In situ measurement of stresses in alumina scales by high temperature x-ray diffraction

High temperature X-ray diffraction with parallel beam geometry was used for the in situ measurement of growth and residual stresses in the alpha -Al2O3 scale on PM 2000 during the growth of the oxide at 1060 degreesC and on cooling and a subsequent reheating. Low compressive stresses were measured during oxidation at 1060 degreesC, decreasing in the first 20 h from -300 to -80 MPa and staying in the range of -80 Wa during the rest of the 100 h oxidation time. On cooling the compressive stresses increased up to values in the range of -3000 to -4000 MPa at room temperature. Upon reheating the sample to 1060 degreesC the stresses decreased to the values measured at high temperatures and increased again on cooling. The parallel beam geometry, achieved on the laboratory X-ray source by a parabolic Gobel-mirror, enables the necessary precision for stress determination at high temperatures avoiding measuring errors caused by inaccurate sample positioning due to thermal dilatation of the heating strip or non symmetric diffraction condition.