- Email: [email protected]
Investigations of grain boundary microchemistry in nickel base superalloys
225
Surface Science 246 (1991) 225-230 North-Holland
Investigations of grain boundary microchemistry in nickel base superalloys Krystyna Stiller ~e~art~e~t Received
of P&&s,
Chalmers university
30 July 1990; accepted
of Tec~~oio~,
for publication
6 August
S-412 96 Giiteborg, Sweden 1990
The grain boundary composition of three Alloy 600 materials and one Alloy 690 material have been investigated using TEM, SIMS and APFIM techniques. The segregation of boron carbon and nitrogen to the grain boundaries and the depletion of chromium at the grain boundaries has been observed in all materials. The influence of grain boundary chemistry and microstructure on the susceptibility of the material to IGSCC is discussed.
1. Introduction
information, as complete as possible, about the chemistry and structure of the grain boundaries.
Nickel base superalloys containing 14-32% Cr and 6-108 Fe are frequently used as tube materials for steam generators in PWR reactors. These tubes are exposed during operation in the reactors to high temperatures in high purity water and suffer under these circumstances from intergranular stress corrosion cracking (IGSCC). This type of corrosion is one of the most severe problems today in the maintenance of steam generators. The present investigation was undertaken in order to contribute to the understanding of the m~hanisms responsible for this type of corrosion. The chemical composition and the microstructure of grain boundaries in materials with slightly different compositions and/or different heat treatments were investigated using APFIM, SIMS and TEM techniques. A combination of different analyzing methods was used in order to obtain Table I Nominal
chemical
Material
composition
2. Experimental Three Alloy 600 materials and one Alloy 690 material have been investigated in this study. The chemical composition of the materials is detailed in table 1, while their heat treatments and crack initiation times are given in table 2. All materials were examined in the as-received condition. In addition Alloy 600 BH was also investigated after its resistance to IGSCC had been tested in an autoclave. The test was performed in high purity water at 365’C using an U-bend tube specimen. Prior to atom probe analyses a Cameca IMS 300 secondary ion mass spectrometer was used for examination of the elemental distribution in all the investigated materials. 0: ions were used as
of the alloys (at%)
Element B
C
Al
Si
Cr
Ti
Fe
Mn
Ni
P
N
0.005 0.005 0.005
0.18 0.09 0.09
0.46 0.53 0.04
0.63 0.69 0.62
11.25 18.06 32.18
0.27 0.33 0.46
8.19 9.47 9.21
0.27 0.78 0.33
72.65 69.94 56.94
0.02 0.02 0.01
0.04 0.08 0.12
ABoy 600 BH and BL Alloy 600 E Alloy 690 PP
0039~6028/91/%03.50
0 1991 - Elsevier Science Publishers
B.V. (North-Holland)
K. Stiller / Grain boundary microchrmtstry
226 Table 2 Heat treatment Material
and crack
Temperature
initiation
times
Time
(“C) BL BH BH* if) E PP
921 1024 1024+365 990+715 1080+715
3-5 min 3-S min 3-5 min+ 13478 h 1 min+12 h 1 min+5 h
Crack initiation time(h) 1051 11920 10528 > 23000
‘) BH* material is the Alloy 600 BH material which investigated after it had been tested in an autocIave 13478 h in 365OC high purity water.
was for
primary ions. APFIM specimens were produced using a standard electropolishing procedure [l]. TEM (Philips 300) was used to investigate the structure of the investigated grain boundaries. TEM investigations were also performed in order to characterize the distribution and composition of precipitates in the investigated materials. The AP analyses were carried out at - ‘70 K using evaporation pulses of 15 or 20% of the DC voltage. The temperature was deliberately chosen to be relatively high in order to minimize the effect of the uneven field evaporation of the materials. It was observed that when analyses were performed at lower temperatures (lower than 60 K) the material field evaporated unevenly giving occasionally 8 ions per flag, thus filling the 8 counters of our TDC. After such an event the specimen voltage had to be increased by - 1% of the DC voltage in order to maintain the evaporation rate (fig. I). This was an indication that big clusters of material had been field evaporated. In the worst case this effect could result in “loss” of the grain boundary during the analysis. The phenomenon was less pronounced at higher temperatures and therefore 70 K was chosen as the investigation temperature.
in Ni hose superaIioys
particles/m”) intragranular precipitates and only a few, quite large (0.1-0.5 pm) grain boundary precipitates. The densities of intergranular precipitates in materials Alloy 600 BH and Alloy 600 E were about the same but much higher (the average spacing - 1 pm) than in the BL material. The grain boundary precipitates in these materials were also smaller (0.01-0.1 pm) than in the BL material. The Alloy 690 PP material contained an almost continuous chain of grain boundary precipitates. The most common type of precipitates in all Alloy 600 materials was M3C,. In addition M,,C, precipitates were also frequently found in Alloy 690 PP. 3.2. SIMS SIMS investigations showed enrichment of B and C at the grain boundaries in all the investigated materials except for the BL material where the grain size was too small (> 10 pm) to make this effect clearly observable. The detected B and C contents were lowest in the E material. The distribution of nitrogen in the alloys was not investigated by SIMS.
-ions
-voltage
9600
9500 9400 9300 9200 9100 9000 8900
3. Results
1 6800
3.1. TEM TEM analyses of materials showed Alloy 600 BL material contained many
that the (3 X 10’
7037
7275 flag
7512
8800
7750
Fig. 1. The AP flag-voltage-ions diagram showing that the specimen voltage had to be increased when 8 ions per flag were collected.
K. Stiller / Grain boundary microchemistry in Ni base superalloys
227
Table 4 Composition of the grain boundary precipitates Material
Precipitate type
Precipitate composition
BL BH
MB
(Cro.,aNio.,F‘%,,Tio.oz %87 (Tio.al&ro.,,2Nio.o,,F%oos)-
M(NC)
(N0.719c0.275%,)0.78
Fig. 2. FIM micrograph showing a grain boundary in Alloy 600 BL material.
3.3. APFZM
The matrix compositions obtained by APFIM analyses (table 3) are in quite good agreement with the nominal compositions. The analyses do not include any contribution from precipitates and therefore the observed carbon content in the matrix is lower than nominal. The analyzed grain boundaries appeared dark in the FIM image (fig. 2) except for one case when this dark contrast was not observed. This was a specimen which in the early stages of the final electropolishing contained precipitates at the grain boundary. Those precipitates were polished away
PP
M23C6
PP
M7C3
and the part of the grain boundary investigated by APFIM was “clean” from particles. 3.4. Grain boundary precipitates Four different types of grain boundary precipitates, namely: M,,&, M,C,, M(CN) and MB, were found in the investigated materials. The detailed composition of these precipitates is shown in the table 4. MB precipitates appeared bright in FIM image (fig. 3) while M,,C,, M,C, and M(CN) were dark with respect to the matrix (fig. 4). The M(CN) precipitate was found in an Alloy 600 BH specimen. This particle was a Ti-rich substoichiometric precipitate (table 4). The boron found in the particle was concentrated to the interface between the particle and the matrix. The Fe content at the region close to the interface particle-matrix (within - 50 A) was higher ( - 15 at%) than in the matrix (- 8 at%). Some enrich-
Table 3 AP matrix composition of the alloys (at’%) Material
Alloy 600 BL Alloy 600 BH Alloy 600 BH * Alloy 600 E Alloy 690 PP
Element B
C
Al
Si + N
Cr
Ti
Fe
Mn
Ni
P
0.00 f 0.00 0.00 * 0.00 0.00 f
0.08 f 0.04 0.11 + 0.08 0.11 f 0.03 0.03 f 0.03 0.05 f 0.02
0.53 * 0.09 0.35 + 0.10 0.38 f 0.05 0.37 f 0.15 0.07 f 0.02
0.56 + 0.10 0.58 + 0.15 0.87 f 0.09 0.78 f 0.20 0.81 f 0.08
17.61 f 0.51 16.52 zt 0.69 16.89 i0.34 18.96 f 0.79 32.95 + 0.38
0.27 f 0.07
8.96 f 0.40 8.31 f 0.53 9.09 * 0.28 9.17 f 0.60 9.83 f 0.25
0.22 + 0.06 0.32 f 0.12 0.29 5 0.05 0.75 * 0.19 0.27 f 0.04
71.77 + 0.61 73.37 f 0.82 72.09 f 0.41 69.49 + 0.92 55.68 f 0.40
0.00 f 0.00 0.00 f
0.00
0.00 f 0.00
0.00 f 0.00
0.45 f 0.11 0.29 f 0.05 0.44 f 0.12 0.33 f
0.00
0.00 * 0.00 0.00 f 0.00 0.00 f 0.00
K. Stiller / Grain boundary microchemistty
228
Fig. 3. FIM micrograph
showing a bright BL material.
MB particle
in the
Fig. 4. F1M micrograph
ment of Si at ‘the region close to the interface particle-matrix was also observed. The M,,C, and M,C, precipitates were found in the PP material. The M,,C, precipitates were elongated 50-100 nm thick particles with the long side parallel to the grain boundary. The investigated M,C, precipitate was a lamellae about 10 A thick along the grain boundary. Both types of precipitates were stoichiometric. The Cr content at the region close to the interface particle-matrix Table 5 Grain boundary composition in Alloy 600 materials (a) Grain boundaries which appeared dark in FIM Material
showing a dark M,, C, particle PP material.
in the
was lower (- 26 at%) than in the matrix (- 33 at%). The MB precipitates were found in a specimen of the BL material. This is a new type of precipitate which has not previously been reported in TEM investigation of Alloy 600. The carbon level of these precipitates was almost zero. An enrichment of P (0.54 at%) was also found at the interface between these precipitates and the matrix.
-
Element B
Afloy 600 BL Alloy 600 BH Alloy 600 BH * Alloy 600 E
(b) Grain
in Ni base superalioys
2.24 * 0.28 5.37 i 0.72 2.10 + 0.37 0.62 + 0.22
boundary
C 0.72 i 0.16 0.72 f 0.27 1.25 f 0.25 1.31 i 0.32
Al
Si
0.75 * 0.16 0.62 f 0.25 0.50 + 0.16 0.54 + 0.28
which did not appeared
0.56 + 0.14 0.58 k 0.25 0.87 rt 0.20 0.78 + 0.29
Cr 15.45 * 0.68 13.31 + 1.09 9.14 * 0.69 16.77 f 1.12
Ti
Fe
Mn
Ni
N
P
0.61 * 0.15 0.21 * 0.15 1.00 + 0.22 0.46 + 0.19
8.84 k 0.54 8.98 &0.92 9.64 Ift 0.86 8.73 + 0.94
0.43 i 0.12 0.52 * 0.23 0.35 * 0.13 1.08 + 0.29
69.94 I 0.87 68.42 i 1.49 74.88 + 1.17 68.70 + 1.44
0.45 + 0.12 0.97 rf 0.30 0.28 k 0.10 0.77 Ifr 0.29
0.00 f 0.00 0.31 + 0.18 0.00 j, 0.00 0.13 + 0.07
dark in FIM
Material
Element B
C
Al
Si
Cr
Ti
Fe
Mn
Ni
N
P
Alloy 600 BH
0.00 _+ 0.00
1.06 + 0.47
0.21 + 0.21
0.58 + 0.30
7.84 + 1.24
0.00 i 0.00
8.26 + 1.48
0.21 * 0.21
81.57 f 2.00
0.27 f 0.21
0.00 i 0.00
K. Stiller / Grain boundary microchemistry in Ni base superalloys
3.5. Precipitate free grain boundary The results from AP analyses of the grain boundaries free from precipitates are summarized in tables 5a and 5b. The average grain boundary composition was calculated from analyses of several grain boundaries in the same material. Because of the very dense distribution of grain boundary precipitates in Alloy 690, no grain boundaries, free from precipitates were found. Even in the case where TEM investigation of the APFIM specimen indicated a precipitate free grain boundary, the APFIM studies revealed the existence of a very thin layer (- 1 nm) of M,C, carbide at the boundary. Segregation of two elements B and C to the grain boundaries was observed in all Alloy 600 materials. However, the degree of enrichment of these elements varied from one boundary to the other in the same material. The amount of nitrogen enrichment at the grain boundaries could only be estimated because of the overlap in mass to charge ratio between N and Si. It was assumed that only nitrogen segregates to the grain boundaries. The N enrichment at the boundary was then calculated by subtracting the Si + N content in the matrix from the Si + N content at the grain boundaries. A Ti enrichment at the grain boundaries in the BL and the BH * materials and a very weak segregation of P in some of the specimens of the BH and the E materials was also observed. The emichment of all elements was well localized to the grain boundary region (within less than 10 nm). AP analyses showed chromium depletion at the grain boundaries in all of the materials. The maximum Cr depletion was observed in the BH material in the grain boundary which was situated between two particles which had been electropolished away during specimen preparation. This was the grain boundary which did no appear dark in the FIM image. The total concentration of the segregants at this boundary was the lowest observed. The chemical composition at this boundary is detailed in table 5b. The width of the Cr depleted region was dependent on its closeness to the grain boundary Cr-rich precipitates. If the investigated part of the grain
229
boundary was close to one of these precipitates (within 50 nm) the Cr depleted region was wider than 80 nm. This was observed in Alloy 600 as well as in Alloy 690. On the other hand, at those parts of the grain boundaries which were further away from precipitates, the Cr depletion was localized to within a few nm of the grain boundary.
4. Discussion The observed Cr depletion was lowest in the materials which contained only few grain boundary precipitates (BL material) or were heat treated for a long time at 715O (E and PP materials). The relatively high diffusivity of chromium in the alloys enables the chromium content to be replenished at the boundaries during the long term heat treatments at low temperature. As a result, a “healed” microstructure (i.e. carbide precipitation without chromium depletion) can be produced. However the observed Cr depletion at the grain boundaries in the E and in the PP materials shows that the heat treatments used were too short in order to complete the healing. The segregation of elements observed by SIMS and APFIM techniques are in good agreement. The detected segregation of B, C and N to grain boundaries in BH, BL and E materials was the result of the heat treatment during manufacturing of these materials. The observed segregation of these elements in BH* material, on the other hand, is also caused by long time exposure of the material to 365” C in water [l]. The B, C and N contents at grain boundaries are slightly different in different materials. However the total concentration of the segregants at the boundaries in BH*, BL and E materials is about the same ( - 3 at%) while the segregant concentration in BH material is much higher ( - 7 at%) It is well known that segregants may modify the grain boundary cohesive strength and in this way affect, for example, hydrogen-associated cracking. According to Seah [2] boron, carbon and nitrogen will increase the boundary cohesion. A segregant content in BH material of about 0.6 of a monolayer (typical segregant content at grain boundaries in this material) gives a maximum
230
K. Stiller / Grain boundary microchemistry
cohesive stress at the boundary that is about 1.11 times higher than that for a clean boundary. The corresponding factor for BL, BH* and E materials will be 1.06. It therefore appears that the segregation in these materials only has a minor effect on the susceptibility to IGSCC in high purity water. The distribution of grain boundary precipitates on the other hand plays a major role in the improvement of the resistivity of these materials to IGSCC. In an environment such as de-aerated water or alkaline liquid, which do not attack chromium depleted areas, a dense distribution of grain boundary precipitates may block the progress of the crack by effective crack blunting [3,4]. Indeed the Alloy 690 PP material which contains a very dense distribution of grain boundary precipitates, shows the best resistivity to IGSCC while Alloy 600 BL material which has only few grain boundary precipitates shows the lowest resistivity to IGSCC. The content of grain boundary segregants in an as-received material (7.1 at%) is very different from that of a material which cracked because of the exposure to 365” C high purity water (3.6 at%). This result indicates that some microstructtral changes in the material must take place during long-time exposure in a steam generator. This is contradictory to the results obtained by Airey [5]. This result also suggests that even if segregants play a minor role in the improvement of susceptibility to IGSCC there might be some critical lower value of B, C and N content at the boundary
in Ni base superalloys
below which the interfacial cohesive energy is lower than the energy needed to blunt the crack. This can result in failure of the material.
5. Conclusions It appears that the segregation of elements and Cr depletion in nickel base superalloys Alloy 600 and Alloy 690 has a minor effect on the susceptibility to IGSCC of these materials in high purity water. l The distribution of grain boundary precipitates on the other hand plays a major role in the improvement of the resistivity of these materials to IGSCC. l
Acknowledgements This work was financially supported by the National Swedish Board for Technical Development, Studsvik Energy, AB Sandvik Steel and the Swedish State Power Board.
References 111K. Stiller, J. Phys (Paris) 8 (1989) 329. VI M.P. Seah, Proc. R. Sot. A 349 (1976) 535. [31J.R. Rice and R. Thomson, Philos. Mag. 73 (1974) 29. [41 SM. Breummer and C.H. Henager, Jr., Ser. Met. 20 (1986) 909. 151G.P. Airey, Corrosion 41 (1985) 2.
Surface Science 246 (1991) 225-230 North-Holland
Investigations of grain boundary microchemistry in nickel base superalloys Krystyna Stiller ~e~art~e~t Received
of P&&s,
Chalmers university
30 July 1990; accepted
of Tec~~oio~,
for publication
6 August
S-412 96 Giiteborg, Sweden 1990
The grain boundary composition of three Alloy 600 materials and one Alloy 690 material have been investigated using TEM, SIMS and APFIM techniques. The segregation of boron carbon and nitrogen to the grain boundaries and the depletion of chromium at the grain boundaries has been observed in all materials. The influence of grain boundary chemistry and microstructure on the susceptibility of the material to IGSCC is discussed.
1. Introduction
information, as complete as possible, about the chemistry and structure of the grain boundaries.
Nickel base superalloys containing 14-32% Cr and 6-108 Fe are frequently used as tube materials for steam generators in PWR reactors. These tubes are exposed during operation in the reactors to high temperatures in high purity water and suffer under these circumstances from intergranular stress corrosion cracking (IGSCC). This type of corrosion is one of the most severe problems today in the maintenance of steam generators. The present investigation was undertaken in order to contribute to the understanding of the m~hanisms responsible for this type of corrosion. The chemical composition and the microstructure of grain boundaries in materials with slightly different compositions and/or different heat treatments were investigated using APFIM, SIMS and TEM techniques. A combination of different analyzing methods was used in order to obtain Table I Nominal
chemical
Material
composition
2. Experimental Three Alloy 600 materials and one Alloy 690 material have been investigated in this study. The chemical composition of the materials is detailed in table 1, while their heat treatments and crack initiation times are given in table 2. All materials were examined in the as-received condition. In addition Alloy 600 BH was also investigated after its resistance to IGSCC had been tested in an autoclave. The test was performed in high purity water at 365’C using an U-bend tube specimen. Prior to atom probe analyses a Cameca IMS 300 secondary ion mass spectrometer was used for examination of the elemental distribution in all the investigated materials. 0: ions were used as
of the alloys (at%)
Element B
C
Al
Si
Cr
Ti
Fe
Mn
Ni
P
N
0.005 0.005 0.005
0.18 0.09 0.09
0.46 0.53 0.04
0.63 0.69 0.62
11.25 18.06 32.18
0.27 0.33 0.46
8.19 9.47 9.21
0.27 0.78 0.33
72.65 69.94 56.94
0.02 0.02 0.01
0.04 0.08 0.12
ABoy 600 BH and BL Alloy 600 E Alloy 690 PP
0039~6028/91/%03.50
0 1991 - Elsevier Science Publishers
B.V. (North-Holland)
K. Stiller / Grain boundary microchrmtstry
226 Table 2 Heat treatment Material
and crack
Temperature
initiation
times
Time
(“C) BL BH BH* if) E PP
921 1024 1024+365 990+715 1080+715
3-5 min 3-S min 3-5 min+ 13478 h 1 min+12 h 1 min+5 h
Crack initiation time(h) 1051 11920 10528 > 23000
‘) BH* material is the Alloy 600 BH material which investigated after it had been tested in an autocIave 13478 h in 365OC high purity water.
was for
primary ions. APFIM specimens were produced using a standard electropolishing procedure [l]. TEM (Philips 300) was used to investigate the structure of the investigated grain boundaries. TEM investigations were also performed in order to characterize the distribution and composition of precipitates in the investigated materials. The AP analyses were carried out at - ‘70 K using evaporation pulses of 15 or 20% of the DC voltage. The temperature was deliberately chosen to be relatively high in order to minimize the effect of the uneven field evaporation of the materials. It was observed that when analyses were performed at lower temperatures (lower than 60 K) the material field evaporated unevenly giving occasionally 8 ions per flag, thus filling the 8 counters of our TDC. After such an event the specimen voltage had to be increased by - 1% of the DC voltage in order to maintain the evaporation rate (fig. I). This was an indication that big clusters of material had been field evaporated. In the worst case this effect could result in “loss” of the grain boundary during the analysis. The phenomenon was less pronounced at higher temperatures and therefore 70 K was chosen as the investigation temperature.
in Ni hose superaIioys
particles/m”) intragranular precipitates and only a few, quite large (0.1-0.5 pm) grain boundary precipitates. The densities of intergranular precipitates in materials Alloy 600 BH and Alloy 600 E were about the same but much higher (the average spacing - 1 pm) than in the BL material. The grain boundary precipitates in these materials were also smaller (0.01-0.1 pm) than in the BL material. The Alloy 690 PP material contained an almost continuous chain of grain boundary precipitates. The most common type of precipitates in all Alloy 600 materials was M3C,. In addition M,,C, precipitates were also frequently found in Alloy 690 PP. 3.2. SIMS SIMS investigations showed enrichment of B and C at the grain boundaries in all the investigated materials except for the BL material where the grain size was too small (> 10 pm) to make this effect clearly observable. The detected B and C contents were lowest in the E material. The distribution of nitrogen in the alloys was not investigated by SIMS.
-ions
-voltage
9600
9500 9400 9300 9200 9100 9000 8900
3. Results
1 6800
3.1. TEM TEM analyses of materials showed Alloy 600 BL material contained many
that the (3 X 10’
7037
7275 flag
7512
8800
7750
Fig. 1. The AP flag-voltage-ions diagram showing that the specimen voltage had to be increased when 8 ions per flag were collected.
K. Stiller / Grain boundary microchemistry in Ni base superalloys
227
Table 4 Composition of the grain boundary precipitates Material
Precipitate type
Precipitate composition
BL BH
MB
(Cro.,aNio.,F‘%,,Tio.oz %87 (Tio.al&ro.,,2Nio.o,,F%oos)-
M(NC)
(N0.719c0.275%,)0.78
Fig. 2. FIM micrograph showing a grain boundary in Alloy 600 BL material.
3.3. APFZM
The matrix compositions obtained by APFIM analyses (table 3) are in quite good agreement with the nominal compositions. The analyses do not include any contribution from precipitates and therefore the observed carbon content in the matrix is lower than nominal. The analyzed grain boundaries appeared dark in the FIM image (fig. 2) except for one case when this dark contrast was not observed. This was a specimen which in the early stages of the final electropolishing contained precipitates at the grain boundary. Those precipitates were polished away
PP
M23C6
PP
M7C3
and the part of the grain boundary investigated by APFIM was “clean” from particles. 3.4. Grain boundary precipitates Four different types of grain boundary precipitates, namely: M,,&, M,C,, M(CN) and MB, were found in the investigated materials. The detailed composition of these precipitates is shown in the table 4. MB precipitates appeared bright in FIM image (fig. 3) while M,,C,, M,C, and M(CN) were dark with respect to the matrix (fig. 4). The M(CN) precipitate was found in an Alloy 600 BH specimen. This particle was a Ti-rich substoichiometric precipitate (table 4). The boron found in the particle was concentrated to the interface between the particle and the matrix. The Fe content at the region close to the interface particle-matrix (within - 50 A) was higher ( - 15 at%) than in the matrix (- 8 at%). Some enrich-
Table 3 AP matrix composition of the alloys (at’%) Material
Alloy 600 BL Alloy 600 BH Alloy 600 BH * Alloy 600 E Alloy 690 PP
Element B
C
Al
Si + N
Cr
Ti
Fe
Mn
Ni
P
0.00 f 0.00 0.00 * 0.00 0.00 f
0.08 f 0.04 0.11 + 0.08 0.11 f 0.03 0.03 f 0.03 0.05 f 0.02
0.53 * 0.09 0.35 + 0.10 0.38 f 0.05 0.37 f 0.15 0.07 f 0.02
0.56 + 0.10 0.58 + 0.15 0.87 f 0.09 0.78 f 0.20 0.81 f 0.08
17.61 f 0.51 16.52 zt 0.69 16.89 i0.34 18.96 f 0.79 32.95 + 0.38
0.27 f 0.07
8.96 f 0.40 8.31 f 0.53 9.09 * 0.28 9.17 f 0.60 9.83 f 0.25
0.22 + 0.06 0.32 f 0.12 0.29 5 0.05 0.75 * 0.19 0.27 f 0.04
71.77 + 0.61 73.37 f 0.82 72.09 f 0.41 69.49 + 0.92 55.68 f 0.40
0.00 f 0.00 0.00 f
0.00
0.00 f 0.00
0.00 f 0.00
0.45 f 0.11 0.29 f 0.05 0.44 f 0.12 0.33 f
0.00
0.00 * 0.00 0.00 f 0.00 0.00 f 0.00
K. Stiller / Grain boundary microchemistty
228
Fig. 3. FIM micrograph
showing a bright BL material.
MB particle
in the
Fig. 4. F1M micrograph
ment of Si at ‘the region close to the interface particle-matrix was also observed. The M,,C, and M,C, precipitates were found in the PP material. The M,,C, precipitates were elongated 50-100 nm thick particles with the long side parallel to the grain boundary. The investigated M,C, precipitate was a lamellae about 10 A thick along the grain boundary. Both types of precipitates were stoichiometric. The Cr content at the region close to the interface particle-matrix Table 5 Grain boundary composition in Alloy 600 materials (a) Grain boundaries which appeared dark in FIM Material
showing a dark M,, C, particle PP material.
in the
was lower (- 26 at%) than in the matrix (- 33 at%). The MB precipitates were found in a specimen of the BL material. This is a new type of precipitate which has not previously been reported in TEM investigation of Alloy 600. The carbon level of these precipitates was almost zero. An enrichment of P (0.54 at%) was also found at the interface between these precipitates and the matrix.
-
Element B
Afloy 600 BL Alloy 600 BH Alloy 600 BH * Alloy 600 E
(b) Grain
in Ni base superalioys
2.24 * 0.28 5.37 i 0.72 2.10 + 0.37 0.62 + 0.22
boundary
C 0.72 i 0.16 0.72 f 0.27 1.25 f 0.25 1.31 i 0.32
Al
Si
0.75 * 0.16 0.62 f 0.25 0.50 + 0.16 0.54 + 0.28
which did not appeared
0.56 + 0.14 0.58 k 0.25 0.87 rt 0.20 0.78 + 0.29
Cr 15.45 * 0.68 13.31 + 1.09 9.14 * 0.69 16.77 f 1.12
Ti
Fe
Mn
Ni
N
P
0.61 * 0.15 0.21 * 0.15 1.00 + 0.22 0.46 + 0.19
8.84 k 0.54 8.98 &0.92 9.64 Ift 0.86 8.73 + 0.94
0.43 i 0.12 0.52 * 0.23 0.35 * 0.13 1.08 + 0.29
69.94 I 0.87 68.42 i 1.49 74.88 + 1.17 68.70 + 1.44
0.45 + 0.12 0.97 rf 0.30 0.28 k 0.10 0.77 Ifr 0.29
0.00 f 0.00 0.31 + 0.18 0.00 j, 0.00 0.13 + 0.07
dark in FIM
Material
Element B
C
Al
Si
Cr
Ti
Fe
Mn
Ni
N
P
Alloy 600 BH
0.00 _+ 0.00
1.06 + 0.47
0.21 + 0.21
0.58 + 0.30
7.84 + 1.24
0.00 i 0.00
8.26 + 1.48
0.21 * 0.21
81.57 f 2.00
0.27 f 0.21
0.00 i 0.00
K. Stiller / Grain boundary microchemistry in Ni base superalloys
3.5. Precipitate free grain boundary The results from AP analyses of the grain boundaries free from precipitates are summarized in tables 5a and 5b. The average grain boundary composition was calculated from analyses of several grain boundaries in the same material. Because of the very dense distribution of grain boundary precipitates in Alloy 690, no grain boundaries, free from precipitates were found. Even in the case where TEM investigation of the APFIM specimen indicated a precipitate free grain boundary, the APFIM studies revealed the existence of a very thin layer (- 1 nm) of M,C, carbide at the boundary. Segregation of two elements B and C to the grain boundaries was observed in all Alloy 600 materials. However, the degree of enrichment of these elements varied from one boundary to the other in the same material. The amount of nitrogen enrichment at the grain boundaries could only be estimated because of the overlap in mass to charge ratio between N and Si. It was assumed that only nitrogen segregates to the grain boundaries. The N enrichment at the boundary was then calculated by subtracting the Si + N content in the matrix from the Si + N content at the grain boundaries. A Ti enrichment at the grain boundaries in the BL and the BH * materials and a very weak segregation of P in some of the specimens of the BH and the E materials was also observed. The emichment of all elements was well localized to the grain boundary region (within less than 10 nm). AP analyses showed chromium depletion at the grain boundaries in all of the materials. The maximum Cr depletion was observed in the BH material in the grain boundary which was situated between two particles which had been electropolished away during specimen preparation. This was the grain boundary which did no appear dark in the FIM image. The total concentration of the segregants at this boundary was the lowest observed. The chemical composition at this boundary is detailed in table 5b. The width of the Cr depleted region was dependent on its closeness to the grain boundary Cr-rich precipitates. If the investigated part of the grain
229
boundary was close to one of these precipitates (within 50 nm) the Cr depleted region was wider than 80 nm. This was observed in Alloy 600 as well as in Alloy 690. On the other hand, at those parts of the grain boundaries which were further away from precipitates, the Cr depletion was localized to within a few nm of the grain boundary.
4. Discussion The observed Cr depletion was lowest in the materials which contained only few grain boundary precipitates (BL material) or were heat treated for a long time at 715O (E and PP materials). The relatively high diffusivity of chromium in the alloys enables the chromium content to be replenished at the boundaries during the long term heat treatments at low temperature. As a result, a “healed” microstructure (i.e. carbide precipitation without chromium depletion) can be produced. However the observed Cr depletion at the grain boundaries in the E and in the PP materials shows that the heat treatments used were too short in order to complete the healing. The segregation of elements observed by SIMS and APFIM techniques are in good agreement. The detected segregation of B, C and N to grain boundaries in BH, BL and E materials was the result of the heat treatment during manufacturing of these materials. The observed segregation of these elements in BH* material, on the other hand, is also caused by long time exposure of the material to 365” C in water [l]. The B, C and N contents at grain boundaries are slightly different in different materials. However the total concentration of the segregants at the boundaries in BH*, BL and E materials is about the same ( - 3 at%) while the segregant concentration in BH material is much higher ( - 7 at%) It is well known that segregants may modify the grain boundary cohesive strength and in this way affect, for example, hydrogen-associated cracking. According to Seah [2] boron, carbon and nitrogen will increase the boundary cohesion. A segregant content in BH material of about 0.6 of a monolayer (typical segregant content at grain boundaries in this material) gives a maximum
230
K. Stiller / Grain boundary microchemistry
cohesive stress at the boundary that is about 1.11 times higher than that for a clean boundary. The corresponding factor for BL, BH* and E materials will be 1.06. It therefore appears that the segregation in these materials only has a minor effect on the susceptibility to IGSCC in high purity water. The distribution of grain boundary precipitates on the other hand plays a major role in the improvement of the resistivity of these materials to IGSCC. In an environment such as de-aerated water or alkaline liquid, which do not attack chromium depleted areas, a dense distribution of grain boundary precipitates may block the progress of the crack by effective crack blunting [3,4]. Indeed the Alloy 690 PP material which contains a very dense distribution of grain boundary precipitates, shows the best resistivity to IGSCC while Alloy 600 BL material which has only few grain boundary precipitates shows the lowest resistivity to IGSCC. The content of grain boundary segregants in an as-received material (7.1 at%) is very different from that of a material which cracked because of the exposure to 365” C high purity water (3.6 at%). This result indicates that some microstructtral changes in the material must take place during long-time exposure in a steam generator. This is contradictory to the results obtained by Airey [5]. This result also suggests that even if segregants play a minor role in the improvement of susceptibility to IGSCC there might be some critical lower value of B, C and N content at the boundary
in Ni base superalloys
below which the interfacial cohesive energy is lower than the energy needed to blunt the crack. This can result in failure of the material.
5. Conclusions It appears that the segregation of elements and Cr depletion in nickel base superalloys Alloy 600 and Alloy 690 has a minor effect on the susceptibility to IGSCC of these materials in high purity water. l The distribution of grain boundary precipitates on the other hand plays a major role in the improvement of the resistivity of these materials to IGSCC. l
Acknowledgements This work was financially supported by the National Swedish Board for Technical Development, Studsvik Energy, AB Sandvik Steel and the Swedish State Power Board.
References 111K. Stiller, J. Phys (Paris) 8 (1989) 329. VI M.P. Seah, Proc. R. Sot. A 349 (1976) 535. [31J.R. Rice and R. Thomson, Philos. Mag. 73 (1974) 29. [41 SM. Breummer and C.H. Henager, Jr., Ser. Met. 20 (1986) 909. 151G.P. Airey, Corrosion 41 (1985) 2.