Views
5 years ago

2 µm - eTheses Repository - University of Birmingham

2 µm - eTheses Repository - University of Birmingham

The reactive preforms

The reactive preforms TOPC20 and MOPC20 infiltrated with alloy IS showed higher θdyn than that found for AOPC20. In contrast, the TiO2 preform TOPC10 with identical chemical composition but finer porosity and grains, showed lower θdyn than TOPC20. Reaction products were only found in TOPC10IS as shown in Figure 4.60, which may explain this variance. In contrast to Garcia-Cordvilla et al. (103) who stated that the reaction layer would partially block the entrance and thereby increase P0 and θdyn, a lower θdyn was found in the present investigations when a reaction layer was formed. This behaviour has to be attributed to the aforementioned exothermic reaction and a resulting increase of the melt temperature, possibly to 1300°C (53) . Applying Equation 1 at this melt temperature, γlv is 0.650 N/m and therefore 26% lower than at 665°C. The lower surface tension of the melt close to the ceramic grain presumably acts as an lubricant, facilitating infiltration and leading to a lower θdyn than observed in TOPC20, where no reactions were found. Even though MOPC20 was rated as reactive, θdyn was significantly higher than in the AO- type preforms. In contrast to the TiO2 preforms, no evidence of reaction products was found in the infiltrated MMC microstructure (Figure 4.60 c) or post SDTA (Figure 4.63 b). Therefore the predicted spinel phase MgAl2O4 was not formed which has to be attributed to its kinetic inhibition (106,140) . This agrees with Jonas et al. (106) who reported that Mg additions had no significant influence for pressure infiltration of Al into Al2O3 preforms. However, others (15,17,140) have proposed that MgAl2O4 formation is the key factor required to facilitate infiltration of Mg-containing alloys in Al2O3. The present investigations concentrated on AlMg alloy (IM) infiltrations into the reactive preforms TOPC10, FATO and MOPC20 as here reactions were most probable. Similar to Jonas et al. (106) , the change of the main alloying element from Si (IS) to Mg (IM) showed only a marginal increase in θdyn of 3° to 5°. In contrast to infiltration with IS, the reaction products Al2O3 and TiAl3 were formed in the TiO2 219

preforms with IM, Figure 4.67. For TOPC10IS, the reaction started after the preform was infiltrated, indicated by the similar infiltration curve gradients for IS (Table 4.5) and IM (Table 4.6). The reduced gradient in FATO indicates that the reaction had already started during the recording of the infiltration curve, at about 3.8 mm infiltration depth, Figure 4.64. This behaviour was attributed to the different grain size of the reactive TiO2 phase, which was between 1 and 10 µm in TOPC10 and below 0.2 µm in FATO. Beyer (53) found spontaneous reaction during infiltration for fine (d50 < 0.5 µm) TiO2 whereas no reactions were found with coarser powders (d50 > 2 µm). Even though reactions in FATO occurred immediately after the melt entered the preform, there was no influence on P0 and, as a consequence, no influence on θdyn. The retardation of the reaction was a result of the relatively high activation energy for the aluminothermic reaction, which was reported as 474 kJ/mol TiO2 (53) . P P0 (MPa) 0 / MPa 8.0 7.2 6.4 5.6 4.8 4.0 3.2 2.4 1.6 0.8 0 MOPC20IM MOPC20IS TOPC20IS AOPC20IS 180 170 160 150 140 130 θ dyn (°) 120 dyn / ° 110 100 90 1 AGPC15IS AGPC15IS 2 3 220 4 5 6 7 SiHg (106 ·m2 /m3 SiHg /10 ) 6m²/m³ Figure 5.6 Threshold pressure P0 as a function of wetting angle θ dyn and the specific surface area per unit volume SiHg. AODY30IS TOPC10IM TOPC10IS TOPC10IS 8 FATOIM FATOIM FATOIS 9 10

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
MODULE 2 - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
eA - eTheses Repository - University of Birmingham
eTheses Repository - University of Birmingham
epinician precepts - eTheses Repository - University of Birmingham
Witch Shoes - eTheses Repository - University of Birmingham
pains of sin - eTheses Repository - University of Birmingham
5% - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
Cymbeline - eTheses Repository - University of Birmingham
1 - eTheses Repository - University of Birmingham
Caroline Winstanley - eTheses Repository - University of Birmingham
B - eTheses Repository - University of Birmingham
here - eTheses Repository - University of Birmingham
View - eTheses Repository - University of Birmingham
J - eTheses Repository - University of Birmingham
Rewriting history - eTheses Repository - University of Birmingham
Four Quartets - eTheses Repository - University of Birmingham
part i - eTheses Repository - University of Birmingham
PIERRE BOAISTUAU - eTheses Repository - University of Birmingham
The Sins of the Sons - eTheses Repository - University of Birmingham
soho depicted - eTheses Repository - University of Birmingham
Table of Contents - eTheses Repository - University of Birmingham
chapter 1 - eTheses Repository - University of Birmingham
THE GOD–MAN - eTheses Repository - University of Birmingham
Engineering texts - eTheses Repository - University of Birmingham