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Microstructure and Processing Ability of ββββ-Solidifying TNM-Based γγγγ-TiAl Alloys
V. Imayev1,2,a, R. Imayev1,b, T. Khismatullin1,c, T. Oleneva1,d, V. Gühter3,e, H.-J. Fecht2,4,f
1Institute for Metals Superplasticity Problems of Russian Academy of Sciences,
Ul. Khalturina 39, 450001 Ufa, Russian Federation
2Institute of Micro- and Nanomaterials, Ulm University, Einstein-Allee 47, 89083 Ulm, Germany
3GfE Metalle und Materialien GmbH, Höfener Str. 45, D-90431, Nürnberg, Germany
4Forschungszentrum Karlsruhe, Institute of Nanotechnology, Weberstraße 5, 76133 Karlsruhe, Germany
ae-mail: [email protected], be-mail: [email protected], ce-mail: [email protected], de-mail: [email protected], ee-mail: [email protected], fe-mail: [email protected]
Keywords: γ-TiAl based alloys, lamellar structure, β(B2) phase, hot workability Abstract. Microstructure and hot workability have been considered for a number of γ-TiAl alloys including β-solidifying TNM alloys. All TNM alloys under study showed improved hot workability in cast condition. As was shown for the Ti-45Al-5Nb-1Mo-0.2B alloy, a critical issue of TNM alloys is room temperature ductility in the conditions with lamellar structure. Introduction
Low processing ability, particularly low hot workability and poor room temperature (RT) ductility and a large scatter in mechanical properties are appear to be the main obstacles for wide industrial application of γ-TiAl based alloys. Fundamentally, it is associated with directed type of interatomic bonding in the γ-TiAl and α2-Ti3Al phase. At the same time, there are other reasons, such as a coarse columnar structure, a sharp casting texture and a high level of dendritic segregation which are often characteristics of ingot-metallurgy γ-TiAl alloys. The TNM alloying design concept has been recently proposed to overcome the mentioned deficiencies [1-5]. This includes the use of alloys solidifying solely via the β-phase alloyed by niobium, molybdenum, boron and other elements. The alloying elements were specially selected, on the one hand, to provide the formation of relatively fine grained (with d≤50 µm) microstructure in a bulk ingot after freezing and cooling, on the other hand, to avoid a serious compromise in high temperature capability. Generally, β-solidifying alloys based on Ti-42…45Al and alloyed by β-stabilizing elements showed wider forging window [6-8], improved rollability in the case of β(B2)+γ+α2 alloys [9] and, on the whole, much higher processing ability in contrast to γ-TiAl alloys solidifying peritectically and based on the (γ+α2) phases. It is due to the relatively fine grained as-cast structure, its improved homogeneity, the presence of the β(B2) phase, which improves workability at elevated temperatures, and insensitivity of the microstructure with respect to small variations in the alloy composition and processing parameters. This paper deals with further development of TNM alloys and its attention is focused on the hot workability and ductility of alloys with different content of β-stabilizing elements and aluminum. Microstructural characterization of different T�M alloys
It is believed that the best balance of mechanical properties in γ-TiAl alloys can be obtained with lamellar microstructures consisting of colonies of γ-TiAl and α2-Ti3Al lamellae. Therefore the first idea concerning the TNM alloys was in the use of the metastable β(B2) phase, which should prevent a strong α grains growth during cooling of γ-TiAl ingot. In this case, the amount of β-stabilizing alloying elements is carefully chosen in order to have the opportunity to dissolve the
Materials Science Forum Vols. 638-642 (2010) pp 235-240Online available since 2010/Jan/12 at www.scientific.net© (2010) Trans Tech Publications, Switzerlanddoi:10.4028/www.scientific.net/MSF.638-642.235
All rights reserved. No part of contents of this paper may be reproduced or transmitted in any form or by any means without the written permission of TTP,www.ttp.net. (ID: 165.123.34.86, University of Pennsylvania Library, Philadelphia, United States of America-10/10/13,11:33:43)
metastable β(B2) phase by final heat treatment. Another idea was to use the alloys with a higher content of β-stabilizing elements and a lower content of aluminum, in which the β(B2) phase is stable. As has been recently demonstrated, such type of alloys can have superplastic properties at T=900-1100°C in the cast condition subjected only to globularization annealing that seems to open up possibilities for sheet rolling in the cast condition [3,4]. Indeed, for example the sheet rolling of low-ductile Ni-based superalloys is feasible under conditions at which the elongation usually does not exceed 30%. There is one more alloy design idea within the TNM concept [3]. It is the alloys with a higher content of β-stabilizing elements and aluminum. These alloys were found to contain a lower amount of the β(B2) and α2 phases that can be favorable for service properties and hot workability. To illustrate the mentioned ideas, three alloys are considered. The alloy compositions and heat treatment regimes are given in Table 1. The ingots were produced by GfE Metalle und Materialien, Germany using VAR technique. Two ingots had a size of ∅120×180 mm, the Ti-47Al-7.1(Nb,Mo,B) ingot was prepared as a 300-gram ingot.
Table 1. The alloy compositions and heat treatment conditions (FC denotes furnace cooling) Alloy [at. %] Heat treatment
Ti-45Al-5Nb-1Mo-0.2B Annealing at T=1300°С (0.5 h), FC, ageing at T=900°С (4 h), FC Ti-43Al-5Nb-2Mo-0.2B Annealing at T=1130°С (20 h) and T=1100°C (50 h), FC Ti-47Al-7.1(Nb,Mo,B) Annealing at T=1200°C (2 h), FC
Fig. 1. BSE images of the Ti-45Al-5Nb-1Mo-0.2B, Ti-43Al-5Nb-2Mo-0.2B and Ti-47Al-7.1(Nb,Mo,B) alloys in (a,c,e) as-cast conditions and (b,d,f) heat treated conditions (see Table 1).
Fig. 1 represents BSE images of the alloys given in Table 1. The heat treatment almost
completely dissolved the metastable β(B2) phase in the Ti-45Al-5Nb-1Mo-0.2B alloy (Figs. 1a,b), that is the idea of using the metastable β(B2) phase for preventing α grain growth during ingot cooling followed by its dissolution is quite realizable. This is in accordance with ref. [2], where the metastable β(B2) phase was almost dissolved in the alloy Ti-43Al-4Nb-1Mo-0.1B by final ageing at T=850°C. One should expect that the obtained microstructural condition can give appropriate service properties and hot workability.
Figs. 1с,d represent the effect of long-term annealing at near the eutectoid temperature, which has been recently revealed in TNM alloys with a higher content of β-stabilizing elements [4]. Owing to the α2⇒β(B2) phase transformation the lamellar (γ+α2) structure was unstable and transformed into globular structure; this annealing leads to an increase of the β(B2) phase amount
a b
e f
c
d
236 THERMEC 2009
and refinement of the microstructure [4]. This type of TNM alloys should be attributed to high-workable γ alloys having high workability and probably rollability.
Figs. 1e,f illustrate the third type of the TNM alloys. In contrast to the Ti-43Al-5Nb-2Mo-0.2B alloy, this alloy is characterized by a lower content of the β(B2) phase in as-cast condition. Annealing at T=1200°C leads to formation of the β(B2) phase precipitations in the interior of (γ+α2) colonies due to the α2⇒β(B2) phase transformation and to destroying the lamellar (γ+α2) structure. EDX analysis not given here showed that these precipitates were stronger than usually reached in molybdenum and niobium. This is because of the fact that the alloy was highly alloyed by these elements and, at the same time, contained a lower amount of the β(B2) phase. One should expect that good service properties and hot workability can be reached in this alloy.
Hot workability
No doubt, that wrought processing for γ-TiAl alloys is more feasible from the economic point of view if the material can be processed “near conventionally”. Meanwhile, temperature gradients throughout the workpiece during hot working should be minimized to reach really homogeneous microstructure in the wrought product. Therefore, isothermal forging is a good alternative way for producing fine grained bulk γ-TiAl materials. In respect of TNM alloys like considered above (at least based on Ti-43Al), forging at temperature below the eutectoid temperature looks quite acceptable because the β(B2) phase amount at T=1000-1100°C is close to that at higher temperatures (up to T=1300°C) [2]. Another thought in favor of isothermal forging below the eutectoid temperature is the fact that Ni-based superalloys can be used as a tool material instead of high-cost Mo-based alloys. To evaluate the hot workability of TNM alloys a number of compression tests was carried out for TNM and other γ alloys at T=1000°C, ε′=5×10-4 s-1 to an engineering strain ε=70%. To do it, small samples of ∅4×8 mm were cut out of 30-gram ingots obtained by arc-melting method.
Table 2 shows the alloy compositions, microstructures of 30-gram γ alloys and the peak flow stress σp obtained via compression tests. Before the compression tests, the binary alloys were HIP’ed at T=1230°C (for detail see elsewhere [10]), the other alloys were heat treated: most ingots were annealed at T=1250°C (τ=0.5 h) then cooled in air and aged at T=900°C (τ=2 h). The alloys Ti-44Al-5Nb-0.2B-0.3C and Ti-42Al-5Nb-0.2B were annealed at Т=1150°С (τ=1 h) and aged at T=900°C (τ=2 h), the alloys Ti-43Al-5Nb-1Mo-0.2B and Ti-43Al-5Nb-2Mo-0.2B were subjected to long-term annealing at T=1100-1130°C followed by furnace cooling and the Ti-47Al-7.1(Nb,Mo,B) alloy was annealed at T=1200°C (τ=2 h) followed by furnace cooling. The mean colony/grain size d was evaluated taking into account the volume fractions of the colonies/grains.
The colony/grain size increased with a decrease of the aluminum content in the case of the binary alloys. d=50-100 µm was obtained in the Ti-(48-50)Al alloys as a result of recrystallization which occurred during HIP’ing [10]. The effect of aluminum on the colony/grain size in the alloys alloyed by niobium and boron should be ascribed to a change in the solidification pathway (at transition from Ti-46Al-5Nb-0.2B to Ti-45Al-5Nb-0.2B) and a decrease of the alpha transus temperature Tα [1]. The effect of niobium on the colony/grain size can be ascribed to lower diffusivity in the alloys with a higher niobium content and to the presence of the metastable β(B2) phase which prevented α grain growth. The effect of alloying elements on the colony/grain size in the multi-component alloys depended on the content of the β-stabilizing alloying elements and Tα. On the whole, the colony/grain size was smaller in the TNM alloys. It is important that almost the same colony/grain size was reached in the lab-scale ingots of the TNM alloys (Figs. 1a-d) [3-5].
To compare the hot workability of the alloys the peak flow stress σp obtained as a result of compression tests was taken into account (Table 2). As a workability criterion the parameter σp√d used by Semiatin et al. was calculated (Fig. 2) [11]. The obtained parameter values correlated with hot working behavior. Particularly, all alloy samples showing the value of σp√d>5 (Ti-(44-47)Al, Ti-46Al-5Nb-0.2B) had rather low workability and were cracked. The samples of the TNM alloys
Materials Science Forum Vols. 638-642 237
showed σp√d≤1.5 and had the highest workability among the alloys under study. The smallest value of σp√d<1 had the alloy with a lower content of aluminum and a higher content of the β-stabilizing elements. Taking into account that the lab-ingots of TNM alloys have almost the same colony/grain size as small ingots [2-5], one can expect that they might be also successfully forged at T=1000°C.
Table 2. Alloy compositions, microstructural characteristics and the peak flow stress σp of different γ alloys (rL, rγ, rγ,β(B2) - the volume fractions of lamellar colonies and equiaxed γ or γ, β(B2) grains)
Alloy [at. %] d [µm] Type of microstructure σp [MPa] Ti-50Al Ti-49Al Ti-48Al Ti-47Al Ti-46Al Ti-45Al Ti-44Al
100 50 50
200 310 530 600
near γ, rL < 10% near γ, rL< 10% duplex, rL =25-30% nearly-lamellar, rγ=20-25% nearly-lamellar, rγ<5% fully-lamellar fully-lamellar
300 260 260 370 360 420 423
Ti-46Al-5Nb-0.2B Ti-45Al-5Nb-0.2B Ti-44Al-5Nb-0.2B Ti-42Al-5Nb-0.2B
550 50 20 8
Fully lamellar Fully lamellar Fully lamellar Near lamellar, rγ<5%
325 388 371 238
Ti-44Al-0.2B Ti-44Al-5Nb-0.2B Ti-44Al-10Nb-0.2B
50 20 15
Fully lamellar Fully lamellar Near lamellar, rγ,β<10%
384 371 301
Ti-45Al-5Nb-1Mo-0.2B Ti-44Al-5Nb-1Mo-0.2B-0.3C Ti-43Al-5Nb-1Mo-0.2B Ti-43Al-5Nb-2Mo-0.2B Ti-47Al-7.1(Nb,Mo,B)
35 20 20 5
25
Near lamellar, rγ,β≈15% Fully lamellar Equiaxed, rL<15% Equiaxed, rL<15% Destroyed lamellar + β(B2) precipitations
225 301 186 225 280
0
1
2
3
4
5
6
7
8
9
10
Fig. 2. Hot workability criterion built for a number of γ-TiAl alloys. TNM alloys showed lowest values of σp×d1/2 that testified to a better hot workability of these alloys. Tensile mechanical properties
The tensile mechanical properties at elevated temperatures were studied for the Ti-43Al-4.5Nb-2Mo-0.2B alloy, which is close to that with the highest workability at T=1000°C (Fig. 2). The Ti-43Al-4.5Nb-2Mo-0.2B ingot with a size of ∅200×320 mm was produced by GfE Metalle und Materialien, Germany using VAR technique. As-cast microstructure consisted of lamellar colonies with a size dc≈20-40 µm and layers of β(B2), γ grains along colony boundaries with dγ,β(B2)≈5-20
σ p×d
1/2 , M
Pa×µ
m1/
2
Ti-
50A
l T
i-49
Al
Ti-
48A
l T
i-47
Al
Ti-
46A
l
Ti-
44A
l
Ti-
46A
l-5N
b-0.
2B
Ti-
45A
l-5N
b-0.
2B
Ti-
44A
l-5N
b-0.
2B
Ti-
42A
l-5N
b-0.
2B
Ti-
44A
l-0.
2B
Ti-
44A
l-5N
b-0.
2B
Ti-
44A
l-10
Nb-
0.2B
Ti-
45A
l-5N
b-1M
o-0.
2B
Ti-
44A
l-5N
b-1M
o-0.
2B-0
.3C
T
i-43
Al-
5Nb-
1Mo-
0.2B
T
i-43
Al-
5Nb-
2Mo-
0.2B
Ti-
45A
l
Ti-
47A
l-7.
1(N
b,M
o,B
)
238 THERMEC 2009
µm. The workpiece of an appropriate size was cut out of the ingot then the flat samples were cut out of the workpiece, ground and polished before testing. All tests were carried out in air. The tensile tests were performed at T=900-1100°C with ε′=8.3×10-4 and 1.7×10-4 s-1, at T=1050°C additionally with ε′=8.3×10-3 and 8.3×10-2 s-1. Fig. 3 represents the temperature and strain rate dependencies of elongation and ultimate tensile strength of the alloy in as-cast condition. δ≈200-310% was reached at T=1050-1100°C. With increasing the strain rate up to ε′≈10-2-10-1 s-1 (at T=1050°C) elongation remained quite high (δ≈50-150%). Microstructure examination of the tested samples showed that high elongation was attained due to occurrence of dynamic recrystallization and the phase transformation α2⇒β(В2) during straining.
Temperature, î Ñ
800 900 1000 1100 12000
100
200
300
400
500
Elo
ngat
ion,
%
0
50
100
150
200
250
300
350
2
1
Temperature, î Ñ
800 900 1000 1100 12000
50
100
150
200
250
300
350
0
100
200
300
400
500
2
1
Strain rate, s-1
10-4 10-3 10-2 10-1
Ulti
mat
e te
nsile
str
engt
h, Ì
Pà
0
100
200
300
400
500
0
50
100
150
200
250
300
350
2
1
Fig. 3. (а,b) Temperature and (c) strain rate (at Т=1050°С) dependencies of elongation and ultimate tensile strength of the alloy Ti-43Al-4.5Nb-2Mo-0.2B in as-cast condition: 1 - elongation, 2 - σUTS.
The tensile mechanical properties at RT were studied for the Ti-45Al-5Nb-1Mo-0.2B alloy in order to understand, whether the TNM concept is effective from the viewpoint of RT ductility. Processing of the alloy and obtained microstructures are described in Table 3.
Table 3. Processing of the Ti-45Al-5Nb-1Mo-0.2B and Ti-43Al-4.5Nb-2Mo-0.2B alloys and obtained microstructures (dс/dγ,β(B2) - colony/grain size, λ - lamellar spacing)
Alloy [at. %] Processing Microstructure
Ti-45Al-5Nb-1Mo-0.2B
1) Annealing at T=1300°С (0.5 h), FC + ageing at T=900°С (4 h), FC
Fully lamellar, d≈60 µm, λ=20-500 nm
2) Annealing at T=1300°С (0.5 h), FC + ageing at T=1100°С (2 h), FC
Lamellar + β(B2), γ, dc≈50 µm, λ=50-1000 nm, dγ,β(B2)≈5-10 µm
3) Forging at T=1000°C (ε′≈10-3 s-1, ε≈70%) + annealing at T=1300°С (0.5 h), FC + ageing at T=1100°С (2 h), FC
Lamellar + γ, β(B2) grains along colony boundaries, dc≈50 µm, λ=50-1500 nm, dγ,β(B2)≈1-10 µm
Fig. 4 represents the tensile mechanical properties obtained for Ti-45Al-5Nb-1Mo-0.2B at T=20
and 800°C. In the heat treated conditions (1 and 2) the alloy demonstrated relatively high ultimate tensile strength, which slightly increased at RT and was the same at T=800°C with increasing the ageing temperature from T=900 to 1100°C. In the thermomechanically treated near lamellar condition (3) the alloy showed lower ultimate tensile strength in comparison with that obtained for other known γ-TiAl alloys in thermomechanically treated lamellar condition [12,13]. This can be ascribed to large lamellar spacings which were obtained in the alloy after ageing at T=1100°C.
Despite the fact that heat treatment resulted in the formation of a homogeneous near lamellar microstructure with a relatively small colony size the alloy showed rather poor RT ductility especially after ageing at T=900°C. Apparently, alloying elements decelerated the kinetics of the α⇒γ phase transformation during cooling that led to the formation of a supersaturated and brittle α2 solid solution and increased internal stresses. An increase in the ageing temperature from T=900 to 1100°C led to the α2⇒γ and α2⇒β(В2) phase transformations that likely equilibrated the material (decreased internal stresses) and resulted in a slight increase of elongation (up to δ=0.8%). Another
a b c
Materials Science Forum Vols. 638-642 239
reason of rather low RT ductility might be associated with the presence of metallurgical defects and rough borides [3]. Thermomechanical treatment broke up the rough borides and reduced the metallurgical defects but elongation was only δ=1%. As was earlier obtained [12], the low-alloyed Ti-44.2Al-2.5(Nb,Cr)-0.4B alloy in the thermomechanically treated lamellar condition gave much higher RT ductility (δ=2.7%) and σUTS=770 MPa. Comparing the Ti-45Al-5Nb-1Mo-0.2B and Ti-44.2Al-3(Nb,Cr,B) alloys, one can conclude that high alloying by niobium and molybdenum is detrimental for RT ductility of γ alloys with lamellar structure. This point needs further verification.
1 2 3
Elo
ngat
ion,
%
0.0
0.2
0.4
0.6
0.8
1.0
1.2
0
200
400
600
800
100020оСδ
σUTS
1 2 30
5
10
15
20
Ulti
mat
e te
nsile
str
engt
h, M
Pa
0
200
400
600
800
1000800оСδ
σUTS
Fig. 4. Tensile mechanical properties of the Ti-45Al-5Nb-1Mo-0.2B alloy: conditions 1-3 are described in Table 3.
Conclusions
The TNM alloy design concept was found to be viable for large ingots with diameter up to 200 mm. Due to relatively fine grained microstructure (d≤50 µm) formed in ingots, they showed improved hot workability at T=1000°C. The Ti-43Al-4.5Nb-2Mo-0.2B alloy showed superplastic properties in as-cast condition at T=900…1100°C (δmax=310%) that seems to open up the possibilities for sheet rolling in as-cast condition. The Ti-45Al-5Nb-1Mo-0.2В alloy with a lamellar structure had reasonable strength, however, the RT ductility was rather low (δ≤1%). Analysis of the obtained results suggests that an increase in the RT ductility of the TNM alloys with lamellar structure can be reached via a decrease in the content of alloying elements. This point needs special attention. Acknowledgements This work was supported by ISTC Project #3073. The 30-gram ingots were produced at the GKSS research center, Germany. The support of this work is gratefully acknowledged.
References
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THERMEC 2009 10.4028/www.scientific.net/MSF.638-642 Microstructure and Processing Ability of β-Solidifying TNM-Based γ-TiAl Alloys 10.4028/www.scientific.net/MSF.638-642.235
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