The yield and fracture behaviour of high-purity iron single crystals at high rates of strain

Abstract

Single crystals of iron were deformed in tension at an impact strain rate of about 1000 per second at temperatures between 20 and -196 °C. The results were compared with those of tests at a conventional rate of strain. The effect of raising the strain rate was to increase the yield stress, to increase the temperature range over which the specimens deformed by twinning and to increase the range of orientation showing brittle failure at -196°C. There was no significant effect of strain rate, however, upon the fracture stress level. A few specimens cleaved at a temperature higher than -196°C in tests at the impact strain rate, but there w as no indication of a ductile-brittle orientation transition at these temperatures. The yield stress, in the absence of twinning, was found to be independent of crystal orientation. The twinning stress at the high strain rate was found to be independent of temperature but was a little higher than at the conventional rate of strain. While twinning always took place on those systems supporting the higher resolved shear stresses under tensile loading, it did so at a value of shear stress on the most favoured twinning system which was not independent of orientation but increased as the tensile axis of the crystal approached [001]. In many tests on crystals which did not fracture twin intersections of the type that would be expected to initiate cleavage were found. It is concluded that the mere presence of such critical intersections is not enough to cause fracture. The records from tests at the impact strain rate, however, provide direct evidence that when cleavage fracture was obtained it was always preceded by twinning.