Transformation behaviour of a Ti₅₀Ni₄₇Fe₃ alloy. III. Martensitic transformation

Abstract

In situ electron microscopic observations of the martensitic transformation and the martensite phase in a Ti₅₀Ni₄₇Fe₃ alloy have indicated that the M₈ temperature in thin foils of the subject alloy is about -98°C. The electrical resistance decreases abruptly at the M₈ temperature. Below the M₈ temperature, martensite plates nucleate and grow, and ‘consume’ the previously formed needle-like domains and their matrix. The crystal structure of the martensitie phase was determined to be of the B_19′ type (monolinic). In addition to (IIT) internal transformation twins, stacking faults on the (001) basal plane and antiphase domains boundaries were observed within the martensite plates. The antiphase domains in the martensite, which are a consequence of atomic ordering rather than electronic ordering (charge density waves–CDWs), are different from those observed in the premartensitic phases. The ‘1/3’ superlattice reflections characteristic of CDW formation are not found in the martensitic phase. The relative orientations of the martensitic phase, needle domains and matrix rhombohedral phase were also determined.

It is suggested that the ‘premartensitic’ effects found in the Ti₅₀Ni⁴⁷Fe₃ alloy are separate, electronically driven structural changes, and not precursory effects, in so far as the martensitic transformation itself ia concerned. Thus the sequence of transformation events (on cooling) in the TiNiFe alloy is as follows: parent phase (B₂)→ incommensurate phase (distorted nubic)→commensurate phase (rhombohedral)→martensitic phase (monolinic, B_19′).