Structural and Phase Transformations and Physical and Mechanical Properties of Cu-Al-Ni Shape Memory Alloys Subjected to Severe Plastic Deformation and Annealing

Using the methods of electron microscopy and X-ray analysis in combination with measurements of the electrical resistance and magnetic susceptibility, the authors have obtained data on the peculiar features of pre-martensitic states and martensitic transformations, as well as subsequent decomposition, in the alloys with shape memory effect of Cu–14wt%Al–3wt%Ni and Cu–13.5wt%Al–3.5wt%Ni. For the first time, we established the microstructure, phase composition, mechanical properties, and microhardness of the alloys obtained in the nanocrystalline state as a result of severe plastic deformation under high pressure torsion and subsequent annealing. A crystallographic model of the martensite nucleation and the rearrangements β1→β1' and β1→γ1ꞌ are proposed based on the analysis of the observed tweed contrast and diffuse scattering in the austenite and the internal defects in the substructure of the martensite.

Superstructures and ordering phenomena in ceramic superconductors

It is now well established that the phase transformation behavior of YBa2Cu3O6+δ is significantly influenced by matrix strain effects, as evidenced by the formation of accommodation twins, the occurrence of diffuse scattering in diffraction patterns, the appearance of tweed contrast in electron micrographs, and the generation of displacive modulation superstructures, all of which have been successfully modeled via simple Monte Carlo simulations. The model is based upon a static lattice formulation with two types of excitations, one of which is a change in oxygen occupancy, and the other a small displacement of both the copper and oxygen sublattices. Results of these simulations show that a displacive superstructure forms very rapidly in a morphology of finely textured domains, followed by domain growth and a more sharply defined modulation wavelength, ultimately evolving into a strong <110> tweed with 5 nm to 7 nm period. What is new about these findings is the revelation that both the small-scale deformation superstructures and coarser tweed morphologies can result from displacive modulations in ordered YBa2Cu3O6+δ and need not be restricted to domain coarsening of the disordered phase. Figures 1 and 2 show a representative image and diffraction pattern for fully-ordered (δ = 1) YBa2Cu3O6+δ associated with a long-period <110> modulation.

An atomic model for premartensitic tweed in Ni-Al as derived from HREM images

The premartensitic microstructures observed in the quenched Ni1-XAlx (B2) phase have been examined at room temperature by HREM. Depending on the composition the material transforms martensitically to a close-packed 7R (5) stacking at Ms≈200 K (x=0.37) or to the 3R (L10) structure at Ms≈290 K (x=0.36). In CTEM images the premartensitic structure is revealed as the well known two-beam tweed contrast. In HREM phase contrast images a random fine-scale mosaic assembly of micromodulated domains is observed (fig. 1). Fig. 2 shows a single domain in which the intensity modulation can clearly be seen. From images taken under different conditions the following observations (among others) could be made: 1) the micromodulation has an averaged incommensurate wavelength of≈1.3 nm, 2) OD reproduces the <110>* diffuse streaks confirming the static nature and the correlation with the tweed image, 3) each domain exhibits a single modulation, 4) the intensity difference is of the order of 60-85%, 5) some images (e.g., fig. 2) reveal dots elongated perpendicular to the wave-vector, 6) in some cases a small average deviation (≈1%) from the cubic symmetry can be measured. These different characteristics could be explained by assuming, per domain, small transverse shuffle plus shear distortions of the {110}<> type and including a decrease in atom displacement from the center to the edge of each domain. The atomic model and one calculated image are presented in fig. 3.