Ultra-high tensile strength via precipitates and enhanced martensite transformation in a FeNiAlC alloy

Aging and quenching at critical temperatures were applied to a hot-rolled (HR) Fe-24.86Ni-5.8Al-0.38C (mass%) dual-phase alloy to obtain B2 precipitates with various volume fractions and sizes. Higher yield strength and stronger strain hardening were achieved in the aged samples compared to these for the HR sample, and the corresponding deformation mechanisms were carefully revealed. The aged samples show stronger hetero-deformation induced hardening compared to that for the HR sample. The amount of phase transformation during tensile tests is much higher for the aged sample compared to that for the HR sample due to the reduced stability of the austenite phase, which can be attributed to the high local stress level induced by undeformable and hard B2 precipitates around the austenite grains and reduction of nickel and aluminum in the austenite phase by diffusion from the austenite phase to the B2 precipitates during aging. Deformation-induced lath martensite with high dislocation density can be observed after tensile deformation, and these transformation-induced dislocations should have great impact on the strain hardening. Moreover, the density of twins becomes much higher for the martensite grains after tensile deformation, and these deformation-induced nanotwins should contribute significantly to the strain hardening, as the dynamic Hall-Petch effect.