THE EFFECT OF RETAINED AUSTENITE ON THE CRYOGENIC MECHANICAL PROPERTIES IN FINE-GRAINED Fe-12Ni-0.25Ti ALLOY

The phenomenological behavior of diffusion-controlled reverted austenite in a fine-grained Fe-12Ni-0.25Ti cryogenic alloy system was investigated. The grain-refined martensite structure was exposed to a temperature within two-phase (a + y) region. The stability of equilibrium austenite was affected by the decomposition temperature. It appeared to be the redistribution of solute elements that controlled the stability. The microstructural appearance of retained austenite was examined using transmission electron microscopy. A preferential distribution of the austenite phase along the martensite lath boundaries was observed. A precipitate-correlated austenite nucleation was also found to occur. The ' beneficial effect of the introducing retained austenite appeared not only in the tensile elongation but also in the Charpy impact toughness at low temperatures. A degradation in the yield stress occurred after an ex~ tended holding at an intermediate temperature or after a moderate time holding at a relatively high temperature. The deterioration of strength was interpreted in terms of retained austenite stability and precipitate averaging. The retained austenite did riot improve KIC at 77°K. Unstable crack propagation developed during the fracture toughness tests of most specimens containing retained austenite phase.