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Microstructure stability of γ-TiAl produced by selective laser melting
Scripta Materialia 130 (2017) 110–113
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Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular article
Microstructure stability of γ-TiAl produced by selective laser melting J. Gussone a,⁎, G. Garces b, J. Haubrich a, A. Stark c, Y.-C. Hagedorn d, N. Schell c, G. Requena a,e a
Department of Metals and Hybrid Materials, Institute of Materials Research, German Aerospace Center, Linder Höhe, 51147, Cologne, Germany Department of Physical Metallurgy, National Center for Metallurgical Research (CENIM-CSIC), Avda. Gregorio del Amo 8, 28040, Madrid, Spain c Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502, Geesthacht, Germany d Fraunhofer Institute for Laser Technology, Steinbachstraße 15, 52074, Aachen, Germany e RWTH Aachen, 52062Aachen, Germany b
a r t i c l e
i n f o
Article history: Received 16 September 2016 Received in revised form 16 November 2016 Accepted 21 November 2016 Available online xxxx Keywords: Selective laser melting X-ray diffraction Titanium aluminides Phase transformation
a b s t r a c t The effects of intrinsic heat-treatment during selective laser melting of Ti-44.8Al-6Nb-1.0Mo-0.1B are studied and compared to extrinsic post-annealing. Phase evolution during heat-treatment of as-built samples studied by in situ synchrotron radiation diffraction showed pronounced intensity changes and peak shifts below the eutectoid temperature, which are explained by stabilization of α2 as well as transformations to γ and β/βο. These observations contrast the marginal intensity changes found for the microstructure thermodynamically stabilized by hot isostatic pressing. The intrinsic heat-treatment of the as-built state leads to graded microstructures characterized by an increase of globular γ and β/βο at the α2/γ colony boundaries. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
γ-TiAl alloys are well-suited for aerospace applications due to their low density, high temperature strength and Young's modulus as well as very good corrosion and oxidation resistance [1]. Their technological maturity has been proven by recent applications, e.g. cast turbine blades made of the Ti-48Al-2Cr-2Nb alloy were introduced in the last two rotor stages of the low-pressure turbine of the GEnX jet engine [2]. However, current products are restricted to conventional processing routes, i.e. casting and forging. The development of suitable processing conditions for additive layer manufacturing (ALM) of γ-TiAl alloys would allow the realisation of more complex geometries, e.g. components with cooling channels and hollow structures, as well as the reduction of lead time and material savings. Although most research activities regarding ALM of TiAl alloys currently deal with electron beam melting (EBM) [3–6], in a few studies TiAl has been processed by selective laser melting (SLM) [7–11] and it has been reported in some cases that crack-free samples could be achieved [7,8]. However, ALM of γ-TiAl is still very demanding. The fast cooling rates typical for ALM induce residual stresses that lead to crack formation, while evaporation of Al results in compositional variations of the alloys. These two effects must be overcome to avoid deficient products [4,10,8]. One promising approach to reduce residual stresses is to increase the temperature of the substrate plate sufficiently during ALM followed by slow cooling after completion of the process [8,11]. This reduces the temperature gradient between the solidifying layers and the lower parts of the built specimens and thus decreases the magnitude ⁎ Corresponding author. E-mail address: [email protected] (J. Gussone).
http://dx.doi.org/10.1016/j.scriptamat.2016.11.028 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
of the thermal stresses. Furthermore, if the temperature is high enough, these thermal stresses can be at least partially relaxed. Another fact is that the microstructures formed are usually far from thermodynamic equilibrium conditions. Fast solidification and cooling rates can result in the formation of metastable phases and microstructural changes can occur during subsequent exposure to high temperatures. Moreover, the heating of solidified layers as new powder layers are successively deposited and molten (termed intrinsic heat treatment), can result in the formation of a graded microstructure in the building direction. The microstructural stability of conventionally produced γ-TiAl alloys and its impact on mechanical behaviour has been examined in a few studies that include in situ experiments to follow microstructural evolution during heat treatment (e.g. [12–15]). These investigations mainly focus on the development of the microstructure from supersaturated α2 obtained by quenching and aging below the eutectoid temperature [12,13,16,17]. The importance of these processes for powder metallurgy and ALM was recently pointed out considering that the fast cooling rates during ALM can be to some extent compared to quenching [18]. Nevertheless, cooling rates during SLM can be orders of magnitude higher than during quenching [19] and SLM as-built microstructures are the result of a more complex thermal history. Short re-melting of the upper layers of deposited material and heating cycles with decreasing temperature take place as the SLM process advances [20–22] likely leading to a vertical gradient of the materials' properties (besides the anisotropy due to the layer-wise building process). Since the present phases and their morphology form the basis of the materials' performance, it is necessary to characterize SLM as-built
J. Gussone et al. / Scripta Materialia 130 (2017) 110–113
microstructures in terms of microstructural constituents, their distribution along the building direction and their stability during high temperature exposure. For this purpose, Ti-44.8Al-6Nb-1.0Mo-0.1B (at.-%), also known as TNBV4 - a similar alloy to TNM-B1 under development for turbine blades -, was manufactured by SLM at the Fraunhofer Institute for Laser Technology, using a laboratory SLM machine and TNBV4 powder with particle sizes mainly between 20 and 60 μm. The baseplate was held at 800 °C during manufacturing of samples with a size of 10 × 10 × 5 mm3. An energy density [23] of approx. 60 J/mm3 was applied as a result of the combination of laser power (P = 80 W), scan velocity (v = 450 mm/s), hatch distance (h = 0.1 mm) and layer thickness (t = 0.03 mm). The microstructure of the SLM samples and their thermal stability was studied for two initial conditions, i.e. asbuilt and after hot isostatic pressing (HIP) carried out at 1200 °C with 200 MPa for 4 h and subsequent cooling at 10 K/min. The HIP treatment is expected to render a thermodynamically more stable microstructure. The microstructures were studied by SEM (JEOL, JSM 6500F) and TEM (JEOL, JEM 2010). Specimens for SEM and TEM observations were prepared by electrolytic polishing using a mixture of 3% perchloric acid, 15% butanol and 82% methanol at −20 °C and 22 V. The thermal stability of the two conditions was studied at about 1 mm from the upper surface by in situ high energy X-ray diffraction (HEXRD) during heating. The experiments were carried out at the beamline P07 of the synchrotron source PETRA III at DESY. A monochromatic beam with an energy of 100 keV was used for HEXRD in transmission mode (sample thickness ≈ 4 mm) with a slit size of 1 × 1 mm2. A modified dilatometer DIL 805A/D (TA Instruments) with an induction coil furnace was used to heat the samples at 20 K/min up to 1200 °C in a high purity argon atmosphere (N 99.999%). 2D diffraction patterns were collected during heating using a Perkin Elmer flat panel detector (XRD 1621) at an acquisition rate of ~1 image / 10 s. The samples were kept fixed during acquisition, and the temperature was controlled by a B-type thermocouple spot-welded next to the position of the incoming beam. Azimuthal integration of the Debye-Scherrer rings was carried out using the software FIT2D [24]. Additionally, the variation of the diffraction reflexes along the building direction (z-direction) was investigated for the as-built condition along the sample height of 5 mm to examine the microstructural gradient induced by the intrinsic heat treatment. Different microstructures can be expected for conventionally produced γ-TiAl alloys depending on the annealing temperature and the cooling rate. Microstructures with combinations of lamellar, Widmanstätten, feathery lamellar, massive γ and supersaturated α2 structures have been found for a large variety of TiAl alloys quenched from the α-field [25]. The SLM as-built microstructure can be described as a very fine near lamellar microstructure (Fig. 1a) with sub-μm-sized equiaxed γ (dark) and β/βο (bright) decorating the α2/γ colony boundaries. TEM reveals lamellar colonies (Fig. 1c) that partially exhibit order domains with phase/antiphase boundaries (bright non-rectilinear lines in Fig. 1d) [26,27]. Despite, and in contrast to quenched γ-TiAl [28,29], the formation of γ-laths within α2 is induced by the additional thermal energy introduced by the combined effect of the intrinsic heat treatment and the preheat temperature of 800 °C during SLM. This is a remarkable difference with respect to α-quenched microstructures, where γ-laths formation is suppressed and, therefore, large supersaturated α2 grains are obtained. The SLM + HIP condition leads mainly to a globular microstructure (Fig. 1b) with a few regions appearing to have originated from the former α2/γ lamellar structure. A comparison of the HEXRD patterns obtained at RT shows broader diffraction peaks for the SLM as-built condition than for SLM + HIP (Fig. 2a). The high coherence strains at antiphase boundaries (Fig. 1d) together with local chemical heterogeneities are responsible for this effect [30] and emphasize the metastability of the SLM as-built condition. A further evidence is the overlap of the γ-(002) and γ-(200) reflexes at RT (Fig. 2a) that reflects the low tetragonality of the γ-phase in its initial condition. This can be understood taking into account that the facecentred-cubic structure (in which (002) and (200) reflexes appear as
111
Fig. 1. Microstructures from the centre of a 5 mm Ti-44.8Al-6Nb-1.0Mo-0.1B alloy sample manufactured by SLM at 800 °C. a) SEM image of SLM as-built condition showing a very fine irregular near-lamellar structure (γ and β/βο decorating α2/γ colony boundaries). b) SEM image of the SLM + HIP condition depicting a fine globular structure with relicts from lamellar colonies. Corresponding TEM images (c,d) show lamellar colonies with adjacent globular grains. Some colonies (d) exhibit domains separated by antiphase boundaries (bright non-rectilinear lines).
a single peak) is an intermediate state during the formation of the lamellar γ structure from supersaturated α2. The f.c.c. lattice finally evolves into the thermodynamically stable face-centred-tetragonal crystal structure by chemical composition changes and ordering reactions [27]. In contrast, at 1175 °C (Fig. 2b), i.e. sufficiently below the eutectoid temperature of TNBV4 (ca. 1200 °C [31]), the diffraction patterns of as-built and HIP conditions do not show these differences anymore indicating that both have reached comparable thermodynamic states. The evolution of both conditions (SLM and SLM + HIP) during heating is shown by the colour coded 2D representation of the reflex intensities during heating from 700 °C to 1200 °C in Fig. 3. In case of the SLM + HIP condition, comparatively small intensity changes beginning at around 1050 °C can be observed: an increase for the β/βο-(110) and β/βο-(100) reflexes, a decrease for α2, especially α2-(101), a decrease of γ-(200) and an increase of α2-(200). On the other hand, the initially broad reflexes of the as-built condition become narrower above ~900 °C. At these temperatures diffusion is activated, which leads to relaxation of coherence strains at the α2/γ interfaces, re-ordering of the phase/antiphase domains and coarsening of the α2/γ colonies. Significant changes are found during heating for the α2-reflexes in the as-built condition. Between 800 and 950 °C there is a decrease in intensity while it increases again significantly at T N 950 °C. Furthermore, peak shifts take place between 900 and 1000 °C. Interestingly, these shifts are more pronounced for α2-(100) and α2-(200), whereas the α2-(101) reflex mainly shows an increase of the peak intensity. Simultaneously, the intensity of the γ-(200) reflex begins to increase strongly at ~950 °C and reaches a maximum at ~1050 °C, consistent with the formation and growth of γ-laths from supersaturated-α2. In previous studies [12,32] the formation of α2/γ lamellae was also observed at 730 °C by TEM during in situ heating of oil-quenched Ti-45Al-7.5Nb with an initial microstructure consisting of supersaturated-α2 with massive γ at α2 grain boundaries and triple points. The displacements of the α2 and γ reflexes between 800 °C and 1000 °C for the as-built condition are a consequence of still sufficiently fast cooling rates during SLM at 800 °C that result in the formation of metastable supersaturated-α2, especially in upper layers of material.
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J. Gussone et al. / Scripta Materialia 130 (2017) 110–113
Fig. 2. Comparison of diffractograms (as-built SLM and SLM + HIP) at room temperature (a) and at 1175 °C (b).
As diffusion becomes relevant during heating, the metastable supersaturated-α2 decomposes into α2 + γ by an element partitioning mechanism, particularly Al atoms diffusing from supersaturated-α2 into γ [33]. Thus, the peak shifts observed are a result of changes in local chemical composition [34] namely a pronounced increase of the lattice parameter a due to the decrease in the Al content between supersaturated-α2 and α2 [35]. On the other hand, the decrease and
increase in the intensity of α2/γ reflexes indicate a partial dissolution and the formation of these phases towards their respective equilibrium conditions. At T N 1050 °C the intensity of the γ-(200) peak decreases again together with the increase of β/βο and α2, similarly to the HIP condition. Small variations of the peak intensities can also be found along the SLM building direction (z) in the as-built condition (Fig. 4a) owing to
Fig. 3. Evolution of reflections during heating from 700 °C to 1200 °C with a constant heating rate of 20 K/min. a) Comparison of as-built SLM condition (left) and after additional HIP treatment at 1200 °C (right). b) Development of selected superlattice reflexes for as-built SLM (left) and SLM + HIP (right).
J. Gussone et al. / Scripta Materialia 130 (2017) 110–113
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Fig. 4. Intensity of selected XRD reflections (a). The z-position corresponds to the sample height (build direction). The position closest to the baseplate (z = 0 mm) corresponds to the condition with the longest in situ heat treatment (ca. 3 h). For this measurement the beam size was reduced to 0.1 × 0.5 mm2 (z, x). SEM images of graded microstructure from corresponding z-positions (b).
a graded microstructure resulting from the intrinsic heat treatment. An increase of the intensities of β/βο and γ reflexes is observed simultaneously with a decrease for α2-(200) as the distance from the last deposited powder layer increases although no pronounced peak shifts of α2 reflexes could be observed as a function of z. SEM images from the corresponding z-positions show an increase in volume fraction of globular β/βο and γ at the colony boundaries (Fig. 4b). During the intrinsic heat treatment element partitioning takes place while α2 transforms into γ (loosing Al) and β/βο (depleting Nb, Mo). Due to the high coherence of the γ/α2 phase boundaries, β/βο does generally not precipitate within the colonies and full stabilization of α2 does not take place since the temperature is too low for long-range diffusion processes. The microstructural gradient constitutes a heterogeneity which may be alleviated, e.g., by heat treatment at T N 800 °C. However, the microstructural instability of the SLM as-built condition may be prone to discontinuous precipitation [36] under heat influence and thus provide unfavourable properties for high temperature applications (i.e. creep behaviour). Therefore, subsequent heat treatments may be required to achieve stable lamellar microstructures [31]. In summary, we showed using HEXRD that significant microstructural transformations are induced by heating the metastable as-built alloy Ti-44.8Al-6Nb-1.0Mo-0.1B up to 1200 °C, which differs strongly from the corresponding annealing experiment of a material that was more stabilized and closer to thermodynamic equilibrium due to HIP at 1200 °C followed by relatively slow cooling. Moreover, the intrinsic heat treatment during SLM leads to a graded microstructure and phase evolutions consistent with the post-annealing below 950 °C. Acknowledgements The authors gratefully acknowledge the beam-time allocation for proposal I-20140200 EC at beamline P07 of the synchrotron source PETRA III at DESY, Hamburg. References [1] H. Clemens, W. Wallgram, S. Kremmer, V. Güther, A. Otto, A. Bartels, Adv. Eng. Mater. 10 (2008) 707–713. [2] S.F. Clark. 787, Propulsion System. Aero Quarterly; Available from: http://www. boeing.com/commercial/aeromagazine/articles/2012_q3/2/. [3] G. Baudana, S. Biamino, B. Klöden, A. Kirchner, T. Weißgärber, B. Kieback, M. Pavese, D. Ugues, P. Fino, C. Badini, Intermetallics 73 (2016) 43–49. [4] J. Schwerdtfeger, C. Körner, Intermetallics 49 (2014) 29–35. [5] S. Biamino, A. Penna, U. Ackelid, S. Sabbadini, O. Tassa, P. Fino, M. Pavese, P. Gennaro, C. Badini, Intermetallics 19 (2011) 776–781.
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Contents lists available at ScienceDirect
Scripta Materialia journal homepage: www.elsevier.com/locate/scriptamat
Regular article
Microstructure stability of γ-TiAl produced by selective laser melting J. Gussone a,⁎, G. Garces b, J. Haubrich a, A. Stark c, Y.-C. Hagedorn d, N. Schell c, G. Requena a,e a
Department of Metals and Hybrid Materials, Institute of Materials Research, German Aerospace Center, Linder Höhe, 51147, Cologne, Germany Department of Physical Metallurgy, National Center for Metallurgical Research (CENIM-CSIC), Avda. Gregorio del Amo 8, 28040, Madrid, Spain c Institute of Materials Research, Helmholtz-Zentrum Geesthacht, Max-Planck-Str. 1, 21502, Geesthacht, Germany d Fraunhofer Institute for Laser Technology, Steinbachstraße 15, 52074, Aachen, Germany e RWTH Aachen, 52062Aachen, Germany b
a r t i c l e
i n f o
Article history: Received 16 September 2016 Received in revised form 16 November 2016 Accepted 21 November 2016 Available online xxxx Keywords: Selective laser melting X-ray diffraction Titanium aluminides Phase transformation
a b s t r a c t The effects of intrinsic heat-treatment during selective laser melting of Ti-44.8Al-6Nb-1.0Mo-0.1B are studied and compared to extrinsic post-annealing. Phase evolution during heat-treatment of as-built samples studied by in situ synchrotron radiation diffraction showed pronounced intensity changes and peak shifts below the eutectoid temperature, which are explained by stabilization of α2 as well as transformations to γ and β/βο. These observations contrast the marginal intensity changes found for the microstructure thermodynamically stabilized by hot isostatic pressing. The intrinsic heat-treatment of the as-built state leads to graded microstructures characterized by an increase of globular γ and β/βο at the α2/γ colony boundaries. © 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
γ-TiAl alloys are well-suited for aerospace applications due to their low density, high temperature strength and Young's modulus as well as very good corrosion and oxidation resistance [1]. Their technological maturity has been proven by recent applications, e.g. cast turbine blades made of the Ti-48Al-2Cr-2Nb alloy were introduced in the last two rotor stages of the low-pressure turbine of the GEnX jet engine [2]. However, current products are restricted to conventional processing routes, i.e. casting and forging. The development of suitable processing conditions for additive layer manufacturing (ALM) of γ-TiAl alloys would allow the realisation of more complex geometries, e.g. components with cooling channels and hollow structures, as well as the reduction of lead time and material savings. Although most research activities regarding ALM of TiAl alloys currently deal with electron beam melting (EBM) [3–6], in a few studies TiAl has been processed by selective laser melting (SLM) [7–11] and it has been reported in some cases that crack-free samples could be achieved [7,8]. However, ALM of γ-TiAl is still very demanding. The fast cooling rates typical for ALM induce residual stresses that lead to crack formation, while evaporation of Al results in compositional variations of the alloys. These two effects must be overcome to avoid deficient products [4,10,8]. One promising approach to reduce residual stresses is to increase the temperature of the substrate plate sufficiently during ALM followed by slow cooling after completion of the process [8,11]. This reduces the temperature gradient between the solidifying layers and the lower parts of the built specimens and thus decreases the magnitude ⁎ Corresponding author. E-mail address: [email protected] (J. Gussone).
http://dx.doi.org/10.1016/j.scriptamat.2016.11.028 1359-6462/© 2016 Acta Materialia Inc. Published by Elsevier Ltd. All rights reserved.
of the thermal stresses. Furthermore, if the temperature is high enough, these thermal stresses can be at least partially relaxed. Another fact is that the microstructures formed are usually far from thermodynamic equilibrium conditions. Fast solidification and cooling rates can result in the formation of metastable phases and microstructural changes can occur during subsequent exposure to high temperatures. Moreover, the heating of solidified layers as new powder layers are successively deposited and molten (termed intrinsic heat treatment), can result in the formation of a graded microstructure in the building direction. The microstructural stability of conventionally produced γ-TiAl alloys and its impact on mechanical behaviour has been examined in a few studies that include in situ experiments to follow microstructural evolution during heat treatment (e.g. [12–15]). These investigations mainly focus on the development of the microstructure from supersaturated α2 obtained by quenching and aging below the eutectoid temperature [12,13,16,17]. The importance of these processes for powder metallurgy and ALM was recently pointed out considering that the fast cooling rates during ALM can be to some extent compared to quenching [18]. Nevertheless, cooling rates during SLM can be orders of magnitude higher than during quenching [19] and SLM as-built microstructures are the result of a more complex thermal history. Short re-melting of the upper layers of deposited material and heating cycles with decreasing temperature take place as the SLM process advances [20–22] likely leading to a vertical gradient of the materials' properties (besides the anisotropy due to the layer-wise building process). Since the present phases and their morphology form the basis of the materials' performance, it is necessary to characterize SLM as-built
J. Gussone et al. / Scripta Materialia 130 (2017) 110–113
microstructures in terms of microstructural constituents, their distribution along the building direction and their stability during high temperature exposure. For this purpose, Ti-44.8Al-6Nb-1.0Mo-0.1B (at.-%), also known as TNBV4 - a similar alloy to TNM-B1 under development for turbine blades -, was manufactured by SLM at the Fraunhofer Institute for Laser Technology, using a laboratory SLM machine and TNBV4 powder with particle sizes mainly between 20 and 60 μm. The baseplate was held at 800 °C during manufacturing of samples with a size of 10 × 10 × 5 mm3. An energy density [23] of approx. 60 J/mm3 was applied as a result of the combination of laser power (P = 80 W), scan velocity (v = 450 mm/s), hatch distance (h = 0.1 mm) and layer thickness (t = 0.03 mm). The microstructure of the SLM samples and their thermal stability was studied for two initial conditions, i.e. asbuilt and after hot isostatic pressing (HIP) carried out at 1200 °C with 200 MPa for 4 h and subsequent cooling at 10 K/min. The HIP treatment is expected to render a thermodynamically more stable microstructure. The microstructures were studied by SEM (JEOL, JSM 6500F) and TEM (JEOL, JEM 2010). Specimens for SEM and TEM observations were prepared by electrolytic polishing using a mixture of 3% perchloric acid, 15% butanol and 82% methanol at −20 °C and 22 V. The thermal stability of the two conditions was studied at about 1 mm from the upper surface by in situ high energy X-ray diffraction (HEXRD) during heating. The experiments were carried out at the beamline P07 of the synchrotron source PETRA III at DESY. A monochromatic beam with an energy of 100 keV was used for HEXRD in transmission mode (sample thickness ≈ 4 mm) with a slit size of 1 × 1 mm2. A modified dilatometer DIL 805A/D (TA Instruments) with an induction coil furnace was used to heat the samples at 20 K/min up to 1200 °C in a high purity argon atmosphere (N 99.999%). 2D diffraction patterns were collected during heating using a Perkin Elmer flat panel detector (XRD 1621) at an acquisition rate of ~1 image / 10 s. The samples were kept fixed during acquisition, and the temperature was controlled by a B-type thermocouple spot-welded next to the position of the incoming beam. Azimuthal integration of the Debye-Scherrer rings was carried out using the software FIT2D [24]. Additionally, the variation of the diffraction reflexes along the building direction (z-direction) was investigated for the as-built condition along the sample height of 5 mm to examine the microstructural gradient induced by the intrinsic heat treatment. Different microstructures can be expected for conventionally produced γ-TiAl alloys depending on the annealing temperature and the cooling rate. Microstructures with combinations of lamellar, Widmanstätten, feathery lamellar, massive γ and supersaturated α2 structures have been found for a large variety of TiAl alloys quenched from the α-field [25]. The SLM as-built microstructure can be described as a very fine near lamellar microstructure (Fig. 1a) with sub-μm-sized equiaxed γ (dark) and β/βο (bright) decorating the α2/γ colony boundaries. TEM reveals lamellar colonies (Fig. 1c) that partially exhibit order domains with phase/antiphase boundaries (bright non-rectilinear lines in Fig. 1d) [26,27]. Despite, and in contrast to quenched γ-TiAl [28,29], the formation of γ-laths within α2 is induced by the additional thermal energy introduced by the combined effect of the intrinsic heat treatment and the preheat temperature of 800 °C during SLM. This is a remarkable difference with respect to α-quenched microstructures, where γ-laths formation is suppressed and, therefore, large supersaturated α2 grains are obtained. The SLM + HIP condition leads mainly to a globular microstructure (Fig. 1b) with a few regions appearing to have originated from the former α2/γ lamellar structure. A comparison of the HEXRD patterns obtained at RT shows broader diffraction peaks for the SLM as-built condition than for SLM + HIP (Fig. 2a). The high coherence strains at antiphase boundaries (Fig. 1d) together with local chemical heterogeneities are responsible for this effect [30] and emphasize the metastability of the SLM as-built condition. A further evidence is the overlap of the γ-(002) and γ-(200) reflexes at RT (Fig. 2a) that reflects the low tetragonality of the γ-phase in its initial condition. This can be understood taking into account that the facecentred-cubic structure (in which (002) and (200) reflexes appear as
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Fig. 1. Microstructures from the centre of a 5 mm Ti-44.8Al-6Nb-1.0Mo-0.1B alloy sample manufactured by SLM at 800 °C. a) SEM image of SLM as-built condition showing a very fine irregular near-lamellar structure (γ and β/βο decorating α2/γ colony boundaries). b) SEM image of the SLM + HIP condition depicting a fine globular structure with relicts from lamellar colonies. Corresponding TEM images (c,d) show lamellar colonies with adjacent globular grains. Some colonies (d) exhibit domains separated by antiphase boundaries (bright non-rectilinear lines).
a single peak) is an intermediate state during the formation of the lamellar γ structure from supersaturated α2. The f.c.c. lattice finally evolves into the thermodynamically stable face-centred-tetragonal crystal structure by chemical composition changes and ordering reactions [27]. In contrast, at 1175 °C (Fig. 2b), i.e. sufficiently below the eutectoid temperature of TNBV4 (ca. 1200 °C [31]), the diffraction patterns of as-built and HIP conditions do not show these differences anymore indicating that both have reached comparable thermodynamic states. The evolution of both conditions (SLM and SLM + HIP) during heating is shown by the colour coded 2D representation of the reflex intensities during heating from 700 °C to 1200 °C in Fig. 3. In case of the SLM + HIP condition, comparatively small intensity changes beginning at around 1050 °C can be observed: an increase for the β/βο-(110) and β/βο-(100) reflexes, a decrease for α2, especially α2-(101), a decrease of γ-(200) and an increase of α2-(200). On the other hand, the initially broad reflexes of the as-built condition become narrower above ~900 °C. At these temperatures diffusion is activated, which leads to relaxation of coherence strains at the α2/γ interfaces, re-ordering of the phase/antiphase domains and coarsening of the α2/γ colonies. Significant changes are found during heating for the α2-reflexes in the as-built condition. Between 800 and 950 °C there is a decrease in intensity while it increases again significantly at T N 950 °C. Furthermore, peak shifts take place between 900 and 1000 °C. Interestingly, these shifts are more pronounced for α2-(100) and α2-(200), whereas the α2-(101) reflex mainly shows an increase of the peak intensity. Simultaneously, the intensity of the γ-(200) reflex begins to increase strongly at ~950 °C and reaches a maximum at ~1050 °C, consistent with the formation and growth of γ-laths from supersaturated-α2. In previous studies [12,32] the formation of α2/γ lamellae was also observed at 730 °C by TEM during in situ heating of oil-quenched Ti-45Al-7.5Nb with an initial microstructure consisting of supersaturated-α2 with massive γ at α2 grain boundaries and triple points. The displacements of the α2 and γ reflexes between 800 °C and 1000 °C for the as-built condition are a consequence of still sufficiently fast cooling rates during SLM at 800 °C that result in the formation of metastable supersaturated-α2, especially in upper layers of material.
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Fig. 2. Comparison of diffractograms (as-built SLM and SLM + HIP) at room temperature (a) and at 1175 °C (b).
As diffusion becomes relevant during heating, the metastable supersaturated-α2 decomposes into α2 + γ by an element partitioning mechanism, particularly Al atoms diffusing from supersaturated-α2 into γ [33]. Thus, the peak shifts observed are a result of changes in local chemical composition [34] namely a pronounced increase of the lattice parameter a due to the decrease in the Al content between supersaturated-α2 and α2 [35]. On the other hand, the decrease and
increase in the intensity of α2/γ reflexes indicate a partial dissolution and the formation of these phases towards their respective equilibrium conditions. At T N 1050 °C the intensity of the γ-(200) peak decreases again together with the increase of β/βο and α2, similarly to the HIP condition. Small variations of the peak intensities can also be found along the SLM building direction (z) in the as-built condition (Fig. 4a) owing to
Fig. 3. Evolution of reflections during heating from 700 °C to 1200 °C with a constant heating rate of 20 K/min. a) Comparison of as-built SLM condition (left) and after additional HIP treatment at 1200 °C (right). b) Development of selected superlattice reflexes for as-built SLM (left) and SLM + HIP (right).
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Fig. 4. Intensity of selected XRD reflections (a). The z-position corresponds to the sample height (build direction). The position closest to the baseplate (z = 0 mm) corresponds to the condition with the longest in situ heat treatment (ca. 3 h). For this measurement the beam size was reduced to 0.1 × 0.5 mm2 (z, x). SEM images of graded microstructure from corresponding z-positions (b).
a graded microstructure resulting from the intrinsic heat treatment. An increase of the intensities of β/βο and γ reflexes is observed simultaneously with a decrease for α2-(200) as the distance from the last deposited powder layer increases although no pronounced peak shifts of α2 reflexes could be observed as a function of z. SEM images from the corresponding z-positions show an increase in volume fraction of globular β/βο and γ at the colony boundaries (Fig. 4b). During the intrinsic heat treatment element partitioning takes place while α2 transforms into γ (loosing Al) and β/βο (depleting Nb, Mo). Due to the high coherence of the γ/α2 phase boundaries, β/βο does generally not precipitate within the colonies and full stabilization of α2 does not take place since the temperature is too low for long-range diffusion processes. The microstructural gradient constitutes a heterogeneity which may be alleviated, e.g., by heat treatment at T N 800 °C. However, the microstructural instability of the SLM as-built condition may be prone to discontinuous precipitation [36] under heat influence and thus provide unfavourable properties for high temperature applications (i.e. creep behaviour). Therefore, subsequent heat treatments may be required to achieve stable lamellar microstructures [31]. In summary, we showed using HEXRD that significant microstructural transformations are induced by heating the metastable as-built alloy Ti-44.8Al-6Nb-1.0Mo-0.1B up to 1200 °C, which differs strongly from the corresponding annealing experiment of a material that was more stabilized and closer to thermodynamic equilibrium due to HIP at 1200 °C followed by relatively slow cooling. Moreover, the intrinsic heat treatment during SLM leads to a graded microstructure and phase evolutions consistent with the post-annealing below 950 °C. Acknowledgements The authors gratefully acknowledge the beam-time allocation for proposal I-20140200 EC at beamline P07 of the synchrotron source PETRA III at DESY, Hamburg. References [1] H. Clemens, W. Wallgram, S. Kremmer, V. Güther, A. Otto, A. Bartels, Adv. Eng. Mater. 10 (2008) 707–713. [2] S.F. Clark. 787, Propulsion System. Aero Quarterly; Available from: http://www. boeing.com/commercial/aeromagazine/articles/2012_q3/2/. [3] G. Baudana, S. Biamino, B. Klöden, A. Kirchner, T. Weißgärber, B. Kieback, M. Pavese, D. Ugues, P. Fino, C. Badini, Intermetallics 73 (2016) 43–49. [4] J. Schwerdtfeger, C. Körner, Intermetallics 49 (2014) 29–35. [5] S. Biamino, A. Penna, U. Ackelid, S. Sabbadini, O. Tassa, P. Fino, M. Pavese, P. Gennaro, C. Badini, Intermetallics 19 (2011) 776–781.
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