Materials Science Forum Vols. 567-568

Abstract: The paper presents the results of research focused on assessment of the influence of loading cycle asymmetry on fatigue limit values. For tests two heats of unalloyed nodular cast iron were used. Test bars made of keel blocks were heat treated in salt bathes (austenitization at 900 °C during 1 hour, isothermal transformation at 380 and 400 °C) and loaded at symmetrical, repeating and pulsating loading cycles at room temperature. Evaluation of fatigue properties was based on the determination of S-N curves in high-cycle region including the fatigue limit assessment for 107 cycles to fracture. Fatigue and static tests were completed by metallographic and quantitative phase analysis. Most important result obtained from the presented study is that the dependence of stress amplitude on mean stress cannot be approached by the linear relation but by general power law with exponent lower than 1 (i.e. the Haigh diagram has convex shape).

Abstract: A large testing programme of a strip steel HSLA grade, microalloyed by vanadium, titanium and niobium, was conducted. The experiment was based on combination of cold rolling, recrystallization annealing, mechanical testing, metallographic examinations and TEM analysis. Flat samples with thickness 3.9 mm were rolled in several passes with the total height reduction 5 to 75 %. Afterwards the laboratory mill products were annealed in the vacuum furnace with the protective gas atmosphere consisting of N2+H2. The annealed samples underwent the mechanical testing. The gained results – hardness, yield stress, tensile strength and their ratio, as well as elongation, were summarized in graph in dependence on relative height reduction before annealing. It was confirmed that by a suitable combination of size of previous cold deformation and parameters of the following recrystallization annealing it is possible to influence a complex of mechanical properties of particular strips. Particular trends of strength and plastic properties correspond to each other and they may be utilized for optimization of terms of heat treatment of the investigated HSLA steel in a cold rolling mill. These trends are caused by structure-forming processes (recrystallization, grain coarsening, changes of the state of precipitates) which were documented by micrographs.

Abstract: Fracture behaviour of two intermetallic alloys based on FeAl and Fe3Al was studied. On the alloys Fe-40Al-1C (at%) and Fe-29.5Al-2.3Cr-0.63Zr-0.2C (at%) (FA06Z), a basic characterization, the fracture toughness tests and fractographic analysis were carried out. Tensile tests and fracture toughness tests were performed at 20, 200, 400 and 600°C. The fracture toughness values range from 26 MPa.m1/2 at 20°C to 42 MPa.m1/2 at 400°C. In addition, Jintegral dependence on
a obtained by potential method was measured. The fractographic analysis showed that samples fractured at 20, 200 and 400°C in the tensile or fracture toughness tests exhibit transgranular cleavage fracture, while at 600°C the ductile dimple fracture predominates.

Abstract: We used mechanical measurements and unbiased statistical morphometry in order to supply the computer model of smooth muscle cells (SMC) and collagen connective tissue of a gastropod Arion sp. with sufficient input data. We identified the elasticity modulus for both living (34 kPa) and formalin-fixed samples (440 kPa). The relative volume proportion of SMC was 57.8%, their numerical density was 425 925 mm-3, surface density of SMC was 405.9 mm-1 and the mean cell volume was 1 358 .m3.

Abstract: The influence of plastic deformation and heat-treatment on the precipitation of Al3(Sc, Zr) particles and the effect of these precipitates on hardening and softening processes of dilute ternary Al-0.2wt.%Sc-0.1wt.%Zr alloy was investigated. Behaviour of two differently prepared alloys (mold cast and prepared by powder metallurgy – PM) was investigated in as-prepared and in cold rolled state. Both alloys exhibit the same peak age hardening, PM one reaches it already during extrusion at 350°C. Both cold rolled alloys are highly resistant against recovery, which proceeds without rapid hardness decrease at high temperatures. Evolution of hardness agrees well with that of resistivity and with TEM observation.

Abstract: The sheet of ZK60 alloy with a thickness of 1mm was prepared from a casting ingot followed by homogenization and warm-rolling. Variations in microstructure and mechanical properties of ZK60 alloy sheets were investigated during T6 treatment. Especially artificial aging after solution heat treatment affected both precipitates distribution and mechanical properties with aging treatment. Variations of mechanical properties were related to precipitates, i.e. rod-shaped ( 1 β ′ ) or disc shaped ( 2 β ′ ) particles. Around the peak of hardness values, regularly distributed rod-shaped ( 1 β ′ ) precipitates were found. The rod-shaped ( 1 β ′ ) precipitates were oriented with a growth direction of [0001]. When over-aged, rod-shaped ( 1 β ′ ) precipitates were expected to decrease and the density of disc-shaped ( 2 β ′ ) precipitates to change. The rod-shaped ( 1 β ′ ) precipitates mainly consist of {Mg, Zn}, while disc-shaped ( 2 β ′ ) precipitates, {Mg, Zn, Zr} or {Mg, Zn}. In this study the optimum T6 treatment was determined as solution treatment at 430 °C for 6 hours and subsequently aging treatment at 175 °C for 18 hours. At this T6 condition the tensile strength, yield strength and elongation are 321MPa, 280MPa and 16%, respectively.

Abstract: The superplastic deformation characteristics of the AZ91, the mostly used magnesium alloy, were investigated at various strain rates in the interval from 3x10-5 to 1x10-2 s-1 and temperature of 420 °C. To prepare superplastic alloys thermo-mechanical treatment was used. Cast materials were heat-treated in two stages, after homogenization at 415 °C for 10 h were submitted to the precipitation annealing at temperature in the range of 200-380 °C for 10 h, and deformed by hot extrusion. Microstructure of samples was observed by the light microscope Olympus. Strain rate sensitivity parameter m has been estimated by the abrupt strain rate changes method. The strong strain rate dependence of the m-parameter was found. The highest elongation to failure, 584%, was found for the samples aged at 380 °C. Possible physical mechanisms of the superplastic flow are discussed.

Abstract: Fracture behaviour and mechanical properties are the key features when a material for given application is supposed to be selected. Advanced glass ceramics composites are perspective structural materials for many applications due to their low production expenses and satisfactory properties even at elevated temperatures. Borosilicate glass matrix composite reinforced by alumina platelets was investigated to describe toughening mechanisms and their changes in a wide range of temperatures (from room temperature up to glassy transition temperature Tg). The dissipation of energy by bridging and/or deflection of propagating crack by alumina platelets uniformly dispersed in the glass matrix were the main toughening mechanisms observed. The alumina platelets have a higher ability to deflect propagating crack in comparison with spherical or rectangular particles having the same volume. Three and four point bend test for Young’s modulus and flexural strength determination was used. Fracture toughness determination was conducted using chevron notch technique. More than 100% increase of fracture toughness was observed when 30% of alumina platelets were added in to borosilicate matrix.

Abstract: With the aid of an ASC-type finishing line (Automatic System Control) and continuous cooling of the rolled stock in water boxes, modern continuous light mills enable thermomechanical (controlled) rolling of SBQ-type rods (Special Bar Quality). During the thermomechanical forming (at different temperatures or cooling rates etc.), one can analyze the quality parameters of roundsection steel bars. In order to obtain better understanding to the forming conditions (temperature, strain and strain rate) and the possibility of cooling (related to its rate and time), an experiment involving continuous and discontinuous testing by means of a Gleeble plastometer was suggested. After its implementation, the attention was focused on the mathematical interpretation of the stressstrain characteristics of the steel and on metallographic observation of microstructural changes. The results achieved are of universal meaning and contribute to the theoretical understanding of controlled forming.

Abstract: The effect of intercritical annealing of 10GN2MFA grade steel on mechanical properties and impact notch toughness is studied in this paper. This type of low alloy banitic steel is used for production of collector bodies of steam generators for nuclear power station of VVER type. This article is focused on optimalization of fundamental utility properties and heat treatment process of the steel under investigation to increase toughness and critical temperature of brittleness. An intecritical annealing has been chosen as a useful tool for such improvement. Mechanical properties and impact notch toughness values after the intercritical annealing and conventional heat treatment regimes are compared. Effect of simulation of stress relieving including minimum and maximum allowable heating regimes are studied too. Intercritical annealing was added to the conventional quality heat treatment process, between quenching (normalizing) and tempering. The application of intercritical annealing improved the impact toughness significantly, an increase in the upper shelf energy and a decrease in transition temperature of steel under investigation.