Volume 99, Issue 9

The kinetics of the isochronal austenite (γ) → ferrite (α) transformation for a series of ultra-low-carbon Fe – C alloys (with a carbon concentration from 0.001 to 0.05 at.%) were investigated for cooling rates in the range of 5 – 20 K min − 1 using high-resolution dilatometry. The rate of formation of the product (ferrite) phase as a function of time exhibits maxima. The first transformation-rate maxima pertain to diffusion-controlled ferrite growth, whereas the last, main transformation-rate maximum corresponds to massive transformation. The thus determined critical temperatures, at and below which massive transformation takes place, are always below the T 0 temperatures, where the Gibbs energy of the (metastable) austenite equals that of (metastable) ferrite of the same carbon concentration, and are also below the ferrite solvus temperatures for carbon concentrations below about 0.01 at.% C and above the ferrite solvus temperatures for carbon concentrations above about 0.01 at.% C.

The sin 2 ψ method is a frequently used, very successful method for data evaluation in X-ray or neutron stress measurements, but besides all its merits it also has a number of disadvantages and shortcomings. One of the shortcomings is the fact that in the general case of an in-plane stress measurement the method is not a correct least-squares fit. This results in an unnecessary loss of accuracy of the calculated stress tensor. The same must be said about some other methods which are similar to the sin 2 ψ method, inasmuch as they also rely on a linear dependence of measured data on sin 2 ψ or some other function of ψ. This is proved by analyzing the mathematical basis of these methods.

In the present work, the liquidus projection of the Fe – Co – C ternary system is plotted in the iron-rich corner using differential thermal analysis, optical microscopy, scanning electron microscopy and X-ray diffraction. This projection contains three primary crystallisation phases (δ(Fe), γ(Fe) and graphite) as well as two univariant lines: peritectic L + δ(Fe) γ(Fe) and eutectic L γ(Fe) + C graphite .

A method for predicting activities as a function of the concentration in liquid solutions for binary FeCo, FeCu, FeNi, FeMn and FePd systems using Padé approximants is presented. Padé approximants of orders 4/0 and 1/3 are proposed for the model using as input variables only the conditions from Henry's law at a given temperature. Results derived from the approximant 4/0 show good agreement with the experimental data in systems with moderate positive and negative deviations from the Raoultian behaviour. On the other hand, the approximant 1/3 shows a satisfactory agreement in the FeCu system which exhibits a high positive deviation from the Raoultian behaviour. A comparison with the results from a Redlich – Kister expansion using the same input conditions is discussed.

Dislocation structures in fatigued [] coplanar double-slip-oriented copper single crystals were studied using the electron channelling contrast technique in scanning electron microscopy, especially focusing on the observations of the microstructures of persistent slip bands and deformation bands. It was found that, at low plastic strain amplitudes (γ pl ≤ 3.4 × 10 – 4 ), the dislocation structures of the [] copper single crystal comprise a majority of elongated veins and a few elongated cells along the primary slip plane (111). As γ pl increases to 6.2 × 10 – 4 , a special persistent slip band structure consisting of irregular dislocation cells, rather than generally-observed ladders in single-slip crystals, was found to form, which is responsible for the occurrence of a quasi-plateau in the cyclic stress – strain curve of the [] crystal. When γ pl ≤ 3.5 × 10 – 3 , different dislocation microstructures in deformation bands, such as wall-like features, dislocation walls, elongated cells and dense dislocation cells, were found in the cyclically saturated [] crystal. Effects of the crystallographic orientation, the applied plastic strain amplitude, the accumulated plastic strain and the strain localization in the crystal on the microstructure of deformation bands are summarized and discussed.

Differential thermal analysis experiments and treatments for 50 h at 1000, 1100 and 1200 °C were performed for a binary Fe30Cr alloy and for six Fe30Cr (0.2 to 2 wt.%) C alloys. A complete metallographic characterization was done for all samples. The solidus and liquidus temperatures, the natures and the surface fractions of the carbide and matrix phases were determined for the three temperatures. They were compared to thermodynamic calculation results. The values and the evolutions of the solidus temperature versus the carbon content are in good agreement between experiment and calculations. It is less true for the carbide volume fractions which were obtained either by image analysis, or from the matrix Cr content. Mismatches were also observed for the liquidus temperature. Comparisons were done with the Co30CrxC and Ni30CrxC systems and with Ta-containing Fe30CrxC alloys.

The microstructure and mechanical properties of Mg3.5Zn0.6Gd and Mg3.5Zn0.6Gd1.5Cu alloys were investigated under different solidification conditions. The microstructure and phase constitutions of alloys were characterized using optical microscopy, scanning electron microscopy, X-ray diffraction and differential thermal analysis. The results indicate that the cooling rate plays an important role in grain refinement and phase constitutions of Mg – Zn – Gd alloys. In the cooling rate range from 4.8 K s − 1 to 18.5 K s − 1 for the Mg – Zn – Gd system, higher content of the icosahedral phase (I-phase) was obtained under the condition of higher cooling rate which accelerated the nucleation and growth of the I-phase. Under the cooling rate of 18.5 K s − 1 , the mechanical properties of the Mg3.5Zn0.6Gd alloy were further improved by the high I-phase content due to its unique icosahedral structure, in addition to the contribution of grain refinement.

Microstructural evolution and hardness changes of as-cast Mg-6Zn-2Al alloys with Sn addition during prolonged thermal exposure to 150 °C and 200 °C were studied using optical microscopy, X-ray diffraction, scanning electron microscopy and hardness tests. Neither obvious grain growth nor new phases were detected in these alloys during thermal exposure. There were initial precipitation hardening peaks followed by long-term hardness stability during thermal exposure to 150 °C, indicating that all the alloys exhibited excellent microstructural stability. There were two peaks in the hardness vs. annealing time curves of these alloys during thermal exposure to 200 °C, which corresponded to the precipitation of MgZn and Mg 2 Sn. The overall hardness of these alloys decreased with prolonged time, resulting from the aggregation and coarsening of Mg 2 Sn.

The in-situ formation of nano-eutectic/primary dendrite bulk composites in Ti-base multicomponent alloy systems allows the design of advanced high strength materials, where a nanostructured matrix is combined with ductile β-Ti solid solution dendrites as a toughening phase. This microstructure can be achieved over a wide range of cooling rates. The multicomponent recipe stabilizes the β-Ti phase and helps to reduce the eutectic spacing to the nanometer scale. The superiority in the mechanical properties, i. e. high room temperature ductility (up to 30 %) as well as high strength (up to 2600 MPa), stems from the possibility to tailor the alloy composition leading to the formation of different volume fractions of dendrites in a nanostructured eutectic matrix. All composites with high volume fraction of dendrites offer a high ductility (∊ p = 8 – 30 %) and a final failure angle in the range of 46 – 51°. The role of the volume fraction of the dendrites in the composite in enhancing the ductility as well as the fracture angle is critically assessed. The deformation and fracture mechanisms are linked to the macroscopic fracture features as well as to impingement of the shear bands leading to rotation of the shear plane and lattice distortion at the atomic level.

The microstructural, mechanical and magnetic properties of FeCr25Co15 alloy were examined after upset followed by torsion. Deformation was carried out at temperatures that corresponded to the appearance of α + γ phases. During the process, the initial lamellar microstructure of the material was transformed into a globular one. In the sample sections parallel to the upsetting direction a gradient microstructure, with a layer of submicron grains in the highest deformation zone, was formed. The thickness of the zone did not significantly depend on the deformation temperature. Measurements of the mechanical properties after magnetic treatment showed that the plasticity and strength of the deformed samples increased in comparison with the high coercive state.

The concentration dependent diffusion coefficient of nitrogen in expanded austenite was determined from the rate of retracting nitrogen from thin initially N-saturated coupons. Nitrogen saturated homogeneous foils of expanded austenite were obtained by nitriding AISI 304 and AISI 316 in pure ammonia at 693 K and 718 K. Denitriding experiments were performed by equilibrating the foils with a successively lower nitrogen activity, as imposed by a gas mixture of ammonia and hydrogen. The concentration depen-dent diffusion coefficient of nitrogen in expanded austenite was approximated in the composition range where nitrogen can be extracted by hydrogen gas at the diffusion temperature. Numerical simulation of the denitriding experiments shows that the thus determined concentration dependent diffusion coefficients are an accurate approximation of the actual diffusivity of nitrogen in expanded austenite.

This study was done to evaluate precipitation annealing of 18 % Ni maraging steel repair welds during aluminium alloy die casting and to predict the prolonged in-service tool life. The emphasis of this study was the influence of post-weld precipitation annealing heat treatment and aluminium die casting thermal cycling on metallurgical and mechanical properties. A series of specimens of 1.2344 tool steel was prepared to which 1.6356 maraging steel was gas tungsten arc weld clad. Analysis of weld microstructure and hardness was made in order to understand the metallurgical processes during heat treatment at elevated temperatures and at prolonged time. The Response Surface model for prediction of hardness after heat treatment was developed. The edges of immersion test specimens are gas tungsten arc weld clad with 1.6356 maraging steel and machined to the final edge geometry. Two specimens were tested in as-welded condition and two in optimally aged condition. Testing was performed on specially developed immersion test apparatus, which enables the simulation of thermal fatigue during aluminium alloy die casting. After completion of a particular number of thermal fatigue cycles the weld microstructure and hardness were evaluated. A hypothetical model for prediction of precipitation annealing time during aluminium alloy die casting was proposed. The results showed that tool heating lasts 30 % of molten metal injection time, preparing conditions for precipitation annealing at 10 % of injection time, and precipitation annealing at 60 % of injection time. These results, together with a finite element model for prediction of tool temperature and response surface model for prediction of hardness after precipitation annealing, enable accurate prediction of maraging steel tool hardness after any particular number of die casting cycles and consequently in-service tool life. The thermal fatigue resistance of maraging steel welds was compared to AISI H13 tool steel. The results showed superior thermal fatique resistance for AISI H13.

This paper reports on the individual activity and selectivity of Fe-, Pd- and Pd – Fe-mordenite catalysts prepared by wet Fe-ion exchange and Pd-ion impregnation for N 2 O reduction using simulated hydrocarbon-selective catalytic reduction operation conditions of nitric acid plants. Laboratory-scale data on the activity, selectivity and characterization of Fe-, Pd-, and Pd – Fe-mordenite catalysts for conversion of N 2 O have been obtained under the simulated hydrocarbon-selective catalytic reduction operating condition in the temperature range of 325 – 400 °C encountered in nitric acid plant tail gases. More specifically, Pd-, Fe- and Pd – Fe-mordenite catalysts with Si/Al ratios of 10.25 – 10.65 were prepared. Wet ion-exchange methodology was used to prepare Fe-mordenite catalyst, whilst Pd-ion impregnation was used to prepare Pd-, and Pd – Fe-mordenite catalysts. Catalytic activities could not be consistently correlated with and explained by X-ray absorption near edge spectroscopy spectra. Further analysis by temperature-programmed desorption of ammonia was performed to explain higher catalytic activity of Pd – Fe-mordenite compared to Fe-, and Pd-mordenites. Higher catalytic activity of Pd – Fe-mordenite is most probably attributable to the presence of strong Lewis type acid centers as manifested by a TPD spectrum peak at about 300 °C.

In the present work, heat-treated hollow nickel fibres were prepared by thermal decomposition from nickel viscose composite. The thermodynamic properties of viscose fibre, namely standard enthalpy of formation, heat capacity, absolute entropy, and standard Gibbs free energy of formation were estimated by bond enthalpy. Thermodynamic analysis showed that viscose fibres could decompose at any reachable temperature, when the kinetic conditions were favourable. The results of thermogravimetric analysis experiments indicated that viscose filling could be removed by heat treatment in air. The thermogravimetric analysis results along with X-ray diffraction, scanning electron microscopy and X-ray energy dispersive spectrum analysis showed that hollow nickel fibre could be prepared by direct thermal decomposition in air flow at low temperatures, e. g. 573 K. Decomposition of the viscose filling could also be carried out at higher temperatures. However, serious oxidation of nickel would also take place during the decomposition. To remove nickel oxide, reduction by hydrogen gas could be applied.

This study was carried out in order to obtain intrinsic kinetic parameters for the reaction between molybdenum disulfide and hydrogen. The experiments were performed using thermogravimetry within the temperature range of 1073 – 1223 K and at atmospheric pressure. The reaction was conducted by applying thin pellets and the data obtained were analyzed using a nucleation and growth kinetic model. The reaction was first order with respect to the gas phase reactant. The activation energy for the reduction process was determined to be 135.4 kJ mol − 1 .