Prakt. Met. Sonderband 41 (2009) 321 
erent contrast is a Z-contrast image, it can be qualitatively said that the phases with the same contrast should have 
e NiAl matrix similar composition. EFTEM/element mapping (Fig. 6b) and STEM/XEDS analysis (Fig. 6¢) sub- 
pping (Fig. 4d) stantiated that the grey particles (A) were V(CO)-compounds, the dark grey particles (B, C) were 
> to the ALLO; VAI(OC)-compounds, the dark particles (D, F) could be identified as Al-oxide, and the bright area 
iAl was a Cr- (E) in the interfacial area was determined to be nickelaluminide. These results confirm the compli- 
e identified as cated chemical reactions during hot pressing. 
> to the C-rich 
4 Discussion 
In comparison to the interfacial shear strength of Al;O3-NiAl composites, which is as high as 
250MPa [7] for complete debonding, the shear strength modified by the hBN and MAX-phase in- 
ire and chem- terlayer is reduced more than 50%. On one hand the low shear strength caused by these interlayers 
r with V,AIC could help them with relieving the thermal residual stress (TRS due to the different CTEs) and 
ee therefore to decrease the fiber degradation during the composites fabrication; but on the other hand 
he interfacial the intensive chemical reactions that occurred during hot pressing and owing to the additional 
chemical elements from those interlayers, could also deteriorate the interface properties and then the 
mechanical performance of the composites. 
At the interface between BN and NiAl the Gibbs free energy of the reaction involving the elements 
Al + BN = AIN + B (1) 
is negative, i.e. there is a general tendency for AIN to replace BN [3]. The present investigations 
substantiate that there is a chemical incompatibility between NiAl, Al,Os fiber and Cr as well as V 
MAX -phase interlayers due to the inter-diffusion of certain elements, like O, C and Ni at high tem- 
peratures. As a result, the MAX-phase interlayers decomposed completely and various reaction 
“46.2 products were formed in the interfacial area. For the hBN case, the dramatic loss of the interfacial 
21.8 shear strength is attributed mainly to the sliding (or delamination) of the hBN basal planes. The 
sliding of the hBN basal planes is primarily caused by the textured hBN interlayer, i.e. the hBN 
0.4 grain orientation distribution is not isotropic. HRTEM observations demonstrated that the hBN 
basal planes near the boundary with the sapphire fibre were more perfectly parallel to the interface 
than those far away from the boundary [3]. Furthermore, debonding occurred mainly in the hBN 
interlayer adjacent to the boundary between fiber and hBN when the fiber was pushed out [3]. The 
observed microstructure and the debonding behaviour support the conclusion that the sliding of the 
Z-contrast image BN coating is mainly caused by the texture of the hBN interlayer. Therefore, texture control of the 
e area defined by hBN interlayer during composite fabrication might impede the sliding of the hBN basal planes, and 
thus improve the interfacial shear strength. 
“sapphire fiber 
terlayer with a 5 Summary 
ie thin continu- 
RN sed of Based on the currently available experimental condition, BN, Cr,AlIC as well as V,AIC have shown 
the chemical incompatibility with NiAl and Al,O3 at high temperatures. These interlayers are not 
erlayer was as- suitable to modify the interface structure and properties of NiAl composites and to improve their 
e crack on the mechanical properties. 
erface structure 
Fig. 6a as a 
ıniform MAX- 
t. Since Fie. 6a