study on segregation behavior of alloying elements in titanium alloys during solidification

112 Pages·2009·9.58 MB·English

STUDY ON SEGREGATION BEHAVIOR OF ALLOYING
ELEMENTS IN TITANIUM ALLOYS DURING SOLIDIFICATION
AKIRA KAWAKAMI
B.E., Kyushu University (Japan), 1987
M.E., Kyushu University (Japan), 1989
A THESIS SUBMITTED IN PARTIAL FULFUFILMENT OF
THE REQUIREMENTS FOR THE DEGREE OF
MASTER OF APPLIED SCIENCE
in
THE FACULTY OF GRADUATE STUDIES
(Department of Metals and Materials Engineering)
We accept this thesis as conforming
to the required standard
THE UNIVERSITY OF BRITISH COLUMBIA
May 2002
© Akira Kawakami, 2002
In presenting this thesis in partial fulfilment of the requirements for an advanced
degree at the University of British Columbia, 1 agree that the Library shall make it
freely available for reference and study. I further agree that permission for extensive
copying of this thesis for scholarly purposes may be granted by the head of my
department or by his or her representatives. It is understood that copying or
publication of this thesis for financial gain shall not be allowed without my written
permission.
Department of
The University of British Columbia
Vancouver, Canada
Date 0T/Z2. /OZ.
DE-6 (2/88)
ABSTRACT
A fundamental study was conducted on segregation behavior of alloying elements in
titanium alloys to clarify the formation mechanism of "beta-flecks", melt-related defects
enriched in beta stabilizing elements, which can cause a decrease in mechanical
performance. Commercial titanium alloys, which are prone to the beta-fleck formation,
such as 10-2-3 (Ti-10%V-2%Fe-3%Al), Ti-17(Ti-5%Al-2%Sn-2%Zr-4%Mo-4%Cr) and 6242
(Ti-6%Al-2%Sn-4%Zr-2%Mo) were used. Solidification parameters, such as dendrite arm
spacing, distribution coefficients and densities of solid/liquid phase during solidification,
were investigated in these alloys.
Electron Probe Micro Analysis (EPMA) revealed that periodicity in distribution profiles
of alloying elements corresponded to the primary or secondary dendrite arm spacing both
in laboratory melted 10-2-3 ingots and in production 10-2-3 and Ti-17 ingots. This result
indicates that EPMA is an effective method to clarify the dimensions of dendrite
structures in titanium alloys(no good etching technique has been demonstrated that
resolves the original dendritic structure). Distribution coefficients of alloying elements in
10-2-3, Ti-17 and 6242 were experimentally obtained using a zone-melting furnace.
Distribution coefficients for iron in 10-2-3, zirconium and molybdenum in 6242 were
deviated from the equilibrium distribution coefficients calculated from the binary phase
diagrams. The fraction solidified (fs) at the initiation of beta-flecks was estimated to be 0.9
in 10-2-3 and Ti-17 using the Scheil equation, in which experimentally obtained
distribution coefficients were used. The density of liquid and solid metal at around the
melting point was estimated with the calculation software "METALS" and it was clarified
that solid metal is heavier than liquid enriched with iron in 10-2-3 and that enriched with
ii
chromium in Ti-17.
The Rayleigh number was calculated to exceed 1 when the periodicity of chromium
observed in a Ti-17 production ingot was assumed to be primary dendrite arm spacing.
This fact suggests that the density-driven downward flow of liquid metal can occur. This
in turn could cause channels perpendicular to the solidification direction and lead to the
formation of beta-flecks, and supports the proposed mechanism. However, there are still
some questions about the mechanism, such as the possibility of fluid flow at the final stage
of solidification and the validity of considering the periodicity as primary dendrite arm
spacing.
iii
Table of Contents
Abstract ii
Table of Contents iv
List of Tables vi
List of Figures vii
List of Symbols ix
Acknowledgements xi
1. INTRODUCTION 1
2. LITERATURE REVIEW 3
2.1 SEGREGATION BEHAVIOR OF ALLOYING ELEMENTS IN TITANIUM
ALLOYS(GENERAL) 3
Solidification microstructures in titanium alloys 3
Dendrite arm spacing 4
Solidification parameters 6
Distribution coefficients 6
Solidus and liquidus temperature 8
Solid state diffusion 8
2.2 BETA-FLECKS IN TITANIUM ALLOYS 10
Features of beta-flecks 10
Effects of beta-flecks on mechanical properties 13
Features of freckles 15
Formation mechanisms of freckles and beta-flecks 17
Freckle criterion and its application to beta-flecks 19
2.3 SUMMARY OF LITERATURE REVIEW 22
3. RESEARCH OBJECTIVES 34
4. EXPERIMENTAL METHODOLOGY 36
4.1 CHOICE OF ALLOYS 36
4.2 EXPERIMENTAL METHODS 37
Segregation behavior of iron in 10-2-3 small ingots melted and cast in an
argon arc melting furnace 37
Segregation behavior of alloying elements in production ingots 38
Segregation behavior of alloying elements in laboratory melted small ingots
using a zone melting furnace 39
iv
5. EXPERIMENTAL RESULTS 43
5.1 SEGREGATION BEHAVIOR OF ALLOYING ELEMENTS IN TITANIUM
ALLOY INGOTS SOLIDIFIED IN DENDRITIC MANNER 43
Segregation behavior of iron in laboratory melted 10-2-3 ingots using an
argon arc furnace 43
Segregation behavior of alloying elements in production ingots 45
5.2 SEGREGATION BEHAVIOR OF ALLOYING ELEMENTS IN ZONE
MELTED TITANIUM ALLOY INGOTS 48
5.3 DENSITY OF BETA-FLECK AND LIQUID METAL CALCULATED USING
"METALS" FOR 10-2-3 AND Ti-17 ALLOYS 54
6. DISCUSSION 74
6.1 DETERMINATION OF DENDRITE ARM SPACING BY EPMA 74
The relationship between dendrite arm spacing and solidification conditions
in laboratory-melted ingots using argon an arc furnace 74
Dendrite arm spacing in production ingots 75
6.2 SEGREGATION COEFFICIENTS OF ALLOYING ELEMENTS IN
COMMERCIAL TITANIUM ALLOYS AND ESTIMATION OF THE
FRACTION SOLIDIFIED AT THE INITIATION OF BETA-FLECKS 77
Segregation coefficients of alloying elements in zone-melted commercial
titanium alloys 77
Estimation of the fraction solidified at the initiation of beta-flecks 79
Estimation of distribution coefficients and fraction solidified at the initiation
of beta-flecks with "pseudo-binary phase approach" 80
6.3 FORMATION MECHANISM OF BETA-FLECKS 82
Possibility of downward flow of liquid metal during solidification 82
Problems in the proposed formation mechanism of beta-flecks 84
7. CONCLUSIONS AND FUTURE WORKS 88
7.1 CONCLUSIONS 88
7.2 RECOMMENDATIONS FOR FUTURE WORKS 90
REFERENCES 92
APPENDIX A: MATHEMATICAL MODEL"METALS" 99
v
List of Tables
Table 1 Distribution coefficients of alloying elements in titanium alloys 7
7,20-23
Table 2 Liquidus and solidus temperatures in titanium alloys g
!3,23,24
Table 3 Melting experiment results with an argon arc furnace. 43
Table 4 Composition variations and distribution coefficients (k) in a zone melted
10-2-3 ingot. 53
Table 5 Composition variations and distribution coefficients (k) in a zone melted
Ti-17 ingot. 53
Table 6 Composition variations and distribution coefficients (k) in a zone melted
6242 ingot. 53
Table 7 Composition variations and distribution coefficients (k) for oxygen and
nitrogen in a zone melted 10-2-3 ingot 47. 53
Table 8 Parameters and values used for pseudo-binary phase approach for Ti-17. 81
Table 9 Parameters and values used for pseudo-binary phase approach for 10-2-3. 81
vi
List of Figures
Figure 1 Solidification maps for (a) Ti-6-4 and (b) Ti-17 alloys 14. 24
Figure 2 Dendrite arm spacing in Ti-17 13. 24
Figure 3 Residual segregation index vs. homogenization parameter for
chromium in steel19. 25
Figure 4 Macrostructure of cross-section of a 10-2-3 ingot27. 25
Figure 5 Optical micrograph and scanning fractograph of fractured 10-2-3 5. 26
Figure 6 Effect of beta-fleck area on LCF fife of 10-2-3 5. 26
Figure 7 Typical microstructure of Ti-6-6-2 with beta flecks 28. 27
Figure 8 Various appearances of freckles in industrial castings 30. 28-29
Figure 9 Schematic diagram of directional solidification and associated thermal
(pT), solutal (pc) and thermosolutal (pT+c) density profiles illustrating the
density inversion theory 30. 29
Figure 10 Schematic illustration depicting freckle formation and associated fluid
flow pattern 29. 30
Figure 11 The mechanism of freckle formation showing the sequence of the
density-driven downward-forming channel to form a freckle 36. 30
Figure 12 Schematic illustration of typical curved growth front found in ingot 29. 31
Figure 13 Modified Rayleigh number vs growth front angle for alloy (a)CMSX-HB,
(b)RENE88, (c)NIM80A, (d)IN718-Si, (e)WASPALOY and (f)MAR-M247 29. 32
Figure 14 Calculated Rayleigh numbers for the directionally solidification
experiments for the SX-1 superalloy as a function of the thermal
parameter G-1/2»R-1/4 42 . 33
Figure 15 Photograph of an argon arc melting furnace in AMPEL. 41
Figure 16 Schematic diagram of an argon arc melting furnace in AMPEL. 42
Figure 17 Macrographs of 10-2-3 ingots melted by an argon arc furnace.
(a)No.l (b)No.2 (c) No.3 (d) No.5 (e) No.7 56,57
Figure 18 Distribution of iron concentration in the horizontal direction in a
10-2-3 lboratory-melted ingot. 58
Figure 19 Distribution of iron concentration in the direction inclined to the
horizontal direction by 30 ° in a 10-2-3 laboratory-melted ingot. 58
Figure 20 Distribution of iron concentration in the direction inclined to the
horizontal direction by 45 ° in a 10-2-3 laboratory-melted ingot. 59
Figure 21 Distribution of iron concentration in the direction inclined to the
horizontal direction by 60 ° in a 10-2-3 laboratory-melted ingot. 59
Figure 22 Evolution of temperature with time during solidification in
Commercially Pure Titanium melted in an argon arc furnace. 60
Figure 23 Macrograph of a Ti-17 production ingot (as-received). 61
Figure 24 Microstructure of a Ti-17 production ingot. 61
Figure 25 Distribution of chromium concentration in a Ti-17 production ingot. 62
Figure 26 Estimated distribution of chromium concentration in a Ti-17 production
ingot just after solidification. 62
Figure 27 Macrograph of a 10-2-3 production ingot (as-received). 63
Figure 28 Distribution of iron concentration in the direction inclined to the
horizontal direction by 60 ° in a 10-2-3 production ingot. 63
Figure 29 Distribution of iron concentration in a 10-2-3 production ingot. 64
Figure 30 Macrographs of zone melted samples (as received). 65
Figure 31 Cross-sectional macrographs of a zone melted 10-2-3 ingot. 66
vii
Figure 32 Cross-sectional macrographs of a zone melted Ti-17 ingot. 67
Figure 33 Cross-sectional macrographs of a zone melted 6242 ingot. 68
Figure 34 Concentration distribution of alloying elements in the longitudinal
direction of a zone melted 10-2-3 ingot. 69
Figure 35 Concentration distribution of alloying elements in the longitudinal
direction of a zone melted Ti-17 ingot. 69
Figure 36 Concentration distribution of alloying elements in the longitudinal
direction of a zone melted 6242 ingot. 70
Figure 37 Concentration distribution of alloying elements in the longitudinal
direction of a zone melted 10-2-3 ingot (original data). 70
Figure 38 Distribution of oxygen concentration in the longitudinal direction of a
zone melted 10-2-3 ingot 47. 71
Figure 39 Distribution of nitrogen concentration in the longitudinal direction of a
zone melted 10-2-3 ingot 47. 71
Figure 40 Effect of iron concentration on density of the liquid and solid phase at
around melting temperature (1905 K) in the 10-2-3 alloy. 72
Figure 41 Effect of temperature on density of the liquid and solid phase in the
10-2-3 alloy. 72
Figure 42 Effect of chromium concentration on density of the liquid and solid
phase at around melting temperature (1914 K) in the Ti-17 alloy. 73
Figure 43 Effect of temperature on density of the liquid and solid phase in the
Ti-17 alloy. 73
Figure 44 Effect of primary dendrite arm spacing on the Rayleigh number in a
Ti-17 production ingot. 87
Figure 45 Effect of primary dendrite arm spacing on the Rayleigh number in a
10-2-3 production ingot. 87
viii
List of Symbols
Symbols
C Solute Concentration (wt.%)
Co Equilibrium Solute Concentration (wt.%)
CM Maximum Solute Concentration of Component i at Time th(wt.%)
Cm Minimum Solute Concentration of Component i at Time th(wt.%)
C°M Maximum Initial Solute Concentration of Component i (wt.%)
C°m Minimum Initial Solute Concentration of Component i (wt.%)
Cave Average Concentration of Alloying Element in the Matrix (wt.%)
Cmax Maximum Concentration of Alloying Element Detected by EDX (wt.%)
Cmin Minimum Concentration of Alloying Element Detected by EDX (wt.%)
CL1,CL2 Liquid Composition in a Phase Diagram (wt.%)
D Interdiffusion Coeficient in the Solid State (m2/s)
s
DT Thermal Diffusivity (m2/s)
f Fraction Solidified
s
g Gravitational Acceleration (m/s2)
G Thermal Gradient (K/m)
GFV Vertical Temperature Gradient (K/m)
GL Temperature Gradient of the Liquidus (K/m)
h Characteristic Linear Dimension (m)
hm Height of the Mushy Zone (m)
k Distribution Coefficient
keq Equilibrium Distribution Coefficient
K Permeability in the Vertical Direction (m2)
Km Mean Permeability (m2)
K Permeability Parallel to the Primary Dendrite Trunks (m2)
y
L Half of the Dendrite Arm Spacing (m)
Nf Cycle Number
R Solidification Rate (K/s)
Ra, RaT/s Rayleigh Number
Racrit Critical Rayleigh Number
Ra* Modified Rayleigh Number
t Time (s)
t Total Solidification Time (s)
s
T Temperature (K)
Tl,T2 Temperature in a Phase Diagram (K)
Tbuik Bulk Alloy Transformation Temperature (K)
TL Liquidus temperature (K)
Ts Solidus Temperature (K)
V Withdrawal Rate (m/s)
ix