Hall–Petch relationship in two-phase TiAl alloys with fully lamellar microstructures

ABSTRACTSpecial ultrafine fully-lamellar microstructures have been found recently in γ-TiAl alloys with 46–48 at.% Al, when they are processed or heat-treated above the α-transus temperature (Tα). Hot-extrusion above Tα also produces a refined colony or grain size. Refined-colony/ultrafine-lamellar (RC/UL) microstructures produce an excellent combination of room-temperature ductility and high-temperature strength in Ti-47Al-2Cr-2Nb (at.%) alloys. UL structures generally have an average interlamellar spacing of 100–200 nm, and have regularly alternating γ and α2 lamellea, such that they are dominated by γ/α2 interfaces with relatively few γ/γ twin boundaries. The focus of this study is how variations in processing parameters or alloy composition affect formation of the UL structure, particularly the α2 component.

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Numerical Simulations of Tensile Deformation Behavior in Two Phase α2 + γ Lamellar Tial Alloys

MRS Proceedings ◽

10.1557/proc-364-169 ◽

1994 ◽

Vol 364 ◽

Author(s):

Bimal K. Kad ◽

Ming Dao ◽

Robert J. Asaro

Keyword(s):

Volume Fraction ◽

Tensile Deformation ◽

Hydrostatic Stress ◽

Fracture Initiation ◽

Two Phase ◽

Tial Alloys ◽

Tensile Deformation Behavior ◽

Mechanical Methods ◽

Fully Lamellar ◽

Stress Buildup

AbstractDeformation microstructures in γ-TiAl + α2Ti3Al based fully lamellar (FL) and nearly lamellar (NL) microstructures have been simulated using micro-mechanical methods. The deformation is extremely inhomogenous resulting in a large accumulation of hydrostatic stresses at the grain boundaries, thereby promoting intergranular fracture initiation. In particular, the increase in ductility with increasing equiaxed γ-grain volume fraction (with compliant deformation characteristics) in nearly lamellar alloys is explained by the reduction of the hydrostatic stress buildup at the boundaries, consequently mitigating fracture.

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Creep of a fine-grained, fully-lamellar, two-phase TiAl alloy at 760{degree}C

10.2172/46701 ◽

1995 ◽

Cited By ~ 1

Author(s):

J.N. Wang ◽

A.J. Schwartz ◽

T.G. Nieh ◽

C.T. Liu ◽

V.K. Sikka ◽

...

Keyword(s):

Tial Alloy ◽

Two Phase ◽

Fine Grained ◽

Fully Lamellar

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Ductile TiAl alloy with a Widmanstätten lamellar structure formed by rapid heating

Journal of Materials Research ◽

10.1557/jmr.2008.0138 ◽

2008 ◽

Vol 23 (4) ◽

pp. 949-953 ◽

Cited By ~ 5

Author(s):

J.P. Cui ◽

M.L. Sui ◽

Y.Y. Cui ◽

D.X. Li

Keyword(s):

Electric Current ◽

Lamellar Structure ◽

Tial Alloy ◽

Rapid Heating ◽

Pulse Treatment ◽

Electric Current Pulse ◽

Microstructural Refinement ◽

Tial Alloys ◽

Refinement Mechanism ◽

Fully Lamellar

Instead of conventional grain-refinement treatments for improving the ductility of fully lamellar TiAl alloys, multiorientational, lamellar, subcolony refinement with good ductility has been achieved simply by using an electric-current pulse treatment. The microstructural refinement mechanism is attributed to the transformation on heating of γ laths in the prior large-grain lamellar structure to Widmanstätten α in several orientations, which on subsequent cooling forms lamellar structure colonies in multiple orientations. This kind of refined multiple-colony lamellar structure was found to enhance the ductility of the TiAl alloy.

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Influence of Step Aging on Creep Behavior and Microstructural Evolution of Fine-Grained Fully Lamellar XD TiAl Alloys

THERMEC 2006 - Materials Science Forum ◽

10.4028/0-87849-428-6.1525 ◽

2007 ◽

pp. 1525-1530

Author(s):

Han Liang Zhu ◽

Dong Yi Seo ◽

Kouichi Maruyama ◽

Peter Au

Keyword(s):

Microstructural Evolution ◽

Creep Behavior ◽

Tial Alloys ◽

Fine Grained ◽

Fully Lamellar

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Microstructural Characterization of α2 + γ Titanium Aluminides

Microscopy and Microanalysis ◽

10.1017/s1431927600010394 ◽

1997 ◽

Vol 3 (S2) ◽

pp. 701-702

Author(s):

D. J. Larson ◽

M. K. Miller

Keyword(s):

Titanium Aluminides ◽

Base Alloy ◽

Significant Proportion ◽

Weight Ratio ◽

High Strength ◽

Microstructural Characterization ◽

Two Phase ◽

Tial Alloys ◽

Boron Doped

Two-phase α2+γ TiAl alloys with microalloying additions, Fig. 1, are of interest due to the high strength-to-weight ratio they can provide in automotive and aircraft applications. In boron-doped α2+γTiAl containing Cr, Nb, and W, the B levels were found to be significantly depleted below the nominal alloy content in both the α2 andγ phases. The boron solubilities in the γ and α2 phases were 0.011 ± 0.005 at. % B and 0.003 ± 0.005 at. % B, respectively in Ti-47% Al-2% Cr-1.8% Nb-0.2% W-0.15 % B that was aged for 2 h at 900°C (base alloy). The majority of the B was in a variety of borides including TiB, TiB2 and a Cr-enriched (Ti,Cr)2B precipitate. With the exception of the smaller (< 50 nm thick) Cr-enriched (Ti,Cr)2B precipitates, Fig. 2, most of the borides were larger than ∼100 nm. A significant proportion of the microalloying additions is in these borides, Table 1.

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Microstructure Transformation of a Refined High Nb Containing TiAl Alloy

Materials Science Forum ◽

10.4028/www.scientific.net/msf.747-748.38 ◽

2013 ◽

Vol 747-748 ◽

pp. 38-43 ◽

Cited By ~ 1

Author(s):

Li Hua Chai ◽

Liang Yang ◽

Jian Peng Zhang ◽

Zhi Yong Zhang ◽

Lai Qi Zhang ◽

...

Keyword(s):

Room Temperature ◽

Tial Alloy ◽

High Strength ◽

Temperature Limit ◽

Tial Alloys ◽

Β Phase ◽

Α Phase ◽

Refined Microstructure ◽

Fully Lamellar ◽

Sem Microstructure

High Nb containing TiAl alloys have been investigated traditionally as potential high temperature structural materials because of their high strength, good oxidation and creep resistance. However, the poor ductility and fracture toughness at room temperature limit their application, which could be improved by controlling microstructure to get refine and homogeneous fully lamellar structure. In this study, a high Nb containing TiAl alloy alloying Mn, B and Y with refined microstructure was produced. The solidification path was analyzed by DSC and SEM microstructure of the alloy was observed, after heating at a certain temperature for 1-24hrs and then quenching in water. The dissolution of β phase was also investigated. The results showed that the β phase could decompose only by heating in single β or near α phase field.

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Elemental Segregation and O-Phase Formation in a Gamma-TiAl Alloy

Materials Science Forum ◽

10.4028/www.scientific.net/msf.941.741 ◽

2018 ◽

Vol 941 ◽

pp. 741-746 ◽

Cited By ~ 1

Author(s):

Heike Gabrisch ◽

Tobias Krekeler ◽

Uwe Lorenz ◽

Marcus Willi Rackel ◽

Martin Ritter ◽

...

Keyword(s):

Tial Alloy ◽

Titanium Aluminides ◽

Orthorhombic Phase ◽

Aircraft Engines ◽

Duplex Microstructure ◽

Two Phase ◽

Chemical Homogeneity ◽

Phase Form ◽

Tial Alloys ◽

O Phase

Titanium aluminides based on the L10 ordered g-phase are promising structural light-weight materials for applications in aircraft engines. Typical compositions for γ-TiAl alloys lie in the range Ti-(44-48)Al (at.-%). For high creep resistance, a two-phase microstructure based on lamellar (α2+γ)-colonies is desirable that may be tuned towards better ductility by introducing pure γ-grains (near lamellar or duplex microstructure).γ-TiAl alloys are often alloyed with niobium for increased oxidation resistance and improved mechanical properties. HEXRD and TEM studies of the alloy Ti-42Al-8.5Nb revealed that the orthorhombic O-phase forms during annealing at 500-650°C. This orthorhombic phase has been known in Nb-rich, Al-lean, α2-based Ti-aluminides since the late 1980ies (Nb> 12.5 at.-%, Al< 31 at.-%) but the finding in γ-based alloys is new.TEM imaging showed that the O-phase is located within α2 lamellae of lamellar (α2+γ)-colonies. O-phase domains and α2 phase form small columnar crystallites based in the α2/γ interface. The columnar crystallites grow parallel to the [0001] direction of the α2 phase and appear as facets when observed along this direction. The evolution of domains and facets with annealing time and the chemical homogeneity of the phases are investigated.The results of STEM imaging show that O-phase domains form during annealing at 550 °C for 8hours or 168 hours. After 168 hours of annealing Nb segregations are observed by EDX mapping within O-phase domains. In comparison, no segregation of niobium is detected after 8 hours of annealing.

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Efect of Cyclic Heat Treatment on Cast Structure of TiAl Alloy

Key Engineering Materials ◽

10.4028/www.scientific.net/kem.586.222 ◽

2013 ◽

Vol 586 ◽

pp. 222-225

Author(s):

Martin Petrenec ◽

Eva Vraspírová ◽

Karel Němec ◽

Milan Heczko

Keyword(s):

Heat Treatment ◽

Columnar Structure ◽

Cyclic Heat Treatment ◽

Two Phase ◽

Cast Structure ◽

Mean Grain Size ◽

Cyclic Heat ◽

The Subject ◽

The Mean ◽

Fully Lamellar

The subject of the article is focused on the refining of the cast structure of gamma TiAl–2Nb alloy using cyclic heat treatment and on the analysis of the grain refining mechanism. Microstructure evolution after applied cycles of heat treatment was characterized using light, laser and electron microscopy and using microhardness tests. Application of five heat treatment cycles during which two phase transformations (eutectoid and alfa-recrystallization reactions) repeatedly took place resulted αin refining of the cast columnar structure. The mean grain-lamellar colony size 0.512 mm was transformed to fully lamellar structure containing gamma and alfa2 phases having the mean grain size 0.229 mm. Lamellae thickness of gamma was not changed while the thickness of alfa2 phase decreased to 78 nm. Refining of alfa2 phase resulted in the increase of the microhardness by 20 %. The recrystallized cast structure obtained by cyclic heat treatment and the knowledge on the mechanisms of the refining the structure were compared and discussed with the literature data.

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