Dislocation densities and internal stress in Ni3Al

Dislocation densities have been measured in Ni3Al(Hf) single crystals after deformation at various temperatures and strains. In addition, effective and athermal stresses have been determined by strain dip tests. A model is proposed for the strength anomaly domain which correlates the athermal part of the stress with the operation of dislocation sources, to compensate for the strong dislocation exhaustion. Dislocation density data, which evidence an anomaly in the density with temperature over the stress anomaly domain, are shown to support this model predictions between 373K and 683K for plastic strains larger than 3x102 but not at yield. In addition, the presence of Hf atoms (3.3 at. %) induces a friction stress close to 63MPa

About the activation volume for cross-slip in Cu at high stresses

The experiment performed by Bonneville and Escaig, designed for the direct measurement of the activation parameters of cross-slip, has been repeated. A denser initial dislocation forest density is obtained with a higher predeformation stress compared with a previous study. The minimum activation volume at yield is 124b(3) when cross-slip is dominant at 24 K, instead of 280b(3) as proposed formerly. Consequently, the activation energy at zero stress is in the range of 0.61 eV, lower than in the previous study. The results are discussed in terms of predictions of linear elasticity models and those from recent atomistic simulations. To understand the discrepancies between the experimental and simulation results the Bonneville Escaig method, as well as the simulation conditions, require further assessment. (C) 2011 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved

Experimental study of Ni3Al slip traces by atomic force microscopy: an evidence of mobile dislocation exhaustion

Slip markings produced on the surfaces of Ni3(Al, Ta) single crystals, plastically deformed at various temperatures in the flow stress anomaly domain, were examined by atomic force microscopy (AFM). A dominant feature is that for all investigated temperatures, the slip traces are rectilinear and correspond to the primary octahedral glide plane. In addition, their lengths drastically decrease when the temperature is raised. This latter result is interpreted as a strong increase of the exhaustion rate of mobile dislocations with increasing temperatures. The consequences of these results in the understanding of Ni3Al flow stress anomaly are discussed

Searching for the proper law of dislocation multiplication in covalent crystals

A simple model based on dislocation theory allows the construction of a fully defined system of differential equations and the calculation of curves that correspond to different mechanical tests such as stress relaxation, the creep test and the imposed strain rate test. Various multiplication and exhaustion rates of mobile dislocations have been considered. The numerical solution of the system reproduces satisfactorily experimental curves obtained in Ge single crystals at 750 K

Dislocation multiplication rate in the early stage of germanium plasticity

A set of constitutive equations describing the plasticity of semiconductors is compared with the results of mechanical tests. Constant strain-rate compression tests are interrupted before the peak of the initial multiplication yield point, i.e. at the moment of intense dislocation multiplication, by transient tests (stress-relaxations and creep tests). Various laws for dislocation multiplication are used in constitutive modelling and their predictions are compared with experimental transient curves. A generalized law is proposed, which perfectly fits all the transient tests data, provided each sample is considered separately. It seems necessary to account for the properties of dislocation sources in the multiplication law, at least at the early stages of plasticity