MARTENSITIC PHASE-TRANSFORMATIONS IN ALKALI-METALS AND COBALT ALLOYS BY NEUTRON-SCATTERING TECHNIQUES

The paper gives a review of a series of neutron scattering experimments revealing static and dynamic precursor phenomena of the structural instability in b.c.c. alkali metals occuring at low temperatures. Inelastic neutron scattering studies in lithium, sodium and potassium showed in all three systems a zone boundary softening of the transverse acoustic phonon branch TA2[110][110] propagating in the [110] direction and polarized in [110]. Moreover, elastic diffuse neutron scattering investigations give evidence for defect formation in the vicinity of the phase transition in sodium and lithium, but also in potassium at low temperatures where, however, no macroscopic phase change occurs. Finally, an experiment on the h.c.p.-to-f.c.c. transformation in cobalt and Co-32wt.%Ni is described, showing the presence of an intermediate modulated structure in the two-phase region. This lastt experiment shows the importance of coherency stresses in martensite nucleation and evolution. © 1990.