The Optimization and Design of a Fully Austenitic, Gamma-Prime Strengthened TRIP Steel for Blast and Fragment Resistance

Current analysis into the property requirements of materials designed for blast and fragment protection has led to the need for high tensile uniform ductility to withstand the pressure wave and high shear localization resistance to withstand fragment penetration. Additionally, it has been shown that steels with retained austenite are able to outperform standard martensitic steels when subjected to fragment simulating projectiles (FSP) in ballistic experiments. Using a systems based, computational materials design approach, a series of prototype precipitation strengthened, fully austenitic steels have been designed to obtain superior performance in blast and fragment protection. The most recent design, TRIP-180, explores optimized transformation induced plasticity (TRIP) to counteract strain softening and thus significantly increase uniform plastic deformation in both tension and shear at high strength (1241 MPa / 180 ksi). The transformation hardening delays the onset of localization, which in tension delays necking, and in shear delays plugging. Through precipitation heat treatment, the matrix composition can be varied to optimize the austenite stability, quantified by the Ms sigma temperature.;Baseline data quantifying the martensitic transformation in shear was obtained through a series of quasi-static torsion experiments performed on TRIP-180. Analysis of the postmortem microstructures allowed for calibration of M_s.;sigma(sh) temperatures with the transformation product morphologiesin the stress-assisted regime, where the plate martensite forms at the same locations as when quenching, and strain-induced regime, where the finely dispersed martensite forms at the intersections of shear bands. Dynamic testing (E = 104/s) identified the optimal austenite stability ( T -- Ms sigma(sh) = 60°C ) required to delay the shear localization instability at higher ultimate shear stress levels (1420 MPa) and larger plastic strains (0.103) than an existing Navy standard, HSLA-100 (970 MPa and 0.090). Even with adiabatic heating reducing the martensitic transformation under adiabatic conditions, the performance indicates that there is sufficient transformation plasticity to alter and delay shear localization. Under ballistic conditions (E = 105/s) TRIP-180's performance shows that optimized austenite stability provides for greater relative FSP V50 performance. In addition, postmortem analysis on the impact craters reveals multiple arrested adiabatic shear bands with evidence of a martensite transformation zone around each band's termination. Several parametric models were updated and calibrated with three-dimensional atom probe data to more precisely predict the evolution of the gamma-prime precipitates and matrix composition. Knowledge of the required optimum Ms sigma(sh) temperature, the optimum yield strength for FSP ballistic performance, and the requirement to thermodynamically suppress a grain boundary cellular precipitation of the eta phase quantitatively defined the design parameters of the current prototype alloy, Blastalloy TRIP 130. Designed with a yield strength of 896 MPa (130 ksi) and a M ssigma(sh) temperature of -40C, the alloy exhibited a yield strength of 852 MPa (123.5 ksi) with superior transformation plasticity and a Ms sigma(sh) temperature of -42°C without grain boundary eta formation given an aging of 20 hrs at 700°C.