Volume 99, Issue 3

Uni-modal and bi-modal microstructures were concurrently observed in a TiO 2 -doped -Al 2 O 3 ceramic material with some SiO 2 impurity. The segregation of Ti and Si to grain boundaries was systematically evaluated to reveal their role at the start of anisotropic grain growth. In general, in the bi-modal area grain boundaries exhibit higher Si excess than those in the uni-modal area. However, the basal plane of anisotropic grains was always found to contain more segregated Si while the non-basal planes were found to contain less Si. Such enrichment of SiO 2 was also found at the basal planes of equiaxed grains in the uni-modal area, which occurred in the initial stage of anisotropic grain growth. Before and after this stage, the grain boundary composition was relatively uniform with the mole ratio of SiO 2 to TiO 2 as 2.3: 1 and 5.8: 1, respectively. The latter should be related to the composition of eutectic liquid formed during the sintering, which is responsible for the development of the bi-modal microstructure.

Statistical thermodynamic analysis was applied to the available phosphorus solubility data set for molten Fe 1 – y Cr y P x given as functions of Cr composition y , temperature T and phosphorus activity a (P) under the assumption that the solubility limit x of P in the molten Fe 1- y Cr y was 0.50 irrespective of y . The evaluated values of the energy parameter representing the extent of stability of P atoms in the molten Fe 1 – y Cr y P x showed an apparently realistic variation pattern with y for the level of y up to 0.465. Further, it was demonstrated by an interpolating estimation using the obtained statistical thermodynamic parameter values at two known levels of y that a (P) vs. x relationships for arbitrary y at specified T could be derived with acceptable accuracy.

The isothermal section at 650°C and the vertical section Fe-(Ga 0.75 Al 0.25 ) of the Fe-rich portion in the Fe – Ga – Al ternary system were determined using optical microscopy, scanning electron microscopy, energy dispersion X-ray spectroscopy, X-ray diffraction and differential thermal analysis. The isothermal section of the Fe-rich portion consists of 4 single-phase regions: A2, B2, D0 3 , D0 19 , and 4 two-phase regions: A2 + B2, A2 + D0 3 , D0 3 + D0 19 and B2 + D0 19 . No fcc L1 2 phase region occurs in the isothermal section of the Fe-rich portion. The vertical section Fe-(Ga 0.75 Al 0.25 ) of the Fe-rich portion consists of 3 single-phase regions: A2, B2 and D0 3 , 4 two-phase regions: A2 + D0 3 , D0 3 + D0 19 , A2 + L1 2 and D0 3 + L1 2 , and 2 three-phase regions: D0 3 + D0 19 + L1 2 and A2 + D0 3 + L1 2 .

An investigation of the isothermal section at 773 K of the Gd – Er – Co ternary system has been carried out by means of X-ray powder diffraction, metallographic analysis and differential thermal analysis techniques. At this temperature, a series of continuous solid solutions forming from the binary compounds in the Gd – Co and Er – Co systems characterized the phase diagram. The Curie temperatures of Gd 1 – xEr x Co 2 ( x = 0, 0.2, 0.4, 0.6, 0.8) alloys have also been determined.

Nd – Fe – B alloys are widely used in many technical applications due to their excellent permanent magnetic properties. YNi 2 B 2 C intermetallic compounds receive much attention because of the coexistence of superconductivity and magnetic ordering phenomena. Er 2 PdSi 3 alloys with hexagonal AlB2-type crystallographic structure are of special interest because of their complex magnetic ordering, Kondo and heavy fermion behavior. The thermophysical properties (electrical conductivity, thermoelectric power, thermal conductivity and viscosity) of these liquid multicomponent intermetallic alloys have been measured in order to supply necessary information for crystal growth and casting processes. Good reproducibility of the results has been obtained.

Knowing how chemical composition and microstructure can influence the thermal expansion of high temperature alloys is of great importance. The behaviour of pure Ni, Co or Fe and of simple 30 wt.% Cr-containing cast alloys based on these elements, carbide-free or containing 0.8 wt.% C, were studied between 500°C and 1200°C. Special attention was given to the possible effect of the carbide and of the dendrite orientation. Thermal expansion coefficients are provided and compared with regard to the microstructure. The presence of Cr in the alloys leads to higher thermal expansions than for pure elements. The interdendritic carbides tend to reduce the alloys' thermal expansion behaviour by obstructing the allotropic dimensional phenomena or by plastically deforming the matrix in compression. The dendritic orientation seems to influence the thermal expansion of carbide-containing alloys.

The interaction of equal channel angular extrusion (ECAE) with precipitation hardening during warm ECAE at 180°C in a Al-4.3 wt.% Mg-1.2 wt.% Cu alloy, annealed at various stages of ageing, was studied by room temperature compression tests and transmission electron microscopy. It was observed that peak aged samples lose much of their strength during ECAE due to fragmentation of precipitates. The highest strength after four ECAE passes (route Bc) at 180°C is obtained with samples in the under-aged condition although in this case dissolution of precipitates into the matrix occurs. After four ECAE passes fine equiaxed grains with an average size of approximately 200 nm with some traces of remnant elongated grains were obtained.

The combustion synthesis of (Ti, V)C-reinforced iron-matrix composites from elemental powders was examined. Examination using scanning electron microscopy indicated that two types of carbide particles were formed having a different size and composition from each other. These were sub-micron size (Ti, V)C particles and pure TiC or TiC-rich particles of a slightly larger particle size. The development of the microstructure and formation of the two types of carbides were explained through an understanding of the reaction mechanism and the thermodynamics of the carbide phases. Increasing the amount of vanadium in the reactants led to a decrease in the carbide and the iron grain size and to a corresponding increase in microhardness.

New measurements of the influence of Cr and V on the melting temperature of the binder phase in Co-based hardmetals are presented. It was shown that both Cr and V affect the melting temperature significantly. Cr decreases the melting temperature by over 100°C, and V by approximately 40°C. This is due to the high solubility of Cr and V in liquid Co, and this solubility was determined by experiments and calculations. The solubility of Cr in solid fcc-Co is also discussed, as well as the composition of the MC, M 7 C 3 and the M 6 C carbides in alloys with Cr and V. The new experimental information is used to obtain a reliable thermodynamic description of the binder phase (Co-rich) in C – Co – Cr – W and C – Co – V – W alloys. Calculated equilibria are compared with experiments.

The Cu – Se system was modeled by means of the CALPHAD technique. Considering two miscibility gaps of the liquid existed on each side of the compound Cu 2 Se, the associate model with the associate species “Cu 2 Se” was adopted to describe the Gibbs energy for the liquid and compared to the random solution model. The optimizing results showed that the thermodynamic model describing the liquid phase as a solution of Cu, Se and a “Cu 2 Se” associate fits well with the experimental data. The intermetallic compounds, Cu 3 Se 2 , α-CuSe, β-CuSe, γ-CuSe and CuSe 2 were treated as stoichiometric compounds. The com-pounds α-Cu 2 Se and β-Cu 2 Se, which have a homogeneity range, were treated as the formulae α-(Cu, Se) 2 Se and β-(Cu, Se) 2 Se by a two-sublattice model. A set of self-consistent thermodynamic parameters of the Cu – Se system was obtained.

Fe – V binary alloys with different vanadium concentrations (4 to 55 at.%) are arc melted and systematically characterized by means of; (1) differential thermal analysis, (2) optical and scanning electron microscopy coupled to an energy dispersive X-ray microprobe analysis, (3) quantitative X-ray fluorescence spectrometry, and (4) X-ray diffraction. The present work exhibits the solidification sequences according to the observed microstructures for Fe – V synthesis alloys. For the first time, a metastable -phase was observed in Fe rich alloy (4 at.% V). The phase transition is clearly shown. It takes place at 1320°C for the alloy V1 (4 at.% V). For higher vanadium contents (55 at.% V maximum), the -phase is no longer observable. The phase transition is also shown. The -phase precipitates directly by a congruent reaction from the -solid solution. It occurs at 1090°C for the alloy V3 (25 at.% V) and increases with the vanadium content (ranging from 25 to 55 at.% V). These results are plotted on the accepted Fe – V phase diagram.

Using X-ray diffraction and back-scattered electron microscopy techniques, microstructural changes and phase decomposition at the ultra-precision machined surface of a Zn – Al based alloy were studied. The effects of the depth of cut on the microstructural changes were discussed in relation to the creep deformation, and the milling-induced microstructural changes. Structural evolution of the chip from the machining was studied in order to understand the ultra-precision machining induced phase decomposition. The effect of ultra-precision machining on the hardness of the surface of the alloy specimen was studied.

The effect of inclusion of trivalent arsenic ions with increasing arsenic concentration on the structural, electrical and thermal properties of the system As x Fe 3 – x O 4 ( x = 0.2, 0.4, 0.6, 0.8 and 1.0), synthesized at ambient temperature have been studied. Thermal measurements from room temperature to 1173 K and DC resistivity measurements with activation energy calculations were used for this purpose. Thermal measurements were carried out on these value added products. This investigation provides evidence for all compounds of the system. In particular: 1) They are stable up to 473 K and further only a small loss in weight was observed for the temperature range of 473 K – 1113 K, 2) In As 0.2 Fe 2.8 O 4 , a strong exothermic peak was observed at around 561 K, which has been shifted to 541 K for AsFe 2 O 4 , this shows the phase transition and sample crystallization. X-ray diffraction was used for characterization and DC resistivity measurement from room temperature to 623 K shows semi conducting behavior. The present work reveals that the incorporation of As 3+ ions in the lattice of magnetite results in orthorhombic symmetry for arsenic composition x = 0.2 to x = 1.0.

Thin film multilayer structures based on MgF 2 , CeO 2 and Al 2 O 3 layers were prepared on a BK7 glass substrate with resistive heating evaporation as an antireflection coating in the 450–750 nm spectral range. Prior to deposition of the multilayer structure, individual layers of the materials used were characterized. A three-layer structure of MgF 2 /CeO 2 /Al 2 O 3 /BK7 was then prepared using the same deposition conditions as for individual layers. Reasonably low reflection was observed for the as-deposited sample with R < 0.8% from 500–750 nm and R ave < 1% over the desired spectrum. Further, low surface roughness and small particle size were seen for the as-deposited sample. Only a few low intensity peaks for the materials were observed with X-ray diffraction. Optical and structural properties were studied as a function of annealing temperature in the range 250°C–450°C. A slight shift of reflection spectra was observed with annealing, however, an increase in particle size (35 nm–70 nm) along with surface roughness was also observed.