Thermal kinetics of SiCp reinforced Al-Zn-Mg-Cu alloy composite

In the present work, the artificial aging kinetics of SiCp particles reinforced AA7075SiCp composite fabricated by stir casting method was investigated. The aging behavior of AA7075-SiCp composite was investigated by Rockwell hardness tests and differential scanning calorimetry (DSC). Results show there are no changes in the sequences of formation and dissolution of precipitate. Reinforced particles are uniformly distributed throughout the matrix. The hardness profile shows increase in hardness with the comparison of AA7075 base alloy. In addition to SiCp in the matrix, precipitation kinetics has changed compared with base alloy since higher dislocations present in composite, hence requires lower activation energy to form ή precipitate and takes less time to reach the maximum hardness. In contrast, the addition of SiCp at low volume percent also showing accelerated aging phenomena in the composite during the aging process. High-resolution transmission electron microscope (HRTEM) micrograph of peak age (T6) condition divulges that enormous fine and plate-like ή (MgZn2) precipitates are uniformly distributed in the composite.


Introduction
Aluminum matrix composite (AMCs) has drawn remarkable attentions because of its superior properties like high strength, good wear resistance, high specific modulus These attractive properties make the AMCs as a candidate material for application in the field of aerospace as well as automobile industries [1]. An enormous number of investigations have been carried out on AMCs to improve properties and increase application. The precipitation sequence of aluminum alloy is as follows: α -supersaturated solid solution (αssss) → GP zones (Zn, Mg) →ή (MgZn2) → η (MgZn2). Thermal treatment is the most significant method to increase the application area of AMCs which are based on precipitation hardening phenomenon [2][3][4][5]. Inspection showed that reinforcement in AMCs does not affect the sequence of precipitation formation but changes the precipitation kinetics of AMCs during the aging process [6,7]. Inception of ceramic particles in the matrix alloys introduced enormous dislocations to the immediate vicinity of the reinforcement. Generation of these dislocations in the matrix is due to the mismatch of coefficient of thermal expansion (CTE) between the matrix and reinforcement when specimens are cool down rapidly from higher to lower temperature. These dislocations act as high diffusivity tracks and nucleation spots of Guinier-Preston (GP) zones, precipitates or different intermediate phases

Most researchers have focused their studies on aluminum-based AMCs reinforced
with Al2O3, SiC, Si3N4, and TiC, etc. [6,[8][9][10]. Among them, silicon carbide (SiC) particles have higher strength, high wear resistance, excellent thermal shock resistance, and low thermal expansion coefficient [10][11][12]. There are several methods to manufacture AMCs such as powder metallurgy [13][14][15], squeeze casting [16], stir casting technique [17][18][19]1], friction stir welding [20,21] and spraying process [22,23]. Wang et al. [24] showed that the addition of SiCp (15 vol. %) in the matrix alloy (AZ91) reduced the peak aging time to 28 hr, higher age hardening efficiency, and faster aging kinetics. Dasgupta et al. [19] found a significant improvement in the hardness of 15 vol.% SiCp-AA7075 composite than that of AA7075 alloy. LEE et al. [25] reported that composite AA7075 with 10 vol.% of SiCp showed higher strength values in all aging conditions. Kumar et al. [26] found that AA7075-SiCp composite showed better mechanical properties with the addition of 2 wt% SiCp in the AA7075 matrix alloy. Bembalge et al. [27] reported that AA6063-SiC composite with 4 wt.% of SiCp showed age-hardening kinetics were accelerated and maximum hardening affect was observed at 150 o C. They attributed this behavior presence of enormous dislocations in the matrix due to mismatch of CTE between the alloy and SiCp. Review of existing scientific literature discovered that the addition of SiCp particles would affect the aging kinetics during the aging process. Evaluation of the process activation energy or thermal diffusion activation energy (TDAE) is the main key to understand the aging kinetics or transformation kinetics.
Lu et al. [28] found that TDAE in the AA7075-SiCp composite is lesser than AA7075 alloy.
They attributed this behavior to increase of dislocation densities in the composite by the addition of SiC particles. Similarly, Min et al. [29] reported that AA6061required higher activation energy to form β´ phases than that of 40 vol. % AA6061-SiCp composite. Jin et al. [10] reported that with increase in solution temperature, TDAE of GP zone formation and dissolution in the composite increases due to lower vacancy concentration at higher temperature. On the other hand thermal diffusion activation energy for S´ phase requires lower activation energy attributed higher vacancy concentration at higher temperature.
However, age hardening kinetics of composites during the aging treatment depends on matrix metal and reinforcing elements such as size, volume fraction, aging temperature etc.
Small particles can load-transfer through the composite interface and achieve enhancement in the strength; while increasing the volume fraction, particles lose their ability to homogeneous distribution and ductility with strength. From the above discussion, it is clear that lots of investigation has done on aging behavior of AA7075-SiCp composite but not enough data available to the evaluation of process activation energy or thermal diffusion activation energy required forming the phases like GP zone, metastable (ή) and stable (η) phases at lower volume fraction. So, in the present work, reinforcement was restricted to 5 vol. % of SiCp to retain their ductility with strength and fluidity of composite melt. Efforts have been made to understand the aging kinetics of AA7075-SiCp composite. The ageing behavior of the AA7075-SiCp composite was investigated by Rockwell hardness tests and differential scanning calorymetry (DSC). Microstructure of composite was examined by optical microscopy (OM), scanning electron microscopy (SEM) and high resolution transmission electron microscopy (HRTEM). Simultaneously, open circuit potential with time has been studied to understand the electrochemical behavior with aging.

Material
AA7075 alloy based metal matrix composites are prepared using stir casting route.
The chemical composition is shown in Table 1. Alloy ingot is cut into small pieces and

Microstructural Analysis
For microstructural analysis of the specimens, specimens were ground with 80 to 1500 emery paper followed by cloth polish using 0.5 µm diamond paste and etched with Keller's reagent (Distilled water 95%, HNO3 2.5%, HCl 1.5%, HF 1%) to reveal grain and grain boundary. Optical microscope (OM) LEICA DM 750M and scanning electron microscope (SEM) ZEISS-SIGMA 300 were used to observe specimens' surface morphology.
For HRTEM study, samples were mechanically thin up to 40 µm followed by 3 mm diameter discs punched from foil, dimpling, and ion milling. To observed microstructural features, precipitates and second phase particles etc., HRTEM (JEM-2100, JEOL make, Japan) at 200kv was used.

Ageing behavior:
The alloy and composite's aging behavior was studied by solution treating them at 470 o C for 1 hour soaking period and followed by water quenching. After that, specimens were artificially aged at 120 o C for different time. Hardness measurement was carried out by digital Rockwell hardness testing machine (SSS instrument, India) at 100 kgf load, dwell time 10 second.

X-ray diffraction (XRD) analysis:
XRD of the base alloy and composite were taken using PANlytical X'pert pro Basic X-ray diffractometer unit CuKα radiation.

Differential scanning calorimetry (DSC) analysis
To identify the precipitation sequence of alloy and composite, DSC study were carried out. The specimens weight of 13 mg approx. were cut from the alloy and composite, solutionized at 470 o C for 1 hr soaking time followed by water quenching. DSC run were initiated at room temperature to 500 o C at various heating rates (5K, 10K, 15K, 20K, 25 K) to study the deviations in activation energy for precipitation using a NETZSCH DSC instrument model 214polyma, Germany. As a reference sample pure aluminium was used and to nitrogen gas was flushed to suppress oxidation.

Open circuit potential with time (OCPT)
For OCPT, aged samples were mechanically ground with emery paper up to 1500 grade. Three (3) electrode systems were used for OCPT measurement where samples were working electrode, Ag and Ag/Cl as reference electrode. Platinum rod serves as counter electrode. OCPT has run for 6 hours in 3.5 wt.% of NaCl neutral solution. The electrochemical measurement was carried out on CHI660E, a product of CH instruments.

Results and discussion
3.1 Microstructural characteristics Fig. 1 (a, b) shows the scanning electron micrograph (SEM) of silicon carbide (SiC) particles of sizes 30 µm (approx.). Fig. 1(c,d) shows energy dispersive X-ray (EDX) analysis of powders and confirms that powders are SiC. Fig. 1(e,f) shows the optical microstructure  SiCp aged at 120 o C for 24 hours were investigated to analyze precipitation morphology. It has been widely reported that the important precipitate in the alloy matrix after aging are ή phases [30,31]. HRTEM morphological study reveals that enormous fine (approx. 100 nm) and plate-like ή precipitates are uniformly distributed in the composite at peak age condition ( Fig. 1a). Fig. 1d shows the corresponding electron diffraction patterns of Al matrix. Fig. 1b represents precipitates present in the grain boundaries. Most of the dislocations are still present in the matrix after aging treatment since the aging temperature 120 o C was not high enough to properly annihilate the dislocations. Fig. 1c reveals the dislocation present near the interface between matrix and SiCp particle. This behavior is attributed to the large differences    Table 3 shows exothermic and endothermic peak temperatures of the as quench specimen of AA7075 at different heating rate. The peak temperature III and IV indicates the formation and dissolution of metastable phase (ή) precipitation from the temperature interval of 220-300 o C. Nucleation of metastable phase requires vacancies. From Table 3 peak temperature V and VI attributed with the formation and dissolution of η phase. It has been noticed that dissolution of η phase is broader than ή phase. This observation indicating that the absorption of energy during the dissolution of equilibrium phase (η) larger than that of metastable phases (ή). which requires more activation energy to transformation.
The dislocation density of the material is associated with the variations in alloy and composite ages [26]. Typically, mismatch strains happen because of differences of CTE among the matrix and reinforcement, while composite is cooled down from the solution temperature. The dislocation density (ρ) and mismatch strain (ε) can be calculated as revealed by Arsenault and Shi [9].
Where, ∆ is the CTEs and ∆ is temperature gradient. Whereas, B, Vf, b, t are the geometric constant, volume fraction of reinforcement, burgers vector and smallest dimensions of the reinforcement respectively. Equation (1) reveals that higher the ΔT value and residual stress, higher the dislocation density [36]. Plastic deformation, strains occurs as a result of the thermal mismatch, resulting in a high density of dislocations, particularly near the SiC particulates. The dislocation density (ρ) can be evaluated by equation (2). The previous research work [37] indicated that the growth of precipitates accelerated at the dislocations because dislocations create free path for the kinetically faster atomic transportation of precipitates comparative to majority transmission in a crystal. Hence, presence of SiC particles in the composite would offers a lot nucleation site of precipitate, resulting shorter in the aging time than that of the AA7075.
ln  (3) and (4). Calculated activation energies are summarized in Fig. 5 (a)-(f). Table 5 shows the diffusion activation energies of AA7075 and AA7075-5 vol. % SiC composite. composite that is more sensitive to ageing temperature. The TDAE of composite is lower than AA7075, indicating that lower energy was required to form the phases, i.e. formation of phases is accelerated that can be attributed to excess nucleation sites provided by dislocations [28]. Therefore, faster the solute atoms diffuse, shorter the aging time to reach the peak aging hardness. Finally, AA7075-SiC composites reach peak aging time in shorter time than the matrix. precipitate into stable η precipitate. The exothermic peak for η precipitation at the temperature 268 o C for AA7075-T7 temper is much broader then AA7075-T6 temper. But the noticeable thing is that for AA7075-T7 temper transformation peak from ή́ to η is still present in overage temper. It can be ascribed, for transformation of phases from one to another requires a higher driving force and this aging temperature acts as a driving force. Higher the aging temperature higher the driving force and lower the aging time. In this case, the aging temperature 120 o C is not high enough to transform the ή into η within the 120 hours of duration i.e. it requires more time to completely transform into η precipitate. The final broad endothermic peak region of all tempers is associated with the dissolution of equilibrium η precipitates. for a period of 6 hours. As compared to alloy tempers at the same stage of immersion, composite tempers showed sudden increase in voltage at the beginning for a shorter time period and after some time it will stabilize with prolongation of dipping time (Fig. 10b). This can be ascribed by the formation of passive layer on the surface of the composite. Fig. 10 (a,b) shows electrochemical noise (EN) amplitude of the base alloy tempers are higher than that of composite tempers. As the same for other composite tempers, fluctuations had seen but less than AQ specimen (Fig. 10b).

Conclusion
In summary, the incorporation of SiC particles in the matrix alloy has significant influences on the aging behavior of AA7075-SiC composite produced by stir casting method. Results can be concluded that the precipitate's activation energy for composite is smaller than matrix alloy. The CTE imbalance between the reinforcement and matrix alloy results in a large number of dislocations adjacent to the ceramic phase. These dislocations serves as high diffusivity route and nucleation sites for precipitates as a result takes less time to grow the precipitate as well as to reach the peak hardness. TEM morphological study at peak age condition also revealed, specimen were fully uniformly distributed with precipitate.
Composites show better electrochemical behavior in 3.5 wt. % of NaCl solution with aging than that of unreinforced alloy.