Effects of Carbon Content, Annealing Condition and Internal Defects on the nucleation, growth and coarsening of P-type Carbides in High Niobium Containing TiAl Alloys

2014 ◽  
Vol 1760 ◽  
Author(s):  
Li Wang ◽  
Florian Pyczak ◽  
Heike Gabrisch ◽  
Uwe Lorenz ◽  
Mathias Münch ◽  
...  
Keyword(s):  
Grain Boundary ◽  
Carbon Content ◽  
Grain Boundaries ◽  
Carbon Solubility ◽  
Tial Alloys ◽  
Free Zone ◽  
Transmission Electron ◽  
Crystallographic Defects ◽  
P Type ◽  
Nucleation Growth

ABSTRACTThe nucleation, growth and coarsening of carbides is investigated in high niobium containing TiAl alloys by diffraction and transmission electron microscopy. Higher carbon content increases the dissolution temperature of carbides. The solubility of carbon is much higher in a γ/α2-phase alloy than in the γ phase alone. Hereby no significant influence of Nb on carbon solubility is found. Crystallographic defects as grain boundaries and dislocations promote carbide nucleation which results in a carbide precipitation sequence starting first at grain boundaries, then at dislocations and only later in the γ matrix away from crystallographic defects. The consumption of carbon by grain boundary carbides or neighboring α2 grains also generates a precipitate free zone in γ grains near the grain boundary.

scholarly journals The Role of Grain Boundary Precipitates during Intergranular Fracture in 6xxx Series Aluminium Alloys

Metals ◽  
2021 ◽  
Vol 11 (6) ◽  
pp. 894
Author(s):  
Inga G. Ringdalen ◽  
Ingvild J. T. Jensen ◽  
Calin D. Marioara ◽  
Jesper Friis
Keyword(s):  
Tensile Strength ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Intergranular Fracture ◽  
Aluminium Alloys ◽  
Density Functional ◽  
Tensile Tests ◽  
Free Zone ◽  
Transmission Electron

During ageing, 6xxx aluminium alloys will develop a microstructure characterised by needle-shaped Mg/Si-rich precipitates in the bulk, precipitate-free zones along the grain boundaries and larger Mg/Si-rich precipitates on the grain boundary. Depending on, among other things, the size of the precipitate-free zone, these alloys are prone to intergranular fracture. The role of the grain boundary precipitates during the initiation and propagation of the intergranular fracture is still not fully understood. Transmission Electron Microscopy has been used to characterise the grain boundaries and grain boundary precipitates. The precipitates were found to be of the β′ type surrounded by a layer of U2 structure. The atomic details of relevant interfaces of Al-β′ were characterised for further investigation. Density Functional Theory simulations were performed on the bulk precipitate structures and on the interfaces obtained experimentally. The decohesion energy of these interfaces was calculated and compared to bulk values. In addition, simulated tensile tests were performed in order to find values for the tensile strength σt. The dependence of the interfacial energy and tensile strength of β′ grain boundary precipitates were found to depend on the orientation and type of interface in addition to the amount of defects on the interface.

scholarly journals Coupling Automated Electron Backscatter Diffraction with Transmission Electron and Atomic Force Microscopies

2000 ◽  
Vol 6 (S2) ◽  
pp. 940-941
Author(s):  
A.J. Schwartz ◽  
M. Kumar ◽  
P.J. Bedrossian ◽  
W.E. King
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Thermomechanical Processing ◽  
Electron Backscatter Diffraction ◽  
Grain Boundary Character Distribution ◽  
Atomic Force ◽  
Transmission Electron ◽  
Electron Backscatter ◽  
Network Engineering ◽  
Backscatter Diffraction

Grain boundary network engineering is an emerging field that encompasses the concept that modifications to conventional thermomechanical processing can result in improved properties through the disruption of the random grain boundary network. Various researchers have reported a correlation between the grain boundary character distribution (defined as the fractions of “special” and “random” grain boundaries) and dramatic improvements in properties such as corrosion and stress corrosion cracking, creep, etc. While much early work in the field emphasized property improvements, the opportunity now exists to elucidate the underlying materials science of grain boundary network engineering. Recent investigations at LLNL have coupled automated electron backscatter diffraction (EBSD) with transmission electron microscopy (TEM)5 and atomic force microscopy (AFM) to elucidate these fundamental mechanisms.An example of the coupling of TEM and EBSD is given in Figures 1-3. The EBSD image in Figure 1 reveals “segmentation” of boundaries from special to random and random to special and low angle grain boundaries in some grains, but not others, resulting from the 15% compression of an Inconel 600 polycrystal.

Observations and implications of grain boundary dislocation networks in high-angle YBa2Cu3O7−δ grain boundaries

1990 ◽  
Vol 5 (5) ◽  
pp. 919-928 ◽  
Author(s):  
S. E. Babcock ◽  
D. C. Larbalestier
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Weak Link ◽  
Coincidence Site Lattice ◽  
Boundary Structure ◽  
High Angle ◽  
Grain Boundary Structure ◽  
Transmission Electron ◽  
Means Of Transmission ◽  
Regular Networks

Regular networks of localized grain boundary dislocations (GBDs) have been imaged by means of transmission electron microscopy in three different types of high-angle grain boundaries in YBa2Cu3O7-δ, implying that these boundaries possess ordered structures upon which a significant periodic strain field is superimposed. The occurrence of these GBD networks is shown to be consistent with the GBD/Structural Unit and Coincidence Site Lattice (CSL)/Near CSL descriptions for grain boundary structure. Thus, these dislocations appear to be intrinsic features of the boundary structure. The spacing of the observed GBDs ranged from ∼10 nm to ∼100 nm. These GBDs make the grain boundaries heterogeneous on a scale that approaches the coherence length and may contribute to their weak-link character by producing the “superconducting micro-bridge” microstructure which has been suggested on the basis of detailed electromagnetic measurements on similar samples.

The Structure of Dislocations in Low-Angle Grain Boundaries in the Diamond Cubic Lattice

1981 ◽  
Vol 5 ◽  
Author(s):  
C.B. Carter
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Stacking Faults ◽  
Burgers Vector ◽  
Cubic Lattice ◽  
Partial Dislocations ◽  
Tilt Boundaries ◽  
P Type ◽  
Twist Boundaries

ABSTRACTDislocations in low-angle tilt boundaries exhibit a wide variety of Burgers vector including a/2<112> a<001> and a<111>. The dislocations are usually dissociated: Shohkley, stair-rod and Frank partial dislocations may each be formed together with associated intrinsic and extrinsic stackingfaults. Dislocations in low-angle {111} twist boundaries are usually assumed to dissociated by a glide mechanism to give two types of extended nodes, known as P–type and K–type, which contain intrinsic and extrinsic stacking-faults respectively. It is shown that dissociation by climb actually occurs for both types of grain boundary.

The Effect of Grain Boundaries on Surface Diffusion Mediated-Planarization of Polycrystalline Cu Films

1995 ◽  
Vol 389 ◽  
Author(s):  
R.A. Brain ◽  
D.S. Gardner ◽  
D.B. Fraser ◽  
H.A. Atwater
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Surface Diffusion ◽  
Cross Sections ◽  
Boundary Surface ◽  
Surface Profile ◽  
Transmission Electron Micrograph ◽  
Cu Films ◽  
Transmission Electron

ABSTRACTIn situ, ultrahigh vacuum anneals were performed to induce Cu reflow at 500°C following deposition of Cu films and a Ta barrier layer on 1 μm wide by 1 μm deep trenches. Transmission electron micrograph cross-sections show profiles which suggest that grain boundaries and surface energy anisotropy significantly affect reflow. The extent of reflow is dependent on the structure of grain boundary-surface intersections, and the surface profile consists of regions of low curvature within grains and with sharp discontinuities in curvature at grain boundaries, a structure that inhibits surface diffusion. We present results showing how the surface diffusion mediated reflow varies with grain boundary groove angle and position, and compare these results with finite-element simulations that model surface diffusion-driven reflow.

Effect of boron on the mechanism of strain transfer across grain boundaries in Ni3Al

1987 ◽  
Vol 2 (4) ◽  
pp. 436-440 ◽  
Author(s):  
G. M. Bond ◽  
I. M. Robertson ◽  
H. K. Birnbaum
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Dislocation Motion ◽  
Strain Transfer ◽  
Undoped Material ◽  
Sudden Failure ◽  
Transmission Electron ◽  
Large Numbers ◽  
Dislocation Sources

The effect of boron on the mechanism of strain transfer across grain boundaries in Ni3Al has been investigated by dynamic recording of events occurring during in-situ straining in the transmission electron microscope. Boundaries in both doped and undoped material can act as effective barriers to dislocation motion, large numbers of dislocations being incorporated into the boundary without any plastic strain occurring in the adjacent grain. In the undoped material, the grain-boundary strain is relieved by the sudden failure of the grain boundary. In the doped material the strain is relieved by the sudden generation and emission of large numbers of dislocations from the grain boundary. This effect may be understood by boron either increasing the grain-boundary cohesion or reducing the stress required to operate grain-boundary dislocation sources, rather than easing the passage of slip dislocations through the grain boundary.

Grain-Boundary Segregation and Phase-Separation Mechanism in Yttria-Stabilized Tetragonal Zirconia Polycrystal

2011 ◽  
Vol 484 ◽  
pp. 82-88
Author(s):  
Koji Matsui ◽  
Hidehiro Yoshida ◽  
Yuichi Ikuhara
Keyword(s):  
Electron Microscopy ◽  
Transmission Electron Microscopy ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Boundary Segregation ◽  
Grain Boundary Segregation ◽  
Cubic Phase ◽  
Phase Boundaries ◽  
Transmission Electron ◽  
Sintering Condition

Microstructure development during sintering in 3 mol% Y2O3-stabilized tetragonal ZrO2 polycrystal (Y-TZP) was systematically investigated in two sintering conditions: (a) 1100-1650°C for 2 h and (b) 1300°C for 0-50 h. In the sintering condition (a), the density and grain size in Y-TZP increased with the increasing sintering temperature. Scanning transmission electron microscopy (STEM) and nanoprobe X-ray energy dispersive spectroscopy (EDS) measurements revealed that the Y3+ ion distribution was nearly homogeneous up to 1300°C, i.e., most of grains were the tetragonal phase, but cubic-phase regions with high Y3+ ion concentration were clearly formed in grain interiors adjacent to the grain boundaries at 1500°C. High-resolution transmission electron microscopy (HRTEM) and nanoprobe EDS measurements revealed that no amorphous or second phase is present along the grain-boundary faces, and Y3+ ions segregated not only along the tetragonal-tetragonal phase boundaries but also along tetragonal-cubic phase boundaries over a width below about 10 nm, respectively. These results indicate that the cubic-phase regions are formed from the grain boundaries and/or the multiple junctions in which Y3+ ions segregated. We termed this process a “grain boundary segregation-induced phase transformation (GBSIPT)” mechanism. In the sintering condition (b), the density was low and the grain-growth rate was much slow. In the specimen sintered at 1300°C for 50 h, the cubic-phase regions were clearly formed in the grain interiors adjacent to the grain boundaries. This behavior shows that the cubic-phase regions were formed without grain growth, which can be explained by the GBSIPT model.

Preferred inclinations of grain boundaries in epitaxially grown bicrystalline thin films of gold

1978 ◽  
Vol 36 (1) ◽  
pp. 434-435
Author(s):  
F. Cosandey ◽  
Y. Komem ◽  
C. L. Bauer
Keyword(s):  
Grain Boundary ◽  
Grain Boundaries ◽  
Boundary Energy ◽  
Boundary Plane ◽  
Boundary Structure ◽  
Structural Elements ◽  
Grain Boundary Energy ◽  
Transmission Electron ◽  
A New Technique ◽  
Misorientation Of Grains

Energy and concomitant structure of grain boundaries are related to inclination of the boundary plane as well as misorientation of grains defining the boundary. Although increasing information is becoming available on variation of grain boundary energy with misorientation, still relatively little is known about variation of grain boundary energy with inclination. The purpose of this research is to examine preferred inclinations of preselected grain boundaries in gold by transmission electron microscopy (TEM) in order to identify principal structural elements and to relate these elements to the energy of special grain boundary configurations.Grain boundaries examined in this research are produced by a new technique involving vapor deposition of gold on common (001) surfaces of bicrystalline substrates of NaCl, characterized by preselected rotation about a common [001] axis, and subsequent epitaxial growth to form a bicrystalline thin film. These films are then removed from their substrates and examined by TEM. The principal advantage of this technique is that the grain boundary is formed during the deposition and growth process, thus resulting in a more perfect boundary structure while eliminating necessity of a separate bonding operation.

Grain Boundary Responses to Local and Applied Stress: An In Situ TEM Deformation Study

2006 ◽  
Vol 976 ◽  
Author(s):  
Bryan Miller ◽  
Jamey Fenske ◽  
Dong Su ◽  
Chung-Ming Li ◽  
Lisa Dougherty ◽  
...  
Keyword(s):  
Electron Microscope ◽  
Grain Boundary ◽  
Grain Boundaries ◽  
Grain Growth ◽  
Transmission Electron Microscope ◽  
In Situ Tem ◽  
Deformation Twins ◽  
Transmission Electron ◽  
Dislocation Interactions

AbstractDeformation experiments at temperatures between 300 and 750 K have been performed in situ in the transmission electron microscope to investigate dislocation interactions and reactions with grain boundaries and other obstacles. Dislocations, both partial and perfect, as well as deformation twins have been observed being emitted from grain boundaries and, in some cases, even the same grain boundary. The ejection of dislocations from the grain boundary can result in its partial or total annihilation. In the latter case, the disintegration of the grain boundary was accompanied by grain growth and a change in misorientation.

The Slip Transfer Process Through Grain Boundaries in HCP Ti

1993 ◽  
Vol 319 ◽  
Author(s):  
J. Shirokoff ◽  
I.M. Robertson ◽  
H.K. Birnbaum
Keyword(s):  
Grain Boundary ◽  
Slip System ◽  
Grain Boundaries ◽  
Slip Systems ◽  
Boundary Dislocation ◽  
Burgers Vector ◽  
Slip Transfer ◽  
Transmission Electron ◽  
Fcc Materials

AbstractInformation on the mechanisms of slip transfer across grain boundaries in an HCP α-Ti alloy has been obtained from deformation experiments performed In situ in the transmission electron microscope. Initially, lattice dislocations are accommodated within the grain boundary until a critical local dislocation density is reached. The boundary then responds by activating slip in the adjoining grain on the slip system experiencing the highest local resolved shear stress and producing the residual grain-boundary dislocation with the smallest Burgers vector. Slip on secondary slip systems may be initiated provided they reduce the magnitude of the Burgers vector of, or eliminate, the residual grainboundary dislocation. The selection rules used to predict the slip system activated by the grain boundary are the same as apply in ordered and disordered FCC materials.

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