An investigation of transformation strengthening in precipitation-hardened Fe-Ni austenite

Abstract

Ausaged Fe-33Ni-3Ti and Fe-34Ni-3Ti-0.5Al austenitic alloys were transformation strengthened through sequential martensite (α′ → γ) and reverse martensitic transfor-mations. An increase in yield strength of 20 to 30 ksi (130 to 210 MPa) was obtained, leading to alloy yield strengths near 180 ksi (1250 MPa) in the ausaged and transforma-tion strengthened condition. The presence of the γ′ precipitates formed on ausaging did not radically change the nature of the shear transformation nor the morphology and sub-structure of the transformation product. The presence of the γ′ precipitates did, how-ever, have the beneficial consequence that the α′→ γ reversion could be accomplished with slow heating rates without loss of transformation strengthening. The yield strength actually increased when a slow heating rate was used, a result attributed to the success-ful pinning of transformation induced defects by the γ′ precipitates and to the occurrence of additional precipitation during heating. The efficient pinning of transformation induced defects had the further consequence that the alloys could be annealed for long periods after reversion to austenite without significant loss of strength.