-like structures.     

Phase transformations of disordered structures of graphite-like Boron Nitride under high-temperature shock compression

The pattern of phase transformations in boron nitride under high-temperature shock compression has been studied using a previously proposed method for high-temperature shock-wave synthesis of high-pressure phases followed by rapid quenching. Fine powders of turbostratic and partially ordered graphite-like BN were used as initial structures. Shock compression was carried out in ring devices at a pressure of 30 GPa and a temperature above 2500 K. A mixture of dense phases (wurtzitic and sphaleritic) was found to form from the graphite-like structures under those conditions; the total yield of those phases and the relative amount of the sphaleritic modification are considerably higher when turbostratic BN is the starting material. Both of the dense phases formed have a nanocrystalline grainstructure. The wurtzitic phase does not transform into the sphaleritic phase under those conditions, which points to cubic BN forming directly from the graphite-like structures.     

Crystallization behavior of amorphous Cu48Ti52: Formation of an intermediate long-period superlattice phase

Amorphous Cu₄₈Ti₅₂ crystallizes during continuous heating by a two-step process involving initial polymorphic transformation to a long period superlattice structure followed by decomposition to the equilibrium phases. From TEM lattice fringe imaging and diffraction data, an antiphase boundary (APB) model for the superlattice structure based on the equilibrium CuTi phase is proposed. The antiphase boundaries have a structure related to the minority equilibrium phase CuTi₂. During longer anneals, oriented precipitates of CuTi₂ form in the CuTi matrix with an orientation relationship rotated 90° from that of the APB structure. The reorientation minimizes strain energy between the lattices.