Superplasticity And Superplastic Forming Of Nano-Sized Si₂N₂O-Sialon Composites

Nano-structured Si₂N₂O-Sialon composites were prepared by hot pressed sintering (HPS) by amorphous nano-sized Si₃N₄ powders. Their mechanical properties, superplasticity and deformation mechanism were investigated. The forging forming of gears was simulated for nano-sized Si₂N₂O-Sialon ceramic. The superplastic forming rules were observed by compare the simulated results and exprimental results.The nano-sized Si₃N₄ ceramic with average grain size 18nm were prepared by hot pressed sintering (HPS) with Y₂O₃ and Al₂O₃ powders prepared by polyacrylamide gel method with average grain size less than 20nm and the AlN powder with average grain size 50nm as the sintering additives. The mass ratio of the nano-sized powders Si₃N₄, AlN, Y₂O₃, Al₂O₃ is 72:14:4:10. The typical nano-structured composites with an average grain size of 80nm were fabricated at the tempreture of 1600℃.The tention strain experimets were preceded for nano-sized Si₂N₂O-Sialon ceramic. The extensibility ratio is 115% at strain rates of 4.7×10-4s-1 and low temperature of 1550℃, this result indicates that nano-sized Si₂N₂O-Sialon ceramic owns excellent superplasticity. Superplastic extrusion can be undertaken at 1550℃with a high velocity of 1mm/s and a big extrusion ratio of 3.57. Based on Deform, simulate the foeging and the combined extrusion of nano-sized Si₂N₂O-Sialon ceramic. Compare the analog results and the exprimental results to obtain the the rule of the superplastic deformation.The tensile deformation of Si₂N₂O-Sialon composite is uniform and without pinch phenomenon. The flow stress is less than 30MPa, stress sensitive is from 1.5 to 2.3. The principle deformation mechanism is considered to be the non-Newtonian viscous flow of grain boundary glassy phase when stress sensitivity is less than 2. At the last stage (more than 80% elongation and 2 stress sensitivity), however, superplastic behavior is considered to be mainly controlled by the solution-precipitation.