Neutron Studies of Pretransitional Fluctuations Above Structural Transitions in Metals

Abstract

Resume. -Les transitions de phase structurales dans les metaux 3-d mettent en jeu des transformations simples (de Burgers) entre les structures bien connues, cfc, cc, hc, et la phase mktastable omiga. Ces changements cristallographiques sont accompagnks de cisaillements macroscopiques variks et de deplacements microscopiques dependant du type de changements de symktrie mis en jeu. La plupart de ces transitions sont du ler ordre, cependant, et l'on continue de chercher jusqu'8 quel point on peut decrire de tels changements de phase sur la base d'une dynamique de rbseau. La diffraction de neutron, associke aux etudes de rayons X, est en principe l'outil ideal pour attaquer ce problkme, et on a mis en evidence dans certains de ces systemes, l'existence de mode mou etlou de fluctuations prktransitionnelles. Cependant, la majoritk de ces systkmes ne prksentent pas de comportement de mode mou dramatique, comme il se manifeste, par exemple, dans Nb3Sn, et dans de nombreux cas, la rkponse dynamique de r6seau apparait normale. I1 est fait aussi brikvement allusion 8 I'influence des defauts sur la diffusion des neutrons.Abstract. -Structural phase transitions in (3-D) metals usually involve simple (Burgers) transformations among the well known structures, fcc, bcc, hcp and metastably, the omega phase. Accompanying these crystallographic changes are various macroscopic shears and microscopic shuffles depending upon what kinds of symmetry changes are ultimately involved. Most of the transitions are first order, however, and it has been of continuing interest to explore the extent to which a lattice dynamical basis can be given to such phase changes. Neutron scattering, coupled with X-ray studies, is in principle an ideal tool for looking at this problem and limited evidence exists for mode softening and/or pretransitional fluctuations in some of these systems. The majority of systems do not, however, exhibit dramatic soft mode behavior, as is seen, for example, in Nb3Sn, and in many cases the lattice dynamical response appears normal. The influence of defects on the neutron scattering is also briefly alluded to.

Domain boundary-dominated systems: adaptive structures and functional twin boundaries

“…Static adaptive states are known to be stable only over a narrow temperature range near the transformation temperature. It should be noted, however, that a strain or quadrupolar glass condition consisting of nanoregions of martensite has been reported to remain frozen to much lower temperatures for some of these alloys with high concentrations of defects [100][101][102], as originally conjectured by Sethna and Krumhansl [108]. In fact, they may be rather common, as there seems to be some development of shape anisotropy in the nanoregion morphology, suggesting the onset of tweed-like characteristics [110,169].…”

“…Early structural investigations of martensitic phase transformations have revealed a pretransitory phenomenon sandwiched between the high-temperature austenite and low-temperature martensite phase regions over a narrow temperature range near the transition temperature [100][101][102][103]. Electron microscopy indicated the presence of elastic tweed precursors, as illustrated in Figure 18 [102].…”