Optical methods were used to measure thermal deformations in commercial 4H- and 6H-SiC wafers. In general, during thermal excursions to 500 degreesC, the radii of curvature of the wafers increased (i.e., the wafers became less bowed). Upon cooling to room temperature, nearly all the wafers retained the high temperature radius of curvature values, a characteristic of thermoplastic deformation. Further cyclic excursions to 500 degreesC did not yield any significant changes in the radii of curvature, thus signifying the irreversibility of the thermal deformation. The magnitude of the relieved stress following thermal deformation at 500 degreesC was estimated to be up to 2.14 MPa. Also observed was a significant difference in the thermoplastic deformation between the off- and on-axis wafers of both polytypes, with the former having more deformation (DeltaR > 3 m) than the latter (DeltaR < 2 m). An on-axis n-type 6H-SiC that was subjected to thermal cycling to 900 degreesC in vacuum also exhibited thermoplastic deformation, with an activation energy of 3.14 +/- 0.8 eV and a relieved stress magnitude of 5.67 MPa.