J.Th.M. De Hosson

Interactions between Lattice Dislocations and Grain Boundaries in Ni3Al Investigated by Means of In Situ TEM and Computer Modelling Experiments

Abstract The interaction between lattice dislocations and grain boundaries in Ni 3 Al has been investigated by means of in situ TEM deformation experiments. The interaction between screw dislocations and a coherent twin boundary could be analyzed in detail. The interaction mechanism found experimentally was compared to the results of a computer modelling study. In the computer modelling study, many-body potentials representing Ni 3 Al were used. The results of the in situ straining indicate that 〈110〉 screw dislocations impinging on a Σ = 3 coherent twin boundary that have a Burgers vector that is parallel to the grain boundary plane can be transmitted to the symmetric slip plane in the other grain under influence of an applied stress. A one-to-one comparison with the results of a computer modelling study of exactly the same system in Ni 3 Al can be made and the experiment agrees with the simulations. Also, observations were made of superlattice intrinsic stacking faults (SISF) that were formed as a result of the interaction between gliding dislocations and the dislocations of a low angle grain boundary (cell wall). The creation of jogs in the line of the gliding dislocation may be the cause of the SISF formation.

Metal-Ceramic Interfaces in Laser Coated Aluminium Alloys

A novel process was developed to firmly coat an aluminum alloy, Al6061, with [alpha]-Al[sub 2]O[sub 3] by means of laser processing. In this approach a mixture of SiO[sub 2] and Al powder was used to inject in the laser melted surface of aluminum. A reaction product [alpha]-Al[sub 2]O[sub 3] layer of a thickness of 100 [mu]m was created which was well bonded to the aluminum surface. Various interfaces, Al/[alpha]-Al[sub 2]O[sub 3], Al/mullite and [alpha]-Al[sub 2]O[sub 3]/mullite, were studied by conventional transmission electron microscopy (CTEM) and high resolution electron microscope (HREM). It turns out that the presence of the Al/mullite interface may be essential to form a well bonded oxide layer and the high Si-content [alpha]-Al[sub 2]O[sub 3] intermediate layer may be wetted better by liquid Al. Investigations of the interface structures and wetting phenomena during laser processing are presented and a simple correlation between wetting phenomena and interface strength is derived.

A reaction coating on aluminium alloys by laser processing

An aluminium oxide layer of 100 µm in thickness has been successfully coated on aluminium alloy 6061 and pure aluminium using a powder mixture of silicon oxide and aluminium by laser processing. A strong Al/Al2O3 interface was formed. The exothermic chemical reaction between SiO2 and Al may promote the metal/oxide wetting and the formation of Al2O3 layer. This new approach of ceramic coating on metals using a chemical reaction of other ceramics with metals may be applied to other systems.

Residual stresses in the surface layer of laser-treated steels

Abstract Although laser treatment of certain metals may enhance the wear performance in general it may result equally well in large residual stresses which affect the wear performance detrimentally. Tensile stresses generated in the surface layer may lead to severe cracking of the material. This paper describes surface stress situations found after laser treatments in samples of different materials (C22, CK60 and pure Fe) with single laser tracks and in samples with multiple overlapping laser tracks. The principal results of the X-ray measurements are the large stress variations inside and outside the laser tracks. The laser treatment gives rise to both tensile and compressive residual stresses. The residual stresses in and around laser tracks have their origin in plastic deformation due to thermal expansion and in volume changes due to phase transformations. The plastic deformation is dictated by the strength of the material and the temperature function forced upon the material by the laser treatment. The residual stress state in multiple laser tracks is of tensile character. Each new laid laser track exerts a tensile force upon the previous laser tracks by which the compressive stresses disappear. Some annealing and/or tempering occurs, leading to a decrease in the absolute value of the stresses.

Microstructure and Mechanical Properties of a Laser Treated Al Alloy

Abstract An Alue5f8Cuue5f8Mg alloy, Al 2024-T3, was exposed to laser treatments at various scan velocities. In this paper the microstructural features and mechanical properties are reported. As far as the mechanical property is concerned a striking observation is a minimum in the hardness value at a laser scan velocity of 1 2 cm/s . Usually an increasing hardness with increasing laser scan velocities is reported in the literature. This remarkable property could be explained based on the microstructural features observed by transmission electron microscopy. It turned out that depending on the laser scan velocity nucleation and growth of precipitates could either be enhanced or suppressed. In addition at low scan velocities helical dislocations are observed which may contribute to an increase of the hardness. After subsequent shot peening, in all cases the formation of precipitates was observed, independent of the laser scan velocities originally applied. This phenomenon of precipitation, induced by shot peening afterwards is most striking at a high concentration of alloying elements in solid solution.

Microstructure of Laser Treated Al Alloys

In this study the microstructures of laser treated ultra pure Al and two Alue5f8Si alloys (Alue5f80.4 Si and Alue5f80.75 Si) were investigated. In ultra pure Al a large number of dislocation loops were found especially at higher laser scan velocities. During annealing only at laser scan velocities above 2 cm/s a large quantity of dislocation loops became visible. Both results indicate that at high laser velocities vacancies are frozen in, but at laser velocities around 1 cm/s there is still enough time at high temperature to reduce the vacancy concentration towards lower super-saturation. In Alue5f8Si alloys the dislocation density rises with higher laser scan velocities probably caused by the smaller distances between the eutectic cell walls. In these alloys entangled dislocation structures were found in contrast to ultra pure Al. For solidification structures consisting of an eutectic material with a high hardness as for an eutectic structure in Alue5f8Si alloys it was found that the hardness can be described by a pile up mechanism, which depends on the difficulty to exert stresses on neighbouring cells due to thick and hard walls. The hardness has been described by a 1/d2 dependence, i.e. it is mainly determined by the small size d, of the solidification structure.

Development of residual stress and surface cracks in laser treated low carbon steel

Scripta ME?~LLJR3iCA Vol. 25, pp. 779-784, 199! Pergamon Press plc et M.~,e~AL[A Printed in the U.S.A. All rights reserved DEVELOPMENT OF RESIDUAL STRESS AND SURFACE CRACKS IN LASER TREATED LOW CARBON STEEL B.A. Van Bmssel, H.J. Hegge and J.Th.M. De Hosson Department of Applied Physics, Materials Science Centre, University of Groningen, Nijenborgh 18, 9747 AG Groningen, The Netherlands R. Delhez, Th.H. de Keijser, N.M. Van der Pers Laboratory of Metallurgy, Rotterdamseweg 137, 2628 AL Delft, The Netherlands (Received January 17, 1991) Introduction Laser surface melting is a powerful technique to produce wear resistant layers. It combines the advantages of local hardening, alloying and high quench rates. The latter may result in new metastable phases with novel tribological properties. Despite these advantages laser melting may produce residual stresses in the surface layer which may affect the wear performance and the sensitivity to fatigue and fracture. As is known from conventional welding technologies [ 11, local heating may cause detrimental stresses. During a laser treatment a similar situation is created and consequently residual stresses are being detected [2,3,41. In former studies however average stress intensities are measured over a laser track. Here results of measurements will be reported which present a better insight in stress variations within and in the neighbourhood of a laser pass. Another aspect of a laser treatment is the occurrence of some cracks in the surface. In principle cracks are preferential sites for fatigue or corrosion fracture [5,6,7]. Here the main interest concentrates on the residual stresses produces by a continuous COz laser treatment and effects on fracture in a low carbon steel CK 22. With respect to cooling rate and heat input a continuous laser can be placed in between a short pulse laser and electric arc welding. Applying a short pulse laser it is possible to produce a shock wave. By rapid heating plastic deformation occurs and even evaporation of a thin surface layer [8,91. Although the interaction time of a continuous laser beam is larger, deformation effects may be expected as is confirmed by the residual stresses found. In welding heat is maintained during a time lapse which is long enough to anneal a large amount of the defects. In contrast to this, the thermal cycle of a laser treatment is much shorter, which may prevent annealing effects. Experiment

Surface Modification by Means of Laser Melting Combined with Shot Peening: A Novel Approach

Abstract Among the available laser applications laser surface melting has turned out to be a powerful technique for the production of wear-resistant layers. Despite the advantages of this process, laser surface melting results in tensile stresses which may assist crack propagation. In this paper it will be shown that shot peening can overcome this drawback effectively. It turned out that a preceding laser treatment of an eutectic aluminium-silicon alloy is able to amplify considerably the effectiveness of the shot peening treatment. In particular the maximum attainable hardness and compressive stress increase upon increasing the quench rate, i.e. upon increasing the laser scan velocity. The high concentration of silicon in solid solution turned out to be the main reason for the enhanced mechanical performance, not only directly through solid solution hardening, but also by precipitation hardening and by a higher dislocation density. The latter contribution is affected indirectly by a changed cross slip behavior.

SPINEL METAL INTERFACES IN LASER COATED STEELS - A TRANSMISSION ELECTRON-MICROSCOPY STUDY

This paper reports on coating a Duplex steel SAF 2205 and stainless steel 304 by bringing a mixture of Cr2O3 and Fe powder into a laser beam. Transmission electron microscopy reveals that in the case of proper bonding between substrate and coating a spinel structure around the composition FeCr2O4 could always be found near the interface. The Duplex steel transforms into a b.c.c. structure, whereas SS304 maintains its f.c.c. structure after laser treatment. Particles with a spinel structure have been observed in both the b.c.c. and the f.c.c. substrate. Crystallographic orientation relationships have been identified for the interfaces of the spinel structure both with the b.c.c. and f.c.c. matrix using electron diffraction. Planar faults have been observed in the spinel particles. No cracks have been found at the spinel/b.c.c. interface, whereas some cracks were detected at the spinel/f.c.c. interface in case there did not exist a crystallographic orientation relationship.

Ne implantation induced transformation in stainless steel

Abstract This paper reports a microstructural investigation of the changes induced by Ne-implantation in stainless steel of the austenitic type. At a critical dose of 2.3·1017/cm2 a martensitic phase transformation was observed. In particular, attention has been paid to the effect of the stress field of neon bubbles on the nucleation of martensite. It is found that the critical size of the nucleus as well as the activation energy is drastically lowered. No definite orientation relationship between martensite and austenite has been detected. This is probably caused by the small size of martensite particles and the deformation in the vicinity of the bubbles.