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Microstructure and properties of laser-clad high-temperature wear-resistant alloys1
Applied Surface Science 140 Ž1999. 19–23
Microstructure and properties of laser-clad high-temperature wear-resistant alloys 1 Yongqiang Yang
)
South China UniÕersity of Technology, Department of Mechatronic Engineering, Guangzhou 510641, China Received 17 December 1997; accepted 26 May 1998
Abstract A 2-kW CO 2 laser with a powder feeder was used to produce alloy coatings with high temperature-wear resistance on the surface of steel substrates. To analyze the microstructure and microchemical composition of the laser-clad layers, a scanning electron microscope ŽSEM. equipped with an energy dispersive X-ray microanalysis system was employed. X-ray diffraction techniques were applied to characterize the phases formed during the cladding process. The results show that the microstructure of the cladding alloy consists mainly of many dispersed particles ŽW2 C, ŽW,Ti.C 1yx , WC., a lamellar eutectic carbide M 12 C, and an Žf.c.c. matrix. Hardness tested at room and high temperature showed that the laser-clad zone has a moderate room temperature hardness and relatively higher elevated temperature hardness. The application of the laser-clad layer to a hot tool was very successful, and its operational life span was prolonged 1 to 4 times. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: High temperature wear-resistant alloy; Laser cladding; Microstructure; Elevated temperature hardness; Hot tool
1. Introduction Laser cladding is one of many laser surface modification techniques. In the laser cladding procedure, not only the clad powder but also the substrate is melted by the laser beam, so a new surface coating whose composition differs from that of either the powder or the substrate is produced. Although there have been some reports w1–3x of applying laser cladding techniques to industrial components, the application of laser-clad high temperature wear-re-
sistant alloys to tools operated at elevated temperatures has not been common. In this paper, a coating produced by laser cladding is used on the surface of hot tools. Our unique technological feature is the use of a powder feeding method to achieve a coating of high temperature alloys. The microstructure, metal phases and microcomposition have been investigated and the hardness at room and elevated temperatures tested and analyzed.
2. Experimental methods )
Tel.rFax: q86 20 87114484; E-mail: [email protected] Sponsored by the National Science Foundation of Guandong province, China. 1
The alloy powders used in laser cladding include a mixture of 70% of a Ni-based high temperature
0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 3 2 0 - 1
Y. Yangr Applied Surface Science 140 (1999) 19–23
20
Table 1 Composition of the powders tested by EDAX Materials
Ni-based alloy WC
Wt.% W
Ni
Fe
Co
Cr
Ti
Mo
Al
16.42 100
48.14
4.54
8.51
5.09
2.68
4.81
9.81
Carbon cannot be tested by EDAX, the amount of C in the complete powder, by chemical analysis, is 1.62 wt.%.
alloy and 30% WC particles. The Ni-based alloy particles are globe-shaped and the WC particles are lump-shaped. The two metal powders were mixed by mechanically. The particulate size is y200 q 300 mesh. The composition is shown in Table 1. The laser cladding device consists of two parts ŽFig. 1.. The first part is the laser optical system, which includes a 2 kW multimode CW CO 2 laser, a GaAs lens of focal length 300 mm, and a nozzle. The melting pool is located 50 mm from the focal point and the laser beam diameter at the melting pool is about 5 mm. The second part is powder feeding system, which includes a powder feeder and a powder-carried copper tube of inner diameter 3 mm. The carrying gas is Argon at a flow rate 0.2 M 3rh, and both parts employed Ar as the shielding gas. The substrate materials were 4Cr5 MoV1 Si and 45 steel, in whose original microstructure the 4Cr5MoV1 Si material presents tempered martensite and 45 steel pearlite q ferrite. The compositions of substrate materials shown in Table 2. The specimen size: 40 = 40 = 9 Žmm..
The devices for investigation and analysis included SEM Žscanning electronic microscope., EDAX with an electron probe diameter of 1 mm and an X-ray diffraction analysis device with a Cu target with E s 30 V, I s 30 mA, and a scanning rate of 48rmin.. Hardness tests were carried out on a microsclerometer and a vacuum high temperature Vickers sclerometer.
3. Results and analysis The multi-pass overlapping method was used to obtain a wide zone of laser-clad material. Its schematic is shown in Fig. 2. Under the conditions of laser power, that is, 1500 W, a powder feeding rate of 10 grmin, a laser beam scanning rate of 3 mmrs, and an overlap of 50%, a surface coating 0.9 mm thick and 40 = 40 mm2 area is produced.
3.1. Microstructure of the laser clad layer
Fig. 1. Schematic diagram of laser cladding configuration.
Metallographic investigation shows that the laser-clad layer does not have a uniform microstructure. The whole layer consists of two zones, an upper zone and a lower zone. The thickness of the upper zone is about 0.6 mm, and that of the lower zone is 0.3 mm. In the upper zone, see Fig. 3a, X-ray and EDAX analysis identifies the metal phases to include the g Žf.c.c. phase, WC, W2 C and ŽW,Ti.C 1yx particles and lamellar eutectic carbide M 12 C. ŽW, Ti. C 1y x is type of MC carbide, and its atom ratio of C to ŽW, Ti. is less than 1. The carbide M 12 C has a ˚ and its molecular forlattice constant a 0 s 10.80 A Ž . Ž mula is Fe,Cr,Ni,Co 6 W,Mo,Ti. 6 C.
Y. Yangr Applied Surface Science 140 (1999) 19–23
21
Table 2 Compositions of 4Cr5 MoV1 Si and 45 steel Steel
Chemical composition
4Cr5 MoV1 Si 45a steel
C
Si
Mn
P
S
Cr
Mo
V
0.38 0.45
1.10 0.21
0.33 0.63
0.020 0.021
0.015 0.018
5.15 0.15
1.43 –
0.55 –
In the lower zone, the main part of the microstructure is similar to that of the upper zone except for a lack of WC, but also the distribution of each phase is different, see Fig. 3b. The amount of W2 C and ŽW, Ti.C 1y x is less than that in the upper zone, but the amount of M 12 C is greater. Another difference between the upper and lower zones is the amount of Fe, see Table 3. There is more Fe in the lower zone. The reason for the difference is the dilution effect of the substrate.
highest, even higher than at room temperature, up to 550–580 Hv0.2. At 8008C, the hardness is 380–550 Hv0.2; at 9508C, 100–200 Hv0.2; and at 10508C, 50–90 Hv0.2. These results demonstrate that the
3.2. Hardness of the laser-clad layers We tested both room temperature and elevated temperature hardness. The results are shown in Figs. 4 and 5. Fig. 4 shows the hardness of the laser-clad specimen from surface to substrate at room temperature. In the laser clad zone Ždistance 0 to 1 mm from the surface. the average hardness is about 520 Hv0.2. In the heat-affected zone, it is different between two substrates. The hardness of the heat-affected zone in the 4Cr5 MoV1 Si substrate is even higher than in the laser-clad zone Žcurve 1.. Curve 2 corresponds to the 45 steel substrate. We tested the elevated temperature hardness 600 to 10508C Žsee Fig. 5.. The hardness at 6008C is the
Fig. 2. Schematic diagram of multi-pass laser cladding.
Fig. 3. SEM photographs of the laser cladding layer, Ža. upper zone, Žb. lower zone.
Y. Yangr Applied Surface Science 140 (1999) 19–23
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Table 3 Composition of the laser clad layer as determined by EDAX Wt.%
Upper zone, Fig. 3 Lower zone, Fig. 4
W
Ni
Fe
Co
Cr
Ti
Mo
Al
33.35 29.75
26.14 21.79
23.55 33.07
5.82 3.70
4.57 4.69
0.98 0.93
2.66 3.54
2.93 2.53
coating has the potential to function at high temperatures.
zone, 0.6 mm and 0.3 mm, respectively, which means that laser beam melts the powders dominantly and the clad ingredient composes the main part of the laser clad layer.
4. Discussion 4.2. Metallurgical analysis of laser clad layer 4.1. The formation of upper zone and lower zone Two zones accurately describe the laser-clad layer. A heat and mass transfer model w4x elucidates the reason why two zones are formed. Suppose the entire energy of the laser beam melts the powder and substrate materials. While the laser beam interacts with the powder, the melting pool is a uniform solution w5x. As the cooling rate is very high during laser cladding, the melted zone solidifies rapidly and releases crystalline latent heat, which will melt the lower substrate material. Then there is no time to uniformly mix this layer with the upper solution. As a result, the upper zone has the dominant composition of the laser cladding and the lower zone that of the substrate material. In this experiment, the thickness of upper zone is larger than that of the lower
Fig. 4. Hardness of the claddings at room temperature.
The evident features of laser cladding are rapid melting and rapid solidifying. When powders and the substrate surface are melted by the laser beam and become liquid, some unmelted WC particles remain that will form crystalline nuclei during subsequent solidification. However, because the nucleation and development of WC needs more C and W2 C and ŽW, Ti.C 1y x is easier to form than WC, in the liquid it will be difficult to form new WC in subsequent solidification. Also because Ti forms carbides more readily than W w6x, almost all Ti exists as carbide. Other elements, Ni, Co and Cr are in the g phase as elements substituting for Fe, and are found less in carbides. The alloying element Mo functions like W and exists as the carbide M 12 C. Carbides absorb most of the W and Ti, which causes the concentration of W and Ti to decrease. Thus the remnant liquid solution undergoes a eutec-
Fig. 5. Hardness of the cladding at elevating temperature.
Y. Yangr Applied Surface Science 140 (1999) 19–23
23
Thus not only can the operational life span of the laser clad hot tool be prolonged but also tool material can be saved.
6. Conclusions
Fig. 6. Schematic diagram of hot tool used in rolling mill.
tic transformation in subsequent cooling. According to the analysis above, the cooling transformations take place in turn: L ™ W2 C q Ž W,Ti . C 1yx L ™ g q M 12 C
Ž 1. Ž 2.
5. The application of high temperature alloy coating to the hot tool of a rolling mill Fig. 6 is a schematic diagram of a hot tool. Its working surface is on the middle of the tool, a width of 20 mm. After the laser cladding procedure, a coating width of 30 mm and thickness of 2 mm, is obtained. The substrate material is 4Cr5 MoV1 Si steel. A 2008C and 2-h tempering treatment was carried out after laser cladding, which relieves stress resulting from the laser cladding procedure and avoids brittle fracture of the laser-clad tool at room temperature. The result in real working surroundings at 10008C shows the capability of the laser cladding tool to be a rolling steel quantity of 1000–1100 tons per pair. This is twice as good as bulk 4Cr5 MoV1 Si tools, and 5 times better than plain-carbon 45 steel tools. By using the technique of laser cladding, the failed tool can be re-used after another laser coating is applied.
Ž1. Under the condition of laser power of 1500 W, powder-feeding rate of 10 grmin, scanning rate of 3 mmrs, a wide laser cladding layer has been produced by multi-pass overlapping method. Ž2. The metal phases of laser cladding layer’s microstructure mainly consist of g-phase Žmatrix., dispersed WC, W2 C, ŽW, Ti.C 1yx particles and eutectic M 12 C carbides. Ž3. The coating produced by laser cladding has moderate room temperature hardness and a relatively higher high temperature hardness. Ž4. Great success was achieved in the application of laser cladding layer of high temperature alloy to a hot tool. The operational life span was significantly prolonged relative to those of bulk 4Cr5 MoV1 Si and 45 steel, respectively.
References w1x O. Moriaki et al., Development of Laser Cladding Process. Proceeding of LAMP ’87, Osaka, 5, 1987, pp. 156–160. w2x T. Takeda, W.M. Steen, D.R.F. West, Instill Clad Alloy Formation by Laser Cladding, Proc. LIM2, Burmingham, UK, March, 1985, Publ. by IFS Publications, Bedford UK. w3x M. MacIntyre, Laser Hardfacing of RB211 Turbine Blade Shroud Interlocks, Proc. 2nd Int. Conf. On Applications of Lasers in Material Processing, Jan. 1983 Los Angeles, USA. w4x L.J. Li, J. Mazumder, in: K. Mukherji, J. Mazumder ŽEds.. , Laser Processing of Materials, Published by TMS-AIME, 1985, pp. 35–50. w5x T. Chande, J. Mazumder, Appl. Phys. Lett. 57 Ž1982. 2226. w6x Y.Q. Li, J.Y. Liu ŽEds.., Space Phase of Grain Boundary of Superalloy, Metallurgical Industry Press, China, 1990.
Microstructure and properties of laser-clad high-temperature wear-resistant alloys 1 Yongqiang Yang
)
South China UniÕersity of Technology, Department of Mechatronic Engineering, Guangzhou 510641, China Received 17 December 1997; accepted 26 May 1998
Abstract A 2-kW CO 2 laser with a powder feeder was used to produce alloy coatings with high temperature-wear resistance on the surface of steel substrates. To analyze the microstructure and microchemical composition of the laser-clad layers, a scanning electron microscope ŽSEM. equipped with an energy dispersive X-ray microanalysis system was employed. X-ray diffraction techniques were applied to characterize the phases formed during the cladding process. The results show that the microstructure of the cladding alloy consists mainly of many dispersed particles ŽW2 C, ŽW,Ti.C 1yx , WC., a lamellar eutectic carbide M 12 C, and an Žf.c.c. matrix. Hardness tested at room and high temperature showed that the laser-clad zone has a moderate room temperature hardness and relatively higher elevated temperature hardness. The application of the laser-clad layer to a hot tool was very successful, and its operational life span was prolonged 1 to 4 times. q 1999 Published by Elsevier Science B.V. All rights reserved. Keywords: High temperature wear-resistant alloy; Laser cladding; Microstructure; Elevated temperature hardness; Hot tool
1. Introduction Laser cladding is one of many laser surface modification techniques. In the laser cladding procedure, not only the clad powder but also the substrate is melted by the laser beam, so a new surface coating whose composition differs from that of either the powder or the substrate is produced. Although there have been some reports w1–3x of applying laser cladding techniques to industrial components, the application of laser-clad high temperature wear-re-
sistant alloys to tools operated at elevated temperatures has not been common. In this paper, a coating produced by laser cladding is used on the surface of hot tools. Our unique technological feature is the use of a powder feeding method to achieve a coating of high temperature alloys. The microstructure, metal phases and microcomposition have been investigated and the hardness at room and elevated temperatures tested and analyzed.
2. Experimental methods )
Tel.rFax: q86 20 87114484; E-mail: [email protected] Sponsored by the National Science Foundation of Guandong province, China. 1
The alloy powders used in laser cladding include a mixture of 70% of a Ni-based high temperature
0169-4332r99r$ - see front matter q 1999 Published by Elsevier Science B.V. All rights reserved. PII: S 0 1 6 9 - 4 3 3 2 Ž 9 8 . 0 0 3 2 0 - 1
Y. Yangr Applied Surface Science 140 (1999) 19–23
20
Table 1 Composition of the powders tested by EDAX Materials
Ni-based alloy WC
Wt.% W
Ni
Fe
Co
Cr
Ti
Mo
Al
16.42 100
48.14
4.54
8.51
5.09
2.68
4.81
9.81
Carbon cannot be tested by EDAX, the amount of C in the complete powder, by chemical analysis, is 1.62 wt.%.
alloy and 30% WC particles. The Ni-based alloy particles are globe-shaped and the WC particles are lump-shaped. The two metal powders were mixed by mechanically. The particulate size is y200 q 300 mesh. The composition is shown in Table 1. The laser cladding device consists of two parts ŽFig. 1.. The first part is the laser optical system, which includes a 2 kW multimode CW CO 2 laser, a GaAs lens of focal length 300 mm, and a nozzle. The melting pool is located 50 mm from the focal point and the laser beam diameter at the melting pool is about 5 mm. The second part is powder feeding system, which includes a powder feeder and a powder-carried copper tube of inner diameter 3 mm. The carrying gas is Argon at a flow rate 0.2 M 3rh, and both parts employed Ar as the shielding gas. The substrate materials were 4Cr5 MoV1 Si and 45 steel, in whose original microstructure the 4Cr5MoV1 Si material presents tempered martensite and 45 steel pearlite q ferrite. The compositions of substrate materials shown in Table 2. The specimen size: 40 = 40 = 9 Žmm..
The devices for investigation and analysis included SEM Žscanning electronic microscope., EDAX with an electron probe diameter of 1 mm and an X-ray diffraction analysis device with a Cu target with E s 30 V, I s 30 mA, and a scanning rate of 48rmin.. Hardness tests were carried out on a microsclerometer and a vacuum high temperature Vickers sclerometer.
3. Results and analysis The multi-pass overlapping method was used to obtain a wide zone of laser-clad material. Its schematic is shown in Fig. 2. Under the conditions of laser power, that is, 1500 W, a powder feeding rate of 10 grmin, a laser beam scanning rate of 3 mmrs, and an overlap of 50%, a surface coating 0.9 mm thick and 40 = 40 mm2 area is produced.
3.1. Microstructure of the laser clad layer
Fig. 1. Schematic diagram of laser cladding configuration.
Metallographic investigation shows that the laser-clad layer does not have a uniform microstructure. The whole layer consists of two zones, an upper zone and a lower zone. The thickness of the upper zone is about 0.6 mm, and that of the lower zone is 0.3 mm. In the upper zone, see Fig. 3a, X-ray and EDAX analysis identifies the metal phases to include the g Žf.c.c. phase, WC, W2 C and ŽW,Ti.C 1yx particles and lamellar eutectic carbide M 12 C. ŽW, Ti. C 1y x is type of MC carbide, and its atom ratio of C to ŽW, Ti. is less than 1. The carbide M 12 C has a ˚ and its molecular forlattice constant a 0 s 10.80 A Ž . Ž mula is Fe,Cr,Ni,Co 6 W,Mo,Ti. 6 C.
Y. Yangr Applied Surface Science 140 (1999) 19–23
21
Table 2 Compositions of 4Cr5 MoV1 Si and 45 steel Steel
Chemical composition
4Cr5 MoV1 Si 45a steel
C
Si
Mn
P
S
Cr
Mo
V
0.38 0.45
1.10 0.21
0.33 0.63
0.020 0.021
0.015 0.018
5.15 0.15
1.43 –
0.55 –
In the lower zone, the main part of the microstructure is similar to that of the upper zone except for a lack of WC, but also the distribution of each phase is different, see Fig. 3b. The amount of W2 C and ŽW, Ti.C 1y x is less than that in the upper zone, but the amount of M 12 C is greater. Another difference between the upper and lower zones is the amount of Fe, see Table 3. There is more Fe in the lower zone. The reason for the difference is the dilution effect of the substrate.
highest, even higher than at room temperature, up to 550–580 Hv0.2. At 8008C, the hardness is 380–550 Hv0.2; at 9508C, 100–200 Hv0.2; and at 10508C, 50–90 Hv0.2. These results demonstrate that the
3.2. Hardness of the laser-clad layers We tested both room temperature and elevated temperature hardness. The results are shown in Figs. 4 and 5. Fig. 4 shows the hardness of the laser-clad specimen from surface to substrate at room temperature. In the laser clad zone Ždistance 0 to 1 mm from the surface. the average hardness is about 520 Hv0.2. In the heat-affected zone, it is different between two substrates. The hardness of the heat-affected zone in the 4Cr5 MoV1 Si substrate is even higher than in the laser-clad zone Žcurve 1.. Curve 2 corresponds to the 45 steel substrate. We tested the elevated temperature hardness 600 to 10508C Žsee Fig. 5.. The hardness at 6008C is the
Fig. 2. Schematic diagram of multi-pass laser cladding.
Fig. 3. SEM photographs of the laser cladding layer, Ža. upper zone, Žb. lower zone.
Y. Yangr Applied Surface Science 140 (1999) 19–23
22
Table 3 Composition of the laser clad layer as determined by EDAX Wt.%
Upper zone, Fig. 3 Lower zone, Fig. 4
W
Ni
Fe
Co
Cr
Ti
Mo
Al
33.35 29.75
26.14 21.79
23.55 33.07
5.82 3.70
4.57 4.69
0.98 0.93
2.66 3.54
2.93 2.53
coating has the potential to function at high temperatures.
zone, 0.6 mm and 0.3 mm, respectively, which means that laser beam melts the powders dominantly and the clad ingredient composes the main part of the laser clad layer.
4. Discussion 4.2. Metallurgical analysis of laser clad layer 4.1. The formation of upper zone and lower zone Two zones accurately describe the laser-clad layer. A heat and mass transfer model w4x elucidates the reason why two zones are formed. Suppose the entire energy of the laser beam melts the powder and substrate materials. While the laser beam interacts with the powder, the melting pool is a uniform solution w5x. As the cooling rate is very high during laser cladding, the melted zone solidifies rapidly and releases crystalline latent heat, which will melt the lower substrate material. Then there is no time to uniformly mix this layer with the upper solution. As a result, the upper zone has the dominant composition of the laser cladding and the lower zone that of the substrate material. In this experiment, the thickness of upper zone is larger than that of the lower
Fig. 4. Hardness of the claddings at room temperature.
The evident features of laser cladding are rapid melting and rapid solidifying. When powders and the substrate surface are melted by the laser beam and become liquid, some unmelted WC particles remain that will form crystalline nuclei during subsequent solidification. However, because the nucleation and development of WC needs more C and W2 C and ŽW, Ti.C 1y x is easier to form than WC, in the liquid it will be difficult to form new WC in subsequent solidification. Also because Ti forms carbides more readily than W w6x, almost all Ti exists as carbide. Other elements, Ni, Co and Cr are in the g phase as elements substituting for Fe, and are found less in carbides. The alloying element Mo functions like W and exists as the carbide M 12 C. Carbides absorb most of the W and Ti, which causes the concentration of W and Ti to decrease. Thus the remnant liquid solution undergoes a eutec-
Fig. 5. Hardness of the cladding at elevating temperature.
Y. Yangr Applied Surface Science 140 (1999) 19–23
23
Thus not only can the operational life span of the laser clad hot tool be prolonged but also tool material can be saved.
6. Conclusions
Fig. 6. Schematic diagram of hot tool used in rolling mill.
tic transformation in subsequent cooling. According to the analysis above, the cooling transformations take place in turn: L ™ W2 C q Ž W,Ti . C 1yx L ™ g q M 12 C
Ž 1. Ž 2.
5. The application of high temperature alloy coating to the hot tool of a rolling mill Fig. 6 is a schematic diagram of a hot tool. Its working surface is on the middle of the tool, a width of 20 mm. After the laser cladding procedure, a coating width of 30 mm and thickness of 2 mm, is obtained. The substrate material is 4Cr5 MoV1 Si steel. A 2008C and 2-h tempering treatment was carried out after laser cladding, which relieves stress resulting from the laser cladding procedure and avoids brittle fracture of the laser-clad tool at room temperature. The result in real working surroundings at 10008C shows the capability of the laser cladding tool to be a rolling steel quantity of 1000–1100 tons per pair. This is twice as good as bulk 4Cr5 MoV1 Si tools, and 5 times better than plain-carbon 45 steel tools. By using the technique of laser cladding, the failed tool can be re-used after another laser coating is applied.
Ž1. Under the condition of laser power of 1500 W, powder-feeding rate of 10 grmin, scanning rate of 3 mmrs, a wide laser cladding layer has been produced by multi-pass overlapping method. Ž2. The metal phases of laser cladding layer’s microstructure mainly consist of g-phase Žmatrix., dispersed WC, W2 C, ŽW, Ti.C 1yx particles and eutectic M 12 C carbides. Ž3. The coating produced by laser cladding has moderate room temperature hardness and a relatively higher high temperature hardness. Ž4. Great success was achieved in the application of laser cladding layer of high temperature alloy to a hot tool. The operational life span was significantly prolonged relative to those of bulk 4Cr5 MoV1 Si and 45 steel, respectively.
References w1x O. Moriaki et al., Development of Laser Cladding Process. Proceeding of LAMP ’87, Osaka, 5, 1987, pp. 156–160. w2x T. Takeda, W.M. Steen, D.R.F. West, Instill Clad Alloy Formation by Laser Cladding, Proc. LIM2, Burmingham, UK, March, 1985, Publ. by IFS Publications, Bedford UK. w3x M. MacIntyre, Laser Hardfacing of RB211 Turbine Blade Shroud Interlocks, Proc. 2nd Int. Conf. On Applications of Lasers in Material Processing, Jan. 1983 Los Angeles, USA. w4x L.J. Li, J. Mazumder, in: K. Mukherji, J. Mazumder ŽEds.. , Laser Processing of Materials, Published by TMS-AIME, 1985, pp. 35–50. w5x T. Chande, J. Mazumder, Appl. Phys. Lett. 57 Ž1982. 2226. w6x Y.Q. Li, J.Y. Liu ŽEds.., Space Phase of Grain Boundary of Superalloy, Metallurgical Industry Press, China, 1990.