Epsilon martensite to austenite reversion and related phenomena in Fe24Mn and Fe24Mn6Si alloys
Abstract
Fe24Mn and Fe24Mn6Si alloys whose Ms temperatures are higher than room temperature have been found to show markedly different epsilon martensite (ɛ) → austenite (γ) reverse transformation behavior in spite of exhibiting a similar γ → ɛ forward transformation behavior. Hereafter, they are referred to as 24Mn and 24Mn6Si, respectively. Closely connected with the one-way and two-way shape memories and microstructure memory, the γ ⇔ ɛ transformations were studied by in-situ transmission electron microscopy (TEM) observations and tension tests. The ɛ → γ reversion was found to occur not only from the outside but also from the inside of ɛ plates in both alloys. The dislocations yielded by the reverse transformation can move easily showing cross slip in 24Mn while small segments of stacking faults remain even at 873 K in 24Mn6Si. This difference in the dislocation substructure in the reversed γ at elevated temperatures leads to cause the reversible γ ⇔ ɛ cyclic transformations in 24Mn6Si and less reversible ones in 24Mn. The in-situ TEM observations of γ → ɛ transformation after annealing at 873 K in 24Mn6Si reveals that stacking faults are piled up to form nano-scale ɛ/γ lamellar plates. On the other hand, γ is stabilized with cyclic γ ⇔ ɛ transformations in 24Mn. Since the dislocation density in the reversed γ differs in the two alloys distinguishably, the strengths of γ at various situations were measured, and the characteristic features observed above are confirmed macroscopically: the recovery of these transformation induced dislocations was studied as a function of annealing temperature. The suitable upper annealing temperature is imperative for obtaining an excellent reversibility in the cyclic γ ⇔ ɛ transformations in 24Mn6Si
- Publication:
-
Acta Materialia
- Pub Date:
- 1998
- DOI:
- 10.1016/S1359-6454(97)00285-1
- Bibcode:
- 1998AcMat..46.1419T