S.T. Mileiko Current Opinion in Solid State and Materials Science 9(2005)219-229 structural materials, a number of questions must be 人143MPa answered. Stress rupture and oxidation resistance at high temperatures as well as crack resistance at room tempera ture are most important ones. At the present time, there can be seen some ways to make oxide-fibre/Ti-Al compos- ites that are not too brittle. An illustration of a possibility of one way to do this is presented in Fig. Il 3.3. Ni-based-matrix: variable interface An effectiveness of the usage of ICM-fibres in Ni-based- matrix was discussed in details in Refs. [16, 19, 32]. The main finding of that research is a synergetic influence of he fibre/matrix interface on the creep behaviour of the composites. This means that the interface determines the composite creep resistance in two ways and the output is Fig. Il. Load versus displacement for 3-point bending of two oxide-fibre/ TiAl-matrix composite specimens of a special macrostructure at room emperature larger than just a linear sum. The first way is sufficiently obvious: increasing the fibre/matrix strength yields increasing stresses in the fibre d way is not so obvious. It emerges from an in between the fibre surface and matrix material The matrix, especially in a liquid state, can cause dam age of the fibre surface and then the fibre being extracted from the matrix and tested as an isolated object reveals an essential decrease in the strength. In the case of sapphire fibre in nickel-based-matrix the fibre degradation of such 65m kind was observed in a number of works [23, 24]. Their authors used this observation to speak out a doubt in the Fig9.SEM micrograph of a cross-section of the sapphire-fibre/Ti-48AI- future of oxide/Ni systems as heat resistant composites However. the behaviour of a fibre in matrix differs from that of a separate fibre and the difference can be impressive Actually, an example of such difference was presented above, in Figs. 5 and 6, where the coating acted as a matrix. Hence, if the fibre/matrix interface is ideal, like the coating is, then strength characteristics of the fibre in matrix will be much higher than those of fibres tested separately. The idea can also be proved by an analysis of results of testing com- osites of a definite fibre/n matrix with interface strength. The analysis is supposed to be based on comparison of experimental results and those obtained by calculations made on a basis of the micro-mechanical model The main problem with this composite system is actu lly the interface bond. From the very beginning of the history of modern composites, poor wetting of oxide (in 40 particular, sapphire) with molten nickel has been known [25]. Also it is now clear that appropriate design of the interface is possible [24], which will yield a sufficiently strong bonding at the interface. The importance of the Fig. 10. Stress to cause I% creep stain for 100 h versus testing temperature interface bonding in designing heat-resistant composites for the sapphire-fibre/Ti-48AI-matrix composite with fibre volume of oxide/Ni-based-matrix is illustrated by experiments in fractions between 0.20 and 0. 25 [20]. hich creep tests [16] have been complemented withstructural materials, a number of questions must be answered. Stress rupture and oxidation resistance at high temperatures as well as crack resistance at room tempera￾ture are most important ones. At the present time, there can be seen some ways to make oxide–fibre/Ti–Al compos￾ites that are not too brittle. An illustration of a possibility of one way to do this is presented in Fig. 11. 3.3. Ni-based-matrix: variable interface An effectiveness of the usage of ICM-fibres in Ni-based￾matrix was discussed in details in Refs. [16,19,32]. The main finding of that research is a synergetic influence of the fibre/matrix interface on the creep behaviour of the composites. This means that the interface determines the composite creep resistance in two ways and the output is larger than just a linear sum. The first way is sufficiently obvious: increasing the fibre/matrix interface strength yields increasing stresses in the fibre. The second way is not so obvious. It emerges from an interaction between the fibre surface and matrix material. The matrix, especially in a liquid state, can cause dam￾age of the fibre surface and then the fibre being extracted from the matrix and tested as an isolated object reveals an essential decrease in the strength. In the case of sapphire fibre in nickel-based-matrix the fibre degradation of such kind was observed in a number of works [23,24]. Their authors used this observation to speak out a doubt in the future of oxide/Ni systems as heat resistant composites. However, the behaviour of a fibre in matrix differs from that of a separate fibre and the difference can be impressive. Actually, an example of such difference was presented above, in Figs. 5 and 6, where the coating acted as a matrix. Hence, if the fibre/matrix interface is ideal, like the coating is, then strength characteristics of the fibre in matrix will be much higher than those of fibres tested separately. The idea can also be proved by an analysis of results of testing com￾posites of a definite fibre/matrix system with changing interface strength. The analysis is supposed to be based on comparison of experimental results and those obtained by calculations made on a basis of the micro-mechanical model. The main problem with this composite system is actu￾ally the interface bond. From the very beginning of the history of modern composites, poor wetting of oxide (in particular, sapphire) with molten nickel has been known [25]. Also it is now clear that appropriate design of the interface is possible [24], which will yield a sufficiently strong bonding at the interface. The importance of the interface bonding in designing heat-resistant composites of oxide/Ni-based-matrix is illustrated by experiments in which creep tests [16] have been complemented with Fig. 9. SEM micrograph of a cross-section of the sapphire–fibre/Ti–48Al– matrix composite [20]. Fig. 10. Stress to cause 1% creep stain for 100 h versus testing temperature for the sapphire–fibre/Ti–48Al–matrix composite with fibre volume fractions between 0.20 and 0.25 [20]. Fig. 11. Load versus displacement for 3-point bending of two oxide–fibre/ TiAl–matrix composite specimens of a special macrostructure at room temperature. S.T. Mileiko / Current Opinion in Solid State and Materials Science 9 (2005) 219–229 225