On the formation of ultra-fine grained Fe-base alloys via phase transformations

This paper concentrates on the formation of ultra-fine grained Fe-base alloys via phase transformations. In particular the manipulation of the microstructure of Fe–Ni–Ti and Fe–Ni–Cr alloys via phase cycling in the Fe–N system was investigated. Transitions between bcc (α-Fe), fcc (γ′-Fe4N) and hcp (ε-FexN) structures were obtained by gaseous nitriding of cold-rolled foils. During cycling the rate of transformation was progressively retarded due to recovery as a result of the nitriding/reduction treatment. The α↔γ′ cycling at 380 °C did not produce significant grain reorientation in neither Fe–Ni–Ti nor Fe–Ni–Cr alloys. After α↔ε cycling at 400 °C the texture of the Fe–Ni–Ti alloy was preserved. Reduction of texture in this alloy was obtained at 500 °C. Grain refinement and grain reorientation took place after α↔ε transformations in FeNiCr alloy at 400 °C. An optimum of five cycles was found for structure refinement to about 100 nm. Increasing the number of cycles leads to further coarsening of the microstructure.