Martensitic Structural Transformation Research Articles

Theory of the bcc-to-9R structural phase transformation of Li.

We present a Landau theory of the martensitic structural phase transformation undergone by bcc Li into the 9R phase. We propose that the transition occurs via incomplete softening of the ((1/3 1) /3 0) ${\ensuremath{\Sigma}}_{4}$ phonon. Even though this branch does not soften to zero frequency (or even close to zero frequency), because the system is strongly anharmonic, a first-order transition can occur. Coupled to this distortion are the two homogeneous strains possessing the soft Zener elastic constant 1/2(${c}_{11}$-${c}_{12}$). These strains shear and stretch the (110) planes. As a result of our studies, we propose that Li may transform into any member of a family of displacement patterns, the 9R stacking sequence corresponding to one solution within this family. In addition, our theory describes the stacking faults that are known to occur in terms of (i) domain-wall structures, which model twin faults, and (ii) a multiple domain-wall solution of the double sine-Gordon equation (an equation that describes the phase-modulation equilibrium condition around defects), which models deformation faults. We also discuss the possible extension of this theory to other \ensuremath{\beta}-cubic structures that undergo martensitic phase transformations, viz., the other alkali metals, and the TiNi(Fe) alloy system.

Magnetoresistance measurements of V 3Si evidence for precursor effects of the martensitic transformation

We report magnetoresistivity measurements on transforming and non-transforming V 3Si. The martensitic structural transformation ( T m ≊ 21 K) is induced by annealing. The transforming sample shows large deviations from the Köhler's rule [Δϱ/ϱ = ƒ(H/ϱ)], at temperatures much higher than T m . They are attributed to precursor effects of the transition, previously observed by other techniques, and are interpreted in terms of the development of different relaxation times for the scattering of the electrons.

Experimental characterization of the flux-line lattice in superconducting V<inf>3</inf>Si

Several microscopic properties of the flux-line lattice (FLL) in three separate single crystals of V 3 Si have been investigated by means of small-angle neutron diffraction. These low-field FLL characterizations have been correlated with the following material and superconducting properties: (A) The real crystal symmetry parallel to the applied magnetic field; (B) The micro-structure as determined by TEM; (C) Magnetic irreversibilities in the mixed state; (D) Reversible flux-line motion in ac response; (E) Martensitic structural transformation observed by X-ray diffraction. The three samples, V 3 Si-MP3, -MP4, and -MP5 possessed different defect structures, and this was manifested foremost in the FLL perfection. At low field (B B\gsim0-.5 T, but a highly mosaic, nearly polycrystalline FLL at lower fields. Sample MP4 contained large (500-1000 A) incoherent precipitates, and showed only a polycrystalline FLL at low field. In both MP3 and MP5, distinct anisotropic correlations were observed between the FLL morphology and the real-crystal direction along the applied field. The FLL perfection was strongly dependent on the growth history. The peak width history dependence for two different scattering geometries can be qualitatively modeled by proposed flux-pinning mechanisms. Quantitative comparisons with critical current measurements, however, are not totally reconcilable.