S. Korte

Discussion of the dependence of the effect of size on the yield stress in hard materials studied by microcompression of MgO

Microcompression has attracted considerable interest in the study of size effects, mainly in soft metals. Little data is available in the literature on experiments on materials with a higher bulk flow stress, although it has been shown that the technique can be successfully employed to suppress cracking due to the small specimen dimensions. Here, microcompressions on MgO were carried out to demonstrate the possibility of individually activating different slip systems. The yield stresses obtained in conjunction with transmission electron microscopy show that both the hard and soft slip system in MgO can be characterised individually. Microcompression is, therefore, a potential alternative to macroscopic testing of brittle materials under confining pressure or at high temperatures. To determine the influence of size on such measurements, results on the two slip systems in MgO and from the literature are compared. It is found that the bulk yield stress of a material might be used to estimate the effect of size on its yield stress at the microscale.

Ductile-brittle transition in micropillar compression of GaAs at room temperature

Experiments have been carried out on how compressive failure of <100> axis GaAs micropillars at room temperature is influenced by their diameter. Slip was observed in all micropillars, often on intersecting slip planes. Cracks could nucleate at these intersections and then grow axially in the sample, with bursts of crack growth. However, GaAs micropillars with diameters less than approximately 1 µm did not split, nor was splitting observed where slip occurred on only one plane. The conditions under which such splitting can occur have been estimated by modifying an existing analysis. This predicts a ductile–brittle transition at a micropillar diameter of approximately 1 µm, consistent with experimental observations.

Influence of test temperature on the size effect in molybdenum small-scale compression pillars

Previous research has shown that body-centred cubic (bcc) metals exhibit a smaller size dependence of strength than what is commonly observed in face-centred cubic (fcc) metals. This work investigates compression testing of focused ion beam-manufactured molybdenum pillars ranging in size from 300 nm to 5 μm, both above and below its critical temperature at 300 and 500 K. At 500 K the size effect is found to be consistent with what is observed in fcc metals, owing to the increased mobility of screw dislocations.