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Microstructural evolution in laser deposited Ni–25at.% Mo alloy
Materials Science and Engineering A347 (2003) 1 /4 www.elsevier.com/locate/msea
Microstructural evolution in laser deposited Ni 25at.% Mo alloy /
R. Banerjee a, C.A. Brice a, S. Banerjee b, H.L. Fraser a,* a
Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA b Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India Received 6 December 2001; received in revised form 10 February 2002
Abstract Ni /25at.% Mo alloys have been successfully deposited using the laser engineered net-shaping (LENSTM) process. The feedstock used for the depositions was a blend of elemental Ni and Mo powders. The as-deposited alloy was homogeneously mixed and exhibited a metastable single phase microstructure consisting of an fcc phase with short range chemical ordering of the 1 1/2 0* type. The effect of the thermodynamic enthalpy of mixing of the elemental powders on the microstructural development and rate of solidification in the alloy has been discussed in this paper. # 2002 Published by Elsevier Science B.V. Keywords: Microstructural evolution; Ni /25at.% Mo alloy; Laser engineered net-shaping
1. Introduction In recent years, direct fabrication of metallic components using the solid freeform fabrication route has been shown to be a viable and promising manufacturing technology. One such rapidly developing process is the Laser Engineered Net Shaping (LENSTM) process. A variety of metals and alloys have been deposited using LENSTM [1,2]. The alloys deposited using this process have primarily been from pre-alloyed powders of the required composition. However, since the LENSTM process uses a powder feedstock, it allows the flexibility to deposit a blend of elemental powders and create an alloy in situ. This is a very attractive proposition since if successful, it could potentially reduce the costs of processing to a large extent. In addition, using elemental powder blends in a system with multiple hoppers also allows the possibility of depositing graded compositions within the same sample. There have been only a limited number of studies on the deposition of alloys from elemental powder blends [3,4]. Takeda et al. [3] and Steen et al. [4] have used this approach to study laser deposited Fe /Cr /Ni and Fe /Co /Al alloys, respec-
* Corresponding author. Tel.: /1-614-292-2708; fax: /1-614-2927523 E-mail address: [email protected] (H.L. Fraser).
tively. The results of initial experiments carried out on laser deposition of alloys using LENSTM from elemental powder blends suggests that one of the important factors determining the microstructure and compositional homogeneity in these alloys is the thermodynamic enthalpy of mixing of the constituent elements [5]. Ni /Mo alloys have a wide variety of applications in the nuclear industry. In addition, the Ni /Mo system appears to be a model system for the study of order / disorder phase transformations in fcc -based structures since it exhibits a range of short and long range chemical ordering effects [6 /10]. It is well established that on quenching at sufficiently high rates from the single phase fcc region, Ni3Mo based alloys produce a shortrange ordered state characterized by diffraction intensity at 1/4 {420} positions, commonly referred to as the 1 1/ 2 0* type short-range ordering [7,9,10]. In addition there have been detailed studies of the evolution of longrange order in these alloys from the short-range ordered condition. Ni3Mo based alloys exhibit a tendency of phase separation into the Ni2Mo (Pt2Mo type) and Ni4Mo (D1a type) long-range ordered structures during the initial stages of ordering [9,10]. Therefore, Ni3Mo appears to be an interesting system to study from the viewpoint of rapid solidification effects. Furthermore, the near-net shape processing of this alloy could be potentially useful for a variety of applications. This
0921-5093/02/$ - see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 6 0 3 - 2
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R. Banerjee et al. / Materials Science and Engineering A347 (2003) 1 /4
paper presents an initial study of the microstructural development in a Ni /25at.% Mo alloy deposited using LENSTM from an elemental powder blend.
2. Experimental procedure The deposition of the Ni /Mo alloys was carried out in a LENSTM unit consisting of a 760 W Nd:YAG laser which produced near-infrared laser radiation at a wavelength of 1.064 mm. The energy density was in the range of 30 000 /100 000 W cm 2 and the power used was 450 W. The oxygen content in the glove box was maintained below 5 ppm during the depositions. The measured powder flow rate was 2.57 g min 1 while the argon volumetric flow rate was maintained at 3 l min 1. The Ni /Mo alloys were deposited from a blend of pure elemental Ni (99.9% elemental basis) and Mo powders (99.95% elemental basis) mixed in the ratio of 75 at.% Ni/25at.% Mo onto Ti alloy substrates. The asdeposited alloys were sectioned using electro-discharge machining (EDM) and subsequently investigated by scanning electron microscopy (SEM) in a Philips XL30 FEG-SEM and by TEM in a Philips CM200 TEM. In addition, X-ray diffraction studies have been carried out on the alloys in a Scintag PAD V diffractometer using CuKa incident radiation. Details of the TEM sample preparation procedure are given elsewhere [11].
3. Results and discussion An X-ray diffraction pattern from the as-deposited Ni /25at.% Mo alloy is shown in Fig. 1. All the peaks in this pattern can be consistently indexed based on a single fcc phase with a lattice parameter of a /0.36 nm. The absence of any superlattice peaks in the diffraction pattern suggests the lack of long-range chemical ordering in this fcc phase. The atomic radii of Ni (/0.125 nm) and Mo ( /0.136 nm) differ by 8.5%. Therefore, as a first approximation Vegard’s law can be applied to
Fig. 1. X-ray diffraction pattern from the LENSTM deposited Ni / 25Mo alloy. All the peaks can be consistently indexed based on a single disordered fcc Ni /25Mo phase.
estimate the lattice parameter of an fcc solid solution of Ni and Mo. The lattice parameter of pure fcc Ni is 0.352 nm. Mo exhibits a stable bcc structure at standard temperature and pressure. However, based on the atomic radius of Mo and close packing of atomic spheres, an effective lattice parameter for a metastable fcc phase of Mo has been estimated. The calculated lattice parameter of fcc Mo is /0.385 nm. Applying Vegard’s law, a substitutional solid solution of Ni / 25at.% Mo should exhibit a lattice parameter which equals, afcc (Ni /25Mo) /0.75afcc (Ni)/0.25afcc (Mo) / 0.36 nm. The lattice parameter of this phase measured from the X-ray diffraction pattern is in excellent agreement with the calculated value. This suggests that the as-deposited LENSTMTM alloy consists of a substitutional solid solution of composition Ni /25at.% Mo. An SEM secondary electron image of the as-deposited microstructure is shown in Fig. 2. A dendritic or cellular morphology is evident in the microstructure. The SEM investigations reveal the alloy to be homogeneously mixed without evidence of any unmelted powder particles. TEM studies corroborate the X-ray diffraction results. A single phase microstructure with a large density of dislocations was observed as shown in the bright field micrograph in Fig. 3(a). The high density of dislocations is likely to be a consequence of the rapid solidification rates encountered during the process of laser deposition. Chemical analysis by EDS in the TEM revealed the composition to be Ni /24.8at.% Mo (Fig. 3(b)) which is almost identical to the composition of the elemental powder blend used for the LENSTM deposition. This not only suggests uniform feeding of the powder blend from the hopper but also a homogeneous mixing of the powders in the melt. In some areas of the
Fig. 2. SEM micrograph from the LENSTM deposited Ni /25Mo alloy. Note the dendritic solidification morphology visible within the deposit.
R. Banerjee et al. / Materials Science and Engineering A347 (2003) 1 /4
Fig. 3. (a) TEM micrograph exhibiting a high density of dislocations in the as-deposited Ni /25Mo alloy. (b) EDS spectrum from the alloy confirming the composition. (c) Fine scale inhomogeneities are visible within the fcc Ni /25Mo matrix.
TEM specimen, fine scale inhomogeneities were observed as shown in Fig. 3(c). The origin of these inhomogeneities is not understood at present. Selected area diffraction (SAD) patterns from the TEM specimen are shown in Fig. 4(a and b). The fundamental reflections in these diffraction patterns can be indexed based on an fcc phase with a lattice parameter of a /0.36 nm in agreement with X-ray diffraction results. Thus, Fig. 4(a) corresponds to the [001] zone axis and Fig. 4(b) to the [112] zone axis of the fcc phase. In addition to fundamental reflections, weak superlattice reflections
3
can be observed in both SAD patterns. The arrow in Fig. 4(a) corresponds to a 1/4{420} reciprocal lattice vector. The superlattice reflections appear at 1/4 {420} and 3/4 {420} positions but is extinct at the 1/2 {420} or {210} position. This is a characteristic feature of the 1 1/2 0* type short range ordering in fcc based structures and has been extensively studied in Ni /Mo alloys [6 / 10]. It should be noted that the 1 1/2 0* type ordering is a thermodynamically metastable state for an alloy of composition Ni /25at.% Mo. A relevant section of the binary Ni /Mo phase diagram is shown in Fig. 5 [12]. Thus, the equilibrium process during cooling would involve precipitation of the NiMo phase in the temperature range 1317 /910 8. Depending on the cooling rate, the conventionally observed metastable processes during cooling are, solidification into the metastable Ni3Mo structure, and phase separation into the Ni2Mo and Ni4Mo phases [9,10]. For high quench rates, Ni3Mo based alloys exhibit a 1 1/2 0* type short-range ordered state [7,9,10]. It is interesting to note that the solidification rate achieved during the deposition of the Ni /25at.% Mo alloy using LENSTM was sufficiently high, so as to suppress the above mentioned equilibrium and metastable processes. Another factor that influences the microstructural evolution is the post-deposition thermal exposure encountered by these alloys due to the multiple passes made by the laser beam. Thus, during the deposition of a new layer of material, there is transfer of heat to the previously deposited underlying layers. Though the effects of such post-deposition heating on the microstructure is the subject of ongoing research, it is likely that the short-range ordering initiated in the LENSTM deposited Ni /25Mo alloy is one such effect. Typical solidification rates during LENSTM deposition are /200 /6000 K s 1 [13]. However, it should be noted that the solidification rate is strongly dependent on the thermal conductivity and geometry of the substrate material, the composition of the deposited alloy, and a variety of different processing parameters. One of the important factors which appears to influence the rate of solidification during laser deposition of alloys from elemental powder blends is the thermodynamic enthalpy of mixing of the constituent elements [14]. The binary Ni /Mo system has a large negative enthalpy of mixing / /2600 cal mol 1 [14]. Therefore, the mixing of elemental Ni and Mo in solution would generate substantial heat. This additional heat is expected to locally raise the temperature of the melt pool. The rate of solidification is proportional to the rate of heat extraction by the substrate, which in turn is proportional to the temperature difference between the melt pool and the surrounding solid substrate. Thus, a rapid local increase in the melt pool temperature is expected to result in a faster solidification rate. The results of the present study clearly indicate that the solidification rate
R. Banerjee et al. / Materials Science and Engineering A347 (2003) 1 /4
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Fig. 4. (a) [001] Zone axis electron diffraction pattern from the LENSTM deposited Ni /25Mo alloy exhibiting weak reflections at the 1/4(420) and 3/ 4(420) positions indicative of {1 1/2 0}* ordering. (b) [112] Zone axis diffraction pattern from the same sample exhibiting characteristic features of {1 1/2 0}* ordering.
the laser deposited alloy as well as in determining the rate of solidification.
Acknowledgements This work has been supported in part by the Air Force Office of Scientific Research (Grant # F49620.01.1.0047) and by Lockheed Martin, LMAC, under the Industrial Partnership Program of the Center for the Accelerated Maturation of Materials (CAMM) at the Ohio State University.
Fig. 5. Relevant section of the binary Ni/Mo phase diagram.
encountered by the LENSTM deposited Ni /Mo alloy is sufficiently high to result in the formation of metastable phases.
4. Summary Ni /Mo alloys of nominal composition Ni /25at.% Mo have been successfully deposited from a blend of 75% elemental Ni powder and 25% elemental Mo powder using the LENSTM process. The laser deposited Ni /Mo alloys exhibited a rapidly solidified microstructure consisting of a single fcc phase with short range chemical ordering of the 1/4 420 type. Absence of the equilibrium phases as well as metastable phases observed during conventional processing in Ni /25at.% Mo alloys suggests a very high solidification rate encountered by the laser deposited alloys. The highly exothermic enthalpy of mixing of elemental Ni and Mo appears to play a significant role in the homogeneous mixing of
References [1] D.M. Keicher, W.D. Miller, Met. Powder Rep. 53 (1998) 26. [2] J. Brooks, C. Robino, T. Headley, S. Goods, M. Griffith, Solid Freeform Fabrication Proceedings, University of Texas, Austin, 1999, p. 375. [3] T. Takeda, W.M. Steen, D.R.F. West, Proceedings of ICALEO’84, 44, 1984, p. 151. [4] W.M. Steen, R.M. Vilar, K.G. Watkins, M.G.S. Ferreira, P. Carvalho, C.L. Sexton, M. Pontinha, M. McMohan, Proceedings of ICALEO’92, 52, 1992, p. 278. [5] K.I. Schwendner, R. Banerjee, P.C. Collins, C.A. Brice, H.L. Fraser, Scripta Mater. 45(10) (2001) 1123. [6] R. De Ridder, G. Van Tendeloo, S. Amelinckx, Acta Crystallogr. Sect. A 32 (1976) 216. [7] S. Banerjee, K. Urban, M. Wilkens, Acta Metall. 32 (1984) 299. [8] J. Mayer, K. Urban, Acta Metall. 33 (1985) 539. [9] U.D. Kulkarni, S. Banerjee, Acta Metall. 36 (1988) 413. [10] S. Banerjee, U.D. Kulkarni, K. Urban, Acta Metall. 37 (1989) 35. [11] R. Banerjee, P.C. Collins, K.I. Schwendner, H.L. Fraser, Metall. Mater. Trans. A 33 (2002) 2129. [12] J.L. Murray, Handbook of Binary Phase Diagrams, 2nd Ed., ASM International, 1990, p. 2637. [13] W. Hofmeister, M. Griffith, M. Ensz, J. Smugeresky, JOM (Journal of the Minerals, Metals, and Material Society) 53(9) (2001) 30. [14] L. Kaufmann, H. Nesor, CALPHAD 2 (1) (1978) 81.
Microstructural evolution in laser deposited Ni 25at.% Mo alloy /
R. Banerjee a, C.A. Brice a, S. Banerjee b, H.L. Fraser a,* a
Department of Materials Science and Engineering, The Ohio State University, 2041 College Road, Columbus, OH 43210, USA b Materials Group, Bhabha Atomic Research Centre, Mumbai 400085, India Received 6 December 2001; received in revised form 10 February 2002
Abstract Ni /25at.% Mo alloys have been successfully deposited using the laser engineered net-shaping (LENSTM) process. The feedstock used for the depositions was a blend of elemental Ni and Mo powders. The as-deposited alloy was homogeneously mixed and exhibited a metastable single phase microstructure consisting of an fcc phase with short range chemical ordering of the 1 1/2 0* type. The effect of the thermodynamic enthalpy of mixing of the elemental powders on the microstructural development and rate of solidification in the alloy has been discussed in this paper. # 2002 Published by Elsevier Science B.V. Keywords: Microstructural evolution; Ni /25at.% Mo alloy; Laser engineered net-shaping
1. Introduction In recent years, direct fabrication of metallic components using the solid freeform fabrication route has been shown to be a viable and promising manufacturing technology. One such rapidly developing process is the Laser Engineered Net Shaping (LENSTM) process. A variety of metals and alloys have been deposited using LENSTM [1,2]. The alloys deposited using this process have primarily been from pre-alloyed powders of the required composition. However, since the LENSTM process uses a powder feedstock, it allows the flexibility to deposit a blend of elemental powders and create an alloy in situ. This is a very attractive proposition since if successful, it could potentially reduce the costs of processing to a large extent. In addition, using elemental powder blends in a system with multiple hoppers also allows the possibility of depositing graded compositions within the same sample. There have been only a limited number of studies on the deposition of alloys from elemental powder blends [3,4]. Takeda et al. [3] and Steen et al. [4] have used this approach to study laser deposited Fe /Cr /Ni and Fe /Co /Al alloys, respec-
* Corresponding author. Tel.: /1-614-292-2708; fax: /1-614-2927523 E-mail address: [email protected] (H.L. Fraser).
tively. The results of initial experiments carried out on laser deposition of alloys using LENSTM from elemental powder blends suggests that one of the important factors determining the microstructure and compositional homogeneity in these alloys is the thermodynamic enthalpy of mixing of the constituent elements [5]. Ni /Mo alloys have a wide variety of applications in the nuclear industry. In addition, the Ni /Mo system appears to be a model system for the study of order / disorder phase transformations in fcc -based structures since it exhibits a range of short and long range chemical ordering effects [6 /10]. It is well established that on quenching at sufficiently high rates from the single phase fcc region, Ni3Mo based alloys produce a shortrange ordered state characterized by diffraction intensity at 1/4 {420} positions, commonly referred to as the 1 1/ 2 0* type short-range ordering [7,9,10]. In addition there have been detailed studies of the evolution of longrange order in these alloys from the short-range ordered condition. Ni3Mo based alloys exhibit a tendency of phase separation into the Ni2Mo (Pt2Mo type) and Ni4Mo (D1a type) long-range ordered structures during the initial stages of ordering [9,10]. Therefore, Ni3Mo appears to be an interesting system to study from the viewpoint of rapid solidification effects. Furthermore, the near-net shape processing of this alloy could be potentially useful for a variety of applications. This
0921-5093/02/$ - see front matter # 2002 Published by Elsevier Science B.V. PII: S 0 9 2 1 - 5 0 9 3 ( 0 2 ) 0 0 6 0 3 - 2
2
R. Banerjee et al. / Materials Science and Engineering A347 (2003) 1 /4
paper presents an initial study of the microstructural development in a Ni /25at.% Mo alloy deposited using LENSTM from an elemental powder blend.
2. Experimental procedure The deposition of the Ni /Mo alloys was carried out in a LENSTM unit consisting of a 760 W Nd:YAG laser which produced near-infrared laser radiation at a wavelength of 1.064 mm. The energy density was in the range of 30 000 /100 000 W cm 2 and the power used was 450 W. The oxygen content in the glove box was maintained below 5 ppm during the depositions. The measured powder flow rate was 2.57 g min 1 while the argon volumetric flow rate was maintained at 3 l min 1. The Ni /Mo alloys were deposited from a blend of pure elemental Ni (99.9% elemental basis) and Mo powders (99.95% elemental basis) mixed in the ratio of 75 at.% Ni/25at.% Mo onto Ti alloy substrates. The asdeposited alloys were sectioned using electro-discharge machining (EDM) and subsequently investigated by scanning electron microscopy (SEM) in a Philips XL30 FEG-SEM and by TEM in a Philips CM200 TEM. In addition, X-ray diffraction studies have been carried out on the alloys in a Scintag PAD V diffractometer using CuKa incident radiation. Details of the TEM sample preparation procedure are given elsewhere [11].
3. Results and discussion An X-ray diffraction pattern from the as-deposited Ni /25at.% Mo alloy is shown in Fig. 1. All the peaks in this pattern can be consistently indexed based on a single fcc phase with a lattice parameter of a /0.36 nm. The absence of any superlattice peaks in the diffraction pattern suggests the lack of long-range chemical ordering in this fcc phase. The atomic radii of Ni (/0.125 nm) and Mo ( /0.136 nm) differ by 8.5%. Therefore, as a first approximation Vegard’s law can be applied to
Fig. 1. X-ray diffraction pattern from the LENSTM deposited Ni / 25Mo alloy. All the peaks can be consistently indexed based on a single disordered fcc Ni /25Mo phase.
estimate the lattice parameter of an fcc solid solution of Ni and Mo. The lattice parameter of pure fcc Ni is 0.352 nm. Mo exhibits a stable bcc structure at standard temperature and pressure. However, based on the atomic radius of Mo and close packing of atomic spheres, an effective lattice parameter for a metastable fcc phase of Mo has been estimated. The calculated lattice parameter of fcc Mo is /0.385 nm. Applying Vegard’s law, a substitutional solid solution of Ni / 25at.% Mo should exhibit a lattice parameter which equals, afcc (Ni /25Mo) /0.75afcc (Ni)/0.25afcc (Mo) / 0.36 nm. The lattice parameter of this phase measured from the X-ray diffraction pattern is in excellent agreement with the calculated value. This suggests that the as-deposited LENSTMTM alloy consists of a substitutional solid solution of composition Ni /25at.% Mo. An SEM secondary electron image of the as-deposited microstructure is shown in Fig. 2. A dendritic or cellular morphology is evident in the microstructure. The SEM investigations reveal the alloy to be homogeneously mixed without evidence of any unmelted powder particles. TEM studies corroborate the X-ray diffraction results. A single phase microstructure with a large density of dislocations was observed as shown in the bright field micrograph in Fig. 3(a). The high density of dislocations is likely to be a consequence of the rapid solidification rates encountered during the process of laser deposition. Chemical analysis by EDS in the TEM revealed the composition to be Ni /24.8at.% Mo (Fig. 3(b)) which is almost identical to the composition of the elemental powder blend used for the LENSTM deposition. This not only suggests uniform feeding of the powder blend from the hopper but also a homogeneous mixing of the powders in the melt. In some areas of the
Fig. 2. SEM micrograph from the LENSTM deposited Ni /25Mo alloy. Note the dendritic solidification morphology visible within the deposit.
R. Banerjee et al. / Materials Science and Engineering A347 (2003) 1 /4
Fig. 3. (a) TEM micrograph exhibiting a high density of dislocations in the as-deposited Ni /25Mo alloy. (b) EDS spectrum from the alloy confirming the composition. (c) Fine scale inhomogeneities are visible within the fcc Ni /25Mo matrix.
TEM specimen, fine scale inhomogeneities were observed as shown in Fig. 3(c). The origin of these inhomogeneities is not understood at present. Selected area diffraction (SAD) patterns from the TEM specimen are shown in Fig. 4(a and b). The fundamental reflections in these diffraction patterns can be indexed based on an fcc phase with a lattice parameter of a /0.36 nm in agreement with X-ray diffraction results. Thus, Fig. 4(a) corresponds to the [001] zone axis and Fig. 4(b) to the [112] zone axis of the fcc phase. In addition to fundamental reflections, weak superlattice reflections
3
can be observed in both SAD patterns. The arrow in Fig. 4(a) corresponds to a 1/4{420} reciprocal lattice vector. The superlattice reflections appear at 1/4 {420} and 3/4 {420} positions but is extinct at the 1/2 {420} or {210} position. This is a characteristic feature of the 1 1/2 0* type short range ordering in fcc based structures and has been extensively studied in Ni /Mo alloys [6 / 10]. It should be noted that the 1 1/2 0* type ordering is a thermodynamically metastable state for an alloy of composition Ni /25at.% Mo. A relevant section of the binary Ni /Mo phase diagram is shown in Fig. 5 [12]. Thus, the equilibrium process during cooling would involve precipitation of the NiMo phase in the temperature range 1317 /910 8. Depending on the cooling rate, the conventionally observed metastable processes during cooling are, solidification into the metastable Ni3Mo structure, and phase separation into the Ni2Mo and Ni4Mo phases [9,10]. For high quench rates, Ni3Mo based alloys exhibit a 1 1/2 0* type short-range ordered state [7,9,10]. It is interesting to note that the solidification rate achieved during the deposition of the Ni /25at.% Mo alloy using LENSTM was sufficiently high, so as to suppress the above mentioned equilibrium and metastable processes. Another factor that influences the microstructural evolution is the post-deposition thermal exposure encountered by these alloys due to the multiple passes made by the laser beam. Thus, during the deposition of a new layer of material, there is transfer of heat to the previously deposited underlying layers. Though the effects of such post-deposition heating on the microstructure is the subject of ongoing research, it is likely that the short-range ordering initiated in the LENSTM deposited Ni /25Mo alloy is one such effect. Typical solidification rates during LENSTM deposition are /200 /6000 K s 1 [13]. However, it should be noted that the solidification rate is strongly dependent on the thermal conductivity and geometry of the substrate material, the composition of the deposited alloy, and a variety of different processing parameters. One of the important factors which appears to influence the rate of solidification during laser deposition of alloys from elemental powder blends is the thermodynamic enthalpy of mixing of the constituent elements [14]. The binary Ni /Mo system has a large negative enthalpy of mixing / /2600 cal mol 1 [14]. Therefore, the mixing of elemental Ni and Mo in solution would generate substantial heat. This additional heat is expected to locally raise the temperature of the melt pool. The rate of solidification is proportional to the rate of heat extraction by the substrate, which in turn is proportional to the temperature difference between the melt pool and the surrounding solid substrate. Thus, a rapid local increase in the melt pool temperature is expected to result in a faster solidification rate. The results of the present study clearly indicate that the solidification rate
R. Banerjee et al. / Materials Science and Engineering A347 (2003) 1 /4
4
Fig. 4. (a) [001] Zone axis electron diffraction pattern from the LENSTM deposited Ni /25Mo alloy exhibiting weak reflections at the 1/4(420) and 3/ 4(420) positions indicative of {1 1/2 0}* ordering. (b) [112] Zone axis diffraction pattern from the same sample exhibiting characteristic features of {1 1/2 0}* ordering.
the laser deposited alloy as well as in determining the rate of solidification.
Acknowledgements This work has been supported in part by the Air Force Office of Scientific Research (Grant # F49620.01.1.0047) and by Lockheed Martin, LMAC, under the Industrial Partnership Program of the Center for the Accelerated Maturation of Materials (CAMM) at the Ohio State University.
Fig. 5. Relevant section of the binary Ni/Mo phase diagram.
encountered by the LENSTM deposited Ni /Mo alloy is sufficiently high to result in the formation of metastable phases.
4. Summary Ni /Mo alloys of nominal composition Ni /25at.% Mo have been successfully deposited from a blend of 75% elemental Ni powder and 25% elemental Mo powder using the LENSTM process. The laser deposited Ni /Mo alloys exhibited a rapidly solidified microstructure consisting of a single fcc phase with short range chemical ordering of the 1/4 420 type. Absence of the equilibrium phases as well as metastable phases observed during conventional processing in Ni /25at.% Mo alloys suggests a very high solidification rate encountered by the laser deposited alloys. The highly exothermic enthalpy of mixing of elemental Ni and Mo appears to play a significant role in the homogeneous mixing of
References [1] D.M. Keicher, W.D. Miller, Met. Powder Rep. 53 (1998) 26. [2] J. Brooks, C. Robino, T. Headley, S. Goods, M. Griffith, Solid Freeform Fabrication Proceedings, University of Texas, Austin, 1999, p. 375. [3] T. Takeda, W.M. Steen, D.R.F. West, Proceedings of ICALEO’84, 44, 1984, p. 151. [4] W.M. Steen, R.M. Vilar, K.G. Watkins, M.G.S. Ferreira, P. Carvalho, C.L. Sexton, M. Pontinha, M. McMohan, Proceedings of ICALEO’92, 52, 1992, p. 278. [5] K.I. Schwendner, R. Banerjee, P.C. Collins, C.A. Brice, H.L. Fraser, Scripta Mater. 45(10) (2001) 1123. [6] R. De Ridder, G. Van Tendeloo, S. Amelinckx, Acta Crystallogr. Sect. A 32 (1976) 216. [7] S. Banerjee, K. Urban, M. Wilkens, Acta Metall. 32 (1984) 299. [8] J. Mayer, K. Urban, Acta Metall. 33 (1985) 539. [9] U.D. Kulkarni, S. Banerjee, Acta Metall. 36 (1988) 413. [10] S. Banerjee, U.D. Kulkarni, K. Urban, Acta Metall. 37 (1989) 35. [11] R. Banerjee, P.C. Collins, K.I. Schwendner, H.L. Fraser, Metall. Mater. Trans. A 33 (2002) 2129. [12] J.L. Murray, Handbook of Binary Phase Diagrams, 2nd Ed., ASM International, 1990, p. 2637. [13] W. Hofmeister, M. Griffith, M. Ensz, J. Smugeresky, JOM (Journal of the Minerals, Metals, and Material Society) 53(9) (2001) 30. [14] L. Kaufmann, H. Nesor, CALPHAD 2 (1) (1978) 81.