Studies on Amorphous Alloy Dispersed Aluminium Matrix Composite Prepared by High Pressure Torsion

Aluminium-based composite reinforced with Cu base amorphous alloy dispersed composite was prepared by means of high pressure torsion between a powder mix of aluminium and amorphous Cu base alloy. The X-ray diffraction pattern of powdered and consolidated composites shows the aluminium phase while the thermal stability of the amorphous alloy was studied with the aid of differential scanning calorimetry (DSC). The microstructural feature of the composite through scanning electron microscope reveals the well-distributed reinforcements in the host aluminium matrix. The hardness measurement on the as prepared composites shows significant increase in hardness with increase in reinforced amorphous alloy. Wear property of the synthesized composites were measured by using ball on plate wear tester which shows increase in wear resistance with increase in reinforced amorphous alloys.


Introduction:
Aluminum (Al)-based metal matrix composites (MMCs) make up a distinct category of advanced engineering materials that provide unique advantages over conventional Al alloys.With the development of new forming methods and the use of low cost particulate material, the use of these composites is increasing in a wide variety of industries.Particulate reinforced composites consist of a uniform distribution of strengthening particles within a matrix.In general, these materials exhibit good wear and erosion resistance, as well as higher stiffness, hardness and strength at a lower density when compared to the unreinforced matrix material (Wielage, Zschunke, Henker, & Steinhauser, 1996).Recently metal matrix composites (MMCs) are increasingly used for critical structural and wear resistant applications because of their excellent strength to weight ratio (Clyne & Withers, 1995) and interesting physical properties.In the last few decades, MMCs, and in particular aluminium (Al)-based MMCs, are receiving increasing application as aerospace and automotive components because of high specific strength and stiffness (Harrington, 1994).Normally MMCs are reinforced with continuous fibers, discontinuous particles or whiskers.However, particle-reinforced MMCs posses some distinct advantages over fiber-reinforced composites in terms of low cost and isotropic mechanical property.In general, reinforcement particulates are prepared independently prior to composite fabrication and are incorporated into the metal matrix at a later stage.The composites so developed are called ex situ MMCs.Al composite have the advantage of low cost over most other metal matrix composites.In addition to the above they offer excellent abrasion resistance, high temperature operation, non-flammability, minimum attack by fuels & solvents, and their ability to be formed and treated on conventional equipments.The performance of MMCs can further be improved by refining the grain structure and the size of reinforcement particles to nano-metric range (Harrington, 1994) and(Yamasakiet al., 2003).Recent investigations find that incorporation of nano-particles into the aluminum matrix could enhance the hardness, the yield and ultimate tensile strength considerably, while the ductility retained (Mazahery, Abdizadeh, & Baharvandi, 2009).
It has been investigated earlier that amorphous materials can be synthesized by mechanical alloying of the elemental powder constituents of different Al-based binary and higher order alloys, e.g.Al-Ti (Lee, Sim, Heo, Cho, & Kwon, 2000), Al-Fe (Kleiner, Bertocco, Khalid, & Beffort, 2005), and Al-Ti-Si (Lv et al., 2001).However, in none of these alloys, amorphous microstructures could be obtained in bulk quantity at relatively slow or normal solidification rate.Cu-Zr based bulk metallic glasses (BMGs) attracted high attention because of their relatively low costs (Pauly, Das, Mattern, Kim, & Eckert, 2009;Louzguine-Luzgin et al., 2012;Zhang, Chen, Zheng, & Chen, 2013).Aluminum has been regarded as useful element to improve the plasticity of the Cu-Zr-based BMGs (Wu et al., 2011).The addition of minor Al quantities (up to 10 at.%) to the Cu-Zr glassy alloys may improve their thermal stability, mechanical properties and glass forming ability (GFA) (Cheung & Shek, 2007;Lee, Jo, & Lee, 2013).Research is therefore in progress to find out other possible Al-based alloy compositions, which can either retain a completely amorphous structure or an amorphous-nanocrystalline aggregate.Such developments are expected to pave the way for fabricating bulk amorphous materials in future.In the present work aluminium based composites synthesized by reinforcing Cu base amorphous alloy by high pressure torsion and the microstructure and mechanical properties have been investigated.

Processing of Amorphous Cu 50 Zr 45 Al 5 Powder
Amorphous Cu 50 Zr 45 Al 5 powder was prepared from commercially available micron sized powders of Cu 50 Zr 45 Al 5 by dry mechanical milling (cryo milling, -196 o C) performed in a planetary ball mill.During milling, the weight ratio of the ball to the powder was maintained at 10:1.The balls and vials are made of tungsten carbide.The rotating speed of the vial was maintained at 300 rpm.Milling was stopped after 4 hr and powder samples were collected.

Processing of Composites
Aluminium powder of 99% purity with an average particle size ~10µm (Loba Chemie Pvt. Ltd., Mumbai) and amorphous Cu 50 Zr 45 Al 5 powder prepared by dry mechanical milling were mixed thoroughly.The different weight percentages of amorphous Cu 50 Zr 45 A l5 powder were mixed with aluminium for composite as shown in the Table :.1 The powder mixture, so prepared, was subsequently compacted in cylindrical (10mm dia.) graphite dies under an applied pressure of 50 MPa in a hot press (Fritschkg Postfech-500428, Germany) at 200 o C. The pressing load on the specimen was applied for 15 min after the desired temperature was attained.The sample was then allowed to cool to room temperature and the cylindrical samples were stripped from the graphite die for characterization.Hot pressing was followed by high pressure torsion (HPT) under an applied pressure of 1GPa was used for full densification and Al-matrix grain refinement.

Characterization
XRD studies on the Cu 50 Zr 45 Al 5 powder prior to and after ball milling were recorded by using Cu-K α radiation.Hot pressed composite samples were also characterized by XRD technique using a Panalytical X'Pert Pro diffractrometer with Cu Kα radiation.Prior to the analyses, the XRD patterns were corrected for the effects of the K α2 radiation.The particle size of the ball milled Cu 50 Zr 45 Al 5 powders were evaluated from the XRD patterns.Microscopic studies to examine the morphology and distribution of particles in the synthesized composites were done by a JEOL 6480 LV scanning electron microscope (SEM).The thermal stability of the amorphous powder, were studied under argon gas atmosphere with the aid of a differential scanning calorimeter (DSC, Mettler, TA400).The flow rate of argon gas was maintained at 80cc/min throughout the experiment.The powder were heated from room temperature to 800 o C at a predetermined heating rate of 10 o C/min and then cooled to room temperature at a cooling rate of 10 o C/min.The density of synthesized aluminium based composites was calculated by using Archimedes principle.
Average values of hardness on different phases of the composite samples were measured with the help of a standard Vickers hardness tester (Type 3212, Zwick, UIm Germany) using a load of 25 gf applied for 15s.Sliding wear resistance of the samples was evaluated using a ball on plate wear testing instrument having a hardened ball (SAE 52100) indenter of 6mm diameter.DUCOM TR-208-M1 ball on plate wear tester was used for evaluating the wear resistance of Al-matrix dispersed with amorphous reinforcement samples.The arrangement of the machine is such that it ensures a precise measurement of the tangential frictional force.This test method involves a ball shaped upper specimen that slides against a rotating disk as a lower specimen under a prescribed set of conditions.The load is applied vertically downward with a motor driven carriage that uses the force/load sensor for feedback to maintain a constant load.The experimental process was carried out at 10 N loads for 15 minutes at 15 rpm at an average humidity of 50-55% and at an average temperature of 25 o C with steel ball.Surface damage caused by wear testing was subsequently analyzed using a scanning electron microscopy to get an idea about the wear mechanism.

Results and Discussion
XRD pattern reveals the phases present and the sequence of their evolution in course of mechanical alloying of the Cu 50 Zr 45 Al 5 elemental powder blend shown in Figure 1.The diffraction pattern shows the peaks of the constituent elements present in the initial mixture either disappear due to alloying, or undergo broadening and reduction in their intensity with increase in the milling time.XRD pattern reveals that the Cu 50 Zr 45 Al 5 powders prior to milling consist of peaks of Al, Cu and Zr.The similar characteristic peaks of respective elements have been reported (Azimi, Shokuhfar, & Zolriasatein, 2014).After 4 h of milling, the powders appear to constitute amorphous solid solution.The XRD diffraction pattern of milled powder shows the amorphization of Cu 50 Zr 45 Al 5 powder.Guoqiang Xie et al. (2010) studied the similar composite and reported the amorphization of milled powder.3 show the XRD patterns of the sample in powdered form and consolidated form prepared by hot pressing respectively.Both XRD pattern revealing the presence of sharp peaks representing the phases of aluminium.The results of XRD analysis of the samples in powdered and consolidated form representing the same structure.Sharifi and Karimzadeh (2011) reported the similar diffraction peaks of aluminium in the synthesized composites.
Figure 4 shows the DSC plot obtained during heating and cooling of green compact from room temperature to 800 o C and from 800 o C to room temperature at a heating/cooling rate of 10 o C/min under argon gas atmosphere.It is seen that, the peak obtained at 475 o C, representing the amorphous-crystalline transformations.Kukuła-Kurzyniec et al. (2014) reported that the crystallization begins in the ribbon at the temperature of 437°C which is higher for the amorphous aluminium based composites.He, Bian, and Chen (2000) determined the DSC curve of bulk metallic glass Zr 52.5 Ni 14.6 Al 10 Cu 17.9 Ti 5 by which the Tg and Tx are reported as 631K and 710K respectively.Inference drawn from the XRD results (Figure 3) investigation suggests the absence of any constituent phase except aluminium in bright areas.Roy, Ghosh, Basumallick, and Basu (2007) reported the SEM micrographs of 10 vol% and 30 vol% reinforced Ti-aluminide content in aluminium matrix prepared by hot pressing at 700°C.
The hardness of the composites has been measured by Vickers hardness tester while densities were calculated by using Archimedes principle.Figure 6 shows the variation of density and hardness with the wt% of reinforcement of the composite samples prepared by hot pressing.On examination of the plots, it is obvious that the samples with 40% of amorphous alloy have demonstrated the highest density, which also coincides with the highest hardness.The lowest density is observed in the samples, containing 0 wt% of amorphous alloy (pure Al) due to absence of reinforcement.It has been found that the hardness of the samples increases remarkably with the increase in reinforcement content.Roy, Basu, and Mallick (2005) reveals the similar results which shows increase in hardness with increase in reinforced content in Ti-aluminide reinforced Al-based insitu metal matrix composites.Wear properties of the designed composites were measured by using ball on plate wear test at an applied load of 10 N at 15 rpm sliding speed on a 6 mm diameter track for 15 minutes.Figure 7 shows the plot for wear depth Vs time for synthesized composites.As the microhardness results shows that hardness of the samples increases with increase in reinforced content.The similar trend was observed in wear test, 40wt % reinforced Al-matrix composite exhibit the high hardness and wear resistance.Roy, Basu, Mallick, Kumar, & Ghosh (2006) represents the similar results in Fe-aluminide reinforced Al matrix composites.

Conclusion
Aluminium matrix composites reinforced with well dispersed amorphous powder prepared by hot pressing followed by high pressure torsion method.XRD pattern of both powdered and consolidated composites shows the phases of aluminium with same structure.DSC reveals that peak obtained at 475°C represents the amorphous to crystalline transformation.Microstructural architecture of amorphous alloy remains same before and after hot pressing followed by high pressure torsion.Microhardness of aluminium matrix composite reinforced with amorphous alloy shows increase in hardness with increase in reinforcement content and similarly increase in wear resistance upto 40vol% reinforcement in synthesized composites.

Figure 1 .
Figure 1.XRD pattern of milled and without milled Cu 50 Zr 45 Al 5 powders

Figure 4 .
Figure 4. DSC curve for the Cu 50 Zr 45 Al 5 mechanically alloyed powders subjected to heating and cooling

Figure 7 .
Figure 7. Plot of wear depth Vs time (in sec) for amorphous Cu 50 Zr 45 Al 5 powder content in Al-matrix

Table 1 .
Different wt% of amorphous alloys in different samples