2 µm - eTheses Repository - University of Birmingham
with T, the actual temperature and Tm the melting point and γlv in N/m. The effect of oxygen on the γlv of liquid aluminium was evaluated by Gourmi and Joud (10) who found that the reduction of the surface tension was proportional to the oxide coverage on the surface. A mean value of 1.050 N/m was found for the surface tension of atomically clean aluminium at 700°C. In contrast, a mean value of 0.871 N/m corresponded to saturation coverage by a homogenous layer (19,20) . The higher values resulted from chemically pure aluminium and the lower values are determined using contaminated conditions which led to oxygen coverage as shown in Figure 2.3. It could be shown that the surface coverage of one monolayer led to a reduction in surface tension of more than 11%. Surface Surface tension tension / N/m(N/m) 1.10 1.05 1.00 0.95 0.90 0.85 0.0 0.5 1.0 1.5 X (Monolayer units) X / monolayer units Figure 2.3 Variation of surface tension of molten Al with level of oxide coverage (20) . Rocher et al. (21) achieved metallic surfaces on Al melts even at relatively high oxygen partial pressures. The thin Al2O3 layer on the liquid melt was removed by additions of K2ZrF6, a well known fluxing agent used in Al melts. The predominant effect to improve cleanliness of the melt is the dissolution of the thin alumina layer coating the liquid metal by the fluorides. The surface tension of a pure liquid melt is significantly affected by binary additions (18) . In general the γlv of Al is observed to decrease with increasing levels of Si, Mg and Sr. Solutes 9
that are most effective in decreasing the surface tension of the solvent typically exhibit surface tensions lower than the solvent (22) . Si, Mg and Sr exhibit lower surface energies as compared to Al and hence also decrease the surface tension (23) . The linear rule of mixture provides a first approximation for the surface tension of the resulting solution in terms of the surface tensions γlv1 and γlv2 of the pure components 1 and 2 respectively (18) . Thus γ N γ + N γ = Equation 2 lv12 1 lv1 2 lv2 where N1 and N2 are the solvent mole fractions of the components 1 and 2 respectively. Experimentally, the surface tension almost always deviates negatively from that predicted by Equation 2. Moreover, the latter is inconsistent with thermodynamics since the liquid surface is found to be enriched with the component with the lower γlv (24) . The addition of Si to the Al melt results in minor reductions in γlv as shown by Koerber and Loehberg (25) . The addition of 12 wt.% Si to pure Al reduces it from 0.84 N/m to 0.83 N/m which is in the range of γlv scatter of contaminated pure Al melts. In presumably more accurate measurements, the latter value was confirmed with an A356 alloy where γlv ranged between 0.801 and 0.889 N/m (19) . Koerber and Loehberg evaluated the effect of other alloying elements on the surface tension of Al melts. It is interesting to note that very low additions of Na in the range of 0.1 wt.% reduce γlv by more than 35%. The Si in Al alloys plays an important role in the production of Al/SiC MMCs. As an alloying element it retards the formation of the unwanted intermetallics Al4C3 and Al4SiC4 (26) which account for the brittle character of the composite materials. Compared to pure Al in oxide ceramic systems, no significant influence of Si additions on composite ductility could be observed. There is a eutectic in the Al-Si system at a Si mass fraction of 11.7 % (Figure 2.4). 10
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
The metal filling the intragranular
the ceramic particles was not visib
etween the dark grey ceramic phases
The windows, one of which is marked
potential interfacial reactions, th
In order to determine the effect of
Infiltration depth L² L² (mm²) /
4.8.12 Microstructure of MMCs with
minor fraction of suboxides with hi
4.9. High pressure die casting infi
In the Y-Z plane section in Figure
4.9.2 Compression of preforms The c
Relative preform compression c pr (
decrease depended on the tooling us
Bending stress σ (MPa) / MPa 500 4
4.10.3 Influence of reinforcement t
Significant deformation developed i
a) b) 2 50 2 50 µm µm 2 50 2 50
5. DISCUSSION First the properties
The measured elastic modulus, Edyn
The MMCs showed similar wear with t
interfacial debonding: Peng et al.
The area Sml was derived using data
MMC. Due to the solidification shri
measurements which resulted in a lo
5.1.5 Influence of reactions No rea
5.2. Preform pore formation The tar
kinetics were reported to be rather
The newly formed water vapour led t
In order to achieve minimum porosit
the present work. These pressures w
indicated by zero values of the fre
influence on the pO2,calc. The lowe
during extended holding and acts as
Compared to Hg, the Al melt may con
preforms with IM, Figure 4.67. For
preform compression, cpr , increase
Specific Specific permeability Perm
Permeability (m²) / m² 1x10 -12 1
As the predominant fluid flow was a
In the CP mode, the Preform 1D code
Local Saturation saturation S () lo
listed in Table 5.1 and 5.3 were us
6. CONCLUSIONS 1. An aqueous proces
anged between 112 and 131° for the
8. REFERENCES 1. Altenpohl, D.: Alu
43. Davis, L.C. and Allison, J.E. :
85. Gennes, P.G. : “Wetting: Stat
127. Corbin, S.F., Lee, J. and Qiao
171. Gmelin, L. : Handbook of Inorg