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Intergranular fracture along migrated boundaries in ordered Ni4Mo
Scripta METALLURGICA
Vol. 22, pp. 1683-1688, 1988 Printed in the U.S.A.
Pergamon Press plc All rights reserved
INTERGRANULAR FRACTUREALONGMIGRATEDBOUNDARIES IN ORDEREDNi4Mo Charlie R. Brooks and MaheshSanganeria Materials Science and Engineering Department The University of Tennessee Knoxville, TN 37996-2200 U.S.A.
(Received July 18, 1988) (Revised August 8, 1988) Introduction [1]
The alloy Ni-20 at. % Mo is FCC and disordered (~ phase) above 868°C, and below this temperature i t forms a superlattice of the Dla type (B phase). Rapid cooling from the ~ region prevents the formation of B. Reheating in the range 700-800oc results in the r e l a t i v e l y rapid formation of the ordered phase. The ordering reaction occurs by preferential arrangementof Ni and Mo atoms on the FCC l a t t i c e , so that the B has a close packed structure and a crystallographic relation to the former ~. Six crystallographic variants of B form and three types of domain boundaries. Thus inside the ~ grains a fine domain structure forms with crystallographic continuity between the domains. I t is only at the former ~ boundaries that high angle boundaries exist between B crystals. Aging results in a coarsening of the domains and accompanying this coarsening is an increase in strength and a drastic reduction in d u c t i l i t y . For example, in the ~ form, the alloy has a d u c t i l i t y of about 60 ~, whereas aging at 775oc for 3 min. reduces the d u c t i l i t y to about 5 ~. Fracture occurs along the former ~, high angle boundaries. Further aging of the alloy results in the migration of the high angle boundaries, with a coarser domain structure forming behind the moving interface [ 2 1 . Eventually, these interfaces contact, so that the original B boundaries become less clear. The microstructural events are shown schematically in Fig. 1, where the scale of the B structure, relative to the original grain size, is exaggerated. As this coarser structure developes, the alloy becomesmore b r i t t l e (e.g., < 1 ~ elongation). With continued aging during these microstructural changes, the fracture surface becomes finer then somewhat coarser [3]. In ordered Ni4Mo, understanding and solving the'embrittlement is of practical interest as the alloy has some attractive mechanical properties [4]. The causes of the embrittlement are not established, but obviously the grain boundary structure plays an important role. For the short aging times, before grain boundary migration, fracture clearly occurs along the high angle boundaries. However,after these boundaries migrate the fracture surface topology becomes f i n e r , and then coarser with longer aging times. However,from the fracture surface topology i t is not clear i f there is a relation between the fracture path and the microstructural features, such as the high angle boundaries and the domain structure. I t was suspected that fracture s t i l l occurs along these migrated, high angle boundaries, but this has never been established. In this paper we show that fracture does occur by separation along the migrated boundaries. Experimental Procedure A high-purity Ni-Mo alloy containing nominally 19,97at. % Mo was fabricated in the form of a plate about 3 mm x 5 mmx 20 mm long. To develop an appropriate ~ grain size for revealingthe migrated boundaries upon aging, the alloy was heated for 1800 s at 1250°C, then cooled in about 1200 s to ]O00°C, then water quenched. It was then aged at 727°C for 720 h, which developed a structure of fine
domains within the former ~ grains and with the high-angle boundaries just beginning to migrate. One side of the plate was metallographically polished then etched to reveal the migrated boundaries. The sample was then placed in a small device so that i t could be loaded in three point bending (Fig. 2). The bending load was slowly applied until cracking was observed with an optical microscope. The sample was examinedwith a scanning electron microscope (SEM) to determine the crack propogation direction relative to the microstructural features. Regions were photographed, the sample loaded additionally to advance the cracks, and the sample again observed in the SEM.
1683 0036-9748/88 $3.00 + .00 Copyright (c) 1988 Pergamon Press plc
1684
INTERGRANULAR FRACTURE IN ORDERED Ni4Mo
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22, No, 10
Results and Discussion Fig. 3 shows regions i l l u s t r a t i n g the migrated boundaries. Note that the position of the o r i g i n a l , straight boundary, prior to migration, can be clearly seen. Also note that the regions inside the grains show l i t t l e contrast with the etchant used, even though the structure consists of very fine domains. Fig. 4 shows at low magnification the structure after deformation sufficient to show cracking. Note that all the grains show extensive slip lines, even though for this heat treatment the alloy w i l l be very b r i t t l e (e.g., 1% elongation). The cracks are clearly following the general region of the high angle, former ~ boundaries. Since the etchant not only revealed the position of the migrated boundary but also the location of the boundary before migration (see Fig. 3), the relation of the crack path relative to these two locations could be observed. Exampleswhich clearly show that cracking occurred along the migrated boundaries are shown in Fig. 5. The fracture surface topology is shown in Fig. 6a. The topology obtained for short aging times(210 min at 727°C), before boundary migration, is shown in Fig. 6b. The results of an Auger spectroscopy analysis of the fracture surface of ordered Ni4Mo have just been reported [5I. Two samples were examined; one was aged so that the structure consisted of only fine domains, with no grain boundary migration. The fracture surface showed only intergranular fracture occurring along the high angle, former ~ boundaries. The other sample was aged so that boundary migration had begun. The fracture surface topology was finer and the relation of the fracture path to the structure less clear. In both samples, sulfur was detected on the fracture surface; the concentration decreased greatly after sputtering the fracture surfaces. I t was postulated that fracture was occurring along the migrated boundaries, and that at the aging temperature the sulfur diffusion rate was s u f f i c i e n t l y high to allow S to maintain segregation to the migrating boundary. The results presented here prove that the Auger analysis was of separated, high angle boundaries. Acknowledgements Appreciation is expressed to Michael Kania for construction and testing of the bending device.
three-point
References 1. 2. 3. 4. 5.
C.R. Brooks, J. E. Spruiell and E. E. Stansbury, Int. Met. Rev. 29, 210 (1984). K. Vasudevan, H. P. Kao, C. R. Brooks and E. E. Stansbury, in Proc. 44th Annual Meeting of the Electron Microscopy Society of America, G. W. Bailey, ed., San Francisco Press, San Francisco, pp. 560 (1986). H.P. Kao, Ph.D. dissertation, The University of Tennessee (1986). H.P. Kao and C. R. Brooks, in High-Temperature Ordered Intermetallic Alloys I I , N. S. Stoloff, C. C. Koch, C. T. Liu and O. Izumi, edts., MaterialsResearch Society, Pittsburgh, pp. 335 (1987). A. Choudhury, H. P. Kao, C. R. Brooks and C. L. White, Scripta Met., 22(7), 1057-1062 (1988).
Vol.
22, No. I0
INTERGRANULAR
I_
FRACTURE
IN O R D E R E D
domain growth
r
Ni4Mo
1685
~i
I'i--domoin coarsening by boundary migration--i" I
all a SRO only 80'7/o elongation
all LRO S ~'0.6 5°70 elongation
Figure 1.
IINNN alt LRO S=0.7 2°70 elongation
all LRO S=0.8 1% elongation
all LRO S=0.9 <4% elongation
Schematic illustration of the sequence of microstructural development during aging of ordered Ni4Mo in the range 700-800oc. S is the long-range order parameter.
ic
L
_
~metalograph mnl .,
~" "
.,e"oenalng
////
° r ,e ced
force
,Ill
viewing direction---~
7 //
~/ ~I~ ~~ ~~-xk.,~) ,ly\
I
side view
Figure 2.
all LRO S=I <1°7o elongation
~ loading screw I
,,,4 cm
|
I
!
top view
Schematic diagram of the three-point bending device used to initiate cracking in the alloy.
1686
INTERGRANULAR FRACTURE IN ORDERED Ni4Mo
Vol. 22, No. i0
Figure 3.
Examples of migrated boundaries (arrows) in ordered Ni4Mo (optical micrograph).
Figure 4.
The metallographically etched surface after s u f f i c i e n t bending to i n i t i a t e cracks (arrows). Note the s l i p traces in a l l the grains.
Vol. 22, No. I0
Figure 5.
INTERGRANULAR FRACTURE IN ORDERED Ni4Mo
1687
Examples of fracture along migrated boundaries in ordered Ni4Mo. The arrows point out the location of the original boundaries prior to migration.
1688
INTERGRANULAR FRACTURE IN ORDERED Ni4Mo
Figure 6.
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22, No. i0
(a) Fracture surface of the sample used in this experiment which had migrated boundaries. (b) Fracture surface of sample of the same alloy aged for short times for which the boundaries had not yet migrated.
Vol. 22, pp. 1683-1688, 1988 Printed in the U.S.A.
Pergamon Press plc All rights reserved
INTERGRANULAR FRACTUREALONGMIGRATEDBOUNDARIES IN ORDEREDNi4Mo Charlie R. Brooks and MaheshSanganeria Materials Science and Engineering Department The University of Tennessee Knoxville, TN 37996-2200 U.S.A.
(Received July 18, 1988) (Revised August 8, 1988) Introduction [1]
The alloy Ni-20 at. % Mo is FCC and disordered (~ phase) above 868°C, and below this temperature i t forms a superlattice of the Dla type (B phase). Rapid cooling from the ~ region prevents the formation of B. Reheating in the range 700-800oc results in the r e l a t i v e l y rapid formation of the ordered phase. The ordering reaction occurs by preferential arrangementof Ni and Mo atoms on the FCC l a t t i c e , so that the B has a close packed structure and a crystallographic relation to the former ~. Six crystallographic variants of B form and three types of domain boundaries. Thus inside the ~ grains a fine domain structure forms with crystallographic continuity between the domains. I t is only at the former ~ boundaries that high angle boundaries exist between B crystals. Aging results in a coarsening of the domains and accompanying this coarsening is an increase in strength and a drastic reduction in d u c t i l i t y . For example, in the ~ form, the alloy has a d u c t i l i t y of about 60 ~, whereas aging at 775oc for 3 min. reduces the d u c t i l i t y to about 5 ~. Fracture occurs along the former ~, high angle boundaries. Further aging of the alloy results in the migration of the high angle boundaries, with a coarser domain structure forming behind the moving interface [ 2 1 . Eventually, these interfaces contact, so that the original B boundaries become less clear. The microstructural events are shown schematically in Fig. 1, where the scale of the B structure, relative to the original grain size, is exaggerated. As this coarser structure developes, the alloy becomesmore b r i t t l e (e.g., < 1 ~ elongation). With continued aging during these microstructural changes, the fracture surface becomes finer then somewhat coarser [3]. In ordered Ni4Mo, understanding and solving the'embrittlement is of practical interest as the alloy has some attractive mechanical properties [4]. The causes of the embrittlement are not established, but obviously the grain boundary structure plays an important role. For the short aging times, before grain boundary migration, fracture clearly occurs along the high angle boundaries. However,after these boundaries migrate the fracture surface topology becomes f i n e r , and then coarser with longer aging times. However,from the fracture surface topology i t is not clear i f there is a relation between the fracture path and the microstructural features, such as the high angle boundaries and the domain structure. I t was suspected that fracture s t i l l occurs along these migrated, high angle boundaries, but this has never been established. In this paper we show that fracture does occur by separation along the migrated boundaries. Experimental Procedure A high-purity Ni-Mo alloy containing nominally 19,97at. % Mo was fabricated in the form of a plate about 3 mm x 5 mmx 20 mm long. To develop an appropriate ~ grain size for revealingthe migrated boundaries upon aging, the alloy was heated for 1800 s at 1250°C, then cooled in about 1200 s to ]O00°C, then water quenched. It was then aged at 727°C for 720 h, which developed a structure of fine
domains within the former ~ grains and with the high-angle boundaries just beginning to migrate. One side of the plate was metallographically polished then etched to reveal the migrated boundaries. The sample was then placed in a small device so that i t could be loaded in three point bending (Fig. 2). The bending load was slowly applied until cracking was observed with an optical microscope. The sample was examinedwith a scanning electron microscope (SEM) to determine the crack propogation direction relative to the microstructural features. Regions were photographed, the sample loaded additionally to advance the cracks, and the sample again observed in the SEM.
1683 0036-9748/88 $3.00 + .00 Copyright (c) 1988 Pergamon Press plc
1684
INTERGRANULAR FRACTURE IN ORDERED Ni4Mo
Vol.
22, No, 10
Results and Discussion Fig. 3 shows regions i l l u s t r a t i n g the migrated boundaries. Note that the position of the o r i g i n a l , straight boundary, prior to migration, can be clearly seen. Also note that the regions inside the grains show l i t t l e contrast with the etchant used, even though the structure consists of very fine domains. Fig. 4 shows at low magnification the structure after deformation sufficient to show cracking. Note that all the grains show extensive slip lines, even though for this heat treatment the alloy w i l l be very b r i t t l e (e.g., 1% elongation). The cracks are clearly following the general region of the high angle, former ~ boundaries. Since the etchant not only revealed the position of the migrated boundary but also the location of the boundary before migration (see Fig. 3), the relation of the crack path relative to these two locations could be observed. Exampleswhich clearly show that cracking occurred along the migrated boundaries are shown in Fig. 5. The fracture surface topology is shown in Fig. 6a. The topology obtained for short aging times(210 min at 727°C), before boundary migration, is shown in Fig. 6b. The results of an Auger spectroscopy analysis of the fracture surface of ordered Ni4Mo have just been reported [5I. Two samples were examined; one was aged so that the structure consisted of only fine domains, with no grain boundary migration. The fracture surface showed only intergranular fracture occurring along the high angle, former ~ boundaries. The other sample was aged so that boundary migration had begun. The fracture surface topology was finer and the relation of the fracture path to the structure less clear. In both samples, sulfur was detected on the fracture surface; the concentration decreased greatly after sputtering the fracture surfaces. I t was postulated that fracture was occurring along the migrated boundaries, and that at the aging temperature the sulfur diffusion rate was s u f f i c i e n t l y high to allow S to maintain segregation to the migrating boundary. The results presented here prove that the Auger analysis was of separated, high angle boundaries. Acknowledgements Appreciation is expressed to Michael Kania for construction and testing of the bending device.
three-point
References 1. 2. 3. 4. 5.
C.R. Brooks, J. E. Spruiell and E. E. Stansbury, Int. Met. Rev. 29, 210 (1984). K. Vasudevan, H. P. Kao, C. R. Brooks and E. E. Stansbury, in Proc. 44th Annual Meeting of the Electron Microscopy Society of America, G. W. Bailey, ed., San Francisco Press, San Francisco, pp. 560 (1986). H.P. Kao, Ph.D. dissertation, The University of Tennessee (1986). H.P. Kao and C. R. Brooks, in High-Temperature Ordered Intermetallic Alloys I I , N. S. Stoloff, C. C. Koch, C. T. Liu and O. Izumi, edts., MaterialsResearch Society, Pittsburgh, pp. 335 (1987). A. Choudhury, H. P. Kao, C. R. Brooks and C. L. White, Scripta Met., 22(7), 1057-1062 (1988).
Vol.
22, No. I0
INTERGRANULAR
I_
FRACTURE
IN O R D E R E D
domain growth
r
Ni4Mo
1685
~i
I'i--domoin coarsening by boundary migration--i" I
all a SRO only 80'7/o elongation
all LRO S ~'0.6 5°70 elongation
Figure 1.
IINNN alt LRO S=0.7 2°70 elongation
all LRO S=0.8 1% elongation
all LRO S=0.9 <4% elongation
Schematic illustration of the sequence of microstructural development during aging of ordered Ni4Mo in the range 700-800oc. S is the long-range order parameter.
ic
L
_
~metalograph mnl .,
~" "
.,e"oenalng
////
° r ,e ced
force
,Ill
viewing direction---~
7 //
~/ ~I~ ~~ ~~-xk.,~) ,ly\
I
side view
Figure 2.
all LRO S=I <1°7o elongation
~ loading screw I
,,,4 cm
|
I
!
top view
Schematic diagram of the three-point bending device used to initiate cracking in the alloy.
1686
INTERGRANULAR FRACTURE IN ORDERED Ni4Mo
Vol. 22, No. i0
Figure 3.
Examples of migrated boundaries (arrows) in ordered Ni4Mo (optical micrograph).
Figure 4.
The metallographically etched surface after s u f f i c i e n t bending to i n i t i a t e cracks (arrows). Note the s l i p traces in a l l the grains.
Vol. 22, No. I0
Figure 5.
INTERGRANULAR FRACTURE IN ORDERED Ni4Mo
1687
Examples of fracture along migrated boundaries in ordered Ni4Mo. The arrows point out the location of the original boundaries prior to migration.
1688
INTERGRANULAR FRACTURE IN ORDERED Ni4Mo
Figure 6.
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22, No. i0
(a) Fracture surface of the sample used in this experiment which had migrated boundaries. (b) Fracture surface of sample of the same alloy aged for short times for which the boundaries had not yet migrated.