scholarly journals Interfacial Properties and Mechanical Behavior of Titanium Aluminides

1997 ◽  
Vol 492 ◽  
Author(s):  
M. H. Yoo ◽  
C. L. Fu

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.

2000 ◽  
Vol 646 ◽  
Author(s):  
Fritz Appel

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.


1994 ◽  
Vol 364 ◽  
Author(s):  
Fritz Appel ◽  
Uwe Lorenz ◽  
Tao Zhang ◽  
Richard Wagner

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.


1995 ◽  
Vol 404 ◽  
Author(s):  
F. Appel ◽  
U. Christoph ◽  
U. Lorenz ◽  
D R. Wagner

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.


2012 ◽  
Author(s):  
Ellen Cerreta ◽  
Saryu Fensin ◽  
Juan P. Escobedo ◽  
George Thompson Gray III ◽  
Adam Farrow ◽  
...  
Keyword(s):  

Nanomaterials ◽  
2021 ◽  
Vol 11 (2) ◽  
pp. 294
Author(s):  
Jiaqing Li ◽  
Cheng Lu ◽  
Long Wang ◽  
Linqing Pei ◽  
Ajit Godbole ◽  
...  

Hydrogen embrittlement (HE) has been extensively studied in bulk materials. However, little is known about the role of H on the plastic deformation and fracture mechanisms of nanoscale materials such as nanowires. In this study, molecular dynamics simulations are employed to study the influence of H segregation on the behavior of intergranular cracks in bicrystalline α-Fe nanowires. The results demonstrate that segregated H atoms have weak embrittling effects on the predicted ductile cracks along the GBs, but favor the cleavage process of intergranular cracks in the theoretically brittle directions. Furthermore, it is revealed that cyclic loading can promote the H accumulation into the GB region ahead of the crack tip and overcome crack trapping, thus inducing a ductile-to-brittle transformation. This information will deepen our understanding on the experimentally-observed H-assisted brittle cleavage failure and have implications for designing new nanocrystalline materials with high resistance to HE.


1996 ◽  
Vol 2 (3) ◽  
pp. 113-128 ◽  
Author(s):  
Sundar Ramamurthy ◽  
Michael P. Mallamaci ◽  
Catherine M. Zimmerman ◽  
C. Barry Carter ◽  
Peter R. Duncombe ◽  
...  

Dense, polycrystalline MgO was infiltrated with monticellite (CaMgSiO4) liquid to study the penetration of liquid along the grain boundaries of MgO. Grain growth was found to be restricted with increasing amounts of liquid. The inter-granular regions were generally found to be comprised of a two-phase mixture: crystalline monticellite and a glassy phase rich in the impurities present in the starting MgO material. MgO grains act as seeding agents for the crystallization of monticellite. The location and composition of the glassy phase with respect to the MgO grains emphasizes the role of intergranular liquid during the devitrification process in “snowplowing” impurities present in the matrix.


2011 ◽  
Vol 528 (3) ◽  
pp. 1162-1172 ◽  
Author(s):  
Byoung Ho Lee ◽  
Sung Hyuk Park ◽  
Seong-Gu Hong ◽  
Kyung-Tae Park ◽  
Chong Soo Lee

2017 ◽  
Vol 27 (1) ◽  
pp. 172-183 ◽  
Author(s):  
K.K. SAXENA ◽  
S.K. JHA ◽  
V. PANCHOLI ◽  
G.P. CHAUDHARI ◽  
D. SRIVASTAVA ◽  
...  

2021 ◽  
Author(s):  
Rogert Sorí ◽  
Raquel Nieto ◽  
Margarida L.R. Liberato ◽  
Luis Gimeno

<p>The regional and global precipitation pattern is highly modulated by the influence of El Niño Southern Oscillation (ENSO), which is considered the most important mode of climate variability on the planet. In this study was investigated the asymmetry of the continental precipitation anomalies during El Niño and La Niña. To do it, a Lagrangian approach already validated was used to determine the proportion of the total Lagrangian precipitation that is of oceanic and terrestrial origin. During both, El Niño and La Niña, the Lagrangian precipitation in regions such as the northeast of South America, the east and west coast of North America, Europe, the south of West Africa, Southeast Asia, and Oceania is generally determined by the oceanic component of the precipitation, while that from terrestrial origin provides a major percentage of the average Lagrangian precipitation towards the interior of the continents. The role of the moisture contribution to precipitation from terrestrial and oceanic origin was evaluated in regions with statistically significant precipitation anomalies during El Niño and La Niña. Two-phase asymmetric behavior of the precipitation was found in regions such the northeast of South America, South Africa, the north of Mexico, and southeast of the United States, etc. principally for December-January-February and June-July-August. For some of these regions was also calculated the anomalies of the precipitation from other datasets to confirm the changes. Besides, for these regions was calculated the anomaly of the Lagrangian precipitation, which agrees in all the cases with the precipitation change. For these regions, it was determined which component of the Lagrangian precipitation, whether oceanic or terrestrial, controlled the precipitation anomalies. A schematic figure represents the extent of the most important seasonal oceanic and terrestrial sources for each subregion during El Niño and La Niña.</p>


2021 ◽  
Vol ahead-of-print (ahead-of-print) ◽  
Author(s):  
Chang Chen ◽  
Zhe Zhang ◽  
Ming Jia

PurposeThe purpose of this study is to examine the effect of stretch goals on unethical behavior and explore the mediating role of ambivalent identification and moderating role of competitive psychological climate.Design/methodology/approachA total of 350 MBA students from Northwestern China completed the two-phase survey. The bootstrapping analysis outlined by Hayes was used to assess a moderated mediation model.FindingsThis study found that stretch goals could trigger employees' unethical behavior via ambivalent identification. Competitive psychological climate intensified the relationship between stretch goals and ambivalent identification. Moreover, such a climate aggravated the indirect effect of stretch goals on unethical behavior via ambivalent identification.Practical implicationsOrganizations and managers should use stretch goals prudently and implement measures to reduce the ethical cost.Originality/valueThis study provides unique contributions by identifying ambivalent identification as an important mediator and competitive psychological climate as a boundary condition of stretch goals' disruptive effect on unethical behavior.


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