Microstructural Influence on Dynamic Properties of Age Hardenable FeMnAl Alloys

Report No. ARL-RP-0321
Authors: MAJ Ryan Andrew Howell, Ph.D.
Date/Pages: April 2011; 262 pages
Abstract: A lightweight castable alloy was sought to reduce the MIL-PRF-32269 class II cast steel perforated armor's weight with the requirement that the material had to be manufactured utilizing existing foundry technology and without incurring large alloy cost increases to meet property requirements. Literature on wrought age hardenable Fe-Mn-Al-C alloys suggested this alloy system could achieve weight reduction through high aluminum concentrations with the highest reported strengths exceeding 2 GPa for a Fe-30Mn-9Al-0.9C composition. Even though ballistic testing had not been conducted on this system, high strain rate data of wrought alloys showed excellent work hardenability; greater than existing ballistic metals. Cast material property information was severely limited, thus, a systematic approach was employed to develop casting and processing techniques and assess related structure property relationships of a nominal silicon modified Fe-30Mn-9Al-0.9C-0.5Mo alloy for ballistic use. Castability was addressed first as this information was crucial for making test coupons and assisting foundries with production of MIL-PRF-32269 ballistic test plates. Four silicon concentrations were investigated for fluidity, microstructure, liquidus, solidus and dendrite coherency point. Silicon was added because it is known to increase fluidity of other ferrous alloys and has also been shown to eliminate a brittle β-Mn phase in wrought Fe-Mn-Al-C alloys. Of the four silicon modified fluidity compositions, two were selected for heat treat property evaluation on the basis of microstructure. Hardness, strength, and ductility were measured (hardness is the only MIL-PRF-32269 measured property). The alloy with the highest ductility was selected for high strain rate evaluation. The strain rate testing results were the final means to lock in the alloy composition and heat treatment for solid plate ballistic testing. While conducting V50 ballistic testing, phosphorus content was correlated to ballistic impact energy. Further testing was conducted to examine phosphorus, quench sensitivity, and aging Charpy V-Notch effects. The culmination of this thesis work resulted with positive ballistic threat testing revealing the alloy investigated here meets the Army's MIL-PRF-32269 ballistic requirements and reduces P900 weight by 13%.
Distribution: Approved for public release
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Last Update / Reviewed: April 1, 2011