The effect of heat treatment on microstructure and tensile properties of cold spray Zr base metal glass/Cu composite
Introduction
Metallic glasses (MG) are generally known for their high strength and high corrosion resistance. As an example, Al and Zr based MG are widely studied due to the glassy phase has better properties as compared to the corresponding crystallized structure [1], [2], [3], [4]. Among metallic glasses, Zr-based bulk glassy alloys are known to present high mechanical resistance and glass-forming ability (GFA). The strength of Zr–MG reaches for example about 2000 MPa at room temperature [5] but of course ductility is low. Zr-based glassy alloys with an improved ability for plastic deformation have been obtained recently in the Zr-rich systems [6]. In addition, a deformation-induced nano-crystallization would probably improve the ductility of metallic glass materials [7]. Zr-alloy series in the Zr–Cu–Al, Zr–Cu–Al–Ni, Zr–Cu–Al–Ag and Zr–Cu–Al–Fe system, have thus been developed [8]. When adding Ti, the super-cooled liquid region (∆ Tx) and the GFA both greatly increase [9], thus, among the Zr-based metallic glasses, Zr–Ti–Al–Cu–Ni series exhibit good GFA and thermal stability. In addition, this class of metallic glasses displays low cost (no precious metals) and low toxicity (i.e., free of Be) [10].
The cold spray process was developed about 30 years ago in the Soviet Union in The Institute of Theoretical and Applied Mechanics by Papyrin et al. [11]. In this process, particles are accelerated to high velocity (higher than a “critical velocity”) in a De Laval type nozzle. Given their kinetic energy, particles are stacked to form the deposit thanks to a huge plastic deformation [12]. Thanks also to the low temperature of the process, materials which are sensitive to phase transformation or oxidizing (i.e. Mg, Zn, Al and Cu) can be deposited without drawbacks linked to the use of molten particles when spraying techniques operating at higher temperature are implemented [13]. Inert gases are often selected as process gas for spraying oxygen sensitive metals.
Thanks to their low cost and good machinability, particles reinforced metal matrix composites show a high application potential. Copper and its alloys are important engineering materials given their high thermal and electrical conductivity [13], [14]. However, Cu-based alloys exhibit relatively poor mechanical properties, which restrict their field of application. Thanks to their high ductility and given their oxygen sensitivity, copper and its alloys were among the early cold spray deposited materials. In order to improve the mechanical properties of copper, TiB2 [15], CNTs [16], quasicrystal [17] and WC [18] among others were selected to act as reinforcing particles. For the particle reinforced composites, the interfacial bonding strength which acts as the function of load transition is very important. Comparing the conventional ceramic reinforcing particles, the MG particles are consisted only by metal, which provide the possibility of diffusion between matrix and reinforcement. Additionally, the Zr–Cu based MG was chosen as the reinforcement, due to its high content of copper which is the matrix metal. So the interfacial strength may be improved by the diffusion process between MG and copper matrix.
In this study, Zr based metal glass particles (MG) were selected to produce composite deposits. As low molar mass gases enhance the particle velocity in the cold spray process and thus the mechanical properties of the deposits, helium gas was selected in this work as the process gas with a system allowing the recycling of this costly medium [19]. NOL (Naval Ordnance Laboratory) ring samples were produced in order to perform tensile tests to determine the mechanical properties of the composite. The ball-on-disk wear test was applied to determine the wear/friction behavior of the MG/Cu composite deposits. The influence of annealing temperature on microstructure, micro-hardness and mechanical properties of the composite was considered aiming at exploring the potential improvements in the mechanical properties. Of course, annealing temperatures were selected below the glass transition temperature (Tg) of Zr–MG to avoid recrystallization.
Section snippets
Experimental procedures
Metal glass and copper powders were prepared in the laboratory by argon gas atomization (Nanoval process). Particles size distribution was determined by laser light scattering (Malvern Masterizer 2000). The raw atomized metal glass powder exhibited an average value of 40.7 μm (d (50)). For the Cu powder, the distribution of particles size was from 7 μm to 80 μm with a mean value of 30 μm (Fig. 1). The MG and Cu powders were mixed with weight proportion Cu: MG = 19:1 in a tumbling mixer for 45 min
Microstructure.
Fig. 5(a) illustrates the typical composite microstructure observed by SEM. The bright particles in the back-scattered image correspond to the MG reinforcements. Their distribution appears relatively uniform with particles size ranging from several μm to about 100 μm. The composition of deposits appears to be in line with that of the blended powder which is approximately 4 wt.% MG. While a significant plastic deformation is observed for Cu particles, it is not the case for the MG particles. The
Conclusion
- (1)
Composite materials consisting of Zr-based MG particles embedded in a Cu-matrix were obtained by cold spray using helium as process gas. The composite porosity is lower than 0.1‰ and MG particles appear to be distributed uniformly in the Cu-matrix. The very low porosity rate of the composite is attributed the high kinetic energy of the MG particles which increases the plastic deformation of the matrix with a peening effect.
- (2)
The composite tensile strength is over 366 MPa at room temperature and
Acknowledgments
One of the authors, N. KANG, is grateful for the financial support provided by the China Scholarship Council.
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