Neutron diffraction study of in situ-reinforced silicon nitride during creep

Two grades of in situ-reinforced silicon nitride (Si 3N4) were tested at high temperature under applied stress using time-of-flight neutron diffraction. The older variety, GS-44, was tested at 1473 K under applied stresses of 100, 125, 150, and 175 MPa. Creep was observed in extensometer data from these tests. The creep exponent was determined from the steady state creep rates as n = 3.18. A recent model for Si3N4 creep was able to match the creep rate vs. applied stress data, but only with the inclusion of an additional empirical stress dependence. The diffraction data indicated that the two lattice parameters of Si3N4 behave in a unique manner, with the c axis expanding while the a axis contracts, in a unit cell volume conserving process. The newer variety, AS800, was seen to remain elastic at 1648 K, thus no creep exponent was obtained. However, the single crystal stiffness tensor was determined from stress-strain data of individual hkl reflections from the diffraction patterns. In addition, for both grades the CTE tensor was determined using single peak data. The temperatures used in these experiments are the highest in literature for time-of-flight neutron diffraction in combination with stress application.