Growth kinetics of Al-Fe intermetallic compounds during annealing treatment of friction stir lap welds

In this study, we explored the growth kinetics of the Al-Fe intermetallic (IM) layer at the joint interface of the St-12/Al-5083 friction stir lap welds during post-weld annealing treatment at 350, 400 and 450 degrees C for 30 to 180 mm. Optical microscope (OM), field emission gun scanning electron microscope (FEG-SEM) and transmission electron microscope (TEM) were employed to investigate the structure of the weld zone. The thickness and composition of the IM layers were evaluated using image analysis system and electron back-scatter diffraction (EBSD), respectively. Moreover, kernel average misorientation (KAM) analysis was performed to evaluate the level of stored energy in the as-welded state. The results showed that the growth kinetics of the IM layer was not governed by a parabolic diffusion law. Presence of the IM compounds as well as high stored energy near the joint interface of the as-welded sample was recognized to be the origin of the observed deviation from the parabolic diffusion law. (c) 2014 Elsevier Inc. All rights reserved.

Reverse precipitation synthesis and characterization of CeO2 nanopowder

Mehdi Mazaheri

In the present work, CeO2 nanopowder was synthesized via a reverse precipitation method using CeCl3 center dot 7H(2)O, NH4OH and sodium dodecyl sulfate as raw materials. The effect of thermal treatment on the crystal growth, surface area and chemical bonds of the powder was discussed. The structural evolutions and morphological characteristics of the nanopowder were investigated using X-ray diffractometery, transmission electron microscopy, scanning electron microscopy, differential thermal analysis and Fourier transform infrared spectroscopy. The results showed that CeO2 with an average particle size of 45 nm is formed. The BET surface area increased to approximate to 41 m(2)/g, with increasing calcination to 300 degrees C; then, it decreased following calcination at higher temperatures. The particles sintered and agglomerateed together after 500 degrees C. The activation energy for CeO2 nanocrystallite growth during calcination was calculated to be about 14.6 kJ/mol. (C) 2009 Elsevier B.V. All rights reserved.

2010

Texture change through film thickness and off-axis accommodation of (002) planes

Alireza Karimi, Akshath Raghu Shetty

We present our recent experimental results on the formation of off-axis texture and crystallographic tilting of crystallites that take place in thin film of transition metal nitrides. For this purpose, the microstructural development of TiAlN film was studied, specially the change in texture with film thickness. Fiber texture was measured using theta-2 theta and pole figure X-ray diffraction (XRD), while scanning electron microscopy (SEM) and transmission electron microscopy (TEM) were used to observe the microstructure and changes in texture with thickness. The sin(2)psi method was applied to determine the stresses on (1 1 1) and (0 0 2) plane. With deposition parameters chosen, the growth texture mechanism is discussed in three different stages of film growth. Surface energy minimization at low thickness leads to the development of (0 0 2) orientation. On the other hand, the competitive growth promotes the growth of (1 1 1) planes parallel to film surface at higher thickness. However, contrary to the prediction of growth models, the (0 0 2) grains are not completely overlapped by (1 1 1) grains at higher thickness. Rather the (0 0 2) grains still constitute the surface, but are tilted away from the substrate normal showing substantial in-plane alignment to allow the (1 1 1) planes remain parallel to film surface. Intrinsic stress along (1 1 1) and (0 0 2) shows a strong dependence with preferred orientation. The stress level in (0 0 2) grains which was compressive at low thickness changes to tensile at higher thickness. This change in the nature of stress allows the (0 0 2) planes to tilt away in order to promote the growth of (111) parallel to film normal and to minimize the overall energy of system due to high compressive stress stored in the (1 1 1) grains. The change in surface morphology with thickness was observed using SEM. An increase in surface roughness with film thickness was observed which indicates the development of (1 1 1) texture parallel to film surface. TEM observations support the XRD results regarding texture change. Film hardness was measured by nanoindentation and a correlation between (1 1 1) texture, stress and hardness is obtained. The results indicate that texture development is a complex interplay between thermodynamic and kinetic forces. An attempt is made to understand this phenomenon of off-axis accommodation of (0 0 2) at higher thicknesses, which is a new result not reported previously. (C) 2011 Elsevier B.V. All rights reserved.

2011

Interaction between liquid aluminium and solid iron Al-rich intermetallics formation

Guillaume Pasche

Intermetallic phases formation is one of the phenomena, which when coupled to thermal and mechanical strains leads to the premature wear of aluminium die casting tools made out of steel. This work focuses on two aspects of this wear problem. The first and major part investigates a simplified chemical system Fe(s)–Al(l) at 700 °C in order to study the formation of the intermetallic compounds. The second part focuses on the limitations of the tool wear by testing different coatings on the steel currently used for die casting. Experiments are performed on two sample types, differentiated by their interface orientation between the two initial phases (vertical and horizontal). Observation are also of two types. Post mortem observations are performed by light microscopy and both scanning and transmission electron microscopy. In situ observation are performed by X-ray tomography, that allows to follow the evolution of the intermetallic layer with time. In the Fe(s)–Al(l) system, one main phase, namely Fe2Al5 , forms with a particular morphology called tongue-like feature in the iron matrix. Tongue tips are generally single crystals. It is also observed that these exhibit a periodical contrast at the nanometre scale. This contrast is found to be due to a slight chemical variation within the existence domain of Fe2Al5. This splitting is linked to the spinodal decomposition of the phase observed at higher temperature. At the interface between tongues and the iron matrix, a layer of about hundred nanometres that surrounds the tongues is observed. It corresponds to the Æ2 FeAl phase. Iron matrix, due to the tongue growth, suffers a plastic deformation that is observed by recrystallisation of the direct tongue surrounding. The induced strain field also leads to porosity formation at the interface between the tongues and the matrix. Pores are then trapped in the Fe2Al5 phase and partially disappear near from the liquid interface. At this interface, a layer of the order of 10μm of Fe4Al13 is observed. The thickness of this layer is stable in the explored time range of 0-4 h. However, intermetallic blocks that detach from the layer are observed in the liquid. This is interpreted as mainly due to the dissolution of the formed layer in the bath to achieve its saturation in Fe. Intermetallic thicknesses and tongue density measurements as functions of time allow to highlight the presence of three main growth regimes. The first at the beginning of the contact corresponds to rapid emergence of the tongues in the matrix. The second is linked to their progressive slow down while they start to thicken. Both thickening and tip growth are in competition from the beginning of this regime. A third regime is however defined and corresponds to a more stable growth during which thickening and tip growth still act in parallel is but their effect is less visible as they vary with the square root of time. The tongue growth observed at the beginning of the reaction is explained by a growth front destabilisation, similarly to the destabilisation of a planar front that leads to dendritic solidification described by Mullins and Sekerka. A calculation based on their model is proposed and confirms the effect of the law aluminium diffusion in iron but is limited by assumptions made for its development. Technological investigation proceeds by modification of the studied system and comparison with the fundamental one. Two ferrous materials and one aluminium alloy are studied. In addition, different coatings deposited by electroless plating on one of the ferrous substrates are tested. A quality coefficient is proposed in order to compare the result of the contact between coating and liquid aluminium. One Co-based coating is revealed as the most promising material that allows to limit the wear of the ferrous part by liquid aluminium.