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Liquid film migration (LFM) in the weld heat affected zone (HAZ) of a ni-base superalloy
Scripta METALLURGICA et M A T E R I A L I A
Vol. 24, pp. 5 3 7 - 5 4 2 , 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 reserved
LIQUID FILM MIGRATION (LFM) IN THE WELD HEAT AFFECTED ZONE (HAZ) OF A NI-BASE SUPERALLOY B.Radhakrlshnan and R.G.Thompson Department of Materials Engineering, University of Alabama at Birmingham, Birmingham, AI 35294 ( R e c e i v e d J u l y 20, 1989) ( R e v i s e d J a n u a r y 3, 1990)
Introduction Alloy 718 is a 7" strengthened nickel-base superalloy(Table i) which is known to suffer from liquation cracking in the heat affected zone(HAZ) of welds(l,2,3,4,5). A careful examination of the grain boundaries in the HAZ of an arc spot weld in alloy 718 (figure i) shows that some of the boundaries appear to exhibit a curvature that changes sign at several points along a boundary facet. Such reverse bending occurs in spite of the natural tendency for the boundaries to remain flat to minimize grain boundary energy. Furthermore, portions of the grains adjacent to the concave side of the boundary appear to etch differently from the rest of the grains. This etching contrast delineates two distinct sets of grain boundaries thus suggesting that the grain boundaries have migrated during the weld thermal cycle. The compositions of these migrated regions are apparently altered during the migration process, as revealed by the etching contrast. The objective of the present study is to establish the mechanism responsible for the migration process.
Methods The alloy 718 samples used in this study were in the form of hot-rolled rods, approximately 5.0mm in diameter. The alloy composltlon(Table I) was within the nominal specifications of the commercial 718 alloy. The gauge-length of the Gleeble samples was approximately 25.0mm. The samples were homogenized at I093"C for 1 hour and aged at 650°C for I0 hours, prior to simulation experiments. The microstructure of the samples prior to thermal cycling consisted of fairly coarse grains of 7 with a dispersion of NbC particles(figure 2). HAZ thermal cycles were simulated using a Duffers 500 Gleeble thermomechanical device. The heating rate used in the simulations (figure 3) was typical of that experienced by the HAZ in common fusion welding processes. The samples were heated to peak temperatures of 1200"C and 1227°C in 8.0 seconds and held isothermally for different lengths of time before quenching in a jet of water. The isothermal hold times were chosen to accentuate the migration process and explore the extent to which LFH occurs at this temperature. The thermal cycle was recorded accurately using the output of a fine wire thermocouple percussion welded to the midsection of the gauge-length. After the simulation experiment, the samples were sectioned transversely at the midpoint of the gauge-length. The transverse sections were prepared for metallography using conventional mounting and polishing techniques and electrolytically etched at room temperature in 10% oxalic acid at 5 volts for 4-6 seconds. The welded junction of the themocouple was seen in all the transverse sections prepared for metallography. Hence, the temperatures recorded were those actually experienced by the transverse sections. Any sample-to-sample variation in temperature due to the presence of a longitudinal temperature gradient was thus eliminated. All the microstructures were recorded at a fixed radial dlstance(0.5 mm) from the periphery, thus eliminating sample-to-sample variation in temperature due to the presence of a radial temperature gradient. The area fraction of the migrated regions was calculated using conventional quantitative microscopy techniques. Scanning electron microscopy was performed using a Phillips 515 SEH. Chemical analysis of the migrated regions was carried out using a Kevex 8000 EDS. Results
The different microstructural stages in the migration of the llquated grain boundaries are seen clearly at a peak temperature of 1227"C, shown in figure 4. The formation of the reversed curvature along the grain boundaries and an adjacent area of solid solution which
537 0 0 3 6 - 9 7 4 8 / 9 0 $ 3 . 0 0 + .00 C o p y r i g h t (c) 1990 P e r g a m o n P r e s s
plc
538
LIQUID
FILM MIGRATION
Vol.
24, No.
3
etches differently from the rest of the matrix are similar to the features in the HAZ of ~he spot weld(flgure I). The carbides show extensive liquation(figure 5), similar to the carbides in the HAZ of the arc spot weld(flgure 1). The area swept out by migration increases with holding time(figure 6), which indicates that migration occurred in the presence of the intergranular liquid. After an isothermal hold of 8.2 seconds, extensive migration of the boundaries has resulted in the formation of several "loops" which enclose the areas swept out by migration. However, in the HAZ of the spot weld, the time available for migration was not sufficient to form these loops. The extent of mlgration(M) is conslderab~y reduced at a peak temperature of 1200"C, as shown in figure 7. The extent of carbide llquatlon is also correspondingly reduced, as seen by comparing figures 7 and 5. Figure 7 shows a thin liquatlon zone at the carbide-matrix interface, which is seen more clearly at a peak temperature Of 1227QC(flgure 5). Figure 8 shows the secondary electron(SE) and the backscattered electron(BSE) images of a migrated region at a peak temperature of 1200"C. Notice that in the BSE image the migrated regions show up in bright contrast against the dark background of the matrix. Quantitative analysis using the EDS(Table 2) shows that the migrated region has 3.6 wt. % or approximately 2.3~at. % excess Nb than the matrix.
Discussion The tendency of intergranular NbC particles to liquate and contribute to cracking in the weld heat affected zone (HAZ) has been well documented(4). Such liquation in alloy 718 can be studied with the help of a pseudo-ternary phase diagram of alloy 718(6). Figure 9 shows the isothermal section through the pseudo-ternary space diagram at a temperature slightly above the terminal eutectlc. Liquatlon can be explained by considering a diffusion couple between NbC and the matrix of composition C M (after the dissolution of the Laves phase present in the cast alloy). A possible diffusion path is shown by the straight line Joining the matrix and NbC. The development of the diffusion couple results in the formation of Nb-rich solids(ranging from C ~ to C~4) in equilibrium with the Nb-rich liquids(ranging from C.. to CT4), as shown by t ~ tle-llnes in the L+7 regions. At least part of the liquid p~duced ~y carbide liquation should be metastable, since the equilibrium volume fraction can be reached only after a prolonged isothermal hold at the peak temperature. This results in the elimination of the excess liquid and the concentration gradients existing in the liquid and the solid, through a dlffuslonal process. It has been shown that the formation of such a metastable liquid along the grain boundaries can cause the migration of the boundarles(7,8,9,10,11). In these experiments, the formation of a metastable liquid was achieved by injecting a ternary solute to the stable grain boundary liquid. However in the weld HAZ the metastable liquid is produced in situ by the constitutional llquatlon of the carbides. It is known that the driving force for such a migration is the presence of coherency stresses at the solid-liquid interface(ll). This coherency stress is due to the mismatch in the lattice parameter between the solute-rich solid in equilibrium with the metastable liquid at the solld-llquld interface and the matrix well away from the interface. The coherency stress results in a concentration gradient across the liquid film and sets up a solute-flux at the solld-llquid interface which causes the boundary to migrate and leave behind an alloyed/dealloyed zone(12). The present study shows that there is Nb enrichment in the region close to the solid-liquid interface compared to regions well away from these interfaces. This difference in Nb concentration is probably the reason for the etching contrast between the grain interior and the region near the migrating boundary. The matrix of alloy 718 consists primarily of Cr, Ni and Fe, all of which have nearly the same atomic radius (13). There is, however, a size discrepancy of about 17% between the matrix atoms and Nb. If we assume that Vegard's law holds for solid solutions of 7 (Cr+Ni+Fe) with Nb, then a 2.3 at. % difference in Nb between the general matrix and the solid-liquld interface will account for a coherency strain of about 0.0023, due to the corresponding mismatch in the lattice parameters. Coherency strains of this magnitude are known to be the driving force for liquid film induced grain boundary migration in several alloy systems (14).
Vol.
24,
No.
3
LIQUID
FILM MIGRATION
539
The occurrence of grain boundary migration in the presence of a metastable grain boundary liquid can significantly affect the rate at which such a liquid can be eliminated, since the kinetics would now be governed by the rate of grain boundary migration. In the absence of grain boundary migration, the grain boundary liquid can be eliminated only by solute diffusion through the matrix which is a slower process. The resolidification kinetics of the liquid are important because the volume fraction and distribution of the liquid, as a function of temperature during cooling of the HAZ, appears to control the hot cracking susceptibility of alloys such as 718 (15). The detection of LFM in the HAZ of alloy 718 is significant because i~ appears to be a signature of the llquatlon process in the HAZ. It is interesting to note that the lowest temperature at which llquatlon and cracking occur in the HAZ of alloy 718 coincides with the lowest temperature at which migration is detected(6). Conclusions i. Liquated grain boundaries in the HAZ of alloy 718, produced by the liquation of NbC particles, are observed to mlgrate in the presence of the liquid and produce microstructures similar to those associated with other liquid film induced migration phenomena. 2. A rough estimate of the coherency strain generated in the matrix suggests that the strain is sufficient to cause this migration. 3. Liquid film migration can explain the presence of several unusual mlcrostructural features in the HAZ of alloy 718 where extensive carbide liquatlon occurs. Acknowledgment The authors wish to thank Dr. C.L.Whlte, Professor, Michigan Technological University, for recognizing the importance of these observations and for his editorial help in the preparation of the manuscript. They also acknowledge the support given to this research by the National Science Foundation under grant DMR-8807915.
ReSerences i. 2. 3. &. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14. 15.
~.A.Owczarski, D.S.Duvall and C.P.Sullivan, Welding Journal, 46, 423-s (1967). E.C.Thompson, Welding Journal, 48, 70-s (1969). P.J.Valdez and J.B.Steinman, Effect of Minor Elements on the Weldability of High-nickel Aloys, Welding Research Council, New York, 93 (1969). R.C.Thompson and S. Genculu, Welding Journal, 62, 337-s (1983). R.Vincent, Acta. Metall., 33, 1205 (1985). B.Radhakrishnan: Ph.D. Thesis, University of Alabama at Birmingham, Dec. 1989. Y.D. Song, S.T. Ahn and D.N.Yoon, Acta Metall. 33, 1907 (1985) Y.J.Baik and D.N.Yoon, Acta Metall. 34, 2039 (1986) W.H.Rhee and D.N.Yoon, Acta Metall., 35, 1447 (1987). H.K.Kang, S.Hackney and D.N.Yoon, Acta Metall., 36, 695 (1988). W.H.Rhee, Y.D.Song and D.N.Yoon, Acta Metall. 35, 57 (1987) M.Hillert, Scripca. Met., 1983, vol. 17, pp. 237-240. Smithells Metals Reference Handbook, Butterworths, 4-24 (1983) C.A.Handwerker, J.W.Cahn, D.N.Yoon and J.E.Blendell, Diffusion in Solids: Recent Developments, TMS/AIME Publication, Warrendale, Pa, 1985. B.Radhakrishnan and R.G.Thompson, Metallography 21, 453 (1988)
Table i. Comoosltlon of alloy 718 Element Cr Fe Ni Nb+Ta Mo A1
Wt.% 18.12 19.07 53.24 5.0 2.28 0.52
Element Sl TI B C S Co
Wt.% 0.II 0.94 0.003 0.03 0.001 0.09
540
LIQUID
FILM MIGRATION
Vol.
24,
Figure l.(a) Mlcrostructure of arc spot weld in wrought alloy 718. showing the fusion zone and HAZ. (b) The boxed HAZ region in (a) showing the reversed curvature of the grain boundaries and the etching contrast behind the migrating boundaries.
1227' "C 1200"C 1200
80Q
6O(3 400
2OO
4
Figure 2. Microstructure of alloy 718 prior to Gleeble thermal simulation, showing the 7 matrix and the N'bC particles. Notice that the grain boundaries are straight,
8
12 16 20 Time in & , ~ o . 4 5 ~
Figure 3. Schematic of the Gleeble thermal cycles used in this study, The samples were heated to the peak temperatures in 8.0 seconds and quenched after various isothermal holds.
No.
5
Vol.
24,
No.
3
LIQUID
FILM
MIGRATION
541
Figure 4. Microstructures of wrought alloy heated to a peak temperature of 1227"C in 8.0 seconds and quenched after an isothermal hold of (a) 1.4 seconds (b) 3.0 seconds and (c) 8.2 seconds. The area swept out by the migrating grains is seen to increase with holding time. At the end of 8.2 seconds, the extensive migration results in the formation of "loops". Notice the similarity in the microstructural features seen in figures 2a,2b and figure lb.
ltt514.
#
7
1312.
t I1 .
3,
10|m7-
3
5 Tim*
Figure 5. Carbide Liquation and grain boundary migration at a peak temperature of 1227°C. The extents of carbide liquation and grain boundary migration appear similar to those present in the HAZ of the spot weld shown in figure I.
7
tn S , a = ~ 4 •
Figure 6. Area fraction of the migrated region as a function of isothermal hold. Notice that the area fraction increases with isothermal hold, indicating that the migration does occur in the presence of liquid. The ranges indicated are for a 99 percent confidence interval.
542
LIQUID
FILM M I G R A T I O N
Figure 7. Grain boundary migration(M) and carbide liquation a~ a peak temperature of 1200°C. Notice tha~ the extent of migration and carbide liquation at 1200°C is less than at 12227"C(Figure 5).
Vol.
24,
Figure 8. Secondary electron (SE) and backscattered electron (BSE) images of a migrating grain boundary. The BSE image indicates Nb enrichment behind ~he migrating boundary.
Table 2. Ouantitative Analysis by EDS Location
Fe
Matrix Migrated Region
Ni
Cr (wt.~)
Ti
Mo
Nb
2.9 (2.6-3.1)
4.1 (3.5-4.3)
20.3 (19.8-20.5)
55.6 20.7 (55.3-56.0) (20.6-21.0)
I.i (1.0-1.2)
19.i (18.5-19.6)
55.0 (54.0-55.7)
1.3 (i.0-1.5)
19.6 (19.4-19.9)
3.2 (2.9-3.3)
The values reported represent the mean for five different locations. within parentheses.
7.7 (6.3-9.5)
The range is given
.%'°';.
r
c~l
al.~
~b ~
La,el
Figure 9. Isothermal section through a pseudo-ternary space diagram of alloy 718, taken just above the ternary eutectlc. The diffusion couple between matrix of composition CM and NbC is shown by the dotted line. C to C. 4 and C I to C_ 4 are the conce~ratlo~s of t~e llqui~ and solid respectively, produced by the operation of the diffusion couple. Notice that CSI to CSA are richer in h~ than the matrix, GM.
No.
3
Vol. 24, pp. 5 3 7 - 5 4 2 , 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 reserved
LIQUID FILM MIGRATION (LFM) IN THE WELD HEAT AFFECTED ZONE (HAZ) OF A NI-BASE SUPERALLOY B.Radhakrlshnan and R.G.Thompson Department of Materials Engineering, University of Alabama at Birmingham, Birmingham, AI 35294 ( R e c e i v e d J u l y 20, 1989) ( R e v i s e d J a n u a r y 3, 1990)
Introduction Alloy 718 is a 7" strengthened nickel-base superalloy(Table i) which is known to suffer from liquation cracking in the heat affected zone(HAZ) of welds(l,2,3,4,5). A careful examination of the grain boundaries in the HAZ of an arc spot weld in alloy 718 (figure i) shows that some of the boundaries appear to exhibit a curvature that changes sign at several points along a boundary facet. Such reverse bending occurs in spite of the natural tendency for the boundaries to remain flat to minimize grain boundary energy. Furthermore, portions of the grains adjacent to the concave side of the boundary appear to etch differently from the rest of the grains. This etching contrast delineates two distinct sets of grain boundaries thus suggesting that the grain boundaries have migrated during the weld thermal cycle. The compositions of these migrated regions are apparently altered during the migration process, as revealed by the etching contrast. The objective of the present study is to establish the mechanism responsible for the migration process.
Methods The alloy 718 samples used in this study were in the form of hot-rolled rods, approximately 5.0mm in diameter. The alloy composltlon(Table I) was within the nominal specifications of the commercial 718 alloy. The gauge-length of the Gleeble samples was approximately 25.0mm. The samples were homogenized at I093"C for 1 hour and aged at 650°C for I0 hours, prior to simulation experiments. The microstructure of the samples prior to thermal cycling consisted of fairly coarse grains of 7 with a dispersion of NbC particles(figure 2). HAZ thermal cycles were simulated using a Duffers 500 Gleeble thermomechanical device. The heating rate used in the simulations (figure 3) was typical of that experienced by the HAZ in common fusion welding processes. The samples were heated to peak temperatures of 1200"C and 1227°C in 8.0 seconds and held isothermally for different lengths of time before quenching in a jet of water. The isothermal hold times were chosen to accentuate the migration process and explore the extent to which LFH occurs at this temperature. The thermal cycle was recorded accurately using the output of a fine wire thermocouple percussion welded to the midsection of the gauge-length. After the simulation experiment, the samples were sectioned transversely at the midpoint of the gauge-length. The transverse sections were prepared for metallography using conventional mounting and polishing techniques and electrolytically etched at room temperature in 10% oxalic acid at 5 volts for 4-6 seconds. The welded junction of the themocouple was seen in all the transverse sections prepared for metallography. Hence, the temperatures recorded were those actually experienced by the transverse sections. Any sample-to-sample variation in temperature due to the presence of a longitudinal temperature gradient was thus eliminated. All the microstructures were recorded at a fixed radial dlstance(0.5 mm) from the periphery, thus eliminating sample-to-sample variation in temperature due to the presence of a radial temperature gradient. The area fraction of the migrated regions was calculated using conventional quantitative microscopy techniques. Scanning electron microscopy was performed using a Phillips 515 SEH. Chemical analysis of the migrated regions was carried out using a Kevex 8000 EDS. Results
The different microstructural stages in the migration of the llquated grain boundaries are seen clearly at a peak temperature of 1227"C, shown in figure 4. The formation of the reversed curvature along the grain boundaries and an adjacent area of solid solution which
537 0 0 3 6 - 9 7 4 8 / 9 0 $ 3 . 0 0 + .00 C o p y r i g h t (c) 1990 P e r g a m o n P r e s s
plc
538
LIQUID
FILM MIGRATION
Vol.
24, No.
3
etches differently from the rest of the matrix are similar to the features in the HAZ of ~he spot weld(flgure I). The carbides show extensive liquation(figure 5), similar to the carbides in the HAZ of the arc spot weld(flgure 1). The area swept out by migration increases with holding time(figure 6), which indicates that migration occurred in the presence of the intergranular liquid. After an isothermal hold of 8.2 seconds, extensive migration of the boundaries has resulted in the formation of several "loops" which enclose the areas swept out by migration. However, in the HAZ of the spot weld, the time available for migration was not sufficient to form these loops. The extent of mlgration(M) is conslderab~y reduced at a peak temperature of 1200"C, as shown in figure 7. The extent of carbide llquatlon is also correspondingly reduced, as seen by comparing figures 7 and 5. Figure 7 shows a thin liquatlon zone at the carbide-matrix interface, which is seen more clearly at a peak temperature Of 1227QC(flgure 5). Figure 8 shows the secondary electron(SE) and the backscattered electron(BSE) images of a migrated region at a peak temperature of 1200"C. Notice that in the BSE image the migrated regions show up in bright contrast against the dark background of the matrix. Quantitative analysis using the EDS(Table 2) shows that the migrated region has 3.6 wt. % or approximately 2.3~at. % excess Nb than the matrix.
Discussion The tendency of intergranular NbC particles to liquate and contribute to cracking in the weld heat affected zone (HAZ) has been well documented(4). Such liquation in alloy 718 can be studied with the help of a pseudo-ternary phase diagram of alloy 718(6). Figure 9 shows the isothermal section through the pseudo-ternary space diagram at a temperature slightly above the terminal eutectlc. Liquatlon can be explained by considering a diffusion couple between NbC and the matrix of composition C M (after the dissolution of the Laves phase present in the cast alloy). A possible diffusion path is shown by the straight line Joining the matrix and NbC. The development of the diffusion couple results in the formation of Nb-rich solids(ranging from C ~ to C~4) in equilibrium with the Nb-rich liquids(ranging from C.. to CT4), as shown by t ~ tle-llnes in the L+7 regions. At least part of the liquid p~duced ~y carbide liquation should be metastable, since the equilibrium volume fraction can be reached only after a prolonged isothermal hold at the peak temperature. This results in the elimination of the excess liquid and the concentration gradients existing in the liquid and the solid, through a dlffuslonal process. It has been shown that the formation of such a metastable liquid along the grain boundaries can cause the migration of the boundarles(7,8,9,10,11). In these experiments, the formation of a metastable liquid was achieved by injecting a ternary solute to the stable grain boundary liquid. However in the weld HAZ the metastable liquid is produced in situ by the constitutional llquatlon of the carbides. It is known that the driving force for such a migration is the presence of coherency stresses at the solid-liquid interface(ll). This coherency stress is due to the mismatch in the lattice parameter between the solute-rich solid in equilibrium with the metastable liquid at the solld-llquld interface and the matrix well away from the interface. The coherency stress results in a concentration gradient across the liquid film and sets up a solute-flux at the solld-llquid interface which causes the boundary to migrate and leave behind an alloyed/dealloyed zone(12). The present study shows that there is Nb enrichment in the region close to the solid-liquid interface compared to regions well away from these interfaces. This difference in Nb concentration is probably the reason for the etching contrast between the grain interior and the region near the migrating boundary. The matrix of alloy 718 consists primarily of Cr, Ni and Fe, all of which have nearly the same atomic radius (13). There is, however, a size discrepancy of about 17% between the matrix atoms and Nb. If we assume that Vegard's law holds for solid solutions of 7 (Cr+Ni+Fe) with Nb, then a 2.3 at. % difference in Nb between the general matrix and the solid-liquld interface will account for a coherency strain of about 0.0023, due to the corresponding mismatch in the lattice parameters. Coherency strains of this magnitude are known to be the driving force for liquid film induced grain boundary migration in several alloy systems (14).
Vol.
24,
No.
3
LIQUID
FILM MIGRATION
539
The occurrence of grain boundary migration in the presence of a metastable grain boundary liquid can significantly affect the rate at which such a liquid can be eliminated, since the kinetics would now be governed by the rate of grain boundary migration. In the absence of grain boundary migration, the grain boundary liquid can be eliminated only by solute diffusion through the matrix which is a slower process. The resolidification kinetics of the liquid are important because the volume fraction and distribution of the liquid, as a function of temperature during cooling of the HAZ, appears to control the hot cracking susceptibility of alloys such as 718 (15). The detection of LFM in the HAZ of alloy 718 is significant because i~ appears to be a signature of the llquatlon process in the HAZ. It is interesting to note that the lowest temperature at which llquatlon and cracking occur in the HAZ of alloy 718 coincides with the lowest temperature at which migration is detected(6). Conclusions i. Liquated grain boundaries in the HAZ of alloy 718, produced by the liquation of NbC particles, are observed to mlgrate in the presence of the liquid and produce microstructures similar to those associated with other liquid film induced migration phenomena. 2. A rough estimate of the coherency strain generated in the matrix suggests that the strain is sufficient to cause this migration. 3. Liquid film migration can explain the presence of several unusual mlcrostructural features in the HAZ of alloy 718 where extensive carbide liquatlon occurs. Acknowledgment The authors wish to thank Dr. C.L.Whlte, Professor, Michigan Technological University, for recognizing the importance of these observations and for his editorial help in the preparation of the manuscript. They also acknowledge the support given to this research by the National Science Foundation under grant DMR-8807915.
ReSerences i. 2. 3. &. 5. 6. 7. 8. 9. i0. ii. 12. 13. 14. 15.
~.A.Owczarski, D.S.Duvall and C.P.Sullivan, Welding Journal, 46, 423-s (1967). E.C.Thompson, Welding Journal, 48, 70-s (1969). P.J.Valdez and J.B.Steinman, Effect of Minor Elements on the Weldability of High-nickel Aloys, Welding Research Council, New York, 93 (1969). R.C.Thompson and S. Genculu, Welding Journal, 62, 337-s (1983). R.Vincent, Acta. Metall., 33, 1205 (1985). B.Radhakrishnan: Ph.D. Thesis, University of Alabama at Birmingham, Dec. 1989. Y.D. Song, S.T. Ahn and D.N.Yoon, Acta Metall. 33, 1907 (1985) Y.J.Baik and D.N.Yoon, Acta Metall. 34, 2039 (1986) W.H.Rhee and D.N.Yoon, Acta Metall., 35, 1447 (1987). H.K.Kang, S.Hackney and D.N.Yoon, Acta Metall., 36, 695 (1988). W.H.Rhee, Y.D.Song and D.N.Yoon, Acta Metall. 35, 57 (1987) M.Hillert, Scripca. Met., 1983, vol. 17, pp. 237-240. Smithells Metals Reference Handbook, Butterworths, 4-24 (1983) C.A.Handwerker, J.W.Cahn, D.N.Yoon and J.E.Blendell, Diffusion in Solids: Recent Developments, TMS/AIME Publication, Warrendale, Pa, 1985. B.Radhakrishnan and R.G.Thompson, Metallography 21, 453 (1988)
Table i. Comoosltlon of alloy 718 Element Cr Fe Ni Nb+Ta Mo A1
Wt.% 18.12 19.07 53.24 5.0 2.28 0.52
Element Sl TI B C S Co
Wt.% 0.II 0.94 0.003 0.03 0.001 0.09
540
LIQUID
FILM MIGRATION
Vol.
24,
Figure l.(a) Mlcrostructure of arc spot weld in wrought alloy 718. showing the fusion zone and HAZ. (b) The boxed HAZ region in (a) showing the reversed curvature of the grain boundaries and the etching contrast behind the migrating boundaries.
1227' "C 1200"C 1200
80Q
6O(3 400
2OO
4
Figure 2. Microstructure of alloy 718 prior to Gleeble thermal simulation, showing the 7 matrix and the N'bC particles. Notice that the grain boundaries are straight,
8
12 16 20 Time in & , ~ o . 4 5 ~
Figure 3. Schematic of the Gleeble thermal cycles used in this study, The samples were heated to the peak temperatures in 8.0 seconds and quenched after various isothermal holds.
No.
5
Vol.
24,
No.
3
LIQUID
FILM
MIGRATION
541
Figure 4. Microstructures of wrought alloy heated to a peak temperature of 1227"C in 8.0 seconds and quenched after an isothermal hold of (a) 1.4 seconds (b) 3.0 seconds and (c) 8.2 seconds. The area swept out by the migrating grains is seen to increase with holding time. At the end of 8.2 seconds, the extensive migration results in the formation of "loops". Notice the similarity in the microstructural features seen in figures 2a,2b and figure lb.
ltt514.
#
7
1312.
t I1 .
3,
10|m7-
3
5 Tim*
Figure 5. Carbide Liquation and grain boundary migration at a peak temperature of 1227°C. The extents of carbide liquation and grain boundary migration appear similar to those present in the HAZ of the spot weld shown in figure I.
7
tn S , a = ~ 4 •
Figure 6. Area fraction of the migrated region as a function of isothermal hold. Notice that the area fraction increases with isothermal hold, indicating that the migration does occur in the presence of liquid. The ranges indicated are for a 99 percent confidence interval.
542
LIQUID
FILM M I G R A T I O N
Figure 7. Grain boundary migration(M) and carbide liquation a~ a peak temperature of 1200°C. Notice tha~ the extent of migration and carbide liquation at 1200°C is less than at 12227"C(Figure 5).
Vol.
24,
Figure 8. Secondary electron (SE) and backscattered electron (BSE) images of a migrating grain boundary. The BSE image indicates Nb enrichment behind ~he migrating boundary.
Table 2. Ouantitative Analysis by EDS Location
Fe
Matrix Migrated Region
Ni
Cr (wt.~)
Ti
Mo
Nb
2.9 (2.6-3.1)
4.1 (3.5-4.3)
20.3 (19.8-20.5)
55.6 20.7 (55.3-56.0) (20.6-21.0)
I.i (1.0-1.2)
19.i (18.5-19.6)
55.0 (54.0-55.7)
1.3 (i.0-1.5)
19.6 (19.4-19.9)
3.2 (2.9-3.3)
The values reported represent the mean for five different locations. within parentheses.
7.7 (6.3-9.5)
The range is given
.%'°';.
r
c~l
al.~
~b ~
La,el
Figure 9. Isothermal section through a pseudo-ternary space diagram of alloy 718, taken just above the ternary eutectlc. The diffusion couple between matrix of composition CM and NbC is shown by the dotted line. C to C. 4 and C I to C_ 4 are the conce~ratlo~s of t~e llqui~ and solid respectively, produced by the operation of the diffusion couple. Notice that CSI to CSA are richer in h~ than the matrix, GM.
No.
3