2 µm - eTheses Repository - University of Birmingham
material, similar to that resulting from the thermal analysis test, KIC was as low as 3 MPa·m 1/2 (163) . This was a result of the brittle character of the intermetallic TiAl3 (53,163) in combination with the presence of 11% porosity (Figure 4.63 a) resulting from the volumetric change due to the reaction products which were predicted by the thermodynamic calculations (Figure 4.3). The KIC values of the MMCs based on the self-fabricated preforms with a bimodal pore structure were higher than the MMCs made using the two purchased preforms: FATO, a hybrid preform with Al2O3 fibres (Saffil) and fine TiO2 particles, and AODY30 in which the predominant porosity was formed by bubbles in the ceramic slurry. The processing of the latter resulted in relatively large spherical porosity as shown in Figure 4.23 in the as- purchased condition (AODY30) and in the MMC (AODY30IS) in Figure 4.58. AODY30IS showed the lowest KIC and strength of all MMCs investigated, but gave superior tribological properties which were attributed to the differences in microstructure. Three main influencing factors were identified for tailoring the mechanical and tribological properties of MMCs at constant reinforcement volume fractions: a) Porosity in the MMC microstructure. b) Interface between the metal phase and the reinforcement c) Small interparticle spacings filled with the metal phase The porosity in the MMC has to be subdivided into pores generated by the melt solidification, which were discussed together with the interfacial aspects in this section, and those resulting from the saturation during infiltration which is discussed in section 5.3. The interparticle spacing is built into the preform during fabrication and is discussed in the following section. 201
5.2. Preform pore formation The target preform porosity range in the present work was between 60 and 70%. Neglecting compression, the size of the porosity measured in the ceramic preforms represents the size of the metal phase after infiltration. It has been shown that the reinforcing effect is superior in terms of mechanical properties for a fine distribution of metal ligaments compared to extensive ceramic and metal ligaments e.g. the MMC of the foamed preform AODY30IS. 5.2.1 Foamed preforms A window diameter of 10 µm and a cell diameter of 120 µm were measured in the microstructure of AODY30 (Figure 4.23). The window size, representing the bottleneck pathway between the single spherical cavities, was confirmed by the mercury intrusion and extrusion results: the main intrusion peak was at 9 µm, and 97% of the mercury was not extruded after pressure release to ambient pressure, which corresponded to a pore diameter of 11 µm, shown in Figure 4.32 and Table 4.2 respectively. The cell sizes assessed on the cross sections of the MMCs shown in Figure 4.58 ranged between 10 µm and more than 500 µm. The range of values resulted from the position of the section: as the bubbles in the ceramics were randomly distributed, the probability of cutting along the diameter of a single sphere was low. Nevertheless, there were larger spheres than the aforementioned 120 µm which indicates inhomogeneities in the preform blowing process. Peng et al. (125) claimed a uniform cell diameter distribution of about 150 µm, but bubbles of up to 350 µm in diameter were visible in their microstructures. This confirms that deviations from the mean cell size of more than 100% are characteristic for this pore forming method. In Equation 35 the relation between pore fraction Vpo and the ratio of window to cell size (k) of foamed preforms is given. According to this model at a k-value of zero, which means a window diameter of zero and therefore a closed cell foam, a Vpo of 0.74 results. Even when 202
Pressure Infiltration Behaviour and
ABSTRACT In the pressure infiltrati
CONTENTS 1. INTRODUCTION 1 2. LITER
4.8.3 Evaluation of infiltration be
Symbol Meaning γRv surface energy
Symbol Meaning TYS tensile yield st
these materials are the detrimental
2. LITERATURE REVIEW 2.1. Materials
changes in the oxide film chemistry
or inside the bulk fluid only. Inte
that are most effective in decreasi
initiation stress of 25 %. Further,
Beffort (36) suggested that even th
einforcement interface and reinforc
It is interesting to note that, for
20 Table 2.1 Compilation of the mec
General models to predict fracture
with values observed by others for
The work of adhesion characterises
and vapour, is difficult to evaluat
system Al-Al2O3 is 10 -49 Pa at 700
In the Al-Cu system, although the p
The heat of reaction ΔGr may be es
al. (100) who found non-wetting beh
capillary or threshold pressure has
using constant gas pressure. Infilt
The superficial velocity v0 in the
The permeability K can be expressed
2.4. Preform fabrication Composites
According to Kniewallner (51) even
2.4.3. Foamed preforms Another inte
structure. This is shown schematica
2.5.1. Gas pressure infiltration (G
MMCs infiltrated with an Al-9Mg or
layer oxide films. The Weber number
Long et al. (50) suggested that v0
3. EXPERIMENTAL PROCEDURE The influ
sintered at 1550°C, which represen
using a AVT-Horn (Aalen, Germany) m
squares fit function within the MAP
areas, SsBET ,of the powders were m
with dimensions of 65 mm x 46 mm x
The preform sintering process was o
in the evaporation of mercury at lo
The compressive strength, σc , of
as the measured mean value 0.23. Th
for 90 s to ensure complete solidif
ottom punch surface. The temperatur
A graphic presentation of the relat
detected. This operation took appro
modulus Edyn of the unreinforced al
calculated using the methods outlin
Positive volume changes were predic
Figure 4.5 Droplet formation of the
with the metal alloy IM: examples a
As shown in Figure 4.9, apart from
4.3.2 Powder specific surface area
The particles of TO and MO were dis
oom temperature and 270°C, with a
obtain usable products when they we
strengths, whereas with 10 and 20 w
strength showed no significant diff
Relative change in dimension s x, s
(a) AOPC20 (b) AGPC15 2 µm (c) TOP
At higher magnification, Figure 4.2
intrusions started at 4 µm and end
As shown in Figure 4.27, the pore s
An overview of the specific values
1.71 to 1.98·10 6 m²/m³. The sim
logarithmic compression behaviour,
The volumetric stiffness Eiso of th
Figure 4.37 shows that the TOPC20 p
unhindered through the gap between
intrusions and the other areas were
4.8.1 Unreinforced matrix propertie
die, Tmelt,die , could not be recor
pressure was recorded as a function
the linear fits for AOPC20, TOPC20
4.8.6 Non destructive testing of MM
X-Y Y-Z Figure 4.51 Virtual cross-s