Papers by Author: Dóra Janovszky

Abstract: The effect of nickel addition was studied in the CuZr system creating alloys with near eutectic composition. Nickel and aluminum have been regarded as useful elements to improve the plasticity, thermal stability of the CuZr-based amorphous alloys. Cu₄₉Zr₄₅Al₆ and (Cu₄₉Zr₄₅Al₆)₉₅Ni₅ were selected because of the good glass-forming ability. After 15 h of milling the structure of the powders was amorphous based on the XRD analysis. By adding nickel, the crystallization temperature (T_x) shifted to higher temperatures compared to CuZrAl alloy. The value of supercooled liquid region was 64 K, which means CuZrAl has a comparatively high glass forming ability.

Abstract: The Cu-Zr-Ag system is characterized by a miscibility gap. The liquid separates into Ag-rich and Cu-Zr rich liquids. Yttrium was added to the Cu-Zr-Ag and Cu-Zr-Ag-Al systems and its influence on liquid immiscibility was studied. This alloying element has been chosen to check the effect of the heat of mixing between silver and the given element. In the case of Ag-Y system it is highly negative (-29 kJ/mol). The liquid becomes immiscible in the Cu-Zr-Ag-Y system. To the effect of Y addition the quaternary liquid decomposed into Ag-Y rich and Cu-Zr rich liquids. The Y addition increased the field of miscibility gap. An amorphous/crystalline composite with 6 mm thickness has been successfully produced by liquid-liquid separation based on preliminary calculation of its composition. The matrix was Cu₃₈Zr₄₈Al₆Ag₈ and the crystalline phases were Ag-Y rich separate spherical droplets.

Abstract: This research work dealt with production of amorphous powder with nominal composition of (Cu₅₅Zr₄₅Al₁₀)₉₇Hf₃ (at%). Combining the mechanical milling and alloying, powder of crystalline Cu-Zr-Al alloy mixed with Hf elemental powder were milled in order to produce a homogenous and amorphous alloy powder The master alloy and the powders milled for different time were analyzed by X-Ray Analysis (XRD) and Scanning Electron Microscopy (SEM). Particle size distribution and hardness were controlled during milling and at the end of procedure. The milling caused dissolving of the hafnium. The 25 h milling time was the optimal to obtain the Hf containing powder with amorphous structure. However, elemental Hf traces with size below 3 µm were still observed in the powder. After 50 h of milling, such impurity elements as iron, nickel, chromium originating from milling tools (vial, balls) were detected.

Abstract: There is an active research work in the field of amorphous alloys since the discovery of metallic glasses, half a century ago [. In contrast with crystalline alloys, amorphous one have unique material properties, e.g. high yield strength, superior elastic limit, high corrosion resistance, unique acoustical properties [2,. Producing amorphous alloys, it is first necessary to quantify the forming of structure and define the amorphous amount. All of the methods to determine the structure have advantages and disadvantages. An accurate determination of amorphous volume fraction can be accomplished by transmission electron microscopy (TEM), this observation is much localized [ and the evaluation is difficult. The most common techniques to determine the amorphous fraction are XRD and DSC methods, which reflect the entire sample [. However, XRD has a detection limit depending on the type of equipment. Apart from this fact, it is an admitted method in researches, if the XRD reflexion shows an amorphous halo. Amorphous fraction transformed to crystalline can be measured by DSC. This method is much sensitive to impurities, especially oxygen, which can influence on the results.

Abstract: The glass forming ability (GFA) of the Cu-Zr-Ag alloy system was investigated on the basis of the thermal stability, and the structural, thermodynamic and kinetic properties of the material. We changed the concentration of the alloys, as we departed from the Cu58Zr42 composition and produced three different eutectic points in the Cu-Zr-Ag ternary system, in accordance with the results published in the respective literature. We produced various test pieces of Cu-Zr-Ag amorphous alloys with different Ag contents (0-70%), by casting the material into wedge-shaped copper moulds. In such ternary alloy system there is only a limited concentration range where amorphous materials can be produced: in the event that the Ag content of the material exceeds 35 at%, no amorphous material structure will develop. In our experiment the maximum temperature range of the supercooled liquid region (ΔTx) was 75 K. The calculated four GFA parameter values are not in perfect harmony and fail to point out the optimal composition available; however, based on γ and the reciprocal value of ω, the best compositions from the GFA aspect are Cu_42.5Zr_37.5Ag₂₀ and Cu₄₀Zr_37.5Ag_22.5. The decrease of the maximum thickness of the bulk metallic glasses is influenced more by the oxygen content than the composition changes within the Cu-Zr-Ag system.

Abstract: This paper reports laser remelting of crystalline Cu based alloys in order to produce amorphous layer on the surface. The as prepared Cu based master alloy ingots were imbedded in a metallic sinking with Wood metal to assure the good thermal conductivity during the laser treatment. The laser remelting of a thin surface layer and a subsequent rapid cooling of it was performed using impulse and continuous mode of Nd:YAG laser. In respectively the impulse mode the laser power and the interaction time were 1.5; 2 kW and 20÷100 ms. In the continuous mode the laser power was 2 kW, and the laser scan speed was 80÷120 mm/s. The characterization of the microstructure of surface layer was performed by XRD, scanning electron microscopy and microhardness measurements.

Abstract: In order to perform a continuous cold rolling of steel sheets, hot rolled coils need to be attached to each other in order to get an endless sheet. This process can be achieved by laser key-hole welding as well. The structure and mechanical properties (e.g. hardness) of the welded joint without added metal, as well its heat affected zone differs significantly from the base metal. During our laboratory experiments the aptitude of cold rolling of joints and their environment, as well as the change in the features of materials was analyzed. During the rolling phases the hardness of the joint is higher than the ground structure, the strip thickness is modified depending on the difference of the hardness during the passes. According to our findings, the significant difference in measured hardness profile and texture structure prior to cold rolling and during cold rolling is minimized due to a recrystallisation annealing treatment following the cold rolling procedure. It is obvious that the normalization before cold rolling has advantageous effect to the properties of the welded joint. Elongation of the normalized samples is bigger than elongation of the non normalized samples.

Abstract: The aim of our research was to comparatively examine Ni content surface layers on amorphisable Cu base alloy produced by different laser surface treatments. Laser surface treatment (LST) techniques, such as laser surface melting, laser alloying and laser cladding, provide a wide range of interesting solutions for the production of wear and corrosion resistant surfaces. [1,2] With LST techniques, the surface can be: i) coated with a layer of another material by laser cladding, ii) the composition of the matrix can be modified by laser alloying. [3] Two kinds of laser surface treatment technologies were used. In the case of coating-melting technology a Ni content surface layer was first developed by galvanization, and then the Ni content layer was melted together with the matrix. In the case of powder blowing technology Ni3Al powder was blown into the layer melted by laser beam and Argon gas. LST was performed using an impulse mode Nd:YAG laser. The laser power and the interaction time were 2 kW and 20÷60 ms. The characterization of the surface layer microstructure was performed by XRD, scanning electron microscopy and microhardness measurements.

Abstract: In this paper CuZrAl amorphous alloys with different Al content were prepared by centrifugal casting. The master alloy ingots were prepared by arc melting the mixture of the constituents on a water cooled copper boat in argon atmosphere. Wedge-shaped samples were prepared from the ingots by centrifugal casting into copper mold. The microstructure of the samples was examined by Scanning Electron Microscope (SEM) and the phases were analyzed by X-ray diffractometry (XRD) to define the amorphous fraction.

Abstract: Bulk Metallic Glasses (BMGs) have been widely investigated due to their excellent physical and chemical properties [1]. The copper based BMG occupies a special place in the family of BMGs since they are relatively low priced. The Cu-Zr-Ag ternary system has been examined on the basis of the ternary phase diagram [2]. We have changed the concentration of the alloys from the Cu58Zr42 to the concentration of the deep eutectic point. Wedge-shaped samples have been cast from the master alloys by centrifugal casting into a copper mould, consequently analyse the influence of the cooling rate on the crystallization. The cooling rate has been estimated from the secondary dendrite arm distances by using a Cu-Sn crystalline alloy. Near the tip of the wedge the samples were amorphous and near the base of the wedge the samples were fully or partially crystallized. The structures of the samples have been characterized by scanning electron microscope and X-ray diffraction.