EFFECTS OF CARBON CONTENT AND AUSAGING ON GAMMA [--] ALPHA' TRANSFORMATION BEHAVIOR AND REVERSE-TRANSFORMED STRUCTURE IN FE-NI-CO-AL-C ALLOYS

The effects of carbon content and ausaging on austenite (gamma) <-- -- > martensite (alpha') transformation behavior and reverse-transformed structure were investigated in Fe-32Ni-12Co-4Al and Fe-(26,28)Ni-12Co- 4Al-0.4C (wt pct) alloys. The M(s) temperature, the hardness of gamma phase, and the tetragonality of alpha' increase with increasing ausagi ng, time, and these values are higher in the carbon-bearing alloys in most cases. The gamma --> alpha' transformation behavior is similar to that of thermoelastic martensite; that is, the width of alpha' plate increases with decreasing temperature in all alloys. The alpha' --> ga mma reverse transformation temperature is lower in the carbon-bearing alloys, which means that the shape memory effect is improved by the ad dition of carbon. The maximum shape recovery of 84 pct is obtained in Fe-28Ni-12Co-4AI-0.4C alloy when the ausaged specimen is deformed at t he M(s) temperature and heated to 1220 K. There are two types of rever se-transformed austenites in the carbon-bearing alloy. One type is the reversed gamma containing many dislocations which were formed when th e gamma/alpha' interface moved reversibly. The plane on which dislocat ions lie is (011)gamma if the twin plane is (112)alpha'. The other typ e of reverse-transformed austenite exhibits gamma islands nucleated wi thin the alpha' plates.