Search

The mechanical behaviour of various types of BeO, Al2O3, and AlN have been investigated at confining pressures up to 1.25 GPa, at 25° C, and at strain rates of 3 to 7×10-5 sec-1. The stress-strain data taken in uniaxial compressive-stress loading indicate the BeO aggregates undergo a transition from brittle fracture at low pressures to plastic flow at high pressures. Depending on the fabrication process, this transition pressure in BeO occurs at 0.4 to 0.7 GPa. Concurrently, the ultimate compressive strength of BeO increases from 1.0 to 1.9 GPa at 0.1 MPa pressure to over 4.0 GPa at 1.O GPa. Alumina remains brittle at all pressures up to 1.25 GPa; its strength increases from 4.5 GPa at 0.1 MPa pressure to over 6.0 GPa at 1.25 GPa. Aluminium nitride behaves similarly to BeO, having a brittle-ductile transition at 0.55 GPa. Its ultimate strength increases from 3.2 GPa at 0.1 MPa pressure to 4.7 GPa at 0.8 GPa. The distortional strain energy (proportional to the area under the stress-strain curve) absorbed by each material during compression at pressure was calculated and compared to available data from the literature. Alumina shows a degraded energy absorption with pressure, but both BeO and AlN yield a strongly enhanced performance at moderate pressures. Beryllium oxide and AlN thus appear to be promising structural materials for certain applications where high strengths and ductilities are required at moderate pressures.

Dolomite single crystals of six different crystallographic orientations were tested in compression under confining pressure at temperatures of 20 to 800° C. The chosen orientations favoured slip or twinning on particular systems. The deformed crystals were analysed by optical and high voltage transmission electron microscopy to determine activated deformation systems, dislocation behaviour, etc., and to assist in interpreting stress-strain data. It is shown that slip on c=(0001) and on $$f \equiv (\bar 1012)$$ ), and twinning on f between 300 and 600° C, are the principal modes of deformation. At low temperatures there is considerable cataclasis, and shear-fracturing must be counted as a significant deformation mechanism. The effects of climb become apparent at temperatures ?600° C.

Mechanical albite and pericline twins were induced in seven feldspars, (Na, Ca) (Al, Si)4O8, in triaxial compression tests at 800° C, 8 to 10 kb confining pressure. Two were initially in a high temperature structural state (disordered) (An1, An59). Three (An32, An39, and An53) were slightly disordered, and two (An76 and An95) had the transitional and primitive anorthite structure, respectively. No microscopic twinning was detected in comparable tests on ordered albite and oligoclase (An1, An20), feldspars in the peristerite range (An11 An14) or in a low structural state labradorite (An51). Other deformational features include lamellae 60 to 80° from the c axis and (010) slip in the opposite sense for (010) twinning.

DOLOMITE, an important rock-forming mineral, is a mixed carbonate possessing trigonal symmetry (R), with alternate layers of Mg and Ca atoms separated by planes containing the carbonate groups. Its ordered structure offers the possibility of extended planar faults of low energy and Barber1has shown evidence for stacking disorder in dolomite of high metamorphic grade. There is no previous evidence, however, for the creation of extended faults as a direct result of deformation processes. We have prepared single crystals, approximately 25 mm2in cross section and 10 mm long, and tested them in compression at temperatures up to 800 °C while subjected to a confining pressure of either 3 or 7 kbar. Specimens were mostly oriented to give a large resolved shear stress on the basal planes while minimising the stress for mechanical twinning on the f ≡ {012} planes. (We use four-digit indices for planes and directions referred to the structural hexagonal unit cell, with c = 16.01 Å and a = 4.81 Å.) It has been shown (optically)2that slip on (0001) and twinning on {012} are common modes of deformation on dolomite. Using TEM methods, Barber1has shown that slip on {012} can also occur in dolomite. The current observations were also made by high voltage transmission electron microscopy, using sections which were thinned first mechanically and then by ion bombardment.