Yield stress anomaly of intermetallic compounds

PhD thesis defended in 2014, Jonathan MICHEL


The intermetallic compounds Ni3Al present, in addition to a clear technological interest since they constitute the hardening phase of superalloys widely used in the aerospace and energy industry, an obvious fundamental interest due to the unusual behaviour of their mechanical properties with temperature, which increase over a given range of temperature. Several elementary dislocation mechanisms have been proposed and modeled in order to explain this anomaly, which they describe with more or less success although they sometimes differ significantly in the description of the deformation processes controlling the kinetics of plastic deformation. Because the proposed mechanisms are generally at the atomic level any direct confrontation with experimental observations could be made. The observations of slip traces at the atomic scale under controlled stress and temperature conditions will make possible for the first time an objective comparison between the different models.


The studies were performed on Ni3 (Al, Ta) single crystals, whose mechanical properties and dislocation microstructures have been fully characterised in previous studies. The slip traces were studied by AFM and STM at three temperatures in the anomaly regime. Using AFM, it is observed that the plastic deformation mainly occurs by the sudden emergences of slip traces, suggesting dislocation burts. Once formed, these slip traces do evolve anymore or very little. The number of cross slip (CS) events and the CS length increase with temperature. Using STM, at the atomic scale, double CS between close and parallel octahedral planes are observed, but double CS events were not imaged once the CS distance is larger than that of the APB in the cube CS plane. Therefore, simple CS event mainly occurs from octahedral planes onto cube CS planes (highlighting in Figure 1), with moving distances on cube planes that are larger than the width of the APB on the cube CS plane, attesting that the so-called Kear-Wilsdorf locks are both stable and mobile on the cube CS planes. Certainly one of the most surprising features is that all observed slip traces are produced by a single superdislocation only, regardless the deformation conditions.

 ni3al3 ni3al4
 (a)  (b)

Figure 1: Slip traces at the nanometer scale on Ni3(Al, Ta) samples deformed at 0.6 % plastic strain at 600 K. (a) topographical AFM image (b) associated step profiles.


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