Dislocation Mechanisms during High Temperature Creep Experiments on MC2 Alloy

Abstract

Abstract.The creep behaviour of MC2 single crystal superalloy has been studied at 1150°C/80 MPa, with an applied load along [001] axis. The resulting dislocation microstructures were examined by transmission electron microscopy. The occurrence of a[010] type dislocations (with a zero Schmid factor) within the ordered γ' precipitates is often observed. It is shown that those dislocations moved by a climb process, based on a mechanism involving two dislocation systems and vacancy exchanges, as proposed in the literature. We calculate the vacancy fluxes associated with such a mechanism and show that the vacancy transportation can be easily insured by a simple diffusion process. This calculation shows that the diffusion and climbing steps do not seem to be the creep rate controlling mechanisms for those situations in MC2 alloy.

High-temperature creep of single-crystal nickel-based superalloy: microstructural changes and effects of thermal cycling

“…The diffusion data show that for T = 950°C and 1150°C, this flux is more than large enough to ensure the transportation of vacancies from system 1 to system 2 and thus can ensure condition ii) as stated above. The thermal diffusion to pre-existing pores, which has been proposed as a alternative vacancy sink [16], was shown to be much more difficult since the distance to be travelled by the vacancies was in this case much larger than the raft thickness [44]. As speculated in the literature [16,30], we may assume that the density of sinks, that is the density of system 2, climbing a[100] dislocations, may be the key point.…”

“…We recently calculated [44] the flux of vacancies generated by their diffusion between the γ/γ' interface (where an oversaturation is due to the activity of system 1 dislocations) and the centre of γ' rafts assumed to be at thermal equilibrium. The diffusion data show that for T = 950°C and 1150°C, this flux is more than large enough to ensure the transportation of vacancies from system 1 to system 2 and thus can ensure condition ii) as stated above.…”

Creep of a nickel-based single-crystal superalloy during very high-temperature jumps followed by synchrotron X-ray diffraction

“…This velocity was previously estimated to be 0.22 nm s À1 in Ref. [37]. During this intermediate stage and supposing that the plastic strain in the z direction goes on, dislocations will travel on the average half-length of a c 0 raft and the plastic strain of the c 0 -rafts in the x or y direction will be:…”

“…Second, the local stress field of these dislocations protects the c 0 precipitates from deformation by ah1 0 0i superdislocations [22,32,33]. Such superdislocations are believed to be responsible for the acceleration of the creep deformation by relaxing coherency stresses and osmotic forces as they allow the removal of a/2h1 1 0i dislocations present at the c/c 0 interfaces [28,[34][35][36][37][38][39]. To a lesser extent, c/c 0 interface dislocations play also a role in the local chemical composition of the c phase during creep tests leading to accelerated microstructural evolutions [40,41], known to be damaging [42,43].…”

“…Stress sensitivity of possible rate-limiting plastic mechanisms of the c 0 phaseAs discussed in Section 4.2, the plastic behavior of the c 0 phase of a superalloy requires the climb of dislocations from different slip systems to exchange vacancies. Depending on the rate-limiting mechanisms (climb mobility of dislocations, flux of vacancies between dislocations[37], entry of dislocations into the c 0 -rafts, multiplication and/exhaustion of dislocations, etc.) and on the different climb systems concerned, we may expect a different dependence of the plastic behavior on the components of the stress tensor within the c 0 -rafts: -if the rate-limiting mechanism concerns dislocations with a a c 0 [0 0 1] Burgers vector, we should detect a strong dependence of the strain rate on the applied stress r a ; -if the strain rate is limited by the mobility of the a c 0 [1 0 0]…”