MARTENSITIC TRANSFORMATIONS IN ULTRA-FINE PARTICLES OF METALS AND ALLOYS

Martensitic transformations in ultra-fine particles of Fe-Ni alloys, pure Co and Co-Fe alloys have been studied by X-ray diffraction and electron microscopy. The diameter of the particles used in the present work is 20-200 nm, three orders of magnitude smaller than that of the particles employed so far in studies of the martensitic transformation of 'fine particles'. The composition range of Fe-Ni alloys is 12-35 at.% Ni and that of Co-Fe is 3-6 at.% Fe, and most of the particles are defect-free single crystals in the as-formed state. The most surprising finding is that the transformation below room temperature is very difficult to induce by simple cooling even in the case where particles of the parent phase contain lattice defects, while the transformation above room temperature occurs normally in most of the particles of Fe-Ni alloys and deformation at room temperature can easily induce the transformation. For pure Co and Co-Fe alloys, the transformation products above room temperature are 2H and 9R structures, although the amounts of the transformation of these structures are small. Deformation of these particles at room temperature induces a 2H structure or other unknown long-period stacking structures from f.c.c. or 9R structures. Stacking faults which are the most favourable nucleation sites for the transformation between close-packed structures are, of course, produced in particles by deformation but, upon cooling below room temperature, no further transformation occurs in these particles. The 4H structure which was reported to be stable at room temperature in Co-Fe bulk specimens is not formed in ultra-fine particles either by cooling below room temperature or by deformation at room temperature.