Xiaole Cheng

Metastable phases formed in the immiscible Co–Ag system by ion mixing

Abstract Assisted by the interfacial free energy, metastable and fcc phases were formed by room temperature 200 keV xenon ion mixing of nano-sized multilayers in the Co–Ag system being immiscible at equilibrium and characterized by a large positive heat of formation. The formation mechanism of metastable phases is discussed in terms of the far-from-equilibrium process of ion mixing, the role of the interfacial free energy in enhancing alloying capability and the effect of valence electrons.

Formation of TaSi2 and associated fractal growth on Si surface upon high current pulsed Ta-ion implantation

Abstract Using a metal vapor vacuum arc ion source, tantalum ion implantation was conducted to synthesize TaSi 2 phases on silicon wafers, and the continuous and stable TaSi 2 layers were directly obtained with neither external heating nor post-annealing. Interestingly, under some specific conditions, the formed TaSi 2 grains organized themselves in a fractal pattern featuring self-similarity. The mechanism of the TaSi 2 formation as well as the growth of the fractal pattern is discussed in terms of the temperature rise caused by the energetic Ta ions beam heating effect and the dose adjustment during high current pulsed Ta-ion implantation far from equilibrium.

Formation of the ErSi2 phase and the associated fractal pattern on the Si surface upon high current Er-ion implantation

Abstract Using a metal vapor vacuum arc ion source, plain and continuous ErSi 2 layers of good crystalline structure were formed on Si surfaces by high current Er-ion implantation. Interestingly, under some specific conditions, the formed ErSi 2 grains organized themselves in a fractal pattern featuring self-similarity. The mechanism of the ErSi 2 formation as well as the growth of the fractal pattern was discussed in terms of the dynamic launching of energetic Er ions into Si, beam heating effect, and the effect of ion fluence during the high current Er-ion implantation of far-from-equilibrium.

Fabrication of PrSi2 layers and associated morphology on Si surface by a single-step high current Pr-ion implantation

Abstract PrSi 2 layers were fabricated on Si wafers with metal vapor vacuum arc ion implantation without in-situ-heating or post-annealing. Under optimal experiment conditions, the obtained PrSi 2 layers were of good crystalline structure and featured plain and continuous morphology. In addition, the formation mechanism of the PrSi 2 layers is also discussed in terms of the temperature rise caused by ion beam heating as well as the variation of ion implantation dose.

On the Supplementation of Magnesium and Usage of Ultrasound Stirring for Fabricating In Situ TiB 2 /A356 Composites with Improved Mechanical Properties

Adding mixed K2TiF6-KBF4 salts into mechanical stirred molten A356 alloy (Al-7Si-0.3Mg) provides a simple method to fabricate in situ TiB2/A356 composites via salts-metal reaction. However, some issues, such as the loss of Mg element in A356 alloy and the clustering of TiB2 particles in the matrix, usually occur during the fabricating process. These issues easily result in the degradation of mechanical properties of composites. In this study, a quantitative analysis revealed that the Mg element was almost consumed after the reaction, and a thermodynamic analysis inferred that the loss of Mg resulted from the reaction between Mg and mixed salts. In order to fabricate in situ TiB2/A356 composites with high strength and good ductility, we proposed a developed approach that consisted of supplementing Mg and using ultrasound stirring during the fabricating process. The introduction of ultrasound into the melt could disperse TiB2 particles uniformly in the matrix, leading to more effective refinement of α-Al grains and modification of Si phase. Compared with the untreated A356, both the strength and ductility of the developed sample were improved significantly, and the coefficient of thermal expansion mismatch strengthening was found as the major contributor to the improvement in the yield strength.

Formation of CeSi2 on the Si surface upon high current pulsed Ce-ion implantation

Abstract Cerium-ion implantation was conducted to synthesize Ce-disilicide films on silicon wafers, using a metal vapor vacuum arc ion source. The continuous CeSi 2 films were directly obtained at relatively low temperature with neither external heating nor post-annealing and the surface morphology varied with the variation of the implantation parameters. The formation mechanism of the CeSi 2 phase is also discussed in terms of the temperature rise caused by ion beam heating and the ion dose in the far-from-equilibrium process of high current pulsed Ce-ion implantation.

Rare Earth Metal Silicides Synthesized by High Current Metal Ion Implantation

The YSi2, LaSi2, CeSi2, PrSi2, NdSi2, SmSi2, GdSi2, TbSi2, DySi2, and ErSi2 layers were formed on Si wafers by respective high current metal‐ion implantation using a metal vacuum vapor arc (MEVVA) ion source and the formation temperature was considerable lower than the critical temperatures (300–350°C) required for the rare earth metal silicides by solid‐state reaction. It was found that the crystalline structures could be improved with increasing slightly the formation temperature as well as the implantation dose. Concerning the growth kinetics, in some cases, fractal patterns were observed on Si surfaces and the branches of the fractals consisted of the grains of respective precipitated silicides. Interestingly, the fractal dimension increased with formation temperature and eventually approached to a value of 2.0, corresponding to a continuous layer, which was required in practical application. The formation mechanism as well as the growth kinetics was discussed in terms of the far‐from‐equilibrium proc...