Jacques Charlier

The structure and mechanical properties of thick cobalt electrodeposits

Cobalt electrodeposits have been produced in chloride solutions. Depending on the electrolysis parameters applied, two different structures for the electrodeposits, namely α and β cobalt were observed. Their characterization included hydrogen content measurement, the relative volume fraction of the α and β phases determined by X-ray diffraction, X-ray diffraction line profile analysis and microstructural investigation by optical microscopy, scanning electron microscopy and transmission electron microscopy. The influence of structure on mechanical properties was examined. The results showed that the ductility properties of the cobalt electrodeposits were highly sensitive to the structure. A higher β phase content that was measured in some deposits did not improve the ductility due to the existence of trapped hydrogen which always exists in such deposits. However, annealing treatments seem to be a promising route to optimize the ductility of cobalt electrodeposits.

Effect of grain size and intergranular stresses on the cyclic behaviour of a ferritic steel

Abstract The effect of the grain size and intergranular stresses on the cyclic stress-strain behaviour of a low carbon steel has been investigated at different strain amplitudes. Annealed and prestrained specimens were submitted to fatigue tests. Optical and scanning electron microscopes were used to analyse the grain size and grain boundary effects on the cyclic behaviour and crack nucleation process. An increase in the grain size reduces the strain amplitude for cyclic hardening. However, a reduction in the grain size increases the resistance to macroyielding, decreases the grain boundary affected zone and changes the crack nucleation mechanism from intergranular to transgranular. An original cyclic model, based on geometrically necessary dislocations theory, is proposed to explain the cyclic stress-strain behaviour observed. The improvement of fatigue life, resulting from grain size reduction and previous cold work, is attributed to a more uniform stress distribution within the grains and to the change in the crack nucleation mechanism.

Effects of heat treatments on the ductility of cobalt electrodeposits

The effects of annealing treatments on the structure and mechanical properties of cobalt electrodeposits have been studied. Annealing temperatures range from 250–800 °C, i.e. below, as well as above, the allotropic α-h c p ⇋ β-f c c transformation temperature (417 °C). The structural characterization included hydrogen content measurement, relative volume fraction of α and β phases determination by X-ray diffraction, and microstructural investigations by optical and electron microscopy. The results showed that an annealing is a very effective means to optimize the ductility of cobalt electrodeposits. The increase of ductility observed after annealing is essentially due to a decrease of the stacking fault density in the deposits. Annealing treatment above the allotropic transformation temperature also produces, in some deposits, an increase of the ductile β-f c c phase content, but this fact does not lead to supplementary improvements of ductility. This is due to the detrimental influence of hydrogen that always exists in these deposits.

Room temperature recrystallization of nanocrystalline thin copper foils

Thin copper foils (100 micrometers thick) are produced by electrolysis on anodized titanium substrates. A sharp distribution of grain diameter is observed around 200 nanometers. The X-ray diffraction pattern shows a slight preferential orientation of the crystals with the (111) planes parallel to the substrate. This X-ray diffraction pattern evolves at room temperature. After 60 days, a preferential orientation of the (200) planes parallel to the substrate is observed. This effect is associated with the recrystallisation of the foil with growth of large copper grains (diameter higher than 5 micrometers) as observed by high resolution transmission electron microscopy. The structural evolution of the copper foils is studied by electron microscopy, X-ray and electron diffractions at different temperatures. The mechanical properties of the foils are also studied as a function of time after electrodeposition.

Influence of heat treatments on the cohesion and adhesion strength of plasma-sprayed deposits

Abstract The aim of our work is to study the influence of subsequent heat treatments on plasma-sprayed deposits. The following coatings have been investigated: Ni−20wt.% Cr, Cr 2 O 3 , (Ni−20wt.%Cr)+(ZrO 2 −8wt.%Y 2 O 3 ). Two steels were chosen as substrate: a chromium ferritic steel and an austenitic steel. Tensile tests were performed to evaluate the cohesion of the coating or the adhesion strength to the substrate. Optical and scanning electron microscopy, and diffusion measurements in the boundary layer by X-ray microanalysis were used to explain the mechanical results. Heat treatments at 500°C do not modify the bond strength of the deposits; on the contrary, heat treatments at a high temperature (800 or 900°C) improve markedly the adhesion strengths of the NiCr and Cr 2 O 3 deposits. Moreover this treatment produces a densification of the Cr 2 O 3 deposit; for (NiCr) +(ZrO 2 −Y 2 O 3 ) the improvement is small.

Influence of the structure on the mechanical properties of electrodeposited metallic thin films

Production of thin (10 to 200 microns thick) metallic (Cu, CO and Ni-P) foils is performed by electrodeposition on various substrates. A competition between substrate-induced and electroplating-induced inhibition of crystal growth appears. Films structures observed by SEM, TEM and X-Ray diffraction are related to the mechanical properties of the films (stress-strain curves, microhardness and working hardening bend test). In some cases, copper thin films with a large number of submicron crystals are obtained. These films recrystallize at room temperature and their mechanical properties are completely modified by this ageing process.

Shot peening treatment characterization by surface acoustic waves

Shot peening is a classical mechanical surface treatment used to improve the properties of components, principally the fatigue and stress corrosion resistance. The possibility of characterization of shot peening treatment efficacy was assessed by leaky Rayleigh SAW generated and detected by a large aperture, broadband and lens-less focused transducer. The broadband pulses propagated in the material surface were processed in order to obtain the dispersion curves, and were compared for different samples that were submitted to different treatment conditions. The results showed that the dispersion curve is sensitive to these treatment parameters. The possibility of depth profiling of the elastic properties changes due the shot peening are discussed on base of the fact that the penetration of SAW depends on the frequency and so the dispersion curve should have information of the elastic properties in different depths. The source of these elastic changes is also analyzed in terms of plastic deformation, residual stresses, roughness and microstructural changes. A comparison of the dispersion curves with a laser based SAW system was also performed showing equivalence of the methods.

Measuring cast iron graphite size by ultrasonic attenuation

Precipitated graphite geometry is one of the main microstructural parameters that contribute to cast iron mechanical properties. In this paper we address the measurement of the graphite size by ultrasonic attenuation. Samples of different cast iron microstructures were characterized by quantitative metallographic techniques and further correlated with ultrasonic parameters. Longitudinal ultrasonic waves were generated and detected by commercial broadband piezoelectric transducers on direct contact and the signals were processed in order remove system effects. For surface acoustic waves, a large aperture lens-less focalized piezoelectric transducer was used. The experimental attenuation spectra were fitted with simple scattering models and both longitudinal and surface wave techniques showed very good quantitative correlations with graphite size, independent of its geometry.

Methods of beam divergence corrections on broadband direct contact ultrasonic technique

The separation of intrinsic attenuation and velocity parameters from extrinsic factors is a complex task in nondestructive ultrasonic characterization of materials. The effect of the ultrasonic beam divergence due to finite source is discussed both theoretically and experimentally in this work. The objective is to evaluate different techniques of attenuation and velocity measurements with beam divergence corrections on direct contact configuration. Both amplitude and phase spectra for signals generated and detected by commercial NDE broadband transducers are to be corrected. The effectiveness of each technique is evaluated with different transducers and sample geometry. The results show that a smaller effective transducer diameter than the nominal must be assumed in theoretical corrections [P. H. Rogers and A. L. Van Buren, J. Acoust. Soc. Am. 55, 724–728 (1974)] and that the experimental determination of calibration correction for a particular transducer by a reference sample can be a good alternative.

Depth profiling of residual stress in steel by laser ultrasonics

Depth profiling of residual stress in steel is of great interest in many practical applications. Conventional techniques are either destructive (x‐ray diffraction and hole drilling) or with obvious restriction (Barkhausen noise). This paper presents a noncontact and nondestructive laser ultrasonics method for determining the depth profile of the residual stress layer in steel samples. It is known that surface acoustic waves penetrate into solids to a depth proportional to the wavelength. They are expected to be dispersive in the presence of gradients in physical properties such as residual stress. Based on the dispersion theory, the depth dependence of an effective velocity v(z), which is related to the stress profile S(z), can be fitted from the frequency dependence of the phase velocity v(w). Due to the advantage of wide bandwidth, laser‐generated and ‐detected surface acoustic waves (SAW) offer a promising nondestructive stress depth profiling method. The profiles obtained by SAW are compared with the stress data obtained by other techniques. a)Also at Service de Metallurgie Physique, Universite Libre de Bruxelles, Belgique.