Band 98, Heft 2

X-ray stress measurement in a specimen with a biaxial stress state is usually done with the -method, where measurements are carried out at different and. For the values the most usual choice is 0°, 45°, 90°. A better choice would be. The reason why is explained in detail by comparing the error calculus for these two cases.

The phase equilibria and thermodynamic properties in the ZrO 2 – DyO 1.5 system have been studied using the experimental methods of X-ray diffraction, scanning electron microscopy, energy dispersive X-ray spectroscopy, differential thermal analysis and high temperature drop calorimetry. The tetragonal + fluorite and fluorite + C-Dy 2 O 3 phase equilibria between 1400 °C and 1700 °C have been determined, together with the enthalpy increments of the materials with 30 mol.% and 50 mol.% DyO 1.5 in the temperature range 200 – 1400 °C. Furthermore, the martensitic transformation temperatures (A s , M s ) have been measured for the samples with 1 and 2 mol.% DyO 1.5 . Finally, thermodynamic assessment has been carried out using the experimental results obtained and literature data.

A range of metal-rich Hf – Ti – Si alloys with Si concentrations up to 35.0 at.% was directionally solidified. Liquid – solid phase equilibria in these alloys have been investigated using scanning electron microscopy, X-ray diffraction, and electron beam microprobe analysis. Four phases were identified: (Hf, Ti) 5 Si 3 , Hf(Ti) 2 Si, β(Hf, Ti, Si), and α(Hf, Ti, Si). Microstructural and microchemical evidence provided a clear definition of the Hf – Ti – Si liquidus surface at the metal-rich end and indicated the existence of the invariant reaction: Liquid + Hf(Ti) 2 Si → (Hf, Ti) 5 Si 3 + β(Hf, Ti, Si) This invariant reaction occurs at a composition of approximately Ti-50Hf-9.5Si, and at a temperature of ∼1420 °C.

The density of liquid Al – Fe and Al – Ni binary alloys have been determined over a wide temperature range by a non-contact technique combining electromagnetic levitation and optical dilatometry. The temperature and composition dependences of the density are analysed. A negative excess volume correlates with the negative enthalpy of mixing, compound forming ability and chemical short-range ordering in liquid Al – Fe and Al – Ni alloys.

In this paper the applicability and the efficiency of in-situ electrical resistance measurements with a four-probe method for monitoring the solidification process are presented. For this purpose a special measurement cell was developed. It enables simultaneous in-situ electrical resistance and temperature measurements during solidification to characterize the L → S phase transformation of metals and alloys. The method was experimentally demonstrated with a eutectic Pb – Sn (i. e. Pb-61.9 wt.% Sn) alloy. Additionally, the L → S phase transformation was monitored and compared with conventional differential thermoanalysis. The results of our investigation show that the simultaneous in-situ electrical resistance and temperature measurement method is able to determine the exact position of the start and end points of the L → S phase transformations from the resistance curves R(t) and the corresponding temperatures.

The magnesium alloys AZ91D, MRI 153M and MRI 230D were investigated in the form of die-cast specimens with a cast-skin. The fine-grained microstructure consists of a dendritic magnesium solid solution and interdentritic precipitates. The cyclic deformation behaviour was characterised in stress-controlled load increase and single step tests by means of mechanical stress – strain hysteresis and temperature measurements. Load increase tests allow a reliable short-time estimation of the endurance limit. With the physically based lifetime calculation method “PHYBAL”, the lifetime of the magnesium alloys can be precisely calculated on the basis of the plastic strain amplitude and the change of the specimen temperature due to cyclic plastic deformation with three experiments. Lifetimes calculated according to “PHYBAL” agree excellently with experimentally determined data.

The tensile deformation behavior at elevated temperatures and the associated mechanisms are investigated for hot isostatically pressed NiAl-28Cr-5.9Mo-0.1Dy fabricated through spray forming. Superplasticity is observed at above 1323 K under an initial strain rate lower than 10 −3 s −1 with an m value of 0.5. The activation energies for superplastic flow are found to be half of that for self-diffusion in NiAl. Grain boundary sliding between NiAl and adjacent NiAl or Cr(Mo) grains with local range accommodation, i. e., dynamic recovery, is suggested to be responsible for the superplastic deformation of the alloy.

The fracture behavior of welded ductile iron is discussed by means of experimental and theoretical studies. Ferritic nodular cast irons were joined by cold arc welding in order to obtain over-matched weldment in which the yield strength of the weld metal is higher than that of the base metal. As a result of the welding process, various regions with different mechanical properties formed in the welded specimens. In the experimental part of the study, the mechanical properties of the different regions in the welded specimens were identified and J-Integral toughness values were determined by fracture toughness tests for different crack locations including the base metal, the heat affected zone and the weld metal. In the numerical part of the study, finite element analyses were conducted by fixing 3-D models precracked at different locations for different crack lengths. In these models, after stress – strain analysis, stress triaxiality and plastic deformation characteristics around the crack tip were determined for each crack location and different crack sizes. It was found that stress triaxiality values increase with growing crack length. Different regions with different strength levels in the weldment affect the extension of plastic deformation in the models. Hindrance to the extension of plastic deformation at the heat affected zone and diffusion line causes an extra increase in stress triaxiality at the crack tip.

There are severe limitations to attaining submicrostructured steels by means of continuous cooling, the achievement of fine grain sizes is limited by the need to dissipate enthalpy during rapid transformation, so that the actual grain size obtained is more than an order of magnitude greater than can be obtained theoretically. Bainitic microstructures obtained by isothermal heat treatment at low temperatures, can overcome the recalescence limitations and at the same time provide a very fine microstructure (nanostructure) that leads to extraordinary mechanical properties. Steel grades for this purpose have been designed by means of phase transformation theory alone and their mechanical properties tested. The results obtained reveal that the microstructure is indeed extremely fine and exhibits high levels of strength accompanied by reasonable ductility and toughness, mainly due to the fine scale of the bainitic ferrite plates achieved. The influence that retained austenite has on the mechanical properties through the TRIP effect is also discussed.

The low stacking fault energy and high carbon content in Hadfield steel make twinning the basic deformation mechanism from the onset of plastic deformation in [1¯11] and [011] oriented single crystals in tension at T = 77 – 300 K. Alloying with aluminium (2.7 Al in wt.%) results in an increase of stacking fault energy from 0.03 J · m 2 to 0.05 J · m −2 and moves twinning to higher degrees of deformation (∊ pl > 15 %). In aluminium-free [1¯23] crystals twinning starts after 20 % strain. For [1¯23], [001] orientations, aluminium additions change the dislocation arrangement from a uniform distribution to a planar dislocation arrangment and also suppress twinning. Intersections of dislocation pile-ups were found to be the governing factor for hardening in the aluminium-alloyed [001] crystals.

Ledeburitic chromium steel X155CrVMo12.1 for cold-work tools was investigated in the quenched and tempered condition. Using appropriate investigative methods the evolution of the microstructure in all conditions was characterized. The change in state of a multicomponent matrix during tempering is crucial for attaining the required properties of this steel. In the material investigated, shape, size, fraction and distribution of eutectic (primary) carbides played a secondary role. The evolution of tempered martensite and precipitation of secondary hardening carbides of M 23 C 6 -type and MC-type from the supersaturated matrix during tempering led to secondary hardening and a secondary hardness peak at about 530 °C. The results show that steel X155CrVMo12.1 also exhibits performance ability at moderately elevated temperatures.