The influence of silicon on j-carbide precipitation in lightweight austenitic Fe-30Mn-9Al-(0.59-1.56)Si-0.9C-0.5Mo cast steels was investigated utilizing transmission electron microscopy, 3D atom-probe tomography, X-ray diffraction, ab initio calculations, and thermodynamic modeling. Increasing the amount of silicon from 0.59 to 1.56 pct Si accelerated formation of the j-carbide precipitates but did not increase the volume fraction. Silicon was shown to increase the activity of carbon in austenite and stabilize the j-carbide at higher temperatures. Increasing the silicon from 0.59 to 1.56 pct increased the partitioning coefficient of carbon from 2.1 to 2.9 for steels aged 60 hours at 803 K (530°C). The increase in strength during aging of Fe-Mn-Al-C steels was found to be a direct function of the increase in the concentration amplitude of carbon during spinodal decomposition. The predicted increase in the yield strength, as determined using a spinodal hardening mechanism, was calculated to be 120 MPa/wt pct Si for specimens aged at 803 K (530°C) for 60 hours and this is in agreement with experimental results. Silicon was shown to partition to the austenite during aging and to slightly reduce the austenite lattice parameter. First-principles calculations show that the Si-C interaction is repulsive and this is the reason for enhanced carbon activity in austenite. The lattice parameter and thermodynamic stability of j-carbide depend on the carbon stoichiometry and on which sublattice the silicon substitutes. Silicon was shown to favor vacancy ordering in j-carbide due to a strong attractive Si-vacancy interaction. It was predicted that Si occupies the Fe sites in nonstoichiometric j-carbide and the formation of Si-vacancy complexes increases the stability as well as the lattice parameter of j-carbide. A comparison of how Si affects the enthalpy of formation for austenite and j-carbide shows that the most energetically favorable position for silicon is in austenite, in agreement with the experimentally measured partitioning ratios.

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Lightweight advanced high strength steels (AHSS) with aluminum contents between 4 and 12 weight percent have been the subject of intense interest in the last decade because of an excellent combination of high strain rate toughness coupled with up to a 17% reduction in density. Fully austenitic cast steels with a nominal composition of Fe-30%Mn-9%Al-0.9%C are almost 15% less dense than quenched and tempered Cr-Mo steels (SAE 4130) with equivalent strengths and dynamic fracture toughness. This article serves as a review of the tensile and high-strain-rate fracture properties associated mainly with silicon additions to this base composition. In the solution-treated condition, cast steels have high work-hardening rates with elongations up to 64%, room-temperature Charpy V-notch (CVN) impact energies up to 200 J, and dynamic fracture toughness over 700 kJ/m 2 . Silicon additions in the range of 0.59-1.56% Si have no significant effect on the mechanical properties of solution-treated steels but increased the tensile strength and hardness during aging. For steels aged at 530°C to an average hardness of 310 Brinell hardness number, HBW, increasing the amount of silicon from 1.07% to 1.56% decreased the room temperature CVN breaking energy from 92 J to 68 J and the dynamic fracture toughness from 376 kJ/m 2 to 265 kJ/m 2 . Notch toughness is a strong function of phosphorus content, decreasing the solutiontreated CVN impact toughness from 200 J in a 0.006% P steel to 28 J in a 0.07% P steel. For age-hardened steels with 1% Si, increasing levels of phosphorus from 0.001% to 0.043% decreased the dynamic fracture toughness from 376 kJ/m 2 to 100 kJ/m 2 .

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The efficiency of removal of solid alumina inclusions by filtration and the distribution of inclusions captured through the thickness of the filter was investigated for an aluminum killed 316 stainless steel casting. A mold design was developed using modeling software to produce two castings that fill simultaneously, one with a filter and the other without a filter. The design was optimized to produce the filtered casting and unfiltered casting from a single ladle pour, while also matching the fill rates and avoiding turbulence and reoxidation during pouring. Samples from the filters and the castings were analyzed using an SEM with EDS and automated feature analysis to measure the efficiency of inclusion removal for a 10 ppi zirconia foam filter. Results showed that inclusion removal efficiency depends strongly on the initial inclusion concentration and that the alumina inclusions are captured within the filter at the filter web-steel interface. This study also documented that inclusion floatation inside the mold cavity plays a role in reducing the inclusion concentration in the casting. The distribution of alumina inclusions captured through the filter thickness was quantified using elemental mapping and the inclusion distribution was found to decrease exponentially, following first-order capture kinetics.