Volume 89, Issue 3

Measurements of heat released upon solid state amorphization (SSA) have been performed to evaluate quantitatively a recently proposed thermodynamic model for solid state amorphization in binary systems. Three types of Ni – Ti systems have been investigated: crystalline Ni/crystalline Ti multilayers with incoherent interfaces, crystalline Ni/crystalline Ti multilayers with coherent interfaces and amorphous Ni/crystalline Ti multilayers. The heat released during SSA has been determined by differential scanning calorimetry (DSC) measurements. For the crystalline/crystalline multilayers with incoherent interfaces the reaction heats measured agree with the reaction heats derived from the thermodynamic model. For the crystalline Ni/crystalline Ti multilayers with coherent interfaces and the amorphous Ni/crystalline Ti multilayers the experimental reaction heats differ 30 to 40 % from the calculated values. The activation energies for SSA in the three Ni/Ti systems have been determined, too, from the DSC measurements. Within experimental accuracy the same value for the activation energy was obtained for the three types of Ni/Ti multilayers. The activation energy was found to be 130 ± 15 kJ/mol. This value agrees with the one obtained previously from X-ray diffraction analysis. The evolution of heat during SSA could be described as a function of annealing time upon isochronal annealing adopting a Johnson-Mehl-Avrami type reaction rate equation.

The influence of alloying levels in the range 0.01 to 0.5 % nitrogen, 4.5 to 6 % molybdenum and 0 to 10 % manganese on precipitation behaviour has been evaluated for austenitic stainless steels with a base composition of 20% Cr, 18 or 25 % Ni and < 0.02 % C. Both precipitation kinetics and the equilibrium conditions have been investigated. Precipitating phases include a number of intermetallic phases in addition to nitrides, silicides and carbides; some of these were transient phases which occurred only in the early stages of precipitation and were not present in the equilibrium structure. Experimental results are compared with calculations of elemental activities and the driving force for precipitation of different phases.

Phase equilibria were studied in the Mo–Ga–N system at 800 °C in order to better understand the metallurgy of Mo contacts to the wide band gap semiconductor GaN. Liquid Ga metal and the solid phases Mo 8 Ga 41 , Mo 3 Ga, and β-Mo 2 N were experimentally observed to be in thermodynamic equilibrium with GaN. Equilibrium between γ-Mo 2 N and GaN can also be inferred. No ternary phases were found in any of the samples examined in this study.

Interdiffusion between titanium and molybdenum has been investigated at high temperatures from 1250 to 1800 °C. A Kirkendall effect was noticed. The interdiffusion coefficients evaluated by the Den Broeder method are strongly dependent on the composition. The Hall method is used to determine the diffusion coefficients at the concentration limits. These coefficients are compared with the impurity diffusion coefficients in the pure metals from the literature, and with coefficients calculated by the Vignes and Birchenhall relation. This binary system does not exhibit an anomalous behaviour. The interdiffusion coefficient at the Kirkendall interface is also calculated by the Heumann method and compared with the results of our previous calculations.

Phase equilibria between the fcc matrix and the hcp nitride phase were installed by long-time heat treatment at 1273 and 1073 K, respectively, in different Fe–Cr–Mn–N alloys. The composition of the phases was determined applying wavelength-dispersive X-ray microanalysis. The results are compared with thermodynamic calculations using published data describing the fcc and the hcp phases in the system Fe–Cr–Mn–N. The agreement is satisfactory at medium concentrations at 1273 K but there are considerable differences at 1073 K.

The solubility of nickel in liquid sodium in the temperature range 773 to 873 K was calculated from corrosion results which were obtained in dynamic non-isothermal corrosion loops. The flow conditions at the nickel specimens were turbulent, and the flowing sodium was of very high purity. The saturation concentrations of Ni and Fe in Na were deduced from the corrosion results using a hydraulic model which relates the corrosion rates to the saturation concentrations of the solid metal in the liquid phase. The calculations resulted in solubilities of Ni in the range of 2.8 · 10 −6 < c sat < 7.6 · 10 −5 mol % in the temperature range. The solubility of iron in sodium and its temperature gradient was found to be much lower. The free energy of solution of nickel in sodium, Δ H sol = – 185 kJ · mol −1 , is higher in this work than in the literature.

Nickel single crystals of different orientations were deformed cyclically into the stage of mechanical saturation and investigated by high resolution X-ray diffraction after unloading from different points of the mechanical hysteresis loop. Characteristic changes of the diffraction profile shape were found indicating a variation of long-range internal stresses within the hysteresis cycle. By an analysis of the diffraction profiles these stresses could be determined for different dislocation structures. For example, the internal stresses arising in persistent slip bands (PSBs) were estimated. From the experimental results the conclusion is drawn that macroyielding occurs not only in PSBs but also in regions with a dislocation structure typical for the [001] sample orientation, provided the strain amplitude exceeds a threshold value.

Copper single crystals subjected to compressive creep at 773 K changed the crystallographic orientation of their longitudinal direction, chosen as a direction of loading. In compressive deformation the change moves the longitudinal direction to the normal of the slip plane. This fact is utilized in the present paper to an investigation of the possible activity of dislocation slip systems on non-compact crystal planes. The results show that the average change of orientation can be interpreted well by an activity of compact 〈110〉 {111} slip systems. Local deviations from the average final orientation can indicate a non-compact glide but a connection with substructure misorientations is expected, too. There are microstructural indications of dislocation cross-slip to {100}, {110} and (probably) {131} non-compact slip planes.

The high temperature creep behavior of two rolled Al–Fe–V–Si dispersion strengthened materials has been studied by means of tensile tests. The microstructure consisted of 16 % and 36 % volume fraction of submicron dispersoids evenly distributed in the aluminum matrix. High apparent stress exponents from 10 to 38, and high apparent activation energies of up to 700 kJ/mol, were observed. A threshold stress - like creep behavior was never observed at any temperature. The creep data was analyzed using Rösler and Arzt’s detachment creep model. The predictions at high temperatures from the model are not accurate, predicting higher creep rates and lower stress exponents than those experimentally measured. A modified version of the detachment creep equation is proposed for predicting the creep behavior of dispersion strengthened materials taking into account the constancy of the microstructure during deformation.

The kinetics of ferrite to austenite transformation has been studied in a high strength low alloy steel containing Nb as the carbide forming element. The growth rate and volume fraction measurements were made using optical microscopy. Three distinct stages of the transformation have been observed. The dissolution of pearlite occurs in the first stage. In the second stage, the growth of austenite occurs by the diffusion of carbon into austenite. However, due to paucity of the carbon diffusion field the transformation at this stage can not continue under a full diffusion field but occurs under a truncated diffusion field. In the third stage, austenite grows into ferrite by the diffusion of Mn in ferrite. The parabolic growth rate constants observed experimentally match well with the calculated values.

Zn-rich Zn–Al–Cu alloys were produced by milling of elemental powders mixtures, Zn-22 wt.% Al-3 wt.% Cu and Zn-20 wt.% Al-10 wt.% Cu, for times of up to 1080 ks. The X-ray diffraction analysis showed the formation of a (aluminum-rich), η (zinc-rich) and ε (CuZn4) phases at the early stage of milling. The presence of ε phase was more evident with the increase of copper content in mixture. In the case of the two alloy compositions, the following phase transformation α + ε → η + τʹ was observed to occur during milling. A mixture of η, α and τʹ phases was found in powder mixtures milled for 1080 ks, which is in agreement with phases predicted for these compositions in the equilibrium Al–Cu–Zn phase diagram. Finally, all these phases were present on a nanometer scale in both mechanically alloyed powders.