Volume 95, Issue 11

Cu 47 Ti 33 Zr 11 Ni 8 X 1 (X = Fe, Si, Sn, Pb) glass-forming alloys are investigated concerning their phase formation, as-cast microstructure, thermal stability and crystallization behavior. The supercooled liquid region Δ T x has a maximum value of 57 K for Cu 47 Ti 33 Zr 11 Ni 8 Si 1 , which is thermally more stable than the other alloys. The reduced glass transition temperature T rg reaches the highest values for the Cu 47 Ti 33 Zr 11 Ni 8 Si 1 and Cu 47 Ti 33 Zr 11 Ni 8 Fe 1 glassy alloys. X-ray diffraction experiments using synchrotron radiation reveal that the first phase formed upon crystallization of fully glassy material has a γ-CuTi-type structure. The final microstructure stable before melting contains a mixture of CuTi, Cu 51 Zr 14, Ti 2 Cu and Cu 2 TiZr intermetallic compounds.

An optimized set of thermodynamic functions to describe the Al –V system has been obtained by a computer-operated least squares method applied to the experimental phase diagram and thermodynamic data available from the literature. Special attention is paid to the modeling of the intermediate phase Al 8 V 5 . Based on the crystal structure data, Al 8 V 5 is described by a sublattice model Al 6/13 ( Al , V) 2/13 (Al, V ) 3/13 V 2/13 in a final treatment. The boldface Al and V mean the normal atoms (i. e., major species) in the sublattices. In order to provide reasonable starting values for the final modeling, three treatments are performed. In the first treatment, Al 8 V 5 is assumed to be a stoichiometric compound; in the second and third ones, it is described by sublattice models Al 6/13 ( Al , V) 2/13 V 5/13 and Al 8/13 (Al, V ) 3/13 V 2/13 , respectively. This step-by-step modeling procedure provides reliable estimates and useful starting values for the parameters at each of the higher levels. The other phases (liquid, fcc (Al), and bcc (V)) are modeled with the Redlich – Kister formula or as stoichiometric phases (Al 21 V 2 , Al 45 V 7 , Al 23 V 4 , and Al 3 V). Comparisons between the calculated and measured phase diagrams as well as thermodynamic quantities indicate that almost all of the reliable experimental information is satisfactorily accounted for by the thermodynamic description of the final treatment. The consistent thermodynamic functions from the first, second, and third treatments are useful for a simplified description of the Al –V system if less accuracy for the Al 8 V 5 phase is required.

A new experimental investigation of the composition of titanium carbonitride (TiC x N y (cF8)) is presented. The C and N contents in equilibrium with graphite and N 2 gas of pressures of 1 bar and 100 bar have been determined at 1430 °C and 1850 °C. Special effort has been made to verify that equilibrium is reached. Using the new experiments a revised thermodynamic description of the Ti –C–N system is presented, including also an assessment of the molar volumes in this system. Calculated phase equilibria are presented and compared with experimental information, and the agreement is discussed. Further, the differences in measured results are discussed.

The rearrangement process assumes that if there is good wetting between liquid and solid phase, solid particles will rearrange themselves under the action of surface tension forces, producing more stable green packing. This paper outlines a computer-based method for calculation of the capillary force during liquid phase sintering based on a two-spherical-particle model and circle approximation for computation of the liquid bridge meniscus. The method was applied to a 3-D study of the rearrangement during liquid phase sintering using a multi-particle model. The results obtained have shown that rearrangement is essentially governed by geometrical factors where uniformity of initial particle packing and a high degree of mixing homogeneity are prerequisites for effective densification.

Using electromagnetic levitation in combination with the oscillating drop technique, the surface tension of liquid Fe-20 wt.% Cu alloy above and below the liquidus temperature was measured. The containerless state during levitation produces substantial undercooling up to 156 K. In the experimentally covered temperature range, the surface tension of Fe-20 wt.% Cu alloy was determined as 1.658 – 2.234×10 – 4 ( T – 1736) N/m. According to Butler’s equation and STCBE software, the surface tension of Fe-20 wt.% Cu alloy was also calculated. A good agreement was found between the calculated and experimentally measured values within the error limits. The different sign of the temperature dependences of the calculated and measured surface tensions is preliminarily discussed.

Phase separation and phase diagram in the Fe-rich corner of the Fe –Al–V alloy system were investigated by means of transmission electron microscopy, scanning electron microscopy and energy-dispersive X-ray spectroscopy. Phase separation into disordered A2 and ordered L2 1 phases was found in a wide composition range in the Fe-rich corner at 923 K, 973 K and 1023 K, and their tie-lines were almost parallel to the direction connecting the Fe corner with the center of the Gibbs triangle. In addition to the A2 + L2 1 phase separation, phase separation into B2 and L2 1 phases was also observed at 1023 K at the side of Fe –Al binary system.

Microstructural changes due to compression creep in gamma Ti–52Al phase at 1100 K and 250 MPa have been investigated using a conventional metallography and transmission electron microscopy. Three distinct microstructural types, the 3D dislocation network, subgrain boundaries and twinning, were found in individual grains after interrupted creep tests. The microstructures always consisted of three fundamental deformation modes: (i) ordinary dislocations, (ii) superdislocations, and (iii) deformation twins that contributed to the overall creep strain from the very beginning of creep. This microstructural evolution and the corresponding metal-type creep behaviour contrasts with the sharp creep rate minima and a lack of superdislocations and deformation twins reported earlier for specimens deformed at 1100 K and applied stresses below 200 MPa. The partitioning of the total strain among fundamental deformation modes is estimated and the link between sharp minima of creep rate and the insufficient activation of <c>-component plastic displacements is suggested.

The evolution of the chemical and phase composition of carbide precipitates in X20CrMoV12.1 steel after longterm service of 56 000 θ (470 – 530 °C at up to 18MPa) and after heat treatment (650 and 800 °C) was investigated using transmission electron microscopy and Auger electron spectroscopy. The precipitates found were mostly of M 23 C 6 type, the S- and v-phase and traces of M 2 C. In service loaded state the presence of M 7 C 3 precipitates was also established. The evolution sequence is obviously M 3 C → M 7 C 3 !M 23 C 6 .

A structural study was performed on the austenitic steels Avesta 254 SMO and Avesta 654 SMO after annealing at 700 °C for 500, 3188, and 6170 h. Both Avesta steels initially show an unexpectedly large amount of the Laves phase, followed by a relatively slow development of the Sigma phase with equilibrium apparently not yet reached after 3188 h. Thermodynamic calculations confirm that the driving forces for alternative precipitates are very similar thus making it easy to form metastable precipitates that only change very slowly to the equilibrium state. TTT calculations also confirm that the Laves phase precipitates earlier than the Sigma phase as the temperature is lowered.

The aim of this article is the characterization of the conditions in the chip forming area during machining of normalized tempering steel C45E (AISI 1045) at high cutting speeds. Especially the effects of the machining process on the microstructure are described. Infeed turning is the chosen production process. The chips and partially formed chips are taken from a real cutting process. The chip formation clearly shows a 3D character. The shear bands can start and end in the chip or mismatches can be seen. The geometrical parameters vary a lot over the chip width. Investigations using a transmission electron microscope show initially an inhomogeneous dislocation structure in the material as a consequence of the deformation caused by the cutting process. Thereafter, the shear bands are formed by the generation of subgrains and microbands, starting from a dislocation cell structure. The material between the bands is only slightly deformed. Martensitic phase transformations were detected in the shear bands.

Copper matrix composites reinforced with 10 vol.% SiC particles were fabricated by a powder metallurgy method. The influence of environmental temperature on friction and wear behavior was studied in the temperature range 25– 390 °C. The incorporation of SiC particles improved the wear resistance at elevated temperature and effectively delayed the occurrence of severe wear at about 100 °C. A compact glaze layer consisted of comminuted SiC particles, fine oxides and Cu, was formed on the worn surface of the SiC/Cu composite. The protective effect of this layer improved the wear resistance of the sliding pair at high temperature. At mild wear region, the detachment of glaze layer was the primary wear mechanism of the SiC/Cu composite. When the environmental temperature was high enough for the subsurface materials to flow over the residual glaze layer, adhesive wear occurred and resulted in rapid failure of the tribological system.

Grain refinement in the as-cast microstructure and the nature of the Sc-bearing primary and secondary phase particles present in as-cast and homogenized Al –Zn –Mg–Cu – Zr base 7010 alloys, containing Sc, have been examined by a combination of light microscopy, scanning electron microscopy (SEM), energy dispersive X-ray spectroscopy (EDXS) and transmission electron microscopy (TEM). It is shown that addition of 0.25 wt.% Sc to the base alloy 7010 gives rise to the desired grain refinement in the as-cast microstructure. Further additions of Sc (investigated up to 0.5 wt.%) reduce the as-cast grain size further, although increased amounts of Sc additions give rise to an increased density of Sc-bearing coarse primary particles that always have an L1 2 crystal structure, dissolve Zr, and survive the homogenization treatment. The compositions of the Scbearing primary phase particles vary depending on their morphologies, i. e. cuboidal and irregular shape. The cuboidal shape of the Sc-bearing primary particles is the dominant one and such particles are mostly located within the grains. The noticeable features of the homogenized microstructure are (1) the changes in the composition of the Scbearing primary particles, (2) the formation of fine, spherical, coherent Al 3 Sc xZr1– x base secondary precipitates having an L1 2 structure and (3) the stabilization of the refined as-cast grain structure.