Volume 94, Issue 2

The formation of dispersoids of nanometer size particles and its consequences on the following properties is discussed: precipitation hardening, stabilisation of grain size, stabilisation of austenite against martensitic, and induction of R-phase transformation. Firstly, a survey is given on the aspects which have to be considered for the production of evenly distributed particles in the size range 1–10 nm by precipitation: pre-nucleation clustering, size of the elementary cell, interfacial structure and defects in the matrix lattice, and in situ transformation of metastable phases. Rules are proposed for the production of nano-dispersoids which are valid for all types of alloys. Interaction of dislocations with the dispersed particles leads to a quantitative understanding of the amount and upper limit of precipitation hardening and related properties: work-hardening, localisation of strain, and crack-growth. “Hardening” of ferromagnetic and superconducting materials can be analysed in a similar manner. Interaction of moving grain boundaries with the particles provides the base for highly retarded grain growth. This is required for many heat treatments, for example, to control grain size and texture. Finally, dispersed particles can impede the motion of the interface of an austenite → martensite transformation. In this way, the transformation temperature of austenite can be lowered considerably in steels as well as in shape memory alloys. On the other hand, stress fields or concentration gradients in the environment of particles may favour the formation of pre-martensitic effects like R-phase transformation. It is attempted to provide a unifying and quantitative approach on the formation of ultrafine dispersoids and their effect on a multitude of properties.

The application of Ga as a base for a solid metal lubricator has been considered. Different methods were applied to investigate properties of liquid Ga in the temperature range 30–300 °C. Some peculiarities of liquid Ga can be explained with structural transformations. It has been shown that Ga is characterized with combination of a high plasticity and strength of a melt. Due to the good adhesion, Ga forms stiff films on the surface of contact bodies. A mixed mode of friction usually exists when Ga is applied as a solid lubricator. Moreover, the friction process is characterized with low friction coefficients, which practically do not depend on load and velocity. Investigations have revealed that Ga is a promising metal for solid lubricators. Properties of these lubricators can be easily controlled by the preparation of suspensions on the base of Ga.

We report in a series of publications new progress on the phase equilibria of the Al–Cu–Fe system around the icosahedral quasicrystalline phase region. Results are obtained by the techniques of differential thermal analysis, magnetothermal analysis, metallography, and X-ray diffraction. This part deals with the liquidus surface, several four-phase planes, involving the icosahedral quasicrystalline phase, and isothermal sections at 900 and 870 °C. It is re-confirmed that the icosahedral quasicrystal (composition approx. Al 62.5 Cu 25 Fe 12.5 ) forms via a ternary peritectic reaction. The reaction temperature is at 882 °C according to the present results. A Scheil reaction scheme is used for explanation of the experimental data.

Part II deals with a set of isopleths, i. e., along the λ–i–φ tie line, ω and i tie line, i and liquid tie line, and with constant Cu content (25 at.%) and Fe (12.5, 12, 11, 10, 7.5, and 5. at.%) contents. These isopleths are elaborated, based on differential thermal analysis, magnetothermal analysis, optical microscopy with polarized light, and scanning electron microscopy. Isothermal sections and liquidus surface projection have been used to verify the consistency of the results.

The phase equilibria of the ternary Al–Cu–Fe system are demonstrated as isothermal sections at 800, 700, 645, 620, 617, 600, 592, and 560 °C. These data are based on metallographic investigation, microscopy with polarized light, and on structure analysis using X-ray diffraction, as well as on isopleths, which were constructed using thermal analyses. A Scheil reaction scheme can be used for the explanation of some novel phase transformations in the solid state.

The Zn-rich corner of the 450 °C isothermal section of the Zn–Fe–Co system was experimentally determined. ζ-FeZn 13 and ζ-CoZn 13 were found to form a continuous solid solution. As a result, the liquid and the ζ phases form a subbinary system at the Zn-rich corner of the ternary system. In addition to the ternary extensions of the binary d-FeZn 10 and d-CoZn 10 phases, a ternary compound, designated as T phase in the present study, has been identified. X-ray powder diffraction studies suggest that the T phase is isomorphous with Γ′-Fe 5 Zn 21 (cF408). The limits of the solid solubility of Fe and Co in their counter δ phases are appreciable, being 3.1 and 4.2 at.%, respectively.

The fatigue behaviour of NiTi wire has been studied. It was found that NiTi wires trained with constant strain could show different fatigue behaviour, depending on the temperature range of training and the temperature range of fatigue. A mechanism for the thermal fatigue behaviour of NiTi is suggested according to the experimental results.

Rapidly solidified ribbons of Cu-0.5-at.% Er and Cu-0.5 at.% Yb alloys have been prepared by melt spinning. An overall assessment of the ribbons has shown that up to three microstructural regions are distinguished: fine equiaxed grains, zone with a columnar structure and coarse equiaxed grains. Their number, type and height are controlled by wheel velocity. The growth behavior of dispersed intermetallic particles depends on the obtained microstructure, i. e., on the ribbon thickness. The large particles in thin ribbons coarsened by bulk diffusion, controlled by the diffusivity of the rare earth (RE) elements, and grow with the cube root of time at all temperatures. In thick ribbons with a high fraction of the coarse equiaxed zone, the coarsening takes place much more rapidly along the grain boundaries, which became a dominant transport mechanism.

The high-temperature oxidation of two Cu–Ti-based alloys (Cu 50 Ti 50 and Cu 50 Ti 40 Al 10 ) produced by rapid solidification was studied in air and in pure oxygen atmosphere by thermogravimetry, X-ray diffraction and electron microscopy. In the temperatures range 650 °C ≤ T ≤ 750 °C the growth of the oxide film is generally diffusion-controlled and its kinetics can be mostly described with parabolic rate law. At lower temperatures (e. g. T = 600 °C) the oxidation reveals cubic kinetics law for both alloys. The two alloys show the same oxidation rate for T < 650 °C, but the alloy with Al possesses a faster oxidation in the temperature range 650–750°C. At high temperatures (> 800°C), Cu 50 Ti 40 Al 10 reveals a higher oxidation resistance. The main oxidation product for both alloys in air as well as in pure oxygen (600–750 °C) is TiO 2 (rutile) from the beginning of the reaction. Oxygen dissolves and diffuses through the oxide and reacts with Ti, counter-diffusing from the bulk of the matrix to the matrix/oxide interface. Cu and Al are rejected from the oxide scale and diffuse to the alloy matrix, as Al enrichment close to the oxide/matrix interface was detected. The metal matrix is microcrystalline at the temperatures of oxidation and consists of the equilibrium phases in these systems (mainly CuTi and Cu 4 Ti 3 ). In the advanced stage of oxidation, due to the oxidation of Ti on the surface the matrix is enriched in Cu and depleted in Ti. The element concentrations measured in the oxide scale, at the metal oxide interface and in the alloy matrix indicate that probably an internal oxidation takes place, as well.

Thermodynamic equilibrium and Gulliver–Scheil as well as diffusion calculations using the DICTRA program have been made to simulate solidification and heat treatment of the nickel-base superalloy SC16. These simulations were validated by comparing them with own and literature experiments concerning directional solidification, heat treatment and differential thermal analysis (DTA) cycles. Equilibrium calculations predicted very well the element distribution in homogenized samples. Gulliver–Scheil based on the CALPHAD method and modified to serve multi-component systems proved to deliver sufficiently correct data for the directional solidification of SC16. The element distribution obtained after cooling below the solvus temperature and by DTA heating and cooling cycles was, however, only described with acceptable precision by applying the diffusion model. All these models require a sound multicomponent thermodynamic data set. Since DICTRA needs much more computing time than the other models it should only be applied wherever its higher precision is actually asked for.

A structural analysis of a Mg-3 wt.% rare earths (RE) matrix alloy and composites reinforced with 30 wt.% SiC particles has been performed. The specimens were obtained by gravity casting of the mechanically mixed suspension. The as-cast Mg-3 wt.% RE alloy was characterized by a dendritic microstructure with a very strong segregation of the alloying elements and the formation of the Mg 12 RE intermetallic phase, located in the interdendritic spaces. Introduction of the SiC particles into the matrix alloy caused the formation of a reaction layer at the interfaces between the components. Transmission electron microscopy analysis allowed the determination of the presence of RE 3 Si 2 precipitates both at the interfaces between the components and in the composite matrix.