| The onset of slip in an initially dislocation-free crystal is a fundamental process that underlies several elemental mechanical properties of crystalline materials. However, the onset of slip and the associated dislocation formation have not been studied satisfactorily or understood completely. The research of this fundamental process should impact the understanding of a number of phenomena that involve dislocations, particularly the brittle-to-ductile transition (BDT), interfacial relaxation, martensitic transformations, and the onset of plasticity in metal whiskers, low dislocation density semiconductor single crystals, and in nano-indentation tests.; Using anisothermal creep test method, the onset of slip is studied in [001] NiAl single crystals that are effectively free of mobile dislocations. Under a constant stress and with a steadily increasing temperature, the onset of slip in [001] NiAl takes place abruptly at a well-defined initiation temperature. The slip vector is identified to be <110> by slip line and TEM observations. The initiation temperature decreases approximately linearly with stress, and increases approximately exponentially with the heating rate at high and low heating rates. There is an intermediate heating rate range in which the onset temperature is independent of the heating rate. The existence of a rate-independent onset temperature cannot be explained by the traditional theories of dislocation formation, but consistent with a cooperative process in dislocation formation.; TEM observations show that the <110> slip is blocked by dislocation locking along both of the two <111> directions in the {lcub}110{rcub} glide plane. The locking is explained by the non-planar splitting of the screw component along the three-fold line directions. HRTEM observations of the locked <110> dislocations end-on show a compact core and rule out climb decomposition as the cause for the locking. The blocking of <110> slip by dislocation locking along two line directions is in contrast to <111> dislocations that are locked only along the screw orientations, and to <100> dislocations that are not locked at all. This explains why <110> slip is more difficult than <111> and <100> slips and the general absence of <110> dislocations in NiAl crystals. An intuitive explanation for the planar core of <100> dislocations is also given. |
