Band 93, Heft 11

The effect of soluble boron on the phase transformations during either cooling and/or isothermal holding has been studied by means of dilatometry. Significant differences in the transformation behaviour were found for all austenite phase transformation reactions. In particular, the morphologies of ferrite and bainite were strongly affected by B alloying. The kinetics was studied in detail for the austenite decomposition reactions. Soluble B was found to be very effective in suppressing the carbide formation. It was also found to interact with the Mn partitioning to the austenite. As a result, the presence of Mn-rich regions in the final microstructures decreased strongly the Ac 1 temperature during reheating. Isothermal transformations in the 450–660 °C temperature range showed that the incubation times for ferrite and pearlite formations were increased. The soluble B was found to affect strongly the nucleation rate. The growth kinetics was slower due to a solute drag effect caused by the partitioning of Mn. The kinetics of bainite formation was not affected by the presence of soluble B. Upper bainite was found to be acicular ferrite in the CMnB steel as a result of the heterogeneous nucleation of ferrite on TiN precipitates.

It is shown that chip formation in high-speed milling of a steel remains unaffected by the presence of a volume fraction of about 10 % ferrite to either martensitic or pearlitic microstructures. Ferrite grains remain visible within the chips and can, therefore, be used for an estimation of the local plastic deformation. In case of the martensite structure, a deformation of 3000 % was measured in the white etching secondary shear zone, while a considerable amount of deformation even takes place within chip segments. The presence of ferrite grains within the white layer is an indication of a thermally-induced phase transformation occurring within the secondary shear zone of the chips.

Variations in structure and heat resistance of niobium alloys after long-term loading (for up to 10 000 h) in vacuum at 1173 and 1373 K are studied. It is concluded that niobium alloys are more heat-resistant while those undergoing solid-solution hardening are more stable structurally. It is shown that long-term strength plots for niobium alloys have kinks which are to be taken into account when predicting life of structurally unstable materials.

An experimental study was done on the effect of rare-earth metals on a high-strength low-alloy steel. The work was focused in deriving the influence on Ce and La on the hot-working behavior. For this purpose, uniaxial hot-compression tests were carried out in a wide range of temperatures and strain rates. The effect of the rare-earth metals was determined by comparison of the characteristic parameters, describing the constitutive equations of the high-temperature response of the steel with, and without, rare-earth metals. The results showed that rare-earth metals were playing a major significant role on hardening mechanisms rather than on softening by dynamic recovery. On the contrary, rare-earth metals were able to delay the onset of dynamic recrystallization. All the present experimental results suggested that the latter roles are played by solid solution strengthening, through a solute drag effect, and not by precipitated particles.

Dry-sliding wear behaviour of bainitic structure (BS) and tempered martensitic structure (TMS) of Ni–Cr–Mo–V steels have been investigated at room temperature, using a pin-on-disc wear testing rig. These two microstructures reveal the rising trend of wear rate with normal applied load and sliding velocities in the initial stages; but once a maxima is attained in wear rate, then wear rate decreases with further increase in applied load or sliding velocity. It is also observed that wear resistance of TMS steel is superior to BS steel, but the difference in wear rate is not significant. The stereo-micrographs of debris generated show the oxidative and metallic modes of wear for both the microstructures. The study of worn surface clearly highlights the oxide formation, smearing of surface and distortion associated with straining and plastic deformation. The micro-ploughing, spalling and subsequent detachment of oxides and metallic surface layers from the worn-out surface of the pin specimen are the dominant mechanisms of the wear under different conditions of applied loads and sliding velocities. The wear rate is very much influenced by the yield strength and hardness of the bainitic and tempered martensitic steels.

A thermodynamic description of the ternary system Co–Nb–Ta is obtained by combining the descriptions of the lower-order systems according to the CALPHAD method. The thermodynamic data set for the binary Co–Nb system is taken from [1], where a new thermodynamic model for the μ-phase was introduced. The μ-phase appears in several transition metal systems, including Co–Ta. The model is applied to the Co–Ta system and extended to the ternary Co–Nb–Ta system. The thermodynamic descriptions of the systems Co–Ta, Nb–Ta and Co–Nb–Ta are reported in the present work.

Recent results of directional solidification experiments with Fe–Co alloys were not consistent with the presently accepted peritectic phase diagram. Therefore, the Fe-rich part of this phase diagram was re-examined. High-temperature gradient plane front solidification was carried out at a low velocity. After quenching of the specimen from the peritectic temperature, the compositions of the phases close to the solid/liquid interface were determined through electron probe microanalysis. Using these results together with previously published phase diagram information, an improved phase diagram is proposed with the peritectic equilibrium at T p = 1774 K, C δ = 17.7, C γ = 18.1 and C L = 19.5 at.% Co.

The B–Co system is investigated via thermodynamic modeling and experiments. In the modeling section, all of the experimental phase diagram and thermodynamic data available from the literature were critically reviewed and assessed by using thermodynamic models for the Gibbs energies of individual phases. In the experimental section, 6 key alloys were prepared by arc melting Co slug and B pieces and annealing at 900 °C for 9 days. Water-quenched samples were analysed by using X-ray diffraction, optical microscopy, electron probe microanalysis and differential thermal analysis techniques. The measured invariant reaction temperatures are: 1136 ± 2 °C for Liquid (L) ↔ (αCo) + Co 3 B, 1157 ± 2 °C for L + Co 2 B ↔ Co 3 B, 1263 ± 2 °C for L ↔ Co 2 B + CoB, and 1358 ± 2 °C for L ↔ CoB + B. A consistent thermodynamic data set for the B–Co system is finally obtained by considering the present experimental results and reliable literature data. Comparisons between the calculated and measured phase diagram and thermodynamic quantities show that all of the accurate experimental information is satisfactorily accounted for by the thermodynamic description.

There is considerable evidence, illustrated here with the example of alkali metals, that the absolute thermoelectric power of a metal is equal to the temperature dependence of its Fermi energy divided by the electronic charge. Some puzzling features, e. g., the positive sign of the value for Li, fall into this pattern. The equality follows exactly from Onsager’s relations, if (1) the pressure or the molar volume is constant along the wire and (2) the ratio of charge carriers to atoms is also constant or if electroneutrality holds at least in the bulk of the material.

The unit cell parameters and the unit cell volumes were investigated for Si- and Ge-containing solid solutions based on the 3d body-centred cubic transition metals. Excepting the high-temperature solid solution δ-Mn(Si), the solubility of Si increases with increasing number of 3d electrons. In contradiction to the large atomic volume of Si, which is substantially larger than that of the 3d transition metals, the unit cell volume of the terminal solid solutions decreases with increasing Si content. For solid solutions showing a large homogeneity range – α-Fe(Si) and α-Fe(Ge) –, the partial atomic volumes of Si and Ge were evaluated. Comparing the quasihomological Al- and Si-containing solid solutions α-Fe(Al) and α-Fe(Si), the higher bond energy occurs between atoms of Si and Fe than between Al and Fe atoms. Taking into consideration the homology of metalloids in the solid solutions α-Fe(Si) and α-Fe(Ge), the higher bond energy can also be expected between Si and Fe than between Ge and Fe atoms.

A continuous S2 silicide ([Ti,Zr] x Si y ) segregation layer was found in the fibre matrix interface of SCS6-SiC/Ti-5.8Al-4Sn-3.5Zr-0.7Nb-0.5Mo-0.35Si (IMI834) composites (composition in wt.%). The S2 layer separates completely the titanium matrix from the reaction layer. It was shown that the S2 layer stabilises the interface during thermal treatment. The thermal stability of composites was tested in which artificial coatings were introduced into the interface. For the artificial coatings a composition and structure of the S2 silicides were aspired. The interface of the composites was investigated with analytical transmission electron microscope in the as-processed condition and annealed at 700 °C for 3 000 h and at 800 °C for 1000 h. The coatings of the same composition and structure can be introduced into the interface of SiC-SCS6/IMI834 composites as those of the S2 silicide obtained by self-segregation. The composites with the artificial coatings have a similar interface stability at 700 °C as those obtained by self-segregation.