Subscribe to the gold package and get unlimited access to Shamra Academy
Register a new userUltra high-temperature deformation in a single crystal superalloy: Meso-scale process simulation and micro-mechanisms
117
0
0.0
(
0
)
Ask ChatGPT about the research
No Arabic abstract
A mesoscale study of a single crystal nickel-base superalloy subjected to an industrially relevant process simulation has revealed the complex interplay between microstructural development and the micromechanical behaviour. As sample gauge volumes were smaller than the length scale of the highly cored structure of the parent material from which they were produced, their subtle composition differences gave rise to differing work hardening rates, influenced by varying secondary dendrite arm spacings, gamma-prime phase solvus temperatures and a topologically inverted gamma/gamma-prime microstructure. The gamma-prime precipitates possessed a characteristic `X morphology, resulting from the simultaneously active solute transport mechanisms of thermally favoured octodendritic growth and N-type rafting, indicating creep-type mechanisms were prevalent. High resolution-electron backscatter diffraction (HR-EBSD) characterisation reveals deformation patterning that follows the gamma/gamma-prime microstructure, with high geometrically necessary dislocation density fields localised to the gamma/gamma-prime interfaces; Orowan looping is evidently the mechanism that mediated plasticity. Examination of the residual elastic stresses indicated the `X gamma-prime precipitate morphology had significantly enhanced the deformation heterogeneity, resulting in stress states within the gamma channels that favour slip, and that encourage further growth of gamma-prime precipitate protrusions. The combination of such localised plasticity and residual stresses are considered to be critical in the formation of the recrystallisation defect in subsequent post-casting homogenisation heat treatments.
rate research
Read More
Deformation twinning in pure aluminum has been considered to be a unique property of nanostructured aluminum. A lingering mystery is whether deformation twinning occurs in coarse-grained or single-crystal aluminum, at scales beyond nanotwins. Here, we present the first experimental demonstration of macro deformation twins in single-crystal aluminum formed under ultrahigh strain-rate ($sim$10$^6$ s$^{-1}$), large shear strain (200$%$) via dynamic equal channel angular pressing. Deformation twinning is rooted in the rate dependences of dislocation motion and twinning, which are coupled, complementary processes during severe plastic deformation under ultrahigh strain rates.
The plastic deformation mechanisms of tungsten carbide at room and elevated temperatures influence the wear and fracture properties of WC-Co hardmetal composite materials. Although the active slip planes and residual defect populations of room-temperature deformed WC have been previously characterised, the relationship between the residual defect structures, including glissile and sessile dislocations and stacking faults, and the active slip modes, which produce slip traces, is not yet clear. Part 1 of this study showed that {10-10} was the primary slip plane at all temperatures and orientations. In the present work, Part 2, crystallographic lattice reorientations of deformed WC micropillar mid-sections were mapped using focused ion beam (FIB) cross-sectioning and electron backscatter diffraction (EBSD). Lattice reorientation axis analysis has been used to discriminate <a> prismatic slip from multiple <c+a> prismatic slip in WC, enabling defect-scale deformation mechanisms to be distinguished, and their contribution to plastic deformation to be assessed, independently of TEM residual defect analysis. In prismatic-oriented pillars, deformation was primarily accommodated by cooperative multiple slip of <c+a> defects at room temperature, and by <a> dislocations at 600 {deg}C. In near-basal oriented pillars, the total slip direction was along <c>. The degree of lattice rotation and plastic buckling in the deformed basal pillar could be explained by prismatic slip constrained by the indenter face and pillar base.
Tungsten carbide cobalt hardmetals are commonly used as cutting tools subject to high operation temperature and pressures, where the mechanical performance of the tungsten carbide phase affects the wear and lifetime of the material. In this study, the mechanical behaviour of the isolated tungsten carbide (WC) phase was investigated using single crystal micropillar compression. Micropillars 1-5 ${mu}$m in diameter, in two crystal orientations, were fabricated using focused ion beam (FIB) machining and subsequently compressed between room temperature and 600 {deg}C. The activated plastic deformation mechanisms were strongly anisotropic and weakly temperature dependent. The flow stresses of basal-oriented pillars were about three times higher than the prismatic pillars, and pillars of both orientations soften slightly with increasing temperature. The basal pillars tended to deform by either unstable cracking or unstable yield, whereas the prismatic pillars deformed by slip-mediated cracking. However, the active deformation mechanisms were also sensitive to pillar size and shape. Slip trace analysis of the deformed pillars showed that {10-10} prismatic planes were the dominant slip plane in WC. Basal slip was also identified as a secondary slip system, activated at high temperatures.
This work is focused on the micromechanical modelling of the low cycle fatigue of the nickel based $gamma/gamma$ superalloy AM1 at high temperature. The nature of the activated slip systems in the different types of channels of the $gamma$ phase is analysed, taking into account the combined effects of the applied and internal stresses. The latter are split into two contributions, misfit stresses and compatibility stresses between the elastic $gamma$ phase and the elasto-plastic $gamma$ phase, which are estimated within a simplified composite approach. Internal stresses may induce slip activity and/or be relaxed by it, which results in a complex sequence of slip activation events in the different channels under increasing applied stress. The consideration of these effects leads to a prediction of the nature and distribution of the active slip systems within the channels in [001] tension, compression and during low cycle fatigue. The resulting microstructural behaviour and its consequences regarding the anisotropic nature of the coalescence of the $gamma$ precipitates are discussed with respect to the available experimental data.
A systematic study of the compression creep properties of a single-crystalline Co-base superalloy (Co-9Al-7.5W-2Ta) was conducted at 950 {deg}C, 975 {deg}C and 1000 {deg}C to reveal the influence of temperature and the resulting diffusion velocity of solutes like Al, W and Ta on the deformation mechanisms. Two creep rate minima are observed at all temperatures indicating that the deformation mechanisms causing these minima are quite similar. Atom-probe tomography analysis reveals elemental segregation to stacking faults, which had formed in the $gammaprime$ phase during creep. Density-functional-theory calculations indicate segregation of W and Ta to the stacking fault and an associated considerable reduction of the stacking fault energy. Since solutes diffuse faster at a higher temperature, segregation can take place more quickly. This results in a significantly faster softening of the alloy, since cutting of the $gammaprime$ precipitate phase by partial dislocations is facilitated through segregation already during the early stages of creep. This is confirmed by transmission electron microscopy analysis. Therefore, not only the smaller precipitate fraction at higher temperatures is responsible for the worse creep properties, but also faster diffusion-assisted shearing of the $gammaprime$ phase by partial dislocations. The understanding of these mechanisms will help in future alloy development by offering new design criteria.
Log in to be able to interact and post comments
comments
Fetching comments
Sign in to be able to follow your search criteria
