Fabrication and Properties of Nano-TiCx / Cu-Cr-Zr Alloy Composites

Nano-sized TiCx/Cu master alloy was prepared by the in situ method of combustion synthesis and hot press consolidation in Cu-TiCNTs system. The nano-TiCx/Cu-Cr-Zr alloy composites were fabricated by dispersing the nano-sized TiCx/Cu master alloy into molten Cu-Cr-Zr alloy via stir casting. It was revealed that the average grain size of TiCx/Cu-Cr-Zr alloy composites was significantly refined from 90 m to 20 m, the Brinell Hardness of nano-TiCx/Cu-Cr-Zr alloy composites increased from 75.1 HB to 86.8 HB, and the wear resistance increased by 12.6%, compared with the Cu-Cr-Zr alloy. The electric conductivity slightly decreased from 64.71% IACS to 52.93% IACS. The improvements of hardness and wear resistance result from nano-TiCx and grain refinement strengthening.


Introduction
Good electrical, thermal conductivity, corrosion resistant and welding performance, in addition to mechanical properties make Cu-Cr-Zr alloy an attractive material for a wide range of applications such as electricity, electronics, machinery, metallurgy and other industrial fields [1][2][3][4] .However, the hardness and wear resistance is a permanent problem for copper based materials.
In order to improve the hardness and wear resistance of Cu-Cr-Zr alloy, several reinforcement methods are put forward.One is alloying, approriate elements are introducted to form a solution; Heat treatment is also used to achieve high strength and high conductivity copper alloy [5,6] .Machining deformation is also an effective method to strengthen Cu-Cr-Zr alloy.Another approach to enhance the hardness and wear resistance of Cu without such detriment to the conductivity is to introduce a hard second phase into the copper to form the production of a copper matrix composite [7] .
Introducing a second phase into the metal, the advantages of both metal and the second phase are preserved [8][9][10][11][12] .According to the theory of conduction, the second phase in metal has weaker influence in scattering of electron compared with the atomic solution [8] .Therefore, the second phase is the best methord in strengthening Cu-Cr-Zr alloy.In situ methord include the following advantages copmpared with ex situ, clean interface, good wettability and high strength combination, besides, the particle is round in shape and uniform in distribution [10,[13][14][15] .Experiments show that the finer of the particles resulting in more number and smaller particle spacing, the better the material performance improvement of a certain volume fraction [16,17] .However, there is hardly any report about in situ ceramic particles, especially in situ nano-sized TiC x enhancing Cu matrix composites [18, 19]   .Therefore, in this study, combustion synthesis (CS) and hot press (HP) are combined to fabricate the nano-sized TiC x -Cu master alloy.Stirring casting is chosen as a main methord to disperse the nano-sized TiC x /Cu master alloy in the Cu-Cr-Zr alloy.The hardness, wear resistance and conductivity of as-cast Cu-Cr-Zr alloy and nano-TiC x /Cu-Cr-Zr alloy composites were investigated in detail.

Materials and Experimental Procedures
The master alloy was fabricated by combustion synthesis of power mixture in Cu-Ti-CNTs system.The starting materials were made of commercial powders of Cu (99.7% purity, ~45Pm), Ti (99.5% purity, ~25Pm) and CNTs (95% purity, 20-30nm in diameter and 30μm in length), which was used to synthetic the master alloy.The reactant powders were mixed in a stainless steel container for 24h to ensure homogeneity, and then pressed into cylindrical compacts (about 28 mm in diameter and 30±5 mm in length) using a stainless steel die to obtain densities of 65±2% theoretical density.uniaxially pressure of 42 MPa just when it was still hot and soft, and the master alloy is ready when it was cooled down to the ambient temperature.The composite was fabricated by dispersing the nano-sized TiC x /Cu master alloy into the Cu-Cr-Zr alloy.The chemical composition of Cu-Cr-Zr alloy is shown in table 1. Cu-Cr-Zr alloy was placed in a graphite crucible and heated in an induction furnace, and then the nano-sized TiC x /Cu master alloy was added into the molten Cu-Cr-Zr alloy which would be protected by carbon powder.Once the nano-sized TiC x /Cu master alloy was molten, poured the melt into the prepared mold.
The phases present of the nano-sized TiC x /Cu master alloy and nano-TiC x /Cu-Cr-Zr alloy composites materials were characterized using X-ray diffraction (XRD, Rigaku D/Max 2500PC, Japan).The bulk samples were then dissolved in a FeCl 3 -HCl distilled water solution to remove the Cu coating on the surfaces of the TiC x grains.The morphologies of the extracted TiC x grains were then observed using a field emission scanning electron microscope (FESEM, JSM 6700F, Japan).The microstructure of as-cast Cu-Cr-Zr alloy and nano-TiC x /Cu-Cr-Zr alloy composites are observed by Olympus optical microscope (XJZ-6, Japan).CNTs is used as carbon source to increase the reactivity in the research, TiC x fabricated by CS is easy to react, and adiabatic temperature is reduced.The main mechanism of thermal explosion in Cu-Ti-CNTs systerm is dissolution-precipitation [15] .With the increase of temperature, the Ti x Cu y compounds are firstly formed due to the solid diffusion between Ti and Cu, once the temperatures reached to the molten point of the Ti x Cu y compounds, the melting of Ti x Cu y compounds occurs, and the Cu-Ti liquid forms and spreads over the CNTs particles.The formation of the Cu-Ti liquid promotes further diffusion of C atoms away from the bulk CNTs particles, as well as the formation of the Cu-Ti-C liquid.As more and more C atoms diffused into the liquids, some TiC x can be formed at the interface between the liquid CNTs particles.With the increase of temperature, the C atoms in the CNTs particle become activated and continuously diffuse into the liquids.CNTs particles become smaller and are gradually consumed, as a result, TiC x particulates are gradually precipitated out from the liquid, finally, the CNTs particles are consumed completely.CNTs are smaller in a single volume and more in number when the total volume is defined.CNTs is propitious to transmit when it is chosen as the carbon source, which is more conducive to react, consequently, the adiabatic temperature is reduced contributing to refine TiC x grains.4(a,b), respectively.The micrographs show the microstructure of as-cast Cu-Cr-Zr alloy and nano-TiC x /Cu-Cr-Zr allloy composites.Fig. 4 (a) shows the grains of as-cast Cu-Cr-Zr alloy is coarse dendrite with the size mainly about in 90Pm.However, the grains of the 4wt.%TiC x /Cu-Cr-Zr alloy composites is equiaxedcrystal with the size about in 20Pm, furthermore, the size is relatively uniform and the grain boundry is more obvious, as shown in fig.4(b).The different morphology of the grains indicated that the incorporation of nano-TiC x refined the grain of Cu-Cr-Zr alloy successfully.Fig. 4 The microstructures of (a) as-cast Cu-Cr-Zr alloy and (b) 4wt.% nano-TiC x /Cu-Cr-Zr alloy composites As known, the morphology change of the dendrites could be an important factor for improving the wear resistance properties.Report has revealed about the nano-sized TiC x refined the Į-Al dendrites as heterogeneous nucleation sites [20] .Here we also suggest that the nano-sized TiC x particles acted as heterogeneous nucleation sties of the Cu crystal during solidification, resulting in a more refined microstructure.More boundaries are formed because of the refined dendrites in the nano-TiC x /Cu-Cr-Zr alloy composites.The high boundary concentration and the ceramic particles play an important role as barriers to the enablement the deformation, which is helpful to improve the hardness and wear resistant.

Hardness and Wear Resistance.
Hardness and wear resistance are the main existing problems of Cu-Cr-Zr alloy so far.The hardness, wear resistance and conductivity experimental results are shown below.On the whole, the dispersion of nano-sized TiC x /Cu master alloy into the Cu-Cr-Zr alloy refined the grains.Under the condition of the pin-on-disc sliding wear, wear resistance is only determined by the hardness.Nano-TiC x plays the role as second hard particles, which improved the hardness.As a result, the wear resistance is improved to a large extent.However, both the grain refinement resulted in the increase of grain boundary and nano-TiC x increase the scattering of free electron movement, interface resistance increased, the conductivity of 4wt.% nano-TiC x /Cu-Cr-Zr alloy composites is reduced to a certain extent.

Conclusions
In summary, nano-sized TiC x -Cu master alloy was fabricated by CS+HP.Nano-TiC x /Cu-Cr-Zr alloy composites were fabricated by dispersing the nano-sized TiC x /Cu master alloy into molten Cu-Cr-Zr alloy via stir casting successfully.With the addition of TiC x , the grain size of the nano-TiC x /Cu-Cr-Zr alloy composites decreased from 90um to 20um.It is beneficial to improve the mechanical property of Cu-Cr-Zr alloy, which Brinell Hardness increased from 75.1 HB to 86.8 HB.Compare with the Cu-Cr-Zr alloy, wear resistance of the nano-TiC x /Cu-Cr-Zr alloy composites increased by 12.6% and the electric conductivity slightly decreased from 64.71% IACS to 52.93% IACS.The improvements of hardness and wear resistance result from nano-TiC x and grain refinement strengthening.

Fig. 1
Fig.1 Schematic of the equipment for the combustion synthesis and hot press consolidation experiment.The combustion synthesis (CS) experiments were conducted in a self-made vacuum thermal explosion furnace as illustrated in Fig.1.The compact was placed in a graphite mould and heated at the heating rate of 30 /min under a protective atmosphere of argon to prevent titanium or carbon loss through oxidation.When the temperature which was measured by Ni-Cr/Ni-Si thermocouples suddenly rose rapidly, indicating that the formation reaction of the ceramics was ignited, the sample was quickly pressed with the

Fig. 2 (
Fig.2 (a) shows the XRD patterns for the master alloy.It can be seen that all the products consist of Cu and TiC x phases, without any intermediate phases, it indicates that the pure composites could be successfully fabricated by the method of combustion synthesis and hot press consolidation.The master alloy is identified as TiC x /Cu master alloy.Fig.2 (b) shows the FESEM images of the extracted TiC x grains in master alloy.As indicated, most of the TiC x grains are typical spherical with sizes about 50nm, and the rests are near spherical.

Fig. 2
Fig.2 a) XRD patterns and b) FESEM images of the TiC x grains for the master alloy According to previous studies, the size of in situ TiC x in particles enhanced Cu matrix composites is almost micron size which is mainly because of the carbon source.However,

Fig. 3
Fig.3 shows the XRD patterns for the as-cast Cu-Cr-Zr alloy and 4wt.% nano-TiC x /Cu-Cr-Zr alloy conposites.It can be seen that the nano-TiC x /Cu-Cr-Zr alloy composites of Cu and TiC x phases, without any intermediate phases, it indicates that the nano-TiC x /Cu-Cr-Zr alloy composites could be successfully fabricated by dispersing nano-sized TiC x /Cu master alloy into Cu-Cr-Zr alloy.

Fig. 3
Fig.3XRD patterns for the as-cast Cu-Cr-Zr alloy and 4wt.% nano-TiC x /Cu-Cr-Zr alloy composites Microstructure of the as-cast Cu-Cr-Zr alloy and 4wt.% nano-TiC x /Cu-Cr-Zr alloy composites are presented in Fig.4(a,b), respectively.The micrographs show the microstructure of as-cast Cu-Cr-Zr alloy and nano-TiC x /Cu-Cr-Zr allloy composites.Fig.4(a) shows the grains of as-cast Cu-Cr-Zr alloy is coarse dendrite with the size mainly about in 90Pm.However, the grains of the 4wt.%TiC x /Cu-Cr-Zr alloy composites is equiaxedcrystal with the size about in 20Pm, furthermore, the size is relatively uniform and the grain boundry is more obvious, as shown in fig.4(b).The different morphology of the grains indicated that the incorporation of nano-TiC x refined the grain of Cu-Cr-Zr alloy successfully.

Fig. 5
Fig.5 The wear volume applied different loads (5N, 10N, 15N) under the counterface of 2.0Pm Al 2 O 3 particles for the as-cast Cu-Cr-Zr alloy, Nano-TiC x /Cu-Cr-Zr alloy composites and nano-sized TiC x -Cu master alloy The volume wear loss of Nano-TiC x /Cu-Cr-Zr alloy composites reduced from 3.674 mm 3 (5N), 5.618 mm 3 (10N) and 8.416 mm 3 (15N) to 3.243 mm 3 (5N), 4.9225 mm 3 (10N and 7.065 mm 3 (15N) respectively compared with as-cast Cu-Cr-Zr alloy, as shown in Fig.5.On the whole, the dispersion of nano-sized TiC x /Cu master alloy into the Cu-Cr-Zr alloy refined the grains.Under the condition of the pin-on-disc sliding wear, wear resistance is only determined by the hardness.Nano-TiC x plays the role as second hard particles, which improved the hardness.As a result, the wear resistance is improved to a large extent.However, both the grain refinement resulted in the increase of grain boundary and nano-TiC x increase the scattering of free electron movement, interface resistance increased, the conductivity of 4wt.% nano-TiC x /Cu-Cr-Zr alloy composites is reduced to a certain extent.

TABLE 1
THE CHEMICAL COMPOSITION OF CU-CR-ZR ALLOY