High Temperature Tensile and Strain Hardening Behaviour of AA5052/9 vol. %ZrB 2 insitu Composite

Various mechanical components such as piston, cylinder blocks, brakes and drums, have to operate under high temperature condition during their service life. Therefore, to meet the demand of high strength materials, a detailed analysis of their synthesis and high temperature tensile behaviour is of utmost importance. Present study is an effort in this direction to develop AA5052/9vol. %ZrB 2 insitu composite by salt-metal reaction technique. An insitu reaction between molten aluminium alloy and two inorganic salts K 2 ZrF 6 and KBF 4 begins at 860°C and continues up to 30 min. The resulting reaction product ZrB 2 is desired reinforcement confirmed by XRD analysis. Microstructural study was performed to analyse grain size, particle morphology, and their distribution in the matrix. Tensile tests were conducted at temperatures ranging from room temperature (RT) to 200°C with an interval of 50ºC. The results revealed the decreasing trend of UTS and YS (0.2% off set) with increase in temperature; however ductility increased with temperature. The composite is able to maintain about 81% of its ambient temperature strength at 150°C and 72% at 200°C. Strain hardening exponent was not significantly affected with temperature and tensile properties were correlated with fractured surface morphology examined under SEM to understand the mechanism.


Introduction
Particulate aluminium matrix composites (PAMCs) are widely used for manufacturing of various mechanical components such as piston, cylinder brakes, discs/drums and piston insert rings, due to their high strength to weight ratio, good thermal and electrical conductivity, good corrosion and wear resistance characteristics 1,2 . PAMCs are synthesized either by exsitu or insitu process. Exsitu involves the addition of externally synthesized reinforcement particles into the melt, whereas, desired reinforcement particles are formed during melting within melt during insitu process. Among these techniques, insitu is preferred because it provides uniform distribution of reinforcement particles, finer particles, excellent bonding between particle and matrix, isometric properties, reaction free interface, and enhanced thermodynamic stability of reinforcement with the matrix 3-6 . Therefore, insitu composites possess superior properties as compared to exsitu composites 7 . PAMCs are generally reinforced with variety of ceramic particles in the form of carbides, oxides, nitrides, and borides [8][9][10][11] . Being an ultra-high temperature ceramic and other characteristics such as high melting point, high hardness, high temperature strength, high wear resistance, good chemical inertness, good thermal and electrical conductivity zirconium diboride (ZrB 2 ) can be a better choice to prepare PAMCs for high temperature applications 12,13 . Moreover, ZrB 2 also has the potential to replace Al 2 O 3 and SiC in many applications 14,15 .
Several workers have studied different systems for high temperature applications to see the effect of temperature on strength retention due to reinforcement. Sahoo and Koczak 16 , prepared Al-4.5wt.%Cu/TiC insitu composites and studied the tensile properties at elevated temperature. They observed that yield strength and tensile strength of composites were improved by 130% and 65% respectively when compared with Al-4.5wt. % Cu matrix alloy processed in the similar conditions. They also observed that composite was able to retain its room temperature strength up to 250°C. Lee et al. 17 investigated the effects of temperature and strain rate on flow properties of carbon-fiber-reinforced 7075 aluminium alloy metal-matrix composite and concluded that flow stress increases with strain rate, but decreases with temperature. Work-hardening rates decrease with increasing strain and temperature. Hoseini and Meratian 18 studied the tensile properties of insitu aluminium alumina composites at ambient as well as at high temperature. They observed that effect of alloying elements on strengthening was more significant at room temperature as compared to composite reinforced with 5 wt.% alumina particles. Whereas, at high temperature (300°C) tensile strength is largely controlled by reinforcement and composite has higher strength due to strain-hardening effect of alumina particles, while alloying elements lose their strengthening effect at high temperature. Yi et al. 19 studied the high temperature mechanical properties of insitu TiB 2p reinforced Al-Si alloy composites and reported that tensile strength of composites were higher than Al-Si master alloy at temperatures ranging from 25° to 400°C. Oñoro 20 studied high temperatures mechanical properties of TiB 2 particles reinforced AMCs based on aluminium alloys (6061 and 7015) up to 500°C. The tensile strengths of the AMCs and the aluminium alloys decreased as the temperature increased, but the ductility was found to increase. Han et al. 21 also investigated the tensile behaviour and fracture mechanism of Al-12Si/4 wt. % TiB 2 composite in a temperature range of 25° to 350°C and observed the improvement in elastic modulus of composites as compared to matrix alloy. At ambient temperature the composite exhibits more yield and tensile strength than unreinforced alloy. However at 200°C and 350°C no significant difference in the strength of composite and alloy was observed. The ductility of the composite was found to be lower than that of the unreinforced matrix alloy at 25° and 200°C, but no major difference was observed at 350°C. Morphology of the fractured surface of Al-Si /TiB 2 composite showed that at 25° and 200°C, the fracture mechanism was dominated by cracking of silicon particles and separated TiB 2 particles. Whereas, de-bonding of the silicon particles coupled with failure of the interface between TiB 2 particles and matrix were the dominant fracture mechanisms at 350°C. Recently, Ram et al. 22 investigated high temperature tensile properties of centrifugally cast in-situ Al-Mg 2 Si functionally graded composites and observed the reduction in strength with increase in temperature. Fracture mode was changed from mixed mode to ductile mode at high temperature.
There is lack of information on the high temperatures tensile behaviour of PAMCs reinforced with nanosize or submicron particles. El-Kady et al. 23 investigated; the tensile properties of A356/Al 2 O 3 nanocomposites at both ambient and elevated temperature. Tensile results revealed that nanocomposites exhibited better mechanical properties than the unreinforced alloy at both ambient and elevated temperatures up to 300°C. Moreover, with increased amount and reduced size of Al 2 O 3 particles both tensile and yield strength was observed to be increased. The information related high temperature strength is very important where nanosize PAMCs are being considered as candidates to replace steel or aluminium alloys for piston liners and cylindrical heads of automobile engines, as well as brake rotors which are used for high temperature industrial applications. Hence, present study is focussed on high-temperature tensile and strain hardening behaviour of AA5052/ 9 vol. % ZrB 2 composite.
Further, tensile results are correlated with morphology of fractured surface to understand the mechanism of failure at elevated temperature.

Raw material, and synthesis of composite
AA5052 aluminium alloy (Al-2.5Mg-0.2Cr) was received from Hindalco Industries, Renukoot, India and two inorganic salts K 2 ZrF 6 and KBF 4 were purchased from Sigma Aldrich, Bangalore, India. Casting was done on stir casting furnace with bottom pouring arrangement. AA5052 alloy has been reinforced with 9 vol. % ZrB 2 particles by insitu synthesis. Detailed procedure for preparing the Al/ZrB 2 composite by insitu synthesis has been discussed in our earlier published work 24 . Fig.1 shows flow chart for synthesizing the composite. Casting ingots were cut and machined to prepare the samples for various characterizations.

Characterization equipment
In order to confirm the formation of ZrB 2 particles XRD (Rigaku, CuK α radiation of wavelength 1.541836 Å) study was carried out. Matrix grain size, morphology and distribution of ZrB 2 particle in the matrix were examined under Optical Microscope (Leitz Metallux-3) and Scanning Electron Microscope (FESEM Quanta 200FEG) respectively. Cylindrical specimens for high temperature tensile testing were prepared according to BS 12-1950 British standards (gauge diameter, 4.5 mm, gauge length, 15.5 mm) and tested on a computerized 100 kN screw-driven Instron TM Universal Testing Machine (model 4206) at temperatures ranging from room temperature (RT) to 200°C at a constant strain rate of 1.07/ 10 -3 s -1 . Three specimens for each composition were tested at different temperatures and average values are reported. Fractured surface were examined under SEM to understand the failure mechanism at high temperature and correlated with the properties.

X-ray diffraction (XRD) study
During the composite synthesis insitu reaction takes place between the molten alloy and salts K 2 ZrF 6 and KBF 4 at 860ºC according to following reactions 3,25 .
(1) XRD pattern of synthesized materials with 0 and 9 vol. % ZrB 2 particles are shown in Fig.2a. Diffraction peaks of ZrB 2 particles are the sign of formation of ZrB 2 particle in the matrix by insitu reaction. For the secondary confirmation, ZrB 2 particles were extracted from the composite sample by dissolving small chips of composite in 10% HCl solution for several days and filtered. Filtered residue was thoroughly washed, dried and examined under XRD machine. Figure 2b shows the XRD pattern of extracted particles in which ZrB 2 peaks can be clearly seen.

Microstructural study
Optical micrographs of aluminium alloy and composite materials are shown in Fig. 3 a-b respectively 26 . Aluminium grains were refined due to insitu formed ZrB 2 particles. Grain size was measured with linear intercept method and found to be 115 µm for alloy and 67 µm for composite material respectively 10 . Insitu formed ZrB 2 particles restrict the growth of Al-rich grains during solidification process which results in refined grain structure 27 . ZrB 2 particle morphology and distribution are examined under SEM. Figure 4 a-b shows SEM micrographs of alloy and composite material. Fig. 4b shows that ZrB 2 particles are uniformly distributed in the matrix 24 ; however, particles are agglomerated at some places due to their finer size. ZrB 2 particles are observed in hexagonal and rectangular morphology as shown in Fig. 4

High temperature tensile behaviour
Tensile tests for composites were conducted at an interval of 50°C up to a temperature of 200°C at a strain rate of 1.07 / 10 -3 s -1 . Figure 5 a-c shows the variation of UTS, YS and percentage elongation with temperature for 9 vol. % ZrB 2 composite. The Experimental results reveal that UTS and YS of composite decrease linearly with increase in operating temperature. Composite exhibits good resistance to high temperature effects in terms of strength. Even at 150°C the UTS of composite is 81% of the ambient temperature strength, and at 200°C also 72% of the ambient temperature strength is retained. It should also be noted that percentage elongation (ductility) increases as the test temperature increases due to thermal softening of the matrix.

High temperature strain hardening behaviour
Influence of ZrB 2 particles on tensile strength at high temperature can be studied in terms of strain hardening. Beyond macroscopic yield, strain hardening behaviour of composite is described by power law which is given by σ = K p n f 28 , where σ and ε ρ are the true stress and true plastic strain respectively. Figure 6a shows true stress (σ) and true plastic strain (ε ρ ) curve on log-log scale for composite at different temperatures 50ºC-200ºC. K is the monotonic strength coefficient (intercept at plastic strain (ε ρ = 1) and n is strain hardening exponent representing slope of the curve. It is interesting to note in Fig. 6b that strain hardening exponent (n) is not significantly influenced by increasing the temperature, which is an indication of good strength retained by the composite even at high temperature. Similar kinds of results are also reported by other workers 29,30 .

Fracture surface analysis
Fractured surface of composite with 9 vol. % ZrB 2 particles are shown in SEM micrographs (Figure 7 a-d). These figures correspond to tests conducted at RT, 100°, 150° and 200°C. The fractured surface morphology at high temperature is quite different from that of the room temperature. At room temperature the fractured surface clearly shows a large crack, initiated by high stresses due to the presence of large clusters of hard ZrB 2 particles as shown in Fig.7a. The major form of fracture, at room temperature, is the rupture of ZrB 2 particles from the matrix. Small size dimples are also seen

Conclusion
From present study following conclusions can be drawn: