First principles FLMTO-GGA electronic structure calculations of the new medium-T C superconductor (MTSC) M gB 2 and related diborides indicate that superconductivity in these compounds is related to the the existence of p x,y -band holes at the Γ point. Based on these calculations, we explain the absence of medium-T C superconductivity for BeB 2 , AlB 2 ScB 2 and Y B 2 . The simulation of a number of M gB 2 -based ternary systems using a supercell approach demonstrates that (i) the electron doping of M gB 2 (i.e., M gB 2−y X y with X = Be, C, N, O) and the creation of isoelectronic defects in the boron sublattice (nonstoichiometric M gB y<2 ) are not favorable for superconductivity, and (ii) a possible way of searching for similar MTSC should be via hole doping of M gB 2 (i.e., M g 1−x M x B 2 with M = Be, Ca, Li, N a, Cu, Zn) or CaB 2 or via creating layered superstructures of the M gB 2 /CaB 2 type. A recent report of superconductivity in Cu doped M gB 2 supports this view.

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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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