1989 Volume 38 Issue 429 Pages 623-629
Thermally induced stresses in the matrix and fibers have been studied by an X-ray diffraction technique during the thermal cycling of an α-Al2O3/Al composite with 50vol% and 20μm diam continuous FP fibers (FP is a tradename of Du Pont Company). The Al matrix contained 1.0wt% Li to wet the fibers. After annealing the composite at 800K for 3hr, the in-situ thermal stress measurements were carried out at temperatures up to 580K by the use of a vacuum resistance furnace mounted on the computer controlled Ω-diffractometer. The results obtained are summarized as follows:
(1) When the composite undergoes a change in temperature, both the matrix and the fibers mechanically deform in such a way as to modify thermal expansion mismatch between two phases.The resultant thermal stress in each phase balanced with each other in the composite.
(2) On cooling the composite from 580K to room temperature (RT), the matrix showed a tensile residual stress of 50MPa. Upon heating the composite from this stress state the matrix tensile deformation was reversed to compressive deformation. On this heating process the matrix showed a strong Bauschinger effect, that is, the matrix yielded during the unloading before being in compression.
(3) On cooling the composite to liquid nitrogen temperature after cooling to RT from 580K, and heating back to RT a compressive residual stress of -30MPa was observed in the matrix. On the subsequent heating process the matrix plastically deformed in compression and a permanent softening due to the stress reversal had been observed in the matrix.
