Synthesis of Ti-Based Bulk Metallic Glass Composites Containing WC Particles

Transcription

1 Materials Transactions, Vol. 46, No. 12 (2005) pp to 2967 Special Issue on Materials Science of Bulk Metallic Glasses #2005 The Japan Institute of Metals Synthesis of Ti-Based Bulk Metallic Glass Composites Containing WC Particles I-Kuan Jeng and Pee-Yew Lee* Institute of Materials Engineering, National Taiwan Ocean University, Keelung, Taiwan 202 The preparation of Ti 50 Cu 28 Ni 15 Sn 7 metallic glass composite powders was accomplished by mechanical alloying of pure Ti, Cu, Ni, Sn and WC for 18 ks. In the ball-milled composites, initial WC particles were homogeneously dispersed in the Ti-based alloy glassy matrix. The metallic glass composite powders exhibited a large supercooled liquid region just below the crystallization temperature. The presence of WC nanoparticles did not change the glass formation ability of amorphous Ti 50 Cu 28 Ni 15 Sn 7 powders. The as-milled Ti 50 Cu 28 Ni 15 Sn 7 and composite powders were consolidated by vacuum hot pressing into compact discs with a diameter and thickness of 10 and 4 mm, respectively. Microstructural analysis showed that the bulk metallic glass composite contained submicron WC particles homogeneously embedded in a highly dense nanocrystalline/amorphous matrix. Incorporation of WC into consolidated composite compacts resulted in a significant increase in hardness. (Received June 21, 2005; Accepted August 22, 2005; Published December 15, 2005) Keywords: mechanical alloying, bulk metallic glass composite, supercooled liquid region, Ti-based alloys, nanoparticles 1. Introduction Recently, new metallic glasses with a wide supercooled liquid region exceeding 20 K have been prepared in a number of Ti-based alloy systems, such as Ti Ni Cu, 1) Ti Ni Cu Al, 2) Ti Zr Ni Cu Al, 3) Ti Ni Cu Sn 4,5) and Ti Ni Cu Si B. 6) The development of these new alloys is expected to expand the applications of bulk metallic glass due to their high specific strength to weight ratio, good ductility and relatively low cost of fabrication. The mechanical properties of these unique materials may be further improved by the presence of ceramic or insoluble metallic particles embedded in the amorphous matrix, which can suppress the propagation of shear bands thereby increasing the toughness and ductility of the glassy matrix composite. 7 9) However, the preparation of a glassy matrix composite by casting often results in partial crystallization at the interface between the amorphous and ceramic phase. Moreover, the differences in densities or melting points among the raw materials of metals and particles make it difficult to prepare cast samples. An alternative way to prepare an amorphous alloy is by mechanical alloying (MA). MA, being a solid-state reaction method, allows the preparation of glassy composites whilst circumventing the difficulties associated with casting. 10) The product material of MA is prepared in powdered form. The consolidation of MA powders into bulk specimens is possible due to the characteristic viscosity minimum in the supercooled liquid region. 11) MA has been used to successfully prepare amorphous Cu- and Zr-based composite powders. 12,13) However, there are no reports concerning the formation of Ti-based composite powders by MA. In this study we have analyzed the feasibility of preparing Ti Cu Ni Sn metallic glass composite powders by MA of elemental powder mixtures with the composition of Ti 50 Cu 28 Ni 15 Sn 7 together with WC particles using a shaker ball mill. WC particles were chosen because of their high thermal stability and low chemical reactivity with the elemental powders involved in the MA process. The consolidation of Ti 50 - *Corresponding author, pylee@mail.ntou.edu.tw Cu 28 Ni 15 Sn 7 metallic glass composite powders into bulk form by a vacuum hot-pressing method was also performed. 2. Experimental Procedure Elemental powders of Ti (99.7%, 325 mesh), Cu (99.9%, 325 mesh), Ni (99.9%, 300 mesh), Sn (99.999%, 100 mesh) and WC (99.999%, 100 mesh) were weighed to yield the desired Ti 50 Cu 28 Ni 15 Sn 7 or WC/Ti 50 Cu 28 Ni 15 Sn 7 composition. A mixture of elemental metallic powder, with a nominal composition of Ti 50 Cu 28 Ni 15 Sn 7 (in at%), was mechanically alloyed with or without addition of WC powder. The milling was performed in a SPEX 8016 shaker ball mill under an atmosphere of Ar. Specific details of the mechanical alloying process are given elsewhere. 14) The asmilled composite powders were consolidated in a vacuum hot-pressing machine at 723 K under a pressure of 1:2 GPa to prepare bulk amorphous discs with a diameter of 10 mm and thickness of 4 mm. The structures of the as-milled and bulk samples were analyzed using an X-ray diffractometer, scanning electron microscopy and transmission electron microscopy. Thermal analysis was investigated using a Dupont 2000 differential scanning calorimeter (DSC), where the sample was heated from room temperature to 600 C under a purified argon atmosphere at a constant heating rate of 40 K/min. The Vickers microhardness of consolidated samples was measured with a Matsuzawa MXT50-UL machine using a static load of 0.5 N. 3. Results and Discussion Figure 1 shows the X-ray diffraction patterns of Ti 50 Cu 28 - Ni 15 Sn 7 with and without the addition of 4, 8 and 12 vol% WC after 18 ks milling. For plain Ti 50 Cu 28 Ni 15 Sn 7 alloy powders, only a broad diffraction peak around 2 ¼ 42 can be observed, which indicates that fully amorphous powders were formed at the end of the MA treatment. The WCcontaining powders display crystalline WC diffraction peaks superimposed on the amorphous broad diffraction peak. Furthermore, the intensity of the WC peak increases with

2 2964 I-K. Jeng and P.-Y. Lee WC peak 710 (d) 12 vol % 12 vol % 709 Intensity[a.u.] 8 vol % 4 vol % Heat Flow [a.u.] (c) 8 vol % (b) 4 vol % (a) 0 vol % T g 0 vol % T x Fig. 1 X-ray diffraction patterns for mechanically alloyed Ti 50 Cu 28 - Ni 15 Sn 7 and composite powders after 18 ks milling. 2θ Temperature, T/K Fig. 3 DSC scans of mechanically alloyed Ti 50 Cu 28 Ni 15 Sn 7 and composite powders after 18 ks milling. (a) (b) Fig. 2 SEM cross-sectional image of as-milled (a) Ti 50 Cu 28 Ni 15 Sn 7 and (b) composite powders containing 8 vol% WC. increasing WC content. These results suggest that Ti 50 Cu 28 - Ni 15 Sn 7 bulk metallic glass composites were successfully prepared after 18 ks milling. The presence of WC particles did not significantly alter the glass formation of amorphous Ti 50 Cu 28 Ni 15 Sn 7 powders. Similar behavior has been reported by Wang et al. 12) who examined Cu 60 Zr 30 Ti 10 with addition of WC by mechanical alloying. Scanning electron microscopy was used to observe the cross-sectional view of as-milled amorphous and composite powders (Fig. 2). Figure 2(a) reveals uniform amorphous powders were obtained after 18 ks milling. For Ti 50 Cu 28 - Ni 15 Sn 7 powders with the addition of 8 vol% WC, crystalline WC particles [i.e. small white particles in Fig. 2(b)] were embedded within the amorphous matrix at the end of the milling process. Differential scanning calorimetry (DSC) was used to investigate the glass transition and crystallization behavior of the as-milled powders. The DSC scans of the as-milled Ti 50 Cu 28 Ni 15 Sn 7 monolithic glass and the three composites with WC particles are shown in Fig. 3. Both the Ti 50 Cu 28 Ni 15 Sn 7 and its composite powders exhibited an endothermic glass transition event followed by a sharp

3 Synthesis of Ti-Based Bulk Metallic Glass Composites Containing WC Particles 2965 Table 1 Thermal stability of amorphous Ti 50 Cu 28 Ni 15 Sn 7 and its composite powders prepared by mechanical alloying. Thermal Property T g T x T x E c Composition (K) (K) (K) (kj/mol) Ti 50 Cu 28 Ni 15 Sn Ti 50 Cu 28 Ni 15 Sn 7 þ 4 vol%wc Ti 50 Cu 28 Ni 15 Sn 7 þ 8 vol%wc Ti 50 Cu 28 Ni 15 Sn 7 þ 12 vol%wc : obtained by a heating rate of 40 K/min. Fig. 5 Microstructures of Ti 50 Cu 28 Ni 15 Sn 7 8 vol% WC bulk metallic glass composite consolidated at 723 K under a pressure of 1.2 GPa. (a) (b) (c) Fig. 4 The outer morphology of the Ti 50 Cu 28 Ni 15 Sn 7 containing 8 vol% WC after vacuum hot pressing at 723 K under a pressure of 1.2 GPa. exothermic peak, indicating the successive stepwise transformations from a supercooled liquid state to crystalline phases. The glass transition (T g ) and crystallization (T x ) temperatures are defined as the onset temperature of the endothermic and exothermic DSC events, respectively. As shown in Fig. 3, both the amorphous Ti 50 Cu 28 Ni 15 Sn 7 and Ti 50 Cu 28 Ni 15 Sn 7 8 vol% WC composite powders exhibited the same supercooled liquid region (T x, i.e., T x T g )of55k. Table 1 summarizes the values of T g, T x and T x for all the samples investigated in the present study. The activation energy (E c ) for crystallization as determined by the Kissinger Method 15) is also listed in Table 1. Intriguingly, the WCcontaining composites exhibit identical (within experimental error) thermal properties to the monolithic glass specimen. These results suggest that the addition of WC particles does not affect the composition or thermal stability of the Ti 50 Cu 28 Ni 15 Sn 7 amorphous matrix. Similar results have also been reported by Eckert et al. 16) in preparing mechanically alloyed W/Zr 55 Al 10 Cu 30 Ni 5 metallic glass composite powders with addition of W of up to 17.5%. Based on the DSC results, the as-milled composite powders were consolidated by vacuum hot pressing into a disk with a diameter of 10 mm and thickness of 4 mm. The powders were hot pressed at 723 K under a pressure of 1.2 GPa for 1.8 ks. Figure 4 shows a typical consolidated sample (Ti 50 Cu 28 Ni 15 Sn 7 with 8 vol% WC addition) of bulk metallic glass composite that exhibited a smooth outer surface and metallic luster. The polished cross-sectional view 200nm Fig. 6 TEM observation of consolidated Ti 50 Cu 28 Ni 15 Sn 7 8 vol% WC samples. (a) Bright field image, (b) SAD pattern of WC nanoparticle, and (c) SAD pattern of BMG matrix. examined by SEM is shown in Fig. 5. The WC particles of submicron size are homogeneously dispersed throughout the sample and no pores or voids were observed. Taken together, these results indicate a bulk metallic glass (BMG) composite with a high densification was successfully prepared. We also examined the microstructure of the bulk metallic glass composite Ti 50 Cu 28 Ni 15 Sn 7 with addition of 8 vol% WC by transmission electron microscopy (TEM). The bright field image shows WC particles with the size smaller than 300 nm were observed. [Fig. 6(a)]. As revealed by the SEM in Fig. 5, the WC nanoparticles were distributed homogeneously within the amorphous matrix. TEM with a higher magnification revealed that the WC nanoparticles embedded within the BMG matrix have irregular shapes and range in size from 20 to 300 nm. Figure 6(c) shows the selected area diffraction (SAD) pattern of the amorphous matrix (bright area in the bottom right corner). A diffuse halo with limited diffraction spots was observed, which is characteristic of an amorphous matrix. However, the SAD pattern of the WC nanoparticles exhibit crystalline diffraction spots. These results imply that the BMG composite was successfully

4 2966 I-K. Jeng and P.-Y. Lee prepared by hot pressing the as-milled powders at 723 K under a pressure of 1.2 GPa. Only limited variation in nanocrystallization occurred during consolidation. The mechanical properties of the BMG composite samples were evaluated by the Vickers microhardness test. Because Ti 50 Cu 28 Ni 15 Sn 7 BMG and its composites prepared in this study were of relatively high density, the influence of porosity can be neglected. The Vickers microhardness of the Ti 50 Cu 28 Ni 15 Sn 7 BMG disc was 6.2 GPa, which is greater than that of mechanically alloyed Zr 55 Al 10 Cu 30 Ni 5 BMG composites (5.6 to 6.1 GPa) and Zr 65 Al 7:5 Cu 17:5 Ni 10 composites (4.9 to 5.7 GPa). 17) Indeed, the Vickers microhardness of the Ti 50 Cu 28 Ni 15 Sn 7 BMG disc is comparable to that of Tibased amorphous ribbons prepared by melt spinning (6.02 to 6.56 GPa). 18) Moreover, the microhardness increased from 6.2 GPa for Ti 50 Cu 28 Ni 15 Sn 7 BMG to 7.2, 8.1 and 9.2 GPa for the composites containing 4, 8 and 12 vol% WC, respectively. No cracks around the indentation either from the corners or sides was observed, which implies the BMG composites had acceptable fracture toughness. In order to better understand the variation in hardness among different mechanically alloyed BMG composites, normalized hardness (denoted as H v, norm ) proposed by Eckert et al. 17) was calculated by dividing the measured hardness with that of the BMG matrix (i.e., 6.2 GPa, hardness of Ti 50 Cu 28 Ni 15 Sn 7 BMG). The normalized Vickers hardness for various BMG composites produced by consolidating the mechanically alloyed powders is illustrated in Fig. 7. The influence of particle content in the glassy matrix on the normalized Vickers hardness can be revealed from the slopes of the linear regression fits (three different fits are shown in Fig. 7). Zr 55 Al 10 Cu 30 Ni 5 -based composites containing Y 2 O 3 or W nanosized particles exhibit only a limited increase in hardness (5% for V f ¼ 10%). Mg 55 Y 15 Cu 30 BMG containing Y 2 O 3 nanosized particles display a moderate increase in hardness (15% for V f ¼ 10%). However, a significant increase in hardness (38% for V f ¼ 10%) was observed in the present study using Ti 50 Cu 28 Ni 15 Sn 7 BMG containing homogeneously dispersed WC particles with a size range of nm. The TEM microstructural investigations suggest that our mechanically alloyed Ti-based BMG composite samples can be modeled as a nanocomposite consisting of nanoparticles embedded in the amorphous matrix. Because it is difficult to envisage any dislocation inside the nanoparticle and amorphous phases, the hardening effect induced by the dislocation motion in both phases of the composites can be neglected. Recent results of finite element analysis of the unit-cell model 19) suggest that, assuming no specific interaction between the nanoparticles and the amorphous matrix, the overall hardness of a nanocomposite can be described by a rule of mixtures based on the volume fraction and the hardness of each phase: Hv ¼ H v,am V f,am þ H v,p V p,m where H and V refer to hardness and volume fraction of each phase. The subscripts am and p denote the amorphous matrix phase and nanoparticle phase, respectively. The linear increase of normalized Vickers hardness as shown in Fig. 7 agrees well with the results expected from such a rule of mixtures. 4. Conclusions In the present study, amorphous Ti 50 Cu 28 Ni 15 Sn 7 and its composite powders were successfully synthesized by mechanical alloying of powder mixtures of pure Ti, Cu, Ni, Sn and WC for 18 ks of milling. No significant change in T g, T x, T x and E c was observed between amorphous Ti 50 Cu 28 - Ni 15 Sn 7 and its composite powders. Our results suggest that the addition of WC particles does not affect the composition and thermal stability of the Ti 50 Cu 28 Ni 15 Sn 7 amorphous matrix. BMG composite compact discs were obtained by consolidating the 5 h as-milled composite powders using a vacuum hot pressing process. The microstructure of Ti 50 Cu 28 Ni 15 Sn 7 BMG with the addition of 8 vol% WC exhibited an amorphous matrix embedded with WC nanoparticles ranging from 20 to 300 nm in size. The Vickers microhardness was 6.2 GPa for Ti 50 Cu 28 Ni 15 Sn 7 BMG and increased to 7.2, 8.1 and 9.2 GPa for the composites containing 4, 8 and 12 vol% WC, respectively. A significant increase in hardness (38%) was achieved for Ti 50 Cu 28 - Ni 15 Sn 7 BMG composites comprising 10 vol% WC. 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