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Hydrides in thermally charged alpha-2 titanium aluminides
Scripta METALLURGICA et M A T E R I A L I A
Vol.
24, pp. 2 1 3 5 - 2 1 3 8 , 1990 P r i n t e d in the U.S.A.
P e r g a m o n Press plc All rights r e s e r v e d
HYDRIDES IN THERMALLY CHARGED ALPHA-2 TITANIUM ALUMINIDES Ming Gao, J. Bart Boodey*, and Robert P. Wel Lehigh University Bethlehem, Pennsylvania 18015 (Received
August
28,
1990)
Introduction Titanium aluminide intermetallics are being considered for use in hydrogen containing environments at high temperatures because of their attractive high-temperature properties and the
improvements in ductility and fracture toughness in recent years [1,2]. The major concern in the use of this class of materials, however, is the potential for embrittlemenC from exposure to hydrogen [3]. Because titanium hydrides are known to adversely affect the strength and ductility of titanium based alloys, the formation of hydrides in titanium aluminldes from hydrogen absorbed during service at high temperatures upon cooling is of particular concern. Limited published work [4] showed that hydrides formed readily, as thin plates in the a2-phase, upon cooling. These hydrides dissociated, or redissolved, at about 973K. The authors provided no information on the structure of the hydrides or their crystallographic relationships with the matrix [4]. To better understand the nature of hydrides in these materials, this study was undertaken to identify the crystal structure of hydrides formed in a thermally charged a 2titanium alumlnlde (TI-24AI-IINb), and to determine the crystallographic relationships between the hydrides and the matrix. Material and ExDerlmental Procedures An alpha-2 (Ti-24AI-IINb) titanium aluminide was used for this study. The chemical composition 9~ this ~^-alloy is given in Table i. It was solution annealed at ll50°C in vacuum (
Chemical composition of ~2-Ti-24AI-IINb alloy (wt%)
A1
~O
Fe
V
0
~
Ti
13.3
21.4
0.08
0
0.084
0.003
Balance
Specimens for transmission electron microscopy (TEM) were in the form of 0.15-mm-thick by 3.0-mm-diameter discs, and for x-ray diffraction, 0.15-mm-thick by 2.5-mm-wide by 12.5-mm-long strips. All of the specimens were thoroughly cleaned in acetone and methanol, using an ultrasonic cleaner, prior to the thermal treatment in hydrogen. The thermal treatment in hydrogen was carried out in a quartz-tube furnace at 1 bar. The furnace was first flushed with high purity (99.99%) argon for 1 h, and then filled with flowing hydrogen. To remove residual surface oxides, the specimens were pretreaced in hydrogen at 700"C for 1 h. Hydrogen charging was then conducted at 500"C for 6 h. Temperature was controlled to within ±5°C during the hydrogen charging thermal treatment. Transmission electron microscopic examinations were performed using a Phillps 400T electron microscope operated at 120 kV. Selected area diffraction (SAD) and centered dark field images were used for phase identification. Specimens for TLM observations were prepared by *Graduate student in Materials Science and Engineering, Lehigh University; employed by t h e Naval Air Development Center, Warminster, PA
2135 0 0 3 6 - 9 7 4 8 / 9 0 $3.00 + .00 C o p y r i g h t (c) 1990 P e r g a m o n Press
plc
2136
HYDRIDES
IN TIT:~uNIUM A L U M I N I D E S
Vol.
2~,
electrochemical thinning in a twln-jet polisher using a solution of 30ml perchloric acid, methyl alcohol and 175mi n-butyl alcohol at 15 V and -AO°C [5].
~,~o. ii
295mi
X-ray diffraction measurements were used to determine the lattice parameters and crystal structures of the matrix and the hydrides. The measurements were made, using an APDI700 Automated Powder Diffractometer operated at 45 kV and 35 mA with CuK~ radiation, at a scanning rate of 120 s/degree in scanning steps of 0.01 degree. These measurements were complemented by SAD data from transmission electron microscopy. Results and Discussions A representative TEM micrograph and t h e corresponding SAD pattern of the =2-Ti-24AI-IIN'b alloy prior to thermal charging are shown in Fig. i. The microstructure of the alloy consists of ~2-martensite plates that contained many stacking faults and dislocations. X-ray diffraction and SAD analyses showed that the ~_-phase is hexagonal and has a DOl_-type crystal structure. Z The lattice parameters were determlned to be a - 0.578 nm and c - 0.~67 nm, which agreed well with those reported in the literature (see Table 2) [6,7].
Z II [012] FIG. i
Representative brlght-fleld TEM micrograph and SAD pattern of an u.-Ti-24Al-llNb alloy showing (a) martensite plates and (b) DOI9 dlffractlon spots (Area A).
Table 2. Comparison of lattice parameters of =9-Ti-24AI-IINb alloy (without hydrogen) from x-ray diffraction measurements (nm)
This Study
Ence & Margolin
[6]
Blackburn
a
0.578
0.576
0.5765
c
0.467
0.465
0.4625
[7]
Following hydrogen charging (to about 2,500 wppm), dense distributions of hydrides were observed in this ag-Ti-24AI-IINb alloy. Typical TLM micrographs showing disclnc= hydride phases are given in Fig.'2. These hydrides were oriented in different but specific crystallographic directions. From the x-ray diffraction and SAD analyses, these hydrides were determined to be body centered tetragonal (bct), with lattice parameters of a - 0.306 nm and c 0.A05 nm, Table 3. This structure corresponds to that of the TiH 2 type hydride reported by Jaffe [8]. In addition, the lattice parameters of the u2-phase were increased by hydrogen charging. The values following charging and the change in relation to those prior to charging (see Table 2) are given in Table 3. A
representative
SAD pattern and the corresponding
superposition of [l120]a 2 and [0Ol]TiH 2
Vol.
24, No.
Ii
HYDRIDES
IN T I T A N I U M
ALUMINIDES
2137
standard projections are shown in Figs 3 and 4, respectively. From the SAD patterns, orienCatlon relationships for the hydrides in the ~2-matrlx were determined (0001)Q2//(II0)TiH 2 and [II20]~2//[O01]TIH2. Table 3. Lattice parameters of the hydride phase and the matrix in a?-Ti-24Al-llNb alloy (after hydrogen charging) from x-ray diffraction measurements (nm)
TIH 2 Type Hydrides
This Study
Jaffa
~2 " Matrix
[8]
After Charging
Expansion due to Hydrogen*
a
0,306
0.312
0.595
2.5%
c
0.405
0.418
0.468
0.27%
*Percent change in relation to a and c values shown in Table 2.
FIG. 2
Representative dr£des
TEM m l c r o g r a p h s o f c r y s t a l l o g r a p h i c a l l y
in the a 2 mlcrostructure
of TI-24AI-IINb
I
i Jh lw
2~'~ :
2~[
hy-
i ~
020
~1
2~2
,,~ ,v
~0
Iw
T
~ --+----o-
•
I]~
Ill
i~0
i-ii
if2
00g 1 ii~
00~ 1 hq
00o ± Ii0
001 ~ lql
002 1 1i"2
-~--=-0~0
FIG. 3
oriented
alloy.
~-_p im
__.~oo
Representative relationSAD pattern showing the crystallographic ship between the matrix and the hydrides. (Diffraction spots from the matrix are indicated by the large spots in the accompanying sketch.)
the as
2138
HYDRIDES
IN T I T A N I U M
ALUMINIDES
Vol.
O
FIG. 4
24,
No,
Ii
al'Ti3A[
Superposltion of standard projections for [I120]~ 2 and Indices for TiH 2 are denoted by underline.
[001]TiH 2.
From the measured lattice parameters and orientation relationships, misfit strains associated with the formation of hydrides can be estimated. The corresponding atomic arrangements of =he hydrides within the matrix can be determined as well. The information would provide insight on the mechanisms for hydride formation in the ~2-titanium aluminides. These calculations are in progress and will be reported in a subsequent paper. Summary To examine hydride ly charged in hydrogen hydrides were observed be of the TiH 2 type, 0.306 nm and c - 0.405 drides were determined
formation in alpha-2 Intermetallics, an ~^-TI-24AI-Nb alloy was thermalat one atmosphere for 6 hours at 500"C. ~ense distributlons of titanium following cooling to room temperature. These hydrides were identified to with a body centered tetragonal structure and lattice parameters a rim. The crystallographic relationships between the matrix and the hyto be (0001)=2//(II0)TiH2 and [II20]=2//[001]TiH 2. Acknowledgement
The authors would like =o thank Dr. Ted Nicholas of AFWAL, Wright Patterson Air Force Base, for providing the material used in this study. Reference~ 1. 2. 3. 4. 5. 6. 7. 8. 9.
H.A. Lipsi~t, "Titanium Aluminides - An Overview', in Proc. of Materials Research Society Svmoosium on Hl~h Temperature Ordered Intermetall~c Al~gys , 39, pp. 351-364 (1985). N.S. Stoloff, "Ordered Alloys for High Temperature Applications", ibld, pp. 3-27. R. Jackson, S.C. Dixon, D.R. Tenney, A.L. Carter and J.R. S=ephens, "Hypersonic Structure and Materials", Aerospace America, pp. 24-30 (Oct. 1987). D,S. Shih, G.K. $carr and G.E. Wasielewski, "Hydrogen Behavior in Ti3AI", 1987 TMS Fall Meeting, Cincinnati, OH (187). M. Blackburn, Trans. ASM, 59, p. 694 (1966). E. Ence and H. Margolin, Trans. AIME, 221, p. 151 (1961). M. Blackburn, Trans. AIME, 239, p. 1200 (1967). R. Jaffe, J. Metals, 8, p. 861 (1956). J.D. Boyd, Trans. ASM, 62, p. 977 (1962).
Vol.
24, pp. 2 1 3 5 - 2 1 3 8 , 1990 P r i n t e d in the U.S.A.
P e r g a m o n Press plc All rights r e s e r v e d
HYDRIDES IN THERMALLY CHARGED ALPHA-2 TITANIUM ALUMINIDES Ming Gao, J. Bart Boodey*, and Robert P. Wel Lehigh University Bethlehem, Pennsylvania 18015 (Received
August
28,
1990)
Introduction Titanium aluminide intermetallics are being considered for use in hydrogen containing environments at high temperatures because of their attractive high-temperature properties and the
improvements in ductility and fracture toughness in recent years [1,2]. The major concern in the use of this class of materials, however, is the potential for embrittlemenC from exposure to hydrogen [3]. Because titanium hydrides are known to adversely affect the strength and ductility of titanium based alloys, the formation of hydrides in titanium aluminldes from hydrogen absorbed during service at high temperatures upon cooling is of particular concern. Limited published work [4] showed that hydrides formed readily, as thin plates in the a2-phase, upon cooling. These hydrides dissociated, or redissolved, at about 973K. The authors provided no information on the structure of the hydrides or their crystallographic relationships with the matrix [4]. To better understand the nature of hydrides in these materials, this study was undertaken to identify the crystal structure of hydrides formed in a thermally charged a 2titanium alumlnlde (TI-24AI-IINb), and to determine the crystallographic relationships between the hydrides and the matrix. Material and ExDerlmental Procedures An alpha-2 (Ti-24AI-IINb) titanium aluminide was used for this study. The chemical composition 9~ this ~^-alloy is given in Table i. It was solution annealed at ll50°C in vacuum (
Chemical composition of ~2-Ti-24AI-IINb alloy (wt%)
A1
~O
Fe
V
0
~
Ti
13.3
21.4
0.08
0
0.084
0.003
Balance
Specimens for transmission electron microscopy (TEM) were in the form of 0.15-mm-thick by 3.0-mm-diameter discs, and for x-ray diffraction, 0.15-mm-thick by 2.5-mm-wide by 12.5-mm-long strips. All of the specimens were thoroughly cleaned in acetone and methanol, using an ultrasonic cleaner, prior to the thermal treatment in hydrogen. The thermal treatment in hydrogen was carried out in a quartz-tube furnace at 1 bar. The furnace was first flushed with high purity (99.99%) argon for 1 h, and then filled with flowing hydrogen. To remove residual surface oxides, the specimens were pretreaced in hydrogen at 700"C for 1 h. Hydrogen charging was then conducted at 500"C for 6 h. Temperature was controlled to within ±5°C during the hydrogen charging thermal treatment. Transmission electron microscopic examinations were performed using a Phillps 400T electron microscope operated at 120 kV. Selected area diffraction (SAD) and centered dark field images were used for phase identification. Specimens for TLM observations were prepared by *Graduate student in Materials Science and Engineering, Lehigh University; employed by t h e Naval Air Development Center, Warminster, PA
2135 0 0 3 6 - 9 7 4 8 / 9 0 $3.00 + .00 C o p y r i g h t (c) 1990 P e r g a m o n Press
plc
2136
HYDRIDES
IN TIT:~uNIUM A L U M I N I D E S
Vol.
2~,
electrochemical thinning in a twln-jet polisher using a solution of 30ml perchloric acid, methyl alcohol and 175mi n-butyl alcohol at 15 V and -AO°C [5].
~,~o. ii
295mi
X-ray diffraction measurements were used to determine the lattice parameters and crystal structures of the matrix and the hydrides. The measurements were made, using an APDI700 Automated Powder Diffractometer operated at 45 kV and 35 mA with CuK~ radiation, at a scanning rate of 120 s/degree in scanning steps of 0.01 degree. These measurements were complemented by SAD data from transmission electron microscopy. Results and Discussions A representative TEM micrograph and t h e corresponding SAD pattern of the =2-Ti-24AI-IIN'b alloy prior to thermal charging are shown in Fig. i. The microstructure of the alloy consists of ~2-martensite plates that contained many stacking faults and dislocations. X-ray diffraction and SAD analyses showed that the ~_-phase is hexagonal and has a DOl_-type crystal structure. Z The lattice parameters were determlned to be a - 0.578 nm and c - 0.~67 nm, which agreed well with those reported in the literature (see Table 2) [6,7].
Z II [012] FIG. i
Representative brlght-fleld TEM micrograph and SAD pattern of an u.-Ti-24Al-llNb alloy showing (a) martensite plates and (b) DOI9 dlffractlon spots (Area A).
Table 2. Comparison of lattice parameters of =9-Ti-24AI-IINb alloy (without hydrogen) from x-ray diffraction measurements (nm)
This Study
Ence & Margolin
[6]
Blackburn
a
0.578
0.576
0.5765
c
0.467
0.465
0.4625
[7]
Following hydrogen charging (to about 2,500 wppm), dense distributions of hydrides were observed in this ag-Ti-24AI-IINb alloy. Typical TLM micrographs showing disclnc= hydride phases are given in Fig.'2. These hydrides were oriented in different but specific crystallographic directions. From the x-ray diffraction and SAD analyses, these hydrides were determined to be body centered tetragonal (bct), with lattice parameters of a - 0.306 nm and c 0.A05 nm, Table 3. This structure corresponds to that of the TiH 2 type hydride reported by Jaffe [8]. In addition, the lattice parameters of the u2-phase were increased by hydrogen charging. The values following charging and the change in relation to those prior to charging (see Table 2) are given in Table 3. A
representative
SAD pattern and the corresponding
superposition of [l120]a 2 and [0Ol]TiH 2
Vol.
24, No.
Ii
HYDRIDES
IN T I T A N I U M
ALUMINIDES
2137
standard projections are shown in Figs 3 and 4, respectively. From the SAD patterns, orienCatlon relationships for the hydrides in the ~2-matrlx were determined (0001)Q2//(II0)TiH 2 and [II20]~2//[O01]TIH2. Table 3. Lattice parameters of the hydride phase and the matrix in a?-Ti-24Al-llNb alloy (after hydrogen charging) from x-ray diffraction measurements (nm)
TIH 2 Type Hydrides
This Study
Jaffa
~2 " Matrix
[8]
After Charging
Expansion due to Hydrogen*
a
0,306
0.312
0.595
2.5%
c
0.405
0.418
0.468
0.27%
*Percent change in relation to a and c values shown in Table 2.
FIG. 2
Representative dr£des
TEM m l c r o g r a p h s o f c r y s t a l l o g r a p h i c a l l y
in the a 2 mlcrostructure
of TI-24AI-IINb
I
i Jh lw
2~'~ :
2~[
hy-
i ~
020
~1
2~2
,,~ ,v
~0
Iw
T
~ --+----o-
•
I]~
Ill
i~0
i-ii
if2
00g 1 ii~
00~ 1 hq
00o ± Ii0
001 ~ lql
002 1 1i"2
-~--=-0~0
FIG. 3
oriented
alloy.
~-_p im
__.~oo
Representative relationSAD pattern showing the crystallographic ship between the matrix and the hydrides. (Diffraction spots from the matrix are indicated by the large spots in the accompanying sketch.)
the as
2138
HYDRIDES
IN T I T A N I U M
ALUMINIDES
Vol.
O
FIG. 4
24,
No,
Ii
al'Ti3A[
Superposltion of standard projections for [I120]~ 2 and Indices for TiH 2 are denoted by underline.
[001]TiH 2.
From the measured lattice parameters and orientation relationships, misfit strains associated with the formation of hydrides can be estimated. The corresponding atomic arrangements of =he hydrides within the matrix can be determined as well. The information would provide insight on the mechanisms for hydride formation in the ~2-titanium aluminides. These calculations are in progress and will be reported in a subsequent paper. Summary To examine hydride ly charged in hydrogen hydrides were observed be of the TiH 2 type, 0.306 nm and c - 0.405 drides were determined
formation in alpha-2 Intermetallics, an ~^-TI-24AI-Nb alloy was thermalat one atmosphere for 6 hours at 500"C. ~ense distributlons of titanium following cooling to room temperature. These hydrides were identified to with a body centered tetragonal structure and lattice parameters a rim. The crystallographic relationships between the matrix and the hyto be (0001)=2//(II0)TiH2 and [II20]=2//[001]TiH 2. Acknowledgement
The authors would like =o thank Dr. Ted Nicholas of AFWAL, Wright Patterson Air Force Base, for providing the material used in this study. Reference~ 1. 2. 3. 4. 5. 6. 7. 8. 9.
H.A. Lipsi~t, "Titanium Aluminides - An Overview', in Proc. of Materials Research Society Svmoosium on Hl~h Temperature Ordered Intermetall~c Al~gys , 39, pp. 351-364 (1985). N.S. Stoloff, "Ordered Alloys for High Temperature Applications", ibld, pp. 3-27. R. Jackson, S.C. Dixon, D.R. Tenney, A.L. Carter and J.R. S=ephens, "Hypersonic Structure and Materials", Aerospace America, pp. 24-30 (Oct. 1987). D,S. Shih, G.K. $carr and G.E. Wasielewski, "Hydrogen Behavior in Ti3AI", 1987 TMS Fall Meeting, Cincinnati, OH (187). M. Blackburn, Trans. ASM, 59, p. 694 (1966). E. Ence and H. Margolin, Trans. AIME, 221, p. 151 (1961). M. Blackburn, Trans. AIME, 239, p. 1200 (1967). R. Jaffe, J. Metals, 8, p. 861 (1956). J.D. Boyd, Trans. ASM, 62, p. 977 (1962).