Directed melt oxidation and nitridation in aluminium alloys: B. S.S. Daniel and V.S. R Murthy oxygen-induced reactions. To reduce the degradation and imt bility between the matrix and come essentia Nicalon SiC fibre preforms are coated to prevent surface oxidation and minimize the interfacial reactions33 During infiltration, filler materials act as secon nucleation sites for the reaction product(AlO, or AIN and refine the composite microstructure (Figure 5), while maintaining a growth direction similar to that of primary growth 22. Secondary nucleation seen in oxide systems is attributed to the reduction of oxide coatings Sic that are inherently present on the filler material (e.g. 5 ur Sio2 or SiC) or that which are formed(ZnO, Mgo vapour deposits)during infiltration. For instance, silica B present on SiC is reduced by the advancing aluminium 3SO2+4A→3Si+2Al2O3 and Al,O, tends to nucleate on the surface of Sic par- ticles. The reaction by-product, silicon, occupies portion of channel space and in extreme cases growth elements(e.g. Zn)that are added to the liquid melt form th front. The deposited Zno is 5 um reduced by the advancing aluminium to give rise to AlO, nucleation Figure 5 Secondary nucleation of (A)AL, 0, on SiC particulate and 3Zn0+ 2Al-Al2O3+ 3Zn (B)AIN oI (Nicalon) fibi In AlO, filler materials, newly formed oxide reactions between the preform and liquid alloys are epitaxial growth, whereas on Sic particles,no nd to enhance the lographic matching was evident". In nitridation wettability. In nitridation, adverse reactions are rather secondary nucleation is verge. limited, however, in an oxidative atmosphere the rein- nous, but a clear mechanism relating to their surface forcements can lose their basic properties due to chemistry is not known Table 2 Mechanical properties of direct melt oxidized composites,> Property Al,O Al composite SiCPAlOyA Al,OAl AlO/Al Uniaxial 2D, 12HSW Youngs modulus(GPa) Shear modulus(GPa) Flexural strength 720±150 540± (3-point bend test) (WM 20)(WM 22)(max 880) (max 620) (MPa vm (max 29) (max 16) 414-2100 0.1370.312 Vickers hardness(GPa 141-15.0 at700°C l18-39.6 Coeff. of thermal expansion 9.3-11.0 Dielectric constant o C-axis of Al O, ⊥ to c-axis of Al2O3 4 awith variation in residual metal content Weibull modulu Materials Design Volume 16 Number 3 1995 159Directed melt oxidation and nitridatjo~ in aiuminium alloys: f3. S. S. Daniel and V. S. R. Murthy Figure 5 Secondary nucleation of (A) A&O, on Sic particulate and (B) AlN on Sic (Nicalon) fibres reactions between the preform and liquid alloys are inevitable and these reactions tend to enhance the wettability. In nitridation, adverse reactions are rather limited, however, in an oxidative atmosphere the rein￾forcements can lose their basic properties due to oxygen-induced reactions. To reduce the degradation and improve the compatibility between the matrix and the fibre, coatings become essential. For instance, Nicalon Sic fibre preforms are coated to prevent surface oxidation and minimize the interfacial reactions33. During infiltration, filler materials act as secondary nucleation sites for the reaction product (A&O, or AlN) and refine the composite microst~cture (r;igure 5), while maintaining a growth direction similar to that of primary growth iLz2 . Secondary nucleation seen in oxide systems is attributed to the reduction of oxide coatings that are inherently present on the filler material (e.g. Si02 or Sic) or that which are formed (ZnO, MgO vapour deposits) during infiltration. For instance, silica present on Sic is reduced by the advancing aluminium via the displacement reaction 3SQ + 4Al-+ 3Si + 2Al,O, (1) and Al,O, tends to nucleate on the surface of Sic par￾ticle.?. The reaction by-product, silicon, occupies a portion of channel space and in extreme cases growth is inhibited due to a ‘choking’ effect3’. Further, the solute elements (e.g. Zn) that are added to the liquid melt form a vapour and are deposited on the filler material surface ahead of the growth front. The deposited ZnO is reduced by the advancing aluminium to give rise to A&O, nucleation23. 3Zn0 f 2Al+ Al& + 3Zn (2) In A&O, filler materials, newly formed oxide exhibits epitaxial growth, whereas on Sic particles, no crystal￾lographic matching was evident34. In nitridation, secondary nucleation is expected to be simply heteroge￾nous, but a clear mechanism relating to their surface chemistry is not known. Table 2 Mechanical properties of direct melt oxidized composites’“*3~‘,4~ Property Al,O,/AI composite without preforma Young’s modulus (GPa) 88-304 Shear modulus @Pa) 41-123 Flexural strength 45-525 (4-point) (MPa) (3-point bend test) Fracture toughness 2.9-9.5 Wa ~‘3 Compressive strength 414-2100 @@a) Poisson’s ratio 0.137-0.312 Vickers hardness (GPa) 1.41-15.0 at 700°C - Thermal conductivity 1 t s-39.6 (Wm-’ K-l) Coeff. of thermal exapansion 9.3-I 1 .o (X 10*/K) Dielectric constant [j to c-axis of A&O, 8.0 I to c-axis of A&O3 6.4 With variation in residual metal content bWeibull modulus 403d Al,OrtAl 301 312 (WMb 20) 5.9 - 8.3 4.9 36 9.0 SiCd SiC~Al~O~lAl Al~O,IAI Uniaxial 2D, I2HSW 324 - 334 720 k 150 540 f 60 (WM 22) (max 880) (max 620) 6.9 27 k3 15 + 1 (max 29) (max 16) - - 6.4 3.7 82 7.5 Materials & Design Volume 16 Number 3 1995 159