H. Neuhäuser

Investigating the Portevin-Le Châtelier effect by the acoustic emission and laser extensometry techniques

Abstract The Portevin–Le Châtelier (PLC) effect is a spectacular effect of dynamic strain aging in many alloys deformed in certain intervals of strain rates and temperatures. The main feature of the PLC effect is a negative strain rate sensitivity of stress, which is linked with stress fluctuations, a macroscopic spatio-temporal localization of plastic deformation (nucleation and propagation of deformation bands) and an intense acoustic emission. Recent theoretical studies have pointed out that cooperative dislocation motion is a necessary condition for plastic instabilities to occur under conditions of negative strain rate sensitivity. In this work the potential of acoustic emission and laser extensometry to monitor in situ cooperative dislocation motion due to PLC effect is reviewed and examined experimentally in an Al–1.5 wt.% Mg alloy. At the conditions of testing, the alloy exhibits a Luders phenomenon followed by the C- and/or B-type of the PLC effect. The results indicate that different dislocation processes are responsible for the Luders phenomenon, the nucleation of PLC bands and the propagation of PLC bands.

Effect of deposition parameters on optical and mechanical properties of MF- and DC-sputtered Nb2O5 films

Abstract Niobium-pentoxide films with thicknesses of 100 nm to 1 μm were deposited by both DC and mid-frequency (MF) magnetron sputtering using a dual magnetron (TwinMag®) configuration. Stress measurements were performed by determining the substrate curvature before and after deposition for various process parameters. By varying the argon or oxygen partial pressure, it could be shown that a minimization of the undesired compressive stress, caused by the particle bombardment, is attainable. It was found that the limits for achieving this strongly depend on the applied process (MF or DC) as well as on the target state (oxide or transition mode). The films were characterized using SEM, AFM, vibrating reed, EPMA, and XRD yielding information concerning morphology, stoichiometry, phase, and surface roughness. Furthermore, ellipsometric measurements allowed the determination of the film thicknesses and the refractive indices ( n 550 nm =2.4) which turned out to be highly correlated to the stress values.

Portevin-LeChatelier effect in strain and stress controlled tensile tests

Abstract The nucleation and propagation of Portevin–LeChatelier (PLC) deformation bands in Cu–15at.%Al have been studied by means of a novel multi-zone scanning laser extensometer providing information on the development of local strain along the main part of the specimen, in addition to the conventional measurement of stress serrations. By means of tensile tests with imposed constant strain rate independent data have been determined on propagation velocity, concentrated strain and width of the bands appearing in three different types of PLC-deformation. The tensile test machine has been softened to permit measurements with imposed constant stress rate. First results will be presented and compared to the strain-controlled ones on Cu–15at%Al.

Vibrating reed apparatus with optical detection and digital signal processing: Application to measurements on thin films

We present an apparatus that permits quasisimultaneous measurements of several vibrating modes in a vibrating reed experiment. Position detection by laser beam deflection offers a simple setup and good sensitivity for higher modes of flexural as well as torsional vibrations. Frequency and damping from free decay data are determined by software which permits high accuracy, especially at low damping. This is particularly interesting for the measurement of the mechanical properties of thin films deposited on a low damping vibrating reed. As an example results on an Al film on a microstructurized silicon resonator are shown.

Propagation of deformation bands investigated by laser scanning extensometry

Abstract Luders band and Portevin–LeChatelier (PLC) band propagation in polycrystalline Cu–0.15at.%Al has been studied by multiple zone laser scanning extensometry during tensile deformation. The dependence of the propagation rate on the external deformation rate is investigated in a wide range ( e =6.67×10 −6 –1.3×10 −2 s−1) at selected temperatures (100–300 °C), in particular for the PLC bands of type B which propagate in a jerky manner. The local strain rates within the bands are determined and the results are compared with recent theoretical modelling of PLC band propagation.

Observation and modelling of propagating Portevin-Le Châtelier deformation bands in Cu-15 at.% Al polycrystals

Abstract The nucleation and propagation of Portevin-Le Châtelier (PLC) deformation bands have been studied by means of a novel multizone laser scanning extensometer providing information on the local development of strain along the main part of the specimen, in addition to the conventional measurement of stress serrations. This permits us to distinguish clearly between the bands of types A, B and C, and to explore their ranges of existence at various temperatures, stresses and strain rates as well as transitions between them along the stress-strain curve. The laser extensometer provides independent data on the propagation velocity, concentrated strain and width of the bands. These experimental data are compared with a recent theoretical space-time analysis of propagating PLC bands, which combines explicitly a physical description of the kinetics of dynamic strain ageing and plastic deformation including the effect of long-range dislocation interactions. The model correctly predicts the above band parameters and their dependences on the deformation rate and specimen thickness.

Mechanical spectroscopy of ordered ferromagnetic Fe3Al intermetallic compounds

Abstract By means of the vibrating-reed technique the temperature and magnetic field dependencies of internal friction and elastic modulus have been measured for ferromagnetic single and polycrystals of D0 3 ordered Fe 3 Al. Specimens with different content of carbon showed the expected variation of the Snoek peak (at around 500 K) which changed its height drastically from the heating to the cooling run, and could be reproduced in its original height by a quenching treatment. This indicates some immobilization and remobilization of carbon probably in carbide precipitates. Quenching and subsequent annealing also causes a slight shift of the position of the Snoek peak while the Zener peak (at about 900 K) remains unchanged. The elastic modulus shows a pronounced Δ E -effect changing with thermal treatments.

On pseudoelasticity in single crystals of the intermetallic compound Fe3Al

Only few notes can be found in literature which report on pseudoelastic effects in Fe{sub 3}Al intermetallic compounds. Reversible deformation on {l_brace}110{r_brace} and {l_brace}112{r_brace} planes was observed in shear and compression experiments by Guedou and Rieu and explained by microtwinning, while Kubin et al. showed by in-situ TEM observations that the motion of the front partial of a superdislocation appeared to cause the slip reversal on {l_brace}110{r_brace} planes. The authors have recently observed such a similar effect during in-situ slip line formation studies of Fe{sub 3}Al crystals with a special crystal orientation deformed in compression at and above room temperature. In the following these observations are described and the possible reasons are discussed in connection with the data from literature.

Influence of stress in thin film modulus measurements by the vibrating reed technique

One method to measure thin film elastic properties is use of the vibrating reed technique to compare stiffness of uncoated and coated cantilever beams. Since the change of frequency is used to calculate the modulus, the change of frequency caused by curvature due to film stress has to be investigated. We present simultaneous measurement of resonant frequencies and curvature to show that curvature can cause large effects, depending on the geometry. An approximate calculation indicates in which cases stress may be neglected. A method to determine stress from combination of torsional and flexural vibrating frequency measurements is proposed.

Elastic and anelastic properties, internal stress and thermal expansion coefficient of cubic boron nitride films on silicon

Abstract The values of Youngs modulus, internal friction, internal stress and linear thermal expansion coefficient of c-BN thin films have been estimated from mechanical spectroscopy (vibrating-reed technique, 175 Hz–12 kHz). The thin films were deposited by reactive sputtering onto microstructurized silicon cantilever substrates. The measured Youngs modulus and thermal expansion coefficient are roughly in accordance with the few literature data on bulk material and with theoretical estimates. The internal stress of approximately 5 GPa is hardly affected by thermal treatments of the film up to 620°C. An interesting anelastic damping peak is detected at approximately 50°C (at 650 Hz) with an activation energy of 0.45 eV which is suggested to arise from the piezo-electric and semiconducting properties of the c-BN film.