Microstructural Stability and Microhardness of Ultrafine Grained and Nanostructured Cu-5vol.%Al2O3 Composite Lumps/Powders Produced by High Energy Mechanical Milling 1
Aamir Mukhtar1, Deliang Zhang1, Charlie Kong2, Paul Munroe2
Waikato Centre for Advanced Materials (WaiCAM), Department of Engineering The University of Waikato, Private Bag 3105, Hamilton, New Zealand Email:
[email protected] Email:
[email protected] Telephone: [+64] (7) 838 4783, Fax: [+64] (7) 838 4835 2 Electron Microscope Unit, The University of New South Wales, Sydney, NSW 2052, Australia Email:
[email protected] Email:
[email protected] metallurgy for processing copper-based composites with a fine dispersion of Al2O3 particles of various sizes, and it has been found that the hardness of nanostructured copper matrix nanocomposite with a dispersion of Al2O3 nanoparticles prepared by HEMM can be clearly higher than that of nanostructured copper prepared by HEMM [12-14]. Since the ultrafine and nanostructured lumps/powders need to be consolidated at elevated temperatures by thermomechanical processing, it is important to understand their microstructural stability at different temperatures. Based on this understanding, optimised process parameters can be selected and used for producing fully dense and high quality bulk UFG and nanostructured materials by consolidating powders with different microstructures. Based on this consideration, we studied the microstructural stability and microhardness changes of bulk UFG Cu-5vol.%Al2O3 composites at different temperatures produced by HEMM of mixtures of Cu and Al2O3 powders. We chose to study Cu-Al2O3 metal matrix composite because this material have a potential to offer high strength, good ductility and high electrical and/or thermal conductivity, which are ideal for application in making resistance welding electrode contacts, electrical switches and microwave and x-ray components.
Abstract—Ultrafine grained lumps and a nanostructured powder of Cu-5vol%Al2O3 composite were produced using two high energy mechanical milling routes respectively. The milled composite materials were heat treated at 150, 300 and 500°C for 1 hour, respectively, to determine the microstructural stability and micohardness changes of the materials as a function of the heat treatment condition. For the Cu-5vol.%Al2O3 composite lumps produced using Route 1 (12 hours milling), after heat treatment at 150°C, the Cu grain sizes decreased from the range of 100-250nm to the range of 50-180nm due to recrystallisation, but its average microhardness also decreased from 224HV to 212HV due to reduction of dislocation density. For the 24 hours milled Cu-5vol%Al2O3 powder produced using Route 2, the Cu grain sizes increase slightly from the range of 40-180nm to the range of 50-200nm, and as the result of this grain coarsening and decrease of dislocation density, the average microhardness decreased from 270HV to 257HV respectively. Further increasing the annealing temperature to 300°C caused the grain sizes of the 12 hours milled lumps to increase to the range of 50-350nm, and those of the 24 hours milled powder to 60-300nm, both resulting in a decrease in the average microhardness to 207HV for the lumps and 248HV for the powder. Increasing the annealing temperature from 300 to 500°C caused a much more significant increase of the Cu grain sizes of both the lumps and the powder, and a significant decrease in the microhardness of the 24 hours milled powder particles to 216HV. However, the microhardness of the lumps decreases very little to 196HV, suggesting the significant reinforcement effect of the Al2O3 nanoparticles.
II.
As starting materials, powders of Cu (99.5% pure; particle size < 63µm) and Al2O3 (99.9% pure; average particle size ~50nm) were used in producing the Cu-5vol.%Al2O3 nanocomposites by HEMM. A hardened steel vial, stainless steel balls and a PM 4000 Restch planetary ball mill with a rotational speed of 400 rpm were used for the milling. The vial containing the balls and 100g of powder mixture was sealed in a glove box filled with high purity argon and the mixture was milled using two different routes. In Route 1, the powder mixture was milled for 12 hours using 60 balls with a diameter of 12.5mm. In Route 2, the powder mixture was first milled for 12 hours using balls with a diameter of 12.5mm, and then, milled for 12 hours using 12 balls with a diameter of 12.5mm
Keywords- ultra fine grains, nanostructure, microhardness, high energy mechanical milling, copper matrix composites.
I.
INTRODUCTION
As one of the severe plastic deformation (SPD) processing processes, high energy mechanical milling (HEMM) has been widely used in producing nanostructured powders [1-3]. Since early 1990s, HEMM has also been used to produce high quality bulk ultrafine grained (UFG) and nanostructured materials from ductile powders such as Cu, Zn and Al powders [4-11]. HEMM is the most widely used technique in powder
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EXPERIMENTAL PROCEDURE
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the XRD pattern, as shown in Fig. 3. This indicates that the grain sizes of the powder decreased after annealing, very likely due to recrystallisation of the heavily deformed Cu matrix. The microstructural refinement as a result of annealing at 150°C was confirmed by TEM examination of the microstructure of the powder particles as shown in Figs. 5(a) and (b). Further increase of the annealing temperature caused little change in the broadness of the Cu peaks of the XRD patterns. For the powders produced using Route 2, the broadness of the Cu peaks changed little after annealing at different temperatures up to 500°C.
and 6 balls with a diameter of 25mm. In both routes, the ball to powder weight ratio was 5:1. For Route 2, the balls were changed in a glove box filled with high purity argon. The analyses and characterization of the samples were performed using X-ray diffractometry (XRD) (Philip’s X-pert system diffractometer using Cu Kα radiation and a copper single crystal monochromator), optical microscopy, scanning electron microcopy (SEM), transmission electron microscopy (TEM), microhardness testing and tube furnace with vacuum reaching 10-6 mbar. The SEM examination was performed using a Hitachi S4000 SEM equipped with an energy dispersive X-ray spectrum analyzer (EDAX), and the TEM characterisation was performed using a Philips/FEI CM200 TEM and thin samples prepared using an FEI nanotech200 dual beam focused ion beam (FIB) microscope. The microhardness was measured using a LECO LM700, Vickers microhardness tester with a load of 25g and a loading duration of 20s.
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RESULTS AND DISCUSSION
During the first 12 hours of milling Cu-5vol.%Al2O3 powder mixture (Route 1) led to formation of lumps with a maximum dimension of almost 6mm, as shown in Fig. 1a [15]. However, using Route 2, under the same milling conditions, the Cu-5vol%Al2O3 larger lumps produced in the first 12 hours of milling were turned into fine Cu-5vol.%Al2O3 powders, as shown in Fig. 1b. The lumps produced after 12 hours of milling had small cavities in them (Fig. 1a). SEM examination and Energy Dispersive X-ray (EDX) elemental mapping of the cross sections of 24h milled Cu-5vol.%Al2O3 powder particles (Fig. 2) confirmed homogenous distribution of Al2O3 nanoparticles in the Cu matrix in each of the powder particles.
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Fig. 3. X-ray diffraction patterns of Cu-5vol.%Al2O3 nanocomposite produced using Route 1 and after annealing at different temperatures.
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Fig. 1: Images of cross sections of Cu-5vol.%Al2O3 composite lumps and powders produced with different milling routes: (a) Route 1 (b) Route 2
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Fig. 2: SEM micrograph and corresponding Energy Dispersive X-ray elemental (Al and Cu respectively) mapping of the cross section of a Cu–5vol.% Al2O3 powder particle produced using Route 2
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Fig. 4. X-ray diffraction patterns of Cu-5vol.%Al2O3 nanocomposite produced using Route 2 and after annealing at different temperatures.
Figs. 3 and 4 show the XRD patterns of Cu-5vol.%Al2O3 nanocomposite lumps/powder produced with Route 1 and Route 2, and after annealing at different temperatures, respectively. The XRD patterns only showed Cu peaks, due to the small fraction and extremely small size of Al2O3 nanoparticles. For the lumps produced using Route 1, annealing at 150°C caused clear broadening of the Cu peaks in
Figs. 5 and 6 show the TEM bright field images of Cu5vol.%Al2O3 nanocomposite lumps/powder produced with Route 1 and Route 2, and after annealing at different temperatures, respectively. For the lumps produced with
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Route 1, TEM examination showed that grains of the Cu matrix decreased from the range of 100-250nm to 50-180nm due to recrystallisation after annealing at 150°C. Increasing the annealing temperature from 150°C to 300°C caused an increase of the grain sizes of the matrix to 50-350nm. Further increasing the annealing temperature to 500°C caused a significant increase of the Cu matrix grain sizes to 80-440nm.
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to 300°C, the sizes of the Cu grains increased to the range of 60-300nm. Further increasing the annealing temperature to 500°C caused more significant coarsening of the Cu grains, with the sizes of the Cu grains increasing to 80-380nm. In comparison with the Cu lumps made using Route 1 and annealed at 500°C, the microstructure of the composite powder produced with Route 2 and annealed at 500°C is much finer, and still in the ultra fine grained scale (grain sizes
