Michael Piston, a doctoral candidate in metallurgical engineering, will defend their dissertation titled “Development of Ultrahigh Strength Lightweight Steels.” Their advisor, Dr. Laura Bartlett, is an associate professor in the materials science and engineering department. The dissertation abstract is provided below.

Steels in the Fe-Mn-Al-C alloy system continue to be of interest as high-strength, low-density alternatives for traditional structural, automotive, and military applications. Additions of aluminum (3-12 wt%) reduces the density of austenitic steels by about 1.4% per 1 wt% Al, while carbon (up to 2 wt%) is over four times more effective than Al for a total density reduction of up to 17% compared to martensitic steels. Fe-Mn-Al-C steels that contain greater than 0.7 wt% C and 7 wt% Al are precipitation hardenable by the homogenous coherent precipitation of nanosized κ-carbide, (Fe,Mn)3AlC, in the temperature range of 450–650 °C. However, κ-carbide alone is insufficient to achieve the hardness requirements (50 HRC) for high hard armor (HHA) without severely reducing impact properties that are crucial for ballistic plate applications. In order to enhance the hardness and strength of these Fe-Mn-Al-C steels, additions of Ni can be added to promote a fine dispersion of heterogeneously precipitated B2-NiAl after cold rolling. However, plate processing cannot utilize cold rolling which excludes a significant number of potential applications for B2-NiAl strengthened Fe-Mn-Al-C.

This research aims to understand the formation mechanisms and kinetics of B2-NiAl precipitation in wrought austenitic Fe-Mn-Al-C steels to design novel compositions that are capable of precipitating B2-NiAl only through hot rolling to achieve higher hardness than in κ-carbide strengthened steels while maintaining adequate impact toughness for use in critical applications. An additional objective is to develop a single composition that can be processed to satisfy both the MIL-DTL-12560K for Class 1 and 2 rolled homogeneous armor (RHA) between 27-42 HRC and the MIL-DTL-46100 specification for high hardness armor (HHA) plate above 50 HRC, while retaining greater than 12 J of toughness in the T-L orientation, and greater than 10% density reduction compared to the martensitic steels currently in use.