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Microstructure and mechanical properties of iron-containing titanium metal-metal composites
International Journal of Refractory Metals & Hard Materials 90 (2020) 105225
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructure and mechanical properties of iron-containing titanium metal-metal composites ⁎
Fangrui Lin, Zhixing Chen, Bin Liu, Yong Liu , Chengshang Zhou
T
⁎
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Titanium-based composite Powder metallurgy Sintering Rotary swaging
Titanium-based metal-metal composites show promises in many applications. In this work, iron-containing titanium-based composites were fabricated by using powder metallurgy method and rotary swaging process. Titanium-based composites with steel addition were sintered at temperature below TieFe eutectic temperature. Increase of the steel addition to 23% resulted in formation of TieFe intermetallic and density decrease in the composites. Further, the as-sintered composites present relatively good workability so hot rotary swaging of the as-sintered composites can be conducted. Steel particles were diffused into the Ti matrix and the composite transformed to a discontinuous β-Ti fiber-reinforced structure. As a result, mechanical properties (in axial direction) of the swaged composites were significantly improved. The tensile strength and elongation reached 1360 MPa and 9.3%, respectively.
1. Introduction Titanium-based composites are a group of attractive structural materials owing to their high mechanical properties, light weight, and corrosion resistance [1]. Using continuous fiber reinforcement is a promising reinforcing strategy. High strength carbon and SiC fibers were considered to be effective reinforcing materials [2]. Fiber-reinforced Ti composites have remarkable combination of mechanical properties. However, due to high reactivity of Ti, brittle phase(s) such as TiC, Ti3Si2 can be formed at the interface, leading to degradation of the performance [3]. Therefore, the interfacial reaction must be carefully controlled [4]. Moreover, manufacturing fiber reinforced Titanium-based composites is complicated and expensive process. The cost issue is mainly according to twofold: high cost of the fiber materials; and difficulties in the post-processing due to the poor workability. Powder metallurgy (PM) process is a promising route to manufacture Ti composites. PM route offers many advantages such as neatnet-shape, cost effectiveness, and refined microstructure [5–7]. For Ti composite fabrication, uniform distribution of reinforcements can be readily achieved by using PM technique [8,9]. For example, in-situ processing using B4C additive is able to produce composites with fine and homogeneously dispersed TiBw and TiCp reinforcements [10]. Iron as a low-cost alloying element has been employed in many Ti alloys [11]. Among different alloying elements, iron serves as a betastabilizer. For fabrication of PM Ti alloy, Liu et al. [12] found that Fe
⁎
could accelerate the sintering process and improve the density of the sintered alloy. This has been also observed by O'Flynn et al. [13] and Esteban et al. [14]. However, the sintering density of PM Ti alloys showed a decreasing trend with increasing of particles size of the Fe addition. This attributes to the TieFe eutectic reaction and imbalanced diffusion rate between Ti and Fe [15]. Recently, Bolzoni et al. [16] employed a 4140 LCH steel powder as alloying additive for fabricating a low-cost Ti alloy. Their result showed that the mechanical properties are comparable to that of wrought Ti alloys. In this study, we introduce a novel Ti metal-metal composite with heterogeneous microstructure consisting of α-Ti matrix and β-Ti reinforcement. It is recognized that high-speed steel (HSS) powder provides alloying elements including Fe W, Mo, Cr, V, which are the β-Ti stabilizers and effective alloying elements [17]. Powder metallurgy route followed with hot rotary swaging were utilized to produce a unique structure with α-Ti matrix and discontinuous β-Ti fiber reinforcement. The microstructure and mechanical properties of the composites were investigated. 2. Experimental As-received titanium powder was purchased from Xi'an Sailong Metal Materials Co., LTD, and high-speed steel (HSS) powder (M2-HSS) was purchased form Höganäs (China) Co., LTD. The particle sizes and chemical compositions of the as-received powders were given in
Corresponding authors. E-mail addresses: [email protected] (Y. Liu), [email protected] (C. Zhou).
https://doi.org/10.1016/j.ijrmhm.2020.105225 Received 30 November 2019; Received in revised form 13 February 2020; Accepted 1 March 2020 Available online 03 March 2020 0263-4368/ © 2020 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 90 (2020) 105225
F. Lin, et al.
Table 1 Chemical compositions and powder parameters of the raw materials. Powders
O, wt%
N, wt%
C, wt%
W, wt%
Mo, wt%
Cr, wt%
V, wt%
Ti, wt%
Fe, wt%
Mean particle size, μm
Ti HSS
0.35 0.09
0.0076 0.012
0.035 0.84
– 5.96
– 5.13
– 4.27
– 4.25
Bal. –
– Bal.
24.4 73.3
temperature above the eutectic point (1085 °C), liquid phase appeared and thus significantly accelerated elemental diffusion [12]. Therefore, sintering temperature should be kept lower than the eutectic temperature to obtain the heterogenous structured composites. Fig. 2 compares the relative densities of the as-sintered composites. Samples with higher HSS addition show lower relative density. For the samples sintered at 1000 °C, the relative densities of composites with 8 wt%, 16 wt%, 23 wt%, 30 wt% HSS additions were 89.1%, 87.1%, 86.6%, 84.3%, respectively. The relative density of the 8 wt% HSS composite sintered at 1200 °C is 96.0%. The density increased with decrease of HSS addition. Fig. 3 shows a morphology and elemental distribution for an HSS particle and surrounding Ti matrix. The sample was prepared by vacuum sintering at 1000 °C for 2 h. From Fig. 3a, pores can be found around the HSS particle. The mechanism of the pore formation in PM TieFe system is mainly due to two factors: Kirkendall diffusion between Fe and Ti, and exothermic eutectic TieFe reaction, which have been discussed in reported literatures [20]. It must be pointed out that the reaction and diffusion between Ti matrix and HSS particles were rather complex cases, because the M2 HSS alloy consisted a number of alloying elements including W, Mo, Cr, V, and C. Carbon diffusion from HSS to Ti matrix occurred and TiC precipitation with size of few microns can be found around the HSS particle. The fine TiC particles could serve as an effective reinforcement that enhanced the properties of strength, hardness, and abrasion resistance [21]. An interesting observation is the presence of W and Mo-rich layer in HSS particles (the bright ring as shown in Fig. 3a). Accumulation of W and Mo at the interface can be related to slow diffusion of W and Mo [22]. According to investigation from O'Flynn [20], it is likely that a thin intermetallic layer consisting of FeTi and Fe2Ti develop at the TieFe interface when it was heated below the eutectic point. The FeTi and Fe2Ti intermetallics may act as a barrier for movement of W and Mo atoms. Fig. 4 compares the XRD patterns of the TMCs with different HSS contents. The composites show both α-Ti and β-Ti phases. The iron peaks can be detected in 23 wt% and 30 wt% HSS/Ti samples, but hardly found in 8 wt% HSS/Ti sample. The intensity changes of α-Ti and β-Ti peaks indicate that the amount of α-Ti phase is decreased and more β-Ti phase forms as increase of HSS addition. Titanium carbide (TiC) phase can be detected in the samples with 16 wt%, 23 wt%, and 30 wt% HSS. Furthermore, FeTi and Fe2Ti intermetallic phases exist in the samples with 23 wt% and 30 wt% HSS addition.
Table 2 Conditions of rotary swaging for HSS/Ti composites. State Pass Pass Pass Pass
1 2 3 4
Deformation degree φ
Final diameter, mm
0.81 1.02 1.38 2.19
20 18 15 10
Table 1. Powder blends with a nominal composition of 8 wt%, 16 wt%, 23 wt%, 30 wt% HSS powder balanced with Ti powder were mixed in a tubular. The mixture was cold-isostatically pressed under a pressure of 200 MPa to cylindrical compacts with approximately 40 mm in diameter. The compacts were then sintered using a vacuum furnace under vacuum of 2 × 10−3 Pa. After the sintering, the as-sintered products (8 wt% HSS addition, 1000 °C sintering for 2 h) were selected for rotary swaging (RS). Rotary swaging was carried out at 900 °C, and the parameters were provided in Table 2. Two samples (denoted composite A and B) were prepared using different deformation ratios. The deformation parameters were given in Table 2. The deformation degree φ was calculated using φ = ln(S0/Sn) relationship, where the S0 is initial cross section (30 mm) and Sn is final cross section [18]. The composite A experienced 3 passes and the final diameter was 15 mm. And composite B experienced 4 passes and the final diameter was 10 mm. After each pass, the bars were re-heated to 900 °C for 20 min. After the rotary swaging, the bars were heated to 900 °C and held for 1 h under vacuum of 2 × 10−3 Pa. Then the bars were cut into dog-bone tensile bars for tensile tests. The tensile bars were machined along the axial direction of the swaged samples. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis were conducted by using a scanning electron microscope (Hitachi 6460, Hitachi Co. Ltd., Japan). The X-ray diffraction (XRD) analysis was carried out using an X-ray diffractometer (SIMENS D500, Bruker, Switzerland). Tensile tests were performed in an Instron 3369 system (Instron Co. Ltd.) at a strain rate of 5 × 10−4 s−1. Densities of the samples were determined using the Archimedes methods. Chemical compositions of Ti and HSS powders were measured by LECO analyzer and ICP-AES analyzer. 3. Results and discussion 3.1. Sintering HSS reinforced Ti composites
3.2. Rotary swaging for the Ti composites In order to examine the effect of sintering temperature on the HSS/ Ti composite, vacuum sintering for 8 wt% HSS/Ti sample were performed at 1000 °C, 1100 °C, 1200 °C for 2 h. Fig. 1 shows the microstructures of the as-sintered samples. In the 1000 °C sintered microstructure, three distinctive regions can be found: HSS particles (bright), β-Ti rich region (bright-grey), and α-Ti rich region (dark-grey). The HSS particles were surrounded by β-Ti rich phase with a thickness of ~50 μm. As Fe is a β phase stabilizer, the formation of β-Ti is correlated to the diffusion of Fe. Moreover, α-Ti rich phase showed a higher porosity compared to the β-Ti rich phase. This is due to the diffused Fe can accelerate sintering [19]. Fig. 1d and f show a typical structure that consists of α and β-Ti phases. When sintering was performed at 1100 °C and 1200 °C, HSS reinforcements fully dissolved into the matrix and homogeneous α + β microstructures were obtained (Fig. 1c and e). At the sintering
Hot rotary swaging was conducted at 900 °C for as-sintered composite. Because the intention of this work is to create a heterogeneous structure, the samples sintered at 1000 °C and with 8 wt% HSS addition (see Fig. 1a), was used for the rotary swaging processing. Although the as-sintered sample has a low relative density (89%) and relatively poor mechanical properties, it is found that the sample is suitable for a thermomechanical processing. Two deformation ratios (φ = 1.38 and 2.19) were employed for the rotary swaging processing. The relative densities and tensile properties of composites A and B are given in Table 3. Because of the severe deformation during the swaging processing, majority of pores in the as-sintered samples were eliminated and near-fully dense composites were obtained. The tensile tests show that: sample A has a tensile strength of 920 MPa and an elongation of 7.7%; sample B has a tensile strength of 1360 MPa and an elongation of 2
International Journal of Refractory Metals & Hard Materials 90 (2020) 105225
F. Lin, et al.
Fig. 1. Microstructures of as-sintered HSS/Ti composite. (a) and (b) 1000 °C × 2 h; (c) and (d) 1100 °C × 2 h; (e) and (f) 1200 °C × 2 h.
Therefore, the β-Ti rich zones, originated from the HSS addition, owns higher strength and hardness due to solid solution strengthening by Fe, W, Mo, Cr and V. The α-Ti rich matrix should possesses better ductility. Moreover, the presence of TiC in the matrix can be also observed, as shown in Fig. 5e. It was found that the size of TiC particles is 1–5 μm. The TiC precipitates dispersed in the Ti matrix and were rearranged along with the swaging direction. Nevertheless, microstructure of the composite mainly consists of two metallic phases: α-Ti matrix and discontinuous fiber-like β-Ti. The swaged composites showed good combination of tensile strength and ductility, which is superior to those of cp-Ti and conventional Ti alloys [23]. Improved mechanical performance should attribute to several aspects. First, rotary swaging significantly improved the density. The sever deformation process eliminated majority of the pores, which leads to significant increase of the relative density from 89% to ~98%. Second, improvement of the mechanical properties can be partially attribute to the heterogeneous structure in the swaged composites. It is noted that the sample obtained by the high deformation ratio (composite B) present a longer and slender β-Ti structure compared to that with lower deformation ration (composite A). As shown in Table 3, the elongated fiber reinforcement offers extra benefit for both ductility and strength compared to composite A reinforced with the short fiber. Higher deformation ratio should lead to strength improvement due to grain refinement. High ductility, however, should be attributed to back-stress hardening that developed from the heterogeneous structure. Wu et al. [24] reported a heterogeneous structure steel, consisting soft micro-grain lamellae and hard ultrafine-grain lamella matrix. Its high ductility-strength combination was explained by back-stress hardening mechanism. Fig. 6 shows fractography of the swaged composite. The Ti matrix exhibits a dimple-like ductile fracture manner (Fig. 6c), and the HSS present a brittle fracture manner (Fig. 6e). The area of Ti-HSS interface presents fine dimples, indicating a strong bonding of the interface.
Fig. 2. Relative densities of as-sintered HSS/Ti composites.
9.3%. Fig. 5 shows both cross section and axial microstructures for composites A and B. It can be found that the swaged composites are very different from the as-sintered structure. Hot swaging processing leads to severe deformation and diffusion of the HSS particles. Likely, the W and Mo-rich region and the intermetallic layer was broken due to the swaging deformation. During the hot swaging and following heat treatment, the elements Fe, W, Mo, Cr, V diffused into the nearby Ti phase, forming β-Ti rich zone. FeTi and Fe2Ti intermetallic phases disappeared also due to diffusion of Fe. As a result, the distinctive HSSTi interface transformed to a FeeTi gradient zone (see Fig. 5e). 3
International Journal of Refractory Metals & Hard Materials 90 (2020) 105225
F. Lin, et al.
Fig. 3. EDS mapping for an HSS particle and surrounding Ti matrix.
Moreover, the TiC precipitations can be found on the facture surface. It can be expected that the hard TiC phase provides additional strengthening for the composites [25].
Table 3 Densities and tensile properties of Ti metal-metal composites after rotary swaging. Samples
Deformation degree
Relative density, %
Tensile strength, MPa
Elongation, %
A B
1.38 2.19
97.3 98.0
920 1360
7.7 9.3
4. Conclusions In conclusion, Fe-containing Ti metal-metal composites were successfully prepared by using PM route and hot rotary swaging. The microstructure, densification, and tensile properties were studied, and following conclusions are drawn:
2. The diffusion of Fe to the Ti matrix occurs during sintering. Assintered microstructure of the composites consists of HSS phase, β-Ti rich phase, and α-Ti rich phase. With increase of HSS content, amount of FeTi and Fe2Ti intermetallics increases and the amount of α-Ti phase decreases.
1. Heterogeneous HSS/Ti composites can be fabricated by press-andsinter method. Sintering at temperature above the TieFe eutectic point improved the densification but results in homogeneous microstructure.
Fig. 4. XRD patterns of as-sintered HSS/Ti Composites with different HSS contents. 4
International Journal of Refractory Metals & Hard Materials 90 (2020) 105225
F. Lin, et al.
Fig. 5. SEM images of swaged Ti metal-metal composites with different deformation: (a) sample A, section; (b) sample A, axial; (c) sample B, section; (d) sample B, axial; (e) sample B, reinforcement area.
Fig. 6. Fracture surface of swaged Ti composite (sample A). (a), (b) Overall fracture; (c) Ti matrix area; (d) Interface area; (e) Reinforcement area; (f) TiC particles. 5
International Journal of Refractory Metals & Hard Materials 90 (2020) 105225
F. Lin, et al.
3. Rotary swaging of as-sintered composite was successfully employed and produced a heterogeneous microstructure with a near full density. The structure consists dissentious fiber β-Ti reinforcement and α-Ti matrix. The relative density, tensile strength, and elongation of the swaged HSS/Ti composite are 98%, 1360 MPa, and 9.3%, respectively.
[7] X. Sun, Y. Han, S. Cao, P. Qiu, W. Lu, Rapid in-situ reaction synthesis of novel TiC and carbon nanotubes reinforced titanium matrix composites, J. Mater. Sci. Technol. 33 (10) (2017) 1165–1171. [8] K. Morsi, V.V. Patel, S. Naraghi, J.E. Garay, Processing of titanium–titanium boride dual matrix composites, J. Mater. Process. Technol. 196 (1–3) (2008) 236–242. [9] W. Zhang, X. Jiao, Y. Yu, J. Yang, Y. Feng, Microstructure and properties of 3.5 vol. % TiBw/Ti6Al4V composite tubes fabricated by hot-hydrostatic extrusion, J. Mater. Sci. Technol. 30 (7) (2014) 710–714. [10] Y. Zhang, J. Sun, R. Vilar, Characterization of (TiB+TiC)/TC4 in situ titanium matrix composites prepared by laser direct deposition, J. Mater. Process. Technol. 211 (4) (2011) 597–601. [11] B.-Y. Chen, K.-S. Hwang, K.-L. Ng, Effect of cooling process on the α phase formation and mechanical properties of sintered Ti–Fe alloys, Mater. Sci. Eng. A 528 (13–14) (2011) 4556–4563. [12] Y. Liu, L.F. Chen, H.P. Tang, C.T. Liu, B. Liu, B.Y. Huang, Design of powder metallurgy titanium alloys and composites, Mater. Sci. Eng. A 418 (1–2) (2006) 25–35. [13] J. O’Flynn, S.F. Corbin, The influence of Fe-40Ti intermetallic master alloy additions on the sintering behaviour of Ti-2.5Fe, J. Alloys Compd. 716 (2017) 184–196. [14] P.G. Esteban, L. Bolzoni, E.M. Ruiz-Navas, E. Gordo, PM processing and characterisation of Ti–7Fe low cost titanium alloys, Powder Metall. 54 (3) (2011) 242–252. [15] L. Bolzoni, E.M. Ruiz-Navas, E. Gordo, Understanding the properties of low-cost iron-containing powder metallurgy titanium alloys, Mater. Des. 110 (2016) 317–323. [16] L. Bolzoni, E.M. Ruiz-Navas, E. Gordo, Quantifying the properties of low-cost powder metallurgy titanium alloys, Mater. Sci. Eng. A 687 (2017) 47–53. [17] J.W. Park, H.C. Lee, S. Lee, Composition, microstructure, hardness, and wear properties of high-speed steel rolls, Metall. Mater. Trans. A 30 (2) (1999) 399–409. [18] R. Kocich, L. Kunčická, A. Macháčková, M. Šofer, Improvement of mechanical and electrical properties of rotary swaged Al-Cu clad composites, Mater. Des. 123 (2017) 137–146. [19] P.G. Esteban, E.M. Ruiz-Navas, E. Gordo, Influence of Fe content and particle size the on the processing and mechanical properties of low-cost Ti–xFe alloys, Mater. Sci. Eng. A 527 (21−22) (2010) 5664–5669. [20] J. O’Flynn, S.F. Corbin, The influence of iron powder size on pore formation, densification and homogenization during blended elemental sintering of Ti–2.5Fe, J. Alloys Compounds 618 (2015) 437–448. [21] Z.Y. Ma, R.S. Mishra, S.C. Tjong, High-temperature creep behavior of TiC particulate reinforced Ti–6Al–4V alloy composite, Acta Mater. 50 (17) (2002) 4293–4302. [22] X. Luo, Y. Yang, Q. Sun, Y. Yu, B. Huang, Y. Chen, Effect of Cu/Mo duplex coating on the interface and property of SiCf/Ti6Al4V composite, Mater. Sci. Eng. A 535 (2012) 6–11. [23] Z.Z. Fang, P. James, S. Pei, K.S.R. Chandran, Y. Zhang, Y. Xia, F. Cao, M. Koopman, M. Free, Powder metallurgy of titanium – past, present, and future, Int. Mater. Rev. 63 (7) (2017) 407–459. [24] X. Wu, P. Jiang, L. Chen, F. Yuan, Y.T. Zhu, Extraordinary strain hardening by gradient structure, Proc. Natl. Acad. Sci. 111 (20) (2014) 7197–7201. [25] Y.-J. Kim, H. Chung, S.-J.L. Kang, Processing and mechanical properties of Ti–6Al–4V/TiC in situ composite fabricated by gas–solid reaction, Mater. Sci. Eng. A 333 (1–2) (2002) 343–350.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Acknowledgements This research was supported by the National Natural Science Funds for Distinguished Young Scholar of China (51625404), the National Key Fundamental Research and Development Project of China (2014CB644002), National Key Research and Development Plan of China (2016YFB0700302). References [1] S.C. Tjong, Y.-W. Mai, Processing-structure-property aspects of particulate- and whisker-reinforced titanium matrix composites, Compos. Sci. Technol. 68 (3–4) (2008) 583–601. [2] K. Naseem, Y. Yang, X. Luo, B. Huang, G. Feng, SEM in situ study on the mechanical behaviour of SiCf/ti composite subjected to axial tensile load, Mater. Sci. Eng. A 528 (13–14) (2011) 4507–4515. [3] L. Hengjun, Y. Yanqing, H. Bin, Y. Meini, L. Xian, Numerical simulation of the densification processing of titanium-matrix coated fiber composites, J. Mater. Process. Technol. 208 (1–3) (2008) 284–288. [4] X. Wang, Y. Yang, X. Luo, W. Zhang, N. Jin, Z. Xiao, G. Feng, Effect of C/Mo duplex coating on the interface and mechanical properties of SiCf/Ti6Al4V composites, Mater. Sci. Eng. A 566 (2013) 47–53. [5] C. Even, C. Arvieu, J.M. Quenisset, Powder route processing of carbon fibres reinforced titanium matrix composites, Compos. Sci. Technol. 68 (6) (2008) 1273–1281. [6] S. Xu, C. Zhou, Y. Liu, B. Liu, K. Li, Microstructure and mechanical properties of Ti15Mo-xTiC composites fabricated by in-situ reactive sintering and hot swaging, J. Alloys Compd. 738 (2018) 188–196.
6
Contents lists available at ScienceDirect
International Journal of Refractory Metals & Hard Materials journal homepage: www.elsevier.com/locate/IJRMHM
Microstructure and mechanical properties of iron-containing titanium metal-metal composites ⁎
Fangrui Lin, Zhixing Chen, Bin Liu, Yong Liu , Chengshang Zhou
T
⁎
State Key Laboratory of Powder Metallurgy, Central South University, Changsha, Hunan 410083, China
A R T I C LE I N FO
A B S T R A C T
Keywords: Titanium-based composite Powder metallurgy Sintering Rotary swaging
Titanium-based metal-metal composites show promises in many applications. In this work, iron-containing titanium-based composites were fabricated by using powder metallurgy method and rotary swaging process. Titanium-based composites with steel addition were sintered at temperature below TieFe eutectic temperature. Increase of the steel addition to 23% resulted in formation of TieFe intermetallic and density decrease in the composites. Further, the as-sintered composites present relatively good workability so hot rotary swaging of the as-sintered composites can be conducted. Steel particles were diffused into the Ti matrix and the composite transformed to a discontinuous β-Ti fiber-reinforced structure. As a result, mechanical properties (in axial direction) of the swaged composites were significantly improved. The tensile strength and elongation reached 1360 MPa and 9.3%, respectively.
1. Introduction Titanium-based composites are a group of attractive structural materials owing to their high mechanical properties, light weight, and corrosion resistance [1]. Using continuous fiber reinforcement is a promising reinforcing strategy. High strength carbon and SiC fibers were considered to be effective reinforcing materials [2]. Fiber-reinforced Ti composites have remarkable combination of mechanical properties. However, due to high reactivity of Ti, brittle phase(s) such as TiC, Ti3Si2 can be formed at the interface, leading to degradation of the performance [3]. Therefore, the interfacial reaction must be carefully controlled [4]. Moreover, manufacturing fiber reinforced Titanium-based composites is complicated and expensive process. The cost issue is mainly according to twofold: high cost of the fiber materials; and difficulties in the post-processing due to the poor workability. Powder metallurgy (PM) process is a promising route to manufacture Ti composites. PM route offers many advantages such as neatnet-shape, cost effectiveness, and refined microstructure [5–7]. For Ti composite fabrication, uniform distribution of reinforcements can be readily achieved by using PM technique [8,9]. For example, in-situ processing using B4C additive is able to produce composites with fine and homogeneously dispersed TiBw and TiCp reinforcements [10]. Iron as a low-cost alloying element has been employed in many Ti alloys [11]. Among different alloying elements, iron serves as a betastabilizer. For fabrication of PM Ti alloy, Liu et al. [12] found that Fe
⁎
could accelerate the sintering process and improve the density of the sintered alloy. This has been also observed by O'Flynn et al. [13] and Esteban et al. [14]. However, the sintering density of PM Ti alloys showed a decreasing trend with increasing of particles size of the Fe addition. This attributes to the TieFe eutectic reaction and imbalanced diffusion rate between Ti and Fe [15]. Recently, Bolzoni et al. [16] employed a 4140 LCH steel powder as alloying additive for fabricating a low-cost Ti alloy. Their result showed that the mechanical properties are comparable to that of wrought Ti alloys. In this study, we introduce a novel Ti metal-metal composite with heterogeneous microstructure consisting of α-Ti matrix and β-Ti reinforcement. It is recognized that high-speed steel (HSS) powder provides alloying elements including Fe W, Mo, Cr, V, which are the β-Ti stabilizers and effective alloying elements [17]. Powder metallurgy route followed with hot rotary swaging were utilized to produce a unique structure with α-Ti matrix and discontinuous β-Ti fiber reinforcement. The microstructure and mechanical properties of the composites were investigated. 2. Experimental As-received titanium powder was purchased from Xi'an Sailong Metal Materials Co., LTD, and high-speed steel (HSS) powder (M2-HSS) was purchased form Höganäs (China) Co., LTD. The particle sizes and chemical compositions of the as-received powders were given in
Corresponding authors. E-mail addresses: [email protected] (Y. Liu), [email protected] (C. Zhou).
https://doi.org/10.1016/j.ijrmhm.2020.105225 Received 30 November 2019; Received in revised form 13 February 2020; Accepted 1 March 2020 Available online 03 March 2020 0263-4368/ © 2020 Elsevier Ltd. All rights reserved.
International Journal of Refractory Metals & Hard Materials 90 (2020) 105225
F. Lin, et al.
Table 1 Chemical compositions and powder parameters of the raw materials. Powders
O, wt%
N, wt%
C, wt%
W, wt%
Mo, wt%
Cr, wt%
V, wt%
Ti, wt%
Fe, wt%
Mean particle size, μm
Ti HSS
0.35 0.09
0.0076 0.012
0.035 0.84
– 5.96
– 5.13
– 4.27
– 4.25
Bal. –
– Bal.
24.4 73.3
temperature above the eutectic point (1085 °C), liquid phase appeared and thus significantly accelerated elemental diffusion [12]. Therefore, sintering temperature should be kept lower than the eutectic temperature to obtain the heterogenous structured composites. Fig. 2 compares the relative densities of the as-sintered composites. Samples with higher HSS addition show lower relative density. For the samples sintered at 1000 °C, the relative densities of composites with 8 wt%, 16 wt%, 23 wt%, 30 wt% HSS additions were 89.1%, 87.1%, 86.6%, 84.3%, respectively. The relative density of the 8 wt% HSS composite sintered at 1200 °C is 96.0%. The density increased with decrease of HSS addition. Fig. 3 shows a morphology and elemental distribution for an HSS particle and surrounding Ti matrix. The sample was prepared by vacuum sintering at 1000 °C for 2 h. From Fig. 3a, pores can be found around the HSS particle. The mechanism of the pore formation in PM TieFe system is mainly due to two factors: Kirkendall diffusion between Fe and Ti, and exothermic eutectic TieFe reaction, which have been discussed in reported literatures [20]. It must be pointed out that the reaction and diffusion between Ti matrix and HSS particles were rather complex cases, because the M2 HSS alloy consisted a number of alloying elements including W, Mo, Cr, V, and C. Carbon diffusion from HSS to Ti matrix occurred and TiC precipitation with size of few microns can be found around the HSS particle. The fine TiC particles could serve as an effective reinforcement that enhanced the properties of strength, hardness, and abrasion resistance [21]. An interesting observation is the presence of W and Mo-rich layer in HSS particles (the bright ring as shown in Fig. 3a). Accumulation of W and Mo at the interface can be related to slow diffusion of W and Mo [22]. According to investigation from O'Flynn [20], it is likely that a thin intermetallic layer consisting of FeTi and Fe2Ti develop at the TieFe interface when it was heated below the eutectic point. The FeTi and Fe2Ti intermetallics may act as a barrier for movement of W and Mo atoms. Fig. 4 compares the XRD patterns of the TMCs with different HSS contents. The composites show both α-Ti and β-Ti phases. The iron peaks can be detected in 23 wt% and 30 wt% HSS/Ti samples, but hardly found in 8 wt% HSS/Ti sample. The intensity changes of α-Ti and β-Ti peaks indicate that the amount of α-Ti phase is decreased and more β-Ti phase forms as increase of HSS addition. Titanium carbide (TiC) phase can be detected in the samples with 16 wt%, 23 wt%, and 30 wt% HSS. Furthermore, FeTi and Fe2Ti intermetallic phases exist in the samples with 23 wt% and 30 wt% HSS addition.
Table 2 Conditions of rotary swaging for HSS/Ti composites. State Pass Pass Pass Pass
1 2 3 4
Deformation degree φ
Final diameter, mm
0.81 1.02 1.38 2.19
20 18 15 10
Table 1. Powder blends with a nominal composition of 8 wt%, 16 wt%, 23 wt%, 30 wt% HSS powder balanced with Ti powder were mixed in a tubular. The mixture was cold-isostatically pressed under a pressure of 200 MPa to cylindrical compacts with approximately 40 mm in diameter. The compacts were then sintered using a vacuum furnace under vacuum of 2 × 10−3 Pa. After the sintering, the as-sintered products (8 wt% HSS addition, 1000 °C sintering for 2 h) were selected for rotary swaging (RS). Rotary swaging was carried out at 900 °C, and the parameters were provided in Table 2. Two samples (denoted composite A and B) were prepared using different deformation ratios. The deformation parameters were given in Table 2. The deformation degree φ was calculated using φ = ln(S0/Sn) relationship, where the S0 is initial cross section (30 mm) and Sn is final cross section [18]. The composite A experienced 3 passes and the final diameter was 15 mm. And composite B experienced 4 passes and the final diameter was 10 mm. After each pass, the bars were re-heated to 900 °C for 20 min. After the rotary swaging, the bars were heated to 900 °C and held for 1 h under vacuum of 2 × 10−3 Pa. Then the bars were cut into dog-bone tensile bars for tensile tests. The tensile bars were machined along the axial direction of the swaged samples. Scanning electron microscopy (SEM) and energy dispersive spectrometer (EDS) analysis were conducted by using a scanning electron microscope (Hitachi 6460, Hitachi Co. Ltd., Japan). The X-ray diffraction (XRD) analysis was carried out using an X-ray diffractometer (SIMENS D500, Bruker, Switzerland). Tensile tests were performed in an Instron 3369 system (Instron Co. Ltd.) at a strain rate of 5 × 10−4 s−1. Densities of the samples were determined using the Archimedes methods. Chemical compositions of Ti and HSS powders were measured by LECO analyzer and ICP-AES analyzer. 3. Results and discussion 3.1. Sintering HSS reinforced Ti composites
3.2. Rotary swaging for the Ti composites In order to examine the effect of sintering temperature on the HSS/ Ti composite, vacuum sintering for 8 wt% HSS/Ti sample were performed at 1000 °C, 1100 °C, 1200 °C for 2 h. Fig. 1 shows the microstructures of the as-sintered samples. In the 1000 °C sintered microstructure, three distinctive regions can be found: HSS particles (bright), β-Ti rich region (bright-grey), and α-Ti rich region (dark-grey). The HSS particles were surrounded by β-Ti rich phase with a thickness of ~50 μm. As Fe is a β phase stabilizer, the formation of β-Ti is correlated to the diffusion of Fe. Moreover, α-Ti rich phase showed a higher porosity compared to the β-Ti rich phase. This is due to the diffused Fe can accelerate sintering [19]. Fig. 1d and f show a typical structure that consists of α and β-Ti phases. When sintering was performed at 1100 °C and 1200 °C, HSS reinforcements fully dissolved into the matrix and homogeneous α + β microstructures were obtained (Fig. 1c and e). At the sintering
Hot rotary swaging was conducted at 900 °C for as-sintered composite. Because the intention of this work is to create a heterogeneous structure, the samples sintered at 1000 °C and with 8 wt% HSS addition (see Fig. 1a), was used for the rotary swaging processing. Although the as-sintered sample has a low relative density (89%) and relatively poor mechanical properties, it is found that the sample is suitable for a thermomechanical processing. Two deformation ratios (φ = 1.38 and 2.19) were employed for the rotary swaging processing. The relative densities and tensile properties of composites A and B are given in Table 3. Because of the severe deformation during the swaging processing, majority of pores in the as-sintered samples were eliminated and near-fully dense composites were obtained. The tensile tests show that: sample A has a tensile strength of 920 MPa and an elongation of 7.7%; sample B has a tensile strength of 1360 MPa and an elongation of 2
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Fig. 1. Microstructures of as-sintered HSS/Ti composite. (a) and (b) 1000 °C × 2 h; (c) and (d) 1100 °C × 2 h; (e) and (f) 1200 °C × 2 h.
Therefore, the β-Ti rich zones, originated from the HSS addition, owns higher strength and hardness due to solid solution strengthening by Fe, W, Mo, Cr and V. The α-Ti rich matrix should possesses better ductility. Moreover, the presence of TiC in the matrix can be also observed, as shown in Fig. 5e. It was found that the size of TiC particles is 1–5 μm. The TiC precipitates dispersed in the Ti matrix and were rearranged along with the swaging direction. Nevertheless, microstructure of the composite mainly consists of two metallic phases: α-Ti matrix and discontinuous fiber-like β-Ti. The swaged composites showed good combination of tensile strength and ductility, which is superior to those of cp-Ti and conventional Ti alloys [23]. Improved mechanical performance should attribute to several aspects. First, rotary swaging significantly improved the density. The sever deformation process eliminated majority of the pores, which leads to significant increase of the relative density from 89% to ~98%. Second, improvement of the mechanical properties can be partially attribute to the heterogeneous structure in the swaged composites. It is noted that the sample obtained by the high deformation ratio (composite B) present a longer and slender β-Ti structure compared to that with lower deformation ration (composite A). As shown in Table 3, the elongated fiber reinforcement offers extra benefit for both ductility and strength compared to composite A reinforced with the short fiber. Higher deformation ratio should lead to strength improvement due to grain refinement. High ductility, however, should be attributed to back-stress hardening that developed from the heterogeneous structure. Wu et al. [24] reported a heterogeneous structure steel, consisting soft micro-grain lamellae and hard ultrafine-grain lamella matrix. Its high ductility-strength combination was explained by back-stress hardening mechanism. Fig. 6 shows fractography of the swaged composite. The Ti matrix exhibits a dimple-like ductile fracture manner (Fig. 6c), and the HSS present a brittle fracture manner (Fig. 6e). The area of Ti-HSS interface presents fine dimples, indicating a strong bonding of the interface.
Fig. 2. Relative densities of as-sintered HSS/Ti composites.
9.3%. Fig. 5 shows both cross section and axial microstructures for composites A and B. It can be found that the swaged composites are very different from the as-sintered structure. Hot swaging processing leads to severe deformation and diffusion of the HSS particles. Likely, the W and Mo-rich region and the intermetallic layer was broken due to the swaging deformation. During the hot swaging and following heat treatment, the elements Fe, W, Mo, Cr, V diffused into the nearby Ti phase, forming β-Ti rich zone. FeTi and Fe2Ti intermetallic phases disappeared also due to diffusion of Fe. As a result, the distinctive HSSTi interface transformed to a FeeTi gradient zone (see Fig. 5e). 3
International Journal of Refractory Metals & Hard Materials 90 (2020) 105225
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Fig. 3. EDS mapping for an HSS particle and surrounding Ti matrix.
Moreover, the TiC precipitations can be found on the facture surface. It can be expected that the hard TiC phase provides additional strengthening for the composites [25].
Table 3 Densities and tensile properties of Ti metal-metal composites after rotary swaging. Samples
Deformation degree
Relative density, %
Tensile strength, MPa
Elongation, %
A B
1.38 2.19
97.3 98.0
920 1360
7.7 9.3
4. Conclusions In conclusion, Fe-containing Ti metal-metal composites were successfully prepared by using PM route and hot rotary swaging. The microstructure, densification, and tensile properties were studied, and following conclusions are drawn:
2. The diffusion of Fe to the Ti matrix occurs during sintering. Assintered microstructure of the composites consists of HSS phase, β-Ti rich phase, and α-Ti rich phase. With increase of HSS content, amount of FeTi and Fe2Ti intermetallics increases and the amount of α-Ti phase decreases.
1. Heterogeneous HSS/Ti composites can be fabricated by press-andsinter method. Sintering at temperature above the TieFe eutectic point improved the densification but results in homogeneous microstructure.
Fig. 4. XRD patterns of as-sintered HSS/Ti Composites with different HSS contents. 4
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Fig. 5. SEM images of swaged Ti metal-metal composites with different deformation: (a) sample A, section; (b) sample A, axial; (c) sample B, section; (d) sample B, axial; (e) sample B, reinforcement area.
Fig. 6. Fracture surface of swaged Ti composite (sample A). (a), (b) Overall fracture; (c) Ti matrix area; (d) Interface area; (e) Reinforcement area; (f) TiC particles. 5
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3. Rotary swaging of as-sintered composite was successfully employed and produced a heterogeneous microstructure with a near full density. The structure consists dissentious fiber β-Ti reinforcement and α-Ti matrix. The relative density, tensile strength, and elongation of the swaged HSS/Ti composite are 98%, 1360 MPa, and 9.3%, respectively.
[7] X. Sun, Y. Han, S. Cao, P. Qiu, W. Lu, Rapid in-situ reaction synthesis of novel TiC and carbon nanotubes reinforced titanium matrix composites, J. Mater. Sci. Technol. 33 (10) (2017) 1165–1171. [8] K. Morsi, V.V. Patel, S. Naraghi, J.E. Garay, Processing of titanium–titanium boride dual matrix composites, J. Mater. Process. Technol. 196 (1–3) (2008) 236–242. [9] W. Zhang, X. Jiao, Y. Yu, J. Yang, Y. Feng, Microstructure and properties of 3.5 vol. % TiBw/Ti6Al4V composite tubes fabricated by hot-hydrostatic extrusion, J. Mater. Sci. Technol. 30 (7) (2014) 710–714. [10] Y. Zhang, J. Sun, R. Vilar, Characterization of (TiB+TiC)/TC4 in situ titanium matrix composites prepared by laser direct deposition, J. Mater. Process. Technol. 211 (4) (2011) 597–601. [11] B.-Y. Chen, K.-S. Hwang, K.-L. Ng, Effect of cooling process on the α phase formation and mechanical properties of sintered Ti–Fe alloys, Mater. Sci. Eng. A 528 (13–14) (2011) 4556–4563. [12] Y. Liu, L.F. Chen, H.P. Tang, C.T. Liu, B. Liu, B.Y. Huang, Design of powder metallurgy titanium alloys and composites, Mater. Sci. Eng. A 418 (1–2) (2006) 25–35. [13] J. O’Flynn, S.F. Corbin, The influence of Fe-40Ti intermetallic master alloy additions on the sintering behaviour of Ti-2.5Fe, J. Alloys Compd. 716 (2017) 184–196. [14] P.G. Esteban, L. Bolzoni, E.M. Ruiz-Navas, E. Gordo, PM processing and characterisation of Ti–7Fe low cost titanium alloys, Powder Metall. 54 (3) (2011) 242–252. [15] L. Bolzoni, E.M. Ruiz-Navas, E. Gordo, Understanding the properties of low-cost iron-containing powder metallurgy titanium alloys, Mater. Des. 110 (2016) 317–323. [16] L. Bolzoni, E.M. Ruiz-Navas, E. Gordo, Quantifying the properties of low-cost powder metallurgy titanium alloys, Mater. Sci. Eng. A 687 (2017) 47–53. [17] J.W. Park, H.C. Lee, S. Lee, Composition, microstructure, hardness, and wear properties of high-speed steel rolls, Metall. Mater. Trans. A 30 (2) (1999) 399–409. [18] R. Kocich, L. Kunčická, A. Macháčková, M. Šofer, Improvement of mechanical and electrical properties of rotary swaged Al-Cu clad composites, Mater. Des. 123 (2017) 137–146. [19] P.G. Esteban, E.M. Ruiz-Navas, E. Gordo, Influence of Fe content and particle size the on the processing and mechanical properties of low-cost Ti–xFe alloys, Mater. Sci. Eng. A 527 (21−22) (2010) 5664–5669. [20] J. O’Flynn, S.F. Corbin, The influence of iron powder size on pore formation, densification and homogenization during blended elemental sintering of Ti–2.5Fe, J. Alloys Compounds 618 (2015) 437–448. [21] Z.Y. Ma, R.S. Mishra, S.C. Tjong, High-temperature creep behavior of TiC particulate reinforced Ti–6Al–4V alloy composite, Acta Mater. 50 (17) (2002) 4293–4302. [22] X. Luo, Y. Yang, Q. Sun, Y. Yu, B. Huang, Y. Chen, Effect of Cu/Mo duplex coating on the interface and property of SiCf/Ti6Al4V composite, Mater. Sci. Eng. A 535 (2012) 6–11. [23] Z.Z. Fang, P. James, S. Pei, K.S.R. Chandran, Y. Zhang, Y. Xia, F. Cao, M. Koopman, M. Free, Powder metallurgy of titanium – past, present, and future, Int. Mater. Rev. 63 (7) (2017) 407–459. [24] X. Wu, P. Jiang, L. Chen, F. Yuan, Y.T. Zhu, Extraordinary strain hardening by gradient structure, Proc. Natl. Acad. Sci. 111 (20) (2014) 7197–7201. [25] Y.-J. Kim, H. Chung, S.-J.L. Kang, Processing and mechanical properties of Ti–6Al–4V/TiC in situ composite fabricated by gas–solid reaction, Mater. Sci. Eng. A 333 (1–2) (2002) 343–350.
Declaration of Competing Interest The authors declare that they have no known competing financial interests or personal relationships that could have appeared to influence the work reported in this paper. The authors declare the following financial interests/personal relationships which may be considered as potential competing interests: Acknowledgements This research was supported by the National Natural Science Funds for Distinguished Young Scholar of China (51625404), the National Key Fundamental Research and Development Project of China (2014CB644002), National Key Research and Development Plan of China (2016YFB0700302). References [1] S.C. Tjong, Y.-W. Mai, Processing-structure-property aspects of particulate- and whisker-reinforced titanium matrix composites, Compos. Sci. Technol. 68 (3–4) (2008) 583–601. [2] K. Naseem, Y. Yang, X. Luo, B. Huang, G. Feng, SEM in situ study on the mechanical behaviour of SiCf/ti composite subjected to axial tensile load, Mater. Sci. Eng. A 528 (13–14) (2011) 4507–4515. [3] L. Hengjun, Y. Yanqing, H. Bin, Y. Meini, L. Xian, Numerical simulation of the densification processing of titanium-matrix coated fiber composites, J. Mater. Process. Technol. 208 (1–3) (2008) 284–288. [4] X. Wang, Y. Yang, X. Luo, W. Zhang, N. Jin, Z. Xiao, G. Feng, Effect of C/Mo duplex coating on the interface and mechanical properties of SiCf/Ti6Al4V composites, Mater. Sci. Eng. A 566 (2013) 47–53. [5] C. Even, C. Arvieu, J.M. Quenisset, Powder route processing of carbon fibres reinforced titanium matrix composites, Compos. Sci. Technol. 68 (6) (2008) 1273–1281. [6] S. Xu, C. Zhou, Y. Liu, B. Liu, K. Li, Microstructure and mechanical properties of Ti15Mo-xTiC composites fabricated by in-situ reactive sintering and hot swaging, J. Alloys Compd. 738 (2018) 188–196.
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