The high-temperature mechanical behavior of single-crystal Ni-base superalloys has been formerly studied by in situ triple-crystal synchrotron X-ray diffractometry (TCD). However, the 1/300 s recording frequency does not allow real-time tests. It is shown here that real-time monitoring is possible with far-field diffractometry in transmission. The use of a far-field camera enables one to follow a diffraction spot with high angular precision and high recording speed. This technique allows measurement of the mechanical response of an AM1 Nibase single-crystal superalloy following steep load jumps and relaxations during high-temperature creep tests. Local crystal misorientation is revealed and rafting (oriented coalescence) is examined. This new technique is compared with TCD, in order to highlight its benefits and drawbacks.

The good mechanical resistance to high temperature creep of [001] oriented single crystal superalloys is due to the properties of the rafts, i.e. platelets of the L12 γ' phase embedded in a γ matrix. At temperatures higher than 900°C, the plastic strain of the rafts results from the climb of dislocation pairs with a total. < 100 > Burgers vector and/or from the climb at the γ/γ' interfaces of 2 ⁄. < 110 > dislocation segments. This climb motion involves the exchange of vacancies between these dislocations and vacancy sinks such as pores and the specimens' surfaces. In this paper we suggest that the entry of. < 100 > into the rafts requires the overcoming of a threshold stress and show that this hypothesis gives a natural explanation to some of the most salient aspects of their mechanical behavior.

The high-temperature creep behavior of a rafted [001] oriented AM1 Ni-based single crystal superalloy was investigated during in situ creep tests on synchrotrons. Experiments were performed at constant temperatures under variable applied stress in order to study the response (plastic strain, load transfer) to stress jumps. Using two different diffraction techniques in transmission (Laue) geometry, it was possible to measure the average lattice parameters of both the γ matrix and the γ′ rafts in the [100] direction at intervals shorter than 300 s. The absolute precision with both diffraction techniques of the constrained transverse mismatch (in the rafts’ plane) is about 10−5. After stress jumps, special attention is given to the evolution of plastic strain within the γ channels. The relaxation of the Von Mises stress at leveled applied stress shows evidence of dislocation multiplication within the γ channels. From the analysis, we showed an interaction between plastic stress and dislocation density of the γ phase.