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Microstructure and tensile properties of L12—Type NiCrAl alloy prepared by vacuum plasma spray forming
ScriptaMctaUurgi~ stIkiat&atia, Vol. 33, No. 7, pp. 1159-l 162,lWS Elscvier Sciencx Lid cqYjli&to1995Aaa~caInc. PrintedintheUSAAllri&b-cd 0956716xt95 $9.50 + .oo
Pergamon
0956-716X(95)00332-0
MICROSTRUCTURE AND TENSILE PROPERTIES OF Ll, - TYPE Ni-Cr-Al ALLOY PREPARED BY VACUUM PLASMA SPRAY FORMING R. Tiwari,’ S. Sampath, B. Gudmundsson,* G. Halada, C. R. Clayton and H. Herman Department of Materials Science and Engineering State University of New York at Stony Brook, Stony Brook, NY 11794-2275 USA (Received April 10,1995) (Revised May 3 1,1995) Introduction
The combination of strength, ductility and oxidation resistance makes single phase y ‘-N&Al(I. 12)a candidate foruseinhigh &npemmm structural applications. An attractive property of this alloy is a substantial increase in yield strength with temperature up to about 700 “C [ 1,2]. Although ordered intermetallic compounds are inherently brittle, addition of boron in amounts of 0.5 at% (0.1 wt%) yields frscture strains up to 30% in polycrystalline N&Al [ I-31. This is due to boron segregation to grain boundaries, leading to increased grain boundaty cohesive strer@, which results in transgranular rather than intergranular failure [4]. Furthermore, alloying with chromium can lead to a further increase in the tensile strength and high temperature corrosion resistance of y ‘-N&Al[5]. Rapid soliditicationprocessing of ordered intermetallics results in a chemically homogeneous and retlned grain structure, which could influence the overall mechanical properties. In fact, rapid solidification processing of boron-doped y ‘- N&Al yields lower ductility in the as-sprayed condition as compared to annealed [2,6]. This is due to the boron distribution in the alloy In the as-sprayed condition boron is uniformly distributed in the alloy and, therefore, does not contribute significantly to the ductility. Upon annealing, boron can di%bse to grain boundaries and duct&e the alloy [2,6]. Vacuum (actually low pressure) plasma spraying (VPS) typically produces near theoretical density (>99.5%), oxi& and stress-free deposits. Due to its ability to retain rapid solidification characteristics, while fabricating thick bulk forms, VPS, is a potential technique for processing of intermetallic compounds. In fact, in an investigation by Chang and coworkers [7], vacuum plasma sprayed boron-doped N&Al alloys (0.75 at%B) showed better mechanical properties than alloys prepared by melt spinning and hot isostatic pressing. It has also been reported by the authors that VPS can produce dense intermetallics and their composites [8, 91.Inthisstudythemicrostructure and tensile properties of a vacuum plasma spray formed Ni-Cr-Al alloy are correlated to the processing and post-deposition annealing.
'F'rexotly atUCAR Carbon Company,Cleveland OH. hesently
at Asea Brown Boveri, Fimpoq
Sweden.
1159
1160
Vol. 33, No. 7
VACUUM PLASMA SPRAY FORMED NiCr-Al ALLOY
Emerimental
Procedure
Single phase inert gas atomized Ni-6.0Cr-19.5Al-0.3Hf-0.25B-0.2C (at%) powder, with a size range of 36 to 5 ~.tm,was supplied by Hoganas AB, Sweden. Plasma spraying was performed using a Plasma Technik vacuum plasma spray unit with the parameters listed in Table I. Deposits, 1.5-2.5 mm thick, were sprayed onto mild steel substrates. Tensile specimens were machined from free-standing deposits, which were obtained by separating from the mild steel substrates atter spraying. The specimens were annealed in flowing argon at 11OO”Cfor2,6and24hoursandfumacecooled. Room temperature tensile tests on the as-sprayed and annealed specimens were conducted under a uniform strain rate of 4xlO?s. The microstrucmre and fracture surfaces of the specimens were examined by means of optical and scanning election microscopy Auger electron microscopy of the fractures surfaces was performed to determine boron distribution in the material. Results and Discussion
Vacuumplasma spray processing yields fully dense y '-N&Al deposits with a lamellar deposit structure built up of splats approximately 100 bun in diameter and S- 10 IMIIin thickness, Fig. 1(a). No grain structure can be resolved in the etched cross-section of the as-sprayed condition. However, a&r annealing at 1100 “C for 2,6 and 24 hours, the lame&u- structure is dissolved and the individual grains can clearly be seen in the slrucmre, as shown in Fig. 1(b). The average grain size for the above annealing treatments was measured to be 5,5.8, and 7 l.tm,respectively The tensile stress-strain behavior of the as-sprayed and annealed deposits is shown in Fig. 2. The room temperature tensile strength of the VPS-processsed y ‘- N&Alalloys decreased from 811 MPa in the as-sprayed condition to 550 h4Pa after annealing at 1100°C for 24 hours; Fig. 2 (a). Further, yield strengths of the VPSpmcessed deposits were measured to 670,560, and 430 after after annealing for 2,6, and 24 hrs at 1lOO”C, respectively. It can also be noted from Fig. 2(a) that no yielding was observed in the as-sprayed condition. The measured yield strength and corresponding grain sizes exhibit Hall-Petch behavior; with a slope k = 3.4 Mpa m* [see Fig. 2(b)] and agrees well with an earlier investigation on y ‘- N&Al alloys [lo]. The as-sprayed material showed no sign of plastic deformation whereas all of the annealed material showed a ductile behaviour. The ductility value increased gradually with annealing time to a maximum of 1.9 % after 24h at 1100°C (Fig. 2). However, these fracture strain values are considerably lower than a conventionally cast
(4
04
Figure 1. Optical micrographs of WS formed deposits(a) as-sprayed (b) annealed.
Vol. 33, No. 7
1161
VACUUM PLASMA SPRAY FORMED Ni-Cr-Al ALLOY
0 (Pa 4.0
5.0
6.0
7.0
8.0
9.0
a
P ; 500 1 ci
250
•l
0
0
4M)2.00
2.25
2.50
2
1
2.75
dD
Strain (%)
(b)
(4
Figure2. (a) Stress-strain behaviourof as-sprayed andannealed VPS formeddeposits.@) Hall-Petch behaviorof yieldstressfor VPS formedandannealed deposits.
boron-doped N&AIwhich show elongationsup to 30 % [2]. This may be attributed to lower starting B content in this composition and/or to B loss during spraying. The fracture surface of as-sprayed and annealed coatings are shown in Figs. 3(a) and (b). It can be seen that the fracture is mainly intergranular in the as-sprayed and annealed conditions. However, some evidence of dimple fxmation at the grain boundaries can be found on the fracture surfaces of the annealed deposits. Results of Auger eleclnm microscopy measurements on the fracture surfaces of the as-sprayed and annealed deposits are given in Figure 4. In the as-sprayed condition, no B was detected at the grain boundaries. The lack of B at grain boumkxies in the as-sprayed condition is a combination of a low B content (0.05 wWo) and a homogeneous boron distribution Rapid solidi&tion during vacuum plasma spraying, yields a chemically homogeneous microstructum and thus, brittle behavior of the y ‘- N&Al deposit is observed. However, upon annealing, B presence is detected on the Cacture surf= at the grain boundaries of the y ‘- N&Al deposit. This ditfusion of B to the grain boundaries results in improved grain boundary strength and increased fractilre strain.
(a) Figure3. Scaming electronmicrographs of fiwtms surf-
(b) (a) as-spayed@) annealed at 1100°C for 2 hrs.
1162
VACUUM
PLASMA
SPRAY FORMED Ni-Cr-Al ALLOY
Vol. 33, No. 7
Conciuaiong
This study illustrates the potential of producing near-net shape structural materials of N&Al intermetallic compounds by vacuum plasma spray forming (WS). The VPS processed N&Al material show rapid solidification characteristics such as fine grain size and high yield strength values. The ductility of the material was relatively low but increased with annealing at 1100 “C to a maximumof1.9%after24hours.Themcreased ductilitywith annealing is due to boron ditlhsion to grain boundaries. As a consequence of grain growth during annealing at 1100°C the yield strength of the material decreases with prolonged annealing due to grain coarsening. The stmngthvalues and corresponding grain size agree well with earlier investigations on N&Al compounds.
The authors wish to thank Mr. Glenn Bancke for help with vacuum plasma spraying and Hogan&s AB, Sweden for providing powder and performing the chemical analysis on the powder. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
N.S. Stoloff, Mat. Reti. Symp. Proc. Vol39,3 (1985). C.C. Koch, M Met. Rev., 33 (1988) 201. K Aolci and 0. Immi, J. Japan. Inst Metals., 43,119O (1979). R Messmer and C.L. Brimt, Acta Metall., 32,1703 (1984). S.G. Huang, E.L. Hall, KM Chang, and RP. I.&m, Metall, Tram, 17A, 1685 (1986). A Choudbary, C.L. White, aud C.R Brooks, Ser. Metall., 20,1061(1986). KW. Chang, Al. Taub, and S.C. Huaug, Mat. Res. Sot. Symp. Proc., Vol39,335 (1985). S.Sampath,RTiwari,B.~ aad H. Heman, Ser. Metall., 25,1425 (1991). S. Sampatb, B. Gudmuadsson, R Tiwari, and H. Herman, Themal Spray Research aud Applications, Ed: T.F.Benmki, ASM hltemational357 (1990). E.M. Schulson,T.P. Weiss,D.B.Viem,andI.Baker,ActaMet.,33,1587(1985). H. Ha&m, L. L’Estrade, and M. Rokon, Pmceed& of the Plasma T&m& Symposium, Lucerne, Switzerland Vol.2 179 (1988).
Pergamon
0956-716X(95)00332-0
MICROSTRUCTURE AND TENSILE PROPERTIES OF Ll, - TYPE Ni-Cr-Al ALLOY PREPARED BY VACUUM PLASMA SPRAY FORMING R. Tiwari,’ S. Sampath, B. Gudmundsson,* G. Halada, C. R. Clayton and H. Herman Department of Materials Science and Engineering State University of New York at Stony Brook, Stony Brook, NY 11794-2275 USA (Received April 10,1995) (Revised May 3 1,1995) Introduction
The combination of strength, ductility and oxidation resistance makes single phase y ‘-N&Al(I. 12)a candidate foruseinhigh &npemmm structural applications. An attractive property of this alloy is a substantial increase in yield strength with temperature up to about 700 “C [ 1,2]. Although ordered intermetallic compounds are inherently brittle, addition of boron in amounts of 0.5 at% (0.1 wt%) yields frscture strains up to 30% in polycrystalline N&Al [ I-31. This is due to boron segregation to grain boundaries, leading to increased grain boundaty cohesive strer@, which results in transgranular rather than intergranular failure [4]. Furthermore, alloying with chromium can lead to a further increase in the tensile strength and high temperature corrosion resistance of y ‘-N&Al[5]. Rapid soliditicationprocessing of ordered intermetallics results in a chemically homogeneous and retlned grain structure, which could influence the overall mechanical properties. In fact, rapid solidification processing of boron-doped y ‘- N&Al yields lower ductility in the as-sprayed condition as compared to annealed [2,6]. This is due to the boron distribution in the alloy In the as-sprayed condition boron is uniformly distributed in the alloy and, therefore, does not contribute significantly to the ductility. Upon annealing, boron can di%bse to grain boundaries and duct&e the alloy [2,6]. Vacuum (actually low pressure) plasma spraying (VPS) typically produces near theoretical density (>99.5%), oxi& and stress-free deposits. Due to its ability to retain rapid solidification characteristics, while fabricating thick bulk forms, VPS, is a potential technique for processing of intermetallic compounds. In fact, in an investigation by Chang and coworkers [7], vacuum plasma sprayed boron-doped N&Al alloys (0.75 at%B) showed better mechanical properties than alloys prepared by melt spinning and hot isostatic pressing. It has also been reported by the authors that VPS can produce dense intermetallics and their composites [8, 91.Inthisstudythemicrostructure and tensile properties of a vacuum plasma spray formed Ni-Cr-Al alloy are correlated to the processing and post-deposition annealing.
'F'rexotly atUCAR Carbon Company,Cleveland OH. hesently
at Asea Brown Boveri, Fimpoq
Sweden.
1159
1160
Vol. 33, No. 7
VACUUM PLASMA SPRAY FORMED NiCr-Al ALLOY
Emerimental
Procedure
Single phase inert gas atomized Ni-6.0Cr-19.5Al-0.3Hf-0.25B-0.2C (at%) powder, with a size range of 36 to 5 ~.tm,was supplied by Hoganas AB, Sweden. Plasma spraying was performed using a Plasma Technik vacuum plasma spray unit with the parameters listed in Table I. Deposits, 1.5-2.5 mm thick, were sprayed onto mild steel substrates. Tensile specimens were machined from free-standing deposits, which were obtained by separating from the mild steel substrates atter spraying. The specimens were annealed in flowing argon at 11OO”Cfor2,6and24hoursandfumacecooled. Room temperature tensile tests on the as-sprayed and annealed specimens were conducted under a uniform strain rate of 4xlO?s. The microstrucmre and fracture surfaces of the specimens were examined by means of optical and scanning election microscopy Auger electron microscopy of the fractures surfaces was performed to determine boron distribution in the material. Results and Discussion
Vacuumplasma spray processing yields fully dense y '-N&Al deposits with a lamellar deposit structure built up of splats approximately 100 bun in diameter and S- 10 IMIIin thickness, Fig. 1(a). No grain structure can be resolved in the etched cross-section of the as-sprayed condition. However, a&r annealing at 1100 “C for 2,6 and 24 hours, the lame&u- structure is dissolved and the individual grains can clearly be seen in the slrucmre, as shown in Fig. 1(b). The average grain size for the above annealing treatments was measured to be 5,5.8, and 7 l.tm,respectively The tensile stress-strain behavior of the as-sprayed and annealed deposits is shown in Fig. 2. The room temperature tensile strength of the VPS-processsed y ‘- N&Alalloys decreased from 811 MPa in the as-sprayed condition to 550 h4Pa after annealing at 1100°C for 24 hours; Fig. 2 (a). Further, yield strengths of the VPSpmcessed deposits were measured to 670,560, and 430 after after annealing for 2,6, and 24 hrs at 1lOO”C, respectively. It can also be noted from Fig. 2(a) that no yielding was observed in the as-sprayed condition. The measured yield strength and corresponding grain sizes exhibit Hall-Petch behavior; with a slope k = 3.4 Mpa m* [see Fig. 2(b)] and agrees well with an earlier investigation on y ‘- N&Al alloys [lo]. The as-sprayed material showed no sign of plastic deformation whereas all of the annealed material showed a ductile behaviour. The ductility value increased gradually with annealing time to a maximum of 1.9 % after 24h at 1100°C (Fig. 2). However, these fracture strain values are considerably lower than a conventionally cast
(4
04
Figure 1. Optical micrographs of WS formed deposits(a) as-sprayed (b) annealed.
Vol. 33, No. 7
1161
VACUUM PLASMA SPRAY FORMED Ni-Cr-Al ALLOY
0 (Pa 4.0
5.0
6.0
7.0
8.0
9.0
a
P ; 500 1 ci
250
•l
0
0
4M)2.00
2.25
2.50
2
1
2.75
dD
Strain (%)
(b)
(4
Figure2. (a) Stress-strain behaviourof as-sprayed andannealed VPS formeddeposits.@) Hall-Petch behaviorof yieldstressfor VPS formedandannealed deposits.
boron-doped N&AIwhich show elongationsup to 30 % [2]. This may be attributed to lower starting B content in this composition and/or to B loss during spraying. The fracture surface of as-sprayed and annealed coatings are shown in Figs. 3(a) and (b). It can be seen that the fracture is mainly intergranular in the as-sprayed and annealed conditions. However, some evidence of dimple fxmation at the grain boundaries can be found on the fracture surfaces of the annealed deposits. Results of Auger eleclnm microscopy measurements on the fracture surfaces of the as-sprayed and annealed deposits are given in Figure 4. In the as-sprayed condition, no B was detected at the grain boundaries. The lack of B at grain boumkxies in the as-sprayed condition is a combination of a low B content (0.05 wWo) and a homogeneous boron distribution Rapid solidi&tion during vacuum plasma spraying, yields a chemically homogeneous microstructum and thus, brittle behavior of the y ‘- N&Al deposit is observed. However, upon annealing, B presence is detected on the Cacture surf= at the grain boundaries of the y ‘- N&Al deposit. This ditfusion of B to the grain boundaries results in improved grain boundary strength and increased fractilre strain.
(a) Figure3. Scaming electronmicrographs of fiwtms surf-
(b) (a) as-spayed@) annealed at 1100°C for 2 hrs.
1162
VACUUM
PLASMA
SPRAY FORMED Ni-Cr-Al ALLOY
Vol. 33, No. 7
Conciuaiong
This study illustrates the potential of producing near-net shape structural materials of N&Al intermetallic compounds by vacuum plasma spray forming (WS). The VPS processed N&Al material show rapid solidification characteristics such as fine grain size and high yield strength values. The ductility of the material was relatively low but increased with annealing at 1100 “C to a maximumof1.9%after24hours.Themcreased ductilitywith annealing is due to boron ditlhsion to grain boundaries. As a consequence of grain growth during annealing at 1100°C the yield strength of the material decreases with prolonged annealing due to grain coarsening. The stmngthvalues and corresponding grain size agree well with earlier investigations on N&Al compounds.
The authors wish to thank Mr. Glenn Bancke for help with vacuum plasma spraying and Hogan&s AB, Sweden for providing powder and performing the chemical analysis on the powder. References 1. 2. 3. 4. 5. 6. 7. 8. 9. 10. 11.
N.S. Stoloff, Mat. Reti. Symp. Proc. Vol39,3 (1985). C.C. Koch, M Met. Rev., 33 (1988) 201. K Aolci and 0. Immi, J. Japan. Inst Metals., 43,119O (1979). R Messmer and C.L. Brimt, Acta Metall., 32,1703 (1984). S.G. Huang, E.L. Hall, KM Chang, and RP. I.&m, Metall, Tram, 17A, 1685 (1986). A Choudbary, C.L. White, aud C.R Brooks, Ser. Metall., 20,1061(1986). KW. Chang, Al. Taub, and S.C. Huaug, Mat. Res. Sot. Symp. Proc., Vol39,335 (1985). S.Sampath,RTiwari,B.~ aad H. Heman, Ser. Metall., 25,1425 (1991). S. Sampatb, B. Gudmuadsson, R Tiwari, and H. Herman, Themal Spray Research aud Applications, Ed: T.F.Benmki, ASM hltemational357 (1990). E.M. Schulson,T.P. Weiss,D.B.Viem,andI.Baker,ActaMet.,33,1587(1985). H. Ha&m, L. L’Estrade, and M. Rokon, Pmceed& of the Plasma T&m& Symposium, Lucerne, Switzerland Vol.2 179 (1988).