Evaluation of the jogged-screw model of creep in equiaxed γ-TiAl: identification of the key substructural parameters

Segregation at stacking faults within the γ′ phase of two Ni-base superalloys following intermediate temperature creep

“…Since the energy of these faults should be very large, it has been further hypothesized that the rate of shearing by Shockley partial pairs is controlled by the elimination of the wrong nearest neighbors via local diffusion -a process termed "reordering." A quantitative model based on this hypothesis has been developed by Kartikeyan, et al [7]. The diffusion mediated reordering process can convert the high energy two layer CSF into a low energy SESF.…”

“…The most widely accepted and employed shearing model is based on the premise that pairs of ½ [110] matrix dislocations cooperatively shear the g', necessitating the formation of an anti-phase boundary between these "superpartials." However, there is considerable evidence in the literature that shearing in the g' phase occurs predominantly by formation of stacking faults (SISF and/or SESF) at intermediate temperatures under creep conditions [1][2][3][4][5][6][7][8]. The formation of these stacking faults are likely to be a key recovery process that relieve the build-up of dislocation content within the g channels.…”

“…The second sample is from a single crystal of CMSX-4 subjected to creep at a temperature of 750°C and a stress of 750 MPa with a tensile axis deviated from the [001] direction by 12.7° along an axis 24.4° from the (100) plane. This misalignment produces a single primary slip system having maximal Schmid factor for the [1][2][3][4][5][6][7][8][9][10][11][12](-111) slip system. The test was interrupted after 8.6% creep strain had been accumulated over 8 h. At the point of interruption, the material had completed primary creep and had been undergoing secondary creep for approximately 2 h. Additional information on the CMSX-4 testing can be obtained from reference [10].…”

“…In this sample, a number of faults within both the g and g' phases, as well as mictrotwins are observed. A grain near [1][2][3][4][5][6][7][8][9][10] orientation was selected for analysis. While in this alloy discriminating g and g' phases using HAADF is difficult due to the similar net atomic number of the two phases, and the relatively weak superlattice fringe contrast in the g', stacking faults in the g' are easy to identify because of the significantly enhanced Z-contrast.…”

Plasticité : rappels de base et aspects microscopiques

“…Barret et Nix [29] ont ainsi proposé une modélisation du fluage à partir du mouvement de dislocations vis saturées en crans. Ce modèle a été plus récemment repris et complété pour s'appliquer à d'autres matériaux (TiAl [30], Ti [31] et Zr [32]) en prenant en compte des crans de hauteurs variables. Enfin, dans certaines situations, la déformation plastique est principalement contrôlée par la montée pure de dislocations.…”