M. Bamberger

Continuous versus interrupted creep in AZ91D magnesium alloy

A qualitative model of the creep process of AZ91D was proposed in the past by the authors on the basis of continuous creep tests, optical and SEM metallography and TEM study. In view of this model, creep resistance in the case of interrupted creep tests is expected to be inferior compared with conventional creep test under the same load and temperature. The results of interrupted creep tests performed in the recent investigation, described in this paper, support the prediction and can be explained on the basis of the above mentioned creep model. The differences between the results of the continuous and interrupted creep test stems from microstructural instability of the alloy. This fact has been verified by heat treating of the alloy and stabilizing the microstructure. The creep behavior of the heat-treated specimens was found to be comparable to that of the as-cast pressure die casting specimen subjected to continuous creep.

Phase investigation in laser surface alloyed steels with TiC

Laser technology enables melting and alloying specimen surfaces without the substrate itself being heated, whereby surfaces with special attributes are obtained with the properties of the substrate remaining unaffected. The surfaces of Armco iron and AISI 1045 steel were laser-alloyed with TiC powder, a CO2 laser of 2.5 kW maximum power being used. Optimal laser and powder-feed parameters were established. Particles of TiC were injected into the molten surface layer, forming a composite material, steel + TiC. The microstructures were investigated metallographically. Some of the particles had partially melted during their passage through the laser beam and had re-solidified, forming small and fine dendrites. Phase identification by X-ray diffraction revealed the presence of α-Fe, martensite, and Fe3C phases, as well as amounts of stochiometric TiC and unknown phases. Identification of phases by TEM and diffraction of electrons revealed the presence of unknown phases, such as tetragonal TiC and (FeTi)C. Mössbauer results show ternary Fe-Ti-C phases, which can be related to the TEM and X-ray diffraction results. A correlation was found between the substrates composition, microstructures, and the different phases present.

Improvement of adhesive bonding strength in sealed anodized aluminium through excimer laser prebond treatment

The feasibility of using an excimer laser as a prebond treatment of sealed anodized aluminium alloys has been demonstrated. Irradiation of sealed chromic acid anodized aluminium by means of pulsed ultraviolet laser (193 nm wavelength, energy density of 0.2–7 J cm−2 per pulse and duration of 24 ns) improved bonding strength by more than 100% compared to that of the sealed anodized but non-treated alloy (using modified epoxy adhesive). The influence of laser treatment on the sealed anodic layer was investigated by various techniques, including scanning electron microscopy, X-ray diffraction and Fourier transform-infrared spectroscopy. Various phenomena, such as morphological changes, ablation, crater formation, melting, gas evaporation, water removal and crystallization, were observed following the laser treatment. The magnitude of these changes was found to depend on both laser-beam energy density and number of pulses.

Laser remelting of cast iron: a Mössbauer study

The Mössbauer spectroscopy in backscattering technique (CXMS) was used to analyse cw-CO2 laser-treated samples of cast iron. The depth profile as revealed by Mössbauer phase analysis is discussed in connection with the initial microstructure.

High-temperature low-cycle fatigue of a nickel-based MAR-M200 + Hf alloy in Ar and Ar + 20% O2 environment

A systematic study was made on the environmental influences of inert gas and oxygen on the low-cycle fatigue failure of MAR-M200 + Hf unidirectionally solidified nickel-based alloy at 975 ‡C. The cyclic load was constant and comprised creep tension and plastic compression. Under an inert argon atmosphere, a pure transgranular fracture free of environmental influences was obtained. Under an Ar + 20% O2 atmosphere the fracture was intergranular along a path through the interdendritic microsegregation zone. In the area close to the propagating crack tip, a uniphase layer was detected, resulting from a process of internal oxidation which developed a preferred oxide-type bonding with the elements making up the principle precipitating phase, γ′, namely titanium and aluminium. The internal oxidation led to changes in the boundary layer between the matrix and γ′ phase, as a result of which the original coherence between them was lost. In order to reduce the surface energy between the precipitate and the matrix, a process of growth and coarsening of γ′ phase took place in these conditions. The internal oxidation and the formation of the uniphase layer increased the brittleness over a comparatively wide area adjacent to the crack tip. As a result, crack branching and blunting took place during propagation in the oxygen-containing atmosphere. The branching and the blunting were instrumental in lengthening the active life of the alloy in the oxygen-containing atmosphere compared with that in the inert atmosphere.

Role of zirconium in the phase formation at the interdendritic zone in nickel-based superalloys

A commercial Alloy 80A and a modified alloy with lower titanium and zirconium contents, e.g. 1.4% and <0.01%, respectively, were subjected to unidirectional solidification followed by a rapid quench in water to preserve the structure at elevated temperatures. The slow solidification results in a coarse dendritic structure with a high enrichment of sulphur (up to 2%), aluminium (up to 4%), titanium (up to 13%) and zirconium in the interdendritic zones. In the commercial Alloy 80A, the zirconium and sulphur present in the interdendritic zone result in a eutectic containing ZrS and NiCrTi-matrix. Excess zirconium forms an NiZr-intermetallic. No TiS is formed in this case. In the absence of zirconium, the titanium enriched in the interdendritic residual melt forms a eutectic of TiS. In both cases the effective solidus temperature corresponding to the composition of the residual melt is 1170°C, compared to 1330 °C, which is the solidus temperature of the bulk material.

Some observations on dendritic arm spacing in Al-Si-Mg and Al-Cu alloy chill castings

The heat flux from a cast cylinder to a steel chill was experimentally determined for a commercial Al-Si-Mg alloy (A356) and for Al-Cu alloys having different copper contents. The relationship between variation of the heat flux with time, initial temperature of the chill, and solute concentration was determined. The heat flux from the casting to the chill in the A356 alloy is higher than in an Al-7.5% Si alloy, but the microstructure of the former is coarser. The time dependence of the heat flux in an Al-Cu alloy is similar to that in A356 and in Al-7.5% Si. Calculated values for the temperature of the casting and the local solidification time as functions of the distance from the chill were obtained with the aid of the heat flux data and a program for calculating the temperature field during solidification. A good fit with experimental measurements was achieved. Measurements of the mean secondary dendritic arm spacing at different distances from the chill resulted in the relationship λ=atf0.43between the local solidification time (tf) and the dendtitic arm spacing (λ), where a is a characteristic of the alloy and of the solute concentration. It is noted from the results that the λ value depends not only on the solidification time but also on the concentration of solute element. Different aspects of the evolution of structure, and some attention to growth with high temperature gradients in the presence of chill is discussed.

Investigation of equilibrium and phase stability in the liquid/solid state in nickel-based wrought superalloys

Isothermal holding experiments between the liquidus temperatures, TL, and the solidus temperatures, TS, were carried out on nickel-based wrought superalloys. It was found that during solidification the elements aluminium, cobalt and tungsten tend to dissolve in the γ matrix whereas titanium, tantalum and molybdenum tend to segregate into the liquid. Molybdenum and titanium cause the formation of brittle σ and η phases, respectively, after prolonged holding at elevated temperatures, in accordance with the results of New-PHACOMP (new phase computation).

Formation and growth of Ni3Sn4 intermediate phase in the Ni - Sn system

Dipping experiments of Ni foils in molten tin were made in the temperature range, 280–310°C. In addition, experiments were conducted while passing a constant current through the system, the foils being used as electrodes. In SEM - EDS observations and analysis the Ni3Sn4 equilibrium intermediate phase was found to grow both as a layer at the solid/liquid interface and as platelets in the melt. Growth kinetics were found to be influenced by the dissolution of Ni in the melt. The voltage between the electrodes vs. time (V(t) curve), measured in-situ, was found to conform to SEM observations. The V(t) curves permit a qualitative evaluation of the layer growth process.

Laser surface alloying of steel with TiC

Laser technology enables a specimen surface to be melted and alloyed without the substrate itself being heated, whereby a surface with special attributes is obtained while the properties of the substrate are unchanged. The surface of Armco iron, AISI 1045, and AISI 1095 steels were laser alloyed with TiC powder, using a CO2 laser of 2.5 kW maximum power. Optimal laser and powder feed parameters were established. Particles of TiC were injected into the molten surface layer, forming a composite material, steel + TiC. The microstructures were investigated metallographically. Some of the particles had partially melted during their passage through the laser beam and had re-solidified, forming small and fine dendrites. Surface hardness increased, mainly due to the presence of these undissolved TiC particles. A new mathematical model describing the alloying process, based on the equation of heat conductance, to the alloying process is presented.