Pavol Zubko

Changes in Mechanical Properties and Microstructure after Quasi-Static and Dynamic Tensile Loading

Dual phase (DP), interstitial free (IF) and advanced high strength low alloy steel (HSLA)sheets have been successfully used for different components of car body. DP and HSLA are used ascrash resistant and IF as cover or “skin” of car body. The development of new vehicles nowadays isbeing driven by the need to simultaneously reduce mass and increase of passenger and pedestriansafety as well as costs saving through cold forming instead of hot forming. Limited publishedinformation is available on the changes in microstructure of these steel grades at different highstrain rates [1-3].This paper deals with changes of mechanical properties, microstructure and fractography of threesteel grades, which were tested at quasi static (10-3 s-1) and high strain rate (3000 s-1). Themicrostructures were characterized in terms of ferrite grain size, aspect ratio of ferrite andelongation of constituent phases. Deformed and undeformed specimens were compared to assess thechanges in the microstructure. The fracture appearance analysis indicates that the fracture patternunder high strain rates is mainly ductile, regardless of steel grades. The microstructure changessignificantly during the deformation process under both quasi-static and dynamic tension in allinvestigated steels. The plastic deformation in ferrite dominates in this process.

Use of Decomposition of Non-Thermoelastic Martensite for Structural Changes of CuZnAl Shape Memory Alloys

Cu22Zn4.6Al shape memory alloy shows memory phenomenon after quenching from dual phase region. Amount of shape memory effect continuously decreases with raised quenching temperature. Decrease is caused by increasing of amount of non-thermoelastic martensite in structure [. Approximately above 700°C there is no effect and martensite is non-thermoleastic. Non-thermoelastic martesite decomposes to alpha and gamma/beta phases at heating at elevated temperatures.[ This effect can be used for modification of structure and thus mechanical and shape memory properties. Some various types of structures were obtained and mechanical and shape memory properties were evaluated. Using of decomposition of non-thermoelastic martensite allows to obtain fine structure with the same amount of alpha phase and martensite as in basic state. This type of structure has better shape memory properties and higher mechanical properties.

Steel Pile-Up Characterization by Atomic Force Microscopy

The Oliver–Pharr method has extensively been adopted for measuring hardness and Young’s modulus by indentation techniques. However, the method assumes that the contact periphery sinks in, which limits the applicability to the materials pile-up [1]. In this work, we characterize the pile-up (shape and height) in steel sheets with different mechanical properties and propose an improved methodology to calculate the real mechanical characteristics of steel sheets with significant pile-up. Pile-up correction of mechanical characteristics is based on ratio of pile-up height and contact depth. Pile-up height was measured by atomic force microscopy (AFM).

Effect of Strain History of Steel Sheets on the Mechanical Characteristics of Individual Microstructural Components by Depth Sensing Indentation

The macroscopic mechanical properties of steel are highly dependent upon microstructure, crystallographic orientation of grains and distribution of each phase present, etc. Nanomechanical testing using depth sensing indentation (DSI) provides a straightforward solution for quantitatively characterizing each of phases in microstructure because it is very powerful technique for characterization of materials in small volumes. Measuring the local properties of each microstructure component separately in multiphase materials gives information that is valuable for the development of new materials and for modelling. The work experimentally analyses the effect of strain history on the mechanical properties of individual components in steel sheets by depth sensing indentation. The measurements were carried out on broken tensile specimens.

Correlation between Hardness and Fatigue Properties

The contribution deals with estimation of tensile properties and fatigue behavior based on hardness measurement. First of all the database of tensile and fatigue properties vs. hardness data was created for a group of steels, from literature survey and performed experiments. Tensile strength, yield strength, ductility and parameters of Ludwig-Hollomon equation in static and cyclic loading were extracted and fitted in relation to the hardness HB. The experimental materials were API 5 L grade steels – X60 and X70 after different deformation exposition. Measured tensile curve (SC) and cyclic deformation curve (CDC) were compared with predicted curves. Hardness was measured in-situ during cyclic loading. The maximum possible hardness values were experimentally determined. The results give a good agreement between estimated and measured data of both static tensile test and fatigue properties.

Structure and Properties of Selected Natural Materials

This paper deals with description of properties of selected natural materials mainly theirs shelter function. The investigated materials are horsetail and walnut shell. Both natural materials have porous shell/tubular structure. Walnut shell provides natural shield cover for fruit with gradient distribution of porosity and membrane function. In case of horse tail, except the gradient distribution of porosity there is also the gradient change of chemical composition along the cross-section.

Prediction of Tensile Properties Based on Hardness Measurement

The contribution deals with prediction of tensile properties based on measurement of microhardness. First of all, the database of stress strain, s-e vs. hardness data was created. Tensile strength, yield strength, ductility and parameters of Ludwig-Hollomon equation σ = σ0+kεn ; k, n were correlated with hardness. Various hardness values found in literature were recalculated to Brinell hardness. In tensile testing measured s-e curves were compared with that obtained from the correlation. The investigated materials were API 5 L grade steels X70 after different deformation exposition. The results give good agreement between compared data. The most difference between estimated and measured curve is in area of yield strength, because of Lüders deformation on investigated steel.

Comparison of the Natural Ageing Behaviour of EN AW 6082 and Lead Free EN AW 6023 Aluminium Alloys

The effect of solution annealing temperature (550 and 570°C) on the Vickers microhardness of the quenched and naturally aged states of EN AW 6082 and lead free aluminium alloy was investigated. The different chemical composition of analysed alloys did not affect the alloys microhardness after quenching. The total strengthening of both alloys was similar due to the same conjunction of the individual strengthening mechanism of alloys matrix after quenching. The microhardness of EN AW 6082 increased with ageing time increasing according to logarithmic dependence because the increment in alloy microhardness decreased with the increase in ageing time. On the contrary, microhardness of lead-free EN AW 6023 did not increase markedly either after 500 hours natural ageing. This alloy have very slow rate of natural ageing, probably due to suppressed diffusion of alloying elements and clusters and/or GP-zone formation in supersaturated solid solution.

Evaluation of Local Mechanical Properties of Thin Coatings Prepared by PVD Evaporation

The contribution deals with evaluation of mechanical properties of thin hard coatings prepared by ARC evaporation on steel substrate K110. In this paper TiN and AlTiCrN are comparatively studied. The thickness of TiN and AlTiCrN coatings was evaluated using the calotest, GDOES and nanohardness was also measured. The results show that AlTiCrN coatings posses higher hardness and Young´s modulus than simple monolayer TiN coatings. The substrate-coating interface was without failures, which confirmed excellent adhesion properties of the system. Due to their specific properties, the coatings appear suitable for use in practical operations.