No abstract

Hot isostatically pressed silicon nitride was produced by densifying Si3N4 powder compacts and reaction‐bonded Si3N4 (RBSN) parts with yttria as a sintering additive. The microstructure was analyzed using scanning electron microscopy, X‐ray diffraction, and density measurements. The influence of the microstructure on fracture strength, creep, and oxidation behavior was investigated. It is assumed that the higher amount of oxygen in the Si3N4 starting powder compared with the RBSN starting material leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in the hot isostatically pressed material. Compared with the hot isostatically pressed RBSN samples therefore, strength decreases whereas the creep rate and the weight gain during oxidation increase.

ChemInform Abstract SEM, XRD, and density measurements are used to investigate the microstructure of hot isostatically pressed silicon nitride produced by densifying Si3N4 powder compacts and reaction-bonded Si3N4 (RBSN) parts with Y2O3 as sintering additive. It is suggested that the higher amount of oxygen in the Si3N4 powder compared with RBSN leads to an increased amount of liquid phase during densification. This results in grain growth and in a larger amount of grain boundary phase in this material. The fine-grained microstructure causes an increased fracture strength. The increased amount of grain boundary phase leads to decreased creep and oxidation resistance.

Fatigue Strength of Fibre-Reinforced Squeeze Cast Piston Alloy GP-AISi 12 CuMgNi Under Elevated TemperaturesOn specimens made of squeeze-cast piston alloy GP-AlSi 12 CuMgNi without reinforcement and A1*03-fibre-reinforced, thermal fatigue was investigated. Several mechanical properties such as tensile and compressive yield strength, incremental-step-method based monotonic and cyclic stress-strain curves in the temperature range of 20 to 400 "C were determined. Moreover, fatigue tests in the range of 5 . lo4 and 2.5 . lo7 were carried out for various stress ratios in the same temperature range on unnotched specimens and S-N-curves were designed. Haigh-diagramms show the mean stress sensitivity of the materials. Obviously the fibre reinforced material is superiour at all regarded temperatures.The considerable gain of material strength obtained by fibrereinforcement entails even better conditions for the use of newly developed pistons under increased thermal load.