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Mechano-synthesis and compaction of titanium–titanium nitride composites
Materials Science and Engineering A 375–377 (2004) 905–910
Mechano-synthesis and compaction of titanium–titanium nitride composites D. Wexler∗ , D. Parker, V. Palm, A. Calka Faculty of Engineering, University of Wollongong, Wollongong, Australia
Abstract Reactive ring grinding was performed on titanium under ammonia and nitrogen atmospheres to form nanostructural precursor powders with varying nitrogen contents. Nitration of Ti during grinding was found to occur more rapidly in N2 than in NH3 , however, it was not clear whether this was related to the strong ability of Ti to induce decomposition of molecular nitrogen into reactive atomic species or to the deleterious effect of Fe contamination levels on reactivity of Ti with N. Depending on the amounts of N and Fe present in the precursor powder a range of phases and microstructures could be produced on uniaxial hot pressing at temperatures below the melting point of pure Ti. With increasing N and Fe contents the transition was from ␣ +  type Ti to multi-component microstructures comprising various combinations of nitrogen-rich ␣-phase, -Ti, FeTi or TiFe2 and spheroidal particles of titanium nitride (TiN). © 2003 Elsevier B.V. All rights reserved. Keywords: Nitrogenated Ti; Monolithic titanium nitride; Reactive mechanical milling
1. Introduction Titanium powder is readily nitrided by reactive milling in nitrogen [1,2] or ammonia [3], producing a highly reactive nanostructural product after extended reaction. In previous work, we reported the synthesis of monolithic Ti–TiN composites through uniaxial hot pressing of precursor nitrided powders [3]. By ending the reaction of Ti with nitrogen or ammonia after an appropriate period and then hot pressing, a homogeneous and uniform mixture of nitrogen-rich Ti and TiN phases can be produced. By controlling nitriding gas pressure changes during milling good control of both the Ti to TiN ratio and final crystallite size distributions can be achieved. Initial investigations revealed that, depending on the degree of nitrogenation of the powder during milling, a range of promising composite microstructures could be produced on uniaxial hot pressing. These comprised dense, semi-spherical titanium nitride (TiN) particles in a multi-component, titanium-based matrix. Such compacts showed high densities and nanoindentation hardnesses in the range of 18–23 GPa. One of the reasons for the ease of densification of the nanostructural powder was that the Fe-based debris associated with me∗
Corresponding author. E-mail address: david [email protected] (D. Wexler).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.219
chanical attrition, aided sintering by dissolving in the Ti and lowering its melting point. For 5–10 g samples milled reactively using a controlled magneto-milling technique [1], Fe contamination levels were found to vary from around 0.3 to 3 wt.% depending on the ball to mass ratio, milling time (30–150 h) and whether the milling mode involved high energy ball-particle impacts or lower energy particle shearing. In the current experiments larger batches of Ti (20–50 g) were nitrided at high rates employing a high energy ring grinding technique. Based on a systematic study of the influence of milling time on powder composition and microstructure after hot pressing, it was hoped to further determine the roles of the different levels of N and steel debris on Ti–TiN composite formation. The ultimate aim of this work was to improve the understanding of which might lead to the development of improved nitrided titanium alloys and TiN based composites, using mechano-synthesis as a method of preparing precursor powders.
2. Experimental Reactive ring grinding of Ti powder (>99.7%) was performed using a Labtech Essa device, comprising a controlled atmosphere cylindrical chamber containing two steel
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Fig. 1. Combustion analyses results as a function of grinding time for 50 g wt. titanium powder samples ring ground in N2 at 400 KPa starting gas pressure and in NH3 at 700 KPa. The gas pressure was topped up to the starting pressure every 0.5 h for grinding in NH3 and every 1 h for grinding in N2 . For the case of grinding in ammonia, the sample size was reduced to 20 g after 2.5 h grinding.
rings and a central solid steel cylinder, with rapid movement in the horizontal plane. The vessel dimensions were 200 mm internal diameter and 50 mm depth. Samples (either 50 or 20 g charge) were ground either in N2 or NH3 . Uniaxial hot pressing of as-milled samples was performed using graphite dies under induction heating and a partial Ar
atmosphere (approximately 0.5 atm). Samples ring ground in NH3 were preheated to around 700 ◦ C and outgassed for ∼1 min then heated at 300 ◦ C/min to either 1500 or 1600 ◦ C under 7 MPa applied pressure and held for 1–5 min. After this the power was shut off and the sample allowed to cool inside the chamber.
Fig. 2. XRD results for samples ring ground in NH3 , (a) crystallite refinement and phase evolution, (b)–(d) selected samples hot pressed at 1600 ◦ C, pre-ground prior to pressing for (b) 2.5 h, (c) 3 h and (d) 3.5 h.
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The structure of as-ground and hot pressed samples was monitored by X-ray diffractometry (XRD) using Cu K␣ radiation. Morphologies were characterised using a Leica Stereoscan 440 scanning electron microscope (SEM) and Fe contamination and nitrogen distribution in as-ground samples investigated using energy dispersive X-ray spectroscopy (EDS, Oxford-Link system). Additional X-ray mapping for elements Ti, N, Fe and Cr was performed using a Cameca SX50 microanalysis system. To evaluate the content of nitrogen and hydrogen in as-ground and annealed samples combustion analysis was used (Carlo Erba Elemental Analyser model 1106).
3. Results and discussion 3.1. Powder synthesis Results of combustion analysis of ring ground (RG) samples are shown in Figs. 1 and 5. They indicate that nitration of Ti during grinding occurred significantly more rapidly in nitrogen gas than in ammonia. For samples ground in NH3 , there was a drop in hydrogen content after 3 h milling. This phenomenon has also been observed for Ti ball milled in NH3 and it is possible that TiH2 formed during the early stages of grinding decomposes to TiN and H2 upon reaction with nitrogen. In samples ring ground in N2 , TiN XRD peaks dominated Ti XRD peaks after 2 h of grinding, and were the only resolvable major peaks present after 3.5 h grinding time. In contrast, in the sample ring ground in NH3 , Ti peaks dominated TiN peaks after 2 h grinding, with additional peaks which could be associated with titanium hydride and/or a bcc Fe-based phase associated with milling media (Fig. 2a). In the latter, a nanoscale structure with approximate crystallite size, as estimated using the Scherer Formula, at below 15 nm after 1.5 h grinding, After 3.5 h, peaks associated with Fe debris, titanium hydride and significant amounts of TiN were present. It was not clear whether the faster rate of nitration in N2 is related to the strong ability of Ti to induce decomposition of molecular nitrogen into reactive atomic species or, conversely for the case of grinding in ammonia, the effect of absorbed H2 on inhibition of reaction with nitrogen. A separate possibility is a detrimental influence of Fe contamination on reactivity with nitrogen for the samples ground in NH3 . Iron (Mn and Cr) contamination levels were significantly higher in samples milled in NH3 , most likely because of the embrittling effect of hydrogen on the steel rings or stainless steel grinding chamber. Furthermore, contamination levels were also significantly higher in samples milled in ammonia for 3 and 3.5 h because the batch size was reduced from 50 to 20 g and contributions of mill debris to the powder product mass were relatively higher. Recent reactive ball milling experiments performed on Ti and B have demonstrated that the formation of TiB2 is delayed when Fe is introduced into the mill, either deliberately as a third el-
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ement, or through increased milling debris associated with milling under a more energetic mode [4]. The effects of Fe on the reactivity of Ti with N will be discussed below. 3.2. Hot pressing of samples ground in NH3 Hot pressing resulted in fully dense monolithic composites containing a range of phases, depending on the degree of nitration and the levels of Fe contamination. X-ray analysis of samples ground for 0.7 and 1.25 h revealed the dominant phase to be ␣-Ti(N), and peak shifts consistent with increasing N levels with increasing grinding times. In samples ground for 2 and 2.5 h additional phases -Ti and TiN were present (Fig. 2b). In the sample ground 3 h the dominant phases were ␣-Ti(N), TiFe and TiN (Fig. 2c) and in the sample ground for 3.5 h the dominant phases were TiN and TiFe2 (Fig. 2d). For samples ring ground in N2 , higher hot pressing temperatures were required for near full
Fig. 3. SEM backscattered images of samples ring ground in N2 . Darkest particles are richest in nitrogen. Average Fe contents (wt.%) are indicated in brackets.
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densification (around 1720 ◦ C). In the latter, the only phases present in detectible quantities were ␣-Ti(N) and TiN. The addition of N to Ti generally results in a stabilisation of the hexagonal ␣-Ti phase while further increases in N might be expected to result in the eventual formation of a Ti–TiN or TiN composite [5]. However, the addition of -stabilising elements, Fe, Mn and Cr, lowers the melting temperature of pure Ti and, in sufficient quantities, promotes eutectoid and eutectic phase transformations [6,7]. SEM backscattered electron images of typical nitrided starting powder and of a selection of hot pressed samples cooled under flowing Ar from the hot pressing temperature are shown in Fig. 3. At low N and Fe concentrations microstructures were of bainitic or Widmanstatten ␣ type (RG
0.7 h, <0.3 wt.% Fe). Elemental mapping revealed segregation of Fe to lath or plate boundaries (Fig. 4a). Iron has low solubility in ␣-Ti yet is a very rapid diffuser in -Ti (particularly at  grain boundaries), for example, the tracer diffusion coefficient of Fe in -Ti at 1000 ◦ C is around 5.6 × 10−8 cm2 s−1 [5] implying diffusion distances in the order of microns if the time to cool to from 1650 ◦ C to this temperature is greater than 0.1 s. Depending on the cooling rate from the -phase through the ␣ +  phase region into the ␣-phase region a Widmanstatten ␣, bainitic or martensitic microstructure will generally be produced. Even at very high cooling rates from the -phase region, transition metal solute species generally diffuse either ahead of or just behind the newly forming ␣ plates, promoting the formation
Fig. 4. Microprobe EDS maps for titanium, nitrogen and iron. (a) sample RG for 0.7 h in NH3 and hot pressed at 1500 ◦ C, (b) sample R.G for 1.25 h in NH3 and hot pressed at 1500 ◦ C, (c) sample RG for 2.5 h in NH3 and hot pressed at 1600 ◦ C.
D. Wexler et al. / Materials Science and Engineering A 375–377 (2004) 905–910
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Fig. 5. Fracture surfaces of (a) sample RG for 1.25 h and hot pressed at 1500 ◦ C, (b) sample RG for 3.5 h and hot pressed at 1600 ◦ C.
of a layer of either retained -phase or particles of eutectoid transformation product, ␣Ti + TiFe, located in the final product in the vicinity of the plate boundaries [5,8,9]. With increasing grinding time in NH3 the nitrogen and iron contents increase to the extent that at 1500–1600 ◦ C, hot pressing is apparently performed in an extended ␣-Ti(N)+ phase region of the phase diagram. In the sample with microstructure comprising near equiaxed ␣+transformed -Ti (1.25 h RG, 0.5 wt.% Fe), the transformed -phase is now bainitic. This is because the cooling rate required to form bainite in Ti alloys decreases with increasing Fe content [5]. At higher Fe and N contents a microstructure comprising equiaxed ␣, equiaxed TiN and retained -Ti (2.5 h RG, 1.5 wt.% Fe) was formed. Examination of the Ti–N binary phase diagram [10] indicates that there is not enough nitrogen present in this material (4 wt.%) to form TiN at the hot pressing temperatures despite the fact that XRD confirms that the phase is present. However, Fe lowers the melting point of Ti and promotes the formation of -Ti. Because N has a low solubility in the -phase, nitrogen levels in the remaining ␣-phase regions are most likely enhanced, in the case of the alloy ring ground for 2.5 h, to the extent that the N concentration reaches that of the ␣-Ti(N)+TiN phase region. In the 20 g samples ring ground in NH3 for more than 2.5 h the following microstructures were produced; equiaxed or spheroidal ␣-Ti and TiN plus FeTi (3 h RG, 20 g batch size, 8.5 wt.% Fe), spheroidal TiN plus Fe2 Ti (3.5 h RG, 20 g batch size, 17 wt.% Fe, 0.6% Cr, 0.16% Mn). Elemental mapping of samples with higher Fe and N content confirmed that the nitrogen was concentrated in the spheroidal particles of ␣-Ti(N) and TiN (Fig. 4b and c). The melting points of both FeTi (1317 ◦ C) and Fe2 Ti (1427 ◦ C) phases [10] are well below the hot pressing temperatures so that the nitrogen-rich phases are in fact liquid phase sintered at relatively high temperatures above the melting points of the particular binders. The reason for the very fine microstruc-
ture observed in the sample containing only TiN and Fe2 Ti appears to be the higher stability and melting range of TiN (2350–3290 ◦ C) compared with that of ␣-Ti(N). As illustrated in Fig. 5, fracture of these sintered samples appears to occur via some combination of cleavage or debonding of the spheroidal phase and/or ductile fracture of the Ti alloy grain boundary phase for samples with lower nitrogen contents (Fig. 5(a)). In contrast to the samples containing Fe, hot pressing of Ti ground for 3 h in N2 (<0.5 wt.% Fe) produced very fine-grained TiN based microstructures (Fig. 3). Materials nitrided in N2 required much higher temperatures for densification (∼1720 ◦ C), although still well below the melting point range of TiN. The trace amounts of Fe present in these samples and detected within thin grain boundary layers were also believed to play a major role in the enhancement of sinterability of these materials.
4. Conclusions Nitration of Ti by reactive ring grinding was found to be more rapid in N2 compared with NH3 . A high rate of steel pickup was also observed during grinding in NH3 , apparently due to the embrittling effect of hydrogen on the grinding chamber and rings. XRD and combustion analysis indicated small amounts of TiN in samples containing 3 wt.% N, with a large fraction formed in samples containing 7–9 wt.% N. Hydride decomposition and evolution of H2 during later stages of grinding appears to be associated with the formation of TiN. Hot pressing resulted in a range of microstructures which may be explained by the presence of both ␣ and  stabilising elements in the precursor powder. Because Fe and Cr stabilise -Ti, a dual phase (␣ + transformed ) microstructure is produced on cooling hot pressed samples containing
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up to 2 wt.% N. The hardness properties of these alloys reflected those in the literature. In hot pressed samples containing 3–7 wt.% N and significant quantities of Fe, some non-equilibrium TiN was formed. This again appears to be related to increased amount of -Ti due to the stabilising effect of Fe, and the low solubility of N in the bcc (and FeTi) phases. The finer microstructure in the H.P. sample containing 9.5 wt% N is believed to arise from the higher stability of TiN compared with ␣-Ti(N). Liquid phase sintering was promoted by the relatively low melting points of -Ti, TiFe and TiFe2 . Acknowledgements This investigation was supported by funding from the Australian Research Council. Grateful thanks to Les More and Alan Richards from BHP Research Laboratories, and to Nick MacKie and Greg Tillman from Wollongong University.
References [1] A. Calka, A.P. Radlinski, Mater. Sci. Eng. A 134 (1991) 1350. [2] M.S. El-Eskandarany, M. Omori, K. Konno, T.J. Sumiyama, T. Hirai, K. Suzuki, Metall. Mater. Trans. A 29A (1998) 1973. [3] D. Wexler, A. Calka, A.Y. Mosbah, J. Alloys Compds. 309 (2000) 201. [4] D. Wexler, A. Fenwick, A. Calka, in: presented at ISMANAM 02, International Sympsonium on Mechanically Alloyed, Nanocrystalline and Metastable Materials, 8–13 Sept., Seoul, Korea 2002. [5] E.W. Collings, The Physical Metallurgy of Titanium Alloys, ASM, Metals Park OH, (1984). [6] J.C. Williams, in: Proceedings of the TMS-AIME Sypmsonium at, Niagra Falls, 20–21 Sept., 1967, TMS-AIME, Warrendale, PA, (1978) p. 191. [7] S. Krishnamurphy, A.G. Jackson, H. Jones, F.H. Froes, Metall. Trans. A 19A (1988) 23. [8] R. Davis, H.M. Flower, D.R. West, J. Mater. Sci. 14 (1979) 712. [9] J.C. Williams, R. Taggart, D.H. Polonis, Metall. Trans. 1 (1970) 2265. [10] T.B. Massalski (Ed.), Binary Alloy Phase Diagrams, ASM International, USA, (1992).
Mechano-synthesis and compaction of titanium–titanium nitride composites D. Wexler∗ , D. Parker, V. Palm, A. Calka Faculty of Engineering, University of Wollongong, Wollongong, Australia
Abstract Reactive ring grinding was performed on titanium under ammonia and nitrogen atmospheres to form nanostructural precursor powders with varying nitrogen contents. Nitration of Ti during grinding was found to occur more rapidly in N2 than in NH3 , however, it was not clear whether this was related to the strong ability of Ti to induce decomposition of molecular nitrogen into reactive atomic species or to the deleterious effect of Fe contamination levels on reactivity of Ti with N. Depending on the amounts of N and Fe present in the precursor powder a range of phases and microstructures could be produced on uniaxial hot pressing at temperatures below the melting point of pure Ti. With increasing N and Fe contents the transition was from ␣ +  type Ti to multi-component microstructures comprising various combinations of nitrogen-rich ␣-phase, -Ti, FeTi or TiFe2 and spheroidal particles of titanium nitride (TiN). © 2003 Elsevier B.V. All rights reserved. Keywords: Nitrogenated Ti; Monolithic titanium nitride; Reactive mechanical milling
1. Introduction Titanium powder is readily nitrided by reactive milling in nitrogen [1,2] or ammonia [3], producing a highly reactive nanostructural product after extended reaction. In previous work, we reported the synthesis of monolithic Ti–TiN composites through uniaxial hot pressing of precursor nitrided powders [3]. By ending the reaction of Ti with nitrogen or ammonia after an appropriate period and then hot pressing, a homogeneous and uniform mixture of nitrogen-rich Ti and TiN phases can be produced. By controlling nitriding gas pressure changes during milling good control of both the Ti to TiN ratio and final crystallite size distributions can be achieved. Initial investigations revealed that, depending on the degree of nitrogenation of the powder during milling, a range of promising composite microstructures could be produced on uniaxial hot pressing. These comprised dense, semi-spherical titanium nitride (TiN) particles in a multi-component, titanium-based matrix. Such compacts showed high densities and nanoindentation hardnesses in the range of 18–23 GPa. One of the reasons for the ease of densification of the nanostructural powder was that the Fe-based debris associated with me∗
Corresponding author. E-mail address: david [email protected] (D. Wexler).
0921-5093/$ – see front matter © 2003 Elsevier B.V. All rights reserved. doi:10.1016/j.msea.2003.10.219
chanical attrition, aided sintering by dissolving in the Ti and lowering its melting point. For 5–10 g samples milled reactively using a controlled magneto-milling technique [1], Fe contamination levels were found to vary from around 0.3 to 3 wt.% depending on the ball to mass ratio, milling time (30–150 h) and whether the milling mode involved high energy ball-particle impacts or lower energy particle shearing. In the current experiments larger batches of Ti (20–50 g) were nitrided at high rates employing a high energy ring grinding technique. Based on a systematic study of the influence of milling time on powder composition and microstructure after hot pressing, it was hoped to further determine the roles of the different levels of N and steel debris on Ti–TiN composite formation. The ultimate aim of this work was to improve the understanding of which might lead to the development of improved nitrided titanium alloys and TiN based composites, using mechano-synthesis as a method of preparing precursor powders.
2. Experimental Reactive ring grinding of Ti powder (>99.7%) was performed using a Labtech Essa device, comprising a controlled atmosphere cylindrical chamber containing two steel
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D. Wexler et al. / Materials Science and Engineering A 375–377 (2004) 905–910
Fig. 1. Combustion analyses results as a function of grinding time for 50 g wt. titanium powder samples ring ground in N2 at 400 KPa starting gas pressure and in NH3 at 700 KPa. The gas pressure was topped up to the starting pressure every 0.5 h for grinding in NH3 and every 1 h for grinding in N2 . For the case of grinding in ammonia, the sample size was reduced to 20 g after 2.5 h grinding.
rings and a central solid steel cylinder, with rapid movement in the horizontal plane. The vessel dimensions were 200 mm internal diameter and 50 mm depth. Samples (either 50 or 20 g charge) were ground either in N2 or NH3 . Uniaxial hot pressing of as-milled samples was performed using graphite dies under induction heating and a partial Ar
atmosphere (approximately 0.5 atm). Samples ring ground in NH3 were preheated to around 700 ◦ C and outgassed for ∼1 min then heated at 300 ◦ C/min to either 1500 or 1600 ◦ C under 7 MPa applied pressure and held for 1–5 min. After this the power was shut off and the sample allowed to cool inside the chamber.
Fig. 2. XRD results for samples ring ground in NH3 , (a) crystallite refinement and phase evolution, (b)–(d) selected samples hot pressed at 1600 ◦ C, pre-ground prior to pressing for (b) 2.5 h, (c) 3 h and (d) 3.5 h.
D. Wexler et al. / Materials Science and Engineering A 375–377 (2004) 905–910
The structure of as-ground and hot pressed samples was monitored by X-ray diffractometry (XRD) using Cu K␣ radiation. Morphologies were characterised using a Leica Stereoscan 440 scanning electron microscope (SEM) and Fe contamination and nitrogen distribution in as-ground samples investigated using energy dispersive X-ray spectroscopy (EDS, Oxford-Link system). Additional X-ray mapping for elements Ti, N, Fe and Cr was performed using a Cameca SX50 microanalysis system. To evaluate the content of nitrogen and hydrogen in as-ground and annealed samples combustion analysis was used (Carlo Erba Elemental Analyser model 1106).
3. Results and discussion 3.1. Powder synthesis Results of combustion analysis of ring ground (RG) samples are shown in Figs. 1 and 5. They indicate that nitration of Ti during grinding occurred significantly more rapidly in nitrogen gas than in ammonia. For samples ground in NH3 , there was a drop in hydrogen content after 3 h milling. This phenomenon has also been observed for Ti ball milled in NH3 and it is possible that TiH2 formed during the early stages of grinding decomposes to TiN and H2 upon reaction with nitrogen. In samples ring ground in N2 , TiN XRD peaks dominated Ti XRD peaks after 2 h of grinding, and were the only resolvable major peaks present after 3.5 h grinding time. In contrast, in the sample ring ground in NH3 , Ti peaks dominated TiN peaks after 2 h grinding, with additional peaks which could be associated with titanium hydride and/or a bcc Fe-based phase associated with milling media (Fig. 2a). In the latter, a nanoscale structure with approximate crystallite size, as estimated using the Scherer Formula, at below 15 nm after 1.5 h grinding, After 3.5 h, peaks associated with Fe debris, titanium hydride and significant amounts of TiN were present. It was not clear whether the faster rate of nitration in N2 is related to the strong ability of Ti to induce decomposition of molecular nitrogen into reactive atomic species or, conversely for the case of grinding in ammonia, the effect of absorbed H2 on inhibition of reaction with nitrogen. A separate possibility is a detrimental influence of Fe contamination on reactivity with nitrogen for the samples ground in NH3 . Iron (Mn and Cr) contamination levels were significantly higher in samples milled in NH3 , most likely because of the embrittling effect of hydrogen on the steel rings or stainless steel grinding chamber. Furthermore, contamination levels were also significantly higher in samples milled in ammonia for 3 and 3.5 h because the batch size was reduced from 50 to 20 g and contributions of mill debris to the powder product mass were relatively higher. Recent reactive ball milling experiments performed on Ti and B have demonstrated that the formation of TiB2 is delayed when Fe is introduced into the mill, either deliberately as a third el-
907
ement, or through increased milling debris associated with milling under a more energetic mode [4]. The effects of Fe on the reactivity of Ti with N will be discussed below. 3.2. Hot pressing of samples ground in NH3 Hot pressing resulted in fully dense monolithic composites containing a range of phases, depending on the degree of nitration and the levels of Fe contamination. X-ray analysis of samples ground for 0.7 and 1.25 h revealed the dominant phase to be ␣-Ti(N), and peak shifts consistent with increasing N levels with increasing grinding times. In samples ground for 2 and 2.5 h additional phases -Ti and TiN were present (Fig. 2b). In the sample ground 3 h the dominant phases were ␣-Ti(N), TiFe and TiN (Fig. 2c) and in the sample ground for 3.5 h the dominant phases were TiN and TiFe2 (Fig. 2d). For samples ring ground in N2 , higher hot pressing temperatures were required for near full
Fig. 3. SEM backscattered images of samples ring ground in N2 . Darkest particles are richest in nitrogen. Average Fe contents (wt.%) are indicated in brackets.
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densification (around 1720 ◦ C). In the latter, the only phases present in detectible quantities were ␣-Ti(N) and TiN. The addition of N to Ti generally results in a stabilisation of the hexagonal ␣-Ti phase while further increases in N might be expected to result in the eventual formation of a Ti–TiN or TiN composite [5]. However, the addition of -stabilising elements, Fe, Mn and Cr, lowers the melting temperature of pure Ti and, in sufficient quantities, promotes eutectoid and eutectic phase transformations [6,7]. SEM backscattered electron images of typical nitrided starting powder and of a selection of hot pressed samples cooled under flowing Ar from the hot pressing temperature are shown in Fig. 3. At low N and Fe concentrations microstructures were of bainitic or Widmanstatten ␣ type (RG
0.7 h, <0.3 wt.% Fe). Elemental mapping revealed segregation of Fe to lath or plate boundaries (Fig. 4a). Iron has low solubility in ␣-Ti yet is a very rapid diffuser in -Ti (particularly at  grain boundaries), for example, the tracer diffusion coefficient of Fe in -Ti at 1000 ◦ C is around 5.6 × 10−8 cm2 s−1 [5] implying diffusion distances in the order of microns if the time to cool to from 1650 ◦ C to this temperature is greater than 0.1 s. Depending on the cooling rate from the -phase through the ␣ +  phase region into the ␣-phase region a Widmanstatten ␣, bainitic or martensitic microstructure will generally be produced. Even at very high cooling rates from the -phase region, transition metal solute species generally diffuse either ahead of or just behind the newly forming ␣ plates, promoting the formation
Fig. 4. Microprobe EDS maps for titanium, nitrogen and iron. (a) sample RG for 0.7 h in NH3 and hot pressed at 1500 ◦ C, (b) sample R.G for 1.25 h in NH3 and hot pressed at 1500 ◦ C, (c) sample RG for 2.5 h in NH3 and hot pressed at 1600 ◦ C.
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Fig. 5. Fracture surfaces of (a) sample RG for 1.25 h and hot pressed at 1500 ◦ C, (b) sample RG for 3.5 h and hot pressed at 1600 ◦ C.
of a layer of either retained -phase or particles of eutectoid transformation product, ␣Ti + TiFe, located in the final product in the vicinity of the plate boundaries [5,8,9]. With increasing grinding time in NH3 the nitrogen and iron contents increase to the extent that at 1500–1600 ◦ C, hot pressing is apparently performed in an extended ␣-Ti(N)+ phase region of the phase diagram. In the sample with microstructure comprising near equiaxed ␣+transformed -Ti (1.25 h RG, 0.5 wt.% Fe), the transformed -phase is now bainitic. This is because the cooling rate required to form bainite in Ti alloys decreases with increasing Fe content [5]. At higher Fe and N contents a microstructure comprising equiaxed ␣, equiaxed TiN and retained -Ti (2.5 h RG, 1.5 wt.% Fe) was formed. Examination of the Ti–N binary phase diagram [10] indicates that there is not enough nitrogen present in this material (4 wt.%) to form TiN at the hot pressing temperatures despite the fact that XRD confirms that the phase is present. However, Fe lowers the melting point of Ti and promotes the formation of -Ti. Because N has a low solubility in the -phase, nitrogen levels in the remaining ␣-phase regions are most likely enhanced, in the case of the alloy ring ground for 2.5 h, to the extent that the N concentration reaches that of the ␣-Ti(N)+TiN phase region. In the 20 g samples ring ground in NH3 for more than 2.5 h the following microstructures were produced; equiaxed or spheroidal ␣-Ti and TiN plus FeTi (3 h RG, 20 g batch size, 8.5 wt.% Fe), spheroidal TiN plus Fe2 Ti (3.5 h RG, 20 g batch size, 17 wt.% Fe, 0.6% Cr, 0.16% Mn). Elemental mapping of samples with higher Fe and N content confirmed that the nitrogen was concentrated in the spheroidal particles of ␣-Ti(N) and TiN (Fig. 4b and c). The melting points of both FeTi (1317 ◦ C) and Fe2 Ti (1427 ◦ C) phases [10] are well below the hot pressing temperatures so that the nitrogen-rich phases are in fact liquid phase sintered at relatively high temperatures above the melting points of the particular binders. The reason for the very fine microstruc-
ture observed in the sample containing only TiN and Fe2 Ti appears to be the higher stability and melting range of TiN (2350–3290 ◦ C) compared with that of ␣-Ti(N). As illustrated in Fig. 5, fracture of these sintered samples appears to occur via some combination of cleavage or debonding of the spheroidal phase and/or ductile fracture of the Ti alloy grain boundary phase for samples with lower nitrogen contents (Fig. 5(a)). In contrast to the samples containing Fe, hot pressing of Ti ground for 3 h in N2 (<0.5 wt.% Fe) produced very fine-grained TiN based microstructures (Fig. 3). Materials nitrided in N2 required much higher temperatures for densification (∼1720 ◦ C), although still well below the melting point range of TiN. The trace amounts of Fe present in these samples and detected within thin grain boundary layers were also believed to play a major role in the enhancement of sinterability of these materials.
4. Conclusions Nitration of Ti by reactive ring grinding was found to be more rapid in N2 compared with NH3 . A high rate of steel pickup was also observed during grinding in NH3 , apparently due to the embrittling effect of hydrogen on the grinding chamber and rings. XRD and combustion analysis indicated small amounts of TiN in samples containing 3 wt.% N, with a large fraction formed in samples containing 7–9 wt.% N. Hydride decomposition and evolution of H2 during later stages of grinding appears to be associated with the formation of TiN. Hot pressing resulted in a range of microstructures which may be explained by the presence of both ␣ and  stabilising elements in the precursor powder. Because Fe and Cr stabilise -Ti, a dual phase (␣ + transformed ) microstructure is produced on cooling hot pressed samples containing
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up to 2 wt.% N. The hardness properties of these alloys reflected those in the literature. In hot pressed samples containing 3–7 wt.% N and significant quantities of Fe, some non-equilibrium TiN was formed. This again appears to be related to increased amount of -Ti due to the stabilising effect of Fe, and the low solubility of N in the bcc (and FeTi) phases. The finer microstructure in the H.P. sample containing 9.5 wt% N is believed to arise from the higher stability of TiN compared with ␣-Ti(N). Liquid phase sintering was promoted by the relatively low melting points of -Ti, TiFe and TiFe2 . Acknowledgements This investigation was supported by funding from the Australian Research Council. Grateful thanks to Les More and Alan Richards from BHP Research Laboratories, and to Nick MacKie and Greg Tillman from Wollongong University.
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