Papers by Author: Cosmin Codrean

Abstract: Components soldered with Sn-based alloys are susceptible to the growth of whiskers. Tin whiskers have been proven to be responsible for equipment failures in a wide range of industries. In order to reduce defects in electronic components a new solder alloy is proposed based on the Sn-Cu alloys. The Sn-Cu-Ga alloy utilised in this study was fabricated as a ribbons using melt-spinning method. These ribbons were then soldered onto electroplated tin layers. Preliminary characterization of the solder alloy is presented in this paper, including scanning electron microscopy, EDX mapping and X-ray diffraction. Key words: tin whiskers, PCB, printed circuit boards, melt spinning, solder, ribbons

Abstract: Development of Fe-based bulk metallic glasses (BMG) with good mechanical and soft magnetic properties has become a major objective in the materials science field. Bulk metallic glasses present an interesting combination of properties. They exhibit very high strength (both in tension and compression), large elastic elongation limit, high hardness, excellent corrosion resistance, and good soft magnetic properties. These properties makes them suitable for many applications like high resistant control cables, pressure vessels, micro-components, pressure sensors, microgears for motors, magnetic cores for power supplies and hard fibers in composite materials. Multi-component Fe₆₀Co₁₄Ga₂P₁₀B₅Si₃Al₃C₃ bulk metallic glass was synthesized in rod form with a diameter of 1 mm by copper mould casting technique using raw industrial materials. The obtained alloy was analyzed by X-ray diffraction (XRD), differential thermal analysis (DTA) and scanning electron microscopy (SEM) techniques, in order to determine the phase constituent, the thermal stability and the fracture surfaces of as-cast samples. The mechanical behaviour was investigated by microhardness and compression tests. The values recorded for hardness and fracture strength includes this alloy in the category of high resistant materials.

Abstract: Ni based amorphous alloys with Si and B, which can also, contains Fe and Cr, prepared by rapid solidification, have low melting temperatures. This fact increases their susceptibility to be joined by welding and brazing. The glass forming ability (GFA) is conditioned also by the crystallization delay, due to certain chemical composition of the alloys. The thermal stability of these alloys was revealed by DTA analysis and structural characteristics were investigated by XRD. Applying an annealing at temperatures between 420°C and 540°C, with 30 minutes maintaining time, allowed the investigation of phase occurred during the crystallization and the estimation of the crystalline grains dimensions.

Abstract: Nickel based amorphous alloys have been developed in ribbons form by planar flow casting method and were subjected to tensile tests. For this purpose, specimens with calibrated area were obtained by electroerosion cutting method and tested at tensile test with different strain rates. Following the results obtained during the tests, the mechanical properties have been determined and compared and the fracture behaviour has been analyzed by scanning electron microscopy.

Abstract: This paper presents some results for numerical simulation of the thermal field developed during rapid cooling of iron base alloy (Fe₇₃Cr₂Ga₄P₁₃Si₅C₃), to obtain bulk metallic glasses. Technology of rapid cooling gives the possibility to obtain bulk metallic alloys with an amorphous structure which can be used for industrial applications like magnetic shields. Validity of the numerical model of the cooling process was confirmed by the X-ray diffraction analysis. Once the model being validate, it can be used to optimize the rapid cooling process and casting mold geometry to be able to investigate the possibility to obtain larger amorphous bulk components.

Abstract: This paper presents a mathematical model that describes the influence of the chemical composition on the glass forming ability of ferromagnetic alloys. Glass forming ability is given by the difference between the glass transition temperature and the primary crystallization temperature of the alloy. The glass forming ability is better as long this difference has a higher value. These temperatures were determined using differential thermal analysis.

Abstract: The paper makes a review about the opportunity, forming conditions and obtaining methods of bulk amorphous alloys and their properties. There are also presented the experiments regarding the obtaining and characterization from mechanical and chemical point of view of some alloys from Fe70 (Cr5-x Nix)5 Ga4 P13 Si5 C3 ( x = 0,1,2,4) family, pointing out the negative effect of the nickel on their resistance characteristics.

Abstract: Tapes of stainless steels were brazed inside a vacuum oven using amorphous or quasi amorphous alloy based on Ni-Cr. By means of different investigation methods (optical microscopy, X ray diffraction) there were emphasized the structural morphology transformations inside brazed joints, revealing the role of process parameters on obtained structure.