Papers by Author: Jaromír Drápala

Abstract: Effects of Cerium (Ce) addition on solidification structure of a low-carbon 42CrMo4 steel was investigated. The addition of up to 0.067 wt.% of Ce in the steel produced greatly improved solidification structure with a suppressed columnar grain zone, finer grain size in an equiaxed grain zone and zero area fraction of casting shrinkage cavity. The added Ce occurred in the steel both in the form of Ce oxy-sulfide inclusions and as dissolved atomic Ce segregated together with other elements at prior austenite grain boundaries and at interdendritic spacing. The Ce oxy-sulfide inclusions were found to play a major role in the observed improved grain structure meanwhile dissolved Ce had pronounced effects on morphology of dendritic networks. The fraction of Ce dissolved in the melt appeared to bring about increase in fluidity of the molten steel, leading to total elimination of interdendritic shrinkage porosity in solidification structure of the steel with added Ce. Ce addition can be considered as a potential solution for grain structure refinement in heavy-weight castings of 42CrMo4 steel grade.

Abstract: A general trend in steelmaking industry is to increase strength, resistance to brittle fracture and fatigue properties of steel products at favorable price. Achieving fine-grained microstructure during austenite to ferrite transformation is a basic prerequisite to improve the mechanical properties. The desired transformation can be achieved in several ways one of which is the use of small non-metallic inclusions as heterogenous nucleation sites during solidification of steel. A great attention is focused on this concept in recent years. Rare earth metals are suitable for the formation of small inclusions similarly as in the case of conventional microalloying elements such as niobium, titanium, vanadium and other. Rare earth metals have a high affinity to oxygen and sulfur. The paper deals with the optimization of microstructure of 42CrMo4 low alloy structural steel used for machine parts exposed to higher stresses. The steel was alloyed with cerium in the form of mischmetal to achive fine-grained microstructure. Operational experiment was proposed and realized in accordance with results of laboratory experiment.

Abstract: This paper deals with FEM analysis of six models that represents human cortical bone, cast TiAl6V4 alloy and porous TiAl6V4 with different pore diameters. Reliable data for the simulations were achieved by meta-analysis that consisted from 53 scientific works. Strain value was chosen with a respect to the frequent daily activities such as walking. According to the FEM analysis of presented models von Mises stress values and stress concentration factors were similar for human cortical bone and porous TiAl6V4.

Abstract: The study of microstructure and fracture surfaces was performed on specimens of reconstruction plate, reconstruction nail and elastic nail. The composition and phase analysis of microstructure was performed by scanning electron microscope (SEM) JEOL JSM - 6490LV equipped with EDS INCA X - ACT probe. Examination of fracture surfaces by SEM confirmed that damage was not simply due to fatigue but contained evidence of corrosion and mechanical fretting as well.

Abstract: Problems of reactive diffusion at the solid phase and melt contact are studied theoretically. The rate constant is a fundamental parameter characterizing the dissolving rate at a certain configuration of experiment. Relationships between the solid phase dissolving rate, i.e. the solid phase interface boundary movement in the melt, and rates of growth of intermetallic phases in the metal (Cu) are observed. This procedure enables the creation of surface and subsurface layers of regulated thickness in metallic materials by means of reactive diffusion. The main intention was an experimental study of copper dissolving in melts of various solder alloys and the related reactive diffusion. We used Sn, SnCu, SnAgCu, SnZn and SnIn alloys as a solder material. The problems that need to be solved preferentially are emphasized. It concerns especially the determination of the rate constant of dissolving and verifying whether the proposed model equations can be used for this constant determination in cases of cylindrical and planar dissolving. Rapid growth of phases in the metal (Cu) and determination of the thickness of layers with these phases pose considerable time demands to X-ray microanalyses (WDX, EDX, BSE, SEM) of specimens after their long-time heating.

Abstract: The study of influence of Fe and Ti alloying elements in the Ni3Al intermetallic alloy on diffusion feature in Ni/Ni3Al-Me (Me = Fe or Ti) joints was performed using the diffusion couple technique. Four sets of diffusion couples of diverse compositions prepared by means of resistance welding or electron beam welding in vacuum were used. The concentrations profiles and diffusivities of welded Ni/Ni3Al-Fe and Ni/Ni3Al-Ti joints were determined after annealing at 1050°C for 100 hours. The concentration profiles were smoothed using suitable types of polynomials. The Kirkendall voids in the area between the Matano plane and the new  interface created after annealing were observed. Metallographic study and qualitative evaluation of voids by means of AFM method revealed different feature of Kirkendall regions in Ni/Ni3Al-Fe couples unlike Ni/Ni3Al-Ti joints.

Abstract: Problems of reactive diffusion at the solid phase and melt contact were studied theoretically and experimentally. The main intention was to calculate the time course of the solid phase dissolving in the case of cylindrical dissolving. These calculations were carried out on the assumption for the rate constant of dissolving K = const. In our work we give heed especially to the dominating process, which is the solid metal A dissolved in the melt B. During the dissolving the melt B saturates with the metal A and the process is influenced by convections which are characteristic for the given experimental configuration. A theoretical description of the kinetics of the solid phase dissolving in the melt will be presented for the case of cylindrical dissolving. The aim is to derive a relation for the interface boundary movement c(t) in dependence on time and a time course of growth of the element A concentration in the melt B. There are problems with accurate determination of the interface boundary movement after certain heating times of specimens, when it is observed experimentally, since intermetallic phases create in the original A metal at both the diffusion and cooling and some phases segregate at the solidifying melt cooling. The main intention was an experimental study of the copper dissolving in the tin melt. Experiments aimed to the determination of the Cu wires (diameters from 0.5 to 3.5 mm) dissolution in the solder melt were carried out at various selected temperatures and times. Rapid growth of phases in the metal A and determination of the thickness of layers with these phases pose considerable time demands to X-ray micro-analyses (WDX, EDX) of specimens after their long-time heating.

Abstract: Problems of reactive diffusion at a solid phase - melt contact were studied theoretically. The main intention was to calculate the time course of the solid phase dissolving in the case of planar dissolving. In our work we give heed especially to the dominating process, which is the solid metal A dissolving in solder melt B. During the dissolving, melt B saturates with metal A, and the process is influenced by convections which are characteristic for a given experimental configuration. A theoretical description of the kinetics of solid phase dissolving in the melt will be presented for the case of planar dissolving. The aim is to derive a relation for the interphase boundary movement (t) depending on time and a time course of growth of the element A concentration in the melt B. There are difficulties in accurate determination of the interphase boundary movement after heating of specimens for certain time intervals. It should be performed experimentally, since intermetallic phases are formed in original metal A both via diffusion and upon cooling and some phases segregate upon cooling of the solidifying melt. The main intention was to study experimentally the copper dissolving in melts of various solder alloys and the related reactive diffusion. We used pure Sn and Sn-Cu, Sn-Ag-Cu, Sn-Sb, Sn-Zn alloys as solder materials. Experiments aimed at the study of a Cu plate dissolving in the solder melt were carried out at various selected temperatures and times. The problems of reactive diffusion were studied both theoretically and experimentally and the problems that have to be solved preferably were emphasized. Concentration profiles of elements and thickness of layers of phases can be determined with SEM and X-ray microanalyses (WDX, EDX) of specimens after their diffusion heating.

Abstract: In this work we give heed especially to the dominating process which is the solid metal A dissolving in the melt B. During the dissolving, the melt B saturates with the metal A and the process is influenced by convections which are characteristic for the given experimental configuration. A theoretical description of the kinetics of the solid phase dissolving in melt will be presented for the case of planar and cylindrical dissolving. The aim is to derive a relation for the interface boundary (t) movement in dependence on time and a time course of growth of the element A concentration in the melt B. There are problems with an accurate determination of the interface boundary movement after certain heating times of specimens, when it is observed experimentally, since intermetallic phases create in the original A metal at both the diffusion and cooling and some phases segregate at the solidifying melt cooling. The rate constant is a fundamental parameter characterizing the dissolving rate at a certain configuration. We present a theoretical description of dissolving of a long metallic cylinder submerged into a melt column and relations for the rate constant determination from the time of the whole metallic cylinder dissolution are derived. In our experiments were performed in which Cu was dissolving in the Sn melt for a Cu cylinder (wire) diameters 0.8÷2.5 mm and the rate constant K (T = 350°C) was determined. Relationships between the solid phase dissolving rate, i.e. the solid phase interface boundary movement (t) in the melt and rates of growth of intermetallic phases in the metal A will be observed. This procedure enables to create surface and subsurface layers of regulated thickness in metallic materials by means of reactive diffusion.

Abstract: When developing new types of materials with special properties for the automotive industry, steels with high content of manganese (up to 25 wt. % Mn) were studied. These steels embody increased failure resistance, high level of deformation hardening, plastic response and high value of energy absorption in crash situations [1,2]. The aim of our work was to study an interaction of a FeMn alloy with pure iron from the view of diffusion processes. For that purpose, diffusion joints Fe/FeMn-steel were created and long-term heated at the temperatures of 800, 900 and 1050 °C. Dependences of diffusivities of Mn on its concentration were calculated from concentration profiles of manganese after diffusion heating applying three analytical methods. In the first method, constant values of diffusivities Di, i = 1,2 for both Fe and steel were taken into account. In the second case a modified Matano-Boltzmann method utilizing polynomials was applied, which simplifies solution in numerous cases. The third method performs the solution of the non-linear equation of diffusion on the assumption of a linear dependence of diffusivity on concentration applying a procedure based on perturbation calculus. The results of the solution will be graphically documented in details together with the description of experiments.