Glide systems of hematite single crystals in deformation experiments

The critical resolved shear stresses (CRSSs) of hematite crystals were determined in compression tests for r-twinning, c-twinning and {a}< m >-slip in the temperature range 25 degrees C to 400 degrees C, at 400 MPa confining pressure, and a strain rate of 10(-5) s(-1) by Hennig-Michaeli, Ch., Siemes, H., 1982. Experimental deformation of hematile crstals betwen 25 degrees C and 400 degrees C at 400 MPa confining pressure. In: Schreyer, W. (Ed.) High Pressure Research in Geoscience, Schweizerbart'sche Verlagsbuchhandlung, Stuttgart, p. 133-150. In the present contribution newly performed experiments on hematite single crystals at temperatures up to 800 degrees C at strain rates of 10(-5) s(-1) and 300 MPa confining pressure extends the knowledge about the CRSS of twin and slip modes. Optical observations, neutron diffraction goniometry, SEM forescatter electron images and electron backscatter diffraction are applied in order to identify the glide modes. Both twinning systems and {a}< m >-slip were confirmed by these methods. Besides the known glide systems the existence of the (c)< a >-slip system could be stated. Mechanical data establish that the CRSS of r-twinning decreases from 140 MPa at 25 degrees C to similar to 5 MPa at 800 degrees C and for {a}< m >-slip from > 560 MPa at 25 degrees C to similar to 40 MPa at 700 degrees C. At room temperature the CRSS for c-twinning is around 90 MPa and at 600 degrees C similar to 60 MPa. The data indicate that the CRSSs above 200 degrees C seem to be between the values for r-twinning and {a}< m >-slip. For (c)< a >-slip only the CRSS at 600 degrees C could be evaluated to similar to 60 MPa. Exact values are difficult to determine because other glide systems are always simultaneously activated. (C) 2007 Elsevier B.V. All rights reserved.