Investigation of the aging behavior and orientation relationships in Fe–31.4Mn–11.4Al–0.89C low-density steel

The aging behavior and orientation relationships in Fe–31.4Mn–11.4Al–0.89C low-density steel were investigated with respect to constituent phases and precipitates, including γ-austenite matrix, β-Mn, and α-precipitate. After aging treatment at 550 °C for various periods of time, the microstructural changes and corresponding mechanical response were characterized by Vickers hardness measurement combined with EBSD and TEM observations. The precipitation sequence during the aging treatment showed that nano-sized κ-carbides firstly precipitated within the γ-austenite matrix at the incipient stage of aging, and induced the primary age hardening. After aging for 300 min, the lath-type β-Mn phase was formed, leading to the dramatic secondary hardening response. The α-precipitates with partial D03 order were subsequently produced at the β-Mn interior, grain/phase boundary region, and the γ-austenite interior after further aging over 10,000 min. The misorientation-angle distribution, Rodrigues–Frank vector space, and orientation relationship stereogram (OR stereogram) from EBSD measurements were employed for analyzing γ-matrix/β-Mn and β-Mn/α-precipitate interphase boundaries, respectively. The OR stereograms showed that the preferred orientation relationships were represented as (111)γ//(221)β−Mn, (011¯)γ//(012¯)β−Mn, (2¯11)γ//(5¯42)β−Mn for γ-matrix/β-Mn interface, and (012)β−Mn//(001)α, (021¯)β−Mn//(010)α, (100)β−Mn//(100)α for β-Mn/α-precipitate interface, respectively. The orientation relationships obtained from the OR stereograms were clarified by checking the deviation angle distributions of interface segments from the ideal orientation relationships, as well as the TEM diffraction patterns at the interface boundaries. In addition, the misorientation distribution between γ-matrix and α-precipitate was examined and compared to conventional fcc/bcc orientation relationships.