Glide systems of hematite single crystals in deformation experiments

Abstract

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°C to 400°C, at 400 MPa confining pressure, and a strain rate of 10 -5 s -1 by Hennig-Michaeli and Siemes, 1982. In the present contribution newly performed experiments on hematite single crystals at temperatures up to 800°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°C to ~5 MPa at 800°C and for {a}<m>-slip from >560 MPa at 25°C to ~40 MPa at 700°C. At room temperature the CRSS for c-twinning is around 90 MPa and at 600°C ~60 MPa. The data indicate that the CRSSs above 200°C seem to be between the values for rtwinning and {a}<m>-slip. For (c)<a>-slip only the CRSS at 600°C could be evaluated to ~60 MPa. Exact values are difficult to determine because other glide systems are always simultaneously activated.

Development of Microstructure and Texture of Hematite Ores Deformed to Large Strain in Torsion: Can Texture Identify the Prevailing Strength and Creep Mechanisms During Deformation?

“…For samples deformed at 1000°C the shear stress (40 MPa) is lower than at 950°C because the critical resolved shear stresses of the glide systems decrease with increasing temperature (Siemes et al [29] ). Again, the texture index for the specimen of the fine-grained block05 is higher (J=3.3) than for the coarse-grained block12 (J=2.2).…”

EBSD analysis of rhombohedral twinning in hematite crystals of naturally deformed iron formations

“…Hematite (Fe 2 O 3 ) is a trigonal mineral, space group R3c, with layers of oxygen atoms in hexagonal closest packing along (0001) separated by Fe 3+ in octahedral coordination occupying two-thirds of the crystallographic sites (Blake et al, 1966). The rhombohedral twinning of hematite might develop as a growth (Heuer, 1966) or mechanical twinning (Heuer, 1966;Bursill & Withers, 1979;Barber & Wenk, 1979;Bursill & Lin, 1989;Bursill et al, 1992;Siemes et al, 2008), but the latter is more commonly recognized for its implication in strain accommodation. It consists of a contact twinning with f0112g as the reflection and the composition plane.…”

“…The f1012g planes are called acute rhomb planes or simply rhomb planes and served as a basis for subsequent studies on twinning of hematite and isostructural crystals such as corundum (Al 2 O 3 ) and ilmenite (FeTiO 3 ). Siemes et al (2008) and Hennig-Michaeli (1977) analyzed rhombohedral twinning in both hematite single crystals and hematite aggregates in deformation experiments to assess how these twins are developed under a stress field. Other authors studied lattice relations between twinned crystals and twinning-related dislocations (see, for example, Bursill & Lin, 1989;Bursill & Withers, 1979).…”

“…As authors do not usually use the same crystallographic notation, four notations are often found for the rhombohedral plane: (1011), (1012), (0111) and (0112). For example, Hennig-Michaeli (1977) utilized r = (1011) and Siemes et al (2008), r = (0112). In this contribution, we use the following notation: f 0 = (1011) for an obtuse positive rhomb plane, f = (0111) for an obtuse negative rhomb plane, r 0 = (1012) for a positive rhomb plane and r = (0112) for a negative rhomb plane or a rhomb plane (Bursill & Withers, 1979;Barber & Wenk, 1979;Bursill & Lin, 1989;Bursill et al, 1992;Siemes et al, 2008).…”

A novel pole figure inversion method: specification of theMTEXalgorithm

“…To demonstrate the features of MTEX, we elaborate on the analysis of preferred crystallographic orientation of a hematite specimen H43C1 which has been interpreted in terms of experimental deformation by Siemes et al (2008). The subject of their communication is the critical resolved shear stresses of hematite crystals and glide modes of hematite.…”