Machinability of nickel-base super alloys: a general review

The toughening behavior of whisker-reinforced ceramics is analyzed in terms of a whisker bridging zone immediately behind the crack tip. This approach is consistent with microscopy observations which reveal that intact bridging whiskers exist behind the crack tip as a result of debonding of the whisker-matrix interface. The theoretical results based on both the stress intensity and the energy change introduced by bridging whiskers reveal the dependence of toughening upon the composition and matrix, interface, and whisker properties. Furthermore, the analytical models of whisker bridging accompanied by very limited pullout accurately describe experimental observations of the toughening behavior in several SiC-whisker-reinforced ceramics. Such analytical descriptions also indicate that increases in whisker size and strength and modification of interface properties will result in further increases in toughness by whisker reinforcement.

Toughening Behavior in Whisker-Reinforced Ceramic Matrix Composites

Ceramic materials in general have a very attractive package of properties: high strength and high stiffness at very high temperatures, chemical inertness, low density, and so on. This attractive package is marred by one deadly flaw, namely, an utter lack of toughness. They are prone to catastrophic failures in the presence of flaws (surface or internal). They are extremely susceptible to thermal shock and are easily damaged during fabrication and/or service. It is therefore understandable that an overriding consideration in ceramic matrix composites (CMCs) is to toughen the ceramics by incorporating fibers in them and thus exploit the attractive high-temperature strength and environmental resistance of ceramic materials without risking a catastrophic failure. It is worth pointing out at the very outset that there are certain basic differences between CMCs and other composites. The general philosophy in nonceramic matrix composites is to have the fiber bear a greater proportion of the applied load. This load partitioning depends on the ratio of fiber and matrix elastic moduli, Ef/Em. In nonceramic matrix composites, this ratio can be very high, while in CMCs, it is rather low and can be as low as unity; think of alumina fiber reinforced alumina matrix composite. Another distinctive point regarding CMCs is that because of limited matrix ductility and generally high fabrication temperature, thermal mismatch between components has a very important bearing on CMC performance. The problem of chemical compatibility between components in CMCs has ramifications similar to those in, say, MMCs. We first describe some of the processing techniques for CMCs, followed by a description of some salient characteristics of CMCs regarding interface and mechanical properties and, in particular, the various possible toughness mechanisms, and finally a description of some applications of CMCs.

Ceramic Matrix Composites

Nickel-based high temperature alloys have excellent physical properties, which make them ideal for use in the manufacture of aerospace components. However, they exhibit poor machinability. Though conventional machining in industries is currently being carried out using carbide tools, there is little scope for improving the material removal rate. Machining, being a major operation, needs to be improved in order to reduce the throughput time. High Speed Machining (HSM) is a promising technique for increasing productivity in this regard. This paper mostly reviews research and development work in the machining of nickel-based high temperature alloys carried out over the last 15 years with the objective of assessing the present scenario. Emphasis is laid on Inconel 718, which is most commonly used. Both turning and milling operations using conventional and High Speed (HS) machining are reviewed herein. HSM is discussed at length in comparison with conventional machining, as it is possible to drastical...

Machinability of nickel-based high temperature alloys

Continuous synthesis and characterization of silicon carbide nanorods

Dense, toughened SiC-whisker-reinforced ceramic matrix (ie Al2O3 and mullite) composites were fabricated by hot-pressing powder-whisker mixtures The densification behavior of the composites was influenced by the temperature, pressing pressure, and whisker content but even at 60 vol pct whisker loading, full density was achieved at 1850 C for the alumina matrix The mechanical performance of these composites was improved In particular, the critical stress intensity factor (equal to about 9 MPa sq rt of m in Al2O3 and equal to about 5 MPa sq rt of m in mullite) exhibited substantial improvements (approximately two-fold increases) over monolithic ceramics In the case of the alumina-20 vol pct SiC whisker composite, the flexural strength was 805 MPa at room temperature and 520 MPa at 1200 C the thermal conductivity was 32 W/mK at room temperature and 10 W/mK at 1000 C 19 references

P.F. Becher

Papers

Development of SiC-whisker-reinforced ceramics