The thermally activated deformation behaviour of single-crystalline microcast aluminium wires

A microcasting process is used to produce high aspect ratio ( > 30) monocrystalline pure aluminium wires with a diameter between 14 and 115 mu m. The role of thermal activation in the plastic deformation of these microwires is measured by means of (single) tensile stress relaxation tests. The microwires deform largely in an intermittent fashion, i.e., through repeated sudden displacement bursts, also during stress relaxation, implying that the bursts can be triggered through thermal activation. By separating the intermittent and the continuous parts of the stress relaxation load vs . time signal, we measure an activation area characteristic of the continuous relaxation mechanism. Haasen plots of the continuous relaxation data suggest the presence of a back-stress on the order of 2.5 MPa, which can be attributed to the thin layer of oxide covering the metal. Smaller (14-25 mu m) diameter crystals oriented for single slip show highly scattered activation area values and a steeper rate of increase of data in the Haasen plot than all other samples, which conform, save for the back-stress, with what is observed in bulk aluminium. Present findings are consistent with data from commensurate aluminium crystals in replicated micro cellular structures. Data of this work show that, in single slip within aluminium crystals, the coupling between the activation area and the flow stress is altered when crystals are below 30 mu m in diameter. The absence of a systematic difference in data between samples produced of 4N or 5N aluminium suggests that this conclusion is characteristic of the pure metal.(c) 2022 The Authors. Published by Elsevier Ltd on behalf of Acta Materialia Inc. This is an open access article under the CC BY license ( http://creativecommons.org/licenses/by/4.0/ )

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.

EPFL2013

High vacuum chemical vapor deposition (HV-CVD) of alumina thin films

Xavier Multone

We analyzed along this work the feasibility to produce high quality alumina thin films by High Vacuum Chemical Vapor Deposition (HV-CVD). We study the influence of various parameters on the growth process and on the film quality, such as substrate temperature, gas flow or pressure. We aim to highlight the advantages and the limitations of the HV-CVD technique. A high vacuum CVD reactor has been designed, built and optimized. A novel effusing source has been designed to guarantee a molecules distribution uniformity of 95% on the substrate surface. The main technical challenges faced were to reach the maximum substrate temperature in vacuum, to control the precursor flux and to control the temperature of the gas lines to avoid condensation. For the first time, films of Al2O3 are obtained by HV-CVD from aluminum isopropoxide as precursor in a novel HV-CVD reactor. The HV-CVD approach stands out of all the other CVD methods as it allows excellent prediction of growth rates and film thickness homogeneities if the decomposition probability of the precursor is once determined. In this work, the latter was determined to increase with increasing substrate temperature to 0.2, further increase of temperature reduces the decomposition probability due to increased desorption rate. Pure alumina films are obtained in a reliable and reproducible way. The presence of oxidizing agents, as decomposition partners, reduces the activation energy of deposition process from 33.1 ± 8.2 kJ/mol to 11.4 ± 5.3 kJ/mol in presence of sufficient oxidizing agent. The Al2O3 films are growing at deposition rate from 1 to 50 nm/min. It has been found that, the lower the deposition temperature, the higher the density of the deposited films therefore the higher refractive index. The index of refraction varies from 1.65 ± 0.01 to 1.35 ± 0.01 at 632 nm wavelength from lowest to highest deposition temperature. At low deposition temperature, Al2O3 can contain residual OH-groups and Al=O entities. This content is independent of the total flow rates of the precursor in the range of 5·1015 molecules/cm2·s to 3·1016 molecules/cm2·s impinging on the substrate. Substrate effects have not been observed in the limited range of tests on natural oxidized silicon, 3 µm SiO2 on silicon, quartz, stainless steel, fused silica and 100 nm silicon nitride on top of silicon. The chemical composition of the deposited alumina films measured by EDX and XPS is nearly stoichiometric with 35 ± 5 at% Al and 65 ± 5 at% O, without carbon contamination. Remaining hydrogen in the films has not been studied in detail, but the difference of OH- absorption peaks by FTIR indicates that, a low temperature deposition, hydrogen incorporation is possible. Concerning the optical properties of the HV-CVD deposited films, absorption is very low from 250 nm to 1800 nm wavelength, but due to the porosity and granular structure of the films, light scattering can take place. But propagation loss smaller than 2 dB/cm were measured in channel waveguides fabricated from the HV-CVD alumina films. Planar and channel waveguides demonstrate good guiding properties at 670 nm and 1.55 µm, as well. This work also opens new possibilities to deposit in situ local structures of or in transparent alumina on large surface. Light induced high vacuum chemical vapor deposition of alumina microstructures by 248 nm excimer laser is presented and shows an activation energy of 5.2 ± 0.4 kJ/mol, much smaller than for thermal deposition. The influence of the fluence and the repetition rate is discussed. Electron beam assisted HV-CVD of alumina is demonstrated and proves the feasibility of in situ structuration under HV-CVD conditions.

EPFL2009

High vacuum chemical vapor deposition (HV-CVD) of alumina thin films

Xavier Multone

EPFL2009

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

Guillaume Pasche

EPFL2013