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CREATION OF HIGH PERFORMANCE TI AND TI-6AL-4V VIA HARMONIC STRUCTURE DESIGN APPROACH

Mie Ota*, Sanjay Kumar Vajpai*, Kazuaki Kurokawa**, Tomoyuki Watanabe**, Kei Ameyama***, Guy Dirras****

*Research Organization of Science and Technology, Ritsumeikan University, Kusatsu, Shiga, Japan

**Graduate School of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan

***Department of Mechanical Engineering, Faculty of Science and Engineering, Ritsumeikan University, Kusatsu, Shiga, Japan

****Laboratoire des Sciences des Procédés et des Matériaux, Université Paris 13, Sorbonne Paris Cité, France

ABSTRACT

Through many years, conventional material developments have emphasized on microstructural refinement and homogeneity. However, "Nano- and Homogeneous" microstructures do not, usually, satisfy the need to be both strong and ductile, due to the plastic instability in the early stage of the deformation. As opposed to such a “nano- and homo-“microstructure design, we have proposed “Harmonic Structure” design. The harmonic structure has a heterogeneous microstructure consisting of bimodal grain size together with a controlled and specific topological distribution of fine and coarse grains. In other words, the harmonic structure is heterogeneous on micro- but homogeneous on macro-scales. In the present work, the harmonic structure design has been applied to pure-Ti and Ti-6Al-4V alloy via a novel powder metallurgy route consisting of controlled severe plastic deformation of the fine-sized powder particles (pre-alloyed in the case of Ti-6Al-4V) via jet milling and subsequent consolidation by SPS. At a macro-scale, the harmonic structure materials exhibited significantly better combination of strength and ductility, under quasi-static tensile loadings, as compared to their homogeneous microstructure counterparts.

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1. INTRODUCTION

Grain refinement is a well-known method of strengthening of metallic materials (Estrin and Vinogradov 2013, Hansen 2004, Meyers, Mishra and Benson (2006)). However, the metals and alloys with homogeneous fine-grained microstructures exhibit improvement in strength at the expense of ductility (Ma 2003, Tsuji, Kamikawa, Ueji, Takata, Koyama and Terada (2008)). As a result, ultra-fine/nano grained materials exhibit very high strength but extremely poor ductility, i.e. poor toughness. Interestingly, however, a bimodal grain size distribution proved to be extremely effective in achieving an attractive combination of high strength together with reasonable ductility (Dirras, Gubicza, Ramtani, Bui and Szilagyi (2010), Fujiwara, Akada, Noro, Yoshita and Ameyama (2008), Wang, Chen, Zhou and Ma (2002), Wang and Ma 2004, Witkin, Lee, Rodriguez, Nutt and Lavernia (2003)). In particular, a novel microstructural design, called “harmonic structure”, has been proposed by Ameyama and co-workers (Sekiguchi, Ono, Fujiwara and Ameyama (2010), Orlov, Fujiwara and Ameyama (2013), Ciuca, Ota, Deng and Ameyama (2013), Sawangrat, Yamaguchi, Vajpai and Ameyama (2014), Zhang, Vajpai, Orlov and Ameyama (2014)). The harmonic structure design corresponds to a heterogeneous and bimodal microstructure. In harmonic structure, the coarse-grained areas (“core”) are surrounded by the three-dimensional continuously connected network (“shell”) of fine-grained matrix. Bulk pure Titanium, pure Copper, SUS329J1 steel, Co-Cr-Mo alloy and SUS304L steel with harmonic structure design were successfully prepared via a controlled mechanical milling of metal/alloy powder particles followed by their consolidation. The harmonic structured materials demonstrated significantly better combination of strength and ductility as compared to their homogeneous fine or coarse-grained counterparts. In our previous work, the harmonic structure was created by controlled mechanical milling of coarse powders via planetary ball mill. However, it would be worth pointing towards the limitations of the proposed powder metallurgy route. Firstly, the controlled mechanical milling via ball mill was effective primarily with coarse particles and it is very difficult to achieve controlled deformation in fine size particles by ball milling. Secondly, the ball milling is also prone to induce some amount of contamination in the milled powders. Therefore, it would be worth making efforts to develop a new powder metallurgy processing method so as the above mentioned issues could be handled effectively. Such developments would immensely enhance the commercial viability of preparing near-net shape components with harmonic structure and improved mechanical properties.

In the present work, “jet milling” is proposed to achieve controlled severe plastic deformation in fine-sized metallic powder particles. In jet milling, highly compressed air or gases are used, usually in a vortex motion, to facilitate the collision of fine particles against each other at a very high velocity (Modoux, Hosek, Pailleres and Authelin (1999)). The jet milling is a relatively clean process. Since no milling media is used in this process, the possibility of contamination is extremely minimal. Furthermore, high purity inert gas can be used, instead of air, to further minimize the risk of oxidation during the milling process. Therefore, the present work deals with the preparation of harmonic structure Ti and Ti-6Al-4V through a powder metallurgy process involving controlled severe plastic deformation of fine-size pure Ti and Ti-6Al-4V alloy powders via jet milling followed by spark plasma sintering. A schematic of the proposed fabrication process of the harmonic structure using jet milling process is shown in Fig. 1. In the present work, the microstructural evolution at every stage of processing has been presented. Furthermore, the effect of milling conditions on the microstructure and mechanical properties of the bulk Ti and Ti-6Al-4V compacts, thus prepared, has also been presented and discussed.

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Fig. 1. An schematic of the proposed fabrication process of harmonic structure design.

2. EXPERIMENTAL PROCEDURE

In this work, gas atomized low oxygen content TILOP grade pure Ti (median particle size: 25.8 μm) and Ti-6Al-4V alloy (median particle size: 83.7 μm) powders, supplied by Osaka Titanium Technologies Co. Ltd., were used as starting material. The powders were milled using Nano Jetmizer apparatus manufactured by Aishin Nano Technologies Co. Ltd. Japan (Figure 2). The mechanism of milling using jet mill is also demonstrated in Figure 2. The milling was carried out using argon gas, at a pressure of 0.9 MPa, to minimize any significant oxidation. The milling was carried out in several passes wherein the powders were collected after every milling pass and recycled for the next milling pass. Each milling pass was completed in 10-15 minutes and the milling time decreased with increasing number of passes. Subsequently, the powders were consolidated by spark plasma sintering (SPS,) using a graphite die with 15 mm internal diameter. The pure Ti and Ti-6Al-4V powders were sintered at 1073 K (800 oC) and 1123 K (850 oC), respectively, for 1.8 ks (30 minutes) under vacuum and 50 MPa applied pressure. The microstructural characteristics of the initial powder, milled powder, and the sintered compacts were observed by scanning electron microscope (SEM) and electron backscattered diffraction (EBSD) technique. The mechanical properties of the bulk material were evaluated by tensile tests. The tensile tests were carried out on specimens having gauge dimensions 3 mm (length) x 1 mm (width) x 1 mm (thickness), using a Shimadzu AGS-10kND tensile testing machine at a strain rate of 5.6x10-4 s-1.

Fig. 2. (a) Nano Jetmizer apparatus, (b) mechanism of milling.

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3. RESULTS AND DISCUSSION

3.1 MICROSTRUCTURAL EVOLUTION DURING JET MILLING PROCESS

Figs. 3 and 4 show the morphology and cross-section, respectively, of the initial and milled pure Ti powders. It can be seen that the initial powders were spherical in shape with almost smooth surface (Fig. 3a) and equiaxed microstructure having grain size in the range of 5 to 10 μm (Fig. 4a). On the other hand, the morphology (Fig. 3b) and the microstructure of the cross-section of the milled powders (Fig. 4b) show that the jet milling led to the formation of a featureless region, having “shell-like” appearance, in the vicinity of the surface of the powder particles whereas the remaining inner part of the milled powder particles, i.e. “core” of the powder, still consisted of coarse equiaxed microstructure. Fig. 5 shows the morphology of the initial and 3-pass jet milled Ti-6Al-4V alloy powders. It can be seen that the initial Ti-6Al-4V powder particles were also spherical in shape, and the subsequent jet milling induced morphological changes were similar to that observed in case of pure Ti powders. These results clearly indicate that the jet milling is very effective in developing controlled milling-induced severe plastic deformation, limited primarily to the near-surface regions of the fine size powder particles, in the form of severely deformed featureless “shell” region. The appearance of such a featureless “shell” region is primarily related with the formation of nanocrystalline structure due to severe plastic deformation (Fujiwara et al. (2008), Zhang et al. (2014)). Hence, the controlled mechanical milling via jet mill successfully created a bimodal microstructure in the milled powder particles, i.e. (i) nanocrystalline “shell” region in the vicinity of the surface of the particle, and (ii) coarse-grained “core” inner region.

Fig. 3. Morphology of pure Ti powders: (a) initial (b) 3-pass

jet milled.

Fig. 4. Cross section of pure Ti powders: (a) initial (b) 3-

pass jet milled.

Fig. 5. Morphology of Ti-6Al-4V alloy powders: (a) Initial

(b) 3-pass jet milled.

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3.2 MICROSTRUCTURE OF THE SINTERED TI AND TI-6AL-4V COMPACTS

Fig. 6 shows the microstructures of the pure Ti compacts prepared from initial and 3-pass jet milled powders. It can be seen that the specimens prepared from the initial powders exhibit coarse equiaxed microstructure (Fig. 6a). On the other hand, it is interesting to note that the milled bimodal powders resulted in equiaxed bimodal grain structure with a peculiar topological distribution, i.e. the regions with coarse grains are embedded in the matrix of fine-equiaxed grains (Figs. 6b & 6c). The sintered compacts prepared from 3-pass jet milled Ti-6Al-4V alloy powders also exhibited similar microstructure, as shown in Fig. 7. Such a peculiar bimodal microstructure, i.e. regions with fine-equiaxed grains form a continuously connected three dimensional network around the regions with coarse grained microstructure, is referred as “Harmonic” structure design wherein the regions with fine-equiaxed and coarse-equiaxed microstructure correspond to the severely deformed “shell” and moderately-deformed/un-deformed “core” regions of the milled powders, respectively. It is also interesting to note that the grain size of the core region in the harmonic structure is somewhat smaller than that of the compacts prepared from the initial as-received Ti powders (Fig. 6a & 6c). Such a microstructural evolution suggests that the effect of jet milling is not only limited to near surface region but also extended to the inner part of the powder particles due to the very small particle sizes. It appears that the relatively finer structure is evolved in the core region due to recrystallization and grain growth during subsequent elevated temperature consolidation. As a result, the grain size of the core and the shell region is not as apparently different as observed in the case of harmonic structure created using coarse powder particles. Nevertheless, as marked in Figs. 6 & 7, the microstructure of the sintered compacts exhibits peculiar characteristics of the harmonic structure design. It would be worth mention that the grain size and the volume fraction of the core and the shell areas can be controlled by varying the milling and consolidation conditions.

Fig. 6: Microstructure of the pure Ti specimens: (a) homogeneous coarse-grained microstructure (b and c) harmonic microstructure.

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Fig. 7: Harmonic structure Ti-6Al-4V prepared from 3-pass jet milled powders.

3.3 MECHANICAL PROPERTIES

Fig. 8 shows the representative nominal stress – nominal strain curves of pure Ti and Ti-6Al-4V alloy specimens with homogeneous coarse-grained, prepared from initial powders, as well as harmonic structure. It can be clearly observed that the sintered compacts with harmonic structure exhibited considerably higher yield strength, tensile strength, total strain-to-fracture, and uniform elongation as compared to their counterparts with coarse-grained homogeneous microstructure prepared from as received powders. Moreover, these results also indicated that the harmonic structured Ti and Ti-6Al-4V exhibit superior set of mechanical properties as compared to their homogeneous coarse-grained or ultra-fine grained counterparts. Therefore, the present results further confirm that the harmonic structure design is extremely effective in achieving higher strength without compromising ductility in the Ti-based alloys, i.e. improved toughness as compared to the conventional homogeneous microstructures. It has been observed that the harmonic structure has the ability to promote uniform distribution of strain during plastic deformation, leading to improved mechanical properties by avoiding or delaying localized plastic instability.

Fig. 8. Stress-Strain curves of homogeneous structure and harmonic structure compacts: (a) Pure Ti (b) Ti-6Al-4V

4. CONCLUSIONS

The harmonic structure was successfully created by controlled milling of fine-sized pure Ti and Ti-6Al-4V alloy powder via jet milling followed by spark plasma sintering. The jet milling led to the milled powder particles with bimodal grain size distribution consisting of severely deformed nano/ultra-fine grained “shell” in the sub-surface region of the particle and coarse-grained “core” inner region. Spark plasma sintering of milled powder resulted in the formation of “harmonic” structure wherein the severely deformed shell regions of the milled powders transformed to a three dimensional network of equiaxed fine-sized grains, enclosing the coarse-grained “core” areas. The pure Ti and Ti-6Al-4V alloy compacts with harmonic structure exhibited a significant enhancement in the strength without compromising with the ductility, i.e. improved toughness as compared to the conventional homogeneous coarse/fine-grained microstructures. The improved mechanical properties of the harmonic structure compacts were

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attributed to the specific topological distribution of high strength fine-grained “shell” and ductile coarse-grained “core” regions; as such a microstructure promotes uniform distribution of strain during plastic deformation and avoids the localized plastic deformation in the early stages of deformation.

ACKNOWLEDGEMENTS

This research was supported by the Japan Science and Technology Agency (JST) under Collaborative Research Based on Industrial Demand "Heterogeneous Structure Control: Towards Innovative Development of Metallic Structural Materials", and by the Grant-in-Aid for Scientific Research on Innovative Area, Bulk Nanostructured Metals through MEXT, Japan (contract No.22102004). These supports are gratefully appreciated.

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