Study on the nanostructure formation mechanism of hypereutectic Al–17.5Si alloy induced by high current pulsed electron beam
Introduction
Nanomaterials are typically characterized by ultrafine grains from 1 to 100 nm (at least one dimension) and large numbers of grain boundaries [1], [2]. These structural features indicate that nanomaterials fundamentally possess unique properties and behavior such as increased strength/hardness, enhanced diffusivity, improved toughness/ductility, reduced elastic modulus, high specific heat, enhanced thermal expansion coefficient, and superior soft magnetic properties [3] compared with conventional coarse-grained materials. A number of unique nanomaterial properties have been discovered in the last 10 years. This discovery has led to the development of various processing techniques for synthesizing bulk nanomaterials, e.g., ultrafine powder consolidation [4], amorphous solid crystallization [5], ball milling [6], [7], electrode position and physical vapor deposition [8], [9], and severe plastic deformation [10]. However, some defects in bulk nanomaterials, such as porosity and contamination, limit the applications of these techniques.
Considering that numerous material failures occur on the surface, the optimization of surface structures and properties has become a feasible and effective method of improving the overall properties and behavior of materials. Hence, many research workers have devoted themselves to investigate the formation of a nanocrystalline surface layer on coarse-grained materials through different methods. In early 2000, Lu et al. [11], [12], [13] successfully made use of surface mechanical attrition treatment to generate a nanocrystalline surface layer without changing the chemical composition of corresponding materials. Consequently, a new concept called surface nanocrystallization [11], [14] has been gradually developed and acknowledged by researchers in the field of nanomaterials; this method is an efficient approach for meeting the specific requirements of material surfaces. High current pulsed electron beam (HCPEB) technique is a novel method for material surface modification [15], [16], [17], [18], [19], [20]. HCPEB is simple, reliable, and has been highly efficient for approximately 10 years. The electron beam causes intense and superfast melting, evaporation, solidification, and even ablation on the surface of target materials and forms thermal stress and shockwaves. Therefore, the depth of the HCPEB modification zone can reach several hundreds of micrometers [21], [22], which greatly meet the modification demands for engineering materials. The combination of the aforementioned influencing factors, which are unique to HCPEB treatment, can cause nanocrystalline formation in near-surface layers of metallic materials [23], [24], [25], [26], [27], [28].
Hypereutectic Al–Si alloys are widely used in aerospace, automobile manufacturing and so on because of their excellent low-expansion performance and high resistance to wear, corrosion, and extreme temperatures [29]. However, the service performance of hypereutectic Al–Si alloys is directly related to the morphological distribution and size of the primary Si particle as well as the binding mold of the primary Si phase and the matrix. For normal casting hypereutectic Al–Si alloys, the precipitation of the coarse primary Si phase destroys the continuity of the matrix and greatly reduces the strength and durability of the alloys. Thus, several rapid solidification methods were adopted in the past to refine the coarse primary Si phase, such as laser surface melting [30], [31], laser surface cladding [32], and high energy beam [33]. The size of the primary phase can also be refined to several micrometers.
In our previous studies [16], nanostructures such as nano-silicon were observed under transmission electron microscope (TEM). However, given the restriction of the visual field under TEM, accurately determining the location and distribution of the nanostructures is difficult. Thus, detailed analysis on the formation and evolution of nanostructure is impossible. In the present study, we adopt scanning electron microscope (SEM) and EBSD technology to analyze the microstructural formation and evolution of the melted layer of HCPEB-treated Al–17.5Si alloys under different number of pulses to obtain the nanostructure formation mechanism.
Section snippets
Material
The experimental material used was an as-cast hypereutectic Al–17.5Si alloy with a composition (wt pct) of 17.5 Si and balance Al. Sample surfaces were subjected to HCPEB treatment to obtain a nanocrystalline surface layer. Before HCPEB treatment, the cast ingot of the Al–17.5 Si alloy was cut into a number of cylinders measuring Φ1 cm × 9 cm. Smooth sample surfaces were achieved by using silicon carbide paper and diamond paste polish.
HCPEB system
HCPEB treatment was performed by using a “Nadezhda-2” HCPEB
Results
Fig. 1 shows the structural features of the initial sample through SEM. The Al–Si eutectic structure of as-cast hypereutectic Al–Si alloys is clearly observed by SEM (Fig. 1). The lamellar-type eutectic Si is embedded in the Al matrix. The lamellar spacing of the lamellar-type eutectic Si is approximately 3–10 μm. Moreover, the coarse primary Si phase at 20–100 μm is distributed randomly on the surface. The average size of the coarse primary Si is approximately 30 μm, as measured by Image-Pro Plus
Discussions
The nanostructures of the Al–17.5 Si alloy, which includes nano-Si crystals in the Si-rich zone and nano-cellular cells in the Al-rich zone, are formed during the rapid solidification process of the HCPEB treatment. Therefore, some factors that affect the nucleation and the growth of the Si and α(Al) phases will play important roles in controlling the formation of the nanostructure. Fig. 5 shows the microstructure characteristics of the zone near the boundary between the Si-rich and Al-rich
Conclusions
EBSD technique was applied to an HCPEB-treated hypereutectic Al–17.5 Si alloy to investigate the effect of increased number of pulses on surface nanostructures. The nano-silicon phase in the Si-rich zone and the nano-cellular cell structure in the Al-rich zone, as well as their corresponding forming mechanisms, are analyzed in detail. The main results are summarized as follows:
- (1)
After HCPEB treatment, the top surface of the melted layer can be divided into three zones: Si-rich zone originating
Acknowledgements
This study is supported by the Fundamental Research Funds for the Central Universities (N130402004), the Program for Excellent Talents in Liaoning Province (LJQ2012021) and Kunming University of Science and Technology Personnel Training Fund (KKSY201452089). The authors gratefully acknowledge the help of Prof. DONG Chuang, Dr. YANG Bo and Postdoctoral ZHANG Yue.
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