Elastic Microstrains during Tension and Creep of Superalloys: Results from In Situ Neutron Diffraction

Abstract

In situ neutron diffraction experiments were conducted to monitor elastic microstrains during tension and creep of a directionally solidified (DS) superalloy PWA1422 and a single crystal (SX) alloy PWA 1484. The rationale was to obtain insight on internal stress evolution, and thereby improved understanding of deformation mechanisms. The misfit between the gamma (γ) and gamma-prime (γ') phases were determined as a function of temperature and deformation. The microstrains in the tension test were used to obtain the critical resolved shear stress (CRSS) of the phases. Results show a dramatic increase in the CRSS of the γ-phase compared to the bulk, and are consistent with dislocations bowing (Orowan type) between narrow γ-channels. A threshold stress is calculated for dislocation activity in the horizontal channel, and is suggested as a minimum stress for initiating any creep or rafting. Microstrains from the creep test at 900°C/425 MPa are consistent with the formation of dislocation networks, but the data do not suggest that the CRSS is exceeded for cutting of the γ'. Rather, rafting is suggested for the onset of tertiary creep. At 1092°C/120 MPa, the far-field creep strain exhibits an unusual behavior in the pre-steady-state low strain (<0.5%) regime. This is also accompanied with creep acceleration and decrease of elastic strain in γ'. The response is accompanied with the formation of a completely rafted microstructure. An explanation is provided in terms of deposition of actual misfit dislocations preferentially on the vertical interfaces.

Micromechanics of primary creep in Ni base superalloys

“…These results together with the neutron diffraction in situ measurements of lattice parameters (under applied stress and after straining of PM alloy RR 1000, Lu et al, 2008) clearly imply that, regardless of grain orientation the first active populations of dislocations are mobilized to cancel out some of the numerous pre-existing internal elastic strain fields generated within the crystals by the misfit of γ' precipitates of all sizes.…”

“…; it varies widely (± 2.10 -3 ) with the chemical composition of the alloy (Pyczak et al 2005(Pyczak et al , 2009 and the temperature (Lu et al, 2008;Royer et al, 1995) and, more subtly, with plastic strain and internal stress fields as pointed out by Kuhn et al (1991) and experimentally measured with great details by Dirand et al (2013).…”

“…In situ measurements of  and ' lattice parameters during monotonic tensile tests, at temperature, were made by use of neutron diffraction techniques and their variations were interpreted as "load transfers" between the matrix and the precipitates in a PM version of RR1000 (Preuss et al, 2008), as well as in the DS-MAR-M200 (Lu et al, 2008) alloys. Making use of an ingenious experimental set-up for carrying out tensile creep tests at 900°C and above, Lu et al (2008) clearly established that a significant reduction of the elastic strain of the  matrix channels along the tensile direction was taking place during the first 10 hours of creep at 900°C under 425 MPa, on a DS MAR-M200 sample, while it was increasing in the transverse direction. The authors correctly attributed these "load transfers" between the matrix and the'cuboids to the presence of dislocations in the ' interfaces relaxing the misfit generated elastic strain fields in the channels normal to the applied uniaxial stress and, on the other hand, aggravating them in the longitudinally oriented channels.…”

In-Situ Synchrotron HEXRD Study on the Micro-Stress Evolution Behavior of a Superalloy during Room-Temperature Compression

“…The lattice strain of (111) reflection along the LD direction and the (200) reflection along the transverse direction (TD) showed an obvious decrease. This phenomenon might be due to the fact that the slip systems of the grains in favorable orientations were activated [ 22 ]. The dislocations began to slip, and part of the internal stress was released, resulting in lattice strain reduction.…”