Views
5 years ago

2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

composites comprising a

composites comprising a Ti3Al matrix with embedded Al2O3 and Ti suboxides of the Ti2O3 and TiO type. In the present study, the resulting CIC composite was reproduced and this was confirmed by differential thermal analysis on TOPC10IS composites (section 4.8.9). In order to alter the reactivity of the ceramics in contact with molten Al, other oxide ceramics were considered. The Al-MgO system exhibited low ΔG values, resulting from the reaction of Al and MgO to the ternary spinel phase MgAl2O4 (Figure 4.2). 5.4.2. Static wetting The sessile drop test was employed to investigate the influence of the substrate chemistry on the static wetting behaviour of the Al-alloys (IM and IS) in contact with Al2O3 (AF), TiO2 (TF) and MgO (MO). Fine grained powders and relatively high sintering temperatures were used to produce substrates of maximum density. Residual porosity ranged between 1.9 and 4.9% (Table 4.1). A vacuum furnace was used for the sessile drop test in which a minimum pressure of 10 -3 Pa could be achieved. The oxygen partial pressure pO2,calc in the sessile drop equipment was estimated using FactSage thermodynamic software and input variables as close as possible to the experimental set-up. Here residual oxygen was gettered using 100 g of Ti-sponge placed directly on the heating coils whose energy input was controlled to give a substrate temperature of 750°C. The furnace volume was 9 dm³ and was assumed to be completely filled with the Ar flushing gas containing 1 volume part per million of oxygen. Calculations were performed first taking into account Ti and the gas, second with the additional 2 g of Al representing the alloy droplet and third with an additional 0.2 g of Mg, the alloying element in IM. As the alloy had no contact with the Ti, the TixAly and TixMgy phases were suppressed. The calculated pO2,calc was 4.5·10 -41 Pa for the first combination and 8.5·10 -42 Pa for the second and third variations. The latter similarity indicates that the Mg addition had no 213

influence on the pO2,calc. The lower value reported for Al indicates that Al oxidation takes place prior to that of Ti. This may be the reason for the oxidation of a liquid alloy IS in sessile drop tests prior to droplet formation. As proposed (83) , oxidation may only be prevented with metals such as Zr. The calculation also indicated that alumina is more stable than magnesia, which further supports the reactivity of the Al-MgO system. Zhen et al. (76) reported the maximum oxygen partial pressure to prevent oxidation of Al melts at 700°C to be 10 -44 Pa. This is lower than the calculated value of 8.5x10 -42 Pa for the system with Al. However, as oxide stability decreases at higher temperatures the value is plausible. In contrast to the alloy IS (Al-12 wt.% Si), a droplet with a bright surface was formed (Figure 4.4) with the alloy IM (Al- 9 wt.% Mg) which was attributed to the different alloying element. Anson et al. (19) reported a shell of Mg vapour surrounding liquid Al-Mg-Si alloys in high vacuum experiments, which protected the melt from oxidation of Al. In the present experiments it has to be assumed that oxidation of the metal droplet was prevented by evaporation of Mg which led to a shielding atmosphere around the droplet and enabled the formation of a droplet. All systems were identified as non-wetting with no change in contact angle during the holding period of 30 min. Sobczak et al. (80) found no decrease in wetting angle in the pure Al-Al2O3 system within this period. They found wetting angles of 120° to 123° which were significantly smaller than those of 148° and 155° found for IM-Al2O3 in the present research. The difference in wetting angle has to be attributed to the difference in the alloy chemistry and pinning of the droplet´s triple line on the substrate by intrusions into the substrate´s microporosity in combination with gravity-driven segregation of an iron rich phase. Figure 4.8 a) shows a micrograph of both phenomena. 214

PDF File 2 - University of Alabama at Birmingham
no· l'2-'1t1 - Repository - Texas A&M University
eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
MODULE 2 - eTheses Repository - University of Birmingham
eA - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
pains of sin - eTheses Repository - University of Birmingham
Witch Shoes - eTheses Repository - University of Birmingham
epinician precepts - eTheses Repository - University of Birmingham
1 - eTheses Repository - University of Birmingham
5% - eTheses Repository - University of Birmingham
Cymbeline - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
Caroline Winstanley - eTheses Repository - University of Birmingham
View - eTheses Repository - University of Birmingham
here - eTheses Repository - University of Birmingham
J - eTheses Repository - University of Birmingham
Engineering texts - eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
THE GOD–MAN - eTheses Repository - University of Birmingham
ford madox brown - eTheses Repository - University of Birmingham
Life on Wings - eTheses Repository - University of Birmingham
Table of Contents - eTheses Repository - University of Birmingham
Four Quartets - eTheses Repository - University of Birmingham
part i - eTheses Repository - University of Birmingham
Rewriting history - eTheses Repository - University of Birmingham
soho depicted - eTheses Repository - University of Birmingham