Quantum confinement of carriers in nanometer-scale materials leads to size-dependent optical and electrical properties that are intermediate between those of molecules and those of extended network solids. The quest in this area of research is to engineer a normally intrinsic property such as a semiconductor bandgap by simply changing the siLe of the material. Semiconductors in this size regime can also show enhanced carrier mobility, nonlinear optical properties, and quantized charge transport. Examples of quantum-confined materials include superlattices,i',2J quantum dots,["'1 and surface-confined systems such as quantum corralsi7] Work on chemically produced nanometer-scale materials has mostly focused on toxic, heavy-metal chalcogenides such as CdS and CdSe.[3,5,61 Since it is difficult to make actual devices out of individual quantum dots, these materials have not been technologically exploited. Here we show that composite films of cop-

No abstract

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.

Crystallographic structures of spheroidal graphite particles (graphite nodules) were examined using transmission electron microscopy (TEM). Structures of graphite nodules were investigated relative to different stages of nodule growth in ductile iron. Curved graphene layers were observed during the early growth of the graphite nodules. Thin layered stacking faults give rise to streaking in the basal reflections, which give rise to curvature of the nodule and growth steps on the surface. Columnar substructures consisting of parallel peripheral subgrains were found in the outside region of graphite nodules that were formed during the intermediate growth stage. Straight planar graphene layers were observed in the columnar substructures. Crystallographic orientation of graphite showed little change through the subgrain and the c-axes of multiple subgrains in a single columnar substructure were parallel. A method for characterizing the crystal structures of graphite based on the selected area diffraction pattern was introduced. Both hexagonal structure and rhombohedral structure were found in the spheroidal graphite particles. Possible crystallographic defects associated with hexagonal-rhombohedral structure transition were discussed. 1. INTRODUCTION The authors have previously reported on quenching experiments where an inoculated liquid ductile iron was examined at various stages of solidification. [1-3]. It was shown that the size of a graphite nodule reflects its growth stage [1-3]. A graphite nodule with a diameter less than ten microns is in contact with liquid phase and regarded as being retained at an early growth stage [1-2]. Smooth surfaces with circumferentially grown steps were observed in the nodules at early growth stages, and an example of a graphite nodule (seven microns diameter) at early growth stage is shown in Fig 1(a). A growth step observed on the surface of the graphite nodule is highlighted by arrow in Fig. 1(a).

No abstract

No abstract

Dynamic strain aging (DSA) and rapid work hardening are typical behaviors observed in medium-Mn transformation-induced plasticity (TRIP) steel. Three alloys with manganese ranging from 10.2 to 13.8 wt pct with calculated room temperature stacking fault energies varying from À 2.1 to 0.7 mJ/m 2 were investigated. Significant serrations were observed in the stress-strain behavior for two of the steels and the addition of 4.6 wt pct chromium was effective in significantly reducing the occurrence of DSA. Addition of chromium to the alloy reduced DSA by precipitation of M 23 (C,N) 6 during batch annealing at 873 K (600°C) for 20 hours. Three distinct DSA mechanisms were identified: one related to manganese ordering in stacking faults associated with e-martensite and austenite interface, with activation energies for the onset and termination of DSA being 145 and 277 kJ/mol. A second mechanism was associated with carbon diffusion in c-austenite where Mn-C bonding added to the total binding energy, and activation energies of 88 and 155 kJ/mol were measured for the onset and termination of DSA. A third mechanism was attributed to dislocation pinning and unpinning by nitrogen in a-ferrite with activation energies of 64 and 123 kJ/mol being identified. Tensile behaviors of the three medium manganese steels were studied in both the hot band and batch annealed after cold working conditions. Ultimate tensile strengths ranged from 1310 to 1404 MPa with total elongation of 24.1 to 34.1 pct. X-ray diffraction (XRD) was used to determine the transformation response of the steels using interrupted tensile tests at room temperature. All three of the processed steels showed evidence of two-stage TRIP where c-austenite first transformed to e-martensite, and subsequently transformed to a-martensite.

No abstract