S.T. Mileiko Current Opinion in Solid State and Materials Science 9(2005)219-229 070 developed before the observation had been done. Reasons for the formation of such interface are not clear: however Matrix we can assume that the nucleation of bonding at some sites of the interface takes place followed by diffusion of necessary elements from the matrix and depletion of the matrix with the elements responsible for the bonding The interface strength measured by a modified push-out technique presented in Fig. 13a starts to decrease with the fibre volume fraction reaches 0. 25, certainly due to Fibre shortage of elements that provide interface bonding. That corresponds to a decrease in the creep resistance, Fig. 13b. The creep-resistance/fibre-V deviates clearly fro lation of the depend 33701q results we need values of the parameters involved in Eq (6). A set of these parameters was chosen from the results of direct independent tests(see Table 1)except written in bold To describe the creep behaviour of composites with large fibre volume fractions and a weak matrix/ matrix interface t'a 20 MPa(a=0.01), a value fibre strength equal to 150 MPa, which is characteristic value for fibres tested separately, can be accepted. The calculation does correspond to the experiment To describe the behaviour of the fibre volume fractions(strong interface), we need, obvi- A-field ously, to adjust the value of a, which describes the inter face strength. However, varying only the value of a does Fig12.TEM images of the fibre/matrix interface in a alumina-YAG- not yield a satisfactory result; to make the dependences for various o to be consistent with the experimental points, it is necessary to assume variations of the fibre strength characteristics together with variations of the observation of the interface microstructure [19] and mea- interface strength. Such assumption is not an artifi surements of strength [2 procedure. The first experimental fact supporting an First of all, it should be emphasized that the fibre/ of the fibre strength characteristics being affected by the matrix interface in Ni-matrix composites(Fig. 12)does matrix via the interface was mentioned above. Secondly drastically differ from what is observed in Mo- or Ti- TEM-observations of the microstructure of the interface based composites(Figs. 2 and 9, respectively). It is inter-( Fig. 12)reveal just a partial contact on the matrix/matrix esting that the interface looks like an island-type model interface in these composites. It should be noted that the Calculation: Weak interface 75 Fibre volume fraction Fibre volume fraction Fig 13. Alumina-YAG-eutectic-matrix/Ni-based-matrix composites: (a) the interface strength versus fibre volume fraction and(b)stress to cause 1% creep strain for 100 h at 1150C versus fibre volume fractionobservation of the interface microstructure [19] and mea￾surements of strength [26]. First of all, it should be emphasized that the fibre/ matrix interface in Ni–matrix composites (Fig. 12) does drastically differ from what is observed in Mo- or Ti￾based composites (Figs. 2 and 9, respectively). It is inter￾esting that the interface looks like an island-type model developed before the observation had been done. Reasons for the formation of such interface are not clear; however, we can assume that the nucleation of bonding at some sites of the interface takes place followed by diffusion of necessary elements from the matrix and depletion of the matrix with the elements responsible for the bonding. The interface strength measured by a modified push-out technique presented in Fig. 13a starts to decrease with the fibre volume fraction reaches 0.25, certainly due to a shortage of elements that provide interface bonding. That corresponds to a decrease in the creep resistance, Fig. 13b. The creep-resistance/fibre-volume-fraction curve deviates clearly from an extrapolation of the dependence observed at low fibre volume fractions. To analyse these results we need values of the parameters involved in Eq. (6). A set of these parameters was chosen from the results of direct independent tests (see Table 1) except for that written in bold. To describe the creep behaviour of composites with large fibre volume fractions and a weak matrix/matrix interface, s* 20 MPa (a = 0.01), a value fibre strength equal to 150 MPa, which is characteristic value for fibres tested separately, can be accepted. The calculation does correspond to the experiment. To describe the behaviour of the composites at low fibre volume fractions (strong interface), we need, obvi￾ously, to adjust the value of a, which describes the inter￾face strength. However, varying only the value of a does not yield a satisfactory result; to make the dependences for various a to be consistent with the experimental points, it is necessary to assume variations of the fibre strength characteristics together with variations of the interface strength. Such assumption is not an artificial procedure. The first experimental fact supporting an idea of the fibre strength characteristics being affected by the matrix via the interface was mentioned above. Secondly, TEM-observations of the microstructure of the interface (Fig. 12) reveal just a partial contact on the matrix/matrix interface in these composites. It should be noted that the Fig. 12. TEM images of the fibre/matrix interface in a alumina–YAG￾eutectic–fibre/Ni-based-matrix composite specimen. Fig. 13. Alumina–YAG-eutectic–matrix/Ni-based-matrix composites: (a) the interface strength versus fibre volume fraction and (b) stress to cause 1% creep strain for 100 h at 1150 C versus fibre volume fraction. 226 S.T. Mileiko / Current Opinion in Solid State and Materials Science 9 (2005) 219–229