- Email: [email protected]
Nanoscale carbide precipitation in the recast layer of a percussion laser-drilled superalloy
Available online at www.sciencedirect.com
Scripta Materialia 61 (2009) 943–946 www.elsevier.com/locate/scriptamat
Nanoscale carbide precipitation in the recast layer of a percussion laser-drilled superalloy Jacquelynn K.M. Garofano, Harris L. Marcus and Mark Aindow* Materials Science and Engineering Program, Department of Chemical Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136, USA Received 17 June 2009; accepted 28 July 2009 Available online 6 August 2009
The microstructure of the recast layer formed during percussion laser drilling of a powder-metallurgy Ni-based superalloy has been studied using electron microscopy in conjunction with focused ion beam serial section tomography. It is shown that the dendritic microsegregation of Ti in the supersaturated gamma recast grains reported previously is accompanied by the formation of nanoscale precipitates in the regions between the dendrite arms. Transmission electron microscopy analysis of extraction replicas has shown that the precipitates are Ti-rich MC-type carbides. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nickel alloys; Microstructure; Percussion laser drilling; Scanning electron microscopy (SEM); Transmission electron microscopy (TEM)
Economic and environmental considerations require that gas turbine engines for aircraft operate at temperatures well above the softening points of the Ni-based superalloys that are used for the hot-zone components. The structural integrity of these components is maintained by using thermal barrier coatings (TBCs) and/or film cooling with compressed air forced through vast networks of fine (<500 lm in diameter) cooling holes [1,2]. For components that require both TBCs and film cooling, the cooling holes are often produced using laser drilling, which enables holes to be drilled through the TBC-coated superalloy [3,4]. This gives a more reproducible hole geometry than conventional techniques such as electrodischarge machining (EDM), wherein the TBC must be applied after drilling, leading to the possibility of hole blockage. Percussion laser drilling is usually preferred for aerospace components as a compromise between single-shot drilling, which is very rapid but leads to excessive debris and extensive metallurgical effects [5], and laser trepanning or femtosecond ablative laser drilling, which produce very high quality holes but with a much longer cycle time [6]. The main metallurgical effects that can occur during percussion laser drilling are the formation of a recast layer and/or a heat-affected zone (HAZ). The
* Corresponding author. Tel.: +1 860 486 2644; fax: +1 860 486 4745; e-mail: [email protected]
recast layer is comprised of molten/vaporized metal that is not ejected from the hole by the vapor generated during the laser pulse but instead resolidifies on the sidewall of the hole. The HAZ consists of a region of the base metal surrounding the hole that has not melted during the drilling but has undergone microstructural changes due to the thermal history in a manner akin to the HAZs formed during laser welding and cladding [7]. It is known that the recast layer and/or HAZ can lead to significant fatigue debits for laser-drilled components, but there is little in the open literature about the microstructural details of the HAZ and recast layers. In recent work by the present authors [8,9], a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and focused ion beam (FIB) microscopy was used to study the details of metallurgical effects in percussion laser-drilled samples of the powder-metallurgy (P/M) Ni-based superalloy IN100. Analysis of samples drilled using typical process conditions led to the surprising result that there was no discernible metallurgical HAZ. Moreover, the recast layer exhibited a coarse polycrystalline c microstructure within which the precipitation of the L12 c0 phase was suppressed completely. The c grains in the recast layer contained a fine cellular substructure and it was shown that these cells correspond to dendritic microsegregation of Ti and Al during the rapid solidification of the recast layer. In the present paper we describe further studies on this cellular substructure
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.07.034
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J. K. M. Garofano et al. / Scripta Materialia 61 (2009) 943–946
using serial-section FIB tomography and TEM of extraction replicas. It is shown that the dendritic microsegregation in the recast layer is accompanied by nanoscale carbide precipitation at the cell boundaries. This precipitation can account for the striking etching/electropolishing effects observed in our previous studies and could have important consequences for the fatigue debits incurred in laser-drilled components. As described in detail elsewhere [8,9], coupons of P/M IN100 (nominal composition: Ni–18.5Co–12.4Cr–5.0Al– 4.3Ti–3.2Mo–0.8V–0.07C–0.06Zr–0.02B all in wt.%) were percussion laser-drilled to partial penetration using four pulses from a Nd:YAG laser giving a hole depth of 4 mm. Secondary electron (SE) SEM images from etched cross-sectional specimens through the holes using a JEOL 6335F field-emission SEM revealed: an abrupt base-metal/ recast interface; the usual c + c0 microstructure in the base metal (e.g. [10,11]) with a hierarchy of c0 precipitates on different length scales (total c0 volume fraction is 60%); no variation in the microstructure of the base metal with distance from the interface, i.e. no HAZ; and a cellular microstructure in the recast layer wherein cells 300–500 nm in diameter are arranged into colonies 2–10 lm in diameter. Further TEM analysis of this cellular microstructure using imaging, selected-area diffraction patterns (SADPs) and energy-dispersive X-ray spectrometry (EDXS) on electropolished foils in an FEI Tecnai T12 TEM showed: no structural differences between the cells and the boundaries between them (all face-centered-cubic (fcc) c phase); no differences in orientation between cells within a colony; and only subtle variations in local chemistry with enrichment of Ti and Al at the cell boundaries. Representative SEM and TEM micrographs from the recast layer are presented in Figure 1. While these data are consistent with the recast layer solidifying as a coarse-grained single-phase c microstructure, with dendritic microsegregation giving the cellular/ colony structure observed experimentally, it seems implausible that such subtle compositional variations would give rise to the pronounced topographical etching/electropolishing effects observed in the SEM and TEM samples. To investigate the origins of these effects, serial-section FIB tomography (e.g. [12,13]) was undertaken to examine the recast structure in greater detail. A cross-sectional sample was processed in an FEI Helios NanoLab 400S dual-beam FIB/SEM. A 2 lm thick protective Pt capping layer was deposited on the surface of the recast layer at the side-wall of a hole and the Ga+ ion beam was used to cut trenches through the Pt cap to produce a Pt-capped pier with a length of 20 lm, a width of 10 lm and a height of approximately 25 lm. The recast microstructure was revealed in the vertical end-wall of the pier, and FIB serial sectioning was performed by using the FEI Slice and View software package to produce a coordinated series of cuts in which the pier was shortened in increments of 50 nm. 200 FIB cuts were made, and at each point a back-scattered electron (BSE) SEM image was obtained using a 5 kV e-beam with a 50 V bias on the in-lens detector to minimize SE contributions to the measured signal. The first and last images in this series are shown in Figure 2a and b, respectively. A 1024 884 pixel frame that was common to all 200 BSE images was selected and the stack of these frames was then
Figure 1. Microstructure of the recast layer: (a) SE FESEM image of cellular colonies; (b) BF TEM image of cells within a colony. The inset SADP in (b) shows that these cells are comprised of supersaturated c and that adjacent cells are at the same orientation (i.e. each colony is a grain).
reconstructed using FEI ResolveRT/Amira software to give three-dimensional representations of the recast microstructure. Two different projections of the reconstructed volume are shown in Figure 2c and d1; these are both from the same 8.5 9.7 10 lm volume of the microstructure and the individual voxels are approximately 8 11 50 nm. These projections revealed more clearly the morphology of the large (>2 lm) dendritic grains, corresponding to the cellular colonies, but also showed that there were fine particles decorating the boundaries of the cells. We note that no such particles had been observed previously, either in the SEM images of the etched cross-sectional metallographic samples, or in the TEM images of the electropolished thin foils. Such nanoscale precipitates can be studied most effectively by using replication to extract them from the metallic phase(s) (e.g. [14]). In this study the particles were extracted using the hybrid extraction replication technique described by Song and Aindow [15]. This method involves: electropolishing using a solution of 10% HCl, 89% methanol and 1% citric acid; etching
1
The animation from which these two projections were taken is included as online Supplementary material.
J. K. M. Garofano et al. / Scripta Materialia 61 (2009) 943–946
945
typical bright-field (BF) TEM images are shown in Figure 3. All of the particles in such regions were extremely small (<20 nm in diameter) and these were located almost exclusively at the cell boundaries as revealed by the thicker ridges formed in the carbon film at these positions. We note that the densities of these particles are much higher than those observed by FIB tomography. This presumably arises because the BSE SEM images used for the tomographic reconstruction give a poorer resolution of the particles than the TEM images. Since the particles observed in the replicas range from about 5 to 20 nm in diameter, one might expect that only the largest of these would be detected/resolved in the BSE SEM images. We also note that while most of the particles appear to be spread evenly around the periphery of the cells, there are some larger clumps of particles present (e.g. at A in Fig. 3a). The SADPs obtained from the recast layer regions of the replicas exhibited sharp continuous rings (e.g. Fig. 3b, inset) indicating that the nanoparticles are crystalline and are oriented randomly with respect to the surrounding metallic phases from which they were extracted. The radii of the first three rings correspond to those expected
Figure 2. FIB tomography of the recast microstructure: (a and b) BSE SEM micrographs obtained from the first and last FIB cuts, respectively, in a 200 slice serial sectioning of a Pt-capped pier milled into the recast; (c and d) perspective views of a 8.5 9.7 10 lm volume reconstructed from the stack of 1024 884 pixel BSE SEM micrographs.
using waterless Kalling’s solution; carbon coating; and then release electroetching using the electropolishing solution. The advantage of this hybrid method is that the carbon replicas reveal clearly the microstructure of the metallic phase(s) in addition to extracting the inert particles. The replicas obtained were examined in an FEI Tecnai T12 TEM operating at an accelerating voltage of 120 kV and equipped with an EDAX r-TEM atmospheric thin window EDXS spectrometer. The TEM data from extraction replicas of the base metal regions (not shown here for reasons of space) were consistent with that shown previously by Song and Aindow for P/M IN100 [15]. Thus, there were three types of particles present: Ti-rich MC-type carbides with the cubic NaCl structure; Cr-rich M23C6-type carbides with the cubic Cr23C6 structure; and (Mo + Cr)-rich M3B2-type borides with the tetragonal U3Si2 structure. The MC-type carbides were equiaxed with diameters of 200 nm and were distributed homogeneously throughout the base metal microstructure. The M23C6-type and M3B2-type particles are coarser (up to 1 lm), have more irregular morphologies, and tend to be located at the c/c and c/c0 boundaries. The regions of the replicas from the recast layer showed a very different distribution of particles, and
Figure 3. BF TEM images showing the morphology and distribution of the particles extracted from the cellular microstructure within the recast layer. The rings in the inset SADP in (b) are consistent with those expected for MC-type carbides with the NaCl structure.
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for 111, 200 and 220 from fcc MC-type carbides with a0 = 0.43 nm. The compositions of the particles were measured using EDXS data obtained from regions containing groups/clusters of particles. The spectra were quantified by excluding the Cu system peaks and the C K-peak, which comes from both the particles and the support, to give the relative concentrations of the remaining species. The average values obtained were: 2Ni–18Cr–66Ti–13Mo (all in at.%); this (Ti, Mo)-rich and (Ni, Co, Al)-lean composition is consistent with what one would expect for MC carbides. In our previous paper it was shown that the recast layer consists of coarse c grains with a fine cellular substructure corresponding to dendritic microsegregation of Ti and Al during the rapid solidification. The present observation of very Ti-rich MC-type carbides at the cell boundaries indicates that the degree of this microsegregation is much greater than had previously been appreciated, with the discrepancy presumably arising because the carbide particles were etched out preferentially during the electropolishing of the TEM foils used for microanalysis of the cell boundaries in the earlier study. No significance can be attributed to the random orientation of the nanoparticles because the larger (>100 nm) MCtype carbides formed in conventionally processed IN100 are also incoherent and oriented randomly. The presence of occasional clusters of nanoparticles could, however, indicate that the carbides nucleate in the Tirich interdendritic liquid ahead of the solidification front, with some of these being ‘‘swept” into clusters in the last pockets of liquid to solidify. It is also interesting to consider the effects of these particles upon the mechanical behavior of the material in the recast layer. The particles could act as strengthening obstacles to dislocation motion, although since the recast is much softer (HV = 170) than the base metal (HV = 440), or even super-solvus processed IN100 (HV = 365), this strengthening is not sufficient to counterbalance the softening effect of suppressing the c0 phase precipitation. As such, one might expect that there would be no inherent fatigue debit associated with the formation of such a structure, but this ignores the possible effects of localization of slip due to the inhomogeneous distribution of the MC-type carbides or of the precipitation of the c0 phase from the supersaturated c0 during elevated temperature service.
The authors thank Robin Bright (University of Connecticut), Dr. Kai Song (Lehigh University) and Dr. Paul Denney of the Connecticut Center for Advanced Technology (CCAT) for helpful discussions, Robert Wright of CCAT for assistance with producing the laser-drilled samples, and Pal O. Pedersen of FEI Company for assistance with FIB tomography. This material is based on research sponsored by CCAT’s National Center for Aerospace Leadership through Grant/Cooperative Agreement Number FA9550-06-1-0397 with the Air Force Office of Scientific Research. The US Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Air Force Research Laboratory or the US Government. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.scriptamat. 2009.07.034. [1] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [2] R.S. Bunker, ASME J. Heat Transf. 127 (2005) 441. [3] C.A. McNally, J. Folkes, I.R. Pashby, Mater. Sci. Tech. 20 (2004) 805. [4] N.B. Dahotre, S.P. Harimkar, Laser Fabrication and Machining of Materials, Springer Verlag, Berlin, 2008. [5] A.K. Dubey, V. Yadava, Int. J. Mach. Tool. Manufact. 48 (2008) 609. [6] Q. Feng, Y.N. Picard, H. Liu, S.M. Yalisove, G. Mourou, T.M. Pollock, Scr. Mater. 53 (2005) 511. [7] W.M. Steen, Laser Materials Processing, Springer Verlag, Berlin, 1991. [8] J.K.M. Garofano, H.L. Marcus, M. Aindow, J. Mater. Sci. 44 (2009) 680. [9] J.K.M. Garofano, H.L. Marcus, M. Aindow, Microsc. Microanal. 14 (S2) (2008) 558. [10] A.M. Wusatowska-Sarnek, G. Ghosh, G.B. Olson, M.J. Blackburn, M. Aindow, J. Mater. Res. 18 (2003) 2653. [11] A.M. Wusatowska-Sarnek, M.J. Blackburn, M. Aindow, Mater. Sci. Eng. A360 (2003) 390. [12] B.J. Inkson, M. Mulvihill, G. Mo¨bus, Scr. Mater. 45 (2001) 753. [13] M.D. Uchic, M.A. Groeber, D.M. Dimiduk, J.P. Simmons, Scr. Mater. 55 (2006) 23. [14] H.R. Zhang, O.A. Ojo, M.C. Chaturvedi, Scr. Mater. 58 (2008) 167. [15] K. Song, M. Aindow, J. Mater. Sci. 40 (2005) 3403.
Scripta Materialia 61 (2009) 943–946 www.elsevier.com/locate/scriptamat
Nanoscale carbide precipitation in the recast layer of a percussion laser-drilled superalloy Jacquelynn K.M. Garofano, Harris L. Marcus and Mark Aindow* Materials Science and Engineering Program, Department of Chemical Materials and Biomolecular Engineering, Institute of Materials Science, University of Connecticut, 97 North Eagleville Road, Storrs, CT 06269-3136, USA Received 17 June 2009; accepted 28 July 2009 Available online 6 August 2009
The microstructure of the recast layer formed during percussion laser drilling of a powder-metallurgy Ni-based superalloy has been studied using electron microscopy in conjunction with focused ion beam serial section tomography. It is shown that the dendritic microsegregation of Ti in the supersaturated gamma recast grains reported previously is accompanied by the formation of nanoscale precipitates in the regions between the dendrite arms. Transmission electron microscopy analysis of extraction replicas has shown that the precipitates are Ti-rich MC-type carbides. Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. Keywords: Nickel alloys; Microstructure; Percussion laser drilling; Scanning electron microscopy (SEM); Transmission electron microscopy (TEM)
Economic and environmental considerations require that gas turbine engines for aircraft operate at temperatures well above the softening points of the Ni-based superalloys that are used for the hot-zone components. The structural integrity of these components is maintained by using thermal barrier coatings (TBCs) and/or film cooling with compressed air forced through vast networks of fine (<500 lm in diameter) cooling holes [1,2]. For components that require both TBCs and film cooling, the cooling holes are often produced using laser drilling, which enables holes to be drilled through the TBC-coated superalloy [3,4]. This gives a more reproducible hole geometry than conventional techniques such as electrodischarge machining (EDM), wherein the TBC must be applied after drilling, leading to the possibility of hole blockage. Percussion laser drilling is usually preferred for aerospace components as a compromise between single-shot drilling, which is very rapid but leads to excessive debris and extensive metallurgical effects [5], and laser trepanning or femtosecond ablative laser drilling, which produce very high quality holes but with a much longer cycle time [6]. The main metallurgical effects that can occur during percussion laser drilling are the formation of a recast layer and/or a heat-affected zone (HAZ). The
* Corresponding author. Tel.: +1 860 486 2644; fax: +1 860 486 4745; e-mail: [email protected]
recast layer is comprised of molten/vaporized metal that is not ejected from the hole by the vapor generated during the laser pulse but instead resolidifies on the sidewall of the hole. The HAZ consists of a region of the base metal surrounding the hole that has not melted during the drilling but has undergone microstructural changes due to the thermal history in a manner akin to the HAZs formed during laser welding and cladding [7]. It is known that the recast layer and/or HAZ can lead to significant fatigue debits for laser-drilled components, but there is little in the open literature about the microstructural details of the HAZ and recast layers. In recent work by the present authors [8,9], a combination of scanning electron microscopy (SEM), transmission electron microscopy (TEM) and focused ion beam (FIB) microscopy was used to study the details of metallurgical effects in percussion laser-drilled samples of the powder-metallurgy (P/M) Ni-based superalloy IN100. Analysis of samples drilled using typical process conditions led to the surprising result that there was no discernible metallurgical HAZ. Moreover, the recast layer exhibited a coarse polycrystalline c microstructure within which the precipitation of the L12 c0 phase was suppressed completely. The c grains in the recast layer contained a fine cellular substructure and it was shown that these cells correspond to dendritic microsegregation of Ti and Al during the rapid solidification of the recast layer. In the present paper we describe further studies on this cellular substructure
1359-6462/$ - see front matter Ó 2009 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved. doi:10.1016/j.scriptamat.2009.07.034
944
J. K. M. Garofano et al. / Scripta Materialia 61 (2009) 943–946
using serial-section FIB tomography and TEM of extraction replicas. It is shown that the dendritic microsegregation in the recast layer is accompanied by nanoscale carbide precipitation at the cell boundaries. This precipitation can account for the striking etching/electropolishing effects observed in our previous studies and could have important consequences for the fatigue debits incurred in laser-drilled components. As described in detail elsewhere [8,9], coupons of P/M IN100 (nominal composition: Ni–18.5Co–12.4Cr–5.0Al– 4.3Ti–3.2Mo–0.8V–0.07C–0.06Zr–0.02B all in wt.%) were percussion laser-drilled to partial penetration using four pulses from a Nd:YAG laser giving a hole depth of 4 mm. Secondary electron (SE) SEM images from etched cross-sectional specimens through the holes using a JEOL 6335F field-emission SEM revealed: an abrupt base-metal/ recast interface; the usual c + c0 microstructure in the base metal (e.g. [10,11]) with a hierarchy of c0 precipitates on different length scales (total c0 volume fraction is 60%); no variation in the microstructure of the base metal with distance from the interface, i.e. no HAZ; and a cellular microstructure in the recast layer wherein cells 300–500 nm in diameter are arranged into colonies 2–10 lm in diameter. Further TEM analysis of this cellular microstructure using imaging, selected-area diffraction patterns (SADPs) and energy-dispersive X-ray spectrometry (EDXS) on electropolished foils in an FEI Tecnai T12 TEM showed: no structural differences between the cells and the boundaries between them (all face-centered-cubic (fcc) c phase); no differences in orientation between cells within a colony; and only subtle variations in local chemistry with enrichment of Ti and Al at the cell boundaries. Representative SEM and TEM micrographs from the recast layer are presented in Figure 1. While these data are consistent with the recast layer solidifying as a coarse-grained single-phase c microstructure, with dendritic microsegregation giving the cellular/ colony structure observed experimentally, it seems implausible that such subtle compositional variations would give rise to the pronounced topographical etching/electropolishing effects observed in the SEM and TEM samples. To investigate the origins of these effects, serial-section FIB tomography (e.g. [12,13]) was undertaken to examine the recast structure in greater detail. A cross-sectional sample was processed in an FEI Helios NanoLab 400S dual-beam FIB/SEM. A 2 lm thick protective Pt capping layer was deposited on the surface of the recast layer at the side-wall of a hole and the Ga+ ion beam was used to cut trenches through the Pt cap to produce a Pt-capped pier with a length of 20 lm, a width of 10 lm and a height of approximately 25 lm. The recast microstructure was revealed in the vertical end-wall of the pier, and FIB serial sectioning was performed by using the FEI Slice and View software package to produce a coordinated series of cuts in which the pier was shortened in increments of 50 nm. 200 FIB cuts were made, and at each point a back-scattered electron (BSE) SEM image was obtained using a 5 kV e-beam with a 50 V bias on the in-lens detector to minimize SE contributions to the measured signal. The first and last images in this series are shown in Figure 2a and b, respectively. A 1024 884 pixel frame that was common to all 200 BSE images was selected and the stack of these frames was then
Figure 1. Microstructure of the recast layer: (a) SE FESEM image of cellular colonies; (b) BF TEM image of cells within a colony. The inset SADP in (b) shows that these cells are comprised of supersaturated c and that adjacent cells are at the same orientation (i.e. each colony is a grain).
reconstructed using FEI ResolveRT/Amira software to give three-dimensional representations of the recast microstructure. Two different projections of the reconstructed volume are shown in Figure 2c and d1; these are both from the same 8.5 9.7 10 lm volume of the microstructure and the individual voxels are approximately 8 11 50 nm. These projections revealed more clearly the morphology of the large (>2 lm) dendritic grains, corresponding to the cellular colonies, but also showed that there were fine particles decorating the boundaries of the cells. We note that no such particles had been observed previously, either in the SEM images of the etched cross-sectional metallographic samples, or in the TEM images of the electropolished thin foils. Such nanoscale precipitates can be studied most effectively by using replication to extract them from the metallic phase(s) (e.g. [14]). In this study the particles were extracted using the hybrid extraction replication technique described by Song and Aindow [15]. This method involves: electropolishing using a solution of 10% HCl, 89% methanol and 1% citric acid; etching
1
The animation from which these two projections were taken is included as online Supplementary material.
J. K. M. Garofano et al. / Scripta Materialia 61 (2009) 943–946
945
typical bright-field (BF) TEM images are shown in Figure 3. All of the particles in such regions were extremely small (<20 nm in diameter) and these were located almost exclusively at the cell boundaries as revealed by the thicker ridges formed in the carbon film at these positions. We note that the densities of these particles are much higher than those observed by FIB tomography. This presumably arises because the BSE SEM images used for the tomographic reconstruction give a poorer resolution of the particles than the TEM images. Since the particles observed in the replicas range from about 5 to 20 nm in diameter, one might expect that only the largest of these would be detected/resolved in the BSE SEM images. We also note that while most of the particles appear to be spread evenly around the periphery of the cells, there are some larger clumps of particles present (e.g. at A in Fig. 3a). The SADPs obtained from the recast layer regions of the replicas exhibited sharp continuous rings (e.g. Fig. 3b, inset) indicating that the nanoparticles are crystalline and are oriented randomly with respect to the surrounding metallic phases from which they were extracted. The radii of the first three rings correspond to those expected
Figure 2. FIB tomography of the recast microstructure: (a and b) BSE SEM micrographs obtained from the first and last FIB cuts, respectively, in a 200 slice serial sectioning of a Pt-capped pier milled into the recast; (c and d) perspective views of a 8.5 9.7 10 lm volume reconstructed from the stack of 1024 884 pixel BSE SEM micrographs.
using waterless Kalling’s solution; carbon coating; and then release electroetching using the electropolishing solution. The advantage of this hybrid method is that the carbon replicas reveal clearly the microstructure of the metallic phase(s) in addition to extracting the inert particles. The replicas obtained were examined in an FEI Tecnai T12 TEM operating at an accelerating voltage of 120 kV and equipped with an EDAX r-TEM atmospheric thin window EDXS spectrometer. The TEM data from extraction replicas of the base metal regions (not shown here for reasons of space) were consistent with that shown previously by Song and Aindow for P/M IN100 [15]. Thus, there were three types of particles present: Ti-rich MC-type carbides with the cubic NaCl structure; Cr-rich M23C6-type carbides with the cubic Cr23C6 structure; and (Mo + Cr)-rich M3B2-type borides with the tetragonal U3Si2 structure. The MC-type carbides were equiaxed with diameters of 200 nm and were distributed homogeneously throughout the base metal microstructure. The M23C6-type and M3B2-type particles are coarser (up to 1 lm), have more irregular morphologies, and tend to be located at the c/c and c/c0 boundaries. The regions of the replicas from the recast layer showed a very different distribution of particles, and
Figure 3. BF TEM images showing the morphology and distribution of the particles extracted from the cellular microstructure within the recast layer. The rings in the inset SADP in (b) are consistent with those expected for MC-type carbides with the NaCl structure.
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J. K. M. Garofano et al. / Scripta Materialia 61 (2009) 943–946
for 111, 200 and 220 from fcc MC-type carbides with a0 = 0.43 nm. The compositions of the particles were measured using EDXS data obtained from regions containing groups/clusters of particles. The spectra were quantified by excluding the Cu system peaks and the C K-peak, which comes from both the particles and the support, to give the relative concentrations of the remaining species. The average values obtained were: 2Ni–18Cr–66Ti–13Mo (all in at.%); this (Ti, Mo)-rich and (Ni, Co, Al)-lean composition is consistent with what one would expect for MC carbides. In our previous paper it was shown that the recast layer consists of coarse c grains with a fine cellular substructure corresponding to dendritic microsegregation of Ti and Al during the rapid solidification. The present observation of very Ti-rich MC-type carbides at the cell boundaries indicates that the degree of this microsegregation is much greater than had previously been appreciated, with the discrepancy presumably arising because the carbide particles were etched out preferentially during the electropolishing of the TEM foils used for microanalysis of the cell boundaries in the earlier study. No significance can be attributed to the random orientation of the nanoparticles because the larger (>100 nm) MCtype carbides formed in conventionally processed IN100 are also incoherent and oriented randomly. The presence of occasional clusters of nanoparticles could, however, indicate that the carbides nucleate in the Tirich interdendritic liquid ahead of the solidification front, with some of these being ‘‘swept” into clusters in the last pockets of liquid to solidify. It is also interesting to consider the effects of these particles upon the mechanical behavior of the material in the recast layer. The particles could act as strengthening obstacles to dislocation motion, although since the recast is much softer (HV = 170) than the base metal (HV = 440), or even super-solvus processed IN100 (HV = 365), this strengthening is not sufficient to counterbalance the softening effect of suppressing the c0 phase precipitation. As such, one might expect that there would be no inherent fatigue debit associated with the formation of such a structure, but this ignores the possible effects of localization of slip due to the inhomogeneous distribution of the MC-type carbides or of the precipitation of the c0 phase from the supersaturated c0 during elevated temperature service.
The authors thank Robin Bright (University of Connecticut), Dr. Kai Song (Lehigh University) and Dr. Paul Denney of the Connecticut Center for Advanced Technology (CCAT) for helpful discussions, Robert Wright of CCAT for assistance with producing the laser-drilled samples, and Pal O. Pedersen of FEI Company for assistance with FIB tomography. This material is based on research sponsored by CCAT’s National Center for Aerospace Leadership through Grant/Cooperative Agreement Number FA9550-06-1-0397 with the Air Force Office of Scientific Research. The US Government is authorized to reproduce and distribute reprints for governmental purposes notwithstanding any copyright notation thereon. The views and conclusions contained herein are those of the authors and should not be interpreted as necessarily representing the official policies or endorsements, either expressed or implied, of the Air Force Research Laboratory or the US Government. Supplementary data associated with this article can be found, in the online version, at doi:10.1016/j.scriptamat. 2009.07.034. [1] N.P. Padture, M. Gell, E.H. Jordan, Science 296 (2002) 280. [2] R.S. Bunker, ASME J. Heat Transf. 127 (2005) 441. [3] C.A. McNally, J. Folkes, I.R. Pashby, Mater. Sci. Tech. 20 (2004) 805. [4] N.B. Dahotre, S.P. Harimkar, Laser Fabrication and Machining of Materials, Springer Verlag, Berlin, 2008. [5] A.K. Dubey, V. Yadava, Int. J. Mach. Tool. Manufact. 48 (2008) 609. [6] Q. Feng, Y.N. Picard, H. Liu, S.M. Yalisove, G. Mourou, T.M. Pollock, Scr. Mater. 53 (2005) 511. [7] W.M. Steen, Laser Materials Processing, Springer Verlag, Berlin, 1991. [8] J.K.M. Garofano, H.L. Marcus, M. Aindow, J. Mater. Sci. 44 (2009) 680. [9] J.K.M. Garofano, H.L. Marcus, M. Aindow, Microsc. Microanal. 14 (S2) (2008) 558. [10] A.M. Wusatowska-Sarnek, G. Ghosh, G.B. Olson, M.J. Blackburn, M. Aindow, J. Mater. Res. 18 (2003) 2653. [11] A.M. Wusatowska-Sarnek, M.J. Blackburn, M. Aindow, Mater. Sci. Eng. A360 (2003) 390. [12] B.J. Inkson, M. Mulvihill, G. Mo¨bus, Scr. Mater. 45 (2001) 753. [13] M.D. Uchic, M.A. Groeber, D.M. Dimiduk, J.P. Simmons, Scr. Mater. 55 (2006) 23. [14] H.R. Zhang, O.A. Ojo, M.C. Chaturvedi, Scr. Mater. 58 (2008) 167. [15] K. Song, M. Aindow, J. Mater. Sci. 40 (2005) 3403.