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Consolidation and mechanical properties of metal and intermetallic matrix nanocomposites produced using high-energy ball milling
V.E. Panin, M.A. Korchagin1, O.I. Lomovsky1, D.V. Dudina1, S.V. Panin,
V.G. Durakov, Yu.I. Pochivalov, and I.V. Stepanova2
Institute of Strength Physics and Materials Science SB RAS, Tomsk, 634021, Russia 1 Institute of Solid State Chemistry and Mechanochemistry SB RAS, Novosibirsk, 630128, Russia 2 Tomsk Polytechnic University, Tomsk, 634050, Russia
Synthesis of nanocomposites in TiB2-Cu, TiB2-Cu-Ni, TiB2-NiTi systems was developed using a combination of high-energy ball milling and self-propagating high-temperature synthesis (SHS). Consolidation behavior of nanocomposite powders, microstructures and mechanical properties of the bulk were investigated in view of developing new materials with high temperature stability and increased strength.
1. Introduction
Development of metal matrix composites containing titanium diboride as a reinforcing phase has attracted much interest recently due to its high melting point, high hardness and substantial thermal and electrical conductivity [1].
Methods of mechanical treatment (ball milling) have shown high efficiency in production composite powders with uniform microstructure, fine particulate inclusions and increased sintering ability [2]. During mechanical treatment high density of interphase boundaries and defects are stored in materials so that nanocomposites of specific kind are formed.
In this work we investigated sintering (consolidation) behavior and evaluated mechanical properties of TiB2-Cu, TiB2-Cu-Ni, TiB2-NiTi composites produced through three stages [3, 4]: preliminary mechanical treatment of elemental powder mixtures, SHS-reaction ignited in the treated mixture and subsequent mechanical treatment of the product of SHS-reaction. Preliminary mechanical treatment allowed SHS to proceed in the systems mixed at the nanolevel and decreased combustion temperature so that submicron or nanoparticles of SHS-product could be formed. Subsequent mechanical treatment decreased the size of inclusions in the matrix down to the nanolevel and created highly defected state in the matrix increasing sintering activity of the composite.
Powder nanocomposites were consolidated using of spark plasma sintering known for possibilities of nanostructure retention in bulk state [5] and hot pressing [6].
2. Experimental
2.1. Synthesis of nanocomposites
To achieve nanocomposite state in powder mixtures mechanical treatment was carried out in high-energy ball mill AGO-2 [7]. To prevent oxidation milling was performed under protective atmosphere.
TiB2-Cu
To produce TiB2-Cu nanocomposites preliminary mechanical treatment of (Ti + 2.1B) + x % Cu elemental powder mixtures was used with subsequent SHS reaction to form titanium diboride [4]. The product of SHS reaction was then mechanically treated to decrease the size of titanium diboride particles. The size of TiB2 particles distributed in copper matrix in the resultant product was 30-50 nm.
As the content of copper matrix in the composites directly produced through SHS-reaction is limited, the advantage of this three-stage processing is the possibility of introduction additional quantity of metal powders during mechanical treatment of the SHS-product. In this work we produced SHS-product with 57 vol. % of titanium diboride and then diluted it by copper to obtain compositions of higher copper content.
TiB2-Cu-Ni
Preparation of compositions with complex Cu-Ni matrix utilized TiB2-43 vol. % Cu SHS-composite and included addition of required quantity of Cu and Ni powders at the stage of mechanical treatment of the product of SHS-reac-tion.
TiB-NiT
Powder mixtures of titanium, nickel and boron were mixed in the ratio corresponding to a desired composition and mechanically treated. SHS-reaction resulted in the formation of composite containing fine titanium diboride particles distributed in NiTi matrix. The product also contained small amounts of Ti2Ni h TiNi3. Unlike composites with copper matrix components of Ti-B-Ni system are not inert to each other. Thus, when heated or mechanically activated Ti-Ni powder mixtures are exothermic enough to obtain titanium nickelide through SHS [8]. This allows producing TiB2 -NiTi composite through parallel reactions in the same
© V.E. Panin, M.A. Korchagin, O.I. Lomovsky, D.V. Dudina, S.V. Panin, VG. Durakov, Yu.I. Pochivalov, and I.V. Stepanova, 2004
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Fig. 1. Microstructure of SPS-compacted Cu - 10 % wt. TiB2
SHS process. Nickel borides were not detected in SHS-product though they exhibit high enthalpies of formation. The product of SHS-reaction was then mechanically treated to produces nanostructures in both matrix and reinforcement phases.
2.2. Consolidation
Spark plasma sintering apparatus (Sumitomo Coal Mining Co., Ltd.) was used with graphite mold of 10 mm diameter. Sintering was performed in vacuum. The applied SPS-pressure was 70 MPa. SPS-temperature varied in the range of 700-950 °C. It should be noted that effective temperature of the sample is usually 50 °C higher than SPS-temperature measured by thermocouple inserted in the wall of the mold. Holding time at the maximum temperature was 5 min.
Hot-pressing was performed at 900 °C using conventional technique.
2.3. Microstructure studies
Microstructures of as-compacted samples were studied by Scanning Electron Microscopy (SEM). To prepare samples for SEM the compacts were polished and etched with (NH4)2S2O8aqueous solution (1: 5).
3. Results and discussion
3.1. Microstructures of TiB2-Cu and TiB2-Ni-Cu SPS-compacts
Unlike compositions of high titanium diboride content (more that 50 vol. %) in which nanograined skeleton was formed under SPS [9], it was impossible to inhibit growth of particles in compositions of low titanium diboride content.
Fig. 2. Microstructure of SPS-compacted Cu - Ni (80/20 at. %) -10 wt. %TiB2
Probably much more molten phases were formed in the latter sample so that growth through liquid phase diffusion was unavoidable and resulted in formation of titanium diboride particles 1-3 ^m in size (Fig. 1).
It was shown [4] that addition of nickel to TiB2-Cu powders resulted in the increase in density and hardness of compacts due to improved wettability of Ni-TiB2-system. So, to obtain composition with complex matrix Cu-Ni with different ratios were obtained and there properties were measured.
Though sintered at higher SPS-temperature Cu-Ni (80/20 at. %) - 10 wt. % TiB2 compact showed diminished growth of titanium diboride particles compared to Cu -10% wt. TiB2 composition due to higher melting temperature of the matrix (Fig. 2).
3.2. Mechanical properties of TiB2-Cu and TiB2-Ni-Cu SPS-compacts
Microhardness, ultimate strength and ductility of the compacts are shown in Table 1. The microhardness changes nonlinearly with changing of the Ni/Cu content ratio in the matrix. Increase of TiB2 content in nickel-free composites causes microhardness increasing but stipulates decreasing wettability and, as a consequence, ultimate strength.
Loading diagrams of nanocomposite specimens of all investigated compositions are shown in Fig. 3. The presented data testifies on high ductility reduction with increasing concentration oftitanium diboride reinforcing phase (Fig. 3, curve 4) or with decreasing of the copper concentration in the matrix (Fig. 3, curves 3, 5, 6). On the other hand, the lack of the nickel in the matrix result in strength reduction (Fig. 3, curves 1, 4).
Table 1
Mechanical properties of TiB2-Cu and TiB2-Ni-Cu SPS-compacts
Composition B2 i H ^ cs) ^ 6 1 о 10 % TiB2 Cu/Ni 60/40 110 % TiB2 Cu/Ni 40/60 10 % TiB2 Cu/Ni 20/80 10 % TiB2 Cu = 100 20 % TiB2 Cu = 100
SPS-temperature, °C 800 800 800 800 700 950
Ultimate strength oB, MPa 1267 1 110 1 130 1250 510 530
Deformation ~8 ~3.5 ~3.2 ~3.4 ~6 ~2.2
Microhardness Hp MPa 4330 6340 6810 7 920 2 860 3 840
Fig. 3. Loading diagrams of SPS-compacted Cu-TiB2 and Cu-Ni-TiB2 composites: Cu - 10 wt. % TiB2(1); (Cu/Ni = 80/20) - 10 wt. % TiB2(2); (Cu/Ni = 20/80) - 10 wt. % TiB2 (3); Cu - 20 wt. % TiB2 (4); (Cu/Ni = = 60/40) - 10 wt. % TiB2 (5); (Cu/Ni = 40/60) - 10 wt. % TiB2(6)
In (Cu/Ni = 80/20) + 10 % TiB2 specimen up to £ ~ ~ 6.5 % formation of strain-induced relief was not observed. Just before fracture microcracks were formed near specimen edge where highest curvature is realized due to barrel-like shape changing under compression (Fig. 4, a, b). Formations of transverse folds (with length of up to 60 ^m) were observed along with beginning of microcracking. It is seen that transverse folds were formed along boundaries of structure elements of nanocomposites. The pattern of strain-induced relief formed on the surface of these specimens at the prefracture stage is shown on Fig. 4(c).
In Cu - 10 wt. % TiB2 specimens the deformation evolution was accompanied by the formation of many microcracks and bands of localized plastic deformation. Their orientation coincided with the normal or made angle of ~ 45° to com-
3.3. Mesoscaleplastic deformation and failure TiB2-Cu and TiB2-Ni-Cu SPS-compacts
The investigations of mesoscale deformation mechanisms of powder nanocomposites, which possess some ductility, are of particular interest. Illustrations will be made using ductile composites: (Cu + Ni + 10% TiB2) and (Cu + 10 % TiB2), see Fig. 3, curves 1, 2.
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Fig. 4. Optical image gained on the surface of (Cu/Ni = 80/20) + 10 % TiB2 specimen under compression (a, c) and main plastic shear distribution in this area (b); 400 X 300 ^m; e - 6.5 (a); 7.6 % (c)
Fig. 5. Main plastic shear distribution (a-c) and optical image of Cu -10 % TiB2 specimen surface under compression (d); 550 x370 ^m; £~ 4
(a), 5.3 (b), 5.8 % (c, d)
Fig. 6. Optical images gained after fracture on Cu + 10 % TiB2 specimen surface; 550 X 370 ^m
pression axis (Fig. 5(a, d). The given deformation mechanism is an effective way of internal stress relaxation, while formation of a great number of such mesodefects does not cause local plastic deformation to reach the macroscale, however it stipulated decreasing stress. Most likely such reaction of material onto external loading is related to low wettability of TiB2 by the matrix material. This determines decreasing of cohesion strength of structure elements of materials under investigation.
With deformation increasing (e ~ 5.3 %) the number of mesofragments being recognized by analysis of strain distribution patterns whose boundaries were determined by microcracks and localized deformation bands is increased while their orientation make an angle of about 70° to the loading axis (Fig. 5(c). In all probability, the similar orientation of mesobands is related to barrel like shape changing of specimens under compression loading. At the prefracture stage the intensity of localized deformation within mesobands is increased that is confirmed by the distribution of main plastic shear (Fig. 5(c)). Specimen failure took place by main crack propagation towards the direction of maximum tangential stresses.
Characteristic pattern of the fractured specimen is shown in Fig. 6. It is evident that deformation evolution occurs non-uniformly: regions with a large number of microcracks and mesobands are alternated with large fragments without pronounced strain-induced relief to be formed on the surface. This result corresponds well to structure investigations performed by scanning electron microscopy.
Promising results were obtained in hot-pressed TiNi -10 wt. % TiB2compacts which possessed very high strength properties (Fig. 7).
4. Conclusions
1. Nanocomposites in TiB2-Cu, TiB2-Cu-Ni, TiB2-NiTi systems were successfully synthesized through combination ofmechanical treatment and SHS. Optimal processing parameters were determined to obtain composite powders with the size of grains less than 100 nm.
2. Optimal Cu/Ni ratio in matrix was determined to be 80/20. Due to improved wetting in TiB2-Ni system addition of nickel to the matrix increases strength of the composites.
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Deformation s, %
Fig. 7. Stress-strain curve for TiNi - 10 wt. % TiB2
Copper is used as a ductile component in the matrix material. Ductility of 10 wt. % TiB2-(Cu/Ni = 80/20) specimens was confirmed by revealing strain-induced relief with transverse folds as main components forming along boundaries of matrix structural elements.
3. Plastic deformation of 10 wt. % TiB2 - Cu specimens under compression is accompanied by the formation and evolution of localized deformation mesobands and microcracks, which are formed in zones of localized deformation oriented towards the direction of maximum tangential stresses. This stipulated low strength of this composition with relatively high ductility.
This work was financially supported by SB RAS integration project No. 93. “Design of Principles and Technologies of Nanostructured State Formation in Surface Layers and Internal Interfaces of Reliable Structural and Functional Materials”.
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