Effect of Coherency Stresses on the Stability of Lamellar (α2+γ) Titanium Aluminides

1995 ◽  
Vol 404 ◽  
Author(s):  
F. Appel ◽  
U. Christoph ◽  
U. Lorenz ◽  
D R. Wagner
Keyword(s):  
Titanium Aluminides ◽  
Lamellar Microstructure ◽  
Structural Features ◽  
Strength Properties ◽  
Two Phase ◽  
Structural Applications ◽  
In Situ Heating ◽  
The Stability ◽  
Lamellar Interfaces

AbstractTwo phase titanium aluminides with a lamellar microstructure of the intermetallic phases α2(Ti3Al) and γ (TiAl) are being developed for high temperature structural applications. Due to differences in lattice parameters and crystal structure, coherency stresses and mismatch structures occur at various types of semicoherent interfaces present in the material. The implications of these structural features on the stability of lamellar microstructures were studied by TEM in situ heating experiments. The investigations revealed that the lamellar interfaces serve as sources for perfect and twinning dislocations. The results will be discussed with respect to the observed degradation of the strength properties of the two phase alloys in the intended service temperature range of 900- 1000 K.

Fracture Behaviour of Two-Phase γ-Titanium Aluminides

1994 ◽  
Vol 364 ◽  
Author(s):  
Fritz Appel ◽  
Uwe Lorenz ◽  
Tao Zhang ◽  
Richard Wagner
Keyword(s):  
Fracture Toughness ◽  
Crack Propagation ◽  
Titanium Aluminides ◽  
Process Zone ◽  
Crack Tip ◽  
Lamellar Microstructure ◽  
High Fracture Toughness ◽  
High Fracture ◽  
Two Phase ◽  
Lamellar Interfaces

AbstractTitanium aluminides with a lamellar microstructure consisting of the intermetallic phases ֱ2 (Ti3Al) and γ(TiAl) suffer from brittleness at ambient temperatures but exhibit at the same time a relatively high fracture toughness. This discrepancy indicates particular processes stabilizing crack propagation in the lamellar microstructure. In this context, the toughening mechanisms were investigated in (α2 + γ) TiAl alloys which contained different volume fractions of lamellar colonies. The fracture toughness for crack propagation parallel or across the lamellar interfaces was estimated by using chevron-notched bending bars. Electron microscope studies were performed to characterize the related processes of crack tip plasticity. Special emphasis was paid to the crystallography of crack propagation and to the interaction of crack tips with lamellar interfaces. Accordingly, the lamellar morphology derives some of its toughness from interface-related processes which stabilize crack propagation by deflecting the crack tip and providing the necessary dislocation sources for crack tip shielding in the process zone ahead of the crack tip.

Influence of ion-milling on the T2 phase in Al-2.5%Li-2.5%Cu alloy

1989 ◽  
Vol 47 ◽  
pp. 288-289
Author(s):  
J. R. Reed ◽  
D. J. Michel ◽  
P. R. Howell
Keyword(s):  
Thin Foil ◽  
Icosahedral Symmetry ◽  
Ion Milling ◽  
Cu Alloy ◽  
Thin Foils ◽  
Heat Treated ◽  
Aluminum Solid Solution ◽  
In Situ Heating ◽  
The Stability

The Al6Li3Cu (T2) phase, which exhibits five-fold or icosahedral symmetry, forms through solid state precipitation in dilute Al-Li-Cu alloys. Recent studies have reported that the T2 phase transforms either during TEM examination of thin foils or following ion-milling of thin foil specimens. Related studies have shown that T2 phase transforms to a microcrystalline array of the TB phase and a dilute aluminum solid solution during in-situ heating in the TEM. The purpose of this paper is to report results from an investigation of the influence of ion-milling on the stability of the T2 phase in dilute Al-Li-Cu alloy.The 3-mm diameter TEM disc specimens were prepared from a specially melted Al-2.5%Li-2.5%Cu alloy produced by conventional procedures. The TEM specimens were solution heat treated 1 h at 550°C and aged 1000 h at 190°C in air to develop the microstructure. The disc specimens were electropolished to achieve electron transparency using a 20:80 (vol. percent) nitric acid: methanol solution at -60°C.

Twinning in Crack Tip Plasticity of Two-Phase Titanium Aluminides

2000 ◽  
Vol 646 ◽  
Author(s):  
Fritz Appel
Keyword(s):  
Titanium Aluminide ◽  
Titanium Aluminides ◽  
Crack Tip ◽  
High Resolution Electron Microscopy ◽  
Strength Properties ◽  
Mechanical Twinning ◽  
Atomic Scale ◽  
Two Phase ◽  
Design Concepts ◽  
Crack Tip Plasticity

ABSTRACTIntermetallic titanium aluminides based on γ(TiAl) are prone to cleavage fracture on low index lattice planes. Unfavourably oriented grains may therefore provide easy crack paths so that the cracks can rapidly grow to a length which is critical for failure. The effect of crack tip plasticity on crack propagation in γ(TiAl) was investigated by conventional and high-resolution electron microscopy. Crack tip shielding due to mechanical twinning was recognized as toughening mechanism, which occur at the atomic scale and apparently is capable to stabilize fastly growing cracks. The potential of the mechanism will be discussed in the context of novel design concepts for improving the strength properties of γ-base titanium aluminide alloys.

Phase separation in Fe9S10 crystal

1986 ◽  
Vol 44 ◽  
pp. 462-463
Author(s):  
Thao A. Nguyen ◽  
Linn W. Hobbs
Keyword(s):  
Low Temperature ◽  
Iron Sulfide ◽  
Temperature Phase ◽  
The Past ◽  
Transmission Electron ◽  
In Situ Heating ◽  
Two Phases ◽  
The Stability ◽  
Low Temperature Phase

The low temperature phase relation of iron sulfide compounds Fe1-xS, with composition ranging from FeS to Fe7S8, has been investigated extensively over the past several decades. Despite these efforts conflicting reports on the stability of low temperature phases still exist and major disagreements between proposed phase diagrams remain unresolved. In this paper we report preliminary findings of our effort to determine whether the low temperature iron sulfide compounds form a homologous series Fen-l,Sn n≥ 8 [1] or a solid solution [2]. We have examined the stability of iron sulfide crystal of composition Fe9S10 using in situ heating experiment and image contrast transmission electron microscopy. We have found that Fe9S10 decomposes to two distinct phases. These two phases are labelled as H and K phases.

Microstructures and Mechanical Properties of NiAl+Mo In-Situ Eutectic Composites

1990 ◽  
Vol 194 ◽  
Author(s):  
P. R. Subramanian ◽  
M. G. Mendiratta ◽  
D. B. Miracle ◽  
D. M. Dimiduk
Keyword(s):  
Mechanical Properties ◽  
Solid Solution ◽  
Room Temperature ◽  
Composite System ◽  
Elevated Temperatures ◽  
Two Phase ◽  
Structural Applications ◽  
Temperature Fracture

AbstractThe quasibinary NiAI-Mo system exhibits a large two-phase field between NiAl and the terminal (Mo) solid solution, and offers the potential for producing in-situ eutectic composites for high-temperature structural applications. The phase stability of this composite system was experimentally evaluated, following long-term exposures at elevated temperatures. Bend strengths as a function of temperature and room-temperature fracture toughness data are presented for selected NiA1-Mo alloys, together with results from fractography observations.

The Mechanical Properties of Near-equiatomic B2/f.c.c. FeNiMnAl Alloys

2013 ◽  
Vol 1516 ◽  
pp. 249-254 ◽  
Author(s):  
Xiaolan Wu ◽  
Ian Baker ◽  
Hong Wu ◽  
Paul R. Munroe
Keyword(s):  
Mechanical Properties ◽  
Yield Strength ◽  
Temperature Dependence ◽  
Orientation Relationship ◽  
Annealing Time ◽  
Two Phase ◽  
Chemical Partitioning ◽  
The Matrix ◽  
In Situ Heating

ABSTRACTTwo types of as-cast microstructures have been observed in a series of near-equiatomic FeNiMnAl alloys: 1) an ultrafine microstructure in Fe30Ni20Mn20Al30 [1] and Fe25Ni25Mn20Al30, which consists of (Fe, Mn)-rich B2-ordered (ordered b.c.c.) and (Ni, Al)-rich L21-ordered (Heusler) phases aligned along <100>; and 2) a fine two-phase microstructure in Fe30Ni20Mn30Al20 and Fe25Ni25Mn30Al20, which consists of alternating (Fe, Mn)-rich f.c.c. and (Ni, Al)-rich B2-ordered platelets with an orientation relationship close to f.c.c (002) // B2 (002); f.c.c. [011] // B2 [001] [2]. The phases in Fe25Ni25Mn20Al30 coarsened upon annealing with no significant change in the chemical partitioning. The hardness behavior was studied as a function of the annealing time at 823 K. AnL21-to-B2 transition, which occurred at 573-623K, was observed using in-situ heating in a TEM. After annealing at 973 K for 100 h, needle-shaped clusters of (Fe, Mn)-rich precipitates were observed along the grain boundaries and in the matrix. The temperature dependence of the yield strength of as-cast Fe25Ni25Mn20Al30 was also studied.

Shark-skin inspired surface engineering on intermetallic titanium aluminides for high temperature applications using the fluorine effect

2011 ◽  
Vol 1295 ◽  
Author(s):  
Raluca Pflumm ◽  
Michael Schütze
Keyword(s):  
High Temperature ◽  
Surface Engineering ◽  
Titanium Aluminides ◽  
Single Digit ◽  
New Developments ◽  
Shark Skin ◽  
The Stability ◽  
Micro Structures ◽  
Temperature Applications

ABSTRACTIncreasing demands on technical components for high-temperature applications (e.g. tur-bine blades) promote new developments not only in the field of alloy design, but also in surface engineering. This paper shows that it is possible to structure the surface of intermetallic titanium aluminides in-situ by locally controlled oxidation of the material due to selective doping with fluorine. The aim is to reproduce a shark-skin pattern (parallel riblets with valleys in between) in order to improve the surface aerodynamics. Riblets with widths in the single digit μm range have been generated. The nucleation process, the aspect ratio and the stability of the generated micro-structures are discussed as a function of the substrate composition and the oxidation conditions.

scholarly journals A Study of Recrystallization and Phase Transitions in Intermetallic Titanium Aluminides by In Situ High-Energy X-Ray Diffraction

2007 ◽  
Vol 539-543 ◽  
pp. 1519-1524 ◽  
Author(s):  
Klaus Dieter Liss ◽  
A. Bartels ◽  
Helmut Clemens ◽  
S. Bystrzanowski ◽  
A. Stark ◽  
...  
Keyword(s):  
Phase Transitions ◽  
Titanium Aluminides ◽  
Strain Relaxation ◽  
High Energy ◽  
Internal Stresses ◽  
X Ray Diffraction ◽  
X Ray ◽  
In Situ Heating ◽  
Increasing Temperature

High-energy synchrotron X-ray diffraction is a novel and powerful tool for bulk studies of materials. In this study, it is applied for the investigation of an intermetallic γ-TiAl based alloy. Not only the diffraction angles, but also the morphology of reflections on the Debye-Scherrer rings are evaluated in order to approach lattice parameters and grain sizes as well as crystallographic relationships. An in-situ heating cycle from room temperature to 1362 °C has been conducted starting from massively transformed γ-TiAl which exhibits high internal stresses. With increasing temperature the occurrence of strain relaxation, chemical and phase separation, domain orientations, phase transitions, recrystallization processes, and subsequent grain growth can be observed. The data obtained by high-energy synchrotron X-ray diffraction, extremely rich in information, are interpreted step by step.

scholarly journals Interfacial Properties and Mechanical Behavior of Titanium Aluminides

1997 ◽  
Vol 492 ◽  
Author(s):  
M. H. Yoo ◽  
C. L. Fu
Keyword(s):  
Titanium Aluminides ◽  
Boundary Energy ◽  
Plastic Anisotropy ◽  
Interfacial Fracture ◽  
Two Phase ◽  
Deformation And Fracture ◽  
Crack Tip Plasticity ◽  
Lamellar Boundary ◽  
Lamellar Interfaces

ABSTRACTThe role of various interfaces in deformation and fracture behavior of two-phase TiAl-Ti3Al alloys is analyzed on the basis of the specific interfacial and surface energies determined from ab initio calculations. The propensity of twinning observed in these alloys is consistent with the low true-twin boundary energy. The strong plastic anisotropy reported in TiAl polysynthetically twinned (PST) crystals is attributed partly to the localized slip along lamellar interfaces, thus lowering the yield stress for soft orientations. Interfacial fracture energies are estimated to be the highest for the α2/γ lamellar boundary and the lowest for the 120° rotational γ/γ boundary. The fracture mode mixity plays an important role in the crack-tip plasticity by ordinary slip and true-twinning, leading to translamellar and interfacial fracture.

Stability of the T2 phase during low-dose ion milling

1990 ◽  
Vol 48 (4) ◽  
pp. 964-965
Author(s):  
J. R. Reed ◽  
D. J. Michel ◽  
P. R. Howell
Keyword(s):  
Ion Beam ◽  
Beam Current ◽  
Ion Milling ◽  
Cu Alloy ◽  
Cu Alloys ◽  
In Situ Heating ◽  
Diffraction Patterns ◽  
The Stability ◽  
Argon Ion

Recent studies have shown that the T2 (Al6CuLi3) phase particles in dilute Al-Li-Cu alloys transform to microcrystalline aggregates during TEM examination, during ion-beam thinning, or during in- situ heating in the TEM. Other studies, however, have noted that the T2 phase particles exhibit an ‘apparent’ five-fold symmetry suggesting that microcrystalline or twinned regions, rather than ‘single crystal’ regions, were responsible for the five-fold diffraction patterns. As a consequence, additional work was considered necessary to investigate further the stability of the T2 phase in dilute Al-Li-Cu alloys.The 3-mm diameter TEM disc specimens were prepared from a specially melted Al-2.5%Li-2.5%Cu alloy produced by conventional procedures. The TEM specimen thermal treatment and electropolishing procedures were previously reported. The electropolished disc specimens were examined in a JEOL 200CX microscope operated at 200 kV. Selected disc specimens containing the T2 phase were then subjected to ion beam thinning in a Gatan precision ion-milling system, operated with an argon ion beam, at accelerating voltage of 6 kV and a beam current of lμA.

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