Volume 88, Issue 10

The present paper describes a general procedure for image-analytical determination of point coordinates of separable objects for a subsequent density estimation. As a particular application, primary dendritic arm spacing is considered. First, a method is introduced for automatic determining of centres of dendrites in directionally solidified matter. This procedure combines well-known methods of image processing with specific ideas which are connected with the shape of dendrites. The result of this automatic method is a point field which is similar to the point field of centres of dendrites. However, because of image noise not all dendrite centres are included in that point field and, on the other hand, artificial points are produced which are not dendrite centres. Therefore, it is recommendable to use a robust method for the determination of dendritic arm spacing from such disturbed point fields. Such a method based on the Voronoi tessellation is suggested here. The application of the whole procedure is demonstrated for some examples.

Precision casting is an attractive method for the net shape production of parts, particularly interesting for expensive alloys like titanium. This paper investigates the possibilities to improve the properties of titanium castings, solely applying thermal treatments. It was shown that in the (α + β) titanium alloy Ti-6Al-4V, which is used for casting applications, the yield stress, the fatigue strength and the creep resistance can be simultaneously improved by producing bi-lamellar microstructures where the β phase is hardened by the precipitation of fine α lamellae. In titanium alloys with a lower volume fraction of the β phase, e. g. in the near-α alloy Ti-6242, localized dissolution of β lamellae may occur, which has a negative influence on the fatigue resistance while the yield stress and the creep resistance are still improved.

This investigation examines the influence of microstructural features on the dry sliding wear characteristics of some plain bearing alloys. Three different alloys were selected for the study: a leaded-tin bronze, an Al bronze and a Zn-based alloy processed under identical conditions. The wear characteristics of the alloys are explained in terms of the load bearing capability, microcracking tendency, thermal stability and lubricating properties of their microconstituents and the sliding conditions. Wear surfaces, subsurface regions and debris are examined to understand the material removal mechanisms during the process of wear.

Giant magnetoresistance (GMR) was observed in pulse-plated Ni 81 Cu 19 /Cu multilayers with a maximum GMR value of about 2 % for Ni – Cu layer thicknesses around 2 to 3 nm. A columnar growth of the multilayers was detected by transmission electron microscopy. The column width (grain size) was the largest for multilayers with the maximum GMR. It could also be established that the multilayer planes are often inclined at angle, which depends on the chemical modulation wavelength, with respect to the substrate plane. This definitely gives rise to a current-perpendicular-to-plane contribution to the GMR, offering a plausible explanation for the location of the maximum of the GMR vs thickness curve. The lattice constant mismatch between the Ni – Cu and Cu layers gives rise to considerable stresses which are relaxed, besides the formation of twinning and dislocation structures, also by an induced periodic lattice distortion (structural modulation) not completely coinciding with the chemical modulation in every direction.

Specimens of the superalloy CoCr22Ni22W14 were deformed in hot tensile tests at temperatures T 1 = 1023 K, T 2 = 1073 K and T 3 = 1123 K with strain rates ε̇ 01 = 3.2 ᐧ 10 −4 s −1 and ε̇ 02 = 3.2 · 10 −5 s −1 followed by microstructure investigations determining the dislocation density quantitatively and characterizing the precipitation state qualitatively. The strain rate sensitivity m was determined from additional strain rate jump tests. Using the obtained microstructural data the evolution of dislocation density and the stress – strain curves have been simulated based on the “one-parameter model” by Kocks, Mecking and Estrin.

The mobilities for diffusion in Fe – Si alloys with bcc and fcc structure have been assessed using the DICTRA software: In the bcc phase the mobilities of both elements are determined as functions of temperature and composition. In the fcc phase the mobility of Si is evaluated as a function of temperature. Using these parameters and the data for carbon diffusion from a database, the kinetics of ferrite growth from Si-containing austenite is calculated and compared with experimental results.

The effect of the weld heat affected zone (HAZ) on microstructure, mechanical properties and hydrogen embrittlement susceptibility of vanadium modified 2.25Cr-1Mo steel (V-Mod Steel) has been studied to examine the viability of using V-Mod steel for reactor applications. The weld HAZ was simulated on the assumption that the submerged arc welding was made for the V-Mod steel with input heat of about 40 kJ/cm. When the weld HAZ was heated to 1123 K, the strength was somewhat decreased, while the 0.2% proof strength exceeded considerably the critical value (415 MPa) established for reactors. Heating the weld HAZ in the region of 1173 to 1473 K had a beneficial effect on the strength and ductile-to-brittle transition temperature. Compared to the base metal, the weld HAZ heated in the region of 1123 to 1673 K exhibited equal or superior hydrogen charged and initial mechanical properties. The results illustrate that the weld HAZ for the V-Mod steel has not only sufficient mechanical properties which could withstand the operation under severely stressed conditions but also beneficial effects on hydrogen embrittlement susceptibility.

The metastable miscibility gap of Fe–Cu alloys in the undercooled liquid state was experimentally studied using a high temperature thermoanalytical method. Applying the melt fluxing technique it was possible to reach undercooling levels up to 250 K. Thus, the miscibility gap could be determined in the wide composition range between 10 at.% and 88 at.% Fe, showing symmetrical shape. Additionally, the specific heat in the liquid and undercooled liquid state was measured. A strong temperature dependence of the liquid specific heat was observed, which can be described in the framework of the self-association model.

Three iron aluminide alloys with the composition Fe-40Al- 0.5X-0.2C-0.05B, in at.% (X = Ti, Zr, Nb), have been investigated by means of optical, scanning and transmission electron microscopy. All three alloys were extruded and exhibited small grain sizes ranging from 12 to 40 μm. The precipitate volume fractions and lattice parameters were consistent with the low predicted solubilities in these alloys. While the Ti- and Nb-containing alloys formed, as expected, TiC and NbC precipitates, the Zr-containing alloy contained a ternary intermetallic phase (Fe 6 Al 6 Zr). In addition, an unidentified Fe–Al–Zr phase with a Zr concentration of approximately 23 at.% as well as ZrC were found. The precipitate morphologies were also very different in the three alloys. Whereas in the Ti-containing alloy the TiC phase tended to occur in the form of needles, the precipitates were more equiaxed in the Zr-containing alloy. The NbC particles were also equiaxed and occurred either as isolated particles in the interior of the grains or as clusters at the grain boundaries.

Small additions of alkaline earth carbonates such as MgCO 3 , CaCO 3 , BaCO 3 and SrCO 3 to (Li 0.62 K 0.38 ) 2 CO 3 can change the basicity, defined as –log( a M20 ), where M represents an alkali metal and a M20 is the net oxide ion activity. Net oxide ion activity is defined as the sum of the alkali and alkaline earth oxides activities dissolved in the melt. If the effect of the alkaline earth additive were to change the basicity of the solvent melt, it ought to be possible to measure that change by a galvanic cell. Therefore, a single zirconia electrolyte cell arrangement has been devised to measure basicity changes at 650 °C, as follows: CO 2 ,O 2 , Au, (Li 0.62 K 0.38 ) 2 CO 3 |ZrO 2 · Y 2 O3| test melt, Au, Ar Overall, the agreement between the thermodynamic model for this cell and the measured cell voltages is reasonably good. To complete the analysis, it was necessary to measure NiO solubilities in (Li 0.62 K 0.38 ) 2 CO 3 melts containing MgCO3 and SrCO3 additions. The cell measurements of basicity and the solubilities are self-consistent.

In the atmospheric corrosion of copper, nickel, and iron, the adsorption of water affects the corrosion rates. Knowledge of water adsorption and metal oxyhydroxide formation is important in understanding the atmospheric corrosion process. The purposes of the present research were (i) to measure the adsorption of water on metal surfaces as a function of temperature and relative humidity (RH) and (ii) to assess Bradley’s polarization model of adsorption. In the present research, the quartz-crystal microbalance (QCM) technique was used to measure the mass changes of copper, nickel, and iron at 0 to 100 % relative humidity and 7 to 90 °C under nitrogen and air environments. Less water was adsorbed on copper, nickel, and iron which form oxides than on gold. The amount of water adsorption was similar on copper, nickel, and iron under N2 and air carrier gases. Functional relationship was first proposed as a way to include dipole/induced dipole interactions between the adsorbents and water layers.