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Contributed talk

Atomic scale imaging of elementary dislocation mechanisms in Ni3Al intermetallic alloys

Joël Bonneville, Université de Poitiers, Chasseneuil - Futuroscope, France
Christophe Coupeau, Université de Poitiers - Institut P', Chasseneuil - Futuroscope, France
Michel Drouet, Université de Poitiers - Institut P', Chasseneuil - Futuroscope, France

Slip traces (ST) have been used in the past in an attempt to identify the dislocation movements controlling plastic deformation, but the observations were limited to the micro-, at best nano-, scale that do not allow a direct comparison with the elementary theoretical dislocation processes. The development since the 90's of scanning probe microscopy now allows imaging surface features at the atomic scale. A decisive step was recently taken with the development of an unique experimental device that allows us to follow by scanning tunnelling microscopy (STM) the developments of sample surfaces under applied plastic strain over a wide temperature range. Atomic scale analysis of ST structures provides a unique opportunity of directly comparing experimental and theoretical elementary dislocation mechanisms.

Ni3(Al,Ta) single crystals were deformed in compression at three temperatures (300 K, 400 K and 600 K) in the temperature domain of the yield stress anomaly (Tp 780 K, Tp is the peak temperature). The observation surface was (111) oriented. It exhibits a 2 x 2 superstructure with a lattice parameter of 0.51 nm, corresponding to the distance between Al atoms. Unfortunately, concomitant observation of atomic structure with ST was never achieved. Nevertheless, the in-plane resolution of the STM images allows us to unambiguously identify atomic scale details on ST.

At all temperatures, ST mainly belong to the primary (111) octahedral planes. Each ST results from a single dislocation. Double cross slip (CS) events between parallel (111) planes are observed at 400 K and 600 K and, in this case, the CS distance onto the (010) CS plane never exceeds the dissociation width of the superdislocations on this plane. The number of CS increases with increasing temperatures, and the majority of CS from (111) planes onto (010) planes are irreversible, i.e., reverse CS from (010) planes onto (111) planes is only observed for incomplete Kear-Wilsdorf (KW) configurations. In other words, once the KW configuration has been reached, superdislocations never cross slip back onto (111) planes, suggesting that the KW configuration is stable on the (010) CS plane. A final remark concerns the slip distance on the (010) CS planes which is always larger than that of the dissociation width of the superdislocations also suggesting that KW configurations are more mobile than expected.

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