Abstract
Four techniques using high-current pulsed electron beam (HCPEB) were proposed to obtain surface nanostructure of metal and alloys. The first method involves the distribution of several fine Mg nanoparticles on the top surface of treated samples by evaporation of pure Mg with low boiling point. The second technique uses superfast heating, melting, and cooling induced by HCPEB irradiation to refine the primary phase or the second phase in alloys to nanosized uniform distributed phases in the matrix, such as the quasicrystal phase in the quasicrystal alloy . The third technique involves the refinement of eutectic silicon phase in hypereutectic Al-15Si alloys to fine particles with the size of several nanometers through solid solution and precipitation refinement. Finally, in the deformation zone induced by HCPEB irradiation, the grain size can be refined to several hundred nanometers, such as the grain size of the hypereutectic Al-15Si alloys in the deformation zone, which can reach ~400 nm after HCPEB treatment for 25 pulses. Therefore, HCPEB technology is an efficient way to obtain surface nanostructure.
1. Introduction
Nanomaterials are typically characterized by ultrafine grains [1]. They fundamentally possess several unique properties and behavior such as increased strength/hardness, enhanced diffusivity, enhanced thermal expansion coefficient, and superior soft magnetic properties [2] compared with the conventional coarse-grained materials. With continuous discovery of the unique properties of nanomaterials in recent years, various processing techniques have been developed to synthesize bulk nanomaterials, such as ultrafine powder consolidation [3], amorphous solid crystallization [4], ball milling [5], and severe plastic deformation [6].
High-current pulsed electron beam (HCPEB) technique, as a novel and valid method [7, 8] used for material surface modification, is long considered to be a very simple, reliable, and highly efficient method. The electron beam generates intense and superfast melting, evaporation, solidification, and even ablation on the surface of target materials together with the formation of thermal stress and shock waves. Therefore, the depth of the HCPEB modification zone can reach several hundred micrometers [9–12], which greatly satisfies the modification demands for engineering materials. The combination of these influencing factors, peculiar to HCPEB treatment, can lead to the nanocrystalline formation in near-surface layers of metallic materials [13, 14].
Hence, four methods involving HCPEB technology were proposed to obtain surface nanocrystalline formation in this paper.
2. Experimental Procedures
2.1. Starting Materials and HCPEB Treatment
The starting materials in this experiment were pure Mg (99.7 wt%), Mg67Zn30Y3 quasicrystal alloy (with a chemical composition of Zn (50.86 wt%), Y (7.02 wt%), and Mg balance), and hypereutectic Al-15Si alloy (with chemical composition of Si (15 wt%) and Al balance). Prior to HCPEB treatment, the samples were cut into Φ10 mm × 9 cm cylinders, with surfaces that were mechanically polished and washed by absolute ethyl alcohol. Then, the sample surfaces were treated through “Nadezhda-2” type HCPEB system, with the following working parameters: accelerating voltage, 23 kV; energy density, 1 J/cm2 to 3 J/cm2; pulse duration, ~1 μs; and pulse number, 10 for pure Mg and quasicrystal alloy Mg67Zn30Y3 and 25 for hypereutectic Al-15Si alloy.
2.2. Microstructural Analysis
The sample surface and crossing section morphologies were analyzed using a field emission gun scanning electron microscope (SEM) (Jeol JSM 6500 F) with EBSD acquisition camera and Channel 5 software. The beam control mode was applied for automatic orientation mapping with a step size of 0.04 μm. Thin films for transmission electron microscopy (TEM) were prepared by grinding, dimpling, and ion-beam thinning, and the microstructure characteristics were observed in a transmission electron microscope (FEI-Tecnai G220).
3. Results and Discussion
The characteristics of the obtained nanostructures according to the four techniques using HCPEB treatment are discussed as follows.
3.1. Surface Characteristics of Pure Mg after HCPEB Treatment
Figure 1 illustrates the surface SEM morphology of HCPEB-treated pure Mg for 10 pulses under energy density of 3 J/cm2. Several separated bulges are evident, and numerous fine particles were observed between and on top of these bulges. The chemical composition of the fine particles is determined to be pure Mg using EDS analysis, and the size of the fine particles ranges from several hundred nanometers to several micrometers. Pure Mg vaporizes on the top surface of the treated sample. The special morphology formation can be explained as follows. First, the surface temperature of the HCPEB-treated Mg reaches boiling point, vaporization occurs at a given depth determined by the energy density of HCPEB, and a considerable amount of bubbles form in the liquid Mg solution. Second, when the bubbles reach the top surface, the abruption of these bubbles induces shock effect on the nearby zone and a large number of fine particles will erupt simultaneously. Finally, these fine erupted particles fall to the top frozen surface under the effect of gravity, and the special surface morphology as seen in Figure 1 is formed.
Thus, vaporization behavior induced by HCPEB treating low-boiling point metal can produce abundant fine particles with sizes ranging from several hundred nanometers to several micrometers.
3.2. Microstructure Characteristics of Mg67Zn30Y3 Quasicrystal Alloy before and after HCPEB Treatment
Figure 2 demonstrates the surface back-scattered electron (BSE) images of Mg67Zn30Y3 quasicrystal alloy in the initial state and after HCPEB treatment for 10 pulses. Figure 2(a) depicts the initial microstructure characteristics of Mg67Zn30Y3 quasicrystal alloy, consisting of gray matrix phase Mg7Zn3, black dendrite α-Mg phase, and white petal-shaped quasicrystal Mg30Zn60Y10 phase [15]. The phase contrast change is mainly attributed to the different contents of Mg element in the different phases. Two types of dendrites are both Mg-rich phases and have few solid solution of Y element. By contrast, the white petal-shaped phase consists of the following: Mg, 37.496%; Zn, 53.022%; and Y, 9.482%. Although a certain deviation from the quasicrystal Mg30Zn60Y10 phase in standard ternary Mg-Zn-Y alloy is reported in [13], the phase should be the quasicrystal Mg30Zn60Y10 from the typical petal morphology, and the deviation is probably caused by the EDS test error. Figure 2(b) illustrates the typical surface morphology of Mg67Zn30Y3 quasicrystal alloy after HCPEB treatment for 10 pulses. The phase boundaries between different phases become clear, and the coarse petal phase disappears. During multiple HCPEB treatments, the top surface of the treated sample undergoes repeated melting and solidification. The chemical composition tends to distribute uniformly as a result of the chemical composition diffusion during the HCPEB process, forming the special morphology (Figure 2(b)). However, determining the accurate size of the quasicrystal Mg30Zn60Y10 phase after 10-pulse treatment by SEM observation is quite difficult.
Figures 2(c) and 2(d) illustrate the TEM observations of the quasicrystal phase of the Mg67Zn30Y3 quasicrystal alloy before and after HCPEB treatment. Figure 2(c) demonstrates the TEM bright field image of the initial sample and corresponding SAED pattern (five-fold symmetry). A black petal-shaped phase is observed under TEM view. Through the identification of the SAED pattern with five-fold symmetry, the petal-shaped phase is considered as quasicrystal Mg30Zn60Y10 with icosahedral structure of quasiperiodic structure arrangement. This result further confirms the observations obtained from SEM. In addition, several fine branches are formed in the vicinity of the large petal-shaped quasicrystal phase, known as the two-fold axis of quasicrystal growth. Figure 2(d) illustrates the TEM bright field image of a 10-pulse-treated sample and corresponding SAED pattern (diffraction rings). After HCPEB treatment for 10 pulses, a considerable amount of nanosized particles with grain size of approximately 10 nm to 30 nm is uniformly distributed in surface layer. The interplanar spacing values corresponding to each diffraction ring were calculated and compared with interplanar spacing (dstan) values of acknowledged quasicrystal Mg30Zn60Y10. Hence, the dispersed metastable nanocrystalline phase is identified to be the quasicrystal phase Mg30Zn60Y10. Thus, a melted layer containing a large number of nanostructured quasicrystal phases is obtained by the HCPEB treatment.
Hence, using HCPEB multiple bombardments, the coarse petal quasicrystal phase Mg30Zn60Y10 can be refined to dispersed distribution nanostructured quasicrystal phase.
3.3. Microstructure Characteristics of Hypereutectic Al-15Si Alloy before and after HCPEB Treatment
Figure 3 depicts the SEM surface morphology of hypereutectic Al-15Si alloy before and after HCPEB treatment under energy density of ~3 J/cm2. For the initial sample shown in Figure 3(a), the Al-Si eutectic structure in as-cast hypereutectic Al-15Si alloy is clearly observed by SEM. In addition, a small number of coarse primary Si phases with size of several ten micrometers are distributed randomly on the surface. For the 25 pulse-treated sample shown in Figure 3(b), the eutectic structure disappears completely after HCPEB treatment and numerous cellular cells (~100 nm wide) are uniformly distributed on the top surface. Additionally, a remarkable amount of Si nanoparticles is precipitated on the boundary of the nanocellular cell structure. Most of the eutectic Si phase of initial sample solid dissolves in the α-Al and forms the supersaturated solid solution. Some of the Si element precipitate from the supersaturated solid solution during the rapid solidification. The estimated size of the Si nanoparticles ranges from 5 nm to 50 nm. Thus, the size of eutectic Si phase can be refined to nm after HCPEB treatment.
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This process is the third way to obtain nanostructure by solid solution and precipitation refinement induced by HCPEB treatment.
3.4. Microstructure Characteristics of Deformation Zone of HCPEB-Treated Hypereutectic Al-15Si Alloy
A deformation zone can be found under the melted layer of the HCPEB-treated sample, but few studies on the grain size distribution in this zone have been reported. Figure 4(a) illustrates the SEM morphology image of this deformation zone (distance from the top surface approximately from 10 μm to 30 μm). Several fine Si particles are distributed in this zone, and the initial eutectic Si phases are deduced to be transformed to these fine particles as a result of microplastic deformation induced by multiple HCPEB shock wave impacts. From the corresponding EBSD orientation map (Figure 4(b)), several large lamellar primary Si phases were subdivided into several grains under multiple plastic deformations induced by the shock wave, and some Si phases release coupled stress as seen in Figure 4(b) and refined to several hundred nm. For the Al matrix, the grains were transformed from coarse dendrite crystal to fine equiaxed crystal with size of ~300 nm (Figure 4(b)). And the preferred orientation for Al phase in this zone is direction, which is parallel to propagation direction of shock wave.
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To show the grain distribution clearly, Figure 5(a) illustrates the EBSD grain map, white lines present the low angel grain boundary (grain orientation difference between 3° and 10°), and black lines present the high angel grain boundary (orientation difference over 10°). Figure 5(b) provides the corresponding grain size distribution map in this zone with an orientation difference of over 3°. Figure 5(a) shows that numerous low-angel grain boundaries are formed resulting from severe plastic deformation induced by the HCPEB shock wave impact. Most of the grains are less than 1 μm in size, and the average grain size is approximately 460 nm from statistical result of all the grain sizes in the deformation zone (Figure 5(b)).
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Hence, the grain size in the deformation zone under melted layer can reach several hundred nanometers after multiple pulses treatment of HCPEB. This is the fourth way to obtain a nanostructure by HCPEB treatment.
4. Conclusions
This work investigated the different techniques involving HCPEB treatment to obtain nanostructure for pure Mg, quasicrystal Mg alloy Mg67Zn30Y3, and hypereutectic Al-15Si alloy. The main results are as follows.(1)A considerable amount of fine particles with sizes from several hundred nanometers to several micrometers are distributed on the top surface of treated pure Mg sample as a result of vaporization behavior induced by HCPEB treating.(2)After 10 pulses of HCPEB treatment for quasicrystal Mg alloy Mg67Zn30Y3, the phase boundaries between different phases become more apparent because of repeated melting and solidification. The surface chemical composition tends to be homogeneously distributed. A large number of nano quasicrystal Mg30Zn60Y10 phases are formed with size of approximately 10 nm to 30 nm. (3)After 25 pulses of HCPEB treatment for hypereutectic Al-15Si alloy, many cellular cells with ~100 nm diameter are uniformly distributed on the top surface. In addition, a number of Si nanoparticles (5 nm to 50 nm) are precipitated on the boundary of the nanocellular cell structure.(4)After 25 pulses of HCPEB treatment for hypereutectic Al-15Si alloy, the grain size in the deformation zone induced by multiple HCPEB shock wave impacts can reach several hundred nanometers.
Acknowledgments
The authors would like to acknowledge the support from the Fundamental Research Funds for the Central Universities (N090602009 and N100402010) and the key projects in the National Science & Technology Pillar Program during the eleventh five-year plan period (2009BAE80B01).