Volume 107, Issue 3

Compound-layer microstructures obtained by gaseous nitriding and nitrocarburising treatments of α-Fe and Fe–N alloy and by subsequent treatments were analysed to obtain new information on the metastable Fe–N and Fe–N–C systems above 853 K at 1 atm. In the case of the binary Fe–N system, invariant temperatures and phase boundaries agree well with the literature data, only the γ/γ + γ' and the γ/γ + ∊ boundaries as determined in the present work by means of electron probe microanalysis are located at N contents up to 1 at.% lower than reported in previous works. In the case of the ternary Fe–N–C system, the invariant eutectoid reaction was found to occur in the range 853 K – 863 K, whereas the invariant transition reaction was found to occur in the range 868 K – 873 K. The obtained data were used to draw isothermal sections of the Fe–N–C system for 853 K and 893 K. The determined temperatures for the invariant eutectoid and transition reactions as well as the extent of the ternary ∊-phase field were compared with phase diagrams calculated on the basis of different thermodynamic descriptions of the Fe–N–C system.

The microstructural development of Fe–N–C compound layers resulting from nitrocarburising and heat treatments applied to Fe–N and Fe–C alloys was investigated. Slow cooling or secondary annealing, instead of quenching, after the end of the nitrocarburising treatment causes decomposition of γ-Fe[N,C] and ∊-Fe 3 (N,C) 1+ x in the compound layer. The resulting decomposition microstructures were analysed by, in particular, light microscopy and electron backscatter diffraction, and X-ray diffraction and electron probe micro-analysis, revealing their constitution and crystallographic properties. The results are discussed in comparison to data obtained from undecomposed, i. e. quenched, specimens and to data reported in the literature.

The isothermal section at 1 050 °C and the liquidus projection in the Co-rich portion of the ternary Co–Ti–Zr system were determined. The microstructures and solidification paths of the as-cast alloys were established experimentally. The constituent phases and their compositions of both as-cast and heat-treated alloys were investigated by means of optical microscopy, X-ray diffraction and electron probe microanalysis. No ternary compound was observed. For the Co-rich portion of the isothermal section at 1 050 °C, there are four three-phase regions, αCo + TiCo 3 + TiCo 2 (h), αCo + TiCo 2 (h) + (Ti,Zr)Co 2 (c), αCo + (Ti,Zr)Co 2 (c) + Zr 6 Co 23 and αCo + Zr 2 Co 11 + Zr 6 Co 23 . For the investigated liquidus projection, there exist six primary crystallization fields: αCo, TiCo 3 , TiCo 2 (h), (Ti,Zr)Co 2 (c), Zr 2 Co 11 and Zr 6 Co 23 ; three transition invariant reactions, L + αCo → TiCo 3 + TiCo 2 (h), L + (Ti,Zr)Co 2 (c) → αCo + TiCo 2 (h), and L + Zr 6 Co 23 → αCo + Zr 2 Co 11 ; and one eutectic invariant reaction of L → αCo + (Ti,Zr)Co 2 (c) + Zr 6 Co 23 .

Ni-based model alloys were used to study the effect of alloying elements, namely Cr, Mo, C and Zr on the occurrence of grain boundary serration. The model alloys were free of aluminum to exclude precipitation of second-phase γ′. Similarly, the carbon content was very low, when present, to prevent precipitation of carbides. A special heat treatment involving slow cooling was used to promote grain boundary serration. No significant sign of serration was observed for Ni-10Cr-10Mo, Ni-20Cr-10Mo and Ni-10Cr-10Mo-0.05C model alloys. However, substantial serration was observed for Ni-10Cr-10Mo-0.5Zr and Ni-20Cr-0.5Zr model alloys. Serrated grain boundaries were observed in the absence of either γ′ or carbides. Zirconium-rich precipitates were recognized at serrated grain boundaries though their involvement in the occurrence of serration was doubtful. A mechanism of grain boundary serration formation is proposed.

The feasibility of drawing with shear of low-carbon steel for wire production was studied. The new approach was shown to be more effective in producing of homogeneous microstructure with refined grain size, reduced porosity and increased mechanical strength. These effects were attributed to a specific “twist” character of material flow.

A new material's forming limit diagram (FLD) must be obtained before it can be used for stampings. Compared to conventional methods, the ISO method with a digital image correlation system provides an efficient way to conduct these tests. However the ISO method is position dependent, so the evaluation procedure for aluminum alloy will be seriously affected by the multiple necking strain distribution, resulting in much error. In this paper, a modified ISO method, which can be used for the evaluation of the aluminum alloy FLD, is proposed, and errors resulting from multiple necking were eliminated through detection of the major necking area. An FLD evaluation software was also programmed. An FLD 0 punch test was conducted to verify the new modified ISO method. Through comparison of the results with the accepted value, the FLD 0 value evaluated from the modified method was determined to be much more accurate than the standard method.

Two-pass hot plane strain compression tests of 6013 aluminum alloy were conducted at different temperatures, strain rates, and holding times. Using the experimental data, four popular metamodel types – Kriging, radial basis function, polynomial regression and artificial neural network – were investigated as potential methods for global modeling of the flow behavior during two-pass deformation. The global model developed from the Kriging method was superior in terms of the accuracy and stability of the prediction. Furthermore, this model was successfully used to predict not only the two-pass deformation behavior beyond the experimental conditions, but also the multipass deformation behavior. It is proved that the Kriging method is an effective and reliable approach to develop a global model for optimization of the hot forming process.

A series of high alumina refractory castables were prepared via casting with tabular alumina aggregates (0.088 – 5 mm), fine powders, and calcium aluminate cement as starting materials. The effect of the median diameter of alumina ( D 50 = 7.26 μm, 5.33 μm, 2.37 μm) on the thermo-mechanical properties of the refractory castables was investigated. The results indicate that the decrease in the alumina particle size from 7.26 μm to 2.37 μm has little influence on both the apparent porosity and bulk density of castables. However, the median pore size of castables fired after 1 600 °C decreased drastically from 4.7 μm to 2.4 μm correspondingly, which led to significant growth in the strength and thermal shock resistance of castables. The cold modulus of rupture and crushing strength were increased by 201 % and 120 %, respectively. At the same time, the hot modulus of rupture and elastic modulus were also increased by 143 % and 127 %, respectively. The residual elastic modulus was enhanced twice after three thermal cycles.

The technological features of processes for the production of steelmaking slags for use in building materials and products are presented, and improvement in the properties of refining slags and slag stabilization via the introduction of alumina flux are demonstrated. The mechanism of slag stabilization by alumina flux is also discussed. In addition, the preparation of Portland cement clinker with a high content of iron oxides from slag with a controlled phase composition is described.

The initiation of dynamic recrystallization during hot deformation, its kinetics according to the Johnson–Mehl–Avrami–Kolmogorov model, and the resultant grain refinement in the Mg-3Gd-1Zn magnesium alloy were studied to elucidate the mechanism of dynamic recrystallization and the opportunities it offers during high-temperature thermomechanical processing of this heat resistant light alloy. The necklace mechanism was found to be responsible for the noticeable grain refinement, during which the mean recrystallized grain size varied slightly. These results were further confirmed based on the kinetics analysis.

Uniform Al 2 O 3 : Tb 3+ samples were successfully fabricated via a hydrothermal method and subsequent thermal decomposition of Tb 3+ doped precursors. Scanning electron microscopy results showed Tb 3+ -doped Al 2 O 3 was stalk-like with an average length about 1.5 μm. Photoluminescence spectra indicated that the 5 D 4 → 7 F 5 (545 nm) electric dipole transition was most intense upon excitation at 227 nm. It is shown that 1.5 mol.% doping concentration of Tb 3+ ions in Al 2 O 3 : Tb 3+ is optimum. According to Dexter's theory, the critical distance between Tb 3+ ions in Al 2 O 3 for energy transfer was determined to be 14 Å.

Ag nanowires deposited at –0.1 V to –0.6 V are single-crystals along [110] direction, and are polycrystalline when deposited at –1.0 V. The large critical nucleus and slow nucleation rate at low potentials could be responsible for the growth of single-crystal. The polycrystalline growth can be related to the small critical nuclei and fast nucleation rate at high potentials. At –0.2 V and different temperatures (15 °C to 60 °C), the preferred growth plane is (220).