Showing papers by "Eduard Arzt published in 1997"

Thermal Vacancies and High-Temperature Mechanical Properties of FeAl

Abstract: The formation and the kinetics of migration of thermal vacancies in Fe61Al39 have been studied by positron lifetime spectroscopy. From the temperature variation of the equilibrium vacancy concentration an effective vacancy formation enthalpy of HFV = 0.98 eV has been determined. From the equilibrium of the thermal vacancy concentration after temperature changes the vacancy migration enthalpy (HMV = 1.7 eV) could be derived. As observed recently, vacancy-type defects give rise to an internal friction maximum at 680 K (2 s—1) after quenching of FeAl. This reorientation effect as well as its recovery by long-range migration of defects is characterized by the above vacancy migration enthalpy. For the defects of the 680 K internal-friction maximum, a complex of a vacancy on the Fe sublattice and an antisite atom is suggested. A direct relationship between the high-temperature mechanical properties and atomic defects arises from the time dependence of the yield stress anomaly in Fe60Al40 measured after fast heating, which fits perfectly with the equilibration kinetics of the thermal vacancy concentration. From this we conclude that the yield stress anomaly is sensitively affected by thermal defects.

Dispersion strengthening of intermetallics

Abstract: Dispersion strengthening has been applied recently to several intermetallic compounds, which otherwise suffer from insufficient creep strength at high temperatures. In this paper an overview is given of new theoretical aspects concerning the interaction between lattice dislocations and dispersoids. Particular complications arise when the dislocations are, due to the atomic order in the matrix, dissociated into superpartials. The development of oxide-dispersion strengthened NiAl, Ni 3 Al and FeAl is briefly described. It is shown that the creep properties can be reasonably well explained by the dislocation models.

Observation of Dislocation DisAppearance in Aluminum Thin Films and Consequences for Thin Film Properties

Abstract: Dislocation structures in Al-Cu thin films have been studied by transmission electron microscopy (TEM). We have observed that the contrast of interface dislocations disappears in the electron beam. We assume that the contrast dissolution is due to the spreading of the dislocation core at the crystalline/amorphous interface or due to a diffusive movement of the dislocation through the oxide. In any case, the relaxation is assumed to be controled by irradiation induced diffusion. As a consequence, the short range stresses and at least partly also the long range stresses of the dislocations relax. This relaxation changes the interaction force between dislocations and may thus significantly affect the mechanical properties of thin films. It is concluded that interaction between interface dislocations may not be responsible for the high temperature strength of aluminum films.

Quantitative Analysis of Electromigration Damage in Al-based Conductor Lines

Abstract: Electromigration damage in Al-based interconnects with three compositions (pure Al, Al–1%Si–0.5%Cu, and Al–2%Cu) was studied quantitatively. Using scanning electron microscopy, the spacings between more than 1000 voids and hillocks were measured. The distribution of the spacings was found to be a function of the composition, the applied current density, and the linewidth. The measurements confirm the existence of a threshold product of current density and diffusion length. In particular, a dependence of this threshold product on the Cu content was found. The results of the analysis show that there are clear correlations between the details of the microscopic damage processes and the lifetime of the conductor lines.

Energy Storage And Recovery In Thin Metal Films On Substrates

Abstract: Dislocation segments which extend through the thickness of a film can move through the film only if dislocation line length is deposited or removed at the film/substrate and film/passivation (if any) interfaces. The dislocation density and, therefore, the energy stored in the film increase during plastic deformation. The reverse process, that is, the reduction of strain energy in the film by the reduction of dislocation line length, is here suggested to be the origin of a number of unexplained features of experimentally obtained stress-temperature curves, including very low (or even “negative”) yield stresses in compression, tensile-compressive flow stress asymmetries, increasing strength with increasing temperature upon heating, and a very strong Bauschinger-like effect which has been seen in thin Cu films. The results of stress-temperature measurements of passivated Cu thin films on silicon substrates are presented.

High-temperature creep in a coarse-grained oxide-dispersion strengthened Ni3Al alloy

Abstract: An oxide-dispersion strengthened (ODS) Ni 3 Al(B)-alloy containing 5 at.% chromium, produced by powder metallurgy, was zone-annealed to produce a coarse-grained microstructure. The creep properties were then investigated between 1000 and 1200°C at creep rates from 10 −7 to 10 −3 s −1 and the resulting experimental data were compared with several models of creep deformation. As expected, the Rosler and Arzt model for the detachment of single dislocations from the dispersoids did not satisfactorily explain the measured data. The model of Gohring and Arzt, which considers superdislocations, as present in ordered Ni 3 Al, provided a much better correlation. The measured creep strengths were approximately five to ten times higher than those of fine-grained ODS-Ni 3 Al; however, they were about 1000 times lower than those of commercial Ni-based alloys such as MA 6000. Suggestions for further development and experiments are made.

Interactions at interface between Cu99Ti1 thin films and polyimide

Abstract: Material characteristics and chemistry of the interface formed between Cu99Ti∼1 films and polyimide were investigated. Plasma pretreatment of the polyimide surface and post-metallization annealing were used to modify the interface. Composition analysis data taken from the interface indicated no significant Cu diffusion into the polyimide layer and accretion of nitrogen at the interface. The formation of Cu and Ti compounds by carbonyl bond breaking, suggested by x-ray photoelectron spectroscopy, could explain suppression of Cu diffusion and drastic increase of adhesion strength.

Mechanical Properties of Al-Cu Films with Various Heat Treatments

Abstract: Precipitation and solid solution hardening have been studied in Al-Cu thin films with Cu content ranging from 0 to 2 wt%. We have changed the precipitate and dislocation structures of films having different Cu concentrations by varying the heat treatments prior to mechanical testing. Pure Al films showed the same values of tensile and compressive yield stresses at a given temperature during stress-temperature cycling. Al-Cu alloy films, however, showed larger stresses in tension than in compression. The compressive flow stress during heating could be changed by a factor of five by the initial heat treatment, but the differences disappeared above 250r.C. Upon cooling from 480r.C, solution hardening as well as precipitation hardening was observed in the Al-Cu films. Solution hardening is independent of Cu concentration, but for precipitation hardening, both the magnitude and the temperature range in which the mechanism is effective are sensitive to the Cu concentration. The microstructure was observed using transmission electron microscopy. The mechanical behavior is consistent with interactions between dislocations and precipitates which arise due to constraints on the film by the substrate.